Note: Descriptions are shown in the official language in which they were submitted.
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MULTI-STAGE LONG-BAND OPTICAL AMPLIFIER WITH
ASE RE-USE
TECHNICAL FIELD
The present relates to multi-stage optical amplifiers, and more specifically
to multi-stage
long-band single-pass rare-earth-doped optical amplifiers using amplified
spontaneous
emission generated in one amplification stage to pump another amplification
stage.
BACKGROUND OF THE ART
Single-pass optical fiber amplifiers consist of a length of rare-earth doped
optical fiber in
which the optical signal to be amplified is propagated. An optical coupler is
used to
couple a pump light in the rare-earth-doped optical fiber to pump the rare-
earth-doped
medium. The signal may either co-propagate or counter-propagate with the pump
light,
or the rare-earth-doped fiber may be pumped from both sides. Isolators are
typically
placed at both ends of the rare-earth-doped optical fiber.
Long-band optical amplifiers are amplifiers used to amplify a signal with a
wavelength
that is offset from the peak of the emission cross-section of the rare-earth
dopant used.
For example, long-band erbium-doped amplifiers are used to amplify optical
signals in
the L-band, i.e. 1570 nm to 1620 nm, while erbium has a peak emission at about
1530 nm. In another example, long-band ytterbium-doped amplifiers are used to
amplify
optical signals with wavelengths of 1064 nm and above while the ytterbium peak
emission is at about 1030 nm.
Long-band amplifiers typically uses absorption of Amplified Spontaneous
Emission
(ASE) generated in the rare-earth-doped fiber as a result of pumping to
further enhance
the gain at the wavelength of the optical signal to be amplified. This
principle usually
requires a relatively long length of rare-earth-doped fiber since the
absorption cross
section at the ASE peak emission is quite lower than the absorption cross
section at the
pump wavelength. U.S. Patents 6,222,670 to Ryu et al. and 6,233,092 to Flood
et al.
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teach long-band amplifier architectures that uses ASE generated in one
amplification
stage as a result of traditional pumping, to pump another amplification stage.
SUMMARY
According to one aspect, there is provided a long-band optical amplifier and
method that
uses a polarization-maintaining rare-earth-doped optical waveguide wherein the
optical
signal to be amplified is polarized along one principal state of polarization
of the optical
waveguide. An optical coupling device, i.e. a polarization combiner/splitter,
uses the
polarization to split the ASE produced in a mid-amplification stage in a first
and a
second part of ASE, used respectively to pump a pre- and a post-amplification
stage,
thereby offering an efficient use of the pump power in the long-band optical
amplifier
and a good noise figure.
By using a polarizer at the output of the long-band optical amplifier, only
the ASE
generated in the polarization state of the optical signal is kept, all other
ASE being
suppressed, thereby improving the noise figure of the amplifier.
A polarization-maintaining fiber coupler may be used as the optical coupling
device,
providing a simple amplification architecture.
According to another aspect, there is provided an long-band rare-earth-doped
optical
amplifier and method for amplifying an optical signal. The optical amplifier
has a pre-, a
mid- and a post-amplification stage. Only the mid-amplification stage is
pumped with a
pump light source. The other two are pumped using ASE generated in the mid-
amplification stage. An optical coupling device is used to couple the three
amplification
stages together and to split the ASE generated in the mid-amplification stage
and
available at one end of the mid-amplification stage. One part of the split ASE
is used to
pump the pre-amplification stage while the other part is used to pump the post-
amplification stage.
In one embodiment, the optical amplifier uses polarization-maintaining optical
waveguides and the optical signal is polarized along one of the principal
polarization
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axis of the waveguides. The optical coupling device may then use a
polarization
combiner/splitter to split the ASE between the pre- and the post-amplification
stage
while not splitting the optical signal.
According to another aspect, there is provided a method for amplifying an
optical signal
in a long-band optical amplifier. The method comprises: (1) propagating the
optical
signal in a first rare-earth-doped amplification waveguide and in a second
rare-earth-
doped amplification waveguide; (2) pumping the second rare-earth-doped
amplification
waveguide with a pump light source to amplify the optical signal propagating
in the
pumped second rare-earth-doped amplification waveguide into an amplified
optical
signal, the pumping generating amplified spontaneous emission in the second
rare-
earth-doped amplification waveguide; (3) splitting the amplified spontaneous
emission in
at least a first and a second part of amplified spontaneous emission; (4)
coupling the
first part in the first rare-earth-doped amplification waveguide for pumping
the first rare-
earth-doped amplification waveguide to pre-amplify the optical signal
propagating
therein; and (5) coupling the amplified optical signal and the second part in
a third rare-
earth-doped amplification waveguide, the second part pumping the third rare-
earth-
doped amplification waveguide for post-amplification of the amplified optical
signal.
According to another aspect, there is provided an optical amplifier for
amplifying an
optical signal in a long-band of the optical amplifier. The optical amplifier
comprises a
pre-amplification stage, a mid-amplification stage and a post-amplification
stage. The
pre-amplification stage has a first rare-earth-doped amplification waveguide.
The optical
signal is to be coupled at an input of the first rare-earth-doped
amplification waveguide
for pre-amplification of the optical signal. The mid-amplification stage has a
second
rare-earth-doped amplification waveguide pumped with a pump light source for
mid-
amplification of the optical signal. The mid-amplification generates amplified
spontaneous emission in the second rare-earth-doped amplification waveguide.
The
post-amplification stage has a third rare-earth-doped amplification waveguide
for post-
amplification of the optical signal. The optical amplifier further comprising
an optical
coupling device connected between the pre-amplification stage, the mid-
amplification
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stage and the post-amplification stage for coupling the optical signal
received from the
pre-amplification stage to the mid-amplification stage, and from the mid-
amplification
stage to the post-amplification stage. The optical coupling device receives
the amplified
spontaneous emission from the mid-amplification stage, and splits it in a
first and a
second part such that the first part is coupled to the pre-amplification stage
for pumping
the pre-amplification stage, and the second part is coupled to the post-
amplification
stage for pumping the post-amplification stage.
According to another aspect, there is provided an optical amplifier for
amplifying an
optical signal in a long-band of the optical amplifier. The optical amplifier
comprises a
pre-amplification stage, a mid-amplification stage and a post-amplification
stage. The
pre-amplification stage has a first rare-earth-doped amplification waveguide.
The optical
signal is to be coupled at an input of the first rare-earth-doped
amplification waveguide
for pre-amplification of the optical signal. The mid-amplification stage has a
second
rare-earth-doped amplification waveguide pumped with a pump light source for
mid-
amplification of the optical signal. The mid-amplification generates amplified
spontaneous emission in the second rare-earth-doped amplification waveguide.
The
post-amplification stage has a third rare-earth-doped amplification waveguide
for post-
amplification of the optical signal. The optical amplifier further comprises
an optical
coupling device for coupling the pre-, mid- and post-amplification stages. The
optical
coupling device has (1) a first port connected to an output of the pre-
amplification stage
and a third port connected to an input of the mid-amplification stage for
coupling the
optical signal received from the pre-amplification stage to the mid-
amplification stage,
and (2) a second port connected to an output of the mid-amplification stage
and a fourth
port connected to an input of the post-amplification stage for coupling the
optical
received from the mid-amplification stage to the post-amplification stage; (3)
the optical
coupling device interconnecting the first, second, third and fourth ports so
as to split the
amplified spontaneous emission received on at least one of the second and the
third
port, in a first and a second part respectively to the first and the fourth
port, such that
the first part is coupled to the pre-amplification stage for pumping the pre-
amplification
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stage, and the second part is coupled to the post-amplification stage for
pumping the
post-amplification stage.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram illustrating an embodiment of a long-band optical
amplifier
wherein a polarization-maintaining fiber coupler is used and wherein the mid-
amplification stage is pumped in co-propagation;
Fig. 2 is a graph showing the transmission spectrum of the example
polarization-
maintaining fiber coupler used in the amplifier of Fig. 1;
Fig. 3 is a block diagram illustrating another embodiment of a long-band
optical amplifier
wherein a polarization-maintaining fiber coupler is used and wherein the mid-
amplification stage is pumped in counter-propagation;
Fig. 4 is a block diagram illustrating yet another embodiment of a long-band
optical
amplifier wherein the polarization-maintaining fiber coupler is replaced by
another
optical coupling device consisting of two polarization beam
combiners/splitters;
Fig. 5 is a block diagram illustrating an optical amplification architecture
used as a
reference for illustrating characteristics of the optical amplifier of Fig. 1;
Fig. 6 is a graph showing the evolution of the optical power in the reference
optical
amplifier of Fig. 5;
Fig. 7 is a graph showing the evolution of the optical power in the optical
amplifier of
Fig. 1;
Fig. 8 is a graph showing the polarization extinction ratio of the ASE at the
output of the
optical amplifiers of Figs. 1 and 5;
Fig. 9 is a graph showing the optical spectrum at the output of the optical
amplifiers of
Figs. 1 and 5, with and without a polarizer;
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Fig. 10 is a graph showing an experimental result of the power at the output
of the
optical amplifiers of Figs. 1 and 5 as a function of the pump power;
Fig. 11 is a graph showing an experimental result of the optical spectrum at
the output
of the optical amplifiers of Figs. 1 and 5; and
Fig. 12 is a block diagram illustrating yet another embodiment of a long-band
optical
amplifier wherein the polarization-maintaining fiber coupler is replaced by a
bulk
polarization beam combiner/splitter cube.
It will be noted that throughout the appended drawings, like features are
identified by
like reference numerals.
DETAILED DESCRIPTION
Now referring to the drawings, Fig. 1 shows an embodiment of a long-band
optical fiber
amplifier 100 which is based on the re-use of ASE generated in one
amplification stage
to pump other amplification stages. The optical amplifier 100 comprises three
single-
pass amplification stages, i.e. a pre-amplification stage 10, a mid-
amplification stage
120 and a post-amplification stage 30. Each amplification stage 10, 120 and 30
uses a
rare-earth doped optical fiber 12, 22, 32 as the amplification medium. The
rare-earth
doped optical fibers 12, 22, 32 are polarization-maintaining fibers.
The optical signal s to be amplified is coupled to the input of the pre-
amplification stage
10 for pre-amplification. The optical signal s is polarized along one of the
principal
states of polarization, in this case the fast axis, of the polarization-
maintaining fibers 12,
22, 32 and this polarization state is maintained throughout the amplification
stages 10,
120 and 30. The output of the pre-amplification stage 10 is connected to the
input of
mid-amplification stage 120 through an optical coupling device, in this case a
polarization-maintaining fiber coupler 44. The output of the pre-amplification
stage 10 is
connected to port 1 of the coupler 44 and the input of the mid-amplification
stage 120 is
connected to port 3 of the coupler 44, such that the optical signal s exiting
the pre-
amplification stage 10 is coupled to the mid-amplification stage 120.
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The mid-amplification stage 120 comprises a pump light source 40, typically a
laser
diode, for pumping the rare-earth-doped optical fiber 22 in order to amplify
the optical
signal s propagating therein. The pump light produced by the pump light source
40 is
coupled into the rare-earth-doped optical fiber 22 at its input, using an
optical fiber
coupler, namely a wavelength division multiplexing coupler 42. The optical
signal s and
the pump light thereby co-propagate in the rare-earth-doped optical fiber 22.
The mid-
amplification stage 120 acts as a high-gain amplification stage. Then, the
output of the
mid-amplification stage 120 is connected to the input of the post-
amplification stage 30
through the coupler 44. The output of the mid-amplification stage 120 is
connected to
port 2 of the coupler 44 and the input of the post-amplification stage 30 is
connected to
port 4 of the coupler 44, such that the optical signal s exiting the mid-
amplification stage
120 is coupled to the post-amplification stage 30. The optical signal s then
propagates
in the post-amplification stage 30 which acts as a power amplifier.
It is noted that in Figs. 1, 2, 4 and 5 an "x" along an optical fiber denotes
a fusion splice
used to connect adjoining optical fibers.
Pumping of the rare-earth-doped optical fiber 22 in the mid-amplification
stage 120
generates ASE in co- and in counter-propagation with the optical signal s.
Since the
pump light is co-propagating with the optical signal s, most of the ASE is
generated near
the input of the mid-amplification stage 120 and co-propagating ASE is largely
re-
absorbed along the rare-earth-doped optical fiber 22. However, most of the
generated
counter-propagating ASE is available in counter-propagation at the input of
the mid-
amplification stage 120, as denoted by arrow 70. The counter-propagating ASE
enters
the coupler 44 on port 3. It is noted that both co- and counter-propagating
ASE
generated in the mid-amplification stage 120 are unpolarized. As will be
explained in
more detail later with reference to Fig. 3, unpolarized ASE entering the
polarization-
maintaining fiber coupler 44 at port 3 is split in power in a first and a
second part of ASE
(ASE, and ASE2) respectively to port 1 and port 4. The splitting ratio is such
that about
30% of the ASE at port 3 is directed to port 1, while about 70% is directed to
port 4.
Consequently, the first part of ASE (ASE1) counter-propagates in the rare-
earth-doped
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optical fiber 12 to act as a pump in the pre-amplification stage 10 for pre-
amplification of
the optical signal s. The second part of ASE (ASE2) co-propagates in the rare-
earth-
doped optical fiber 32 and acts as a pump in the post-amplification stage 30
for power
amplification of the optical signal s. A single pump light source 40 is thus
required for
amplification using three amplification stages.
At the output of the post-amplification stage 30, the optical signal s is
still polarized
along the fast axis while the residual ASE is polarized mostly along the slow
axis.
Optionally, the use of a polarizer 46 aligned along the fast axis and placed
at the output
of the post-amplification stage 30 is used to suppress ASE along the slow
axis, leaving
only ASE along the fast axis. This is used to reduce the noise figure of the
optical
amplifier 100.
It is noted that, in Fig. 1, the use of a polarization-maintaining fiber
coupler 44 provides
an ASE splitting ratio of about 30/70 between port 1 and 4 but this ratio can
be modified
by modifying the characteristics of the polarization-maintaining fiber coupler
44 in
manufacture. This may be used to optimize the optical amplifier 100.
It is noted that the rare-earth element used in the doping of the rare-earth-
doped optical
fibers 12, 22, 32 may vary and that the optical amplifier 100 may then be used
to
amplify optical signals with different wavelengths. Examples of possible rare-
earth
elements are erbium and ytterbium. For example, if erbium is used, the long-
band
amplifier 100 may be used to amplify an optical signal having a wavelength
between
about 1565 to 1625 nm. In the following description, it will be supposed that
ytterbium-
doped silica optical fibers are used to amplify an optical signal at a
wavelength of about
1064 nm. The emission cross-section of an ytterbium-doped silica optical fiber
has a
peak at about 1030 nanometers and such an optical fiber offers a maximum gain
at
about this wavelength when pumped at 976 nm. Amplification at 1064 nm and more
is
considered long-band amplification.
Fig. 2 is a graph showing the transmission spectrum of an example polarization-
maintaining fiber coupler 44 used in the amplifier of Fig. 1. The polarization-
maintaining
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fiber coupler 44 is made for optimal behavior at 1064 nm, which corresponds to
the
wavelength of the optical signal to be amplified in this case. Trace 202 shows
the
transmission spectrum between port 1 and port 2 of the coupler 44, or
equivalently
between port 4 and port 3, for light polarized along the slow axis. Trace 204
shows the
transmission spectrum between port 1 and port 3 of the coupler 44, or
equivalently
between port 4 and port 2, for light polarized along the slow axis. Trace 206
shows the
transmission spectrum between port 1 and port 2 of the coupler 44, or
equivalently
between port 4 and port 3, for light polarized along the fast axis. Trace 208
shows the
transmission spectrum between port 1 and port 3 of the coupler 44, or
equivalently
between port 4 and port 2, for light polarized along the fast axis. In this
case, the
coupler 44 is a PM combiner from Gooch & Housego (formely SIFAM Fibre Optics
in
Torquay, UK), model # FFP-8M3264G10, based on fused fiber coupler technology.
It can be seen that light polarized along the slow axis at port 1 is mostly
transmitted to
port 2, regardless of the wavelength. The same is true from port 4 to port 3,
from port 2
to port 1 and from port 3 to port 4. Light at and around 1064 nm and polarized
along the
fast axis at port 1 is mostly transmitted to port 3. The same is true from
port 4 to port 2,
from port 3 to port 1 and from port 2 to port 4. However, light at port 1
having a
wavelength close to the 1030-nm ASE emission peak of ytterbium and polarized
along
the fast axis is split among ports 2 and 3, with a splitting ratio of about
50/50. The same
is true from port 4 to ports 2 and 3, from port 2 to 4 and 1 and from port 3
to ports 1 and
4.
Accordingly, now referring back to Fig. 1, unpolarized counter-propagating ASE
at port
3 of the coupler 44 is split with a ratio of about 30/70 between port 1 and 4.
ASE
transmitted to port 1 is mostly polarized along the fast axis. ASE transmitted
to port 4 is
mostly polarized along the slow axis. Optical signal at about 1064 nm at port
1 is
transmitted to port 3 with an insertion loss of about 0.15 dB and similarly
from port 2 to
4.
Fig. 3 shows another embodiment of a long-band optical fiber amplifier 200.
The optical
amplifier 300 is in most points similar to the optical amplifier 100 of Fig. 1
and similar
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components will not be herein repetitively described. The difference between
the optical
amplifiers 100 and 300 lies in the pumping of the mid-amplification stage 320
of the
optical amplifier 300. In this case, pumping is made in counter-propagation
instead of
co-propagation. The pump light source 40 and the wavelength division
multiplexing
coupler 42 are thus simply placed between port 2 of the coupler 44 and the
output of the
rare-earth-doped optical fiber 22 in order to couple pump light in counter-
propagation in
the rare-earth-doped optical fiber 22. Consequently, most of the ASE generated
in the
mid-amplification stage 320 and available for pumping the other states 10, 30
is co-
propagating ASE available at the output of the mid-amplification stage 320, as
denoted
by arrow 72. Unpolarized ASE entering the polarization-maintaining fiber
coupler 44 at
port 2 is split in power in a first and a second part of ASE (ASE, and ASE2)
respectively
to port 1 and port 4. Since the ASE enters the coupler 44 at port 2 instead of
port 3, the
splitting ratio is different from the one of Fig. 1. The splitting ratio is
such that about 70%
of the ASE at port 2 is directed to port 1, while about 30% is directed to
port 4.
It is noted that the splitting ratio of Fig. 1 is typically more optimal that
the one of Fig. 3
since pre-amplification typically requires less pump power that post-
amplification. This is
due to the fact that the post-amplification stage 30 is a power amplifier
where the optical
signal is powerful enough to saturate the gain. The pre-amplifier is rather a
low signal
amplifier and less pump power is required to generate gain. Both options are
nonetheless available.
It is noted that the polarization-maintaining fiber coupler 44 of Fig. 1 may
be replaced by
other optical coupling devices, such as other types of polarization
combiners/splitters,
either fused fiber core or bulk. A suitable optical coupling device may be
obtained, for
example, by combining two polarization beam combiners/splitters 52 and 54 as
shown
in Fig. 4. The optical amplifier 400 is in most points similar to the optical
amplifier 100 of
Fig. 1 and similar components will not be herein repetitively described. A
first fiber-
pigtailed bulk polarization beam combiner/splitter 52 is placed between the
pre-
amplification stage 10 and the mid-amplification stage 120 and a second fiber-
pigtailed
bulk polarization beam combiner/splitter 52 is placed between the mid-
amplification
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stage 120 and the post-amplification stage 30. The polarization beam
combiner/splitter
52 splits counter-propagating ASE received from the mid-amplification stage
120
between the pre-amplification stage 10 and one end of an optical fiber 56 also
connected to the polarization beam combiner/splitter 52. The other end of the
optical
fiber 56 is connected to the polarization beam combiner/splitter 54 and
received ASE is
transmitted to the post-amplification stage 30.
It is noted that, in this configuration where bulk polarization beam
combiners/splitters
are used, the counter-propagating ASE is split 50/50 between the pre- and post-
amplification stages. This may not be the optimal ratio since pre-
amplification typically
requires less pump power than post-amplification. However the ASE at the
output of the
optical amplifier 400 has a significant polarization extinction ratio since
bulk polarization
beam combiners/splitters have a broad spectral response, and therefore the use
a
polarizer at the output of the optical amplifier 400 allows a significant
reduction of the
noise figure if the signal is co-propagating with the pump.
Figs. 6 to 9 show numerical simulations that were performed in order to show
the
behavior of the optical amplifier 100. The optical amplifier 100 is now
compared to a
reference long-band amplifier 500 which is based on an architecture that is
more similar
to conventional architectures. The reference optical amplifier 500 is shown in
Fig. 5 and
has a first amplification stage 510 consisting of a polarization-maintaining
rare-earth-
doped optical fiber 512, in cascade with a second amplification stage 530
consisting of
a polarization-maintaining rare-earth-doped optical fiber 532 pumped in co-
propagation
using a pump light source 40 and a wavelength division multiplexing coupler
42.
Counter-propagating ASE generated in the second amplification stage 530 is
used to
pump the first amplification stage 510. The optical signal is polarized and
propagates
along the fast axis.
More specifically, both amplifiers use a highly-ytterbium-doped polarization
maintaining
optical fiber of the model 529C23 manufactured at the Institut National
d'Optique in
Quebec City, Canada. The pump light source 40 is a laser diode pump having a
power
of 450 mW at 976 nm. The input power level of the optical signal s to be
amplified is
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200 pW at a central wavelength of 1064 nm and the optical signal s is
polarized along
the fast axis. The fiber lengths of each stage in each amplifier were
optimized to
maximize the overall gain of each amplifier. The resulting lengths are as
follows: In the
optical amplifier 100 of Fig. 1, the length of the first rare-earth-doped
optical fiber 12 is
5.51 m, the length of the second rare-earth-doped optical fiber 22 is 4.73 m
and the
length of the third rare-earth-doped optical fiber 32 is 3.88 m. In the
reference optical
amplifier 500 of Fig. 5, the length of the first rare-earth-doped optical
fiber 512 is 7.48 m
and the length of the second rare-earth-doped optical fiber 532 is 5.52 m. The
particular optical fiber used in this case has a numerical aperture of 0.14, a
mode field
diameter of 6.1 pm at 1060 nm, a theoretical cutoff wavelength of 950 nm and
peak
absorption at 976 nm of 530 dB/m.
Fig. 6 shows the evolution of the optical power in the reference optical
amplifier 500 of
Fig. 5. Trace 602 shows the evolution of the signal propagating along the fast
axis, trace
604 shows the evolution of the co-propagating ASE along the slow axis, trace
606
shows the evolution of the co-propagating ASE along the fast axis, trace 608
shows the
evolution of the counter-propagating ASE along the slow axis and trace 610
shows the
evolution of the counter-propagating ASE along the fast axis. The overall gain
of the
reference optical amplifier 500 is 29.31 dB.
Fig. 7 shows the evolution of the optical power in the optical amplifier 100
of Fig. 1.
Trace 702 shows the evolution of the signal propagating along the fast axis,
trace 704
shows the evolution of the co-propagating ASE along the slow axis, trace 706
shows
the evolution of the co-propagating ASE along the fast axis, trace 708 shows
the
evolution of the counter-propagating ASE along the slow axis and trace 710
shows the
evolution of the counter-propagating ASE along the fast axis. The overall gain
of the
optical amplifier 100 is 29.06 dB. It can be seen on Figs. 6 and 7 that most
of the ASE
produced in the mid-amplification stage 120 is coupled to the post-
amplification stage
30, while the remaining is coupled to the pre-amplification stage 10. The
overall gain of
the optical amplifier 100 is a bit lower than that of the optical amplifier
500. This is
mostly due to the insertion loss of the coupler 44 which is 0.46 dB.
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Fig. 8 shows the polarization extinction ratio of the slow axis over the fast
axis, of the
ASE at the output of the optical amplifiers 100 and 500 of Figs. 1 and 5.
Trace 802
shows the variation in wavelength of the polarization extinction ratio of the
ASE in the
reference optical amplifier 500 of Fig. 5, while trace 804 shows the
polarization
extinction ratio in the optical amplifier 100 of Fig. 1. It can be seen that,
in the optical
amplifier 100 of Fig. 1, most of the ASE at the output of the amplifier 100 is
polarized
along the slow axis. This ASE can be suppressed using the polarizer 46. For
comparison, in the reference optical amplifier 500, the polarization
extinction ratio of the
ASE at the output is 0 dB over the entire wavelength range. This is due to the
absence
of any polarization selective component in the reference optical amplifier
500.
Fig. 9 shows the optical spectrum at the output of the optical amplifiers 100
and 500 of
Figs. 1 and 5, with and without the use of a polarizer 46 at their output. The
benefit of
using the polarizer 46 can be observed. Trace 902 shows the optical spectrum
at the
output of the reference optical amplifier 500 without the polarizer 46. The
signal to
noise ratio is 11.67 dB. Trace 904 shows the optical spectrum at the output of
the
reference optical amplifier 500 with the polarizer 46. The signal to noise
ratio is
14.68 dB, for an increase of 3.01 dB.
Trace 906 shows the optical spectrum at the output of the optical amplifier
100 without
the polarizer 46. The signal to noise ratio is 12.13 dB. Trace 908 shows the
optical
spectrum at the output of the optical amplifier 100 with the polarizer 46. The
signal to
noise ratio is 16.05 dB, for an increase of 3.92 dB. The improvement in signal
to noise
ratio between the optical amplifier 100 of Fig. 1 and the reference optical
amplifier 500
is of 1.37 dB when considering the use of the polarizer 46.
Figs. 10 and 11 show experimental results obtained with the optical amplifiers
100 and
500 of Figs. 1 and 5. It is noted that the results are in excellent agreement
with the
predictions made from the numerical simulations. Fig. 10 shows the power at
the output
of the optical amplifiers 100 and 500 as a function of the pump power. Trace
1002
shows the output power of the reference optical amplifier 500, while trace
1004 shows
the output power of the optical amplifier 100. Fig. 11 shows the optical
spectra at the
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output of the optical amplifiers 100 and 500. Trace 1102 shows the optical
spectrum at
the output of the reference optical amplifier 500, while trace 1104 shows the
optical
spectrum at the output of the optical amplifier 100. Essentially, what is
shown by the
experimental results is that the efficiency of the two architectures is
comparable but that
the ASE noise in the optical amplifier 100 is reduced compared to the
reference optical
amplifier 500.
Fig. 12 shows yet another embodiment of a long-band optical fiber amplifier
1200
wherein the configuration of Fig. 1 is used but the polarization-maintaining
fiber coupler
44 is replaced by a bulk polarization beam combiner/splitter cube 1244 that is
fiber
pigtailed on its four functional faces to provide ports 1, 2, 3 and 4. Port 1
and 3
correspond to two opposite faces of the cube, while ports 2 and 4 correspond
to the two
other opposite faces. The optical signal s received at port 1 is polarized
along the p-
polarization state of the polarization beam combiner/splitter cube. In this
configuration,
ASE generated in the mid-amplification stage 120 and received on port 3 of the
polarization beam combiner/splitter cube is split 50/50 between port 1 and
port 4, i.e.
between the pre-amplification stage 10 and the post-amplification stage 30.
ASE
polarized along the s-polarization state is transmitted to the post-
amplification stage 30
while ASE polarized along the p-polarization state is transmitted to the pre-
amplification
stage 10. Consequently, ASE coupled in co-propagation in the post-
amplification stage
30 is mostly polarized along the slow propagation axis while the signal is
polarized
along the fast propagation axis, providing a high polarization extinction
ratio of the ASE
at the output of the optical amplifier. In this configuration, the use of the
polarizer 46
therefore significantly improves the noise figure.
It is noted that the optical amplifier architectures described herein may be
used to
amplify continuous, modulated or pulsed optical signals. For example, the
optical
amplifiers described herein may be used in nanosecond pulsed laser sources or
ultrashort pulsed laser sources.
It should be understood that while rare-earth-doped optical fibers are use in
the
illustrated embodiments, other types of optical waveguides may also be used.
For
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example, planar rare-earth-doped waveguides may be used. Since planar
waveguides
are typically shorter, rare-earth dopant concentration may be increased to
achieve a
similar amplification gain.
It is also noted that the rare-earth-doped optical fibers 12, 22 and 32 may
use different
dopants and base materials. In the illustrated case, the amplification
waveguides 12, 22
and 32 are ytterbium-doped silica optical fibers but it is noted that
ytterbium-doped
chalcogenide optical fibers may also be used and that the waveguide may
include other
dopants. The concentration of ytterbium may also vary. Furthermore, it is
noted that
ytterbium may be replaced by another rare-earth dopant, such as erbium for
example,
for long-band amplification of an optical signal at a different wavelength.
Increasing/reducing ytterbium concentration in the amplification waveguide may
also be
used to reduce/increase the length of the amplification waveguide.
Furthermore, in the illustrated case, a polarization combiner/splitter, e.g. a
polarization-
maintaining fiber coupler, is used to split the ASE power generated in the mid-
stage
among the pre- and the post-amplification stages. It is however noted that, if
other
means are used for splitting and coupling of the ASE power, the rare-earth-
doped
optical waveguides may not be necessarily polarization-maintaining and,
similarly, the
optical signal may not be polarized.
The embodiments described above are intended to be exemplary only. The scope
of
the invention is therefore intended to be limited solely by the appended
claims.
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