Abstract:
Two data signals are subjected to a binary differential phase modulation and transmitted as POLMUX multiplex signals. The types of modulation are selected in such a way that they are orthogonal in relation to each other and do not influence each other during the demodulation of the signals. There is thus no need to regulate the polarization on the receiving side.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is the US National Stage of International Application No. PCT/EP2005/054685, filed Sep. 20, 2005 and claims the benefit thereof. The International Application claims the benefits of German application No. 102004047028.6 DE filed Sep. 28, 2004, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION  
       [0002]     The invention relates to a method and to arrangements for the optical transmission of data signals via differential phase modulation in a polarization multiplex method.  
       SUMMARY OF INVENTION  
       [0003]     A method suitable for the transmission of optical signals in which two different data signals are transmitted in polarization planes which are orthogonal with respect to one another is referred to as a polarization multiplex method (POLMUX). The technology commonly in use today employs adjustable polarization controllers with polarization filters connected downstream which allow optimum separation of the signals in both polarization planes. Such a method is however quite complicated and has a greater hardware resource requirement on the receive side.  
         [0004]     In patent application DE 102 43 141 A1 a method is described for the transmission of two four-stage phase-modulated DQPSK signals in the polarization multiplex method. The modulated DQPSK signals are separated on the receive side by means of a polarization splitter. Following a compensation operation on the DQPSK signals, the conversion into binary signals takes place. A multi-dimensional filter is used for the compensation operation.  
         [0005]     In the article “Secure optical communication utilizing PSK modulation, wavelength and polarization agility”, 2003 IEEE Military Communications Conference, MILCOM 2003, Boston, Mass., October 13-16, Aviv Salamon et al. describe a system essentially corresponding to the previously mentioned method, but restrict themselves therein to QPSK (four-stage) phase modulation.  
         [0006]     In the article “Conventional DPSK Versus Symmetrical DPSK: Comparison of Dispersion Tolerances” by Jin Wang in IEEE Photonics Technology Letters; Vol. 16, No. 6, Jun. 2004, pages 1585 to 1587, a comparison between conventional DPSK phase states 0 and π and symmetrical DPSK with the phase states π/2 and −π/2 between successive bits is described.  
         [0007]     An object of the invention is therefore to set down a polarization multiplex method which both has good transmission characteristics and can also be implemented with a smaller resource requirement.  
         [0008]     Such a method and arrangements for the send side and for the receive side for implementing the method are described in the independent claims.  
         [0009]     Advantageous developments of the invention are set down in the dependent claims.  
         [0010]     The invention is based on the idea of transmitting two data signals in one channel (at one wavelength) using two modulations which are orthogonal with respect to one another. This saves having to separate the polarized signals on the receive side or complex compensation facilities.  
         [0011]     The use of differential phase modulation, which exhibits a considerably enhanced quality of transmission compared with the previously widely used amplitude modulation, is particularly advantageous in achieving a good quality of transmission. Differential phase modulation, DPSK—Differential Phase Shift Keying, is in particular insensitive to changes in the characteristics of the transmission path and the transmission equipment since successive modulation sections (bits) are each compared with one another during the demodulation process. With regard to DPSK, demodulators can be implemented particularly easily without them requiring auxiliary frequencies for the demodulation.  
         [0012]     Two data signals can also be modulated orthogonally with respect to one another in the case of phase modulation. Auxiliary frequencies are however required for the demodulation in this case.  
         [0013]     For reasons of stability, it is advantageous if both the modulators in the transmitter and also the demodulators in the receiver employ a symmetrical construction as far as possible; for example, so-called “balanced” demodulators are used on the receive side which moreover exhibit an enhanced performance, in particular an enhanced signal-to-noise ratio. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The invention will now be described in detail with reference to embodiments.  
         [0015]     In the drawings:  
         [0016]      FIG. 1  shows a schematic diagram of a transmission system,  
         [0017]      FIG. 2  shows a diagram of the differential phase modulation and a diagram of the π/2 differential phase modulation,  
         [0018]      FIG. 3  shows the schematic diagram of a π/2 phase modulator/differential phase modulator,  
         [0019]      FIG. 4  shows a schematic diagram of a parallel π/2 phase modulator,  
         [0020]      FIG. 5  shows a schematic diagram of a differential phase demodulator,  
         [0021]      FIG. 6  shows a schematic diagram of a π/2 differential phase demodulator.  
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0022]      FIG. 1  shows the complete transmission system reduced to its essential elements with a transmitter unit TR, a transmission channel CH and a receiver unit RE.  
         [0023]     Two data signals DS 1  and DS 2  are to be transmitted on one channel of a POLMUX system (polarization multiplex system) with two polarizations P0° and P90° which are each orthogonal with respect to one another. An optical carrier signal FT generated by a laser for example is converted by the first data signal DS 1  in a conventional manner in a first modulator MOD 1  by means of two-stage differential phase modulation, referred to as DBPSK (B-binary), into a first transmission signal DPS 1 .  
         [0024]     The same carrier signal FT is delivered to a second modulator MOD 2  and modulated with the second data signal DS 2  DBPSK. However, a modulation “orthogonal” with respect to the first modulation is used, which is referred to here as binary π/2 differential phase modulation, π/2 DBPSK for short, (as π/2 phase modulation in the case of phase modulation). The modulated “π/2 transmission signal” is polarized orthogonally with respect to the other POLMUX signal and combined with the first transmission signal DPS 1  as a second transmission signal DPS 2  to form a polarization multiplex signal PMS (POLMUX signal) and is transmitted by way of the POLMUX channel CH.  
         [0025]     On the receive side, no polarization setting and splitting into two polarized signals DPS 1  and DPS 2  is required because the orthogonally modulated transmission signals do not affect one another during the demodulation. The received POLMUX signal is split into two POLMUX partial signals (preferably of the same power) TS 1  and TS 2  which are fed directly to a demodulator DEM 1  or DEM 2  respectively. The first demodulator DEM 1  is designed for conventional differential phase modulation (or phase modulation) and retrieves the first data signal DS 1  from the first POLMUX partial signal TS 1 . The second transmission signal causes no interference in this situation on account of its orthogonal modulation.  
         [0026]     The second data signal DS 2  is retrieved from the second POLMUX partial signal TS 2 . The second demodulator DEM 2  must therefore take into account the special demodulation of the second transmission signal DPS 2  in order to be able to demodulate the latter, and on the other hand deal with the portions of the first transmission signal DPS 1  in such a way that they do not affect the modulation of the second transmission signal DPS 2 .  
         [0027]     Transmission by means of orthogonal phase or differential phase modulation will first be generally described with reference to  FIG. 2 . The left-hand image symbolizes the transmission of binary data using the known phase or differential phase modulation. With regard to binary phase modulation, the information is present in two different phase states of the carrier signal and in the case of differential phase modulation it is present in the phase differential of successive modulation sections; the two phase states are symbolized by the phase 0° and the phase 180°, for example. With regard to differential phase modulation, the bit sequence of the data signal, in other words the sequence of the binary states 0 and 1 to which phase differences of successive modulation sections are assigned is decisive. A logical 0 can thus for example be transmitted by no change in the phase and a logical 1 by a 180° phase change.  
         [0028]     The modulation in the lower signal branch of the transmitter unit TR is more complicated. This will be explained with reference to the π/2 DBPSK. It is assumed that a first modulation section (odd) B 1  exhibits the phase 0°. Let the next (even) bit again be a logical 0, which would result in retention of the phase during the next modulation section B 2  in the case of standard DBPSK. With regard to the π/2 DBPSK used, however, the phase (counterclockwise, for example) is changed by 90° and the second modulation section B 2  is transmitted with a phase of 90°. For the next modulation section, the phase is first changed by a further counterclockwise 90°. If a logical 0 is then transmitted again, this phase of 180° remains in effect; if however a logical 1 is transmitted, then an additional phase change of 180° takes place; a modulation section B 3  with a phase differential of 360°=0° is then transmitted etc. All the ODD modulation sections B 1 , B 3 , B 5 , . . . therefore exhibit phases of 0° or 180° and all the EVEN modulation sections B 2 , B 4 , B 4 , . . . exhibit phases of 90° and 270°. As will be explained, as a result of these phase differences the second transmission signal DPS 2  has no influence on the demodulation of the first data signal DS 1  and vice versa.  
         [0029]     First, however, the basic structure of a π/2 DPSK modulator will be described with reference to  FIG. 3 . The second data signal DS 2  is fed to a conventional differential phase modulator (or phase modulator) MOD (whereby the inversion of every second bit in the modulo-2 adder M2A will be dealt with later) which converts it into a standard differential phase signal. Every second bit—controlled by a switchover signal US—after passing through a delay element DL—causes an additional phase change of 90° (−π/4; +π/4 or 0; π/2) in a phase modulator PHM connected downstream of the modulator MOD, such that the odd bits are assigned to the phase states 0° and 180°, while the even bits are assigned to the phases 90° and 270°. The modulators MOD and PHM can be replaced by a 4-phase modulator, given appropriate control facilities.  
         [0030]     The previously described rotation of the phase by π/2 in each case can be achieved by setting two different phase changes (or delay times) and by inverting every second bit of the data signal, as is described in the following.  
         [0031]     The coding rule in a general form is as follows:  
               a   k     =       b   k     ⁢     a     k   -   1       ⁢     exp   ⁡     (     j   ⁢     π   2       )                 (   1   )             
 
 where b k ε{−1; 1 } are the information bits and a k ε{1; exp(jπ/2}; −1; exp(−jπ2)} are the transmitted symbols for the k-th transmission interval. 
 
         [0032]     By introducing the carrier frequency f 0 , the transmitted signal pulse p(t) and the signal period T, the following results for the transmitted signal in the k-th modulation interval: 
 
(2) s   k ( t )= Re{a   k   p ( a   k   p ( t−kT )exp( j 2 πf   0   t )}. 
 
         [0033]     By changing the coding rule according to (1) appropriately, the following results: 
        if k is even  
               a   k     =         b   k     ⁢         a     k   -   1       ⁡     (     -   1     )       k     ⁢     exp   ⁡     [       j   ⁡     (       2   ⁢   k     +   1     )       ⁢     π   2       ]         =     {             b   k     ⁢     a     k   -   1       ⁢     exp   ⁡     (     j   ⁢     π   2       )                     -     b   k       ⁢     a     k   -   1       ⁢     exp   ⁡     (       -   j     ⁢     π   2       )               }               (   3   )             
    if k is odd        
 
         [0036]     By introducing modified transmission symbols ã k ε{−1;1} as 
 
(4)  ã   k   =b   k   ã   k−1 (−1) k  
 
 and modified signal pulses 
        if k is even  
                   p   ~     k     ⁡     (   t   )       =     {             p   ⁡     (   t   )       ⁢     exp   ⁡     (     j   ⁢     π   4       )                     p   ⁡     (   t   )       ⁢     exp   ⁡     (       -   j     ⁢     π   4       )               }             (   5   )             
    if k is odd 
 
 the transmitted signal can be described as 
 
(6)  s   k ( t )= Re{ã   k   {tilde over (p)}   k ( t−-kT ) exp( j 2 πf   0   t )}. 
       
 
         [0039]     As expected, this results in a π/2 DPSK modulator whereby the same result is achieved by means of two different settings of the phase modulator instead of a continual rotation in the same direction.  
         [0040]      FIG. 4  shows a symmetrically constructed π/2 DBPSK modulator. A laser LA again generates the carrier signal FT which is split in a splitter SP 1  onto an upper signal branch ZM 1  and a lower signal branch ZM 2  of the π/2 DBPSK modulator. The upper modulator MOD 1  or the lower modulator MOD 2  is switched to active alternately by the switchover signal US by a precoder EC by way of the optical switch OS. The inverter IN here assumes the function of the modulo-2 adder MA 2  from  FIG. 3 . The phases of the modulation sections in each signal branch are 0° or 180°. The phase change of 90° between successive modulation sections is achieved here by means of two fixed phase final control elements (or delay elements) DL 1  and DL 2  which are connected in the upper signal branch and the lower signal branch respectively. The alternately generated output signals are combined by a second splitter (combiner) SP 2  to form the second transmission signal DPS 2 . The illustration should be regarded as a symbolic representation, the delay can also be achieved by means of an appropriately designed modulator or can also be achieved by only one fixed phase final control element DL 1  with a phase displacement of the carrier signal corresponding to π/2 in one signal branch. Symmetrical arrangements generally have the advantage of a greater stability.  
         [0041]      FIG. 5  shows a known “balanced” demodulator arrangement for DBPSK. This corresponds to an interferometer with symmetrical outputs. The polarization multiplex signal PMS received, which contains both transmission signals DPS 1  and DPS 2 , is split in a first splitter SP 3  onto an upper signal branch ZD 1  and a lower signal branch ZD 2 , whereby the phases of these partial signals are different. The series connection of a delay element TB with a delay time TB of one bit length and of a phase final control element PE is arranged in the upper signal branch. The partial signals are overlaid in the output-side splitter. The phase differentials between the overlaid partial signals at the demodulator outputs are 0° and 180°, such that they are added to another in the case of a particular phase difference and are subtracted from one another in the case of a phase difference changed by 180°. This means that as a result of the balanced structure the upper photodiode FD 1  will generate a greater voltage while the lower photodiode FD 2  generates a smaller (no) voltage and in the other situation the upper photodiode will generate a smaller voltage while the lower photodiode delivers a greater voltage. These “analog” signals are converted back into binary data signals by means of a following comparator circuit VG (differential amplifier) and a low-pass TP connected upstream or downstream of the latter. The orthogonal signal portion of the second transmission signal DPS 2  generates voltages of equal sizes in both photodiodes at 90° phase displacement and thus makes no contribution to the demodulated signal.  
         [0042]     In order to achieve optimum demodulation, a precise setting of the phase in both signal branches ZD 1 , ZD 2  of the demodulator is required which is effected here by the phase final control element PE and a phase control signal PHC obtained in a phase control unit PHR. The illustration should be regarded as a symbolic representation, the delay element and phase final control element can be implemented as desired.  
         [0043]      FIG. 6  shows a schematic diagram of the π/2 DBPSK demodulator which differs from the demodulator represented in  FIG. 5  solely in the fact that the phase final control element PE 2  effects a phase displacement of 90° which compensates for the transmitter-side phase shift of the transmission signal DPS 2  by π/2 such that the second transmission signal appears at the outputs of the output-side splitter SP 4  as an overlay on a standard differential phase modulated signal. A phase control unit PHR should once again be present in order to maintain exact phase relationships.  
         [0044]     In principal, the additional phase rotation can also be reversed in a demodulator corresponding to the modulator in order to then perform the demodulation in a conventional DBPSK demodulator. This would however require a bit synchronization and an additional phase modulator with corresponding control facilities. The successive modulation sections of the first transmission signal DPS 1  are phase-shifted by 90° with respect to one another by the phase final control element PE 2  and thus make no contribution to the demodulated signal.  
         [0045]     It should also be noted that this type of transmission exhibits an enhanced signal-to-noise ratio compared with the standard quaternary differential phase modulation as a result of the differential detection and an increased tolerance to phase noise as a result of the maximum phase margin of 180°.