Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2001-01-25
2002-05-07
Tarcza, Thomas H. (Department: 3662)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S341300, C359S337000
Reexamination Certificate
active
06384963
ABSTRACT:
TECHNICAL FIELD
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
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.
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.
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 ~55 &mgr;m
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.
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
g
L, where c is the speed of light in a vacuum, n
g
is the group velocity within the laser cavity and L is the length of the cavity.
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.
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.
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.
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 Lightwa
Ackerman David
Bacher Kenneth L.
Dautremont-Smith William
Du Mei
Rottwitt Karsten
Koba Esq. Wendy W.
Lucent Technologies - Inc.
Sommer Andrew R
Tarcza Thomas H.
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