Abstract:
A dispersion compensator provides dispersion compensation to a WDM optical signal having a plurality of channels located at different wavelengths and traveling in an optical transmission path. The dispersion compensator includes an optical splitter adapted to receive the WDM optical signal. The optical splitter has first and second output ports such that a subset of the plurality of channels are directed along the first output port and remaining ones of the plurality of channels are directed along the second output port. A dispersion compensating element is coupled to the first output port and a multiplexing element having a first input port is coupled to second output port of the optical splitter. The multiplexing element also has a second input port coupled to the dispersion compensating element and an output port on which the subset of channels and the remaining ones of the channels are recombined.

Description:
FIELD OF THE INVENTION 
     The invention relates to the optical transmission of information and, more particularly, to a method and apparatus for compensating for chromatic dispersion that accrues over optical fiber transmission systems. 
     BACKGROUND OF THE INVENTION 
     The availability of high performance optical amplifiers such as the Erbium-Doped Fiber-Amplifier (EDFA) has renewed interest in the use of wavelength division multiplexing (WDM) for optical transmission systems. In a WDM transmission system, two or more optical data carrying channels are combined onto a common path for transmission to a remote receiver. Typically, in a long-haul optical fiber system, the set of wavelength channels would be amplified simultaneously in an optical amplifier based repeater. The Erbium-Doped Fiber-Amplifier is particularly useful for this purpose because of its ability to amplify multiple wavelength channels without crosstalk penalty. 
     Typically, it is advantageous to operate long-haul transmission systems at high data rates per channel. For example, useful data rates include multiples of the Synchronous Digital Hierarchy (SDH) standard, i.e., 2.5 and 10 Gb/s. As the bit rates increase through the gigabit per second range, the optical powers launched into the transmission fiber need to approach 1 mW per channel. As was demonstrated by Bergano et al. (European Conference on Optical Communications, Brussels, Belgium, paper Th.A.3.1, September 1995) the Non-Return-to-Zero (NRZ) transmission format is particularly useful for transmitting large amounts of data over optically amplified fiber paths. However, NRZ channels operating over long distances require sufficient control over the total amount of chromatic dispersion to ensure low dispersion penalties. Accordingly, the preferred transmission medium for such a system is dispersion shifted optical fibers. 
     Crosstalk, or the mixing of channels through the slight nonlinearity in the transmission fiber, may arise from the combination of long distance, low dispersion and high channel power. The transmission of many WDM channels over transoceanic distances may be limited by nonlinear interactions between channels, which in turn is affected by the amount of dispersion. This subject was reviewed by Tkach et al. (Journal of Lightwave Technology in Vol. 13, No. 5, May 1995 pp. 841-849). As discussed in Tkach et al., this problem may be overcome by a technique known as dispersion mapping, in which the generation of mixing products is reduced by offsetting the zero dispersion wavelength of the transmission fiber from the operating wavelengths of the transmitter. This technique employs a series of amplifier sections having dispersion shifted fiber spans with either positive or negative dispersion. The dispersion accumulates over multiple fiber spans of approximately 500 to 1000 km. The fiber spans of either positive or negative sign are followed by a dispersion-compensating fiber having dispersion of the opposite sign. This subsequent section of fiber is sufficient to reduce the average dispersion (averaged over the total length of the transmission system) substantially to zero. That is, a fiber of high negative (positive) dispersion permits compensation by a length of positive (negative) transmission fiber. 
     The efficacy of the dispersion mapping technique is limited because the amount of dispersion that occurs in a typical optical fiber depends on the operating wavelength that is employed. That is, only one given wavelength can operate at average zero dispersion. The wavelength dependence of the dispersion coefficient is sometimes referred to as the dispersion slope of the fiber. Accordingly, because of this characteristic of the dispersion slope, the various channels employed in a WDM system cannot all operate at the wavelength of average zero dispersion. This limitation can be overcome to a limited degree by using individual channel dispersion compensation at the receiver. However, since these systems are subject to nonlinear penalty, the ability to correct for the non-zero dispersion at the receiver terminal is limited. 
     One method and apparatus for managing dispersion in a WDM optical transmission system is shown in U.S. application Ser. No. 08/759,493. In this reference the usable optical bandwidth of the transmission system is divided into sub-bands that individually undergo dispersion compensation before being recombined. Accordingly, in comparison to other dispersion mapping techniques, more WDM data channels reside near a wavelength corresponding to the average zero dispersion wavelength. Unfortunately, this arrangement makes it difficult to upgrade the transmission system by adding more channels since the splitting/recombining elements that produce the sub-bands are designed for particular wavelengths and thus the channel wavelengths can only be changed by replacing the splitting/recombining elements. 
     Accordingly, there is a need for a dispersion compensator for an optical transmission system that allows the system to be upgraded by adding more channels. 
     SUMMARY OF THE INVENTION 
     In accordance with the method of the present invention, dispersion compensation is provided to a WDM optical signal having a plurality of channels located at different wavelengths and traveling in an optical transmission path. The method begins by selecting a subset of the plurality of channels that fall below a prescribed performance threshold when no dispersion compensation is performed. Next, dispersion compensation is provided to each of the subset of channels without compensating for dispersion in the remaining channels. 
     Since the present invention only provides dispersion compensation to a subset of channels, system upgrading can be conveniently performed in those wavebands corresponding to the remaining channels that do not undergo dispersion compensation. 
     In accordance with one aspect of the invention, the prescribed performance threshold is a Q-value for the optical transmission system. Since in some transmission systems the Q-value is relatively high over a wide bandwidth that encompasses all but the outermost channels, the selected subset of channels may be limited to the outermost channels. In such a case system upgrading can be performed over the wide bandwidth region that encompasses all but the outermost channels. 
     In accordance with another aspect of the invention, a dispersion compensator provides dispersion compensation to a WDM optical signal having a plurality of channels located at different wavelengths and traveling in an optical transmission path. The dispersion compensator includes an optical splitter adapted to receive the WDM optical signal. The optical splitter has first and second output ports such that a subset of the plurality of channels are directed along the first output port and remaining ones of the plurality of channels are directed along the second output port. A dispersion compensating element is coupled to the first output port and a multiplexing element having a first input port is coupled to second output port of the optical splitter. The multiplexing element also has a second input port coupled to the dispersion compensating element and an output port on which the subset of channels and the remaining ones of the channels are recombined. 
     In accordance with another aspect of the invention, the optical splitter includes an optical circulator. The optical splitter may also include at least one fiber Bragg grating coupled to a port of the circulator and reflecting thereto the subset of the plurality of channels that are to undergo dispersion compensation. 
     In accordance with yet another aspect of the invention, the multiplexing element is an optical circulator. 
     In accordance with another aspect of the invention, the optical splitter includes a wavelength routing device. 
     In accordance with another aspect of the invention, the dispersion compensator may be a single-mode optical fiber or a fiber Bragg grating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a simplified block diagram of an optical fiber transmission system in which the present invention may be employed. 
     FIG. 2 shows a graph of the Q-value as a function of wavelength for an optical transmission system in which λ 0  is the zero dispersion wavelength. 
     FIG. 3 shows one embodiment of the dispersion compensator constructed in accordance with the present invention. 
     FIG. 4 shows an arbitrary Q-value as a function of wavelength for an optical transmission system. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a simplified block diagram of an exemplary optical fiber transmission system in accordance with the present invention. The system includes an optical transmission path  100 , a transmitting terminal  101 , and a receiving terminal  102 . The transmitting terminal  101  provides an optical data signal that is to be transmitted to the remote receiving terminal via the optical fiber transmission path  100 . The optical signal presented by the terminal  101  to the transmission path  100  may comprise a plurality of WDM optical carriers each carrying an SDH signal. FIG. 1 shows a single period of the dispersion map consisting of optical amplifiers  103 , spans of transmission fiber  104 , and dispersions compensator  105 . In a typical long-haul system, this series of components constituting the dispersion map period might be repeated a number of times over the length of the system. The optical amplifiers  103  may be EDFAs, for example, which amplify optical signals in the 1550 nm wavelength band. In one embodiment of the invention the transmission fibers  104  may be dispersion-shifted single-mode fibers with an average zero dispersion wavelength higher than the operating wavelengths of the system. For example, the transmission fibers  104  may be similar to those used in Bergano et al. (European Conference on Optical Communications, Brussels, Belgium, paper Th.A.3.1, September 1995), in which the transmission fiber had an average zero dispersion wavelength of 1580 nm and a dispersion slope of about 0.073 ps/km-nm 2 . 
     A simple linearized chromatic dispersion relationship between the signal wavelength λ sig  and the dispersion D is given in equation 1: 
     
       
           D=SL (λ sig −λ 0 )  (1) 
       
     
     where the dispersion D is measured in units of ps/nm, the dispersion slope S is measured in units of ps/km-nm 2 , and the average zero dispersion wavelength λ 0  of the transmission fiber is measured in units of nm. As equation 1 clearly indicates, the point of minimum dispersion only occurs at one particular wavelength λ 0 . Accordingly, as disclosed in. U.S. Pat. No. 5,559,920, if a set of WDM channels were transmitted along the transmission path  100 , a dispersion compensating fiber could only translate one channel back to the zero dispersion wavelength. The remaining channels would accumulate dispersion. As previously noted, this problem can be alleviated by dividing the system bandwidth into sub-bands that individually undergo dispersion compensation before being re-combined. The sub-bands may constitute individual channels or a group of channels. As a result, more WDM data channels reside near a wavelength corresponding to the average zero dispersion wavelength. One problem with this technique is that it is difficult to upgrade the transmission system by adding more channels since the splitting/recombining elements that produce the sub-bands are designed for particular wavelengths and thus the channel wavelengths can only be changed by replacing the splitting/recombining elements. 
     The present inventor has recognized that in many circumstances it is not necessary to provide dispersion compensation to each and every sub-band or channel. This conclusion can be understood with reference to FIG. 2, which shows the Q-value (or equivalently, the bit error rate) as a function of wavelength or channel number over the system bandwidth when complete dispersion compensation is only provided at the single wavelength λ 0 . As is well known to those of ordinary skill in the art, the Q-value is a measure of transmission performance that is equivalent to a signal-to-noise measured at the electrical decision threshold point in the receiving terminal. Since complete dispersion compensation is only achieved at λ 0  , the Q-value falls off for those wavelengths increasingly remote from λ 0  as a result of the signal penalty arising from accumulated dispersion. Only when the Q-value falls below some threshold value (e.g., Q 1  in FIG. 2) is the system impairment sufficiently great to warrant the provision of dispersion compensation. For example, if in FIG. 2 channel λ 8  is the zero dispersion wavelength and the Q-value of channels λ 3 -λ 14  remain above the desired threshold Q 1 , then in accordance with the present invention, it only becomes necessary to provide dispersion compensation to wavelengths λ 1 -λ 2  and λ 15 -λ 16 . Since dispersion compensation need not be provided to the waveband encompassed by channels λ 3 -λ 14 , additional channels can be inserted in this waveband when an upgrade to the system is required. 
     FIG. 3 shows one embodiment of a chromatic dispersion compensator  300  constructed in accordance with the present invention, which compensates for the dispersion of channels λ 1 -λ 2  and λ 15 -λ 16  in a WDM optical signal. Dispersion compensator  300  includes a first dispersion compensating stage  302 , which compensates for dispersion in channels λ 1  and λ 2 , and dispersion compensating stage  304 , which compensates for dispersion in channels λ 15  and λ 16 . Dispersion compensating stage  302  includes first and second optical circulators  305  and  307 . The circulator  305  has ports  4 ,  5 , and  6  in which port  5  is located downstream from port  4  and port  6  is located downstream from port  5 . That is, optical energy is transported through the circulator  305  in a clockwise direction. Similarly, circulator  307  has ports  7 ,  8 , and  9  in which port  8  is located downstream from port  7  and port  9  is located downstream from port  8 . That is, optical energy is transported through the circulator  307  in a counterclockwise direction 
     In operation, the WDM channels λ 1 -λ 16  enter port  4  of circulator  305 . The WDM channels λ 1 -λ 16  exit the circulator  305  on port  5  of circulator  305  and enter first and second fiber reflection gratings  306  and  308  that are configured to reflect channels λ 1  and λ 2 , respectively, while transmitting the remaining channels. Channels λ 1  and λ 2  reenter port  5  of circulator  305  after being reflected by fiber Bragg gratings  306  and  308  and subsequently exit port  6  of circulator  305 . Channels λ 1  and λ 2  then enter a dispersion compensating element  314 , which may be, for example, a dispersion compensating fiber or a dispersion compensating grating. Channels λ 1  and λ 2  are then directed to port  8  of circulator  307  while the remaining wavelengths that are transmitted through gratings  306 ,  308 ,  314 , and  316  e.g., λ 3 -λ 16 , are directed to port  7  of circulator  307 . Channels λ 1 -λ 2  are directed to port  7  and are reflected from fiber Bragg gratings  314  and  316 , which, like fiber Bragg gratings  306  and  308 , are configured to reflect channels λ 1 -λ 2 . Finally, channels λ 1 -λ 2  and λ 3 -λ 16  are recombined in circulator  307  so the complete WDM signal exits on port  9  of circulator  307 . 
     Dispersion compensating stage  304  operates similar to dispersion compensating stage  302 . That is, WDM channels λ 1 -λ 16  enter port  10  of circulator  309 . The WDM channels λ 1 -λ 16  exit the circulator  309  on port  11  of circulator  309  and enter first and second fiber reflection gratings  310  and  312  that are configured to reflect channels λ 15  and λ 16 , respectively, while transmitting the remaining channels. Channels λ 15  and λ 16  reenter port  11  of circulator  309  after being reflected by fiber Bragg gratings  310  and  312  and subsequently exit port  12  of circulator  309 . Channels λ 15  and λ 16  then enter dispersion compensating element  316 , which may be, for example, a dispersion compensating fiber or a dispersion compensating grating. Channels λ 15  and λ 16  are then directed to port  16  of circulator  311  while the remaining wavelengths that are transmitted through gratings  310 ,  312 ,  318 , and  320  e.g., λ 1 -λ 14 , are directed to port  14  of circulator  311 . Channels λ 15 -λ 16  are directed to port  16  and are reflected from fiber Bragg gratings  318  and  320 , which, like fiber Bragg gratings  310  and  312 , are configured to reflect channels λ 15 -λ 16 . Finally, channels λ 1 -λ 14  and λ 15 -λ 16  are recombined in circulator  311  so that the complete WDM signal exits on port  15  of circulator  311 . Dispersion compensator  300  has thus overall provided dispersion compensation to channels λ 1 , λ 2 , λ 15 , and λ 16  of the WDM signal. 
     It should be noted that while the embodiment of the invention shown in FIG. 3 provides dispersion compensation to channels λ 1 -λ 2  and λ 15 -λ 16 , the invention more generally may provide dispersion compensation to any subset of the channels that form the WDM optical signal. For example, dispersion compensation may be provided to channels other than the edge channels λ 1 -λ 2  and λ 15 -λ 16 . FIG. 4 shows an arbitrary example of the Q-value as function of wavelength in which, due to the dispersion slope of the transmission path, Q falls below its desired value at channels λ 1 , λ 6 , and λ 16 . Accordingly, in this example, the inventive dispersion compensator would provide dispersion compensation to channels λ 1 , λ 6 , and λ 16 . In such a case, the dispersion compensator may have up to three stages so that a different amount of dispersion compensation can be imparted to each channel. Of course, if the amount of dispersion compensation required for any two or more channels is about the same, these channels can undergo dispersion compensation in the same stage. That is, the maximum number stages that is required will be less than or equal to the number of channels undergoing dispersion compensation. 
     The embodiment of the invention shown in FIG. 3 employs an unchirped fiber Bragg grating for each channel that is to undergo dispersion compensation. More generally, however, the fiber Bragg gratings that are employed may be linearly chirped gratings that reflect the channel or channels that are to undergo dispersion compensation. Accordingly, the number of distinct fiber Bragg gratings that are employed may be less than the total number of channels being reflected. 
     In some embodiments of the invention, one or more the dispersion compensating stages may employ an optical amplifier located downstream from the dispersion compensating element. For example, in FIG. 3, dispersion compensating stage  304  includes optical amplifier  322 . The optical amplifier is employed to overcome any excessive loss that the channels may experience in the dispersion compensating element. 
     In some optical transmission systems the channel spacing may be very small. That is, the system may have a very high channel density. In such systems it may be difficult to separate out the individual channels that are to undergo dispersion compensation. For example, a 64 channel system having a 50 GHz channel spacing (corresponding to about 0.4 nm in the 1550 spectral region) cannot employ readily available components to separate out individual wavelengths. In these cases the dispersion compensator may be preceded with an interleaver/deinterleaver so that the channel spacing can be increased. For example, the previously mentioned 64 channels could be converted by a deinterleaver into two 32 channels signals in which the channel spacing in each signal is 100 GHz per channel instead of the original 50 GHz per channel. Each signal then undergoes dispersion compensation with its own dispersion compensator of the type shown in FIG.  3 . Once the dispersion compensation process is completed, the two signals are directed to an interleaver in which they are remultiplexed to produce the original 64 channel signal with a 50 GHz channel spacing. 
     In the previously described embodiments of the invention it was assumed that the transmissions fiber had a negative dispersion and that the dispersion compensator had a positive dispersion. Of course, those skilled in the art will recognize that the invention alternatively could operate in connection with a transmission fiber having a positive dispersion and a dispersion compensator that has negative dispersion. 
     Although various embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention. For example, while the embodiment of the invention shown in FIG. 3 employs circulators that serve as add-drop multiplexers that split and recombine the signal, one of ordinary skill in the art will recognize that many other elements such as wavelength routing devices may be alternatively employed.