Abstract:
A method and apparatus is provided for transmitting an optical signal through an optical fiber. The apparatus includes an optical signal source, which generates an optical signal having a plurality of optical channels onto which data is modulated. Each of the optical channels is defined by a different carrier wavelength. A phase modulator imparts phase modulation to the plurality of optical channels so that channels nearest a zero dispersion wavelength of the optical fiber are more closely spaced to one another than channels farthest in wavelength from the zero dispersion wavelength of the optical fiber.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to optical transmission systems, and more particularly to a transmitter for transmitting wavelength division multiplexed optical signals through an optical communication system. 
     BACKGROUND OF THE INVENTION 
     Signals on optical fiber transmission lines characterized by large bit rate distance products, such as undersea or transcontinental terrestrial lightwave transmission systems and which employ optical amplifiers are subject to a host of impairments that accumulate along its length. The source of these impairments within a single data channel include amplified spontaneous emission (ASE) noise generated in the erbium-doped fiber amplifiers (EDFAs), polarization dependent gain caused by hole burning in the EDFAs, polarization dependent loss (PDL) in the passive components, nonlinear effects resulting from the dependence of the refractive index of single-mode fiber on the intensity of the light propagating therethrough, and chromatic dispersion which causes different optical frequencies to travel at different group velocities. In addition, for wavelength division multiplexed (WDM) systems in which a plurality of optical channels are transmitted on the same optical fiber, crosstalk between channels caused by the fiber&#39;s nonlinear index and incomplete channel selection at the receiving terminal must be considered. 
     WDM systems often employ transmitters that impart phase modulation to the carrier wavelengths. The phase modulation increases the spectral width of each channel so that its peak intensity is reduced, thus reducing the adverse effects of nonlinear interactions. Such transmitters may also include a polarization scrambler for varying the polarization states of the carrier wavelengths. As discussed in more detail in U.S. Pat. No. 5,526,162, for example, polarization scrambling is advantageous because it minimizes the effects of polarization hole burning. Polarization scrambling is accomplished by splitting the signal into two equal orthogonal components, applying phase modulation to one of the components and then recombining the components. 
     Unfortunately, the phase modulation imparted to the carrier wavelengths to reduce nonlinear interactions and polarization hole burning limits the number of channels that fit within a given bandwidth because phase modulation increases the spectral width of each channel. Of course, to increase transmission capacity, it is desirable to use as many channels as possible over the limited bandwidth that is available, thus requiring that the spectral width of the individual channels be kept to a minimum. 
     Accordingly, it would be desirable to impart sufficient phase modulation to the optical carrier wavelengths in a WDM transmission system to substantially reduce nonlinear interactions and polarization hole burning while avoiding any excess phase modulation so that the spacing between individual channels is reduced to increase system capacity. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an apparatus is provided for transmitting an optical signal through an optical fiber. The apparatus includes an optical signal source, which generates an optical signal having a plurality of optical channels onto which data is modulated. Each of the optical channels is defined by a different carrier wavelength. A phase modulator imparts phase modulation to the plurality of optical channels so that channels nearest a zero dispersion wavelength of the optical fiber are more closely spaced to one another than channels farthest in wavelength from the zero dispersion wavelength of the optical fiber. 
     The optical channels closest to the zero dispersion wavelength exhibit relatively low nonlinearities and hence do not require as a great a reduction in intensity as channels more remote from the zero dispersion wavelength. That is, these channels require less spectral broadening by phase modulation to reduce their peak intensities. Since those optical channels closest to the zero dispersion wavelength require the least amount of phase modulation, the present invention advantageously reduces any excess phase modulation so that the individual channels may be more closely situated to one another, increasing transmission capacity within the operating bandwidth. 
     In accordance with another aspect of the invention a method is provided for transmitting an optical signal. In particular, an optical signal is generated which has a plurality of optical channels onto which data is modulated. Each of the optical channels is defined by a different carrier wavelength. Phase modulation is imparted to each of the plurality of optical channels based on its spectral distance from a zero dispersion wavelength of the optical fiber. In one particular embodiment of the invention, the phase modulation is arranged so that increasing phase modulation is imparted to optical channels increasingly remote in wavelength from the zero dispersion wavelength of the optical fiber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a simplified block diagram of an exemplary wavelength division multiplexed (WDM) transmission system in accordance with the present invention. 
     FIG. 2 shows the spectral locations of six WDM channels relative to the zero dispersion wavelength for a conventional WDM transmitter in which the channel spacing is uniform. 
     FIG. 3 shows spectral locations for ten WDM channels relative to the zero dispersion wavelength for a WDM transmitter constructed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a simplified block diagram of an exemplary wavelength division multiplexed (WDM) transmission system in accordance with the present invention. The transmission system serves to transmit a plurality of optical channels over a single path from a transmitting terminal to a remotely located receiving terminal. Specifically, a plurality of transmitters  101   1 ,  101   2 , . . .  101   N  transmit data signals on a set of wavelengths λ 1 , λ 2 , . . . λ N , which are respectively carried on a plurality of paths  102   1 ,  102   2 , . . .  102   N . The data channels are combined onto a single path by a signal combiner  103 . The functionality of the signal combiner can be performed by a variety of devices such as a wavelength independent directional coupler or a wavelength router of the type described in U.S. Pat. Nos. 5,002,350 and 5,412,744. The combined set of optical channels are transmitted to a plurality of remote receiving terminals  104  via optical transmission path  105 , which could, for example, include erbium-doped fiber amplifiers  106  optically coupling individual spans of single-mode optical fibers  107 . A 1×N splitter  108  and a plurality of optical bandpass filters  109  demultiplexes the optical channels at the remote or receiving end of the system. As those skilled in the art will appreciate, the functionality of splitter  108  and bandpass filters  109  may be achieved in a number of different ways. For example, the previously mentioned wavelength router that is employed as a multiplexer in the transmitter may also be used as a demultiplexer in the receiver. 
     Because of the dispersion slope of optical fiber, only one wavelength or channel can be located at the zero dispersion wavelength of the transmission line. Typically, as shown in FIG. 2, the zero dispersion wavelength is located near the center of the usable bandwidth. For example, the six WDM channels shown in FIG. 2 are centered about the zero dispersion wavelength λ 0 . 
     As previously mentioned, the plurality of transmitters  101   1 ,  101   2 , . . . ,  101   N  each typically includes a phase modulator and/or a polarization scrambler. In either case phase modulation is imparted to the optical signals. For example, the transmitters  101   1 ,  101   2 , . . . ,  101   N  may be so-called synchronously modulated transmitters in which phase and polarization modulation are imparted at the rate at which data is modulated onto the signals. An example of a synchronously modulated transmitter is disclosed in U.S. Appl. Ser. No. 08/771,097 filed on Dec. 20, 1996, now Ser. No. 09/776,942. 
     FIG. 4 shows transmitter  101   N  employing synchronous polarization and optical phase modulation The transmitter disclosed in application Ser. No. 09/776,942 illustrates a laser  400  that produces a continuous wave (CW) optical signal  401 . The optical signal  401  is transmitted to a data modulator  402  that modulates the signal to impart information thereto in a well known fashion, producing a modulated optical information signal  403 . The data modulator  402  receives the data to be imparted to the optical signal  401  from a data source  404  and modulates the optical signal  401  at a frequency determined by a clock  405 . The optical information signal  403  is transmitted from the data modulator  402  to optical phase modulator  406 , amplitude modulator  407 , and finally to polarization modulator  413 . The clock  405  drives the three modulation stages via a series of variable delay elements  408 ,  409  and  414 , which are used to selectively adjust the delay of the modulation imparted by modulators  406 ,  407  and  413  relative to the phase of the data modulation imparted by modulator  402 . In accordance with the present invention, the amplitude modulator  407  is driven by the clock  405  so that the intensity of the optical information signal is re-modulated at a rate equal to the rate at which data is imparted to the optical signal  401 . An amplitude adjustment mechanism  410  is employed to set the modulation depth that amplitude modulator  410  imparts on signal  413 . 
     Prior to phase modulation (whether resulting from polarization scrambling or direct phase modulation), the optical channels in a typical long-haul transmission system have an inherent spectral width of approximately 20 GHz for a 10 Gb/s data rate, in comparison to a spectral width of about 60 Ghz after phase modulation for the same data rate, which substantially reduces the transmission capacity of the system. 
     The present inventor has realized that phase modulation does not have to be applied in equal amounts to all of the channels. Rather, the inventor has recognized that the channel at or nearest to the zero dispersion wavelength requires the least amount of phase modulation and the channel most remote from the zero dispersion wavelength requires the maximum amount of the phase modulation. In other words, the amount of phase modulation that is required depends on the channel&#39;s distance from the zero dispersion wavelength. Specifically, as the distance between the channel and the zero dispersion wavelength of the fiber increases, the required amount of phase modulation increases. 
     Minimal phase modulation is required for signals nearest the zero dispersion wavelength because at this wavelength nonlinearities are low. Accordingly, since the adverse effects of nonlinearities arise at high intensities, it is not necessary to reduce the channel intensity by spectral broadening (i.e., phase modulation) when nonlinearities are low. As nonlinearities increase for channels at increasing distance from the zero dispersion wavelength, correspondingly more phase modulation is required. 
     In accordance with the present invention, a nonuniform distribution of phase modulation is employed across the operating bandwidth of a WDM transmission system. The centermost channels undergo the least amount of phase modulation and the outermost channels undergo the most amount of phase modulation. Accordingly, as shown in FIG. 3, the centermost channels (e.g., channels  5  and  6  in FIG. 3) may be relatively closely spaced to one another while the outermost channels (e.g., channels  1  and  2  and channels  9  and  10 ) require the most spectral separation between them. The present invention thus employs nonuniform channel spacing across the operating bandwidth, with the centermost channels being more closely spaced to one another than the outermost channels. In contrast, known WDM transmission systems employ either a constant channel spacing or a random or quasi-random distribution of channel spacings in an attempt to reduce four-wave mixing. 
     The present invention imparts no more phase modulation to a given channel than is required to reduce nonlinear interactions, allowing more channels to occupy a given bandwidth. A primary advantage of the present invention over known WDM transmission systems is therefore that the capacity of such systems can be increased, possibly by as much as 50% or more.