Abstract:
A method of optically equalizing a multi-level (amplitude or phase) optical signal through the effect of an optical equalizer wherein the optical equalizer (OEQ) is placed at either a transmission end or a receiver end of the optical communications link and a tap delay characteristic of the OEQ need not be determined by symbol spacing, rather it may advantageously be adjusted to desirably compensate non-linear mapping performed in the modulation process or simultaneous operation on a plurality of wavelength division multiplexed (WDM) channels.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/919,696 filed on Mar. 23, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to the field of optical communications and in particular to an optical equalizer for multi-level signal formats which may be positioned at a receiving end of an optical transmission system. 
       BACKGROUND OF THE INVENTION 
       [0003]    Non-binary optical symbol constellations for example, differential quadrature phase shift keying (DQPSK) formats are generated by mapping a multitude of binary electric data streams onto a single optical wavelength through the effect of an optical modulator. Unfortunately, such modulators are oftentimes bandwidth limited. 
       SUMMARY OF THE INVENTION 
       [0004]    An advance is made in the art according to the principles of the present invention whereby transmitter-induced, optical modulator bandwidth limitations are mitigated by optically equalizing a multi-level (amplitude or phase) optical signal through the effect of an optical equalizer. 
         [0005]    According to an aspect of the invention—and in sharp contrast to the teachings of the prior art and in particular binary on/off keying systems wherein an optical equalizer (OEQ) should be placed at a transmitter end of an optical communications link—optical equalization according to the present invention may be advantageously placed at either a transmission end or a receiver end of the optical communications link. 
         [0006]    According to another aspect of the invention, a tap delay characteristic of the OEQ need not be determined by symbol spacing, rather it may advantageously be adjusted to desirably compensate non-linear mapping performed in the modulation process or simultaneous operation on a plurality of wavelength division multiplexed (WDM) channels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0007]    A more complete understanding of the present invention may be realized by reference to the accompanying drawings in which: 
           [0008]      FIG. 1  is a schematic of a multi-level Mach-Zehnder modulator along with an optical equalizer according to the present invention; 
           [0009]      FIG. 2  is a schematic of an optical equalizer constructed on a single optical chip according to the present invention; and 
           [0010]      FIG. 3  is a schematic of an optical transmission system including a multi-level Mach-Zehnder modulator along with an optical equalizer according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    The following merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. 
         [0012]    Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. 
         [0013]    Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
         [0014]    Thus, for example, it will be appreciated by those skilled in the art that the diagrams herein represent conceptual views of illustrative structures embodying the principles of the invention. 
         [0015]    With initial reference to  FIG. 1 , those skilled in the art will quickly recognize the well-known multi-level (nested), two-armed, Mach-Zehnder modulator  110  which receives an input optical signal from input waveguide  120  and splits that signal through the effect of splitter/coupler  118  such that the split portions are directed into the two arms of the Mach-Zehnder modulator. As can be appreciated by those skilled in the art, each of the two arms includes a separate Mach-Zehnder structure  112 ,  114  thereby producing the nested modulator structure. Shown in this  FIG. 1 , each of the nested Mach-Zehnder structures themselves include one or more phase shifters  111 ,  113 ,  115 ,  117  positioned within their arms. Advantageously, this modulator may produce multi-level signals such as quadrature or phase-shift keyed signals. 
         [0016]    Modulated light produced by the nested Mach-Zehnder modulators  112 ,  114  is recombined through the effect of coupler  119  and subsequently output via output waveguide  130  where it is received by optical equalizer  140  and subsequently output. As noted earlier, prior art teachings dictated that the OEQ needed to be placed on a transmitter side of a transmission link—before the addition of optical noise. 
         [0017]    Turning now to  FIG. 2 , there is shown a schematic of an optical equalizer chip  200  constructed according to the teachings of the present invention. More particularly, optical equalizer chip  200  which may be advantageously constructed from well-understood Si:SiO 2  processes includes an input fiber  210 , an output fiber  240  and a two-tap optical equalizer  220 . As shown in this  FIG. 2 , the two-tap optical equalizer  220  includes a pair of cascaded Mach-Zehnder structures each having a pair of adjustable couplers  221 ,  222 ,  231 ,  232  and a pair of unequal length arms  223 ,  224 ,  233 ,  234  respectively which results in an adjustable phase within each of the two taps. Wire bond pads (not shown) permit the application of DC control voltages to the adjustable couplers which generally permits the control of the magnitude of impulses entering and exiting each of the two taps. 
         [0018]    In a preferred embodiment, the differential delay exhibited between the two taps is substantially 0.75 T, where T is symbol period of an input signal applied to the input fiber  210  of the equalizer  200 . Accordingly, for a 100G system, the differential delay for a system employing the equalizer shown in  FIG. 2  would be represented by 
         [0000]    
       
         
           
             
               
                 1 
                 
                   107 
                    
                   
                       
                   
                    
                   
                     Gb 
                     / 
                     s 
                   
                 
               
                
               0.75 
             
             = 
             
               7 
                
               
                   
               
                
               
                 ps 
                 . 
               
             
           
         
       
     
         [0000]    Notably, and according to the present invention, the equalizer tap delay as measured in time is closer to the bit period of the optical signal than its symbol period. Lastly, it is noted that while the example shown and described has involved a two tap equalizer, those skilled in the art will quickly recognize that optical equalizers having more than two taps may be used as well according to the present invention. 
         [0019]    Advantageously, and according to a further aspect of the present invention, the optical equalization may be performed on multiple channels simultaneously. For example, consider the equalizer shown in  FIG. 2 , wherein a multi-wavelength, wavelength division multiplexed (WDM) signal is applied to the input fiber  210 . If each of the channels present in the WDM signal applied exhibited substantially the same impairment then one optical equalizer such as that shown in  FIG. 2  would compensate all of the WDM channels simultaneously if the tap time spacing was substantially equal to N/(WDM Channel Spacing). 
         [0020]    With reference now to  FIG. 3 , there it shows an experimental setup for an optical transmission system employing optical equalization according to the present invention  300 . Shown therein are ten (10) distributed feedback (DFB) lasers operating at the ITU frequency grid from 192.2 to 193.1 THz (1552 to 1560 nm) the outputs of which are combined using an arrayed waveguide grating (AWG) multiplexer  315 . 
         [0021]    All of the channels are simultaneously modulated using a double-nested LiNbO 3  Mach-Zehnder modulator (MZM). For the purposes of demonstration, both in-phase (I) and quadrature (Q) signals were generated by multiplexing four copies of a pseudo random bit sequence, generating a QPSK signal. 
         [0022]    After pre-compensation, the signal was launched into a transmission span and post-compensated  330  and subsequently equalized by optical equalizer according to the present invention. As indicated by its placement in this  FIG. 3 , the optical equalizer  340  is positioned at the receiving end of the transmission span. Accordingly, it affects the optical signal after additional optical noise is added to the transmitted optical signal. We have shown that QPSK signals may be effectively equalized after the addition of optical noise—in sharp contrast to the prior art teachings. Those skilled in the art will recognize that equalizing at a receiving end of a transmission link is advantageous because it facilitates feedback control from the measured received signal performance—among others. 
         [0023]    At this point, while we have discussed and described our invention using some specific examples, those skilled in the art will recognize that our teachings are not so limited. In particular, while we have shown the optical equalization functions Accordingly, our invention should be only limited by the scope of the claims attached hereto.