Abstract:
A method and apparatus is provided for transmitting an optical signal having a total number of channels that are dividable into a prescribed number of wavebands. The method includes the step of generating a first series of optical signals corresponding to each of the wavebands. The first series of optical signals includes a plurality of optical channels, which are sequentially numbered from 1 to N from lowest to highest wavelength. Within each waveband, 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 within each waveband 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 within each waveband 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.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to wavelength division multiplexed transmission systems, and more particularly to a transmitter employed in wavelength division multiplexed 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. 
     U.S. application Ser. No. 09/031,018, now U.S. Pat. No. 6,134,033 discloses an optical transmitter that generates a WDM signal having even-numbered channels in a state of polarization (SOP) orthogonal to the SOP of the odd-numbered channels. This arrangement advantageously limits the four-wave mixing products that can be generated in the transmitter and the optical transmission path to which it is typically coupled. 
     Wavelength division multiplexed systems must also employ dispersion management techniques. As the per channel data rates of such system increase, the interplay of dispersion and fiber nonlinearity needs to be more carefully managed. Typically the transmission line is designed to have an average dispersion value of zero. In the case of WDM systems with a non-zero dispersion slope, however, only one channel can be arranged to have an average dispersion of zero. The remaining channels will have some net nonzero dispersion due to the dispersion slope of the optical fibers forming the transmission line. One technique for overcoming this limitation at intermediate points along the transmission path (i.e., in the undersea plant in undersea transmission systems) in WDM systems is disclosed in U.S. application Ser. No. 08/759,493, now. U.S. Pat. No. 6,137,604. As discussed therein, it is useful to divide the usable optical bandwidth of the transmission system into sub-bands that individually undergo dispersion compensation before being re-combined. In comparison to other dispersion compensation techniques, more WDM data channels reside near a wavelength corresponding to the average zero dispersion wavelength. However, to implement in a straightforward manner this dispersion management technique at the transmitting terminal, in connection with a signal that has SOPs which are pairwise orthogonal, requires a dispersion compensator that maintains the SOPs of the optical signal. 
     Unfortunately, a transmitter that offers both dispersion compensation and a signal in which adjacent channels have orthogonal SOPs is difficult to provide because of the unavailability of a simple and inexpensive means for performing dispersion compensation in a polarization maintaining environment. 
     Accordingly, it would be desirable to provide a transmitter that generates a dispersion-compensated WDM optical signal having SOPs that are pairwise orthogonal without the need for a dispersion compensating element that does not change the SOP of the optical signal. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method and apparatus is provided for transmitting an optical signal having a total number of channels that are dividable into a prescribed number of wavebands. The method includes the step of generating a first series of optical signals corresponding to each of the wavebands. The first series of optical signals includes a plurality of optical channels, which are sequentially numbered from 1 to N from lowest to highest wavelength. Within each waveband, 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 within each waveband 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 within each waveband 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 within a given waveband contained in the optical signal which is transmitted in accordance with the present invention. 
     FIG. 2 shows the total bandwidth of an exemplary WDM optical signal employed in the present invention. 
     FIG. 3 shows a simplified block diagram of an illustrative embodiment of an optical transmitter unit. 
     FIG. 4 shows a simplified block diagram of an illustrative embodiment of an optical transmitter constructed in accordance with the present invention. 
     FIG. 5 shows the WDM optical signal provided by the optical transmitter shown in FIG.  4 . 
     FIG. 6 shows an alternative embodiment of the invention shown in FIG. 4 in which chirped fiber gratings are employed. 
     FIG. 7 shows further details of one particular embodiment of one of the optical sources shown in FIG. 3, which employs synchronous amplitude and optical phase modulation. 
     FIG. 8 shows an exemplary optical communication system that may incorporate the transmitter shown in FIG.  4 . 
    
    
     DETAILED DESCRIPTION 
     In accordance with the present invention, a WDM optical signal is provided in which the odd-numbered channels within a given portion of the signal&#39;s bandwidth have SOPs that are substantially orthogonal to the SOPs of the even-numbered channels within that portion of the bandwidth. FIG. 1 illustrates this orthogonal relationship 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. The reduction of four-wave mixing in this manner is discussed in more detail in Bergano et al., “320 Gb/s WDM Transmission over 7,200 km Using Large Mode Fiber Spans and Chirped Return to Zero Signals,” OFC &#39;98 PD12. 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. 
     It should be 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. 
     FIG. 2 shows the total bandwidth of an exemplary WDM optical signal employed in the present invention. The optical signal comprises 64 channels that are divided into eight wavebands  20   1 ,  20   2 , . . .  20   8 . For reasons that will be explained below, each waveband is separated by a guard band that will typically have a spectral width greater than the separation between adjacent channels within any given waveband. While the WDM signal shown in FIG. 2 comprises 64 channels, one of ordinary skill in the art will recognize that the present invention encompasses a WDM signal comprising any number of channels. Moreover, the channels may be divided into any desired number of wavebands that may or may not each incorporate the same number of channels. 
     FIG. 3 is a simplified block diagram of an optical transmitter unit  300  that produces a single one of the wavebands  20   1 ,  20   2 , . . .  20   8  shown in FIG.  2 . The eight wavebands  20   1 ,  20   2 , . . .  20   8  that make up the WDM signal will each be produced by a transmitter unit similar to transmitter unit  300 . Accordingly, as detailed below in connection with FIG. 4, the optical transmitter constructed in accordance with the present invention will comprise a series of such transmitter units, with the number of transmitter units corresponding to the number of wavebands that are employed. 
     As shown, optical transmitter unit  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 optical data signals  302   1 ,  302   2 , . . .  302   N  each having a different wavelength λ 1 , λ 2 , . . . λ N , respectively, thus defining the N optical channels of the given waveband. The N optical channels belong to one of the wavebands shown in FIG.  2 . Optical sources  301   1 ,  301   2 , . . .  301   N  may be adapted such that optical data 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 chaniel wavelengths are uniformly spaced by, for example, 0.3 nm. However, as previously mentioned, in other applications of the invention it may be desirable to utilize non-uniform channel wavelength spacing. 
     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  301   1 ,  301   3 , . . .  301   N−1  and a second set of even-numbered optical sources  301   2 ,  301   4 , . . .  301   N , where N is an even integer. 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. Output signals  311  and  313  are directed to a polarization combiner  315  for combining the N/2 channels of output signals  311  and  313 . 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 one of the wavebands  20   1 ,  20   2 , . . .  20   N  shown in FIG.  2 . 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 a simplified block diagram of an optical transmitter  40  constructed in accordance with the present invention. The optical transmitter  40  comprises a plurality of the transmitter units  42  of the type shown in FIG.  3  and produces the WDM optical signal shown in FIG. 2, which in this illustrative example comprises 64 channels divided into 8 wavebands of 8 channels each. For clarity of description, the transmitting sources shown in FIG. 4 are denoted T x,y , where x refers to the waveband number and y refers to the particular channel within waveband x. That is, for example, T 7,6  refers to the sixth channel in waveband seven, or equivalently, channel number 54. 
     In accordance with the present invention, dispersion compensation is provided on a waveband by waveband basis at the transmitter (It should be noted that this process will typically provide a pre-compensation of dispersion that will generally be followed by additional dispersion compensation at intermediate points along the transmission path). This is advantageous because, as previously mentioned, due to the dispersion slope of the fiber, only one given wavelength can operate at average zero dispersion. Accordingly, the various channels employed in a WDM system cannot all operate at the wavelength of average zero dispersion. For this reason, as shown in U.S. application Ser. No. 08/759,493, now U.S. Pat. No. 6,137,604, it is useful to divide the usable optical bandwidth of the transmission system into sub-bands that individually undergo dispersion compensation before being re-combined. In comparison to other dispersion compensation techniques, more WDM data channels reside near a wavelength corresponding to the average zero dispersion wavelength. Moreover, since in the present invention the dispersion compensation is provided downstream from the individual transmitting units which arrange the SOPs of the channels, the dispersion compensators advantageously do not need to function in a polarization maintaining environment. 
     As shown in FIG. 4, the output signal  45   1  produced by polarization combiner  415   1  of transmitting unit  42   1  is directed to dispersion compensating element  44   1 . Output signal  45   1 , corresponds to waveband  20   1  shown in FIG.  2 . Similarly, the output signal  45   2  produced by polarization combiner  415   2  of transmitting unit  42   2  (not shown) is directed to dispersion compensating element  44   2 . In this manner dispersion compensation is provided to output signals  45   1 ,  45   2 , . . .  45   8  by polarization combiners  415   1 ,  415   2 , . . .  415   8 , respectively. Finally, dispersion compensated signals  47   1 ,  47   2 , . . .  47   8  are directed to a polarization independent power combiner  48  for multiplexing the signals on output fiber  49 . 
     FIG. 5 shows the resulting WDM optical signal provided by the inventive transmitter shown in FIG. 4 at some arbitrary instant in time. The channels corresponding to only the first two wavebands  20   1 , and  20   2  are shown. Within each waveband the odd-numbered channels have SOPs that are substantially orthogonal to the SOPs of the even-numbered channels. That is, the channels within each waveband are arranged in the same manner as the signal shown in FIG.  1 . Thus, the preferred substantially orthogonal relationship between SOPs of the odd and even-numbered channels advantageously limits the four-wave mixing products that can be generated among the different channels within a waveband. However, the SOPs of one waveband are uncorrelated with the SOPs of any other waveband. In other words, the orthogonal relationship does not apply to different channels in different wavebands. For example, as shown in FIG. 5, the SOP of channel  8  in waveband  20   1 , is not necessarily orthogonal to the SOP of channel  9  in waveband  20   2 , and in fact channels  8  and  9  may, as indicated, have substantially the same SOPs. As a result, adjacent channels belonging to different wavebands may in fact produce significant four-wave mixing products. For this reason guard bands are provided between wavebands. The guard bands ensure that the spectral separation between adjacent channels in different wavebands is sufficiently great to minimize the channel interaction from the amplitude of the resulting four-wave mixing products even when the channels have the same SOP. For example, in one particular embodiment of the invention the channel spacing within a waveband may be selected to be 0.3 nm while the guard band may have a spacing of 0.6 nm. 
     The dispersion compensating elements  44  shown in FIG. 4 may be any appropriate element that imparts dispersion, such as a single mode fiber, for example. The invention also contemplates the use of other dispersion compensating elements such as the chirped fiber grating arrangement shown in FIG.  6 . The fiber grating may be linearly chirped, or alternatively, it may have a higher order chirp. In FIG. 6, the output signals  45   1 ,  45   2 , . . .  45   8  are directed to the respective input ports  62  of a three port circulator  60   1 ,  60   2 , . . .  60   8 . The output signals exit the circulators  60   1 ,  60   2 , . . .  60   8  on output ports  64  and enter chirped fiber gratings  66   1 ,  66   2 , . . .  66   3 . The signals reflected by the gratings are returned to the respective circulators  60   1 ,  60   2 , . . .  60   8  via ports  64  and exit the circulators on ports  68  where, as in the previous embodiment of the invention, they are directed to power combiner  48 . 
     Since different wavelengths will penetrate a different number of layers into the chirped fiber gratings before being reflected, the amount of delay imparted to the wavelengths of the optical signals will also be different for different wavelengths. If a quadratically chirped fiber grating is employed, the correct amount of dispersion can be imparted to each of channels in the wavebands. In this case the only limitation on the bandwidth of the individual wavebands will result from the maximum length of fiber grating that can be fabricated. In fact, if a sufficiently long fiber grating can be manufactured (e.g., on the order of ten meters), then it will not be necessary to divide the WDM signal into a large number of bands to provide dispersion compensation. In some cases only two (or even possibly one) bands will be required, reducing by a commensurate amount the number of optical transmitter units that are required. 
     FIG. 7 shows the pertinent details of one particular embodiment of the optical sources  301   1 ,  301   2 , . . .  301   N  shown in FIG. 3 for synchronously imparting data, amplitude and phase modulation to the optical signals. 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. One of ordinary skill in the art will recognize that the invention is also applicable to optical transmitters that employ various modulation formats such as solitons, for example. 
     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  305  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. 8 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  40  shown in FIG.  4 . 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. 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.