Note: Descriptions are shown in the official language in which they were submitted.
CA 02352378 2004-09-O1
CHALCOGENIDE GLASS BASED RAMAN OPTICAL AMPLIFIER
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to Raman optical amplifiers.
Discussion of the Related Art
To compensate attenuation, optical communication systems often provide
for amplification of optical signals at regular intervals along optical
transmission
fibers. The amplification may be produced by amplifiers based on rare-earth
elements such as erbium and ytterbium or by amplifiers based on the Raman
effect.
Rare-earth amplifiers have limited bandwidth due to their reliance on selected
atomic level transitions. Amplification occurs at discrete wavelengths that
correspond to the selected atomic transitions. Broadband erbium doped fiber
amplifiers are somewhat improved rare earth amplifiers so that these rare-
earth
amplifiers can power some wavelength division multiplexed (WDM) optical
networks. On the other hand, Raman amplifiers are naturally tunable and
capable
of providing amplification at wavelengths in a broad optical band. In such an
amplifier, an amplification wavelength is simply selected by tuning a pump
laser to
produce a wavelength capable of producing stimulated Raman emission at the
selected wavelength. Raman amplifiers can cover a much wider spectral range
than
rare-earth based amplifiers. Furthermore, Raman amplifiers have effectively
lower
noise levels than rare-earth amplifiers. These advantages make Raman
amplifiers
desirable for Iong haul WDM systems where the transmission bandwidth may be
broad.
Nevertheless, conventional Raman fiber amplifiers provide relatively low
gain. In such amplifiers, an optical signal often has to propagate through a
long
and heavily pumped amplifier fiber to receive adequate amplification. For
example, to produce a 20-dB amplification, some conventional Raman fiber
amplifiers use 10 to 100 kilometers (km) of amplifier fiber and 300 to 1,000
milli-
Watts (mW) of pump light. High pump light powers require expensive pump lasers
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and incur higher operating costs for pump lasers. Raman amplifiers based on
shorter amplifier fibers and lower pumping powers are desirable.
BRIEF SUMMARY OF THE INVENTION
In one aspect, the invention features an optical amplifier including a
chalcogenide glass optical waveguide with optical input and output ports, a
pump
optical waveguide, and a wavelength-tunable pump laser. The pump optical
waveguide couples the wavelength-tunable pump laser to the chalcogenide glass
optical waveguide.
In a second aspect, the invention features a method of amplifying light. The
method includes tuning a wavelength-tunable pump laser to produce pump light
with a wavelength capable of causing Raman amplification in a chalcogenide
glass
optical waveguide in response to light of a selected wavelength being received
in
the chalcogenide glass optical waveguide. The method also includes delivering
the
pump light to the chalcogenide glass optical waveguide, and receiving input
light
with the selected wavelength in the chalcogenide glass optical waveguide.
In a third aspect, the invention features an optical communication system.
The system includes a plurality of silica glass optical fibers and at least
one Raman
amplifier coupled between two of the silica glass optical fibers. The Raman
amplifier of the present invention includes a chalcogenide glass optical
waveguide
connecting the two of the silica optical fibers, a pump optical waveguide, and
a
wavelength-tunable pump laser. The pump optical waveguide couples the pump
laser to the chalcogenide glass optical waveguide.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
Figure 1 shows one embodiment of a Raman amplifier;
Figure 2 shows an alternate embodiment of a Raman amplifier;
Figure 3 is a cross-sectional view of a chalcogenide glass fiber used in some
embodiments of the Raman amplifiers of Figures 1 and 2;
Figure 4 shows a portion of an optical communications network that uses
the Raman amplifiers of Figure for 2;
Figure 5 is a flow chart showing a process for amplifying light with the
Raman amplifiers of Figure 1 or 2;
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Figures 6 and 7 graphically illustrate the relationship between the
wavelengths of pump light and input light in the process of Figure 5;
Figure 8 shows a apparatus for drawing a chalcogenide glass fiber for use in
some embodiments of the Raman amplifiers of Figures 1 and 2; and
Figure 9 is a flow chart showing one process for making chalcogenide glass
fibers for use in some embodiments of the Raman amplifiers of Figures 1 and 2.
DETAILED DESCRIPTION OF THE INVENTION
Various embodiments provide improved Raman amplification by using
optical amplification media made of chalcogenide glass instead of optical
amplification media made of silica or other oxide glasses.
Figure 1 shows one embodiment of a Raman amplifier 10 in which a
wavelength- tunable pump laser 12 couples to an input port of a chalcogenide
glass
amplifier waveguide 14 via a 2x1 optical connector 16. The wavelength-
tunability
of the pump laser 12 enables amplification of light belonging to a wide band
of
wavelengths unlike conventional rare-earth based amplifiers in which the
amplification wavelength is not tunable and pumping sources are thus, not
wavelength tunable. In various embodiments, the amplifier medium 14 is either
an
optical fiber or a planar waveguide.
The optical connector 16 also connects an input waveguide 18, e.g., a silica
optical transmission fiber, to the input port of the chalcogenide glass
amplifier
waveguide 14. An output port of the chalcogenide glass amplifier waveguide 14
couples to an output waveguide 20, e.g., another silica optical transmission
fiber,
via an opticaLcoupler 22. In some embodiments, the coupler 22 selectively
filters
out light at wavelengths produced by the pump laser 12 so that pump light is
not
transmitted to the output waveguide 20.
By using an amplification medium of chalcogenide glass, Raman amplifier
10 improves the gain over gains available from silica-glass Raman amplifiers.
The
origin of the improvement can be understood from an approximate equation for a
waveguide's Raman gain, G. The equation states that G = K'eglL. Here, "g" is
the
Raman gain coefficient, L is the length of the amplifier waveguide, and I is
the
pump light intensity. The Raman gain cross section is proportional to the Kerr
coefficient, n2. Thus, the Raman gain (G) depends exponentially on the product
of
the Kerr coefficient, n2, and the pump light intensity, I.
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The exponential dependence on nZ implies that many chalcogenide glasses
will produce much larger Raman gains than silica-glasses, because the n2's of
those
chalcogenide glasses are much larger than the n2's of silica glasses. For
example,
some chalcogenide glasses based on Se compounds have n2's that are about 50 to
1,000 times as large as the n2's of silica glasses, i.e., at least 50 or 200
times the n2
of undoped silica glass. The Raman amplifier 10 uses one of the high n2
chalcogenide glasses for the optical core of amplifier waveguide 14.
The equation for gain (G) also provides guidance for determining the length
of the chalcogenide glass fiber, because the total amplification depends on
the
product LInz. For example, to produce the same amplification as a silica-based
Raman amplifier for which LI is about (10 kilometers) (500 mW), the Raman
amplifier 10 only needs to have a length times power value of about 25,000 to
5,000 meter-mW. Various amplifiers use less than S00 meters of chalcogenide
fiber and a pump source that produces less than 500-mW of pump light due to
the
increased n2 of chalcogenide glasses. For example, a 100-meter length of
chalcogenide fiber and a 50-250 mW pump source is able to produce as much
amplification as 10-kilometer (km) of silica-based Raman amplifier fiber and a
500-mW pump source.
Herein, chalcogenide glass is defined to be an amorphous material that
transmits visible and near infrared light and includes a compound of selenium
(Se),
tellurium (Te), and/or sulfur (S) with one or more other elements. The
combined
molar percentage of Se, Te, and/or S is typically at least 25 percent. In the
compounds, examples of the other elements include germanium (Ge), arsenic
(As),
tin (Sb), thallium (Tl), lead (Pb), phosphorous (P), gallium (Ga), indium
(In),
lanthium (La), silicon (Si), chlorine (Cl), bromine (Br), iodine (I), and a
rare earth
element. The chalcogenide glasses are not oxide glass unlike standard silica
optical
glasses.
Figure 2 shows an alternate embodiment of a Raman amplifier 10' in which
wavelength-tunable pump laser 12 couples to one end of chalcogenide glass
amplifier waveguide 14 and input waveguide 18 couples to the other end of the
chalcogenide glass amplifier waveguide 14. In the amplifier 10', pump light
and
input light counter- propagate in the amplifier waveguide 14 so that pump
light
does not appear in output waveguide 20.
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,
In some embodiments of amplifier 10 or 10' shown in Figures 1 and 2,
waveguide 14 is a chalcogenide glass optical fiber. Figure 3 is a cross-
sectional
view of a chalcogenide glass amplifier fiber 26 used in such embodiments. The
fiber 26 includes a chalcogenide glass core 27 and a chalcogenide glass
cladding
5 28. The core 27 has a diameter of about 2-14 microns (p,m) and a preferable
diameter of less than about 5 ftm. The cladding 28 has an outer diameter of
about
120-130 ~trri.
Core 27 and cladding 28 are made of chalcogenide glasses with different
chemical compositions so that a jump in index of refraction occurs at the core-
cladding interface. To provide for total internal reflection of light
propagating in
amplifier fiber 26, the core 27 has an index of refraction, n~~, that is
higher than
the index of refraction, nyadding, of the cladding 28. To insure single-mode
operation in the amplifier fiber 26, the fiber 26 is single modal, has
fractional jump
in index of refraction at the core-cladding interface, i.e., ~ _ [n~o~ -
n~,,aa~~gl~n
claddings of between 1 and 5 percent, and has a V"um~r of less than about 2.4.
Here,
unumber = (nD~~Hncorc2 ' ncladdingz)~ , D is the core diameter, and ~, is the
wavelength
of the light propagating in the amplifier fiber 26. In WDM systems, ~, is
between
about 1.3 and 1.6 microns.
As an example, the core 27 may be made of As~,Se~ glass, which has an
index of about 2.7, and the cladding 28 may be made of As~,S~, which has an
index of about 2.4. Then, D = 1.25, and the core 27 has a diameter of less
than
about 3 microns to insure single-mode propagation of the light whose
wavelength
is about 1.5 microns.
In some embodiments, core 27 is made of As4o.4oySebo-6oyS~ooy~
Ge2gSe~,Sb~z, GeZ5Se65~~, Te8_,o, or As5oSe35Cu,5, and cladding 28 is made of
As~_
40xSe60-60xs 100x
The choice of chalcogenide glass for the core 27 depends on the desired
Raman gain, G. The gain depends on the Ken coefficient of the core's glass.
As~Se~ glass has a large Kerr coefficient, which improves the amplifier's
gain.
For long amplifier fibers, the gain also depends 2-photon absorption that
generate
pump light tosses. Low 2-photon absorption rates occur, increasing the overall
gain, if the chosen core glass has a bandgap that is larger than twice the
energy of
the desired pump light photons. U.S. Patent No. 6,208,792, issued March 27,
2001.
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6
describes methods for choosing the glass composition for the core 27.
The choice of mohar percentage of sulfur (S), i.e., 100x, in the cladding 28
depends on the above-discussed constraints on V"um~r and 0, which insure
single-
s mode operation. The index of refraction of Asao_4oxSe6o-~S,oox varies
approximately linearly from 2.7 to 2.4 as 100x varies from 0 to 100 percent so
that
the molar percentage "100x" in the cladding 28 can be chosen to satisfy the
constraints for single modal operation.
Figure 4 shows an optical communications network 30 that uses variable
wavelength, chalcogenide glass, Raman amplifiers 32; e.g., amplifiers 10, 10'
of
Figures 1 and 2. The amplifiers 32 are regularly spaced between sequential
segments 34-36 of an optical transmission fiber, e.g., segments made of multi-
modal silica-glass fibers. The segments 34-36 form a transmission pathway
optically connecting optical transmitter 38 to optical receiver 40. The
lengths of
the segments 34-36 of transmission fiber are short enough to insure that
accumulated attenuations are less than about 20 decibels (dB) before the next
stage
of amplification. For example, modern silica-based transmission fibers produce
an
attenuation of about 0.2 dB per kilometer (km) for wavelengths between about
1.3
and 1.6 ~.. For such fibers, individual segments 34-36 are not longer than
about 80
km.
Figure 5 is a flow chart for a process 50 that amplifies light with a Raman
amplifier 10, 10' of Figure 1 or 2. Prior to receiving input light, the
process 50
tunes wavelength-tunable pump laser 12 to produce pump light whose wavelength
is capable of causing Raman amplification in chalcogenide glass waveguide 14
in
response to input light of a selected wavelength (step 51). The tuning may be
performed by an operator or a programmable computer 24 that operates the
wavelength-tunable pump laser 12. If the computer 24 controls the pump laser
14,
the computer 24 looks up an appropriate pump light wavelength in a database
look
up table in response to an external request to amplify input light with the
selected
wavelength.
The choice of pump light wavelength depends on phonon spectrum of the
chalcogenide glass and the selected wavelength to be amplified. In a Raman
amplifier output light is produced by Raman events stimulated by the input
light
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LENZ 14-21 '7
signal. The stimulated events occur due to the background intensity of pump
light.
In a stimulated Raman event, a pump photon produces both a stimulated photon
and a stimulated phonon. Thus, the original energy and momentum of the pump
photon is divided between the stimulated photon and phonon. This dividing of
the
original energy and momentum implies that the stimulated light has a longer
wavelength than the pump light that produced the stimulated light. Since the
stimulated light ',as the same wavelength as the input light, the pump photon
must
have an energy equal to the sum of the energy of the input photon plus the
energy
of the stimulated phonon. Thus, the pump light wavelength has a shift with
respect
to the input light wavelength, which is caused by phonon production.
Similar to other phonon-related properties, the size of the wavelength shift
between the pump and input light depends on physical characteristics of the
amplifier glass. The pump light wavelength is chosen to equal the input light
wavelength minus the phonon-related wavelength shift associated with the
particular chalcogenide glass used in amplifier waveguide 14. Phonon-related
wavelength shifts are known and easily measurable by methods known to those of
skill in the art.
The choice of pump light wavelength has some freedom due to the breadth
of the Raman scattering cross section. The scattering cross section gives the
probability of a Raman event as a function of the shift between wavenumbers of
pump and input light.
Figure 6 illustrates the Raman scattering cross section of As~S6o glass as a
function of wavenumber shift, ~k. The wavenumber shift satisfies: ~k = kP"mp -
ki"p"c where kp"~, and k;ap"~ are wavenumbers of the pump and input light,
respectively. The Raman scattering cross section has an approximately linear
dependence on ~k and has a peak at ~k = 348 cm I. Choosing the wavenumber of
the pump light, kP",~, to equal the wavenumber of the input light, k;nP"~,
plus 348
cm~l provides a high probability of Raman scattering and strong Raman
amplification in As~S~ glass waveguides.
But, other pump light wavelengths for which Raman scattering cross
sections are large, e.g., at least half the maximum cross section, are also
possible
choices for the pump light wavelength. Thus, the Raman cross section defines a
window "w" of available choices for the pump light wavelength, ~,pUMp. Figure
7
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illustrates the window ''w" of pump light wavelengths, /'I,pUMp, for a
selected input
light wavelength, ~,~pUT. The phonon-induced wavelength shift is also shown
for
the chalcogenide glass used in the particular amplifier waveguide. In the
example,
the window "w"is smaller than the optical transmission band for input light,
e.g.,
the transmission band may be a complete set of channels of a DWDM network.
The wavelength-tunable pump laser 12 may be retuned to produce a new pump
wavelength, ~1.'pUMp, for amplifying a later-received input signal with a new
wavelength, ~,'~,pUT, that is outside of the original window "w"'.
Compositions of amplifier media, i.e., waveguide 14 of Figures 1 and 2, can
be selected to produce amplification in a broader band of wavelengths for a
particular choice of pump light wavelength. One way to broaden the
amplification
band entails making the amplifier waveguide of a mixture of two or more binary
chalcogenide compounds with different phonon-induced Raman shifts. For
example, the amplifier waveguide may be a mixture of As4oSso glass and
As4oSe6o
glass, which have respective Raman wavelength shifts of 85 and 55 nanometers
(nm). For such a ternary mixture, the total Raman scattering cross section is
a sum
of the individual scattering cross sections for the binary glasses in the
mixture.
This can result in a total scattering cross section that no longer has a
linear
dependence on shifts as shown in Figure 6. Rather the mixture may have
multiple
peaks so that input signals with wavelengths for which the Raman cross section
has
a value at least half as large as one of the peak values would be amplified by
the
same pump light wavelength.
Referring again to Figure 5, process 50 delivers pump light from
wavelength-tunable pump laser 12 to chalcogenide glass amplifier waveguide 14
after choosing the pump light wavelength (step 52). The chalcogenide glass
amplifier waveguide 14 receives input light, e.g., a sequence of digital
optical
pulses, with the selected input light wavelength from input waveguide 18 (step
53).
The input light and simultaneously delivered pump light produce stimulated
Raman
emission in the amplifier waveguide 14 causing amplification of the input
light
therein. The process 50 sends light from the chalcogenide glass amplifier
waveguide 14 to an output, e.g., output waveguide 20 or simply from an end of
the
amplifier waveguide 14 (step 54). The process 50 may also pass the light from
the
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9
amplifier waveguide 14 through a filter that selectively removes pump light
prior to
sending the light to the output waveguide 20.
Figure 8 is a cross-sectional view of an apparatus 70 for drawing
chalcogenide glass amplifier fiber 26 shown in Figure 3. The apparatus 70
includes
inner and outer cylinders 72, 74 for holding separate chalcogenide glass
preforms
76, 78 for making the fiber's core 24 and cladding 26, respectively. The inner
and
outer cylinders 72, 74 are concentric and have inner diameters of about 5-20
mm
and 10-100 mm, respectively. The cylinders 72, 74 are made of quartz,
platinum,
or an alloy of platinum. The glass preform 76 has the composition of fiber
core 27
and a rod-like shape that enables sliding the preform 76 into the inner
cylinder 72. .
The glass preform 78 has the composition of fiber cladding 28 and a tubular
shape
that enables sliding the preform 78 into the separate tubular space between
the
inner and outer cylinders 72, 74.
Each cylinder 72, 74 tapers at a lower end to form a tubular draw port 80,
82. Herein, upper and lower make reference to directions with respect the
direction "z" of gravity. The draw ports 80, 82 of the inner cylinder 72 have
inner
diameters of about 0.1-20 mm and 0.2-30 mm, respectively. The tower end of the
inner port 80 is 0.5-5 mm upward from the lower end of the outer draw port 82.
Concentric and vertical relative alignments between inner and outer
cylinders 72, 74 are achieved through matched conical seating sections 84, 86.
The
seating sections 84, 86 also seal the upper region between the inner and outer
cylinders 72, 74 from the external ambient atmosphere, i.e., the seal prevents
external gases from entering the region above the preform 78.
Adjustable ports 88-91 enable control of gas pressures in the regions above
glass preforms 76, 78 during fiber drawing as well as introduction of gases
into and
removal of gases from these regions. Similarly, a removable plug 92 may be
positioned to close ports 80, 82 and seal space below the glass preforms 76,
78
thereby stopping the escape of glass during melting.
Outer cylinder 74 is supported by a cylindrical metal body 94, i.e., made of
an Iconel alloy or platinum. The metal body 94 is tapered at the lower end to
physically retain the outer cylinder 74 from falling. The body 94 forms a
thermal
contact between outer cylinder 74 and an adjustable heater 96.
One construction for drawing apparatus 70 is described in U.S. Patent
No. 5,900,036, issued May 4, 1999. Alternate apparatus and
CA 02352378 2004-09-O1
processes for drawing chalcogenide fibers are described in U.S. Patents
5,879,426 and
6,021,649, issued March 9, 1999 and February 8, 2000, respectively.
Figure 9 is a flow chart for a process 100 of making chalcogenide glass
fibers with drawing apparatus 70 of Figure 8. The process 100 positions
separate
5 glass preforms 76, 78 for fiber core 27 and cladding 28 in cylinders 72, 74
and
seats sections 84, 86 to seal the region above cylinders 72, 74 from external
gases
(step 102). Then, the process 100 positions the outer cylinder 74 in metal
body 94
of heater 96 and closes ports 80, 82 with plug 92 (step 104). Then, ports 88-
91 are
used to replace atmospheres in regions above the glass preforms 76, 78 with an
10 inert gas such as nitrogen or helium (step 105). The heater 96 is also
regulated to
slowly heat the foams 76, 78 to temperatures that cause the chalcogenide
glasses of
the preforms 76, 78 to melt (step 106). The process 100 maintains the melts
from
the preforms 76, 78 above melting temperatures for a sufficient period to
remove
bubbles from the melts, e.g., 0.25-6 hours (step 108). Then the process 100
cools
the melts to a draw temperature over a period of 2-10 minutes (step 110). At
the
draw temperature, the chalcogenide materials have viscosities of 103 to 10'
poises.
Next, ports 88-91 are operated to fix gas pressures over the core melt
located inside inner cylinder 72 and the cladding melt located between inner
and
outer cylinders 72, 74 to selected draw pressures (112). The relative gas
pressures
over the two chalcogenide glass melts determine the relative diameters of core
27
and cladding 28 produced by the draw. Fibers with relatively thinner cores are
produced by maintaining a relatively lower gas pressure over the core glass
melt
located in the inner cylinder 72 than over the Madding glass melt located
between
the cylinders 72, 74. After regulating the draw pressures, the plug 92 is
removed
and fiber is drawn from ports 80, 82 (step 114). Gas pressures are maintained
at
values of about 0.01 to 30 pounds per square inch above ambient external
pressures
to produce draw rates of about 1 to 10 meters of fiber per minute.
At a draw temperature of 345°C, draw pressures of 25.4 mm of water
and
0.5 pounds-per-square-inch over respective melts of core and cladding glass
produce a chalcogenide fiber with a As~S58Se2 core 27 having a diameter of 14
ltm
and a As2S3 cladding 28 having an outer diameter of 1301.tm. To make a fiber
of
these dimensions, port 82 has an inner diameter of about 5 mm, and port 80 has
an
inner diameter of about 1 mm and a low end positioned about 0.5 mm upward from
CA 02352378 2001-07-04
LENZ 14-21 11
the lower end of the port 82. These draw conditions produce a draw rate of
about
three meters of fiber per minute.
Other embodiments of the invention will be apparent to those skilled in the
art in light of the specification, drawings, and claims of this application.
