Abstract:
A gain-clamped, optical amplifier includes a rare-earth doped fiber and a source of pump energy coupled to the rare-earth doped fiber. The doped fiber serves as a gain medium that is optically pumped by the pump source. A wavelength-selective optical feedback loop is coupled between input and output ports of the rare-earth doped fiber. The feedback loop supports a compensating laser signal, which is a pulsed signal located at a wavelength different from a signal wavelength.

Description:
RELATED APPLICATIONS 
     This is continuation of application Ser. No. 09,031,018, filed Feb. 26, 1998, now U.S. Pat. No. 6,134,033. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to wavelength division multiplexed transmission systems, and more particularly to a transmitter employed in wavelength division mulitplexed transmission systems which increases spectral efficiency by reducing four-wave mixing. 
     BACKGROUND OF THE INVENTION 
     Wavelength-division multiplexing is expected to be increasingly utilized in undersea and transcontinental terrestrial optical transmission systems due, in part, to the large bandwidth capacity such multiplexing provides. One way of increasing the total transmission capacity of such systems is to more efficiently use the available spectral bandwidth such as by decreasing the spacing between adjacent ones of the multiplexed channels. Unfortunately, wavelength division multiplexed transmission systems are susceptible to performance limitations due to polarization dependent effects such as cross-talk between the multiplexed channels. Cross-talk, which is primarily caused by the non-linear index of refraction of optical transmission fibers, increases as the channel spacing decreases. Four-wave mixing is one significant deleterious effect that produces cross-talk. Accordingly, reducing four-wave mixing while simultaneously increasing spectral efficiency would be desirable in wavelength division multiplexed optical transmission systems. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method and apparatus is provided for transmitting an optical signal. The method includes the step of generating an optical signal that includes a plurality of optical channels, which are sequentially numbered from 1 to N from lowest to highest wavelength. A state-of-polarization of predetermined odd-numbered channels is oriented to be substantially orthogonal to a state of polarization of predetermined even-numbered channels by directing the predetermined odd-numbered channels and the predetermined even-numbered channels through orthogonally polarizing inputs of a polarization coupler. The odd-numbered channels and the even-numbered channels may be directed through first and second wavelength combiners, respectively, prior to orienting their states of polarization. The orthogonal relationship between the states of polarization of odd and even-numbered channels advantageously limits the four-wave mixing products that can be generated in the optical transmitter and the optical transmission path to which it is typically coupled. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the polarization states of channels contained in the optical signal which is transmitted in accordance with the present invention. 
     FIG. 2 shows the results of a demonstration that four-wave mixing is reduced when adjacent channels are transmitted with orthogonal SOPs. 
     FIG. 3 shows a simplified block diagram of an illustrative embodiment of an optical transmitter constructed in accordance with the present invention. 
     FIG. 4 shows further details of one particular embodiment of one of the optical transmitters shown in FIG. 3, which employs synchronous amplitude and optical phase modulation. 
     FIG. 5 shows an exemplary optical communication system that may incorporate the transmitters shown in FIGS.  2 - 3 . 
    
    
     DETAILED DESCRIPTION 
     In accordance with the present invention, a WDM optical signal is provided in which the odd-numbered channels have SOPs that are substantially orthogonal to the SOPs of the even-numbered channels. FIG. 1 illustrates this orthogonal relationship of SOPs for the channels in optical output signal at some arbitrary instant in time. The preferred substantially orthogonal relationship between SOPs of odd and even-numbered channels advantageously limits the four-wave mixing products that can be generated in the optical transmission path. Referring to FIG. 1, it will be evident that this desirable result is achieved because neighboring channels, for example channels λ 1  and λ 2 , are substantially precluded from interacting due to their orthogonal SOPs. Channels sharing the same SOP, for example channels λ 1  and λ 3 , are separated far enough apart in wavelength such that the amplitude of resultant mixing products is minimal. 
     FIG. 2 demonstrates that four-wave mixing is reduced when adjacent channels are transmitted with orthogonal SOPs. The FIG. shows the results of a measurement of degenerate four wave mixing in a 500 km amplifier chain. In FIG.  2 ( a ), light from two CW lasers was launched into an amplifier chain having the same SOP. In FIG.  2 ( b ), light from the two CW lasers were launched into an amplifier chain with orthogonal SOPs. The amplifier chain includes eleven EDFA spans with 45 km of transmission fiber. The average launched power into the transmission spans was +10 dBm. The reduced sidebands in FIG.  2 ( b ) relative to FIG.  2 ( a ) clearly indicate that the effect of four wave mixing is less when the light from the two lasers is launched with orthogonal SOPs. 
     FIG. 3 is a simplified block diagram of an optical transmitter  300  constructed in accordance with the principles of the invention. Transmitter  300  produces WDM optical signal shown in FIG.  1 . As shown, optical transmitter  300  includes a plurality of optical sources  301   1 ,  301   2 , . . .  301   N . The plurality of optical sources  301   1 ,  301   2 , . . .  301   N  which could be, for example, wavelength-tunable semiconductor lasers, are utilized to generate a plurality of continuous-wave optical signals  302   1 ,  302   2 , . . .  302   N  each having a different wavelength λ 1 , λ 2 , . . . . λ N , respectively, thus defining a plurality of N optical channels. Optical sources  301   1 ,  301   2 , . . .  301   N  may be adapted such that optical channels  302   1 ,  302   2 , . . .  302   N  have substantially identical optical power. One or more of the optical sources  301  may be adapted so that optical channels  302  carry information supplied by data sources (not shown) using conventional techniques. For discussion purposes, the channels may be sequentially numbered 1, 2, . . . N, from lowest to highest wavelength. In this illustrative example of the invention the channel wavelengths are uniformly spaced by, for example, 1 nm. However, as previously mentioned, in other applications of the invention it may be desirable to utilize non-uniform channel wavelength spacing. 
     The elements shown in FIG. 3 may be coupled using conventional means, for example, optical fibers, which could include polarization maintaining optical fibers where appropriate. Optical transmitter  300  is typically coupled, for example, to an optical transmission path and optical receiver (not shown) to form an optical transmission system. It is noted at the onset that the term “channel” as used herein refers to any optical phenomena that is defined by a unique wavelength. Thus, the term channel may refer to a component of a wavelength division multiplexed optical signal having a plurality of components, where each component has a different wavelength. Moreover, as used herein, the term channel may refer to a monochromatic optical signal. 
     The plurality of optical sources  301   1 ,  301   2 , . . .  301   N , are arranged in sequential order so that optical channels  302   1 ,  302   2 , . . .  302   N  are produced in ascending (or descending) wavelength order from λ 1  to λ N . As shown in FIG. 3, the optical sources  301  are grouped into two sets, a first set of odd-numbered optical sources  3011 ,  3013 , . . .  301   N−1  and a second set of even-numbered optical sources  301   2 ,  301   4 , . . .  301   N , where N is even. That is, the first set of optical sources produces, in sequential order, the odd-numbered wavelengths λ 1 , λ 3 , . . . λ N−1  while the second set of optical sources produces, in sequential order, the even-numbered wavelengths λ 2 , λ 4 , . . . λ N  Even-numbered wavelengths are directed to a first wavelength combiner  307  while the odd-numbered wavelengths are directed to a second wavelength combiner  308 . The wavelength combiners  307  and  308  may comprise, for example, directional couplers, star couplers or wavelength routers. In preferred embodiments of the invention, each set of optical sources imparts a large degree of polarization (i.e., nearly unity) to the signals so that the signals can be subsequently passed through a polarizer without distortion. The orientation of the polarization may be arbitrarily chosen as long as its value is substantially the same among the channels produced by each set of transmitters. If significant loss and distortion can be tolerated, however, the optical sources need not impart a large degree of polarization. The following discussion assumes that a degree of polarization near unity is imparted to the optical signals. 
     Wavelength combiner  307  forms an output signal  311  comprising N/2 optical channels with each channel being in substantially the same polarization state. Similarly, wavelength combiner  308  forms an output signal  313  comprising N/2 optical channels with each channel being in substantially the same polarization state. The polarization states of output signals  311  and  313  may or may not be the same. Output signals  311  and  313  are directed to a polarization combiner  315  for combining the N/2 channels of output signals  311  and  333 . The N/2 channels of output signal  311  are polarized by polarization combiner  315  in a first polarization state and the N/2 channels of output signal  313  are polarized by polarization combiner  315  in a second polarization state that is orthogonal to the first polarization state. The resulting output from the polarization combiner  315  is the desired optical signal  305  shown in FIG.  1 . That is, polarization combiner  315  provides an output signal in which adjacent channels are orthogonally polarized. One of ordinary skill in the art will recognize that the multiplexing functionality of the polarization combiner  315  may in the alternative be accomplished by a conventional directional coupler in which the SOP&#39;s are carefully adjusted. 
     FIG. 4 shows the pertinent details of one particular embodiment of a transmitter show in FIG. 2 for synchronously imparting data, amplitude and phase modulation to the optical signals generated by the optical sources  301  of FIG.  3 . As shown, data modulator  485  receives data to be imparted to the optical signal  402  from data source  480  and modulates the optical signal  402  at a frequency determined by clock  475 . The clock  475  also drives amplitude modulator  419  via a variable delay line, for example phase shifter  420 . Similarly, clock  475  drives phase modulator  422  via variable delay line  425 , which may also be a phase shifter, for example. In operation, the clock  475  causes the rate of amplitude and phase modulation to be frequency and phase locked to the rate of data modulation. Variable delay lines  420  and  425  are utilized to adjust the relative timing among the data, amplitude and phase modulation. The manner in which clock  475  drives data modulator  485 , amplitude modulator  419 , and phase modulator  422  and the operational details of variable delay lines  420  and  425  are further described in U.S. Pat. No. 5,526,162. While FIG. 3 shows one particular modulation format that may be used in connection with the present invention, one of ordinary skill in the art will recognize that the invention is also applicable to optical transmitters that employ other modulation formats such as solitons, for example. 
     The present invention offers a number of advantages over other optical transmission techniques. For example, transmitters that employ polarization multiplexing with solitons (see U.S. Pat. No. 5,111,322) and transmitters that employ polarization scrambling in WDM systems (see co-pending U.S. patent application Ser. No. 08/355,798) require that the optical channels all operate off a common clock. In other words, such systems require that the channels have a very well defined, fixed phase relationship. In contrast, the present invention does not require a well-defined electrical phase relationship among the channels so that a common clock need not be provided for all the optical channels. As a result, transmitters constructed in accordance with the present invention can use the Synchronous Digital Hierarchy (SDH) input channels, in which the channels may use a common clock frequency but with a random and time-varying phase. Different channels may therefore operate at different bits rates, if desired. 
     It should be recognized that the pair-wise orthogonal relationship of the optical channels provided in accordance with the present invention will not be maintained over the entire transmission path of the system because of an unavoidable degree of polarization mode dispersion (PMD). However, since current communication systems use relatively small channel spacings and optical fibers having a PMD less than about 0.1 ps/{square root over (km)}, the correlation between the polarization states of the channels will be high for nearest neighbors. Since nonlinear mixing primarily occurs between neighboring channels, the present technique will nevertheless substantially reduce the effects of four-wave mixing. Moreover, although the degree of polarization of optical signal  105  will be small, PMD may increase it. But again, if low PMD fibers and a large number of channels are employed, the degree of polarization should remain small. If this re-polarization causes excess noise to accumulate from polarization hole-burning in the optical amplifiers, then, in accordance with U.S. Pat. Nos. 5,309,530 and 5,309,535, a relatively slow speed polarization scrambler may be placed at the output of polarization coupler  315 . 
     FIG. 5 shows a simplified block diagram of an exemplary optical fiber transmission system that employs the transmitter of the present invention. The system includes an optical transmission path  500 , a transmitting terminal  501 , and a receiving terminal  502 . The transmitting terminal  501  corresponds to the transmitter  300  shown in FIG.  3 . The optical signal presented by the terminal  501  to the transmission path  500  may comprise a plurality of WDM optical carriers each carrying an SDH signal. The transmission path may include dispersion compensators  505 . The transmission path  500  also includes optical amplifiers (not shown), which may be EDFAs, for example, which amplify optical signals in the  1550  wavelength band. In one embodiment of the invention the transmission fibers may be dispersion shifted single-mode fibers with an average zero dispersion wavelength higher than the operating wavelengths of the system.