Abstract:
Improved wavelength division multiplexed optical links and signal processing methods that employ cross tap equalization to reduce crosstalk arising from inadequate optical filtering. After optical filtering and photodetection, the wavelength division multiplexed signals adjacent either side of a signal whose crosstalk is to be reduced are sampled, weighted and are then subtracted from the signal whose crosstalk is to be reduced. This is done using adaptive correlation, performed simultaneously on all signals, each with respect to its respective adjacent channels. The present invention enables more closely spaced optical carriers to be used in the wavelength division multiplexed optical link.

Description:
BACKGROUND 
     The present invention relates generally to wavelength division multiplexed optical links, and more particularly, to wavelength division multiplexed optical links and signal processing methods that are limited by crosstalk arising from transmission non-idealities and from inadequate optical filtering at the receiver optical demultiplexer. 
     Previous techniques used to reduce the wavelength spacing of wavelength division multiplexed optical links have involved improving optical filtering schemes to separate more narrowly spaced wavelength division multiplexed channels. However, this has proved difficult for channel spacing below 50 GHz. 
     Accordingly, it is an objective of the present invention to provide for wavelength division multiplexed optical links and signal processing methods that use cross tap equalization to reduce crosstalk arising from inadequate optical filtering and thus enable narrower optical channel spacing. 
     SUMMARY OF THE INVENTION 
     To accomplish the above and other objectives, the present invention provides for improved wavelength division multiplexed optical links and signal processing methods that employ cross tap equalization to reduce crosstalk arising from inadequate optical filtering. In practicing the present invention, after optical filtering and photodetection, the wavelength division multiplexed signals adjacent to either side of a signal whose crosstalk is to be reduced are sampled, weighted and are then subtracted from the signal whose crosstalk is to be reduced. This is done using adaptive correlation, and is performed simultaneously on all signals, each with respect to its respective adjacent channels. Thus, crosstalk is removed electrically after photodetection. The present invention permits use of more closely spaced optical carriers to be used in the wavelength division multiplexed optical link. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural element, and in which: 
     FIG. 1 illustrates an exemplary wavelength division multiplexed optical link in accordance with the principles of the present invention; and 
     FIG. 2 illustrates an exemplary signal processing method in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing figures, FIG. 1 illustrates an exemplary wavelength division multiplexed optical link  10  in accordance with the principles of the present invention. The exemplary wavelength division multiplexed optical link  10  shown in FIG. 1 is designed for use with on-off keyed (baseband) signals. Higher order modulation requires a different arrangement, although the principles of the present invention are used in the same manner therein. 
     The exemplary wavelength division multiplexed optical link  10  comprises a wavelength division multiplexed (WDM) optical filter  11  that receives a wavelength division multiplexed input signal containing optical signals at a plurality of different optical frequencies, and which already may have crosstalk between adjacent optical signals. The WDM optical filter  11  separates out each of the individual wavelength division multiplexed signals, identified as A-E. This filtering may also be insufficiently sharp and thus introduce more crosstalk. Each of the outputs of the WDM optical filter  11  is coupled to an optical-to-electrical converter  12 , such as a photodetector  12 , for example. The optical-to-electrical converters  12  converts the respective optical signals into corresponding electrical signals. 
     Each of the electrical signals output by the optical-electrical converters  12  is simultaneously adaptively correlated with signals from respective adjacent channels. For simplicity, and by way of illustration, FIG. 1 shows that channel “C” is the signal whose crosstalk is to be reduced, and that this signal is adaptively correlated with signals from its two adjacent channels “B” and “D” using an adaptive correlator  20 . 
     To accomplish this, the two adjacent channels (channels “B” and “D”, for example) are thresholded using digital decision devices  13 . The digital decision device  13  is essentially an A/D converter that makes threshold decision, in this case a high or low decision as to whether the applied signal is above or below the threshold, and outputs a corresponding high or low signal. This thresholding yields a good approximation of B and D, marked ˜B and ˜D in FIG.  1 . The thresholded signals of the two adjacent channels are weighted by respective weighting factors (−a 3 , −a 4 ) using two multipliers  14 . Computation of the weighting factors is performed in two correlators and is discussed below. 
     The signal whose crosstalk is to be reduced (channel “C”) is delayed in a delay element  15  and input to a summing device  16 . The length of this delay line is such that the delay from the optical to electrical converters  12  to the summing device  16  is the same for B, C and D. The summing device  16  subtracts the weighted signals derived from the two adjacent channels from the signal whose crosstalk is to be reduced (channel “C”). The summing device  16  generates an output signal having reduced crosstalk. This processing happens for each of the signals output by the WDM optical filter  11 . The adaptive correlation used in the present invention permits use of more closely spaced optical carriers to be used in the wavelength division multiplexed optical link. 
     In essence, and as is illustrated by the respective equations adjacent the outputs of the optical-to-electrical converters  12 , because of crosstalk, the channel “C” has a small amount of crosstalk from channels “B” and “D”. This is illustrated by the factors a 3  and a 4  that modify the signals from the “B” and “D” channels. Similarly, the crosstalk associated with the “B” and “D” channels are shown. 
     Weighting factors (−a 3 , −a 4 ) for the signals of the two adjacent channels are determined by adaptively correlating their signals with the output signal from the channel whose crosstalk is to be reduced. This is done in an adaptive portion of the adaptive correlator  20 , shown at the lower part of FIG.  1 . The signal output from the summing device  16  is thresholded in a digital decision device  21 . The signal output from the summing device  16  is subtracted from the thresholded signal output by the digital decision device  21  in a second summing device  22  to produce an error signal. 
     The error signal output from the second summing device  22  is input to two correlators  26  comprising a multiplier  24  and an integrator  25 . The multipliers  24  respectively multiply the output signal from the summing device  22  by the approximations of the desired outputs of the two adjacent channels. The approximations of the adjacent channels are delayed  23  such that the delay from the optical to electrical converters  12  to the multipliers  24  is the same for channels B, C and D. Outputs of the respective multipliers  24  are each integrated in the integrator  25  to produce the respective weighting factors (−a 3 , −a 4 ). 
     Referring now to FIG. 2, it illustrates an exemplary signal processing method  30  in accordance with the principles of the present invention for reducing crosstalk between channels of a wavelength division multiplexed optical link  10 . The exemplary signal processing method  30  comprises the following steps. 
     A wavelength division multiplexed signal is filtered  31  using a wavelength division multiplexed optical filter to produce a plurality of filtered output signals (A-E). Each respective filtered output signal is converted  32  to an electrical signal. Each of the electrical signals are simultaneously adaptively correlated  33  with respective adjacent signals. 
     The adaptive correlation  33  of the signals is accomplished by thresholding  34  the signals adjacent to the signal whose crosstalk is to be reduced, weighting  35  the thresholded signals, and subtracting  36  the weighted signals derived from the two adjacent channels from the signal whose crosstalk is to be reduced. This produces an output signal that has reduced crosstalk. 
     The weighting factors for the signals of the two adjacent channels are determined by adaptively correlating  40  the adjacent channel signals with the output signal from the channel whose crosstalk is to be reduced. This is achieved by thresholding  41  the reduced crosstalk signal, subtracting  42  the reduced crosstalk signal from the thresholded reduced crosstalk signal to produce an error signal. The error signal is correlated  43  with the signals of the two adjacent channels using multiplication  44  and integration  45  to produce the respective weighting factors. 
     Thus, a wavelength division multiplexed optical link and signal processing method that uses cross tap equalization to allow more closely spaced optical carriers to be used has been disclosed. It is to be understood that the above-described embodiment is merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.