U.S. patent application number 09/769083 was filed with the patent office on 2001-11-01 for optical communication system with co-propagating pump radiation for raman amplification.
Invention is credited to Ackerman, David, Bacher, Kenneth L., Dautremont-Smith, William, Du, Mei, Rottwitt, Karsten, Stentz, Andrew John, Strasser, Thomas A., Zhang, Liming.
Application Number | 20010036004 09/769083 |
Document ID | / |
Family ID | 26882423 |
Filed Date | 2001-11-01 |
United States Patent
Application |
20010036004 |
Kind Code |
A1 |
Ackerman, David ; et
al. |
November 1, 2001 |
Optical communication system with co-propagating pump radiation for
raman amplification
Abstract
A fiber Raman amplifier is configured to use a co-propagating
Raman pump source, which may be beneficial in a variety of system
configurations (for example, in bidirectional communication
systems). By carefully configuring the pump source characteristics,
sufficient optical gain can be achieved in the co-propagating
arrangement, the characteristics including: (1) using an optical
pump power of at least 50 mW, (2) having a relatively large
spectral bandwidth within the pump (to suppress SBS); and (3) a
frequency difference between all longitudinal pump modes of each
pump laser being separated by at least the walk-off frequency
between the pump laser frequency and the signal frequency, and all
intense longitudinal modes between different pump lasers being
separated by at least the electrical bandwidth of the communication
system.
Inventors: |
Ackerman, David; (Hopewell,
NJ) ; Bacher, Kenneth L.; (Macungie, PA) ;
Dautremont-Smith, William; (Orefield, PA) ; Du,
Mei; (Scotch Plains, NJ) ; Rottwitt, Karsten;
(Basking Ridge, NJ) ; Stentz, Andrew John;
(Clinton, NJ) ; Strasser, Thomas A.; (Warren,
NJ) ; Zhang, Liming; (Marlboro, NJ) |
Correspondence
Address: |
Wendy W. Koba Esq.
P.O. Box 556
Springtown
PA
18081
US
|
Family ID: |
26882423 |
Appl. No.: |
09/769083 |
Filed: |
January 25, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60186797 |
Mar 3, 2000 |
|
|
|
Current U.S.
Class: |
359/334 ;
359/341.3 |
Current CPC
Class: |
H01S 3/094096 20130101;
H01S 3/09408 20130101; H04B 10/2916 20130101; H01S 2301/03
20130101; H01S 3/06754 20130101; H01S 3/302 20130101; H01S 5/1096
20130101; H01S 3/094003 20130101 |
Class at
Publication: |
359/334 ;
359/341.3 |
International
Class: |
H01S 003/00 |
Claims
What is claimed is:
1. An article comprising an optical fiber Raman amplifier
comprising an input port responsive to an optical signal to be
amplified; an output port for providing an exit path from said
Raman amplifier for an amplified optical signal; an optical fiber
path for support Raman gain disposed between said input port and
said output port; and a Raman pump source coupled to said input
port with said optical signal to be amplified, said Raman pump
source including at least one pump laser for providing an optical
pump to co-propagate with said optical signal through said optical
fiber path, said Raman pump source exhibiting a relatively high
output power, relatively large spectral width, a frequency
difference between all longitudinal modes of each pump laser being
separated by at least the walk-off frequency between the pump laser
frequency and the signal frequency, and all intense longitudinal
modes between different pump lasers being separated by at least the
electrical bandwidth of the communication system.
2. Article according to claim 1, wherein the optical pump comprises
a power of greater than or equal to 50 mW.
3. Article according to claim 1, wherein the walk-off frequency is
approximately 5 GHz.
4. Article according to claim 1, wherein the walk-off frequency is
approximately 1 GHz.
5. Article according to claim 1, wherein the intense longitudinal
mode is defined as a mode having sufficient intensity to generate
substantial Raman gain.
6. Article according to claim 5 wherein each intense longitudinal
mode exhibits at least 10 dB more power than the remaining
modes.
7. Article according to claim 1 wherein the Raman pump source
comprises a single mode distributed feedback (DFB) laser with an
additional RF tone for increasing the spectral bandwidth of the
output from said single mode DFB laser.
8. Article according to claim 1, wherein the Raman pump source
comprises at least two distributed feedback (DFB) single mode
lasers, each comprising a different center frequency, with an RF
tone added to each DFB laser to provide a relatively large spectral
bandwidth and the frequency spacing between said at least two DFB
lasers selected to exceed the system electrical bandwidth.
9. Article according to claim 8 wherein pairs of DFB lasers are
modulated 180.degree. out of phase to minimize amplitude
modulation.
10. Article according to claim 1, wherein the Raman pump source
comprises at least one multimode distributed feedback (DFB)
laser.
11. Article according to claim 10, wherein the multimode DFB laser
comprises a relatively short grating region, positioned near an
output facet of said laser so as to allow for multimode modes of
the optical signal to lase simultaneously.
12. Article according to claim 11 wherein the mode spacing between
the intense longitudinal modes is greater than the walk-off
frequency of pump and the signal in the Raman amplifier.
13. Article according to claim 10 wherein the Raman pump source
comprises a pair of multimode DFB lasers, offset in wavelength to
separate the mode beating frequency between DFB lasers by at least
the electrical bandwidth of the system
14. Article according to claim 1 wherein the Raman pump source
comprises at least one distributed Bragg reflector (DBR) laser
including a signal source applied to an area disposed above an
included grating of said at least one DBR laser to broaden the
spectral width of said at least one DBR laser.
15. Article according to claim 14 wherein the signal source
comprises a high frequency signal source.
16. Article according to claim 14 wherein the signal source
comprises a noise signal source.
17. Article according to claim 1 wherein the Raman pump source
comprises at least one Fabry-Perot laser exhibiting a predetermined
frequency separation, between all existing longitudinal modes,
equal to at least the walk-off frequency between said at least one
Fabry-Perot laser and the input optical signal.
18. Article according to claim 17 wherein the pump source comprises
at least two Fabry-Perot lasers, each exhibiting a different center
frequency, with a frequency difference between the intense
longitudinal modes of different Fabry-Perot lasers being separated
by at least the electrical bandwidth of the communication
system.
19. Article according to claim 18 wherein the at least two
Fabry-Perot lasers are temperature tuned to maintain a
predetermined separation in center frequency between said at least
two Fabry-Perot lasers.
20. Article according to claim 17 wherein the at least one
Fabry-Perot laser further comprises a pair of external fiber Bragg
gratings to generate a mode spacing between the at least one
Fabry-Perot pump frequency and the input optical signal frequency,
the mode spacing being at least equal to the walk-off frequency
between said pump frequency and said input optical signal
frequency.
21. Article according to claim 1 wherein the optical fiber path
comprises dispersion compensating fiber, the optical signal and
Raman pump source being coupled to said dispersion compensating
fiber.
22. Article according to claim 21 wherein the Raman gain fiber
exhibits a lower dispersion slope at the pump wavelength to reduce
mode partitioning noise when utilized with multi-longitudinal mode
pump lasers.
23. Article according to claim 22 wherein the dispersion slope is
less than 5 ps/nm km at the pump wavelength.
24. Article according to clam 1, wherein the optical fiber path is
selected such that the zero dispersion wavelength is not centered
between the pump and signal wavelengths, the placement of the said
zero dispersion wavelength for reducing pump-signal four wave
mixing effects.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority of Provisional
Application No. 60/186,797, filed Mar. 3, 2000.
TECHNICAL FIELD
[0002] The present invention relates to a Raman amplified optical
communication system and, more particularly, to an optical
communication system utilizing co-propagating Raman amplification
with Raman pump sources particularly designed to overcome known
pump-signal crosstalk problems.
BACKGROUND OF THE INVENTION
[0003] The subject of Raman amplification is well known in the
literature. Stimulated Raman amplification is a nonlinear optical
process in which an intense pump wave is injected into an optical
fiber that is carrying one or more optical signals. In fused silica
fibers, if the pump wave is of a frequency approximately 13 THz
greater than the signal waves (i.e., if the pump wavelength is
approximately 100 nm shorter than the signal wavelength in the
vicinity of 1500 nm), the pump will amplify the signal(s) via
stimulated Raman scattering. If the amplification is made to occur
in the transmission fiber itself, the amplifier is referred to as a
"distributed amplifier". Such distributed amplification has been
found to improve the performance of a communication system, as
discussed in the article "Capacity upgrades of transmission systems
by Raman amplification" by P. Hansen et al. appearing in IEEE Phot.
Tech. Lett., Vol. 9, 1997, at page 262. For example, if a pump wave
is injected into one end of the fiber in a direction that is
counter-propagating with respect to the information signals, the
signals will be amplified before their signal-to-noise ratio
degrades to an unacceptable level. The performance of such an
amplifier is often characterized in terms of its "effective" or
"equivalent" noise figure and its on/off gain. The effective noise
figure is defined as the noise figure that an equivalent
post-amplifier would have in order to achieve the same noise
performance as the distributed Raman amplifier (see, for example,
"Rayleigh scattering limitations in distributed Raman
pre-amplifiers", by P. Hansen et al., IEEE Phot. Tech. Lett. Vol.
10, 1998, at page 159). Experimentally, the effective noise figure
may be found by measuring the noise figure of a span utilizing
counter-propagating Raman amplification and then subtracting (in
decibels) the passive loss of the span. The on/off gain of a
distributed Raman amplifier is defined as the difference (in
decibels) between the output signal power with the Raman pump "on"
to that with the pump "off". Alternatively, a lumped or "discrete"
amplifier can be constructed with a local length of Raman gain
fiber.
[0004] It is well known in the prior art that Raman gain generated
with a polarized pump wave is, in general, polarization dependent.
This phenomenon is discussed in detail in an article entitled
"Polarization effects in fiber Raman and Brilloiun lasers" by R. H.
Stolen et al., appearing in IEEE J. Quantum Electronics, Vol.
QE-15, 1979, at p. 1157. Given that the vast majority of fiber
optic communication systems utilize non-polarization maintaining
fibers, an optical signal's state of polarization at any given
point is not generally known and is subject to capricious
variations. For these reasons, it is desirable to minimize
polarization-dependent loss and gain within the communication
system. It has also been shown that the polarization dependence of
Raman amplifiers can be significantly reduced by polarization
multiplexing polarized Raman sources, as disclosed in U.S. Pat. No.
4,881,790, issued to L. F. Mollenauer et al. on Nov. 21, 1989.
[0005] Significant pump powers are required to generate substantial
on/off Raman gain in conventional transmission fibers. For example,
approximately 300 mW of power is required from a monochromatic pump
to generate 15 dB of on/off Raman gain in transmission fibers with
.about.55 .mu.m.sup.2 effective areas. It is also known that these
pump powers are significantly higher than the threshold for
stimulated Brilloiun scattering (SBS) for pump sources with
spectral widths less than 25 MHz, as discussed in the article
"Optical Power Handling Capacity of Low Loss Optical Fibers as
determined by Stimulated Raman and Brilloiun Scattering", by R. G.
Smith, appearing in Appl. Optics, Vol. 11, 1972, at page 2489.
Stimulated Brilloiun scattering is a well-known nonlinear optical
process in which the pump light couples to an acoustic wave and is
retro-reflected. This retro-reflection may prohibit the penetration
of the Raman pump significantly deep into the transmission fiber,
inhibiting the generation of Raman gain.
[0006] The threshold for SBS can be substantially increased by
broadening the spectral width of the Raman pump source, as
discussed in the above-cited Mollenauer et al. patent. In
particular, one method for broadening the spectral width and thus
suppressing SBS is by frequency dithering of the laser source.
Another mechanism for broadening the spectral width of a laser is
to allow the device to lase in more than one longitudinal mode of
the laser cavity. The frequency spacing of the longitudinal modes
of a laser is defined by the relation c/2 n.sub.gL, where c is the
speed of light in a vacuum, n.sub.g is the group velocity within
the laser cavity and L is the length of the cavity.
[0007] Certain types of semiconductor lasers are preferred for use
as Raman pump sources. The most common types of semiconductor pump
lasers are Fabry-Perot (FP) lasers, and FP lasers locked to
external fiber Bragg gratings. These types of pump sources are
discussed in an article entitled "Broadband lossless DCF using
Raman amplification pumped by multichannel WDM laser diodes" by
Emori et al. appearing in Elec. Lett, Vo. 34, 1998 at p. 2145. It
is typical for the external fiber Bragg gratings to be located
approximately 1 m from the semiconductor laser.
[0008] It is known that when light from a laser, lasing in multiple
longitudinal modes, is passed through a dispersive delay line (such
as an optical fiber), noise components referred to as mode
partitioning noise are generated at frequencies typically less than
a few GHz. See, for example, "Laser Mode Partitioning Noise in
Lightwave Systems Using Dispersive Optical Fiber", by R. Wentworth
et al., appearing in J. of Lightwave Technology, Vol. 10, No. 1,
1992 at pp. 84-89. It is also known that single-longitudinal-mode
semiconductor lasers are typically used as signal sources. Common
types are distributed feedback (DFB) lasers and distributed Bragg
reflector (DBR) lasers.
[0009] The Raman amplification process is known as an extremely
fast nonlinear optical process. For this reason, intensity
fluctuations in the pump may result in fluctuations in the Raman
gain. These gain fluctuations may then impress noise upon the
optical signals, degrading the performance of the communication
system. For the purposes of understanding the teaching of the
present invention, this effect will be referred to as the
"pump-signal crosstalk". It is known that, at sufficiently high
frequencies, the signal and pump will "walk off" with respect to
one another, due to dispersion within the fiber. It is also known
that the use of a strictly counter-propagating pump geometry, that
is, where the direction of propagation of all Raman pumps is
opposite to that of all signals, is effective in reducing
degradations from pump-signal crosstalk. This amplifier geometry is
discussed in detail in an article entitled "Properties of Fiber
Raman Amplifiers and their Applicability to Digital Optical
Communication Systems" by Y. Aoki, appearing in J. Lightwave
Technology, Vol. 6, No. 7, 1988 at pages 1225-29. In
counter-propagating pump geometries, the transit time through the
amplifying fiber is used to average the pump intensity fluctuations
such that "quiet" amplification may be achieved. It is also known
that the counter-propagating pump geometries serve to reduce the
polarization dependence of the Raman gain.
[0010] Another potential source of noise in Raman amplified systems
arises in systems transmitting information in multiple signal
wavelengths, where the multiple signals will more quickly deplete
the power in the Raman pump. See, for example, "Crosstalk in Fiber
Raman Amplification for WDM Systems", W. Jiang et al., J. of
Lightwave Technology, Vol. 7, No. 9, 1989 at pp. 1407-111. In this
situation, the information imposed on one signal wavelength is
impressed upon a signal at the same or a different wavelength via
the Raman gain process. For the purposes of understanding the
teaching of the present invention, this effect will be referred to
as the "signal-pump-signal crosstalk". This source of noise is also
greatly reduced in counter-propagating pump geometries where the
transit time through the amplifying fiber is used to reduce the
effects of any pump intensity fluctuations.
[0011] It is also known that due to unusual noise sources, such as
pump-signal crosstalk and signal-pump-signal crosstalk, it is often
necessary to characterize the noise performance of Raman amplifiers
with electrical noise figure measurements, characterizing the
effective noise figure as a function of electrical frequency.
[0012] There are potential system advantages to the use of
co-propagating Raman amplification, including increasing the
signal-to-noise ratios of the amplified signals, minimizing
excursions of the signal powers as a function of length, and
allowing for the bi-directional propagation of signals within the
same distributed Raman amplifier. However, a problem with these
co-propagating Raman amplifiers is that they are more susceptible
to both pump-signal crosstalk and signal-pump-signal crosstalk.
[0013] An exemplary prior art co-propagating Raman amplifier
arrangement is discussed in the article "Wide-Bandwidth and
Long-Distance WDM Transmission using Highly Gain Flattened Hybrid
Amplifiers" by S. Kawai et al., appearing in IEEE Phot. Tech.
Lett., Vol. 11, No. 7, 1999 at pp. 886-888. However, the on/off
Raman gain of this particular configuration is exceedingly low
(i.e., approximately 4 dB)--a region where the above-mentioned
problems would be minimal.
[0014] Thus, a need remains for a co-propagation Raman
amplification system that provides a sufficient on/off gain to be a
useful device, while not exhibiting undesirable levels of
pump-signal crosstalk and signal-pump-signal crosstalk.
SUMMARY OF THE INVENTION
[0015] The need remaining in the prior art is addressed by the
present invention, which relates to Raman amplified optical
communication system and, more particularly to an optical
communication system utilizing co-propagating Raman amplification
with Raman pump sources particularly designed to overcome
pump-signal crosstalk problems in co-propagating systems.
[0016] In accordance with the present invention, an optimized Raman
pump source is utilized that produces at least 50 mW of output
power, sufficient spectral width to suppress SBS, and is configured
such that the frequency difference between all intense longitudinal
pump modes (regardless of polarization) are separated by at least
the electrical bandwidth of the communication system, or at least
the walk-off frequency, where "walk-off frequency" is defined as
the lowest frequency at which the pump-signal crosstalk is no
longer a significant factor in degrading the performance of the
Raman amplifier.
[0017] In various embodiments, the pump source may comprise one or
more frequency-dithered DFB lasers, multi-longitudinal mode DFB
lasers, DBR lasers, frequency-offset FP lasers, or FP lasers locked
to a Fabry-Perot fiber Bragg grating reflector.
[0018] In one embodiment of the present invention, the pump source
may be injected into the input of dispersion-compensating fiber at
the input of a discrete Raman amplifier to generate co-propagating
Raman amplification, where the effects of both pump-signal
crosstalk and signal-pump-signal crosstalk are minimized.
[0019] Advantageously, the pump sources of the present invention
may be used in either a distributed Raman amplifier application or
a discrete Raman amplifier application.
[0020] Other and further embodiments of the present invention will
become apparent during the course of the following discussion and
by reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] Referring now to the drawings,
[0022] FIG. 1 contains plots of the electrical effective noise
figure spectra of distributed Raman amplifiers pumped with
conventional FP semiconductor pump lasers locked to fiber Bragg
gratings in both co- and counter-propagating geometries, where the
solid lines are experimentally measured and dashed lines are
calculated values;
[0023] FIG. 2 contains plots of relative intensity noise (RIN) of
the light from an FP semiconductor laser before and after
propagating through approximately 40 km lengths of TrueWave
Minus.RTM., TrueWave Plus.RTM. and TrueWave Reduced Slope.RTM.
fibers with group-velocity dispersions at pump wavelengths of
-11.8, -5.2 and 0.8 ps/nm-km, respectively;
[0024] FIG. 3 contains plots of optical spectra generated from two
FP semiconductor lasers that have been polarization
multiplexed;
[0025] FIG. 4 contains higher resolution plots of the spectra of
the longitudinal modes of the two FP semiconductor lasers of FIG.
3;
[0026] FIG. 5 contains plots of amplitude noise of the FP lasers
associated with FIG. 3, before propagation through a transmission
fiber at a temperature of 25.degree. C. for both lasers, and after
propagation through 20 km of TrueWave Minus.RTM. fiber at
temperatures of 20.degree., 25.degree., 30.degree. and 40.degree.
C. for one FP laser, the other held at 25.degree. C. to provide an
offset in their center wavelengths;
[0027] FIG. 6 contains plots of the electrical effective noise
figure spectra of distributed Raman amplifiers pumped with the FP
semiconductor pump lasers of FIG. 3, where the solid lines are
associated with measured values and dashed lines with calculated
values;
[0028] FIG. 7 contains plots of the optical spectra generated from
the two FP semiconductor lasers of FIG. 3, with one pump maintained
at a lower temperature to offset the two laser center
wavelengths;
[0029] FIG. 8 contains plots of the electrical effective noise
figure spectra of distributed Raman amplifiers pumped with the FP
semiconductor lasers of FIG. 7, with the solid lines associated
with experimentally measured values and the dashed lines associated
with calculated values; and
[0030] FIG. 9 contains plots of the optical spectra of an optical
signal at a wavelength of 1570 nm that has been amplified by a
co-propagating Raman pump at a wavelength of 1470 nm with a Raman
gain of 4.5 dB in lengths of standard single mode fiber (SSMF) and
TrueWave Plus.RTM. fiber.
DETAILED DESCRIPTION
[0031] As discussed above, there are many significant differences
in performance when comparing a Raman amplifier utilizing a
counter-propagating pump source to a Raman amplifier utilizing a
co-propagating pump source. The problems associated with co-pumped
configurations need to be addressed, where one problem in
particular--pump-signal crosstalk--is often not an issue in a
counter-propagating arrangement since the signal has a transient
time through the amplifier and the strong averaging effect reduces
the crosstalk. However, in a co-pumping configuration, the signal
and the pump travel together and the only averaging effect is the
dispersive delay related to the walk-off between the pump and the
signal.
[0032] In the course of studying the pump-signal crosstalk problem
in association with the present invention, it has been found that a
first type of noise originates from the intrinsic amplitude noise
of the pump. In particular and unlike the case for the
counter-propagating geometry, the mode beating noise in the pump
will couple to the signal in the forward (co-pumped) direction.
This type of noise can degrade the signal-to-noise ratio (SNR),
causing a power penalty in the co-pumped Raman amplifier
configuration. FIG. 1 illustrates the electrical noise figure
measurement, which is used to identify this noise contribution at
different frequencies. That is, for the plots of FIG. 1, the
electrical effective noise figure spectra of distributed Raman
amplifiers pumped with conventional FP semiconductor pump lasers
locked to fiber Bragg gratings was measured. The wavelength of the
Raman pump was chosen to be 1450 nm and the wavelength of the
optical signal was 1560 nm and an on/off Raman gain of 11.7 dB was
achieved. The solid lines illustrate the experimentally measured
values, while the dashed lines are theoretical values. In
particular, the theoretical values are calculated based solely on
noise from signal-spontaneous beating, that is, excluding all
"excess" noise sources that will be discussed in detail hereinbelow
in association with the teaching of the present invention. The
theoretical value ignores noise from pump-signal crosstalk (i.e.,
presumes a "perfect" pump).
[0033] As shown, there is substantial agreement between the
theoretical and measured values of the effective noise figure for
the counter-propagating arrangement, supporting the theory that
counter-pumping effectively eliminates pump-signal crosstalk. In
contrast, as illustrated in FIG. 1, a significant deviation exists
between the experimental results and the theoretical values for a
co-propagating pump configuration, where this deviation is
attributed to pump-signal crosstalk. As shown clearly in FIG. 1,
pump-signal walk-off effectively averages this source of noise for
frequencies greater than approximately 5 GHz (the value of 5 GHz
being exemplary only and associated with this particular pump
module, Raman gain and group velocity difference between the pump
and signal; for other amplifiers the walk-off frequency may be as
low as 1 GHz). For the purposes of understanding the teaching of
the present invention, this frequency (above which the pump-signal
crosstalk is no longer a significant factor in degrading the
performance of the Raman amplifier) will be referred to as the
"walk-off frequency". In particular, this frequency will depend on
the relative group velocities of the pump and signal wavelengths in
the gain fiber, as well as the magnitude of the Raman gain. Thus,
an optical pump source suitable for use in a co-propagating
geometry in accordance with the present invention should only
exhibit longitudinal modes separated by at least the system
walk-off frequency.
[0034] FIG. 2 contains plots of the relative intensity noise (RIN)
of the light from a FP semiconductor laser before and after
propagating through .about.40 km of TrueWave Minus.RTM., TrueWave
Plus.RTM. and TrueWave Reduced Slope.RTM. fibers with
group-velocity dispersions at the pump wavelengths of -11.8, -5.2
and 0.8 ps/nm-km, respectively. The low frequency noise is due to
mode-partitioning noise. Referring to FIG. 2, it is shown that the
noise is higher after propagation through fibers of higher
dispersion. This source of noise will then be transferred to a
co-propagating signal through Raman amplification. Heretofore, this
noise source was not discovered and, therefore, problems associated
with this high dispersion were not addressed.
[0035] The plots of the optical spectra generated from two FP
semiconductor pump lasers that have been polarization multiplexed,
at a center wavelength of approximately 1470 nm, are shown in FIG.
3 and denoted as "Pump 1" and "Pump 2". The individual longitudinal
modes cannot be distinguished in these plots. A higher resolution
of these plots is contained in FIG. 4, where these plots show that
the two FP lasers have slightly different longitudinal mode
spacings such that there are inevitably wavelengths at which modes
of the two lasers overlap. As the temperature of one of the diodes
is increased, it has been found that the modes of the laser shift
to longer wavelengths, thereby changing the relative spacing
between various longitudinal modes of the two lasers.
[0036] FIG. 5 contains a plot (labeled "A") of the amplitude noise
from the same two FP lasers, the amplitude noise measured, at a
laser temperature of 25.degree. C. as in FIG. 3, before propagating
through a significant length of fiber. It is evident that this plot
is free of any noise spikes. This is the case since the
semiconductor pumps have been polarization multiplexed, and the
orthogonal light beams will not interfere with each other. Also
shown in FIG. 5 are plots of the amplitude noise after propagation
through 20 km of TrueWave Minus.RTM. fiber with both FP lasers
having a temperature of 25.degree. C. (plot B), and the one FP
laser held at 25.degree. C., with the other FP laser at
temperatures of 20, 30 and 40.degree. C., respectively (plots C, D
and E, respectively). In each instance, there are noise spikes
after propagation through the fiber, where the frequency of the
spikes varies with the temperature of the FP lasers. These noise
spikes are caused by the mixing of polarization components as they
propagate through the optical fiber. The frequencies of the spikes
will vary with temperature, due to the shifting of the laser mode
frequencies with temperature. As with the mode spacing discussed
above, the movement of these noise spikes with temperature is a
heretofore undisclosed phenomenon. It is also to be noted that
mode-partitioning noise, as discussed above with FIG. 2, is also
present in the plots as shown in FIG. 5 after propagation through
the transmission fiber.
[0037] FIG. 6 contains plots of the electrical effective noise
figure spectra of distributed Raman amplifiers pumped with the
conventional FP semiconductor pump lasers whose spectra are plotted
in FIG. 3. Data is shown for both a co- and counter-pumped
amplifier. A pump power of 260 mW was used to generate the spectra
of FIG. 6, generating an on/off Raman gain of 12.3 dB. The solid
lines are experimentally measured values, while the dashed lines
are theoretical values. The theoretical values were calculated
based only on noise from signal-spontaneous beating; that is,
excluding all "excess" noise sources that are the subject of this
invention. It is to be noted that good agreement is found between
the measured noise figure and the theoretical noise figure in the
counter-propagating geometry, indicating that again the
counter-propagating geometry has eliminated any "excess" noise
features. In the co-propagating geometry, however, an extremely
large noise spike at .about.17 GHz is observed. This noise feature
is due to mode beating among the longitudinal modes of the
polarization multiplexed laser diodes, whose spectra are plotted in
FIG. 3. Upon propagation, the polarization states of the two laser
diodes are mixed, mode beating occurs and the resulting amplitude
noise is transferred to the signal. It is important to note that
this type of noise can appear above the "walk-off" frequency (5 GHz
in this case) and it cannot be "averaged out" by fiber dispersion.
This data illustrates the importance of configuring Raman pump
sources for co-propagating Raman amplifiers in accordance with the
present invention such that the frequency difference between all
intense longitudinal modes of different, regardless of
polarization, are separated by at least the electrical bandwidth of
the communication system.
[0038] FIG. 7 contains plots of the optical spectra of the same
lasers as used to generate FIG. 3, except that one pump has been
temperature-tuned to a shorter wavelength. As shown, most of the
intense longitudinal modes of the two lasers no longer overlap,
although there remains some overlap of the modes at much lower
intensities. FIG. 8 contains plots of the electrical effective
noise figure spectra of distributed Raman amplifiers pumped with
the FP semiconductor pump lasers whose spectra are plotted in FIG.
7. The data is shown for both a co- and a counter-pumped amplifier.
A pump power of 260 mW was used, generating an on/off Raman gain of
12.3 dB. As before, the solid lines are associated with
experimentally measured values and the dashed lines are associated
with theoretical values. For the co-propagating pump geometry, good
agreement is shown between the experimental and theoretical values.
In comparing these results to those shown in FIG. 6, it is seen
that the noise spike evident in FIG. 6 is essentially eliminated by
temperature tuning the pumps such that the frequency difference
between all intense longitudinal modes of the different
lasers--regardless of polarization--are separated by an amount
greater than the electrical bandwidth of the system (in this case,
22 GHz). It should be noted that temperature tuning is not
essential for achieving this pump configuration. Semiconductor
diodes with the appropriate wavelengths exhibiting off-set center
wavelengths could be used.
[0039] FIG. 9 contains plots of the optical spectra of a signal at
1570 nm after it has co-propagated with a Raman pump at a
wavelength of 1470 nm and experienced an on/off Raman gain of 4.5
dB. As shown, there are significant four-wave mixing sidebands
generated by the mixing of the pump modes with the signal. The
magnitude of the sidebands can be substantially reduced by the use
of a single mode fiber, where the zero-dispersion wavelength
.lambda..sub.0 is near 1300 nm, rather than a fiber such as
TrueWave Plus.RTM., with a .lambda..sub.0 that is nearly centered
between the pump wavelength and the signal wavelength. It is to be
noted that the difference in group velocity of the pump with
respect to the signal can reduce the four-wave mixing
efficiencies.
[0040] A number of different semiconductor pump sources could
potentially meet the criteria outlined above for use in a
co-propagating Raman amplifier. In accordance with the teachings of
the present invention, an acceptable pump source for a
co-propagating geometry is characterized by: (1) producing an
output power of at least 50 mW; (2) having sufficient spectral
width to suppress SBS; and (3) designed such that the frequency
difference between all intense longitudinal modes of different
lasers, regardless of polarization, are separated by at least the
electrical bandwidth of the communication system, and that the
frequency different between all intense longitudinal modes of one
laser are separated by the walk-off frequency between the pump and
the signals.
[0041] A frequency-dithered distributed feedback (DFB) laser can be
modified, as described below, to exhibit the necessary attributes
of a pump source for a co-propagating system geometry in accordance
with the present invention. In particular, when used as a pump
source in the co-propagating Raman amplifier, a conventional DFB
laser is configured to produce a single longitudinal mode. One
exemplary DFB laser that is useful in the co-propagating system of
the present invention is disclosed in U.S. Pat. No. 5,111,475,
"Analog Optical Fiber Communication System and Laser Adapted for
Use in such a System, D. Ackerman et al, issued May 5, 1992, and
assigned to the assignee of the present invention. In order to
provide frequency dithering, a small-amplitude RF tone is added to
the laser drive current, thus broadening the laser linewidth and
suppressing the SBS. A suitable choice of the offset between the
laser gain and the DFB wavelength will maximize the FM to AM
efficiency; that is, the ratio of the amount of frequency to
amplitude modulation induced by the RF tone. In accordance with the
present invention, multiple frequency-dithered DFB lasers can be
used as a co-propagating pump source, where the frequency spacing
between adjacent DFB lasers needs to exceed the electrical
bandwidth of the communication system. Additionally, pairs of DFBs
can be modulated 180.degree. out of phase such that the impact of
any residual amplitude modulation is minimized.
[0042] Multimode DFB lasers are designed with the grating extending
along a significant portion of the cavity length so that one (of
only two possible) longitudinal modes satisfies the laser
conditions on roundtrip gain and phase change at the laser's
operating conditions. Decreasing the length of the grating region
relative to the total cavity length, and positioning the grating
toward the output facet of the laser, can allow several cavity
modes near in wavelength to satisfy the lasing conditions
simultaneously, achieving wavelength-stabilized multimode
operation. It can be shown that a stabilized multimode spectrum
from a DFB laser can increase the SBS threshold from a few
milliwatts to greater than 120 milliwatts. The small ratio of
grating length to total cavity length will also decrease the
sensitivity of the laser's performance to the phase of the grating
relative to the laser HR facet.
[0043] A distributed Bragg reflector (DBR) laser, like a DFB laser,
uses a grating integrated into the laser cavity to control the
laser wavelength. Unlike the DFB, however, the laser material above
the grating in a DBR laser is biased separately from the rest of
the laser cavity. This separate biasing allows the carrier density,
and thus the index of refraction of the material incorporating the
grating, to be controlled by the bias applied to the grating
section of the laser. Since the stabilized wavelength of the laser
is dependent on the "optical period" of the grating (i.e., the
physical period multiplied by the index of refraction), the lasing
wavelength can be controlled by the bias applied to this grating
section. A selective area growth (SAG) may be used in the grating
region to prevent this section from being absorbing at the
designated lasing wavelength.
[0044] While a bias on the grating section can be used to adjust
the lasing wavelength, any high frequency variation in this bias
will broaden the linewidth of the output. The efficiency of the
linewidth broadening can be two to three orders of magnitude
greater than that of a standard DFB laser. The laser linewidth
required for a co-propagating pump in accordance with the present
invention may thus be obtained by adding noise to the bias signal
to the grating section, rather than requiring a separate dithering
circuit, as discussed above. One other alternative is to connect
the grating section bias to the laser bias using an adjustable
resistance element. The output wavelength can then be tuned to
better match the desired target wavelength (by adjusting the
resistance), while the Johnson noise from the resistor will provide
the bias variation to broaden the laser linewidth.
[0045] As discussed above in association with the figures, simple
Fabry-Perot (FP) lasers can be used as sources for a co-propagating
Raman amplifier, with temperature-tuning applied to provide the
desired frequency difference between all intense longitudinal
modes. It should be noted that temperature tuning is not essential
for achieving this pump configuration, since semiconductor diodes
with separated center wavelengths at a given temperature could be
used instead. Additionally, a pair of fiber Bragg gratings can be
used to "lock" the wavelength of a simple FP laser. The pair of
Bragg gratings are located approximately one meter from the
semiconductor laser, while being sufficiently close to each other
such that the mode spacing of the cavity created by the Bragg
gratings is at least as large as the electrical bandwidth of the
communication system. The Bragg grating located further from the
semiconductor cavity is required to exhibit a reflectivity greater
than that of the other Bragg grating.
[0046] Typically, the lowest (or nearly lowest) signal powers are
present in a communication system at the point where the signals
exit the transmission fiber and enter a discrete optical amplifier.
It is common for the signals to be amplified in the first stage of
an erbium-doped fiber amplifier (EDFA), and then to pass through a
length of dispersion-compensating fiber. In accordance with the
present invention, it is contemplated to remove the first stage
EDFA, instead directly injected the communication signals into the
dispersion compensating fiber with a co-propagating Raman pump. The
low signal powers and high dispersion slope of the dispersion
compensating fiber make this location in a communication system an
ideal point for the application of co-propagating Raman
amplification. In addition, the properties of the
dispersion-compensating fiber may be optimized for Raman gain, as
well as their dispersion characteristics.
[0047] It should be noted that while the above-described sources
are particularly well-suited for use as Raman pump sources in a
co-propagating amplifier arrangement (either as a discrete Raman
amplifier or a distributed Raman amplifier), they are equally
applicable to a counter-propagating geometry, due to their high
output power and wavelength stability. Further, these sources may
be useful as sources in conventional erbium-doped fiber amplifier
arrangements.
* * * * *