Abstract:
An optical communication system having a transmitter in which a pair of optical signals having different frequencies are modulated using a duobinary encoding scheme, and then multiplexed using polarization division multiplexing. Advantageously, the frequency difference between the two signals can be less than the data rate conveyed by each signal, resulting in a narrow spectral bandwidth, while still allowing demultiplexing at a receiver using simple bandpass filters and without the need of any form of polarization tracking. A receiver has a beam splitter for splitting the received optical signal into two portions which are each directed, via respective bandpass filters centred at slightly different frequencies, to respective detectors. Advantageously, the frequency difference between the frequencies at which the bandpass filters are centred can be less than the data rate of a detected signal. The receiver does not require any polarization tracking or balancing, and accordingly is straightforward to implement

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an optical communication system in which an optical beam is modulated in accordance with data in a transmitter, and the modulated optical beam is then transmitted to a remote receiver which recovers the data. The invention has particular, but not exclusive, application to a so-called 40G optical communication network at which data is communicated along a data pipe at a rate of 40 Gigabits per second (Gbps) or more. 
         [0003]    2. The Background Art 
         [0004]    In recent years, the need to increase data rates in optical communication to the benchmark figures of 40 Gbps and 100 Gbps has prompted much research. One problem with increasing data rates is the consequent increase in frequency bandwidth, which is problematic due to increased dispersion in optical fibers and also because an increase in frequency bandwidth requires a greater frequency spacing of data channels in a wavelength division multiplexing (WDM) system. 
         [0005]    The use of optical duobinary modulation, in which a data signal is added to a one-bit delayed version of itself to generate a three level signal, has attracted attention due to its narrow bandwidth in comparison with a binary non-return-to-zero (NRZ) modulated signal. In practice, optical duobinary modulation typically employs a precoder to perform differential encoding in order to prevent error propagation. In order to maintain the bandwidth advantage when using such a precoder, one binary logic level output by the precoder is converted to a low amplitude state of the optical signal while the other binary logic level output by the precoder is converted to high amplitude states of the optical signal having opposite phases. At the receiver, conveniently the low amplitude state is converted to one binary logic value while both the high amplitude states are converted to the other binary logic value to recover the original data signal. 
         [0006]    Other modulation techniques deployed include phase-shift keying (DPSK) and quadrature phase shift keying (QPSK), particularly in differential format. In addition, polarization division multiplexing has been used to further increase the data rates by employing two optical signals at the same frequency but with orthogonal polarizations. Polarization division multiplexing typically requires, however, a complex receiver due to the difficulty in separating the two optical signals at the receiver with acceptable levels of crosstalk. 
       SUMMARY OF THE INVENTION 
       [0007]    One aspect of the present invention provides for a transmitter in which a pair of optical signals having different frequencies are modulated using a duobinary encoding scheme, and then multiplexed using polarization division multiplexing. Advantageously, the frequency difference between the two signals can be less than the data rate conveyed by each signal, resulting in a narrow spectral bandwidth, while still allowing demultiplexing at a receiver using simple bandpass filters and without the need of any form of polarization tracking. 
         [0008]    Another aspect of the invention provides for a receiver having a wavelength-dependent beam splitter arrangement for splitting a received optical signal into two portions which are each directed to respective detectors. A first spectral component at a first frequency is preferentially split into the first portion, and a second spectral component at a second frequency is preferentially split into the second portion. Advantageously, the frequency difference between the first and second frequencies can be less than the data rate of a detected signal. The receiver does not require any polarization tracking or balancing, and accordingly is straightforward to implement. 
         [0009]    A further aspect of the invention provides a Dense Wavelength Division Multiplexing (DWDM) optical communication system in which a plurality of transmitters generate a modulated optical signal by using polarization division modulation to combine two optical signals at slightly different frequencies, modulated in accordance with a duobinary encoding scheme, to generate respective optical data signals. The optical data signals are combined using wavelength division multiplexing, and transmitted over an optical fibre to a demultiplexer which demultiplexes the optical data signals. Each optical data signal is then split into two portions, and each portion is directed via a respective bandpass filter to a respective detector. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a block diagram showing the main components of an optical communication system forming a preferred embodiment of the invention; 
           [0011]      FIG. 2  is a graph showing the variation of electric field for an optical signal output by a Mach-Zehnder modulator with variation of an applied electrical potential; 
           [0012]      FIG. 3  is a graph showing an exemplary waveform input into a low pass filter forming part of a transmitter of the optical communication system of  FIG. 1 ; 
           [0013]      FIG. 4  is a graph showing the waveform output by the low-pass filter in response to the exemplary input waveform illustrated in  FIG. 3 ; 
           [0014]      FIG. 5  is a graph showing an exemplary frequency spectrum of the output of a transmitter of the optical communication system illustrated in  FIG. 1 ; 
           [0015]      FIG. 6  is a graph showing transmissivity against frequency for a bandpass filter in a receiver forming part of the optical communication system illustrated in  FIG. 1 ; 
           [0016]      FIG. 7  is a block diagram showing the main components of a first alternative receiver for the optical communication system illustrated in  FIG. 1 ; 
           [0017]      FIG. 8  is a block diagram showing the main components of a second alternative receiver for the optical communication system illustrated in  FIG. 1 ; and 
           [0018]      FIG. 9  is a block diagram of a DWDM optical communication system including optical communication in accordance with the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    Details of the present invention will now be described, including exemplary aspects and embodiments thereof. Referring to the drawings and the following description, like reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of exemplary embodiments in a highly simplified diagrammatic manner. Moreover, the drawings are not intended to depict every feature of actual embodiments nor the relative dimensions of the depicted elements, and are not drawn to scale. 
         [0020]    As shown in  FIG. 1 , in an optical communication system according to the present invention a transmitter  1  transmits a modulated optical signal through an optical fiber  3  to a receiver  5 . The optical signal is modulated in accordance with first and second data signals which are input to respective precoders  7   a ,  7   b  of the transmitter  1 . In this embodiment, each data signal conveys data serially at a data rate of 22 Gigabits per second (Gbps). The pair of data signals may be formed from a single data signal at 44 Gbps. 
         [0021]    Each of the precoders  7  performs differential encoding. In particular, in each precoder the input data signal is inverted and then input into one input of an exclusive-OR gate, and the output of the exclusive-OR gate for each clock cycle is input into the other input of the exclusive-OR gate for the following clock cycle. The output of the exclusive-OR gate also forms the output of the precoder  7 . 
         [0022]    The output of each precoder  7   a ,  7   b  is input to a respective 2V π  drive circuit  9   a ,  9   b , with each 2V π  drive circuit  9  applying control voltages to a corresponding Mach-Zehnder modulator  13 . As those skilled in the art will appreciate, a Mach-Zehnder modulator splits a received coherent optical signal into two light beams which are directed through respective arms of the Mach-Zehnder modulator and then recombined. A variable optical path difference is introduced into one or both of the light paths in order to vary the amplitude of the recombined optical signal. 
         [0023]    In this embodiment, each 2V π , drive circuit  9  has a pair of V π  drive circuits, with the output of each V π  drive circuit being input, via a respective low-pass filter  11 , to an electrode associated with a respective arm of corresponding Mach-Zehnder modulator (MZM)  13 . One of the V π  drive circuits is driven by the output of the corresponding precoder  7  while the other of the V π  drive circuits is driven by the inverse of the output of the corresponding precoder  7  so that differential driving is performed. Each Mach-Zehnder modulator  13  is biased at a level where the optical path difference between the two paths is 180°, resulting in a null output as the light travelling down one path destructively interferes with the light travelling down the other path. The 2V π  drive circuits  9  are configured such that a potential difference of amplitude V is applied across the electrodes associated with the arms of the MZM  13 , with the polarity of the applied voltage dependent on the binary logic level output by the corresponding precoder  7 . The application of the potential difference V with one polarity results in a maximum amplitude of the recombined optical signal output by the MZM  13  with a first phase while the application of the potential difference V with the other polarity results in a maximum amplitude of the recombined optical signal output by the MZM  13  at a second phase which is 180° out of phase with the first phase. In other words, as illustrated in  FIG. 2 , the electric field strengths E of the recombined optical signal output by the MZM  13  when the potential difference V is applied with opposite polarities are of equal amplitude but opposite sign. 
         [0024]    The low-pass filters  11  are configured such that the output of each low-pass filter  11  substantially corresponds to the average of the voltage levels output by the corresponding 2V π  drive circuit  9  for the last two data bits. Accordingly, if the output of a V π  drive circuit  9  corresponds to a sequence of two different bits, then the voltage output by the low-pass filter is effectively zero, whereas if the two bits are the same then the voltage output by the low pass filter corresponds to the input voltage. This is a conventional way of implementing a duobinary encoding scheme. 
         [0025]    In this embodiment, the low-pass filters  11  are 5 th  order Bessel filters which provide a substantially flat group delay up to 13.4 GHz.  FIGS. 3 and 4  respectively show an exemplary input to a low-pass filter  11  and the corresponding output of the low-pass filter  11 . 
         [0026]    First and second lasers  15   a ,  15   b  output coherent light beams which are input to respective ones of the modulators  13   a ,  13   b . In this embodiment, the first laser  15   a  outputs a coherent optical beam at a first wavelength λ 1  and the second laser  15   b  outputs a coherent light beam at a second wavelength λ 2 , with the frequency difference between the two laser equal to 16 GHz. This frequency difference is therefore less than the data rate of one of the data signals. Further, the outputs of the first and second lasers  15   a ,  15   b  have linear polarizations which are mutually orthogonal to each other. A polarization beam combiner  17  combines the two outputs of the MZMs to form the output signal of the transmitter  1 , and this output signal is coupled into the optical fibre  3 . The different polarization states of the outputs of the MZMs reduces interference between the data of the first and second data signals.  FIG. 5  shows the frequency spectrum of an exemplary output of the transmitter  1 . It will be seen that there are two local maxima, which correspond to the wavelengths of the first and second lasers  9 . 
         [0027]    Table 1 illustrates states of the transmitter  1  for an exemplary data string. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 States of components of the transmitter 
               
               
                 1 for an exemplary data string. 
               
             
          
           
               
                 Clock Cycle 
                 −1 
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
               
               
                   
               
               
                 Data 
                   
                 0 
                 1 
                 0 
                 1 
                 1 
                 1 
               
               
                 Precoder Output 
                 0 
                 1 
                 1 
                 0 
                 0 
                 0 
                 0 
               
               
                 Drive Circuit Output 
                 −V 
                 V 
                 V 
                 −V 
                 −V 
                 −V 
                 −V 
               
               
                 Low-pass Filter Output 
                   
                 0 
                 V 
                 0 
                 −V 
                 −V 
                 −V 
               
               
                 MZM Output 
                   
                 0 
                 E 
                 0 
                 −E 
                 −E 
                 −E 
               
               
                   
               
             
          
         
       
     
         [0028]    In table 1 it will be seem that the output of the MZM  13  corresponds to a duobinary encoded version of the data signal in which the binary logic state “1” is represented by an electric field amplitude E at two phases which are 180° out of phase with each other. Accordingly, a spectral component at wavelength λ 1  is modulated in accordance with the first data signal and a spectral component at wavelength λ 2  is modulated in accordance with the second data signal. At the receiver, a data signal can be recovered simply by detecting the amplitude of the electric field strength at the corresponding wavelength. 
         [0029]    Returning to  FIG. 1 , after passing through the optical fiber  3 , the signal output by the transmitter  1  is input to the receiver  5  where it is split into two equal portions by a beam splitter  19 . In this embodiment, the beam splitter  19  is wavelength insensitive so that the spectral distributions of each of the split portions are the same. One split portion is input to a first bandpass filter  21   a  and the other split portion is input to a second bandpass filter  21   b . The first bandpass filter  21   a  is centred at λ 1  while the second bandpass filter  21   b  is centred at λ 2 . The first and second bandpass filters  21   a , 21   b  both have a 3 dB bandwidth of 16 GHz, so that light transmitted by the first bandpass filter  21   a  generally originates from the first laser  15   a  and light transmitted by the second bandpass filter  21   b  generally originates from the second laser  15   b .  FIG. 6  illustrated how the transmissivity of a bandpass filter  21  varies with frequency. 
         [0030]    The light transmitted by the first bandpass filter  21   a  is detected by a first detector  23   a  to recover the first data signal and the light transmitted by the second bandpass filter  21   b  is detected by a second detector  23   b  to recover the second data signal. 
         [0031]    It will be appreciated that the light output from each bandpass filter  21  could be amplified using an optical amplifier prior to detection. 
         [0032]    In an embodiment, the components of the transmitter  1  are formed in an integrated optical circuit, and similarly the components of the detector  5  are formed in an integrated optical circuit. 
         [0033]    In the receiver  5  discussed above, the beam splitter  19  and the first and second bandpass filters  21   a , 21   b  form a wavelength-dependent beam splitting arrangement. Other forms of wavelength-dependent beam splitting arrangements are possible. For example, as shown in  FIG. 7 , in an alternative embodiment a receiver  5 ′ has a wavelength-dependent beam splitter arrangement in the form of an optical de-interleaves  27  which directs a first optical signal predominantly comprising a first spectral component to a first detector  23   a  and a second optical signal predominantly comprising a second spectral component to a second detector  23   b . More generally, as shown in  FIG. 8 , in an embodiment a receiver  5 ″ has a wavelength-dependent beam splitter arrangement in the form of a wavelength demultiplexer  29 . 
         [0034]    Due to the narrow bandwidths of the transmitted optical signals, transmitters and receivers according to the present invention are well suited to a DWDM optical communication system. In a DWDM, multiple channels at different wavelength are multiplexed into a single fiber communications window, usually the window around 1550 nm to take advantage of the devices available at that wavelength. As shown in  FIG. 9 , a plurality of transmitters as described above each output an optical signal having two components centred at slightly different frequencies, with the frequencies used in one transmitter  1  being spaced from the frequencies used in all the other transmitters  1 . The output signals are input to a wavelength multiplexer  31  which combines the output signals, and the combined output signal is transmitted through the optical fiber  3 . Following transmission through the optical fiber  3 , the transmitted signal is demultiplexed by the wavelength demultiplexer  33  to recover the optical signals having two components at slightly different frequencies, and these optical signals are input into respective receivers  5  as described above. 
         [0035]    In the embodiment illustrated in  FIG. 1 , two lasers  9   a ,  9   b  are used having orthogonal linear polarizations. It will be appreciated that differences in the polarization state could be used, for example orthogonal circular polarizations. Alternatively, two lasers emitting light beams having identical polarizations could be used, with the polarization state of one light beam being altered prior to combining with the other light beam in the polarization beam combiner. It will be further appreciated that a single laser can be used to generate two light beams at slightly different frequencies.