Note: Descriptions are shown in the official language in which they were submitted.
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Communication System and Split-band Amplifying Apparatus Using a
Depressed-Profile Fiber Amplifier
s RELATED APPLICATIONS
This application is related to U.S. Application numbers
10/095,303, filed 8 March 2002, 10/186,561, filed 28 June 2002,
and 10/346,960, filed 17 January 2003.
so FIELD OF THE INVENTION
The present invention relates generally to communication systems
and split-band amplifying apparatuses that amplify optical
signals whose wavelength band covers a short wavelength band
such as the S;band and a long wavelength band such as the C-
15 band, and more particularly to a communication system and a
split-band amplifying apparatus employing depressed-profile
fiber for amplifying the short wavelength band.
BACKGROUND OF THE INVENTION
2o The problem of amplifying optical signals for long distance
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
25 with a laser at a wavelength of 980 nm or 1480 nm. The doped,
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
so amplifier into a laser. Early EDFAs could provide 30 to 40 dB
of gain in the C-band extending between 1530 to 1565 nm with.
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noise figures of less than 5 dB. Recently, EDFAs have been
developed that can provide similar performance in the L-band
(1565 to 1625 nm) as well as in the C-band.
There is great interest in developing a broad or wide band
amplifier that can amplify optical signals spanning the C- and
L-bands and shorter wavelengths in the so-called "S-band" or
"short-band". Although poorly defined at present, the S-band is
considered to cover wavelengths between about 1425 nm and about
so 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.
Unfortunately, at present no efficient mechanism exists for
i5 suppressing ASE at 1530 nm and longer wavelengths in an EDFA.
The prior art offers various types of waveguides and fibers in
which an EDFA can be produced. Most waveguides are designed to
prevent injected light from coupling out via mechanisms such as
2o 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
25 Electronics, Vol. QE-18, No. 10, October 1982, pp. 1467-72.
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
so leak light at long wavelengths, as discussed above, and are more
sensitive to bending than other fibers.
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In producing an EDFA for the S-band the relatively high losses
and low gains over the S-band render the selection of fiber and
fiber profile even more difficult. In fact, the problems are so
s 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
to EDFA with discrete Raman Amplifier", ECOC-X001, Post Deadline
Paper. In Ishikawa et al.'s experimental setup, the length of
each EDA is 4.5 meters. The absorption of each suppressing
filter at 1.53 [am is about 30 dB and the insertion losses of
each suppressing filter at 1.48 ~,m and 0.98 ~m are about 2 dB
15 and 1 dB respectively. The pumping configuration is bi-
directional, using a 0.98 hum wavelength to keep a high inversion
of more than D>-0.7 (D, 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
20 1518.7 nm with 9 dB gain tilt.
This method is relatively complicated and not cost-effective, as
it requires five EDFAs, four ASE suppressing filters and high
pump power. Also, each of the ASE suppressing filters used in
2s Ishikawa et al.'s method introduces an additional insertion loss
of 1-2 dB. The total additional insertion loss is thus about' 4-
8 dB.
In U.S. Patent No. 6,049,417 Srivastava et al. teach a wide band
30 optical amplifier which employs a split-band architecture. This
amplifier splits an optical signal into several independent sub-
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bands that then pass in parallel through separate branches of
the optical amplifier. Each branch may be optimized for the
sub-band that traverses it. In one embodiment Srivastava et al.
teach to equip one of the branches with a number of S-band EDFAs
and a number of gain equalization filters (GEFs) interposed
between the S-band EDFAs to obtain amplification in the S-band.
The other sub-bands in this embodiment have EDFAs for amplifying
the C- and L-bands respectively. Unfortunately, the S-band
branch of this wide band amplifier suffers from similar
so disadvantages as discussed above in conjunction with Ishikawa.
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-[gym Wavelength Region"
by Tadashi Kasamatsu, et. al., in IEEE Photonics Technology
Letters, Vol. 13, No. 1, January 2001, pg. 31-33 and references
therein. While good optical performance has been obtained using
2o TDFAs, this performance has only been possible using complex,
non-standard and/or expensive pumping 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.
Optical amplifiers (such as EDFAs, TDFAs, Raman amplifiers,
semiconductor amplifiers, etc...) are used for several purposes in
a telecommunications network. The most important use is to
so compensate for span loss (transmission fiber loss accumulated
over tens or hundreds of km), in which case the amplifier is
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typically called an "in-line amplifier". In-line amplifiers
must provide a small-to-moderate amount of optical power per
optical channel (typically 0.1-10 mW), but must also exhibit low
noise figure and good gain flatness in the case of WDM networks.
s The latter two requirements result from the accumulated effects
of a long cascade of amplifiers over a long fiber link of
hundreds to thousands of km in length.
Optical amplifiers are also used as pre-amplifiers. Pre-
so amplifiers are typically used in order to improve the
sensitivity of receivers, in ways that are well known in the
art. Typically, the pre-amplifier is located just before the
signal receiver in order to increase the signal strength
(optical power) to a level well above the (electronic, or
z5 thermal) detector noise. Pre-amplifiers should exhibit very
good noise figure, though they do not need to operate at high
powers or with flattened gain profiles because they typically
are used to amplify one or a small number of optical channels.
2o Optical amplifiers are also used as power-amplifiers. Power
amplifiers are used in ways well known in the art to provide
high optical power. Typically, they are operated with
relatively high input signal strengths (i.e. are saturated) with
good input signal-to-noise ratios, and therefore do not need
25 very good noise figures. Also, they typically do not need very
high gain. Power amplifiers are used when a large number of WDM
channels are present, even when, for example, each channel needs
only a moderate level of power. Power amplifiers are also used
preceding long/lossy links in order to pre-compensate for the
so upcoming losses.
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Finally, optical amplifiers are used for many other applications
inside communications networks. Some examples are: power
boosting prior-to or after splitting a signal into many parallel
outputs, compensating for lossy network modules such as cross-
connects and switches, providing high enough optical powers to
pump nonlinear devices such as optically driven optical switches
or optical wavelength converters.
In view of the above, it would be an advance in the art to
to provide a wide band amplifier that amplifies optical signals
spanning the S-, C- and L-bands and exhibits high efficiency in
the S-band. Specifically, it would be an advance to provide
such wide band amplifier that amplifies optical signals in the
S-band without requiring many filters and takes full advantage
Of a minimum number of pump sources. In addition, it would be
an advance in the art to provide an optical communication system
that can take advantage of EDFAs for amplifying signals in the
S-band. In particular, it would be advantageous to provide low-
cost S-band EDFAs for use in such communication systems to
2o achieve low-cost pre-amplification, power-boosting and in-line
amplification.
OBJECTS AND ADVANTAGES .
In view of the shortcomings of the prior art, it is a primary
2s object of the present invention to provide a wide band amplifier
for optical signals spanning a long wavelength band such as the
C- and/or L-band and a short wavelength band such as the S-band.
In particular, it is an object of the invention to provide a
wide band amplifier that can use Er-doped fiber amplifiers
30 (EDFAs) in conjunction with an efficient pumping arrangement.
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It is a further object of the invention to provide a wide band
amplifier that uses a minimum number of parts especially in the
S-band.
It is another object of the invention is to provide a method for
designing wide band fiber amplifiers.
It is yet another object of the present invention to provide an
optical communication system that can transmit and amplify
to signals in the S-band of wavelengths. Specifically, the optical
communication system employs an Er-doped fiber amplifier (EDFA)
to amplify signals contained in the S-band in a controlled
fashion to enable pre-amplification, power-boosting and in-line
amplification.
These and other advantages of the present invention will become
apparent upon reading the following description.
SUN~QARY
2o The objects and advantages of the invention are achieved in a
split-band amplifying apparatus having a first section for
amplifying a long wavelength band of an optical signal and a
second section equipped with a fiber amplifier for amplifying a
short wavelength band of the optical signal. The fiber
z5 amplifier in the second section has a core with a core cross-
section and a refractive index no. An active material is doped
in the core. The core is surrounded by a depressed cladding
that has a depressed cladding cross-section and a refractive
index nl. The depressed cladding is surrounded by a secondary
3o cladding that has a secondary cladding cross-section and a
refractive index n2. A pump source is provided for pumping the
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active material to a high relative inversion D, such that the
active material exhibits positive gains in the short wavelength
band and high gains in the long wavelength band. The core
cross-section, the depressed cladding cross-section and
refractive indices no, nl, and n2 are selected to produce a roll-
off loss curve about a cutoff wavelength 7~~, such that the roll-
off loss curve yields losses at least comparable to the high
gains in the long wavelength band and losses substantially
smaller than the positive gains in the short wavelength band.
so In a preferred embodiment the active material is Erbium such
that the fiber amplifier is a first Erbium-doped fiber amplifier
(EDFA).
The short and long wavelength bands can be selected based on the
application. For example, in telecommunications the short
wavelength band can be chosen to contain at least a portion of
the S-band and the long wavelength band can be chosen to contain
at least a portion of the C-band. With this choice of
wavelength bands the cutoff wavelength ~,~ is set at a crossover
2o wavelength between the S-band and the C-band, e.g., at about
1530 nm. Of course, the long wavelength band can contain an
even wider range of wavelengths, e.g., it can also contain at
least a portion of the L-band.
In the preferred embodiment, the first section of the split-band
amplifying apparatus that is designed for amplifying the long
wavelength band has a second Erbium-doped fiber amplifier.
Thus, both first and second sections utilize second and first
EDFAs respectively for amplifying the optical signal.
so Furthermore, a common pump source can be used for delivering
pump radiation to the first and second EDFAs. For example, the
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common pump source can be a laser diode delivering pump
radiation at about 980 nm.
The split-band amplifying apparatus can be designed in many
different ways. In one embodiment the first and second sections
of the apparatus share an overlapping segment. In this
embodiment the overlapping segment can contain the second EDFA
for amplifying the long wavelength band. Alternatively, the
first and second sections can be separate and form separate
~.o branches of the apparatus.
In accordance with a method of the invention, the split-band
amplifying apparatus is used to amplify optical signals spanning
the short and long wavelength bands. When first and second
is sections of the apparatus use EDFAs for amplifying the optical
signal, it is convenient to co-pump and/or counter-pump the
EDFAs. The EDFAs can be co-pumped and/or counter-pumped from
the same source or from separate sources.
2o The present invention further provides an optical communication
system employing a signal source for providing a signal in the
S-band of wavelengths, or simply in the S-band. The
communication system includes an optical fiber that transmits a
signal in the S-band and one or more optical amplifiers that
as typically is in the form of a fiber amplifier. The fiber
amplifier has a core defined by a core cross-section and a
refractive index no. Erbium is doped into the core of the fiber
amplifier for amplifying the signal. The fiber amplifier has a
depressed cladding surrounding the core and a secondary cladding
3o surrounding the depressed cladding. The depressed cladding has
a depressed cladding cross-section and a refractive index nl,~and
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the secondary cladding has a secondary cladding cross-section
and a refractive index nz. The fiber amplifier has a pump source
for pumping the Erbium contained in the core to a level of high
relative inversion D~ in this state the Erbium can amplify the
signal. In particular, the pumping causes Erbium to exhibit
positive gains in the S-band and high gains in a long wavelength
band longer than the S-band, i.e., in the C- and L-Bands. The
core cross-section, the depressed cladding cross-section and the
refractive indices no, n1, and n2 are selected to obtain losses
so 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 the optical communication system the fiber amplifier
s5 functions as a pre-amplifier, a power-boosting amplifier or an
in-line amplifier. The optical communication system can be a
Wavelength-Division-Multiplexed (WDM) communication system,
e.g., a Dense WDM (DWDM) system. The WDM system has a
communication fiber, typically a long section or span of
2o communication fiber, between a Wavelength-Division-Multiplexes
or WDM multiplexes and a WDM demultiplexer. The WDM multiplexes
multiplexes a number of information-bearing signals and launches
them over the communication fiber from a transmitting end. The
WDM demultiplexer demultiplexes the signals arriving on the
25 communication fiber at the receiving end. When the fiber
amplifier is used as a pre-amplifier, it is preferably installed
after the WDM demultiplexer. The fiber amplifier can also be
installed between the WDM multiplexes and demultiplexer to serve
the function of a power-boosting amplifier or an in-line
so amplifier. Of course, the fiber amplifier can also serve the
function of a pre-amplifier in this position.
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In a multiplexed system, such as the WDM communication system
the signal source preferably comprises a laser belonging to a
laser array. 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 radiation at about 980 nm. Alternative sources delivering
pump radiation at about 980 nm or other Er pump bands at
wavelengths shorter than the wavelengths contained in the S-band
1o can also be used.
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 depressed-profile fiber
and its guided and unguided modes according to the
invention.
2o Fig. 2 is a graph illustrating a typical index profile in the
fiber of Fig. 1.
Fig. 3 is a graph illustrating x as a function of the ratio s
for various values of the parameter p;
Fig. 4 is a graph illustrating appropriate selection of the
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 an isometric view of an S-band EDFA adapted for use
in accordance with the invention.
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Fig. 6 is a diagram of a split-band amplifying apparatus with
an S-band EDFA and a C-band EDFA and having an
overlapping segment.
Fig. 7 is a diagram of a split-band amplifying apparatus
having non-overlapping first and second sections.
Fig. 8 is a diagram illustrating the use of EDFAs in an
optical communication system.
Fig. 9 is a diagram illustrating the use of EDFAs as pre-
amplifiers at the receive end of an optical
so communication system.
Fig. 10 is a diagram illustrating a portion of an optical
communication system employing EDFAs for amplifying
sub-bands of the S-band.
DETAINED 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 Figs.
1-2. Fig. 1 is a diagram illustrating a portion of a cross-
2o 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<_rl and r>-r1. 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 refraction n~. The graph positioned
above the partial cross-section of fiber 10 illustrates an
3o average index profile 20 defining a W-profile in fiber 10. In
the present embodiment fiber 10 is a single mode fiber.
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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 that 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 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
so gains in the short wavelength band, making it impossible to
effectively amplify signals in the short wavelength band.
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. When using Erbium, 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 EDFA can be
2o 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 pump Thulium at 1.48
microns, though very high intensities would be needed, possibly
3o as high as 100 mW. 100mW at 1.48 microns is easily obtainable
with commercially available high quality diode pumps with about
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500mW at 1480nm and nearby wavelengths. Another good pump
wavelength is 1530nm where high power sources, up to Watts, are
available.
s 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, 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 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 the
fiber is engineered with a fundamental mode cut-off between 1.9
2o microns and the shorter wavelength of desired operation, and if
the cut-off is selected 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 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.
Fig. 2 illustrates a W-profile 20A as is obtained with normal
3o manufacturing techniques. For the purposes of the invention it
is sufficient that the radially varying index of core 12 have an
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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 of
core 12 is significantly higher than index nz of depressed
cladding 14 and index n~ of secondary cladding 16. The selection
of appropriate values of indices no, nl, nz and radii ro, r1, rz
is made to achieve certain guiding properties of fiber 10, as
required by the instant invention. Specifically, profile 20 is
engineered to have a fundamental mode cutoff wavelength 7~~ such
1o that light in the fundamental mode at wavelengths smaller than ~,
is retained in pore 12 while light in fundamental mode at
wavelength ~,~ or longer wavelengths is lost to secondary cladding
16 over a short distance. This 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) 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~,C for fiber
10 is set in accordance to selection rules for cross-sections
and refractive indices no, nl and n2 of fiber 10 as derived from
Maxwell's equations. In the weak 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
a5 vector equations can be replaced with a scalar equation. The
scalar ~1 represents the 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,
3o Princeton, 1970 , and D. Marcuse, "Theory of Dielectric Optical
Waveguides" (Academic, New York, 1974).
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For convenience, let us define the following parameters:
uo = uo2 - y~22 and ul = ra22 - fZl2
10
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 pore 12 the scalar field ~ is thus:
= Jo (K r) , 0<-r<_ro (region I)
where K is an eigenvalue that needs to be determined, and Jo is
the ~eroth Bessel's function.
Inside depressed cladding 14, the scalar field ~t is:
~l = A Ko ((3 r) + B Io ((3 r) , ro<_r<_rl (region II) (3)
2o where A and B are constants to be determined,
~2 =(uo2 -~-u12)(2,~~~)2 _~.2, and Ko and Io are the modified Bessel's
functions. Here ~, is the vacuum wavelength of the light.
In secondary cladding 16, we obtain:
= C Ko (~y r) , r>_rl (region III) (4)
Here C is another constant, and Y~ -u~~(2~ ~~'~l K~ . A, B, C, and K
are found using the boundary conditions, which require that
so and its first derivative are both continuous at ro and rl.
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It can be shown that fundamental mode cutoff wavelength 7~~ is a
wavelength ~, at which y = 0. (See for example, Cohen et al., IEEE
J. Quant. Electron. QE-18 (1982) 1467-1472.)
For additional convenience, let us define the following
parameters:
2~c uo~o
~ . p = Ui/UO. S = -rn-ro. (5)
c
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:
p Jo(x) K1(px) Ii(Psx) - p Jo(x) Ii(px) E1(psx)
- J1(x) W (psx)Io(px) - JO x) I1(Psx) Ko(px) - 0. (6)
Three observations should be made regarding the parameter x.
2o 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:
s2 >- 1 + 1/p2. (7)
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
so large enough that it is easy to couple light into and out of
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core 12. (A smaller core 12 also makes the nonlinear effects
stronger, which is often a disadvantage.) Therefore, since x -
2~uoro/~,~, preferably x >- 1. This implies that p >- 0.224 or, in
terms of the refractive indices .~(n2 -rai ~/(no -Yr~) >_0.224 .
Third, it is evident from Fig. 3 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
so small effect on the value of fundamental mode cutoff wavelength
Therefore, it is convenient to use the rule s >_ 1 + 1/p, or
in terms of the refractive indices:
>_ I + ~(f2o - h2 )l \j2' ~1
Y '~o
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 7~~. First, ~,~ can be pre-selected, e.g. a wavelength
2o close to 1530 nm, and then convenient values are selected for uo
and r~. 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 ~,~.
2s Typically, all values of p are larger than 0.224. In addition,
the rule of equation 8 is used to further narrow the range of
suitable values of p and s.
Finally, the values of s and p have an additional limitation.
so Namely, they must be selected so that core 12 of fiber 10 has a
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great enough loss, e.g., 100 dB/m or even 200 dB/m or more at a
wavelength ~,>7~~. To find the loss at wavelength 7~>~~. the fiber
modes for light having wavelength 7~>7~~ are required.
Equations (2), (3), and (4) specify the fundamental mode when
When ~,>7~~, the function yl is oscillatory, rather than
exponentially decaying, in secondary cladding 16. Therefore
when ~,>~,~, Eq. ( 4 ) is replaced by:
to ~t = C Jo (qr) + D No (qr) , r>_rl (region III) (9)
where No (also called YO) is the zeroth Neumann function,
qa = KZ -uo2(~~ /~)2 , and C and D are constants to be determined.
There are two key items to note regarding the modes for 7~,>7~~.
First, there are five unknowns (A, B, C, D, and t~) and four
boundary conditions (continuity of ~ and d~/dr at ro and r1).
The equations are underconstrained: K may be chosen to be any
value between 0 and (2~/~,) uo2 +u12 . Thus, there is a continuum of
2o states for each 7~>~,~, corresponding to the continuum of values
that tc may have. This situation is quite different from the
case 7~<~,~, where four unknowns (A, B, C, and ~c) are 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 do
not correspond to the situation that is physically realized.
This is a result of Eq. (9) containing both incoming and
so outgoing waves, whereas in practice only outgoing waves are
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present (the light at wavelength ~,>~,~ 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 ~,~. First, for a
given wavelength 7~, find the value of ~ that minimizes CZ + D~.
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 ~ in the fiber and the quantum mechanical wave
to 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.)
Second, once K is found in the above manner, the outgoing waves
s5 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 7~>~,~ propagating in
core 12 to be attenuated along the length of the fiber. If the
2o beam has power P, then the change in power P with distance
along fiber 10 is described by the equation:
dP
(10)
25 The loss is given by the coefficient A, which is approximately:
~, C~ + DZ . ( )
11
2 0 ro
4~c h ~ rdr t~*tJ~
0
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The loss A, having units of m 1, can be converted to a loss (3 in
units of dB/m, using the, relation:
(3 = 10 loglo (e) ~A . (12)
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, if it remains in secondary cladding
16. In some cases this will be sufficient. In other cases
so light from secondary cladding 16 can be out-coupled or absorbed,
as necessary.
Another method for calculating the losses involves calculating
the complex propagation constant of the leaky fundamental mode
i5 of fiber 10. Zeaky 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 leaky mode. The
complex propagation constant, or its equivalent that is the
20 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
25 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
Fig. 1, but have variations from the ideal as shown by graph 20A
in Fig. 2 of the actual refractive index profile obtained in
so practice. In particular, the most common method of single-mode
fiber manufacture today involves the MOCVD process, which
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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 can
again give fundamental mode cutoff wavelength 7~~ 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.
to G~lhen Eq. (11) is used to estimate the loss, refractive indices
no, nl, 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 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.
2o Here is the prerequisite for the fiber to have fundamental mode
cutoff wavelength 7~,~. Zet 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 wavelength ~,~ if
(see B. Simon, Ann. Phys. 97 (1976), pp. 279):
tar R
f d6 f YdY~n2 (Y, 6) - n22 ~ <_ 0 . ( 13 )
0 0
Note that given the profile of Fig. 1, Eq. (13) becomes:
~ roe uo2 - ~ ( ru - ro2 ) ul2 ~ S . ( 14 )
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which is equivalent to Eq. (7) above.
Fundamental mode cutoff wavelength 7~~ is the largest 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 the smallest outgoing
so 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 ~,~ and losses can therefore be designed by adjusting
the profile n(r,6), which is equivalent to adjusting the cross-
sections and refractive indices of core 12, depressed cladding
14 and secondary cladding 16.
The rules presented above will enable a person skilled in the
2o art to set fundamental mode cutoff wavelength ~,~ by making a
selection of ro, rl, no, nl and n~ . This selection of ro, rl, no.
ni and n2 provides distributed ASE suppression over the length of
the fiber 10 and results in a family of loss curves 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 objectives of the present
invention, as discussed below.
Referring back to Fig. 1, superposed on average index profile 20
3o is an intensity distribution of a guided fundamental mode 22 at
a first wavelength ~,1<7~~. First wavelength ~,1 is contained
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within a short wavelength band, e.g., the S-band. A fundamental
mode 24 that is no longer guided by fiber 10 is also superposed
on index profile 20. Mode 24 is at cutoff wavelength ~,~. An
intensity distribution of another mode 26 that is not guided by
fiber 10 and exhibits an oscillating intensity distribution
beyond core 12 and depressed cladding 14 is also shown.
Radiation in mode 26 has a second wavelength ~,~, which is longer
than cutoff wavelength ~,~<7~2 and is contained in a long
wavelength band, e.g., in the C- or L-band.
Fig. 4 illustrates a gain profile 44 of active material 18, in
this case Erbium, when pumped to a high relative inversion D.
The S-band is designated by reference 42 and long wavelength
band is designated by reference 46. A crossover wavelength
between S-band 42 and long wavelength band 46 is also indicated.
Gain profile 44 exhibits high. gains in long wavelength band 46
and positive gains in S-band 42. In particular, high gains in
long wavelength band 46 include a peak 48 at about 1530 nm that
is very close to crossover wavelength 7~Cross.
In this embodiment the cross-sections or radii of core 12,
depressed cladding 14 and refractive indices no, nl, and n2 are
selected to place cutoff wavelength ~,~ right at peak 48 just
within long wavelength band 46. Additionally, the value of
2s index no of core 12 is selected to obtain a roll-off loss curve
38 about cutoff wavelength 7~~ 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
3o substantially smaller than the positive gains in S-band 42.
Roll-off loss curve 38 drops below the positive gains indicated
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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 S-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. For more information on selecting
appropriate roll-off loss curves the reader is referred to U.S.
Patent Application 10/095,303 filed on March 8th, 2002.
so A split-band amplifying apparatus of the invention takes
advantage of W-profile fiber designed in accordance with the
above rules. In fact, the apparatus of invention preferably
uses w-profile fiber designed in accordance with the above rules
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 L-band or a select
portion or portions of these two bands. Preferably, the host
material of fiber 10 is silicate-containing glass such as
alumino-germanosilicate glass, lanthanum doped germanosilicate
2o glass, aluminum/lanthanum co-doped germanosilicate glass, or
phosphorus doped germanosilicate glass. For example, the split-
band amplifying apparatus can use an Er-doped amplifier ~8
(EDFA) using alumino-germanosilicate glass as the host material,
as shown in Fig. 5. In this example, 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 n1 and a
secondary cladding 74 of index n2. EDFA 68 has a protective
jacket 76 surrounding secondary cladding 74 to offer mechanical
stability and to protect EDFA 68 against external influences.
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An optical signal 78 at a first wavelength ~,1 contained within S-
band 42 is delivered to EDFA 68 for amplification from a fiber
80. For example, optical signal 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 light 88 from a pump source 86
to EDFA 68. Pump source 86, preferably a laser diode, provides
pump light 88 at a pump wavelength ~,p of about 980 nm for pumping
so the Er ions in core 70 to achieve a high level of relative
population inversion D. Parameter D varies from D=-1 indicating
no population inversion to D=1 signifying complete population
inversion. When 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 light 88 is
2o determined such that it ensures a relative inversion of D>-0.7 in
the Er ions.
Pump light 88 and signal light 78 are combined in combiner 84
and both delivered to EDFA 68 by fiber 80. More particularly,
both optical signal and pump light 78, 88 are coupled into core
70 from fiber 80.
Core 70 and claddings 72, 74 all have circular cross sections in
this embodiment. The cross sections and indices n~, nl, n2 are
so selected in accordance with the method of invention to set
cutoff wavelength ?~,~ near 1525 nm. In other words, cutoff
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wavelength ~,~ is selected to be between S-band 42 and long
wavelength band 46 or the C-band and L-band.
It is important that index no of core 70 be chosen to provide for
s a large negative slope in effective index neff with respect to
wavelength, preferably about .008/1,000 nm, near cutoff
wavelength ~,~. As a result, the roll-off loss curve exhibits a
rapid decrease for wavelengths below cutoff wavelength ~
ensuring that the losses in S-band 42 are lower than the
to positive gains. The losses produced by this roll-off loss curve
increase rapidly for wavelengths larger than cutoff wavelength
Thus, the losses produced in the C- and L-bands 46 are at
least comparable to the high gains.
15 Designing EDFA 68 in accordance with the invention will ensure
that optical signal 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 42 will typically be on the order of 1 dB per meter (or,
2o depending on fiber design and on which wavelengths within the S-
band are of greatest interest, 0.2-5 dB/meter) above the losses
and thus, to obtain sufficient amplification of optical signal
78, EDFA 68 requires a certain length L. The esmaller the
difference between the positive gains and losses in the S-band
2s 42, the longer length L has to be to provide for sufficient
amplification of optical signal 78. For a useful S-band
amplification of 25 dB, the length L may need to be about 5
meters to over 100 meters.
3o Fig. 6 illustrates a split-band amplifying apparatus 100 in
accordance with the invention. Apparatus 100 has an input fiber
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102 for receiving an optical signal 104 to be amplified.
Optical signal 104 spans a long wavelength band 106 and a short
wavelength band 108 separated by a crossover wavelength 7~~ross as
indicated. Apparatus 100 also has a pump input fiber 112 for
s receiving a pump radiation 116 from a pump source 114. In the
present embodiment, pump source 114 is a laser diode emitting
pump radiation 116 at a pump wavelength 7~p=980 nm.
Apparatus 100 has a segment 118 shared by a first section 130
so and a second section 132 of apparatus 100. In other words,
segment 118 is where first and second sections 130, 132 overlap.
Second section 132 has a fiber amplifier 138 positioned beyond
segment 118 for amplifying short wavelength band 108 of optical
signal 104. Preferably, fiber amplifier 138 is a first EDFA.
s5 Segment 118 has an amplifier 122 for amplifying long wavelength
band 106 of optical signal 104. It should be noted that
amplifier 122 does not have to be an EDFA or even a fiber
amplifier.
2o Amplifier 122 should provide some amplification (or at least
zero amplification, but no loss) to short wavelength band 108
while providing useful gain for long wavelength band 106. In a
preferred version of apparatus 100, amplifier 122 should provide
enough amplification of short wavelength band 108 so as to
25 compensate for any losses experienced by short wavelength band
108 in components preceding fiber amplifier 138. In a further
preferred version of apparatus 100, amplifier 122 should provide
low-noise amplification of short wavelength band 108 so as to
minimise the noise impact of any losses experienced by short
3o wavelength band 108 in components preceding fiber amplifier 138.
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Segment 118 is also equipped with a coupler 120 and a coupler
124. Coupler 120 is any suitable optical coupling device, e.g.,
a WDM coupler capable of joining optical signal 104 and pump
radiation 116 and launching them through amplifier 122 to
achieve amplification of optical signal 104. In the present
embodiment amplifier 122 is a second EDFA and long wavelength
band 106 spans the C-band. Coupler 124 is any suitable optical
splitter, e.g., a WDM coupler capable of splitting signal 104
into two signals 104A and 104B. In the present example coupler
124 is a 50/50 WDM coupler such that signal 104 is split into
signals 104A, 104B of equal intensity and spectral content.
Alternatively, a coupler with a different splitting ratio or a
band combiner that separates the short wavelength band 108 from
the long wavelength band 106 can be used.
Coupler 124 is connected to first section 130 and to second,
section 132. In particular, coupler 124 is set to send signal
104A along first section 130 and signal 104B along second
section 132. First section 130 contains a delay element 126,
2o which can be a certain length of fiber for compensating delays
in time of travel between first section 130 and second section
132. First and second sections 130, 132 are recombined by a
coupler 128, which is preferably a band combiner for combining
signals 104A and 104B in the short and long wavelength bands
108, 106 while rejecting radiation at other wavelengths such as
a residual pump radiation 116'.
Second section 132 is equipped with a coupler 134 for out-
coupling residual pump radiation 116' out of section 132.
3o Coupler 134 is followed by an isolator 136 and first EDFA 138.
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Isolator 136 is set to pass signal 104B to first EDFA 138 and
block any radiation from propagating back to segment 118.
First EDFA 138 is constructed in accordance with the above-
described rules to amplify signal 104B in short wavelength band
108, i.e., in the S-band in this embodiment. A pump source 140
is provided for generating pump radiation 142 for first EDFA
138. Pump source 140 is a diode laser emitting pump radiation
142 at a pump wavelength 7~p=980 nm. An input fiber 144 and a
to coupler 146 are provided for delivering pump radiation 142 to
first EDFA 138. Coupler 146 is connected such that pump
radiation 142 is delivered from the opposite direction to that
of advancing signal 104B. In other words, pump radiation 142 is
counter-propagating to signal 104B in first EDFA 138. It should
be noted that any residual pump radiation 142' is blocked from
propagating back to segment 118 by coupler 134 and also by
isolator 136. Counter-propagation of the pump provides for
highly efficient pump-to-signal conversion efficiency in
amplifier 138.
Coupler 146 is set to pass signal 104B amplified by first EDFA
138 within short wavelength band 108, i.e., the S-band, to
coupler 128. Coupler 128 re-comlaines signal 104A amplified in
long wavelength band 106 and signal 104B amplified in short
wavelength band 108 to reconstitute an amplified optical signal
104'. An optional filter 148, e.g., a gain flattening filter
(GFF), can be provided to equalize amplified optical signal
104' .
3o During operation, optical signal 104 enters apparatus 100 via
input fiber 102 and is delivered together with pump signal 116
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to second EDFA 122 in segment 118 along the same direction (in
co-propagating geometry). Second EDFA 122 amplifies the portion
of signal 104 in long wavelength band 106, and in preferred
embodiments also provides a small degree of amplification for
the portion of signal 104 in short wavelength band 108.
Coupler 124 splits signal 104 amplified in long wavelength band
106 into signals 104A and 1048 and sends them into remaining,
non-overlapping portions of sections 130 and 132, respectively.
to Signal 104A experiences no further amplification in section 130.
Meanwhile, the portion of signal 104B spanning short wavelength
band 108 is amplified by first EDFA 138.
Most of pump radiation 116 is preferably used up in second EDFA
122 and any residual pump radiation 116' which is potentially
detrimental to the operation of pump source 140 of first EDFA
138 is separated out by coupler 134. Similarly, coupler 134 and
isolator 136 block any residual pump radiation 142' from
propagating back and interfering with the operation of pump
~o source 114.
Delay element 126 ensures that amplified signals 104A and 1048
are recombined with the appropriate group delay into signal 104'
in combiner 128. Optional filter 148 adjusts the amplification
level of signal 104' across entire wavelength band, i.e., across
short and long wavelength bands 108, 106 or the S- and C-bands
in this case.
Apparatus 100 is very efficient and uses a minimum number of
3o elements to amplify signal 104. It should also be noted that
short and long wavelength bands 108, 106 can be selected based
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on the application. Although in telecommunications short
wavelength band 108 will typically contain at least a portion of
the S-band, other short wavelength bands can be accommodated as
well by changing the parameters of first EDFA 138 in accordance
s with the above-described rules. In telecommunications
application long wavelength band 106 will contain at least a
portion of the C-band or the L-band or both. Of course, long
wavelength band 106 can span any desired long wavelength band by
selecting appropriate amplifier 122 or even replacing amplifier
l0 122 with a number of amplifiers in series or in parallel.
An additional advantage of apparatus 100 is its good noise
figure. Specifically, the configuration of elements in
apparatus 100 yields a good noise figure because the input
s5 signals directly enter a co-pumped EDFA with no distributed
losses. In general, it is best for noise performance if the
gain happens before any losses. (Losses that occur before gain
add 1000 to noise figure.) It is also beneficial that shared
amplifier 122 is co-pumped, because this enables high levels of
2o inversion. It is also beneficial when shared amplifier 122
length is chosen such that the inversion level of the amplifier
is nearly 1000 because of minimal ASE at all wavelengths. With
near perfect inversion, this stage of amplification will
contribute minimally to the noise figure. Using standard Erbium-
a5 doped fibers, this approach can only provide small (about 5 dB)
amounts of S-band gain without sacrificing inversion. If the
fiber length is increased beyond the optimal length, then the
increased inversion would negatively impact the noise figure
while limiting the S-band gain (note, however that the C-band
so gain will continue to increase for a while.) Further increasing
of the length of the fiber beyond the optimal length would
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ultimately lead to a reduction of the S-band gain, and
eventually to net loss in the S-band. This latter condition (S-
band loss) is the condition most commonly experienced in EDFAs
optimized for operation in the C-band.
It is also beneficial for the noise figure that shared amplifier
122 is of traditional EDFA variety, with no fundamental mode
cutoff. For this reason, it is unlikely to have any distributed
losses other than those caused by Erbium absorption (i.e. by
to <100o inversion). This lack of distributed losses further
promotes a good noise figure.
The basic design of apparatus 100 splits the low-noise and
amplified .signal into two branches. This splitting (and its
associated losses) occur after the initial small amplification,
and therefore has less impact on noise figure than may have
occurred had the splitting occurred before the initial small
amplification. Additionally, any imperfect noise figure
inherent in S-band EDFA 138 in the S-band branch is effectively
2o de-magnified by the good-noise-figure gain of the initial small
amplification.
The split-band amplifying apparatus can be designed in many
different ways. Fig. 7 illustrates another split-band
amplifying apparatus 150 having separate first and second
sections 152, 154 and using one common pump source 156. In this
embodiment common pump source 156 is also a diode laser emitting
pump radiation 116 at pump wavelength ~,p=980 nm. In fact,
elements of apparatus 150 corresponding to those of apparatus
100 are labeled by the same reference numbers.
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Optical signal 104 is received by a coupler 158. Coupler 158 is
set to divide signal 104 into a first signal 160 spanning short
wavelength band 108 and a second signal 162 spanning long
wavelength band 106. First signal 160 is delivered by a fiber
164 belonging to first section 152 to a coupler 166. Coupler
166 also receives pump radiation 116 from laser diode 156. The
output of coupler 166 combines pump radiation 116 and first
signal 160 and delivers them to fiber amplifier 168 of first
section 152.
to
Fiber amplifier is designed in accordance with the above-
described rules to amplify signal 160. In other words fiber
amplifier 168 is designed to amplify short wavelength band 108.
Preferably, fiber amplifier 168 is a first EDFA. First EDFA 168
is followed by a coupler 170 which is set to pass amplified
signal 160' to a fiber 172 belonging to first section 152 and to
pass pump radiation 116 to a coupler 174 belonging to second
section 154.
2o Meanwhile, second signal 162 is delivered by a fiber 176
belonging to second section 154 to coupler 174. Coupler 174
combines pump radiation 116 undepleted by first EDFA 168 with
second signal 162 and delivers them to amplifier 178 designed
for amplifying long wavelength band 106. In this embodiment
amplifier 178 is a second fiber amplifier, in particular a
second EDFA. Second EDFA 178 amplifies signal 162 to yield an
amplified signal 162'.
A combiner 180 combines amplified signal 160' from first branch
152 and amplified signal 162' from second branch 154. Combiner
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180 is followed by an isolator 182 which passes only re-combined
signal 104' and removes any residual pump radiation 116.
Apparatus 150 is very efficient in its use of pump source since
it takes advantage of single common pump source 156. In
particular, S-band fiber amplifier 168 is operated with the full
pump power available in a co-pumping configuration, thereby
enabling the highest inversion possible for the best S-band
amplification performance. When an S-band EDFA is operated with
1o very high inversion D, it naturally absorbs the pump radiation
inefficiently, resulting in significant amounts of "wasted" pump
light. Apparatus 150 takes advantage of this otherwise "wasted"
pump light for pumping amplifier 178. Of course, a person
skilled in the art will appreciate that apparatus 150 can be re-
organized to use two pump sources and/or accommodate co-pumped
and counter-pumped geometries. Furthermore, the fiber
amplifiers can include several fiber amplifier sections
separated by isolators as necessary. Use of "mid-span" isolation
can improve noise figure and also net gain of an amplifier by
2o eliminating backwards-propagating ASE that would otherwise rob
power from the amplifier, depleting inversion, and thereby
saturating the amplifier.
Fig. 8 illustrates an optical communication system 200 using a
signal source 202A that provides a signal 204A in the S-band.
Source 202A is a diode laser belonging to an array of sources
202A through 202H. In fact, communication system 200 is a
Wavelength-Division-Multiplexed (WDM) or even a dense WDM (DWDM)
or coarse WDM (CWDM) communication system in which all eight
so sources 202A~through 202H generate signals 204A through 204H at
eight different wavelengths or in eight wavelength channels
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within the S-band. We note here that while in this example we
describe a WDM system 200 with just eight channels, WDM systems
can use any number of channels from 2 to over 1, 000, as is well
known in the art. The array of sources 202A through 202H is
controlled by a laser array control 210. Control 210 may
include any suitable electronic or optical pumping mechanisms
for driving diode lasers 202A through 202H.
A set of optics 206A through 206H, in this case lenses, is
so provided for focusing and in-coupling signals 204A through 204H
into a WDM multiplexes 208. Multiplexes 208 combines signals
204A through 204H and launches them in accordance with well-
known WDM protocols through a first span 212A of a common
communication fiber 212. Span 212A of communication fiber 212
spans a large distance, e.g., several tens of kilometers.
Preferably, communication fiber 212 is selected among fibers
that exhibit a low, but sufficient amount of dispersion in the
S-band to prevent non-linear effects from affecting signals 204A
through 204H. For example, fiber 212 has a dispersion of 3 to
10 ps/nm~km, but no less than 1 ps/nm~km in the S-band. In
addition, it is advantageous that fiber 212 exhibit low losses
in the S-band, e.g., 0.2 to 0.3 dB/km.
Fiber 212 extends between WDM multiplexes 208 and a WDM
demultiplexer 228. After span 212A, fiber 212 is coupled with a
fiber 214 in a wavelength combines 216. Fiber 214 is used to
couple a pump radiation 218 obtained from a pump source 220 to
an EDFA 222 installed after span 212A. Pump source 220, is a
laser diode emitting pump radiation 218 at a pump wavelength ~
of about 980 nm for pumping the Er ions in the core of EDFA 222
to a high level of relative population inversion D. Source 220
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is controlled by a pump laser control 224 that can include any
suitable electronic or optical pumping mechanisms. A set of
optics 226, in this case a lens is used to in-couple pump
radiation 218 into fiber 214.
EDFA 222 is connected to a second span 212B of fiber 212. In
fact, system 200 can have a number of spans and a number of
EDFAs installed at certain intervals between these spans. In
the present embodiment, span 212B terminates at WDM
to demultiplexer 228. WDM demultiplexer 228 separates signals 204A
through 204H by wavelength and delivers them to respective
receivers 230A through 230H with the aid of optics 232A through
232H. In this embodiment optics 232A through 232H are lenses.
A receiver control 236 is in communication with receivers 230A
through 230H. Control 236 can include any signal level
adjustments and other functionalities including signal
processing functions required for detecting and processing
signals 204A through 204H.
2o In the embodiment shown, EDFA 222 is employed by communication
system 200 as an in-line amplifier. Specifically EDFA 222 has a
length Z tailored to provide sufficient amount of gain in the S-
band to amplify signals 204A through 204H. For example, the
length L can be in the range of 5 to 50 meters. At the same
time, EDFA 222 is adjusted to exhibit a low noise figure, e.g.,
6 dB or lower and a flat gain spectrum. Suitable gain
flattening filters (not shown) can be used, e.g., in the middle
of EDFA 222 to ensure a gain-flattened spectrum, as will be
appreciated by those skilled in the art.
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Alternatively, EDFA 222 can be used as a power-boosting
amplifier. For this purpose, the power of pump source 220 is
increased to raise the saturation power of EDFA 222. At the
same time, other design changes and optimizations can be made.
s For example, the Er doping concentration and length L can be
increased in order to increase the efficiency of absorption of
pump radiation 218 at the expense of noise figure. Also, EDFA
222 can be designed for cladding pumping in this configuration.
Cladding pumping is a well-known technique in the field of C-
lo band EDFA manufacture. A low-cost, high-power, wide stripe 980
nm diode pump laser is used as pump source 220 when EDFA 222 is
designed for cladding pumping. S-band EDFA 222 can also be
optimized for high power by eliminating the gain-flattening
filters, reducing the amount of gain flattening or moving the
s5 gain-flattening filter to an earlier stage in embodiments where
EDFA 222 is made up of a number of segments (multi-stage EDFA).
This will increase the overall efficiency of EDFA 222 and
therefore the saturation power because it reduces the internal
losses of EDFA 222. Typically, power amplifiers are used under
2o heavy saturation or sometimes with a single channel, so that
gain-flattening is not as important for power amplifiers as it
is for in-line amplifiers.
System 200 can also use EDFA 222 as a pre-amplifier of signals
2s 204A through 204H. In this case, length L is reduced to less
than 50 meters, or even less than 10 meters and the noise figure
is carefully controlled by a combination of fiber design
parameters and gain flattening filers. A pre-amplifier is
typically used for a single signal channel and at low optical
so power levels, but with moderate requirements for gain.
Therefore, gain flattening is typically not required, and
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amplifier efficiency is not important. The key design criterion
for pre-amplifiers is the noise figure.
Fig. 9 illustrates a portion of another optical communication
s system 250 in which a number of EDFAs 252A through 252H in
accordance with the invention are used as pre-amplifiers.
System 250 has a WDM demultiplexer 254 for demultiplexing a
number of signals 256A through 256H delivered over a
communication fiber 260. A set of lenses 258A through 258H are
to positioned to focus signals 256A through 256H demultiplexed by
WDM demultiplexer 254 on corresponding receivers 260A through
260H. As in the previous embodiment, receivers 260A through
260H are controlled by a receiver control 262.
15 In this embodiment, EDFAs 252 are installed after a WDM
demultiplexer 254. This is the preferred location for EDFAs 252
when they are designed to function as pre-amplifiers. That is
because in this position each EDFA 252A through 252H processes
only one of signals 256A through 256H and can thus be better
2o tuned to achieve low noise pre-amplification.
Fig. 10 illustrates a portion of yet another optical
communication system 300 employing S-band EDFAs 302A and 302B
according to the invention for different portions or sub-bands
25 of the S-band. EDFAs 302A, 302B are installed between two spans
304A, 304B. A WDM demultiplexer or splitter 306 and WDM
multiplexes or combines 308 are used to deliver the appropriate
portions of the S-band wavelengths to each EDFA 302A, 302B. In
this embodiment, EDFA 302A is optimized for amplifying a sub-
so band of wavelengths between 1460 and 1490 nm and EDFA 302B is
optimized for amplifying a sub-band of wavelengths between 1490
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and 1520 nm. Of course, more than two EDFAs could be used for
amplifying even smaller portions of the S-band, as desired.
Communication system 300 can use a wider total range of
wavelengths in the S-band.
It should be noted that S-band EDFAs 302A, 302B can be combined
with C-band and/or Z-band EDFAs if communication system 300 is
designed to transmit signals over some or all of these bands.
In general, a person skilled in the art will recognise that the
1o use of S-band EDFAs in conjunction with C-band or h-band EDFAs
and/or with two or more sub-bands within the S-band can be
implemented at any position within communication system 300, and
is not limited to use as in-line amplifiers, pre-amplifiers and
power amplifiers.
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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