Abstract:
A device for processing a digital signal of an optical transmission system is described. The device comprises: a polarization beam splitter for receiving the signal, a phase shifter for shifting the phase of the signal at the orthogonal output of the polarization beam splitter, and means for combining the signal at the parallel output of the polarization beam splitter and the shifted signal.

Description:
TECHNICAL FIELD 
   The invention relates to a device and a method for processing a digital signal of an optical transmission system. The invention is based on a priority application EP 04 292 104.9 which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   It is known to use a bit-to-bit polarization-interleaved signal in an optical transmission system. It is also known that this format is very sensitive to polarization-dependent losses (PDL) because two adjacent pulses are not subject to the same attenuation during their transmission. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a device and a method for processing e.g. a bit-to-bit polarization-interleaved signal without the disadvantage of polarization-dependent losses. 
   The invention solves this object with a polarization beam splitter for receiving the signal, a phase shifter for shifting the phase of the signal at the orthogonal output of the polarization beam splitter, and means for combining the signal at the parallel output of the polarization beam splitter and the shifted signal. 
   The invention provides a converter for an alternated-polarized signal into a single-polarized signal. The output of the converter therefore provides a single-polarized data stream that may be further processed in order to compensate the polarization-dependent losses. 
   The advantage of the invention is the fact that it is more effective to compensate polarization-dependent losses of a single-polarized data stream. Therefore, the invention decreases the efforts to overcome these polarization-dependent losses e.g. in connection with a bit-to-bit polarization-interleaved signal. 
   In an embodiment of the invention, a delay filter is provided. With this delay filter, the polarization-dependent losses of the single-polarized data stream can be compensated. 
   The invention is particularly advantageous in connection with a digital signal according to the differential phased shift keying (DPSK) format. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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 feature 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. 
       FIG. 1  shows a schematic circuit diagram of a first device for processing a digital signal of an optical transmission system according to the invention, 
       FIG. 2  shows a schematic circuit diagram of the first device of  FIG. 1  and a second device, and 
       FIG. 3  shows a schematic circuit diagram of the second device of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In  FIG. 1 , a first device  10  is provided that comprises a passive optical polarization beam splitter  11  followed by a passive optical phase shifter  12  at its orthogonal output. 
   The phase shifter  12  shifts the phase of the signal at the orthogonal output of the polarization beam splitter  11  by n/2. The signal at the parallel output of the polarization beam splitter  11  remains unchanged. The output of the phase shifter  12  and the parallel output of the polarization beam splitter  11  are then combined. 
   The first device  11  may be incorporated in a receiver of an optical transmission system. 
   A digital signal A in a bit-to-bit polarization-interleaved format is received at the input of the polarization beam splitter  11 . The signal A comprises subsequent pulses wherein the orientation of a pulse indicates the polarization state of this pulse. The subsequent pulses of the signal A, therefore, have alternating polarizations. 
   After passing the polarization beam splitter  11 , a signal B 1  is present at the parallel output of the polarization beam splitter  11 , and a signal B 2  is present at the orthogonal output of the polarization beam splitter  11 . Again, the orientation of a pulse of the signals B 1 , B 2  indicates the polarization state of this pulse. 
   All pulses of the signal B 1  have the same polarization. The same is valid for the pulses of the signal B 2 . However, the polarization of the pulses of the signal B 1  is different from the polarization of the pulses of the signal B 2 . 
   After passing the phase shifter  12 , the signal B 2  has changed into the signal B 2 ′. All pulses of the signal B 2 ′ still have the same polarization. However, compared to the signal B 2 , the polarization of the pulses of the signal B 2 ′ has changed. This polarization of the signal B 2 ′ is now similar to the polarization of the signal B 1 . 
   As already described, the signal B 1  at the parallel output of the polarization beam splitter  11  and the signal B 2 ′ at the output of the phase shifter  12  are then combined. The result is a signal C that has the same polarization as the signals B 1  and B 2 ′. 
   As a result, the first device  10  converts the alternated-polarized signal A into the single-polarized signal C. The first device  10 , therefore, provides an alternated-to-single-polarization conversion of the signal A. The resulting signal C may then be processed as a single-polarized data stream. 
   In  FIG. 2 , the first device  10  is shown being followed by a second device  20 . The second device  20  comprises a passive optical delay filter  21 , in particular a Mach Zehnder filter. The delay filter  21  comprises a first path that does not change the received signal. A second path of the delay filter  21  delays its received signal by one bit. The two paths are then combined into the output of the delay filter  21 . 
   In connection with  FIG. 3 , the second device  20  is explained in more details. It is assumed that the second device  20  receives the signal C from the first device  10 . Furthermore, it is assumed that the subsequent pulses of this signal C are affected by polarization-dependent losses (PDL). Therefore, the signal at the input of the second device is characterized as a signal C*. The polarization-dependent losses are indicated in this signal C* by the different heights of the subsequent pulses. 
   In the mentioned second path of the delay filter  21  of the second device  20 , the signal C* is delayed by one bit resulting in a signal D*. Then, the signal C* of the first path and the signal D* are combined into a signal E at the output of the second device  20 . 
   A comparison shows that the signal D* still incorporates the described polarization-dependent losses. However, compared to the signal C*, the heights of the subsequent pulses of the signal D* are reversed. 
   This reversed sequence of the pulses of the signal D* has the consequence that the combination of this signal D* with the signal C* leads to a compensation of the polarization-dependent losses. Therefore, the signal E at the output of the second device  20  is not affected anymore by polarization-dependent losses. This is indicated in  FIG. 3  by the same heights of the subsequent pulses of the signal E. 
   The described first and second devices  10 ,  20  may be used in connection with the demodulation of a digital signal according to the differential phased shift keying (DPSK) format. Then, devices and algorithms known from single-polarized applications may be used for receiving and processing this such signal. Furthermore, the required devices, in particular the Mach Zehnder filter, may be carried out in the known PLC technology (PLC=planar lightwave circuit).