Note: Descriptions are shown in the official language in which they were submitted.
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Amplifiers and Light Sources Employing S-Band Erbium-Doped
Fiber and L-band Thulium-Doped Fiber With Distributed
Suppression of Amplified Spontaneous Emission(ASE)
RELATED APPLICATIONS
This application is related to U.S. Application numbers
09/825,148, filed 2 April 2001, 10/095,303, filed 8 March
2002, 10/163,557, filed 5 June 2002, 10/194,680, filed 12 July
2002, and 10/348,802, filed 21 January 2003.
~.o
FIELD OF THE INVENTION
The present invention relates generally to fiber amplifiers
with a W-profile, and in particular to S-band Er-doped fiber
amplifiers with depressed cladding and distributed suppression
of amplified spontaneous emissions (ASE) in the C- and L-
bands, to a Tm-doped fiber amplifier for amplification in the
L-band, to a method for fabricating such fibers, to a method
for designing such fiber amplifiers, and to light sources
employing such fiber amplifiers for producing broadband and
2o narrowband light in the S-band.
BACKGROUND OF THE INVENTION
Optical waveguides are designed to guide light of various
modes and polarization states contained within a range of
wavelengths in a controlled fashion. Single-mode optical
fiber is the most common waveguide for long-distance delivery
of light. Other waveguides, such as diffused waveguides, ion-
exchanged waveguides, strip-loaded waveguides, planar
waveguides, and polymer waveguides are commonly used for
so guiding light over short distances and especially for
combining or separating light of different wavelengths,
optical frequency mixing in nonlinear optical materials,
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modulating light and integrating many functions and operations
into a small space.
In essence, a waveguide is a high refractive index material,
usually referred to as the core in an optical fiber, immersed
in a lower index material or structure, usually referred to as
the cladding, such that light injected into the high index
material within an acceptance cone is generally confined to
propagate through it. The confinement is achieved because at
so the interface between the high and low index materials the
light undergoes total internal reflection (TIR) back into the
high index material.
The performance of fiber amplifiers depends on a number of
s5 parameters including pumping efficiency, level of population
inversion of the ions in the active core, amplified
spontaneous emission (ASE) competing with the useful amplified
signal, cross-sections and refractive indices of the active
core and of the cladding surrounding the active core. In many
2o fiber amplifiers ASE is a major obstacle to effective
amplification of the desired signal and thus ASE has to be
suppressed.
The problem of amplifying optical signals for long distance
2s transmission was successfully addressed by the development of
Erbium doped fiber amplifiers (EDFAs). An EDFA consists of a
length of silica fiber with the core doped with ionized atoms
(Er3+) of the rare earth element Erbium. The fiber is pumped
with a laser at a wavelength of 980 nm or 1480 nm. The doped,
3o pumped fiber is optically coupled with the transmission fiber
so that the input signal is combined with the pump signal in
the doped fiber. An isolator is generally needed at the input
and/or output to prevent reflections that would convert the
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amplifier into a laser. Early EDFAs could provide 30 to 40 dB
of gain in C-band extending between 1530 to 1565 nm with noise
figures of less than 5 dB. Recently, EDFAs have been
developed that can provide 25 dB of gain in the L-band (1565
to 1625 nm) as well as in the C-band.
There is great interest in the telecommunications industry to
make use of the optical spectrum range with wavelengths
shorter than those currently achievable with conventional C-
1o band and L-band EDFAs. This wavelength range, commonly called
the "S-band" or "short-band" is poorly defined because there
is no consensus on the preferred amplifier technology. In
general, however, the S-band is considered to cover
wavelengths between about 1425 nm and about 1525 nm.
The gain in the S-band typically observed in EDFAs is limited
by several factors, including incomplete inversion of the
active Erbium ions and by amplified spontaneous emissions
(ASE) or lasing from the high gain peak near 1530 nm.
2o Unfortunately, at present no efficient mechanism exist for
suppressing ASE at 1530 nm and longer wavelengths in an EDFA.
Most waveguides are designed to prevent injected light from
coupling out via mechanisms such as evanescent wave out-
coupling (tunneling), scattering, bending losses and leaky-
mode losses. A general study of these mechanisms can be found
in the literature such as L.G. Cohen et al., "Radiating Leaky-
Mode Losses in Single-Mode Lightguides with Depressed-Index
Claddings", IEEE Journal of Quantum Electronics, Vol. QE-18,
so No. 10, October 1982, pp. 1467-72. In this reference the
authors describe the propagation of light in more complex
lightguides with claddings having a variation in the
refractive index also referred to as depressed-clad fibers.
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L.G. Cohen et al. teach that varying the cladding profile can
improve various quality parameters of the guided modes while
simultaneously maintaining low losses. Moreover, they observe
that depressed-index claddings produce high losses to the
fundamental mode at long wavelengths. Further, they determine
that W-profile fibers with high index core, low index inner
cladding and intermediate index outer cladding have a certain
cutoff wavelength above which fundamental mode losses from the
to core escalate. These losses do not produce very high
attenuation rates and, in fact, the authors study the guiding
behavior of the fiber near this cutoff wavelength to suggest
ways of reducing losses.
l5 U.S. Pat. Nos. 5,892,615 and 6,118,575 teach the use of W-
profile fibers similar to those described by L.G. Cohen, or QC
fibers to suppress unwanted frequencies and thus achieve
higher output power in a cladding pumped laser. Such fibers
naturally leak light at long wavelengths, as discussed above,
2o and are more sensitive to bending than other fibers. In fact,
when bent the curvature spoils the W or QC fiber's ability to
guide light by total internal reflection. The longer the
wavelength, the deeper its evanescent field penetrates out of
the core of the fiber, and the more likely the light at that
25 wavelength will be lost from the core of the bent fiber.
Hence, bending the fiber cuts off the unpreferred lower
frequencies (longer wavelengths), such as the Raman scattered
wavelengths, at rates of hundreds of dB per meter.
so Unfortunately, the bending of profiled fibers is not a very
controllable and reproducible manner of achieving well-defined
cutoff losses. To achieve a particular curvature the fiber
has to be bent, e.g., by winding it around a spool at just the
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right radius. Different fibers manufactured at different
times exhibit variation in their refractive index profiles as
well as core and cladding thicknesses. Therefore, the right
radius of curvature for the fibers will differ from fiber to
fiber. Hence, this approach to obtaining high attenuation
rates is not practical in manufacturing.
Moreover, the relatively high absorption losses and low gains
over the S-band render the selection of fiber and fiber
to profile in producing an EDFA that amplifies signals in the S-
band very difficult. In fact, the problems are so severe that
the prior art teaches interposition of external filters
between EDFA sections to produce an S-band EDFA.
For example, Ishikawa et al. disclose a method of fabricating
an S-band EDFA by cascading five stages of silica-based EDFA
and four ASE suppressing filters in Ishikawa et al., "Novel
1500 nm-Band EDFA with discrete Raman Amplifier", ECOC-2001,
Post Deadline Paper. In Ishikawa et al.'s experimental setup,
zo the length of each EDA is 4.5 meters. The absorption of each
suppressing filter at 1.53 ~m is about 30 dB and the insertion
losses of each suppressing filter at 1.48 ~m and 0.98 ~m are
about 2 dB and 1 dB respectively. The pumping configuration
is bi-directional, using a 0.98 ~m wavelength to keep a high
population inversion of more than D>-0.7 (D refers to relative
inversion). The forward and backward pumping powers are the
same and the total pumping power is 480 mW. Ishikawa et al.
show a maximum gain of 25 dB at 1518.7 nm with 9 dB gain tilt.
3o In a similar vein, U.S. Pat. No. 5,260,823 to Payne et al.
teaches an EDFA with shaped spectral gain using gain-shaping
filters. The inventors take advantage of the fact that the
EDFA is distributed to interpose a number of the gain-shaping
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filters along the length of the EDFA, rather than just placing
one filter at the end of the fiber. Yet another example of an
approach using a number of filters at discrete locations in a
wide band optical amplifier is taught by Srivastava et al. in
s U.S. Pat. No. 6,049,417. In this approach, the amplifier
employs a split-band architecture where the optical signal is
split into several independent sub-bands, which then pass in
parallel through separate branches of the optical amplifier.
The amplification performance of each branch is optimized for
to the sub-band which traverses it.
Unfortunately, these prior methods are complicated and not
cost-effective, as they require a number of filters.
Specifically, in the case of Ishikawa et al., five EDFAs, four
15 ASE suppressing filters, and high pump power are required.
Also, each of the ASE suppressing filters used by either
method introduces an additional insertion loss of 1-2 dB. The
total additional insertion loss is thus about 4-8 dB.
2o Another approach to providing amplification in the S-band has
focused on fiber amplifiers using Thulium as the lasing medium
doped into a Fluoride fiber core (TDFAs). See, for example,
"Gain-Shifted Dual-Wavelength-Pumped Thulium-Doped-Fiber
Amplifier for WDM Signals in the 1.48-1.51-~.m Wavelength
2s Region" by Tadashi. Kasamatsu, et. al., in IEEE Photonics
Technology Zetters, Vol. 13, N~. 1, January 2001, pg. 31-33
and references therein. While good optical performance has
been obtained using TDFAs, this performance has only been
possible using complex, non-standard and/or expensive pumping
3o schemes. Also, TDFAs suffer from the problems inherent to
their Fluoride fiber host material, namely high fiber cost,
poor reliability and difficulty splicing to standard silica
fibers used elsewhere in the amplifier system.
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Still other approaches to producing amplification systems
based on rare-earth doped fiber amplifiers and cascaded
amplifiers or pre-amplifiers followed by amplifiers are
s described in U.S. Patents 5,867,305 5,933,271 and 6,081,369
to Waarts et al. and in U.S. Patent 5,696,782 to Harter et al.
The teachings in these patents focus on deriving high peak
power pulses at high energy levels. The amplifiers described
in these patents are not suitable for producing broadband and
1o narrowband sources for the S-band.
In view of the aforementioned difficulties in obtaining high
attenuation rates in W-profile fibers as well as the selection
of fiber and fiber profile in producing an EDFA that amplifies
15 signals in the S-band, more recent prior art teaches
distributed suppression of ASE at wavelengths longer than a
cutoff wavelength in fiber amplifiers such as EDFAs. This is
achieved by engineering fiber parameters including the index
profile and cross sections of the core and cladding layer
2o including the use of a V~1-profile refractive index. The
approach is discussed in more detail in the above-referenced
U.S. Patent Application 10/095,303.
Although effective in suppressing ASE at wavelengths longer
25 than the cutoff wavelength, the EDFA's cross-section enables
the coupling of radiation at wavelengths below the cutoff
wavelength between the core and the cladding. This effect,
also known as cladding mode resonance, produces artifacts or
cladding mode coupling losses in the short wavelength range of
3o interest where the signal is to be amplified. For a general
discussion of cladding mode coupling losses the reader is
referred to Akira Tomita et al., "Mode Coupling Zoss in
Single-Mode Fibers with Depressed Inner Cladding", Journal of
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Lightwave Technology, Vol. LT-1, No. 3, September 1983, pp.
449-452.
Cladding mode loss is a problem encountered in fiber Bragg
s gratings. One solution is to extend a photosensitive region
in the core beyond the core to suppress cladding mode losses
as taught in U.S. Pat. No. 6,351,588 to Bhatia et al, entitled
"Fiber Bragg Grating with Cladding Mode Suppression". U.S.
Pat. No. 6,009,222 to Dong et al. also teaches to take
so advantage of a W-profile refractive index to confine the core
mode and cladding modes thus reducing their overlap and
coupling. Related alternatives to confining the core mode to
suppress cladding mode losses are found in U.S. Pat. No.
5,852,690 to Haggans et al. and U.S. Pat. No. 6,005,999 to
15 Singh et al.
Unfortunately, the approaches which are useful in suppressing
cladding mode losses and avoiding cladding mode resonance in
fiber Bragg gratings can not be applied to fiber amplifiers.
ao That is because of fundamental differences in fabrication,
construction and operating parameters between fiber Bragg
gratings and fiber amplifiers with distributed suppression of
ASE.
25 Clearly, there is a need for a fiber amplifier with
distributed suppression of ASE at wavelengths longer than a
cutoff wavelength that is able to suppress cladding mode
resonance or the coupling of radiation between the core and
cladding at wavelengths shorter than the cutoff wavelength.
3o It would be particularly useful to provide an EDFA having
these capabilities where the wavelengths below the cutoff
wavelength are contained in the S-band. Moreover, it would be
an advance in the art to provide a fiber amplifier exhibiting
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net gains over the S-band with low pump power and without
requiring external filters. In particular, it would be an
advance to provide an EDFA with distributed ASE suppression in
the C-band and L-band or substantially at 1530 nm and longer
wavelengths over the whole length of a fiber amplifier. It
would be also a welcome advance in the art to provide a method
of designing such fiber amplifiers with net gain over the S-
band as well as reliable narrowband and broadband light
sources employing such fiber amplifiers that can be used for
so testing optical components, measuring the performance of
optical components and generating signals in the S-band.
OBJECTS AND ADVANTAGES
It is a primary object of the present invention to provide a
fiber amplifier that yields losses exceeding any high gains in
a long wavelength band and at the same time yields losses
substantially smaller than any positive gains in a short
wavelength band. In particular, it is an object of the
invention to provide an Er-doped fiber amplifier (EDFA) in
2o which the long wavelength band is the C-band and L-band and
the short wavelength band is the S-band. More specifically,
the EDFA is to provide suppression of amplified spontaneous
emission (ASE) near 1525 nm and above and ensure positive
gains of at least 15 dB over the S-band.
It is an object of the invention to provide such fiber
amplifier in a W-profile (or depressed cladding) fiber and use
the fiber's index profile to eliminate the need for external
filters and reduce the required pump power by controlling a
3o roll-off loss curve.
It is another object of the present invention to provide a
fiber amplifier with distributed suppression of amplified
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spontaneous emissions (ASE) above a certain cutoff wavelength
and suppression of cladding mode loss at wavelengths shorter
than the cutoff wavelength. In particular, it is an object of
the invention to provide an Erbium-doped fiber amplifier
having these capabilities.
It is a further obj ect of the invention to provide short-pass
fibers that use a depressed cladding geometry to define a
cutoff wavelength and an associated roll-off loss curve. In
to particular, the invention provides a reliable method for
drawing fibers that contain various types of dopants,
including active materials such as rare earth ions.
It is another object of the invention to provide reliable
i5 narrowband and broadband light sources in the S-band of
wavelengths with Er-doped fibers or Erbium-doped fiber
amplifiers (EDFAs).
It is yet another object of the invention to provide a
2o Thulium-doped silica fiber having its strongest gain at a
range from about 1.6 ~m to about 1.7 Vim.
These and numerous other advantages of the present invention
will become apparent upon reading the following description.
SUML~1ARY
The objects and advantages of the invention are achieved by a
source generating light in an S-band of wavelengths using a W-
profile fiber having a core doped with an active material such
3o as Neodymium, Erbium or Thulium ions. The fiber core has a
certain cross section and a refractive index no. An active
material or lasant is doped into the core for amplifying
light, e.g., any information-bearing light beam. The fiber's
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core is surrounded by a depressed cladding having a depressed
cladding cross-section and a refractive index nl.
Furthermore, the fiber has a secondary cladding surrounding
the depressed cladding. The secondary cladding has a
secondary cladding cross-section and a refractive index n2.
In an embodiment, a pump source is provided for pumping the
Erbium contained in the core to a high relative inversion D,
such that the Erbium exhibits positive gains in the S-band and
high gains in a long wavelength band longer than the S-band.
1o The core cross-section, the depressed cladding cross-section,
and the refractive indices no, nl, and n2 are selected to
produce losses at least comparable to the high gains in the
long wavelength band and losses substantially smaller than the
positive gains in the S-band.
In another embodiment, the core cross-section, the depressed
cladding cross-section and the refractive indices no, nl, and
n2 are selected to obtain a roll-off loss curve about a cutoff
wavelength ~,~. The roll-off loss curve yields losses at least
2o comparable to the high gains in the long wavelength band and
losses substantially smaller than the positive gains in the
short wavelength band.
In order to obtain the desired roll-off loss curve the
refractive index no in the core is selected such that an
effective index neff experienced by a mode of radiation which
is guided, e.g., the fundamental mode at wavelength shorter
than the cutoff wavelength, is large. In particular,
refractive index no is selected such that the slope of the
3o effective index neff experienced by the confined mode is
maximized, thereby maximizing a roll-off slope of the roll-off
loss curve before the cutoff wavelength 7~,~. Preferably, the
refractive index no is selected such that the slope of the
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effective index neff is in the range of .002/100nm to
.008/1000nm. In another preferred embodiment, the refractive
index no of the core is chosen such that the roll-off slope of
the roll-off loss curve is greater than or about equal to the
s maximum slope of the gain spectrum. In this embodiment, it is
possible to select a cutoff wavelength such that the
distributed loss exceeds the gain for all wavelengths in the
long wavelength band, but that the gain exceeds the
distributed loss for all wavelengths in the short wavelength
1o band.
Depending on the design of the roll-off loss curve, the cutoff
wavelength 7~~ can be contained in the long wavelength band or
in the short wavelength band, or between the short and long
is wavelength bands.
It is important that the selection of the cross-sections,
i.e., the radii, and the selection of indices of refraction be
not performed merely to establish a ratio of radii or
2o refractive indices, but to fix absolute differences between
them. Thus, it is preferable that the refractive index no of
the core differ from the refractive index n~ of the secondary
cladding by about 0.005 to about 0.03. Also; the refractive
index n1 of the depressed cladding should differ from the
25 refractive index n2 of the secondary cladding by about -0.004
to about -0.02.
In the preferred embodiment the fiber amplifier uses Er as the
active material, i.e., it is an Er-doped fiber amplifier
30 (EDFA) doped with a concentration of 0. 1 o wt. of Er. In this
case it is further preferred that the short wavelength band is
selected to be at least a portion of the S-band and the long
wavelength band is selected to be at least a portion of the C-
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band and/or L-band. Further, it is advantageous to set the
cutoff wavelength 7~~ near 1525 nm in this embodiment. The
host material used by the fiber amplifier is preferably a
silicate-containing glass such as alumino-germanosilicate
glass or phosphorus doped germanosilicate glass.
The pump source providing the pump radiation to invert the
population in the Er ions can be any suitable pump source.
For example, the pump source is a laser diode emitting pump
to radiation at about 980 nm. Alternative sources delivering
pump radiation at about 980 nm can also be used. It is
preferred that pumping is in-core pumping.
The fiber amplifier of the invention can be used in fibers of
s5 various cross-sectional profiles. For example, the core-cross
section can have the shape of a circle, an ellipse, a polygon
or another more complex shape. The same is true for the
depressed cladding cross-section. The circular cross-sections
can be used if no preferential polarization is to be amplified
2o by the fiber amplifier. The eliptical cross-section can be
used when a particular polarization is to be maintained during
amplification over an orthogonal polarization.
For proper operation of the fiber amplifier it is important
25 that the pump source provide pump radi-ation at a sufficient
intensity to ensure a high relative inversion D, specifically
D>-0.7. This is especially important in the preferred
embodiment where the active material is Er.
3o Fiber amplifiers designed in accordance with the invention can
be used in any situation where high gains are produced in a
long wavelength band adjacent a short wavelength band in which
the signal to be amplified is contained. In these situations
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the ASE from the long wavelength band will tend to prevent
amplification of signals in the short wavelength band,
especially when the positive gains in the short wavelength
band are low in comparison to the high gains in the adjacent
long wavelength band. The design is particularly useful in
EDFAs to amplify signals in the short wavelength S-band. For
this purpose the cutoff wavelength ~,~ is preferably set at
1525 nm and the roll-off loss curve is selected to yield
losses of at least 100 dB in the C-band and L-band to suppress
1o ASE from the 1530 nm gain peak. Meanwhile, the roll-off loss
curve is also adjusted to yield losses in the S-band which are
smaller by at least 5 dB than the positive gains in the S-band
to allow for signal amplification. This relationship will
ensure at least a 5 dB amplification in the S-band.
In accordance With another embodiment of the invention several
fiber amplifiers made according to the method can be used to
amplify signals in the short wavelength band, e.g., the S-
band. The length L of each of the fiber amplifiers can be
2o varied to obtain the desired amount of gain for separate
portions of the S-band.
In some embodiments the arrangerilent for suppressing coupling
between the active core and the cladding is a material
distributed in the cladding. The material can be a scattering
material or an absorbing material. For example, a rare earth
element can be used as the absorbing material.
Preferably, the cladding has a depressed cladding having a
3o depressed cladding cross-section and a refractive index nl and
a secondary cladding having a secondary cladding cross-section
and a refractive index n~. The scattering or absorbing
material is distributed in the secondary cladding. The
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radiation propagating in the active core occupies a mode
having a mode diameter. The mode diameter extends from the
active core into the cladding. It is important that the
material be distributed outside the mode diameter of the
s radiation.
In some embodiments the arrangement for suppressing coupling
between the active core and the cladding is a non-phase-
matched length section in the fiber amplifier. The non-phase-
to matched length section is built such that coupling of the
radiation between the active core and the cladding is not
phase matched. In these embodiments the core has a core
cross-section and a refractive index no and the cladding has a
cladding cross-section and a refractive index n~laa. The non-
15 phase-matched length section is formed by a predetermined
selection of the core cross-section, cladding cross-section
and refractive indices no, rl~lad~ Preferably, the cladding has
a depressed cladding having a depressed cladding cross-section
and refractive index nl, a secondary cladding having a
2o secondary cladding cross-section and a refractive index n2.
The non-phase-matched length section is formed by a
predetermined selection of the cross-sections and refractive
indices no, nl, n2. Even more preferably, the cladding has an
outer cladding having an outer cladding cross-section and a
2s refractive index n3 and n3 is selected such that n3<n2.
The fiber amplifier can contain any suitable active medium in
its active core. For example, the active core can be doped
with Neodymium, Erbium, or Thulium ions. V~lhen using Erbium,
3o the fiber amplifier is an EDFA and in one advantageous
embodiment its cutoff wavelength ~,~ is set near 1525 nm.
Thus, the EDFA is pumped by a pump source delivering radiation
at a pump wavelength near 980 nm. Under these conditions the
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EDFA can be used for amplifying signals in the short
wavelength range falling within the S-band.
In another example, Thulium is doped into fused-silica fibers.
Although the Thulium gain is typically thought to be at 1.9
microns, and indeed that is the peak of the gain, the
wavelength range over which gain is possible stretches from
1.5 microns to 2.1 microns. The typical Thulium pump
wavelength is 0.78 microns. However, it is also possible to
1o pump Thulium at 1.48 microns, though very high intensities
would be needed, possibly as high as 100 mW. 100mV~1 at 1.48
microns is easily obtainable with commercially available high
quality diode pumps with about 500mW at 1480nm and nearby
wavelengths. Another good pump wavelength is 1530nm where
high power sources, up to watts, are available.
The gain cross-section and the upper-laser-level lifetime of
the Thulium ion are similar to those of the Erbium ion which
is conventionally used to make 1.5 micron amplifiers. Thus
2o the threshold for gain is similar - several milliwatts of pump
power are required.
The Thulium ion could be used on the short-wavelength end of
its gain region in exactly the same way as the Erbium ion. By
pumping with an intense pump (30 mV~l or so) it is possible to
reach inversion even at short wavelengths. However, before
high gain is reached at a short wavelength such as 1.6
microns, there will be overwhelming superfluorescence near 1.9
microns.
A useful amplifier can be made at the shorter wavelength if
the fiber is designed with a fundamental mode cut-off between
1.9 microns and the shorter wavelength of desired operation,
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and if the cut-off is such that the increase in loss at longer
wavelengths exceeds the increase in gain due to the higher
cross-section. This technique makes it possible to build
useful amplifiers in the wavelength range between about 1.6 to
s 1.8 microns. Since telecommunication fiber is highly
transmissive in this range, it is anticipated that amplifiers
that work in this wavelength range will be highly desirable.
In accordance with the invention fiber amplifiers can be
to designed to suppress cladding mode loss. This is done in
fibers where an appropriate index profile in the active core
and cladding is established to set a cutoff wavelength ~
Cutoff wavelength 7~,C is set such that the fiber amplifier
exhibits positive gains in a short wavelength range below the
is cutoff wavelength ~,~. The coupling of radiation in the short
wavelength range between the core and cladding is suppressed.
This is achieved by distributing a material that scatters or
absorbs the radiation in the cladding of the fiber amplifier.
Preferably, the material is located outside the mode diameter
20 of the radiation propagating through the active core. In
another embodiment, the coupling is suppressed by preventing
phase matching such that the coupling of radiation between the
core and cladding is not phase matched. This can be achieved
by engineering the cross-sections and refractive indices of
25 the core and cladding in accordance with the invention.
When using the source as a narrowband. source a wavelength
selecting mechanism is provided for selecting an output
wavelength of the light. This mechanism can be a feedback
3o mechanism such as a fiber Bragg grating. In other embodiments
the wavelength selecting mechanism is a filter selected from
the group consisting of tilted etalons, strain-tuned fiber
Bragg gratings, temperature-tuned fiber Bragg gratings,
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interferometers, arrays waveguide gratings, diffraction
gratings and tunable coupled cavity reflectors.
Alternatively, or in combination with the feedback mechanism
or filter an additional pump source adjustment for tuning the
s high relative inversion D can be used to select the output
wavelength. In yet another alternative, or in combination
with the previous mechanism or mechanisms, a coiling diameter
of the fiber can be used to select the output wavelength. The
coiling diameter can be constant or variable, e. g. , it can be
to continuously variable.
The fiber of the source can be placed within an optical
cavity, e.g., in cases where it is desired that the fiber
operate as a laser for producing light at a specific narrow
15 output wavelength. Preferably the cavity is a ring cavity.
In one embodiment of the source, a master oscillator is used
for seeding the fiber. The master oscillator can be any
suitable optical source such as a distributed feedback laser,
2o a Fabry-Perot laser, an external cavity diode laser, a
distributed Bragg reflector laser, a vertical cavity surface
emitting laser, a semiconductor laser, a fiber laser or a
broadband source.
25 In a preferred embodiment, the fiber is broken up into two
sections. The first section of the fiber has a first coiling
diameter and the second section has a second coiling diameter
larger than the first coiling diameter. The first section,
whose emission spectrum is centered at a shorter wavelength,
3o is positioned before the second section whose emission
spectrum is centered at a longer wavelength. In this
configuration the output from the first section is used to
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seed the second section. In some embodiments an isolator is
installed between the two sections.
In another embodiment the first section is designed such that
the core cross-section, the depressed cladding cross-section,
and the refractive indices no, n1, and n2 produce a first
cutoff wavelength 7~~1. Meanwhile, the core cross-section, the
depressed cladding cross-section, and the refractive indices
no, nl, and n2 in the second section are designed to produce a
to second cutoff wavelength ~,~2 that is longer than the first
cutoff wavelength 7~,~1. In this embodiment the first section
produces an emission spectrum centered at a shorter wavelength
and the second section produces an emission spectrum centered
at a longer wavelength. Once again, the first section is
positioned before the second section for seeding the second
section. An isolator can be installed between the two
sections in this embodiment.
The pump source for pumping the Erbium in the core of the
2o fiber is preferably a laser diode. For example, one can use a
laser diode providing pump light at about 980 nm. In
accordance with the method of the instant invention it is
preferable to use a counter-propagating pumping arrangement
to pump the Erbium. In other words, the pump light is
counter-propagating with respect to the output light.
The source of the invention can be used for testing and
measuring purposes as well as for generating output light in
the S-band. The source can be operated in a continuous mode
so or in a pulsed mode, as desired. The output light generated
by the fiber can also be combined with light outside the S-
band, e.g., with light in the C- and L-bands.
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A detailed description of the invention and the preferred and
alternative embodiments is presented below in reference to the
attached drawing figures.
BRIEF DESCRIPTION OF THE FIGURES
Fig. 1 is a diagram illustrating a W-profile fiber and guided
and unguided modes according to the invention.
Fig. 2 is a graph illustrating a typical index profile in
the
fiber of Fig. 1.
so Fig. is a graph illustrating the selection of appropriate
3
core index no to ensure that the effective index
experienced by a guided mode in the short wavelength
band of interest is maximized.
Fig. 4 is a graph illustrating appropriate selection of the
s5 core index to obtain a suitable roll-off loss curve
in
an Er-doped fiber amplifier (EDFA) in accordance with
the invention.
Fig. 5 is a graph of the absorption and gain cross sections
of Er ions in alumino-germanosilicate glass.
2o Fig. 6 is an isometric view of an EDFA operated in accordance
with the invention.
Fig. 7 are graphs of net gain in a 6 meter long alumino-
germanosilicate EDFA doped at 0.1o wt. with a mode
overlap factor h=0.5 at various inversion values D.
25 Fig. 8 are graphs of net gain spectra in an alumino-
germanosilicate EDFA at inversion values between D=0.4
and D=1 for fiber lengths between 5 meters and 13
meters chosen to maintain 45 dB gain at 1530 nm.
Fig. 9 are graphs of gain spectra for a 15 meter long
so alumino-germanosilicate EDFA at inversion values
between D=0.6 and D=1.
Fig. 10 is a diagram illustrating the use of three EDFA
amplifiers to amplify three portions of the S-band.
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Fig. 11 is a graph illustrating the gain spectra for the three
EDFAs of Fig. 10
Fig. 12 illustrates the cross-section of another fiber
amplifier with an elliptical core and depressed
cladding.
Fig. 13 is a diagram illustrating a partial cross-section of a
fiber amplifier in accordance with the invention and
illustrating a core mode and a cladding mode.
Fig. 14 is a graph illustrating a typical index profile in the
so fiber of Fig. 13.
Fig. 15 is a graph illustrating the effects of cladding mode
losses in the fiber of Fig. 13.
Fig. 16 are graphs illustrating the effects of an absorbing
polymer material embedded in outer cladding of the
fiber of Fig. 13.
Fig. 17 is a diagram illustrating a partial cross-section of
another fiber amplifier according to the invention.
Fig. 18 is a graph illustrating the phase-matching condition
between core modes and cladding modes in the fiber
2o amplifier of Fig. 17.
Fig. 19 are graphs of power levels of radiation in core mode
and cladding mode.
Figs. 20A&B are graphs illustrating the effective index neff
experienced by the core mode and cladding modes.
Figs. 21A&B are cross-sectional view of alternative fiber
amplifiers in accordance with the invention.
Fig. 22 is a diagram illustrating the use of an EDFA in a
fixed narrowband source according to the invention.
Fig. 23 is a graph illustrating the typical shaped of the ASE
3o emission spectrum of the EDFA used in the source of
Fig. 22.
Fig. 24 is a diagram illustrating the use of an EDFA in an
alternative source according to the invention.
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Fig. 25 is a diagram illustrating a source using a single EDFA
in a ring cavity.
Fig. 26 is a diagram illustrating a source using two EDFAs in
a parallel configuration in a ring cavity.
s Fig. 27 is a diagram illustrating a source using an EDFA's
coiling diameter for output wavelength tuning.
Fig. 28 is a diagram illustrating a source using two fiber
sections in accordance with the invention.
Fig. 29 is a graph illustrating the effects of seeding EDFA
to having a longer wavelength emission spectrum by an
EDFA having a shorter wavelength emission spectrum.
Fig. 30 is a graph illustrating the ASE emission spectra for
EDFAs at several coiling diameters.
Fig. 31 is a graph illustrating the effect of using different
15 pump power levels in two EDFA sections separated by an
isolator on the total ASE emission spectrum.
Fig. 32 is a diagram of a source with two EDFAs having
different coiling diameters separated by an isolator.
Fig. 33 is a diagram of a source with two EDFAs having
2o different coiling diameters and employing a single
pump source.
Fig. 34 is a diagram illustrating an S-band source using a
master oscillator.
Fig. 35 is a diagram illustrating the use of an S-band source
25 in a testing or measuring application in accordance
with the invention.
Fig. 36 is a diagram illustrating the pulling of a preform
into a short-pass fiber with a depressed-profile.
Fig. 37A is a graph illustrating the transverse portion of the
3o refractive index profile in the preform of Fig. 36.
Fig. 37B is a graph illustrating the longitudinal portion of
the refractive index profile in the preform of Fig.
36.
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Fig. 38 are graphs of exemplary roll-off loss curves obtained
with the method of the invention.
Fig. 39 shows the fluorescence spectrum of Thulium in fused
silica.
DETAILED DESCRIPTION
The instant invention will be best understood by first
reviewing the principles of generating a roll-off loss curve
in a depressed profile or W-profile fiber 10 as illustrated in
to Figs. 1-4. Fig. 1 is a diagram illustrating a portion of a
cross-section of a fiber 10 having a core 12 surrounded by a
depressed cladding 14. Depressed cladding 14 is surrounded by
a secondary cladding 16. Core 12 has a circular cross-
section, as do depressed cladding 14 and secondary cladding
16. A region I associated with core 12 extends from 0_<r<_ro,
depressed cladding 14 and secondary cladding 16 occupy regions
II, III extending between ro<_r_<r1 and r>-rl. Core 12 has an
index of refraction no, depressed cladding 14 has an index of
refraction nl and secondary cladding 16 has an index of
2o refraction n2. The graph positioned above the partial cross-
section of fiber 10 illustrates an average index profile 20
defining a W-profile in fiber 10. In the present embodiment
fiber 10 is a single mode fiber.
Fiber 10 has an active material 18 doped in core 12. Active
material 18 is a lasing medium such as a rare earth ion or any
other lasant which exhibits high gains in a long wavelength
band and positive gains in a short wavelength band.
Specifically, when pumped to a high relative inversion D, the
so high gains of active material 18 in the long wavelength band
cause amplified spontaneous emissions (ASE) or lasing which
reduces the population inversion of lasant 18 and thus reduces
the positive gains in the short wavelength band, making it
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impossible to effectively amplify signals in the short
wavelength band.
Fig. 2 illustrates a W-profile 20A as is obtained with normal
s manufacturing techniques. For the purposes of the invention
it is sufficient that the radially varying index of core 12
have an average value equal to no. Likewise, it is sufficient
that indices of depressed cladding 14 and secondary cladding
16 average out to the values n1 and n~ . The average index no
to of core 12 is significantly higher than index n1 of depressed
cladding 14 and index n2 of secondary cladding 16. The
selection of appropriate values of indices no, nl, n2 and radii
ro, r1, r2 is made to achieve certain guiding properties of
fiber 10, as required by the instant invention. Specifically,
15 profile 20 is engineered to have a fundamental mode cutoff
wavelength 7~,~ such that light in the fundamental mode at
wavelengths smaller than ~,~ is retained in core 12 while light
in fundamental mode at wavelength 7~,~ or longer wavelengths is
lost to secondary cladding 16 over a short distance. This
20 objective is accomplished by appropriately engineering W-
profile 20A.
Fundamental mode cutoff wavelength ?~~ of fiber 10 is a
wavelength at which the fundamental mode (the LPol mode)
25 transitions from low-losses to high losses in core 12, i.e.,
is cut off from core 12. First, the fundamental mode cutoff
wavelength 7~~ for fiber 10 is set in accordance to selection
rules for cross-sections and refractive indices no, n1 and n2
of fiber 10 as derived from Maxwell' s equations . In the weak
3o guiding approximation (which is valid when the indices of
refraction of core 12 and claddings 14, 16 are all relatively
close to each other), the Maxwell vector equations can be
replaced with a scalar equation. The scalar ~ represents the
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strength of the transverse electric field in the fiber. For
more information, see for example G. Agrawal, "Nonlinear Fiber
Optics" (Academic, San Diego, 1995), D. Marcuse, "Light
Transmission Optics" (Van Nostrand, Princeton, 1972), and D.
Marcuse, "Theory of Dielectric Optical Waveguides" (Academic,
New York, 1974 ) .
For convenience, let us define the following parameters:
io uo = not - n22 and ul = n22 - ~2 ( 1 )
The scalar field ~ inside fiber 10 satisfies a wave equation
whose solutions are Bessel functions and modified Bessel
functions. For the fundamental mode supported by fiber 10,
inside core 12 is thus:
W = Jo (K r) . ~~r~ro (region I) (2)
where K is an eigenvalue that needs to be determined, and Jo
2o is the wroth Bessel's function.
Inside depressed cladding 14, the scalar field W is:
= A Ko (~3 r) + B Io ((3 r) , ro_<r<-r1 (region II) (3)
where A and B are constants to be determined,
~3' _ (uoz + u12)(2~l ~,)2 -K2, and Ko and Io are the modified Bessel' s
functions. Here 7~ is the vacuum wavelength of the light.
so In secondary cladding 16, we obtain:
= C Ko ('y r ) , r>_rl ( region III ) ( 4 )
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Here C is another constant, and Yz =uoz(2~ ~~)2 -KZ . A, B, C,
and x are found using the boundary conditions, which require
that ~ and its first derivative are both continuous at ro and
rl~
It can be shown that fundamental mode cutoff wavelength ~,C is
a wavelength ~, at which 'y = 0 . ( See for example, Cohen et al . ,
IEEE J. Quant. Electron. QE-18 (1982) 1467-1472.)
so For additional convenience, let us define the following
parameters:
2~ uo~o
x=~ p = ul/uo, s = rllro. (5)
Now, fundamental mode cutoff wavelength ~,~ can be determined
if parameter x is determined. That determination can be made
with the aid of algebra known to a person skilled in the art,
since parameter x is the root of the following equation:
2o p Jo (x) Kl (px) Ii (psx) - p Jo (x) Ii (px) W (psx)
- J1(x) W (psx)Io(px) - J1(x) I1(psx) Ko(px) - 0.
(6)
Three observations should be made regarding the parameter x.
First, x does not exist for all values of s and p. For
example, for p = 1 and s<_ ~, there is no x that satisfies Eq.
(6). This means that all wavelengths are guided in core 12 in
this regime~ The criterion that Eq. (6) have a solution is:
3o s2 >_ 1 + 1 /p2 . ( 7 )
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Second, for practical applications x cannot be too small.
This is because, according to Eq. (5), the parameter x is
proportional to radius ro of core 12, and the radius has to be
large enough that it is easy to couple light into and out of
s core 12. (A smaller core 12 also makes the nonlinear effects
stronger, which is often a disadvantage.) Therefore, since x
- 2~cu0ro/7~,~, preferably x >- 1. This implies that p >_ 0.224 or,
in terms of the refractive indices (~ -~Zl ~/(~eo -n2) >_~.224 .
1o Third, it is evident from Fig.7 that for larger values of s,
the value of x only weakly depends on s. Thus it is
advantageous to have a fiber in this region of parameter
space, since a manufacturing flaw producing an error in s will
have a small effect on the value of fundamental mode cutoff
15 wavelength ~,~. Therefore, it is convenient to use the rule s
>_ 1 + 1/p, or in terms of the refractive indices:
>_ 1 + (t2o - y2z ll (j2z ~1 ~ '
ro
2o The selection of cross sections and refractive indices of core
12, depressed cladding 14 and outer cladding 16 is guided by
the above rules in setting the appropriate fundamental mode
cutoff wavelength ~,C. First, 7~C can be pre-selected, e.g. a
wavelength close to 1530 nm, and then convenient values are
25 selected for uo and ro. Based on these choices x is computed
from equation 5, and conveniently x>-1 (otherwise the previous
choices can be adjusted). Then, suitable values of s and p
are found using equation 6. A range of values for p and s
will yield desired 7~~. Typically, all values of p are larger
3o than 0.224. In addition, the rule of equation 8 is used to
further narrow the range of suitable values of p and s.
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Finally, the values of s and p have an additional limitation.
Namely, they must be selected so that core 12 of fiber 10 has
a great enough loss, e.g., 5 dB/m or even 100 dB/m or more at
a wavelength ~,>~,~. To find the loss at wavelength ?~>7~~, the
s fiber modes for light having wavelength 7~>7~,~ are required.
Equations (2), (3), and (4) specify the fundamental mode when
When 7~>~,~, the function 1~I is oscillatory, rather than
exponentially decaying, in secondary cladding 16. Therefore
to when ~,>7~~, Eq. (4) is replaced by:
= C Jo(qr) + D No(qr), r>_rl (region III) (9)
where No (also called Yo) is the wroth Neumann function,
15 fZ =Kz -uoz(~~./~)2 , and C and D are constants to be determined.
There are two key items to note regarding the modes for ~,>
First, there are five unknowns (A, B, C, D, and tc) and four
boundary conditions ( continuity of ~I and dt~/dr at ro and r~ ) .
2o The equations are underconstrained: K may be chosen to be any
value between 0 and (2?sl i1,) uo2 +ul2 . Thus, there is a continuum
of states for each 7~,>7~~, corresponding to the continuum of
values that K may have. This situation is quite ditterent
from the case 7~<7~~, where four unknowns (A, B, C, and K) are
25 fixed by the four boundary conditions, resulting in K being a
discrete eigenvalue having a unique value at each ~,<~,~.
Second, the modes specified by Eqs. (2), (3), and (9) are
eigenmodes of the fiber, e.g. a W-fiber; however, these modes
so do not correspond to the situation that is physically
realized. This is a result of Eq. (9) containing both
incoming and outgoing waves, whereas in practice only outgoing
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waves are present (the light at wavelength ~,>7~~ originally
propagating in core 12 radiates out).
Nevertheless, the modes of Eqs. (2), (3), and (9) can be used
to estimate the losses at wavelengths greater than 7~~. First,
for a given wavelength 7~, find the value of K that minimizes
C2 + D2. This corresponds to the mode that is the most long-
lived within the core. (An analogy can be made between the
wave equation for the scalar ~I in the fiber and the quantum
so mechanical wave equation for a particle in a potential well.
Then the quantum mechanical results can be borrowed. See for
example David Bohm, "Quantum Theory", Dover 1989, Chapter 12
X14 - 22 . )
s5 Second, once K is found in the above manner, the outgoing
waves can be computed from Eq. (9). These outgoing waves give
a reasonable estimation of the loss from core 12 into
secondary cladding 18, even when no incoming waves are
present. These outgoing waves will cause beam at wavelength
20 ~,>~,~ propagating in core 12 to be attenuated along the length
of the fiber. If the beam has power P, then the change in
power P with distance ~ along fiber 10 is described by the
equation:
dY _
25 ~ ~ . (10)
The loss is given by the coefficient 11, which is
approximately:
30 ~_ ~, CZ+D' . (11)
4?c2j~.o r°
f rdr t~ t/~
0
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The loss 11, having units of m l, can be converted to a loss (3
in units dB/m, using the relation:
s ~3 = 10 loglo (e) ~A
Here the term "loss" refers to radiation that leaks out of
core 12 into secondary cladding 16. In fact, the radiation
may not be truly lost from fiber 10 itself, if it remains in
so secondary cladding 16. In some cases this will be sufficient.
In other cases light from secondary cladding 16 can be out-
coupled, as necessary.
Another method for calculating the losses involves calculating
15 the complex propagation constant of the leaky fundamental mode
of fiber 10. Leaky modes are discussed in, for example, D.
Marcuse, "Theory of Dielectric Optical Waveguides" (Academic,
New York, 1974) Chapter 1. The loss is related to the
imaginary part of the complex propagation constant of the
20 leaky mode. The complex propagation constant, or its
equivalent that is the complex effective index of refraction,
may be computed using commercially available software, such as
that obtainable from Optiwave Corporation of Nepean, ON,
Canada.
In some cases it may be preferable to numerically solve for
the modes of a given fiber rather than use the Bessel function
approach outlined above, since real fibers do not have the
idealized step index profile indicated by profile 20 shown in
so Fig. 1, but have variations from the ideal as shown by graph
20A in Fig. 2 of the actual refractive index profile obtained
in practice. In particular, the most common method of single-
mode fiber manufacture today involves the MOCVD process, which
CA 02478314 2004-09-07
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typically leaves an index dip in the center of core 12.
' Numerical solutions can, more easily than the method described
above, take into account the actual variations in refractive
index as a function of radius. Such numerical calculations
s can again give fundamental mode cutoff wavelength ~,C and fiber
losses as a function of fiber parameters including cross-
sections and refractive indices, allowing fiber 10 to be
designed to exhibit the desired features.
so When Eq. (11) is used to estimate the loss, refractive indices
no, n1, and n2 will in general be average indices of refraction
of profile 20, since the actual indices of refraction will
vary somewhat as a function of radius (see profile 20A).
Also, the index of refraction n is not necessarily radially
is symmetric. If the cross section of fiber 10 is described by
polar coordinates r and 8 the refractive index may depend upon
the angle 8 as well as the radius r. Thus, n = n(r,9). Such
an asymmetric fiber may be desirable for polarization
maintenance, for example.
Here is the prerequisite for the fiber to have fundamental
mode cutoff wavelength ~,~. Let R be a radius large enough
that the index at radius R has substantially leveled off to
the value n2. Then fiber 10 will have fundamental mode cutoff
2s wavelength ~,~ if (see B. Simon, Ann. Phys. 97 (1976), pp.
279)
2~t R
f aef Y~r(~Z~Y,e>-~~2)<_0 . (13)
0 0
3o Note that given the profile of Fig. 1, Eq. (13) becomes:
rat uo2 - ~ ( r1~ - ro2 ) u12 <_ 0 , ( 14 )
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which is equivalent to Eq. (7) above.
Fundamental mode cutoff wavelength ~.~ is the largest
s wavelength for which there is an eigenmode that is localized
in region I. The losses for wavelengths above cutoff
wavelength ~,~ can be determined, for example, by (i) solving
for the modes that are not localized but include incoming and
outgoing waves, (ii) for each wavelength finding the mode with
Zo the smallest outgoing intensity, and (iii) using this outgoing
intensity to estimate the loss. As discussed above, other
methods are also available to a person skilled in the art for
calculating losses. In general, fiber 10 with a desired
fundamental mode cutoff wavelength 7~~ and losses can therefore
is be designed by adjusting the profile n(r,~), which is
equivalent to adjusting the cross-sections and refractive
indices of core 12, depressed cladding 14 and secondary
cladding 16.
2o The rules presented above will enable a person skilled in the
art can to set fundamental mode cutoff wavelength ~,C by making
a selection of ro, r1, no, nl and n2. This selection of ro, r1,
no, nl and n2 provide distributed ASE suppression over the
length of the fiber 10 and result in a family of loss curves
2s with different roll-offs (with respect to wavelength).
Therefore, additional constraints have to be placed on the
selection of ro, rl, no, nl and n~ to achieve the obj ectives of
the present invention, as discussed below.
3o Referring back to Fig. 1, superposed on average index profile
20 is an intensity distribution of a guided fundamental mode
22 at a first wavelength 7~1<~,~. First wavelength 7~,1 is
contained within a short wavelength band. A fundamental mode
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24 which is no longer guided by fiber 10 is also superposed on
index profile 20. Mode 24 is at cutoff wavelength 7~~. An
intensity distribution of another mode 26 which is not guided
by fiber 10 and exhibits an oscillating intensity distribution
s beyond core 12 and depressed cladding 14 is also shown.
Radiation in mode 26 has a second wavelength ~,2, which is
longer than cutoff wavelength 7~~<~,~ and is contained in a long
wavelength band.
so The graphs in Fig. 3 are plots of wavelength versus an
effective index neff experienced by guided mode 22 whose
wavelength ~,l is contained within a short wavelength band 42
and of non-guided mode 24 at cutoff wavelength 7~~ for three
choices of the value of index no of core 12. Specifically, at
is a lowest value of index nol of core 12, the effective index
neff experienced by mode 22 is described by graph 28. Graph 28
illustrates a relatively low value of effective index neff over
short wavelength band 42, i.e., over the entire range of
wavelengths ~,1 at which mode 22 is guided. In addition, the
2o value of neff remains very low in a region of interest 40 below
cutoff wavelength 7~,~. The choice of an intermediate value of
index not of core 12 produces graph 30. In this graph neff 1s
higher than in graph 28 over the entire short wavelength band
42. Still, the value of neff is low in region of interest 40.
25 A choice of a large value of index no3 produces graph 32,
which increases neff experienced by mode 22 over entire short
wavelength band 42 including region of~ interest 40. Given
such large value of refractive index no3 effective index neff
exhibits a large negative slope right before cutoff wavelength
30 ~,C in region of interest 40. Preferably, the value of
refractive index no3 is large enough such that this roll-off
slope is in the range of .002/1000 nm to .008/1000 nm. In a
preferred embodiment, the refractive index no of the core is
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at least 0.50 larger than the refractive index n2 of the
secondary cladding. Of course, a person skilled in the art
will realize that index no of core 12 can not be made
arbitrarily large to continue increasing the negative slope of
neff before 7~,~ due to material constraints .
Fig. 4 illustrates a gain profile 44 of active material 18
when pumped to a high relative inversion D. Short wavelength
band is designated by reference 42, as in Fig. 3, and long
1o wavelength band is designated by reference 46. Gain profile
44 exhibits high gains in long wavelength band 46 and positive
gains in short wavelength band 42. In particular, high gains
in long wavelength band 46 include a peak 48 very close to
short wavelength band 42.
In this embodiment the cross-sections or radii of core 12,
depressed cladding 14 and refractive indices no, ni, and n2 are
selected to place cutoff wavelength 7~~ right at peak 48.
Additionally, the value of index no of core 12 is selected to
obtain a roll-off loss curve 38 about cutoff wavelength 7~C set
at peak 48 of high gains in long wavelength band 46. More
particularly, roll-off loss curve 38 is selected to yield
losses at least comparable to the high gains in long
wavelength band 46 while yielding losses substantially smaller
than the positive gains in short wavelength band 42. Roll-off
loss curve 38 drops below the positive gains indicated by
profile 44 because of its rapid decrease or large positive
slope to the left for wavelengths below cutoff wavelength ~
The gains thus exceed losses across entire short wavelength
3o band 42, as better visualized by hatched area 50. Preferably,
roll-off loss curve 38 is such that the gains exceed the
losses in short wavelength band 42 by at least 5 dB.
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Curve 38 is obtained when neff experienced by guided mode 22 is
high and the slope of neff j ust below 7~C has a large negative
slope. In other words, curve 38 is obtained by selecting
index no3 for core 12. Roll-off loss curves obtained with
lower indices not and nol in core 12 are indicated by
references 36 and 34 respectively. Because neff and its slope
below ~,~ experienced by mode 22 can not be maximized by
choosing indices lower than no3, the roll-off slope is smaller
for curves 36 and 34 and thus the losses they introduce in
to short wavelength band 42 remain above the positive gains. As
long as losses exceed gains no useful amplification can be
produced by active material 18 in short wavelength band 42.
The W-profile fiber designed in accordance with the above
is rules finds its preferred embodiment when active material 18
is Er and the short wavelength band is the S-band or a select
portion of the S-band while the long wavelength band covers
the C-band and/or the Z-band or a select portion or portions
of these two bands. Preferably, the host material of fiber 10
2o is silicate-containing glass such as alumino-germanosilicate
glass or phosphorus doped germanosilicate glass.
Fig. 5 shows the wavelength dependent absorption cross-section
60 and wavelength dependent emission cross section 62 of Er-
25 doped alumino-germanosilicate glass. Other Er-doped glasses
have qualitatively similar gain (emission) and absorption
spectra. Note that the gain extends to wavelengths shorter
than 1450 nm, but the absorption cross section is much greater
than the emission cross section for all wavelengths with a
3o short wavelength band 64, in this case the S-band extending
from about 1425 nm to about 1525 nm. Specifically, absorption
cross section is much above emission cross section near 1500
nm. This indicates that high levels of relative population
CA 02478314 2004-09-07
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inversion D is required for Er to yield substantial net gain
in S-band 64. A long wavelength band 66, in this case the C-
band and the L-band extend from 1525 nm to 1600 nm and beyond.
The C- and L-bands exhibit high gains, especially in the C-
band at a peak wavelength of about 1530 nm. The choice of
alumino-germanosilicate glass or phosphorus doped
germanosilicate glass is preferred because when Er is doped
into these host materials the emission cross section is
increased in comparison to standard glass fiber. Other glass
so compositions which boost the emission cross section in S-band
64 relative to emission cross section 62 at the emission peak
near 1530 nm can also be used.
Fig. 6 shows an Er-doped fiber amplifier 68 (EDFA) using
alumino-germanosilicate glass as the host material. EDFA 68
is doped with a concentration of 0.1o wt. of Er in a core 70
of index no. Core 70 is surrounded by a depressed cladding 72
of index nl and a secondary cladding 74 of index n2. EDFA 68
has a protective jacket 76 surrounding secondary cladding 74
2o to offer mechanical stability and to protect EDFA 68 against
external influences.
A signal radiation 78 at a first wavelength 7~1 contained
within S-band 64 is delivered to EDFA 68 for amplification
25 from a fiber 80. For example, signal radiation 78 can be an
information-bearing signal requiring amplification.
Fiber 80 is coupled with a fiber 82 in a wavelength combiner
84. Fiber 82 is used to couple a pump radiation 88 from a
3o pump source 86 to EDFA 68. Pump source 86, preferably a laser
diode, provides pump radiation 88 at a pump wavelength 7~p of
about 980 nm for pumping the Er ions in core 70 to achieve a
high level of relative population inversion D. Parameter D
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varies from D=-1 indicating no population inversion to D=1
signifying complete population inversion. V~lhen D=0, exactly
half of the Er ions are in the excited energy state or
manifold of states, while half remain in the ground energy
manifold. In this case, EDFA 68 is approximately transparent
(for wavelengths near the 3-level transition at 1530 nm). For
non-uniformly inverted EDFAs, parameter D is considered as the
average value of inversion. In the present embodiment, the
intensity of pump radiation 88 is determined such that it
so ensures a relative inversion of D>-0.7 in the Er ions.
Pump radiation 88 and signal radiation 78 are combined in
combiner 84 and both delivered to EDFA 68 by fiber 90. More
particularly, both signal and pump radiation 78, 88 are
is coupled into core 70 from fiber 90.
Core 70 and claddings 72, 74 all have circular cross sections
in this embodiment. The cross sections and indices no, nl, n2
are selected in accordance with the method of invention to set
2o cutoff wavelength ~,~ near 1525 nm (see Fig. 5). In other
words, cutoff wavelength 7~~ is selected to be between short
wavelength band 64 or the S-band and the long wavelength band
66 or the C-band and L-band.
25 It is important that index n~ of core 70 be chosen to provide
for a large negative slope in effective index neff, preferably
about .008/1,000 nm, near cutoff wavelength 7~~. As a result,
the roll-off loss curve exhibits a rapid decrease for
wavelengths below cutoff wavelength ~,~ ensuring that the
so losses in S-band 64 are lower than the positive gains. The
losses produced by this roll-off loss curve increase rapidly
for wavelengths larger than cutoff wavelength ~,~. Thus, the
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losses produced in the C- and L-bands 66 are at least
comparable to the high gains.
Designing EDFA 68 in accordance with the invention will ensure
that signal radiation 78 at 7~1 is amplified while ASE at any
wavelength 7~2 in the C- and L-bands 66, and especially at
~,2=1530 nm is rejected into cladding 74 as shown. Positive
gains in S-band 64 will typically be on the order of 25 dB
above the losses and thus, to obtain sufficient amplification
so of signal radiation 78, EDFA 68 requires a certain length L.
The smaller the difference between the positive gains and
losses in the S-band 64, the longer length L has to be to
provide for sufficient amplification of signal radiation 78.
In the present embodiment L is about 6 meters.
l5
Fig. 7 shows the net gain (gain minus absorption) of EDFA 68
for L=6 meters and with a typical mode-overlap factor, r=0.5
without the benefit of the roll-off loss curve. The family of
curves represent various levels of inversion, from D=0.6
2o through D=1. Note that as the level of inversion is increased,
the net gain increases for all wavelengths. For full
inversion (D=1), the S-band 64 net gain ranges from 5-25 dB
over 1470-1520 nm, the C-band 66 net gain exceeds 30 dB, and
the 1530 nm gain peak exhibits over 55 dB of net gain. This
25 condition is, in practice, very difficult to achieve because
lasing at 1530 nm, and/or significant amplified spontaneous
emission (ASE) would occur at significantly lower values of
net gain (about 45 dB or lower), thereby limiting the
achievable level of inversion. The middle curve (D=0.8, with
so 900 of Er ions in the excited energy manifold) corresponds
approximately to this ASE-limited situation, with about 25 dB
of net gain within the C-band 66 and only about 10 dB of net
gain within the S-band 64.
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In prior art EDFAs this situation gets worse (for the S-band)
when a C-band EDFA is optimized for efficiency (dB gain per
unit pump power). This optimization results in somewhat
s longer (or more highly doped) fibers which become ASE-limited
(45 dB net gain at 1530 nm) with lower levels of inversion.
In summary, most EDFAs in use today operate with incomplete
inversion because of 1530 nm-ASE combined with the requirement
for good overall efficiency.
The relationship between the level of inversion, D, and the
net gain in the S-band 64 relative to the net gain in the C-
band 66 is shown in Fig. 8. A family of curves representing
the net gain spectra for EDFA 68 without the benefit of the
roll-off loss curve at inversion levels between D=0.4 and D=1,
and L between 5 meters and 13 meters is shown. Lengths L were
chosen in order to maintain 45 dB of net gain at 1530 nm, as
this situation corresponds approximately to the onset of ASE.
Note that the higher levels of inversion D favor gain in S-
2o band 64, while more moderate (D=0.4-0.6) levels of inversion
result in minimal gain-slope within the C-band 66. In other
words, an EDFA designed for use within the S-band 64 should
have nearly complete inversion, unlike an EDFA optimized for
use within the C-band 66. For this reason, in the preferred
embodiment the invention is maintained in the range D>-0.7.
Referring still to Fig. 8, one observes that the gain in S-
band 64 can not exceed ~5 dB at 1470 nm and ~20 dB at 1520 nm
if the 1530 nm gain is limited to 45 dB. To achieve higher
3o gain the length L of EDFA 68 has to be increased, while
maintaining a high level of inversion would to produce larger
gain in S-band 64. Fig. 9 shows the net gain spectra for EDFA
68 at inversion levels between D=0.6 and D=1, when length L is
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increased to 15 meters. V~lhile gain in S-band 64 exceeds 20 dB
for a bandwidth exceeding 30 nm, the 1530 nm gain is in excess
of >100 dB for D>0.7. Now, with the aid of the roll-off loss
curve engineered in EDFA 68 in accordance with the invention,
the losses at 1530 nm can be comparable or larger than this
gain, thus preventing ASE or lasing.
Fig. 10 illustrates an embodiment in which three EDFAs 102,
104, 106 doped with Er at 0.1o wt. all engineered in
to accordance with the invention are provided to amplify three
portions of the S-band. Four wavelength combiners 108, 110,
112, 114 are used to connect EDFAs 102, 104, 106 in accordance
with well-known splicing and wavelength combining procedures
to separately amplify the three portions of the S-band. EDFA
102 has a length of 10 meters and a cutoff wavelength ~,~ at
1520 nm, EDFA 104 has a length of 33 meters and a cutoff
wavelength 7~~ at 1490 nm, and EDFA 106 has a length of 143
meters with a cutoff wavelength ~,C at 1460 nm. EDFA 102
amplifies input in the 1490-1520 nm range, EDFA 104 amplifies
2o input in the 1460-1485 nm range and EDFA 106 amplifies input
in the 1435-1455 nm range. All EDFAs 102, 104, 106 are
engineered for the largest possible slope of neff. i.e.,
.008/1000 nm, near their respective cutoff wavelengths and the
indices of refraction are: no=+0.011 and nl=-0.0053. Fig. 11
illustrates the net gain spectra for these three EDFAs when
pumping is sufficiently strong to obtain an inversion D=0.9.
Note that they cover about 80 nm of total bandwidth in the S-
band and provide gain exceeding 15 dB over this 80 nm
bandwidth.
It should be noted that cutoff wavelength in this embodiment
is placed in the short wavelength band for EDFAs 104 and 106.
In fact, cutoff wavelength can also be placed in the long
CA 02478314 2004-09-07
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wavelength band, if desired. The choice of exactly where to
place the cutoff wavelength can be made by the designer once
the slope of the roll-off is known and the amount of high
gains in the long wavelength band to be matched or exceeded
are known.
Fiber amplifiers according to the invention can be used in
fibers whose cores and cladding layers have cross-sections
other than circular. For example, Fig. 12 illustrates the
so cross-section of a fiber amplifier 120 engineered according to
the invention and whose core 122 is elliptical. Depressed
cladding 124 is also elliptical while secondary cladding 126
has a circular cross section. These elliptical cross sections
are advantageous when radiation in one polarization rather
than the other polarization is to be maintained during
amplification.
Fig. 13 is a diagram illustrating a partial cross-section of a
fiber amplifier 210 with a core mode and a cladding mode. The
2o fiber amplifier 210 has an active core 212 surrounded by a
depressed cladding 214, which is surrounded by a secondary
cladding 216. Core 212 as a circular cross section, as do
depressed cladding 214 and secondary cladding 216. In
addition, an outer cladding 220 of circular cross-section
surrounds secondary cladding 216.
A region I associated with core 212 extends from 0<_r<_ro, while
depressed cladding 214 and secondary cladding 216 occupy
regions II, III extending between ro r<_rl and rl<-r<r2. Outer
3o cladding 220 is associated with a region IV extending from
r>r~. Core 212 has an index of refraction no, depressed
cladding 214 has an index of refraction n1 and secondary
cladding 216 has an index of refraction n2. Outer cladding
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220 has an index of refraction n3. The graph positioned above
the partial cross-section of fiber amplifier 210 illustrates
an average index profile 222 defined in fiber amplifier 210.
In the present embodiment fiber amplifier 210 is a single mode
fiber amplifier.
Fiber amplifier 210 has an active material 218 doped in core
212. Active material 218 is a lasing medium such as a rare
earth ion or any other lasant that exhibits high gains in a
so long wavelength band and positive gains in a short wavelength
band. Specifically, when pumped to a high relative inversion
D, the high gains of active material 218 in the long
wavelength band cause amplified spontaneous emissions (ASE) or
lasing which reduces the population inversion of lasant 218
is and thus reduces the positive gains in the short wavelength
band.
Superposed on average index profile 222 is an intensity
distribution of radiation in a guided fundamental core mode
20 224 at a first wavelength 7~1 where ~,1<7~~. First wavelength
is contained within a short wavelength band where active
material 218 exhibits positive gains. An intensity
distribution of radiation in a cladding mode 226 that exhibits
an oscillating intensity distribution beyond core 212 and
25 depressed cladding 214 is also shown. There is an overlap
between core mode 224 and cladding mode 226 as indicated by
hatched areas A. However, as with all modes of waveguide
structures, these modes are orthogonal (cladding mode 226 is
anti-symmetric in electric field) in the ideal case. Hence,
3o ideally there is no coupling between core mode 224 and
cladding mode 226. However, all real waveguides have
imperfections, inhomogeneities, scattering centers and
perturbations which break the orthogonality and enable
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coupling between core and cladding modes. In fact, the three
main causes of coupling in fiber amplifier 210 are
manufacturing defects, bending or coiling of fiber amplifier
210 as necessary for packaging purposes, and micro bends and
stresses which are pre-existing (e. g., frozen in during
manufacturing) or paused during packaging. Clearly, it is
beneficial to reduce these causes for coupling as far as
possible.
so Fig. 14 illustrates a refractive index profile 222A as is
obtained with normal manufacturing techniques. For the
purposes of the invention it is sufficient that the radially
varying index of core 212 have an average value equal to no.
Likewise, it is sufficient that indices of depressed cladding
s5 214, secondary cladding 216 and outer cladding 220 average out
to the values nl, nz, n3. The average index no of core 212 is
significantly higher than index nl of depressed cladding 214
and index n2 of secondary cladding 216. In this embodiment,
the average index n3 of outer cladding 220 is higher than all
20 other indices, although this need not be so.
The selection of appropriate values of indices no, nl, n2 and
radii ro, rl, r2 is made to achieve certain guiding properties
of fiber amplifier 210, as required by the instant invention.
2s In particular, index profile 222A is established in core 212
and in the first two cladding layers, i.e., depressed cladding
layer 214 and secondary cladding layer 216 such that radiation
in core 212 exhibits a loss above a cutoff wavelength ?~~ and
positive gains in a short wavelength range below the cutoff
3o wavelength ~,~. In a preferred embodiment, index profile 222A
is engineered to have a fundamental mode cutoff wavelength
such that radiation in fundamental mode 224 at wavelengths
smaller than ~,~ is retained in core 212 while radiation in
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fundamental mode 224 at wavelength ~,~ or longer wavelengths is
lost to secondary cladding 216 over a short distance. An
exemplary engineering method of the refractive index profile
222A will now be discussed.
In Fig. 13, the cutoff wavelength ~,~ is set such that core 212
exhibits a loss above cutoff wavelength ~,C and positive gains
due to active material 218 in a short wavelength range below
the cutoff wavelength 7~~. This selection of ro, rl, no, n1 and
to n2 provides distributed ASE suppression at wavelengths longer
than cutoff wavelength 7~,~ over the length of fiber amplifier
210. Superposed on average index profile 222 is the
intensity distribution of radiation in guided fundamental pore
mode 224 at a first wavelength ~,1 where ~,1<~,~ and the intensity
Of radiation in cladding mode 226. Radiation in core mode 224
and in cladding mode 226 propagates at first wavelength ~,1.
In other words, single mode fiber amplifier 210 allows for
discrete modes, such as mode 226 to propagate in secondary
cladding 216. Substantial power can then be transferred from
2o core mode 224 to cladding modes such as cladding mode 226 when
the phase velocities of core mode 224 and cladding mode 226
become identical. For a theoretical teaching on the cladding
mode coupling effect the reader is referred to Akira Tomita et
al., "Mode Coupling Loss in Single-Mode Fibers with Depressed
Inner Cladding", Journal of Lightwave Technology, Vol. LT-1,
No. 3, September 1983, pp. 449-452.
The transfer of power from core mode 224 to cladding mode 226
causes losses from core 212 at wavelength 7~,1. Thus, a signal
3o at 7~1 within the short wavelength band is not able to take
advantage of the full positive gains of active material 218 at
As used herein, these losses are referred to as cladding
mode losses. In certain cases, some power is also transferred
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back from cladding mode 226 to core mode 224 when coupling
exists between core mode 224 and cladding mode 226. As used
herein, this condition is referred to as cladding mode
resonance.
The general effect of cladding mode losses sustained by fiber
amplifier 210 is shown in Fig. 15. In this example Erbium is
used as active material 18 and the short wavelength band is
within the S-band. Specifically, graph 228 shows the gains of
Zo Erbium around its peak 230 at about 1530 nm. The design of
refractive index profile of fiber 210 sets cutoff wavelength
just below 1530 nm, e.g., at 1525 nm and produces a loss
curve 232. Loss curve 232 indicates that the losses above
cutoff wavelength ~,~ increase rapidly. Thus, any ASE due to
the gains of Erbium at 1530 nm and at longer wavelengths is
effectively suppressed. Meanwhile, in short wavelength band
234 below cutoff wavelength ~,~ Erbium exhibits gains above the
losses produced by loss curve 232. In other words, the Erbium
has positive gains in short wavelength band 234 and is
2o therefore able to amplify signals in short wavelength band
234.
Due to coupling between fundamental mode 224 and cladding mode
226 at wavelength 7~1 there is a loss peak 236 in short
wavelength band 234 centered at 7~1. The size of loss peak 236
is not drawn to scale and is indicated in dashed lines. It
should be noted that in practice there can be a number of
wavelengths within short wavelength band 234 at which coupling
between core mode and cladding mode occurs producing
s
so corresponding loss peaks. Also, it should be noted that
coupling between fundamental core modes and cladding modes at
wavelengths longer than 7~,~ can take place as well. For
example, core mode and cladding mode coupling occurs at
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The corresponding cladding mode resonance 238 is indicated in
dashed lines. Because ASE in the wavelength range spanning
is suppressed, this coupling is not as detrimental to the
function of fiber amplifier 210. Still, in a preferred
embodiment, cladding mode coupling at wavelengths longer than
~,~ should also be avoided.
Clearly, loss peak 236 reduces the effectiveness of fiber
amplifier 210 at wavelength 7~1. Therefore, in accordance with
so the invention, loss peak 236 is suppressed by suppressing
cladding mode loss in fiber amplifier 210. In the general
case, as well as in this embodiment, this object is achieved
by providing an arrangement for suppressing the coupling of
radiation in the short wavelength' range between active core
212 and secondary cladding 216. In the embodiment of Fig. 13,
the arrangement for suppressing coupling employs a material
240 distributed in outer cladding 220.
Material 240 is a scattering material or an absorbing
2o material. In either case, material 240 is embedded in outer
cladding 220 at a distance where core mode 224 is negligibly
small. In particular, core mode 224 has a mode diameter D
extending from core 212 into the cladding, i.e., into
depressed cladding 214 and secondary cladding 216. Material
240 is distributed outside the mode diameter of core mode 224.
Thus, core mode 224 does not exhibit appreciable intensity in
the region where material a240 is deposited within outer
cladding 220. This means that in single mode fiber amplifier
210 material 240 should be embedded several tens of microns
so away from core 212. It should be noted that outer cladding
220 can be made up entirely of material 240 if outer cladding
220 commences at a distance where core mode 224 is negligibly
small.
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In the embodiment where material 240 is an absorber, it can be
a rare earth element doped into outer cladding 220. Suitable
materials include Erbium, Cobalt, Samarium and other suitable
absorbers. Material 240 can be embedded in outer oladding 220
using any suitable fabrication technique. For example, in a
typical manufacturing process employing the "sleeving
technique" a sleeve of pure silica that is to be pulled over
secondary cladding 216 can be provided with a layer of doped
to material 240 prior to the sleeving process. Specifically, a
layer of doped material 240 coated onto the inner surface
prior to the sleeving process can be employed. Modified
Chemical Vapor Deposition (MCVD) and solution doping, followed
by sintering can be used to create the proper layer of
is absorbing material 240.
In another embodiment, material 240 is any suitable scattering
material, such as an inhomogeneous acrylate layer or other
material exhibiting rapid variations in the refractive index
2o and/or geometry. Scattering material can employ two
scattering effects. First, it can scatter radiation in
cladding mode 226 that is phasematched with core mode 224 into
an assortment of other cladding modes. Typically there will
be a large number (usually hundreds) of other cladding modes
25 into which radiation of cladding mode 226 can be scattered.
This effect is substantially equivalent to absorption loss as
far as cladding mode 226 is concerned. Alternatively,
radiation in cladding mode 226 can be perturbed in phase in a
random fashion by scattering material 240. This effect is
3o substantially similar to preventing phase matching between
core mode 224 and cladding mode 226. By preventing phase
matching the accumulation of cladding mode loss over a long
distance of fiber amplifier 210 is thus prevented.
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The effect of using material 240 in outer cladding 220 is
illustrated in Fig. 15. Specifically, by using material 240
loss peak 236 at 7~1 is reduced to a smaller loss peak 236'
indicated in solid line. Fig. 16 illustrates the experimental
results of using absorbing material 240 in the form of a
polymer buffer in outer cladding 220 of fiber amplifier 210.
In this case, the host material of fiber 210 is silicate-
containing glass such as alumino-germanosilicate glass or
so phosphorus doped germanosilicate glass. Graph 242 indicates
the gain experienced by a signal in fiber 210 without material
240 in outer cladding 220 and graph 244 indicates the gain
obtained with material 240. In these oases both material 240
and outer cladding 220 are made of polymer materials with
differing loss characteristics. Clearly, the dip in gain
associated with loss peak 236 is removed with the aid of
absorbing material 240. Thus, fiber amplifier 210 of present
invention provides distributed suppression of amplified
spontaneous emissions (ASE) above cutoff wavelength 7~,~ and
2o suppresses cladding mode loss at wavelengths shorter than
cutoff wavelength ~,~, i.e., wavelengths in short wavelength
range 234 such as wavelength ~,1 in particular. It should be
noted that the presence of absorbing material 240 in outer
cladding 220 also suppresses cladding mode effects at 7~
Fig. 17 illustrates a partial cross-section of another fiber
amplifier 200 in accordance with the invention. Parts of
fiber amplifier 200 corresponding to those of fiber amplifier
210 are referenced by the same reference numbers. In fiber
3o amplifier 100 the arrangement for suppressing coupling between
core mode 224 and cladding mode 226 is a non-phase-matched
length section of fiber amplifier 200. In the non-phase-
matched length section outer cladding 220 has a lower
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refractive index n3 than all other indices. Most importantly,
refractive index n3 is lower than refractive index n2 of
secondary cladding 216, i.e., n3<n2. This condition ensures
that radiation in core mode 224 and cladding mode 226 are not
s phase matched. Appropriate material for outer cladding 220 to
ensure such low refractive index n3 is silicone, Teflon,
Fluorine-doped silica and other low-index materials such as
those used in dual clad fibers well known to those skilled in
the art.
Prevention of phase matching and the selection of the value of
refractive index n3 will be better understood by referring to
the graphs in Fig. 18. Graph 2102 illustrates the normalized
propagation constant of radiation in core mode 224 plotted
is versus inverse of the wavelength (i.e., optical frequency,
which is also proportional to the k-vector) for n3>-nz. Graph
2104 illustrates the normalized propagation constant of
radiation in cladding mode 226 also plotted versus inverse of
the wavelength for n3>-n2. (The condition n3>-n2 is typical for
2o telecommunications fibers which use acrylate as the typical
outer cladding also referred to as buffer.) At 1/71 graphs
2102 and 2104 intersect indicating phasematching and hence
cladding mode loss.
25 Graphs 2106 and 2108 in Fig. 19 illustrate the power level of
radiation normalized to the value 1 (1000 power level) in core
mode 224 and cladding mode 226, respectively. Graphs 2106 and
2108 are observed for the phasematched condition and are
graphed as a function of length of fiber amplifier 200
so assuming an ideal case in which no power is lost or gained
(i.e., no amplification). The power level of core mode 224
represented by graph 2106 starts at the high power value of 1
and undergoes sinusoidal oscillations between 1 and 0. In
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contrast, the power level of cladding mode 226 starts at the
low power value of 0 and undergoes sinusoidal oscillation
between 0 and 1. Clearly, power is transferred from core mode
224 to cladding mode 226 during the first part of the
s oscillation and back from the cladding mode 226 to core mode
224 during the second part of the oscillation.
In practice, outer cladding 220 has a loss of a finite value oc
per unit length of fiber amplifier 200 while the loss in core
l0 212 is negligible. Therefore, the power in core mode 224 will
not manage to be coupled completely into cladding mode 226.
Under these conditions, the power level in core mode 224 will
follow a graph 2106' and the power level in cladding mode 226
will follow a graph 2108' as shown for an intermediate value
15 Of oG. At a large value of oc the power levels will follow
graphs 2106" and 2108". The cladding mode loss prevents
appreciable power from building up in cladding mode 226,
thereby reducing the coupling of power from core mode 224 to
cladding mode 226. In fact, the loss of power y from core
2o mode 224 to cladding mode 226 can be described by the
following equation:
Y- 8.7cz (15)
a
25 where c2 is the speed of light squared. From this equation it
is evident that increasing the loss oc experienced by cladding
mode 226 decreases the loss experienced by core mode 224. For
a detailed derivation of the equation the reader is referred
to Akira Tomita et al., "Mode Coupling Loss in Single-Mode
3o Fibers with Depressed Inner Cladding", Journal of Lightwave
Technology, Vol. LT-1, No. 3, September 1983, pp. 449-452.
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Now, changing the refractive index n3 of outer cladding 220
has the effect of shifting the phasematching wavelength 7~,1 and
can be used to eliminate coupling of radiation from core mode
224 to cladding mode 226 in accordance with the invention.
Graphs 2110 and 2110' in Figs. 20A and 20B illustrate the
effective index neff experienced by core mode 224 when n3>n~ or
n3<n~, respectively. Because a change in n3 does not affect
core mode 224 appreciably, graphs 2110 and 2110' are almost
identical. The effective indices of a number of cladding
to modes, including cladding mode 226 are indicated by lines 2112
and 2112', respectively.
In Fig. 21A the condition n3>n~ dictates that the effective
indices of cladding modes can exceed n2. In fact, the
is effective index of core mode 224 intersects with the effective
index of cladding mode 226 at intersection point 2114 in the
short wavelength range below cutoff wavelength
Furthermore, effective index of core mode 224 also intersects
with the effective indices of two additional cladding modes in
2o this case. Therefore, cladding mode losses due to coupling
between core mode 224 and cladding mode 226 as well as
coupling between core mode 224 and the two additional cladding
modes exist. The coupling behavior is as indicated by graphs
2106', 2106" and 2108', 2108" in Fig. 19 (depending on the
2s value of cladding loss cc) and causes the undesired cladding
mode loss.
On the other hand, when n2>n3 the effective indices of
cladding modes cannot exceed n2, as shown in Fig. 20B. Thus,
3o the effective index of 'core mode 224 does not intersect with
any cladding modes below cutoff wavelength 7~~. Therefore,
there is no coupling between core mode 224 and cladding mode
226 or any other cladding mode below cutoff wavelength ?~,~. In
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fact, the intersection point 2114' between core mode 224 and
cladding mode 226 occurs above cutoff wavelength ~,~ in the
long wavelength range in which ASE is being suppressed by the
design of fiber amplifier 200, as discussed above. The same
is true for coupling from core mode 224 to the other cladding
modes.
The phasematching principle is used in accordance with the
invention by introducing a non-phase-matched length section Z
so of fiber amplifier 200 in which n3<n2 to suppress cladding
mode loss. Referring to Fig. 6, in this embodiment, fiber
amplifier 68 is similarly designed as fiber amplifier 200.
Fiber amplifier 68 is used in a system 1200 to amplify a
signal 78 at wavelength 7~1 propagating through a fiber 80.
System 1200 has a pump source 86 providing a pump radiation 88
at wavelength 7~p. Pump radiation 88 is coupled from source 86
into a fiber 82.
A fiber coupler 84 receives fibers 80 and 84 and couples them
2o into a single output fiber 90. Output fiber 90 is connected
to fiber amplifier 68.
During operation, signal 78 and pump radiation 88 are combined
in coupler 84 and launched together through output fiber 90.
Fiber 90 delivers signal 78 and radiation 88 to active core 70
of fiber amplifier 68. In accordance with the above-described
principles, signal 78 is amplified in core 70. Meanwhile,
pump radiation 88 is depleted in passing through core 70, as
indicated. In fact, at the end of non-phase matched section L
3o there may be little pump radiation remaining in fiber
amplifier 68.
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ASE radiation at a wavelength 7~2 is generated as a by-product
of pumping active core 70. Wavelength ~,2 is longer than
cutoff wavelength 7~,~ of fiber amplifier 68 and is therefore
lost into outer cladding 76. At the same time, some of signal
78, which travels in core mode, is also lost into outer
cladding 76 because of cladding mode losses. However, since
non phase-matched length section L has an index n3 lower than
n2, the amount of loss of signal 78 to outer cladding 76 is
minimized.
to
System 1200 using non-phase-matched length section L of fiber
amplifier 68 is thus capable of suppressing mode loss at
wavelengths shorter than the cutoff wavelength. In fact,
fiber amplifier 68 can be effectively employed in various
optical systems.
In another alternative embodiment, the use of a non-phase-
matched length section and the use of an absorbing or
scattering materials can be combined in one.fiber amplifier.
2o For example, the scattering or absorbing material may
constitute a part of the outer cladding or the entire outer
cladding in such alternative embodiments.
Yet another embodiment in accordance with the invention
employs a non-phase-matched length section L which prevents
phase matching between core and cladding modes by varying the
cross-sectional profile of a fiber amplifier 2150 as shown in
Figs. 21A and 21B. Fig. 21A shows the cross-section of fiber
amplifier 2150 at a position L=xl. Fibex amplifier 2150 has
3o an active core 2152 surrounded by a cladding 2154 having a
varying cladding index n~laa ~ A minimum value of n~laa is
indicated by line 2156. A graph of index profile 2158 showing
the variation of n as a function of radius r is shown above
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fiber amplifier 2150. A person skilled in the art will
appreciate that, in general, n~laa can vary as a function of
radius r and azimuthal angle 8, i. e. , n~lad-nclad (r~ B) .
s At position L=x2 the cross section of fiber amplifier 2150 is
different, as shown in Fig. 21B. In particular, index profile
2158' remains the same as index profile 2158 in and near
active core 2152 to ensure the same cutoff wavelength 7~~ and
loss curve for longer wavelengths are the same at positions x1
to and x~. However, the portion of index profile 2158' further
away from core 2152 within cladding 2154 exhibits a different
curvature and minimum value than index profile 2158.
Specifically, the location of the new minimum value of n~laa in
index profile 2158' is indicated by line 2156'. Because of
15 this variation of index profile from 2158 at xl to 2158' at
x2, the wavelength for which cladding mode loss is
phasematched at position xl is different from the wavelength
for which cladding mode loss is phasematched at position x2.
Therefore, phase matching between core mode and cladding modes
2o in fiber amplifier 2150 is prevented.
As mentioned heretofore, the fiber amplifier of the present
invention can contain any suitable active medium in its active
core. For example, the active core can be doped with
2s Neodymium, Erbium, or Thulium ions. When using Erbium, the
fiber amplifier is an EDFA and in one advantageous embodiment
its cutoff wavelength 7~,~ is set near 1525 nm. Specifically,
Erbium 18 acts as a lasing medium and exhibits high gains in a
long wavelength band including the C- and L-bands. Erbium 18
3o also has positive gains in a short or S-band of wavelengths
shorter than the wavelengths in the C- and L-bands. When
pumped to a high relative inversion D, the high gains of
Erbium 18 in the long wavelength band cause amplified
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spontaneous emissions (ASE) or lasing which reduces the
population inversion of Erbium 18 and thus reduces the
positive gains in the S-band. As discussed before, by
selecting the core cross-section, the depressed cladding
s cross-section, and the refractive indices no, nl, and n2 to
produce losses at least comparable to the high gains in the
long wavelength band and losses substantially smaller than the
positive gains in said S-band, the W-index profile of the
inventive fiber nevertheless enables the fiber to effectively
to amplifying signals in the S-band.
In another example, Thulium is doped into fused-silica fibers.
Although the Thulium gain is typically thought to be at 1.9
microns, and indeed that is the peak of the gain, the
i5 wavelength range over which gain is possible stretches from
1.5 microns to 2.1 microns. The typical Thulium pump
wavelength is 0.78 microns. However, it is also possible to
pump Thulium at 1.48 microns or about 1.5 microns, though very
high intensities would be needed, possibly as high as 100 mW.
Graphs A and B in Fig. 39 show that the Thulium has
fluorescent emission from 1.6 to 2 Vim. The shape of the
fluorescence spectrum is very similar to that of the gain
spectrum, except that the gain will be at a slightly longer
wavelength than the fluorescence. If Thulium acts as an ideal
ion, as do Erbium and Ytterbium, then gain should be possible
to stretch from 1.5 ~zm to 2.1 ~.un. The peak of the gain will
be between 1.8 and 1.9 microns. The gain cross-section and
the upper-laser-level lifetime of the Thulium ion are similar
3o to those of the Erbium ion. Thus, the threshold for gain is
similar - several milliwatts of pump power are required.
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The Thulium 3+ ion could be used on the short-wavelength end
of its gain region in exactly the same way as the Erbium ion.
By pumping with an intense pump (30 mW or so) it is possible
to reach inversion even at short wavelengths. However, before
high gain is reached at a short wavelength such as 1.6
microns, there will be overwhelming superfluorescence near 1.9
microns.
A useful amplifier can be made at the shorter wavelength if
so the fiber is designed with a fundamental mode cut-off between
1.9 microns and the shorter wavelength of desired operation,
and if the cut-off is such that the increase in loss at longer
wavelengths exceeds the increase in gain due to the higher
cross-section. In an embodiment, the long wavelength band is
is about 1.7 to 2.1 microns, the short wavelength band is the L-
band, which is roughly 1.6 to 1.8 microns, the cut-off
wavelength is about 1.7 to 1.9 microns, and the pump
wavelength is about 1.48 to 1.5 microns. The cut-off
wavelength is selected such that the increase in loss at
20 longer wavelengths exceeds the increase in gain due to the
higher cross-section. This technique makes it possible to
build useful amplifiers in the wavelength range between about
1.6 to 1.8 microns. Since telecommunication fiber is highly
transmissive in this range, it is anticipated that amplifiers
a5 that work in this wavelength range will be highly desirable.
Fig. 22 illustrates a source 300 of light in the S-band
employing a fiber 302 doped with Erbium 306 and constructed to
form an EDFA in accordance with the above principles.
so Specifically, fiber 302 has a core 304 doped with Erbium 306,
a depressed cladding 308 surrounding core 304 and a secondary
cladding 310 surrounding depressed cladding 308.
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Source 300 has a pump source 312 for providing a pump light
314. Pump source 312 is preferably a diode laser emitting
pump light 314 at a wavelength of about 980 nm. An optic 316
in the form of a lens is provided for coupling pump light 314
into a fiber 318. A coupler 320 is provided for coupling pump
light 314 from fiber 318 into a fiber 324. Fiber 324 is
joined to one end of fiber 302 in accordance with any suitable
fiber splicing technique known to those skilled in the art
such that fiber 324 delivers pump light 314 into core 304 of
so fiber 302.
Pump source 312 is controlled by a pump control 322 such that
source 312 delivers pump light 314 for pumping Erbium 306 in
core 304 to a high relative inversion D. The relative
i5 inversion D is sufficiently high when Erbium 306 exhibits
positive gains in the S-band and high gains in the long
wavelength band, i.e., the L- and C-bands. The cross-sections
and refractive indices, no, nl, n~ of core 304, depressed
cladding 308 and secondary cladding 310 are selected in
2o accordance with the above rules. In particular, the cross-
sections and refractive indices no, nl, n2 are selected to
produce losses at least comparable to the high gains in the L-
and C-bands and losses substantially smaller than the positive
gains in the S-band.
Fiber 324 passes through coupler 320 and is terminated by a
wavelength-selecting device 326. In the present embodiment
device 326 is a wavelength-selecting feedback mechanism in the
form of a fiber Bragg grating. Fiber Bragg grating 326 is a
3o wavelength-selecting feedback mechanism because the portion of
light that it is tuned to reflect propagates through fiber 324
back into fiber 302. Of course, other mechanisms can also be
used. For example, another advantageous wavelength-selecting
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feedback mechanism is a tunable free-space diffraction grating
configured to retro-reflect light at the desired output
wavelength.
At its other end fiber 302 is joined with a fiber 328 that is
terminated by an output coupler 330. Once again, any suitable
fiber joining technique can be employed to join the end of
fiber 302 to fiber 328. The junction is such that light
propagating through core 304 of fiber 302 is freely coupled
so between fiber 302 and fiber 328. Output coupler 330 is any
suitable optical coupling device fox passing an output light
332. For example output coupler 330 can be a cleaved end of
fiber 328, i.e., a cleaved output facet, a wavelength coupler,
a free-space reflector, a fiber Bragg grating, a 2x2 fused
fiber coupler used in conjunction with a broadband reflector.
In fact, any output coupling device used to couple output
light from a fiber laser can be used as output coupler 330
including a diffraction grating. ~In fact, as will be
appreciated by a person skilled in the art, a diffraction
2o grating can be used to serve the function of wavelength
selecting device 326 and output coupler 330.
During operation pump laser control 322 is turned on to
provide pump light 314 to fiber 302 suchthat Erbium 306 is
pumped to a high relative inversion D. As a result, Erbium
306 exhibits .positive gains in the S-band and high gains in
the L- and C-bands. The selection of cross-sections and
refractive indices no, n1, n2 of core 304, depressed cladding
308 and secondary cladding 310 in accordance with the
so invention cause losses at least comparable to the high gains
in the L- and C-bands and losses substantially smaller than
the positive gains in S-band 342. Therefore, fiber 302
exhibits a net optical gain spectrum that extends several tens
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of nanometers below fundamental mode cutoff wavelength
within S-band 342.
Fig. 23 shows a shortest wavelength 7~Short and a longest
wavelength ~l,long for which the gain is positive define a net
gain bandwidth 390. Shortest and longest wavelengths 7~Short~
7~,long are determined by design parameters of fiber 302
including a roll-off loss curve below cutoff wavelength
doping concentration, and distribution of Erbium 306 in core
so 304, and average degree of inversion D over the length of
fiber 302. Changes in the length of fiber 302 do not impact
shortest and longest wavelengths ~shortr long for which the
gain is positive as long as inversion D remains oonstant.
Changes in the length of fiber 302, however, impact the amount
is of gain within net gain bandwidth 390 contained between
shortest and longest wavelengths 7~Short. ~lon~ ~ On the other
hand, changes in the power of pump light 314 and its direction
as well as single-end or dual-end pumping directly affect the
average degree of inversion D in fiber 302. The present
2o embodiment employs single-end pumping in which pump light 314
and output light 332 are co-propagating (propagate in the same
direction). Higher inversion D produces higher gain (or lower
loss) at all wavelengths within S-band 342 and can also expand
net gain bandwidth 390 between shortest and longest
25 wavelengths ~l.Shorti ~long~
Even when fiber 302 does not receive a signal light for
amplification (e.g., signal light 78 as illustrated in Fig. 6)
it still creates an optical output. Unavoidable fluorescence
3o also referred to as spontaneous emission (SE) occurs due to
the natural radiative decay of excited (pumped) atoms of
Erbium 306 back down to ground state. The spontaneous
emission process happens in exact proportion to the spectrum
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of the "emission cross section" (often called the gain cross
section, due to their correspondence). In fact, even if
population inversion has not been achieved, spontaneous
emission still occurs. Some of this spontaneous emission
s generates light within S-band 342, and some of this light
overlaps with a mode guided by fiber 302. More specifically,
some of the light produced by spontaneous emission is trapped
in core 304 of fiber 302 and travels along its core 304 in a
guided mode . Of that trapped light the portion that overlaps
1o with net gain bandwidth 390 of fiber 302 is amplified. Light
outside net gain bandwidth 390 is generally not amplified and
is lost by direct attenuation, absorption by non-inverted
atoms of Erbium 306 and loss to secondary cladding 310 among
other. In this case, ASE is guided in core 304 and amplified
15 by fiber 302.
As will be appreciated by those skilled in the art, the
spectral shape of the ASE is determined both by the spectral
shape of the spontaneous emission (which is related to the
2o emission cross section) and also by the spectral shape of the
net gain bandwidth 390. Net gain bandwidth 390 is related to
the emission cross section, absorption cross section, degree
of inversion D and the spectral shape of the losses dictated
by roll-off loss curve produced by the selection of cross
25 sections and refractive indices of fiber 302. However, the
spectral shape of ASE is not merely the product of the
spontaneous emission spectrum and the spectrum associated with
net gain bandwidth 390, as would be expected if all of the
spontaneous emission happened at one end of fiber 302 and all
30 of the amplification occurred at a different location in fiber
302. Rather, the ASE output from fiber 302 is the
superposition of the amplified bits of spontaneous emission
originating at each and all locations within fiber 302.
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In general, wavelengths at which there are high gains and high
losses exhibits higher ASE power than wavelengths with low
gains and low losses, even if the net gain is the same.
Typically, due to the shape of the emission cross section of
Erbium 306, longer wavelengths within net gain bandwidth 390
exhibit higher gains than shorter wavelengths. Also,
typically, longer wavelengths exhibit higher losses than
shorter wavelengths. The higher losses are due to the shape
Zo of the absorption cross section of Erbium 306 and the shape of
the roll-off loss curve. Hence, the ASE spectrum of S-band
amplifier constituted by fiber 302 is often biased towards
these longer wavelengths, even though the longest of these
wavelengths may experience net loss. Typically, the shorter
s5 wavelengths of the ASE emission spectrum exhibit small
positive net gains, through not much ASE power.
As a result of the above phenomena the following rules should
be observed when constructing source 300. First, one should
2o select a peak wavelength 7~peax within net gain bandwidth 390.
Then, the cross sections and refractive indices of co°re 304,
depressed cladding 308 and secondary cladding 310 should be
selected to set cutoff wavelength ~,C about 10-20 nm above
peak ~ The exact distance between 7~peax and ~,~ should be
25 adjusted depending on the steepness of roll-off loss curve 38
(see Fig. 4). In particular, when roll-off loss curve 38 is
steep then ~,~ should be set only about 10 nm above speak ~ ~n
the other hand, for a less steep roll-off loss curve a cutoff
wavelength 7~'~ should be set up to 20 nm above 7~.peak~. The
3o general shape of the ASE emission spectrum has the shape of
the net gain spectrum within net gain bandwidth 390 as
indicated by graph 392 for steep roll-off loss curve and by
graph 392' for less steep roll-off loss curve.
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Next, one should determine the desired power level and
bandwidth of source 300. To obtain output light 332 at a high
power level fiber 302 is lengthened. The doping concentration
of Erbium 306 in core 304 can be kept the same or even
increased to further aid in increasing the power level of
light 332. Then, the power level of pump light 314 is
increased, e.g., to obtain 100-200 dB absorption of pump light
314 in fiber 302. For example, pump light 314 is delivered at
so a power level such that fiber 302 absorbs up to 90% of pump
light 314. On the other hand, to obtain output light 332
spanning a wide bandwidth, fiber 302 is kept short and the
power level of pump light 314 is decreased.
Thus, there exists a tradeoff between power and bandwidth.
This is because for the high gains and wide amplification
bandwidths that can be achieved in doped EDFAs the ASE process
is quite efficient. The typical way of further increasing the
power of an EDFA is to pump harder and/or lengthen the EDFA.
2o This approach works well up to a point. However, the fiber
length and pumping cannot be increased as much as desired due
to the high efficiency of the ASE process. Once a significant
ASE power builds up in an EDFA, e.g., up to net gains of 40
dB, the ASE process begins to rob the population of Erbium
atoms in the excited state, thereby reducing the degree of
inversion D. Reduced inversion D causes a reduction of
spontaneous emission and a significantly reduced amount of net
gain. The S-band EDFA is particularly sensitive to reductions
in inversion D because of the unfavorable radio of emission
so cross section to absorption cross section within the S-band.
This interplay between ASE and gain limits the available power
and/or bandwidth of ASE within the S-band when a single EDFA
section is used. Therefore, if sufficient power over the
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desired bandwidth cannot be achieved with fiber 302, then
severa l fibers analogous to fiber 302 can be used in
combination. Further details of such combinations are
described below.
As fiber 302 is being pumped by pump light 314, source 300
generates ASE emission spectrum 392 centered about peak
wavelength 7l,peak. Lasing operation of source 300 is obtained
with the aid of fiber Bragg grating 326. Specifically, fiber
to Bragg grating 326 is set to reflect an output wavelength 7~.output
within ASE emission spectrum 392. At the same time, output
coupler 330 is set to pass a fraction of light 332 at
wavelength ~output~ After many round trips between fiber Bragg
grating 326 and output coupler 330, light at output dominates
i5 over ASE emission spectrum 392. Therefore, source 300 emits
output light 332 having a narrowband spectrum 334 centered at
wavelength output through output coupler 330. Pump source
control 322 can operate in a continuous mode or in a pulsed
mode. Therefore, output light 332 can be delivered in pulses
20 or continuously, as desired.
Fig. 24 illustrates an alternative embodiment of a source 340
in which parts corresponding to those of source 300 are
referenced by the same reference numerals. Source 340 differs
25 from source 300 in that it has a wavelength selecting
mechanism 342 and a control 344 for adjusting the wavelength
reflected bank to fiber 302 by mechanism 342. V~lavelength
selecting mechanism 342 is a wavelength filter such as a
tilted etalon, a strain-tuned fiber Bragg grating, a
3o temperature-tuned fiber Bragg grating, an interferometer, an
array of waveguide gratings, a diffraction gratings or a
tunable coupled cavity reflector. Correspondingly, control
344 is a mechanism for controlling stain, temperature,
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inclination angle or other required tuning parameter of filter
342, as will be appreciated by a person skilled in the art.
By controlling the wavelength band reflected by filter 342 an
output wavelength 7~,~utput of light 332 is selected as in source
300. Of course, output coupler 330 is adjusted to pass light
332 at the selected output wavelength output ~ One can also
select several output wavelengths within ASE emission spectrum
392 (see Fig. 23).
Alternatively, or in combination with output wavelength
selection performed with the aid of filer 342, pump source
control 322 of source 340 can also be used to adjust the
output wavelength of light 332 by tuning the level of relative
is inversion D. This is achieved by tuning the power delivered
by control 322 to pump source 312. Changing the power level
applied to pump source 312 adjusts the intensity of pump light
314, hence tuning the level of relative inversion D, as will
be appreciated by a person skilled in the art.
Fig. 25 illustrates a preferred embodiment of a source 360
according to the invention using a single EDFA 364. Source
360 does not require the use of reflectors by virtue of having
a fiber ring cavity 362 with an output coupler 366. In this
embodiment, a wavelength filter 368 installed in ring cavity
362 serves as a wavelength selecting mechanism. Filter 368
can be an adjustable filter, preferably a diffraction grating
used in conjunction with an optical circulator or a
temperature controlled fiber Bragg grating with a suitable
3o control mechanism (not shown), an acousto-optic transmission
filter (AOTF) or even a tunable etalon. Fiber ring cavity 362
also has an isolator 370 for controlling back-reflections and
preventing output light 372 containing the light fraction at
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output or the ASE from propagating in both directions around
ring 362. During operation EDFA 364 is pumped by a pump
source (not shown) and operated in accordance with the
principles described above.
Fig. 26 illustrates a source 361 also using a fiber ring
cavity 363. In order to obtain a broader ASE emission
spectrum, source 361 employs two EDFAs 365, 367 connected in
parallel between two couplers 371, 373. A third coupler 371
1o is employed for deriving output light 377 from ring cavity
363. The pump sources providing pump light to EDFAs 365, 367
are not shown in this embodiment for reasons of clarity.
EDFAs 365, 367 have different ASE emission spectra. These ASE
i5 emission spectra can be controlled by any of the above-
discussed mechanisms, including different fiber parameters
(cross sections, refractive indices and roll-off loss curves),
lengths and inversion levels set by the intensity of pump
light (not shown). Preferably, the ASE emission spectra of
2o EDFAs 365, 367 are chosen to have their peak wavelengths at
different locations within the S-band to thus span a wider
total ASE emission spectrum. Thus, source 361 is able to
provide a broader ASE emission spectrum and offers a wider
range of wavelengths within which the output wavelength 7~output
25 is selected by filter 369. Furthermore, source 361 also has
an isolator 375 for controlling back-reflections and
preventing output light 377 containing the light fraction at
output Or the ASE from propagating in both directions around
ring cavity 361.
During operation wavelength filter 369 sets output wavelength
output of light 377 within the total ASE emission spectrum
provided by EDFAs 365, 367. Light 377 is outcoupled from ring
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cavity 363 through output coupler 371, as shown. It should be
noted that more than two EDFAs can be used to cover a still
broader ASE emission spectrum. In fact, filter 369 can be
left out completely in some embodiments to outcouple light 377
s covering the wide bandwidth afforded by the parallel-
configured EDFAs thus yielding a broadband source.
Source 361 can be easily adapted to cover more than just the
S-band. For example, another EDFA covering the C- or L-band
so of wavelengths, or in fact several additional EDFAs, can be
connected in parallel with EDFAs 365 and 367 and their outputs
combined.
Fig. 27 illustrates yet another embodiment of a source 380
15 that uses a single EDFA 382. Source 380 has a pump source 384
for providing pump light 386. A lens 388 focuses pump light
386 into a fiber 390, which is coupled to EDFA 382 by a
coupler 392.
2o EDFA 382 is coiled at a constant coiling diameter CD. To
provide for mechanical stability, EDFA 382 can be coiled about
a spool of diameter CD (not shown). In fact, the strain
introduced into EDFA 382 by coiling diameter CD serves as the
wavelength-selecting mechanism in this embodiment. That is
25 because coiling diameter CD produces a desired ASE emission
spectrum in EDFA 382. Specifically, selecting a larger
coiling diameter CD, e.g., CD' as indicated, shifts the
maximum of the ASE emission spectrum of EDFA 382 to longer
wavelengths within the S-band.
EDFA 382 is terminated by an angle cleaved facet 394 or other
non-reflective termination that prevents back reflection for
better stability of source 380. Thus, angle cleaved facet 394
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ensures that a sufficient amount of stable and low-noise
output light 402 is emitted by EDFA 382 to an output coupler
396. An isolator 398 is interposed between EDFA 382 and
output coupler 396 to prevent back-coupling of light 402 into
EDFA 382.
In this embodiment output coupler 396 has an additional tap
400 for deriving a small amount of output light 402, e.g.
about 10, for output monitoring. A photodetector 404, in this
to case a photodiode, is provided for measuring tapped output
light 402.
Source 380 can be used as a fixed source or as a tunable
source. In particular, source 380 can be rendered tunable by
providing a mechanism for altering coiling diameter CD.
Alternatively, source 380 can be rendered broadband by
widening the ASE emission spectrum of EDFA 382, e.g., by
selecting a less steep roll-off loss curve, as discussed
above.
Source 380 employs a counter-propagating pumping geometry
where pump light 386 is injected from a direction opposite to
the direction in which output light 402 is derived from EDFA
382. This approach is preferred to co-pumping arrangements
used in the above-described embodiments where the pump light
is delivered along the same direction as the direction along
which output light is derived.
A preferred embodiment of a source 410 is shown in Fig. 28.
3o Source 410 uses two EDFA sections (which may or may not belong
to the same piece of fiber) specifically a first section 412
and a second section 414. These two sections have different
ASE emission spectra. In this case the ASE emission spectra
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are set by first and second coiling diameters CD1 and CD2 of
sections 412, 414 respectively. Specifically, first section
412 has a smaller coiling diameter and second section 414 has
a larger coiling diameter, CD1<CD2. Thus, the maximum of ASE
emission spectrum of first section 412 is at a shorter peak
wavelength ~l,peak than the maximum of ASE emission spectrum of
second section 414.
As in the previous embodiment, an angle cleaved facet 416
so prevents back reflection of output light 428. EDFA sections
412, 414 are pumped by pump light 418 delivered from a pump
source 420 in a counter-propagating pumping arrangement. In
particular, pump light 418 is focused by a lens 422 into a
fiber 424 and a coupler 426 couples pump light 418 from fiber
424 into EDFA sections 412, 414.
An isolator 430 ensures that output light 428 is not coupled
back into EDFA sections 412, 414. An output coupler 432 is
provided for outcoupling output light 428. Output coupler 432
2o has a tap 434 for tapping a small portion of output light 428
and a photodetector 436 for monitoring the tapped portion of
output light 428.
It is important to note that in source 410 first section 412
is positioned before second section 414 such that first
section 412 seeds second section 414. In other words, the ASE
from first section 412 propagates into second section 414 and
output light 428 is derived from second section 414. The
reasons for this arrangement is that second section 414 offers
so positive net gain for light at wavelengths generated by first
section 412. First section 412, however, does not offer
positive net gain and hence does not amplify light at
wavelengths generated by second section 414. In other words,
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first section 412, which emits light centered around a shorter
peak wavelength speak Can be used to seed second section 414
but not vice versa. Still differently put, the two-coil
design of source 410 does not cause significant depletion of
inversion D in second section 414, while reversing the order
of sections 412 and 414 would and would hence degrade the
operation of source 410.
Fig. 29 illustrates the ASE emission spectrum of first section
l0 412 at first coiling diameter CD1=2.2 inches and of second
section 414 at second coiling diameter CD2=2.9 inches. Fig.
29 also shows the total ASE emission spectrum obtained when
section 412 seeds second section 414.
Based on the above principle, a number of EDFAs of different
coiling diameters can be used in series from smallest diameter
(shortest peak wavelength 7~peak) to largest diameter (longest
peak wavelength 7~peak) to construct a still broader bandwidth
source in accordance with the invention. Fig. 30 illustrates
2o ASE emission spectra of five EDFAs having increasing coiling
diameters ranging from 2.25 inches to 2.90 inches. Using
these EDFAs in series makes it possible to construct a source
spanning a wavelength range covering most of the S-band, i.e.,
from about 1460 nm to about 1525 nm.
Yet another method for broadening the ASE emission spectrum of
an EDFA is by providing a continuously variable coiling
diameter CD along the length of the EDFA. The coiling
diameter should be increasing for seeding reasons, as
3o explained above. A continuously variable coiling diameter can
be produced, e.g., by winding the EDFA around a cone.
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Fig. 32 illustrates yet another embodiment of a source 440
employing an EDFA having a first section 442 and a second
section 444. First section 442 has a smaller first coiling
diameter and is used to seed second section 444 having a
s larger second coiling diameter. Source 440 has two separate
pump sources 446, 448 with associated lenses 450, 452, fibers
454, 456 and couplers 458, 460 for delivering pump light 462
to first section 442 and pump light 464 to second section 444.
to Source 440 has an angle cleaved facet 466 terminating first
section 442. Source 440 employs an isolator 468 between first
section 442 and second section 444 for stabilization. A
tunable filter 470 installed after isolator 468 and before
second section 444 is used to tune output wavelength .output of
15 output light 472. Conveniently, coupler 460 is used as output
coupler for light 472 in this embodiment.
Source 440 enables the operator to quickly and easily adjust
the levels of pump power delivered by pump light 462 and 464
2o to sections 442 and 444 for tuning of output light 472. In
fact, Fig. 31 illustrates how the use different levels of pump
power in first and second sections 442, 444 tunes the total
ASE emission spectrum for coiling diameters of first and
second sections 442, 444 equal to 2.25 and 2.5 inches
25 respectively.
Fig. 33 illustrates a source 480 employing a similar
arrangement as source 440. Corresponding parts of source 480
are referenced by the same reference numerals. Source 480
so uses a single pump source 482 for delivering pump light 484 to
both sections 442, 444. This is done with the aid of lens
486, coupler 488 and fibers 490, 492 as shown. The coupling
ratio of coupler 488 between fibers 490 and 492 can be
~o
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adjusted to control the levels of pump power delivered by pump
light 484 to section 442 and section 444. The methods to
adjust this coupling ratio are well-known to those skilled in
the art.
Of course, an EDFA in accordance with the invention can also
be seeded by other means than a preceding EDFA.section. Fig.
34 illustrates in a simplified diagram a source 500 in which
an EDFA 502 is seeded by a master oscillator 504. Master
oscillator 504 can be any suitable source such as a
distributed feedback laser, Fabry-Perot laser, external cavity
diode laser, distributed Bragg reflector laser, vertical
cavity surface emitting laser, semiconductor laser, a fiber
laser or a broadband source. Input light 506 from master
i5 oscillator 504 is coupled into EDFA 502 by a lens 508. Output
light 510 can be derived directly from EDFA 502 or with the
aid of any suitable output coupling mechanism.
Alternatively, the sections of EDFA fiber in any of the
2o preceding embodiments using coiling diameter to control the
ASE emission spectrum and the peak wavelength can take
advantage of appropriate selection of core cross-section,
depressed cladding cross-section, and refractive indices no,
n1, and n2. Specifically, in the first section a first cutoff
25 wavelength 7~C1 is produced by appropriate selection of these
parameters. In the second section a second cutoff wavelength
longer than said first cutoff wavelength ~,~1 is produced.
Then, the first section is positioned before the second
section for seeding the second section in the same manner as
so discussed above. Preferably, .an isolator is positioned
between these two sections. Of course, an additional
adjustment of the ASE emission spectrum of the two sections
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can be performed by coiling the first and second sections as
necessary.
Fig. 35 illustrates the use of a source 520 according to the
s invention to test a device under test 522 (DUT) for
performance characteristics in the S-band. An optical
spectrum analyser 524 is provided to measure the response of
DUT 522. Source 520 generates test light 526 by using any of
the above described configurations. Zight 526 can span a wide
so band or be tuned to a particular output wavelength 7~output~ as
required for testing DUT 522.
The method of producing short-pass fibers in accordance with
the invention will be discussed with reference to Fig. 3~,
15 which shows a preform 600 for a depressed cladding fiber
designed for pulling a short-pass fiber 620. Preform 600 has
a core 612 surrounded by a depressed cladding 614. A
secondary cladding 616 surrounds depressed cladding 614.
Preform 600 is made of primary glass constituent Si02 and is
2o manufactured by hydrolysis, oxidation, sol-gel or any other
suitable method.
Core 612 has a core cross-section that is circular and is
described by a core radius r~. Depressed cladding 614 and
2s secondary cladding 616 have corresponding circular cross-
sections described by radii rd~ and rs~, respectively. Core
612 has a refractive index no, depressed cladding 614 has a
refractive index n1 and secondary cladding 616 has a
refractive index n~. Refractive index no of core 612 is the
3o highest, while refractive index nl of depressed cladding 614
is the lowest. In the present embodiment, refractive index no
is attained by doping core 612 with index-raising dopant such
as germanium or aluminum. Refractive index nl is attained by
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doping depressed cladding 614 with an index-lowering dopant
such as boron or fluorine. Secondary cladding 616 remains
undoped and its refractive index n2 is that of the primary
glass constituent SiO~. The incorporation of index-raising
s dopant ions in core 612 and index-lowering dopant ions in
depressed cladding 614 is performed in accordance with the
hydrolysis or oxidation processes.
Fig. 37A is a graph illustrating a typical refractive index
so profile 622 obtained in practice in preform 600 as a function
of radius (r), i.e., the transverse portion of index profile
622. The incorporation of index-raising dopant ions in core
612 and index-lowering dopant ions in depressed cladding 614
in either the hydrolysis or oxidation processes is controlled
15 by the equilibria established during dopant reaction,
deposition, and sintering. As a result, a depression 624 in
refractive index is present in core 612 of preform 600. The
equilibria further cause a sawtooth pattern 626 in refractive
index to manifest in depressed cladding 614. Thus, refractive
2o index no of core 612 is in fact an average refractive index.
Likewise, refractive index n1 of depressed cladding 614 is
also an average refractive index. Meanwhile, refractive index
n2 of secondary cladding 616 is also an average refractive
index. The actual value of the refractive index in secondary
25 cladding 616 exhibits comparatively low variability as a
function of radius because refractive index n2 is not attained
by doping.
In addition to exhibiting radial variation, actual refractive
3o index profile 622 also varies as a function of position along
an axis 627 of preform 600. In other words, profile 622 has a
longitudinal portion varying along the length of preform 600.
Preform 600 has a total length L and the variation of the
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refractive index at preselected radii as a function of length
is illustrated in the graphs of Fig. 37B. Once again,
refractive indices no and n1 are average refractive indices
while the actual refractive index values exhibit a large
variation. Meanwhile, refractive index n~ is also an average
refractive index while the actual refractive index value
remains relatively constant. Depending on the specifics of
the manufacturing processes, the actual refractive index
values in core 612 and depressed cladding 614 exhibit
so tolerance ranges TRH and TRH that may approach up to 20% along
the length of axis 627.
Fig. 36 also shows how short-pass fiber 620 is obtained by
drawing or pulling preform 600 from an initial cross sectional
s5 area Ao to a final total cross sectional area Af. Initial and
final cross sectional areas are equal to:
Ao = ~cr~ , and
A f = 9zr,,a ,
where r2 is the radius of secondary cladding in pulled short-
pass fiber 620. A drawing ratio DR is defined as the ratio of
the radius of the fiber to the radius of the preform:
DR = ra
Since the refractive indices are nearly preserved during the
pulling process, the average index no of core 612 is
significantly higher than the average index nl of depressed
so cladding 614 and average index n~ of secondary cladding 16 in
pulled short-pass fiber 620. The drawing ratio DR by which
preform 600 is to be pulled to obtain radii ro, rl, r~
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corresponding to core 612, depressed cladding 614 and
secondary cladding 616 in pulled short-pass fiber 620 is made
to achieve certain guiding properties in short-pass fiber 620.
Specifically, the indices and radii are selected to produce a
fundamental mode cutoff wavelength ~,~ such that light in the
fundamental mode at wavelengths smaller than ~,~ is retained in
core 612 while light in fundamental mode at wavelength ~,~ or
longer wavelengths is lost to secondary cladding 616 over a
short distance. This objective is accomplished by ensuring
so that pulled short-pass fiber 620 exhibits the appropriate
average refractive indices no, nl, n~ and cross-sections or
radii ro, r1, r2. In other words,- this goal is accomplished by
appropriately engineering refractive index profile 622 and
cross-sections of core 612, depressed cladding 614 and
15 secondary cladding 616, or, still differently put, by
obtaining the appropriate W-profile, such as the index profile
20 shown in Fig. 1, in short-pass fiber 620.
In any practical short-pass fiber 620 the depressed cladding
2o cross-section has to be larger than the core cross-section.
This is ensured by selecting the core cross-section A~ smaller
than depressed cladding cross-section Ads in preform 600.
Specifically, in preform 600 core cross-section is equal to:
25 A~ =~r2, (16)
while the depressed cladding cross section is equal to:
Aa~ _ ?Lyre _ ~a) . ( 17 )
The aerial ratio established between core and depressed
cladding cross-sections (AC/Ad~) is preserved during the
pulling of preform 600 into short-pass fiber 620. Likewise,
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the values of average refractive indices no, nl, n2 are nearly
preserved during the pulling.
The values of ro, rl, no, n1 and n2 in short-pass fiber 620 not
only define fundamental mode cutoff wavelength ~,~ but also
define a roll-off loss curve with respect to wavelength. Fig.
38 illustrates an exemplary family of loss curves 640 for the
same fundamental mode cutoff wavelength ~,~. It has been found
that only the cutoff wavelength ~,~ gets displaced during the
so pulling of short-pass fiber 620 from preform 600. In other
words, the overall shapes of roll-off loss curves 640 are
basically preserved.
Unfortunately, the actual fundamental cutoff wavelength will
15 differ from point to point along axis 627 due to the variation
in index profile 622 along axis 627. In fact, given that
tolerances TR1 and TRZ for refractive indices no and nl vary up
to 20o (see Fig. 37B), the actual cutoff wavelength can
fluctuate by up to about 200. Therefore, it is clearly not
2o feasible to simply pull preform 600 at the computed drawing
ratio DR to produce short-pass fiber 620 with the calculated
fundamental mode cutoff wavelength ?~~.
Thus, in accordance with the method of invention, a minimum
2s fundamental mode cutoff wavelength 7~m is set before pulling
preform 600. Specifically, minimum cutoff wavelength 7~,m is
set to be the smallest possible value that cutoff wavelength
can assume at any point along axis 627 in pulled short-pass
fiber 620.
Furthermore, core cross-section A~ and depressed cladding
cross-section Ads are measured in preform 600 before pulling.
In the present embodiment, where core 612 and depressed
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cladding 614 are circular this is done by measuring radii r~,
rdC and using the equations given above. Preferably, the
values of radii r~, rd~ are measured at a number of locations
along axis 627 to obtain average values.
The longitudinal portion of refractive index profile 622 in
core 612 and depressed cladding 614 is measured along axis 627
of preform 600 as well. This is conveniently performed by
taking measurements of actual values of refractive indices no,
so nz at regular intervals along axis 627 and at a number of
radii to thus obtain the average values of refractive indices
no, nl. Such measurements can be performed by deflection
tomography, which is well known in the art of optical fiber
preform characterization. It is also convenient to plot the
s5 measurements of refractive indices no, nl in the form of a
graph of average values at each point along axis 627, similar
to the graph shown in Fig. 37A.
In accordance with the invention drawing ratio DR is derived
2o from measured core cross-section A~, depressed cladding cross-
section AdC and the variation in indices no, n1 determined in
preform 600. In particular, drawing ratio DR is set to
achieve a final value of core cross-section A'~. In this
embodiment final value of core cross-section A'~ is defined by
25 the final core radius ro to be obtained in short-pass fiber
620. This is done such that, given a final depressed cladding
cross section A'd~ and indices no, nl final core radius ro
defines fundamental mode cutoff wavelength 7~~ such that ~,~
at all points along axis 627. Preferably, ~,m is set at least
30 5 nm below a lowest value of fundamental cutoff wavelength
along axis 627. This is done as a precaution so that
subsequent fine adjustment of fundamental mode cutoff
wavelength ~,~ in pulled short-pass fiber 620 is still possible
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by standard techniques, such as stressing or bending of the
fiber.
Referring back to Fig. 36, in the next step preform 600 is
pulled by drawing ratio DR determined in accordance with the
above-defined rules. It is important to get as close as
possible to the desired core radius ro during this pull. For
example, when core radius ro is within 0.50 of the desired
core radius, the error in cutoff wavelength 7~~ is within 0.5%.
1o This corresponds to a 5 nm error in cutoff wavelength ~,~ when
operating at a wavelength of 1.0 micron and an 8 nm error when
operating at a wavelength of 1525 nm. This type of error is
barely acceptable for short-pass .fibers used for S-band
amplification with Er-doped fiber, and is more than adequate
is for short-pass fibers used for amplification at 980 nm with
Nd-doped fiber.
In accordance with a preferred embodiment a short pilot
section or test section 634 of preform 600 is pulled first by
zo drawing ratio DR. The pulling of test section 34, sometimes
also referred to as pilot draw, aids in eliminating
systematical errors. That is because the process of pulling
can modify index profile 622. For example, the pulling
process tends to produce a smoothing of index profile 622 due
25 to melting of the glass during the pulling process. Melting
tends to shift actual fundamental mode cutoff wavelength ~,C to
shorter or longer wavelengths depending on the details of the
design of refractive index profile 622 of perform 600 and any
index raising or lowering materials it uses. For example,
so index-lowering dopants consisting of small atoms such as
Fluorine diffuse easily in softened silica glass. Thus, an
index-lowering dopant such as Fluorine diffuses into core 612
and shifts cutoff wavelength ~,~ to a shorter wavelength. This
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is problematic when depressed cladding 614 is deep and wide,
so that significant diffusion into core 612 occurs without
appreciably affecting the average refractive index of
depressed cladding 614. Thus, the pilot draw is useful
because the smoothing of profile 622 cannot be calculated or
modeled with sufficient accuracy to determine its effect on
fundamental mode cutoff wavelength ~,~. _
After pulling of test section 634 fundamental mode cutoff
to wavelength 7~~ is determined in pulled test section 634. This
is preferably done at several points along axis 627. It is
important to choose test section 634 long enough to be
representative of preform 600 and hence of short-pass fiber
620 that will be pulled from preform 600. For this reason
test section 634 should be chosen to be between a few percent
and up to 20 percent of length L. The cutoff wavelength may
be determined experimentally as the wavelength at which light
is lost from the core at a significantly high rate, for
example, at 10 dB/m or 40 dB/m.
Based on the deviation of fundamental mode cutoff wavelength
measured in pulled test section 634 drawing ratio DR is
adjusted to an adjusted drawing ratio DR' as follows:
DR = DR ~ desired ~,~
measured ~,~
Then the remainder of preform 600 is pulled by adjusted
drawing ratio DR' to produce short-pass fiber 620.
so This preferred embodiment of the method is especially useful
in situations when preform 600 exhibits a sufficiently high
uniformity such as about 0.5o for radii or indices. At this
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level of uniformity compensation for dopant diffusion and
other systematic effects is very effective. However, even in
cases where such uniformity is not present, it is still
convenient to pull test section 634 and measure the deviation
of fundamental mode cutoff wavelength 7~~ in test section 634
to determine adjusted drawing ratio DR' for pulling the
remainder of preform 600.
In an alternative embodiment, drawing ratio DR is varied as
so the preform is pulled to compensate for the variations in
refractive index graphed in Fig. 37B. This variable drawing
ratio DR(~) is used to obtain an approximately constant cutoff
wavelength 7~~ along the length of the fiber. As above, the
variable drawing ratio DR(z) may be multiplied by a factor
15 ~,~ (desired) /?~,~ (measured) once the cutoff wavelength of the
test section is measured, to compensate for systematic errors
due to the pulling process.
In some embodiments of the method secondary cladding cross-
2o section is adjusted before the pulling step. In the present
embodiment this is done by increasing or decreasing secondary
cladding radius rs~. For example, radius rSC of secondary
cladding 616 is augmented by a rod-in-tube (also sometimes
called "sleeving") technique or outside vapor deposition
25 (OVD) . Alternatively, radius rs~ of secondary cladding 616 is
reduced by a technique such as etching. For more information
on these techniques the reader is referred to Erbium-Doped
Fiber Amplifiers Fundamentals and Technology by P. C. Becker,
N. A. Olsson, and J. R. Simpson, chapter 2 (Optical Fiber
3o Fabrication), published by Academic Press, pp. 13-42. The
necessity to augment or reduce secondary cladding cross-
section before pulling it arises when the pulled fiber is
supposed to have a certain, e.g., standard, outside diameter
CA 02478314 2004-09-07
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(OD), such as 125 +/-1 microns or 80 +/-1 microns. It is
important to maintain standard fiber OD when low loss splicing
to standard single mode fiber is required. The remainder of
the method is performed in accordance with the above-discussed
s principles for designing short-pass fiber 620.
The method of invention can be used for pulling short-pass
fibers to obtain ~5 nm control of fundamental mode cutoff
wavelength ~,~ in performs with random variations in index or
to cross sections of up to 200. Without the method such
variations cause >100 nm unpredictable shifts in fundamental
mode cutoff wavelength 7~,~. This advantage can be obtained
even when systematic shifts are present.
s5 Fundamental cutoff wavelength ~,~ in pulled short-pass fiber
620 can be further adjusted by stressing or coiling fiber 620
in accordance with well-known principles. That is because the
fundamental mode cutoff wavelength 7~~ gets displaced during
coiling of short-pass fiber 620 relative to the fundamental
ao mode cutoff wavelength of short-pass fiber 620 when straight.
Meanwhile, the overall shape of roll-off loss curves 640, as
shown in Fig. 38 is basically preserved. Hence, one can use
the coiling diameter of short-pass fiber 620 to make fine-
tuning adjustments to fundamental mode cutoff wavelength
25 after short-pass fiber 620 has been drawn from preform 600.
This can be done to compensate for slight errors in selecting
the correct drawing ratio DR, or to compensate for slight
errors in pulling to an adjusted drawing ratio DR'. This can
also be done to compensate for slight shifts in fundamental
3o mode cutoff wavelength 7~,~ resulting from diffusion of dopants
in short-pass fiber 620 during the pulling process, or to
compensate for slight variations in the longitudinal portion
of refractive index profile 622 of perform 600 along axis 627.
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Alternatively, it is also possible to determine drawing ratio
DR with a particular coiling radius in mind. This is
frequently the case when short-pass fiber 620 is to be
packaged in a box of prescribed dimensions. The effect of
coiling on cutoff wavelength 7~~ is considered in determining
drawing ratio DR in these situations. Specifically, since
decreasing the coiling diameter (smaller coil) shifts cutoff
wavelength 7~~ to shorter wavelengths the amount of shift for
2o the desired coiling diameter has to be added when setting
minimum fundamental mode cutoff wavelength ~,m before drawing
preform 600. For example, coiling a fiber at a diameter of 50
nm shifts fundamental mode cutoff wavelength 7~,m by about 20 nm
to 200 nm as compared with fundamental mode cutoff wavelength
7~,~, for the same fiber when straight. This shift increases
approximately in proportion to the curvature (inverse of the
diameter) of the fiber. The magnitude of this shift depends
on the mode field diameter (MFD) of the fiber and also depends
on the outside diameter (OD) of the fiber. In general, a
2o fiber with a larger MFD is more sensitive to coiling (due tc
the increased stresses produced for a given bend diameter).
The sensitivity to bending of a particular fiber design (i.e.,
given MFD, OD, etc.) can be measured at the pilot draw stage.
In the embodiments discussed above the cross-sections
described by radii r~, rd~ and rSC in preform 600 exhibit only
small variation along the length of preform 600 or along axis
627 and hence do not cause significant variations of final
cross-sections described by radius ro or radii rl, r2 in pulled
so short-pass fiber 620 along axis 627. In fact, when the
variations in drawing ratio DR, radii and indices (i.e., the
corresponding tolerances in DR, radii and indices) remain
within .3% in preform 600 the resulting pulled fiber will have
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sufficient performance to amplify signals in the S-band. For
other bands, such as the C- and L-bands the tolerances are
even greater.
A person skilled in the art will realise that the method of
invention can be employed for pulling any type of short-pass
fiber. In particular, it is possible to pull fibers with
active cores, e.g., Er or Tm doped cores. The same steps as
described above can be used in pulling such fibers, and the
so additional effects on the refractive indices introduced by the
active dopants will typically be automatically included in the
measurements of refractive index profile and fundamental mode
cutoff wavelength 7~~.
s5 It will be clear to one skilled in the art that the above
embodiments may be altered in many ways without departing from
the scope of the invention. Accordingly, the scope of the
invention should be determined by the following claims and
their legal equivalents.
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