Abstract:
A method of transmitting digital signals over an optical transmission system is described. The method comprises the following steps: generating a signal according to the non-return-to-zero (NRZ) format, forwarding the NRZ signal to an optical delay filter, and using an output signal of the optical delay filter as a signal according to the modified duobinary return-to-zero (MD-RZ) format.

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to a method of transmitting digital signals over an optical transmission system. The invention is based on a priority application EP 04 290 971.3 which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     In the publication of P. J. Winzer and S. Chandrasekhar, Return-to-zero modulation with electrically continuously tunable duty cycle using single NRZ modulator, Electronics Letters, Vol. 33, No. 11, 29 May 2003, pages 859-860, an electrical input signal is encoded and then split. The resulting two paths are electrically delayed with respect to each other and are then applied to a dual-drive Mach Zehnder modulator. The output signal of the Mach Zehnder modulator represents a signal according to a so-called chirped modified duobinary return-to-zero format, i.e. a chirped MD-RZ signal.  
         [0003]     The afore-described transmission system requires a dual-drive Mach Zehnder modulator and an electrical delay circuit.  
       SUMMARY OF THE INVENTION  
       [0004]     It is an object of the present invention to provide a method of transmitting digital signals over an optical transmission system using a MD-RZ signal but requiring less components compared to the prior art.  
         [0005]     The invention solves this object by a method of transmitting digital signals over an optical transmission system comprising the following steps: generating a signal according to the non-return-to-zero format, forwarding the NRZ signal to an optical delay filter, and using an output signal of the optical delay filter as a signal according to the modified duobinary return-to-zero format, by an optical transmitter for an optical transmission system for transmitting digital signals comprising: means for generating a signal according to the non-return-to-zero format, an optical delay filter for receiving the NRZ signal and for generating an output signal according to the modified duobinary return-to-zero format and by an optical transmission system for transmitting digital signals comprising: means for generating a signal according to the non-return-to-zero format, an optical delay filter for receiving the NRZ signal and for generating an output signal according to the modified duobinary return-to-zero format.  
         [0006]     The invention first generates a signal according to the non-return-to-zero (NRZ) format. Then, this NRZ signal is forwarded to an optical delay filter. And then, the invention uses an output signal of the optical delay filter as a signal according to the modified duobinary return-to-zero (MD-RZ) format.  
         [0007]     Compared to the prior art, the invention generates the MD-RZ signal without requiring a dual-drive optical modulator or an electrical delay circuit. As a result, the invention provides the advantage that the MD-RZ signal is generated with less components.  
         [0008]     In an advantageous embodiment of the invention, the optical modulator is influenced by the input signal at only one input port, and the output signal of the optical modulator is used as a chirped NRZ signal. In another advantageous embodiment of the invention, a dual-driven optical modulator is used, the optical modulator is influenced by the input signal at two input ports, and the output signal of the optical modulator is used as a chirp-free NRZ signal. Thus, the MD-RZ signal is available with a phase being not constant or being constant.  
         [0009]     In an advantageous embodiment of the invention, the optical delay filter is a Mach Zehnder filter wherein the MD-RZ signal is provided at a destructive port of the Mach Zehnder filter. Thus, a very effective solution is provided for generating the MD-RZ signal.  
         [0010]     In another advantageous embodiment of the invention, the optical delay filter has a time delay xT, with x being a value between 0 and 0,5 and T being the time duration of a time cell of a single binary digit. In particular, in connection with a bit rate of 43,06 GBit/sec, the optical delay filter has a time delay of 0,43T. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Further features, applications and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention which are shown in the drawings. There, all described and shown features themselves or in any combination represent the subject matter of the invention, independently of their wording in the description or their representation in the drawings and independently of their combination in the claims or the dependencies of the claims.  
         [0012]      FIG. 1  shows a schematic block diagram of a first embodiment of a method of transmitting digital signals over an optical transmission system according to the invention,  
         [0013]      FIG. 2  shows a schematic block diagram of a second embodiment of a method of transmitting digital signals over an optical transmission system according to the invention, and  
         [0014]      FIG. 3  shows a schematic block diagram of a third embodiment of a method of transmitting digital signals over an optical transmission system according to the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0015]     In  FIG. 1 , a block  10  is followed by a block  15 . Both blocks  10 ,  15  may be part of an optical transmitter or of an optical transmission system for transmitting digital signals.  
         [0016]     The block  10  comprises a light emitting source  11  for generating an optical signal, and an optical modulator  12  for receiving and influencing the optical signal. An electrical encoder  13  is provided which receives a binary input signal E with a given bit rate, e.g. 43,06 GBit/sec, and which influences the optical modulator  12  with an electrical output signal E′.  
         [0017]     The optical modulator  12  outputs an optical signal depending on the signal E′ received from the encoder  13 . The encoder  13  comprises an algorithm which influences the optical modulator  12  such that an optical signal is generated according to a so-called chirped non-return-to-zero (NRZ) format, i.e. a NRZ signal with a phase which is not constant. This output signal of the modulator  12  is characterized by the abbreviation Cd-NRZ in  FIG. 1 .  
         [0018]     The block  15  comprises a passive optical delay filter  16 , in particular a Mach Zehnder filter. The optical delay filter  16  receives the output signal of the optical modulator  12 , The two channels of the optical delay filter  16  have a given optical delay time xT, with x being a value between 0 and 0,5 and T being the time duration of a time cell of a single binary digit.  
         [0019]     With e.g. the above-mentioned given bit rate of 43,06 GBit/sec and a grid value of 100 GHz according to the ITU standard (ITU=international telecommunication union), the value x is selected to be 0,43, i.e. the time delay between the two channels of the optical delay filter  16  is 0,43T.  
         [0020]     The optical delay filter  16  provides two output ports, a constructive port CP and a destructive port DP. At the destructive port DP of the optical delay filter  16 , a chirped modified duobinary return-to-zero signal is present. This signal at the destructive port DP is characterized by the abbreviation Cd-MD-RZ in  FIG. 1 .  
         [0021]     In  FIG. 2 , a block  20  is followed by a block  25 . Both blocks  20 ,  25  may be part of an optical transmitter or an optical transmission system for transmitting digital signals.  
         [0022]     The block  20  of  FIG. 2  is similar to the block  10  of  FIG. 1 . The only difference is the replacement of the optical modulator  12  of  FIG. 1  by a dual-driver optical modulator  22  in  FIG. 2 . Both ports of the optical modulator  22  of  FIG. 2  are supplied with the encoded electrical signal E′ generated by the encoder  13 .  
         [0023]     The encoder  13  of  FIG. 2  comprises an algorithm which influences the two ports of the optical modulator  22  such that a signal according to a so-called chirp-free non-return-to-zero (NRZ) format is generated at the output of the optical modulator  22 , i.e. a NRZ signal with a constant phase. This output signal of the modulator  22  is characterized by the abbreviation Cf-NRZ in  FIG. 2 .  
         [0024]     The block  25  of  FIG. 2  is identical to the block  15  of  FIG. 1 . It comprises the passive optical delay filter  16 , in particular a Mach Zehnder filter. The time delay of the optical delay filter  16  is selected to be 0,43T. At the destructive port DP of the optical delay filter  16 , a chirp-free modified duobinary return-to-zero signal is present. This signal at the destructive port DP is characterized by the abbreviation Cf-MD-RZ in  FIG. 2 . The Cd-MD-RZ signal of  FIG. 1  and the Cf-MD-RZ signal of  FIG. 2  are identical with regard to their intensities, but they are different with regard to their phases. The phase of the Cd-MD-RZ signal is chirped, i.e. it is not constant, whereas the phase of the Cf-MD-RZ signal is chirp-free, i.e. it is constant.  
         [0025]     The above described methods of generating a Cd-MD-RZ signal or a Cf-MD-RZ signal have proven to be very effective, in particular they do not require a dual-drive optical modulator or an electrical delay circuit.  
         [0026]     In  FIG. 3 , a Cd-NRZ signal according to  FIG. 1  or a Cf-NRZ signal according to  FIG. 2  is generated several times for different wavelengths λ 1  to λ 2N+1  of the light emitting source  11 . For that purpose, either an equal number of identical blocks  10  or an equal number of identical blocks  20  is provided. With regard to the blocks  10  or the blocks  20 , reference is made to  FIG. 1  or  2  and the respective descriptions. In  FIG. 3 , the possibility of using either the blocks  10  or the blocks  20  is shown by the dashed lines of the encoded signal E′.  
         [0027]     The output signals of the blocks  10  or  20 , i.e. the Cd-MD-RZ signals or the Cf-MD-RZ signals corresponding to the different wavelengths λ 1  to λ 2N+1 , are multiplexed by two multiplexers  31 ,  32  relating to the two polarizations of the optical signals. The output signals of these two multiplexers  31 ,  32  are forwarded to a block  35  which comprises a passive optical delay filter  36 , in particular a Mach Zehnder filter.  
         [0028]     The optical delay filter  36  of  FIG. 3  is similar to the optical delay filter  16  of  FIGS. 1 and 2  with the only difference that the optical delay filter  36  of  FIG. 3  receives not only one, but two input signals, i.e. the two output signals of the two multiplexers  31 ,  32 .  
         [0029]     At the destructive port DP of the optical delay filter  36 , a Cd-MD-RZ signal or a Df-MD-RZ signal is present, depending on whether the blocks  10  or the blocks  20  are present.  
         [0030]     The method of generating the Cd-MD-RZ signal or the Cf-MD-RZ signal for different wavelengths λ 1  to λ 2N+1  of the light emitting source has proven to be very effective, in particular it only requires a single optical delay filter.