Abstract:
The polarization-multiplexed signal contains two data signals that are orthogonally polarized in relation to one another. Their carrier signals are derived from the same source and thus have the same wavelength. The phase difference between the carrier signals, is adjusted or regulated in such a way that it corresponds to 90°. The phase difference of the carrier signals permits the susceptibility to polarization mode dispersion to be significantly reduced.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is the US National Stage of International Application No. PCT/EP2005/050353, filed Jan. 27, 2005 and claims the benefit thereof. The International Application claims the benefits of German application No. 102004005718.4 DE filed Feb. 5, 2004 both of the applications are incorporated by reference herein in their entirety. 
   FIELD OF INVENTION 
   The invention relates to an improved method for optical transmission of a polarization division multiplexed signal. 
   BACKGROUND OF INVENTION 
   The transmission of data in polarization division multiplex whereby two optical data signals have the same wavelength with orthogonal polarizations is a highly promising method of doubling transmission capacity without having to place more exacting requirements on the transmission link or signal-to-noise-ratio. 
   However, a disadvantage of polarization division multiplex is susceptibility to polarization mode dispersion (PMD) which results in mutual interference between the transmission channels. Although the effect of PMD can be reduced by PMD compensation measures, compensation is required for each channel of a wavelength division multiplex system; it is also complex/costly and does not always produce the desired results. The use of PMD-optimized fibers likewise provides an improvement, but is only possible for new networks. 
   SUMMARY OF INVENTION 
   New possibilities with the object of reducing PMD interference susceptibility and therefore mutual interference of the optical data signals during transmission of a PolMUX signal are therefore required. 
   This object is achieved by the independent claim. 
   Advantageous further developments of the method are set forth in the dependent claims. 
   The method is simple to implement. The carrier signals, derived from the same laser source, of the two optical data signals (PolMUX channels) are mutually phase shifted by a constant 90°. Obviously the two carrier signals therefore also have exactly the same frequency and their phase difference remains constant during transmission. The phase can be adjusted at the transmitter end by different devices such as phase modulators and delay elements. 
   Also advantageous is the use of a phase control arrangement which ensures a constant phase difference between the carrier signals irrespective of the environmental conditions and component tolerances. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail using examples and with reference to the accompanying drawings in which: 
       FIG. 1  shows a circuit diagram of the transmit arrangement, 
       FIG. 2  shows a circuit diagram with phase control, 
       FIG. 3  shows a phase difference measuring arrangement, 
       FIG. 4  shows another phase difference measuring arrangement and 
       FIG. 5  shows an arrangement for phase difference measurement by analyzing orthogonal signal components. 
   

   DETAILED DESCRIPTION OF INVENTION 
     FIG. 1  is a circuit diagram of the transmit arrangement. The method can also be implemented by any desired variants of this arrangement. A constant wave (CW) optical signal normally generated by a laser is fed via an input  1  to a polarization splitter  2  which splits it into two orthogonal carrier signals CW X  and CW Y  of equal amplitude but having planes of polarization differing by 90° (the arrows indicate the relevant polarization). The orthogonal carrier signal CW X  is fed via a first optical fiber  3  to a first modulator  5  where it is intensity modulated with a first data signal DS 1 . The second orthogonal carrier signal CW Y  is fed via a second fiber  4  and a phase shifter  6  to a second modulator  7  where it is intensity modulated with a second data signal DS 2 . The optical data signals OS 1  and OS 2  produced at the outputs of the modulators and which are orthogonally polarized relative to one another and whose carrier signals are phase shifted by 90° are combined in a polarization combiner  8  to form a polarization division multiplex signal (PolMUX signal) PMS and fed out at output  9 . Both the phase shift between the two carrier signals and the polarization can likewise be adjusted after the modulators. 
     FIG. 2  shows such a variant in which the carrier signal CW is first split, in a power splitter  13 , into two equal components CW 1  and CW 2  which are modulated as carrier signals with data signals DS 1  and DS 2  respectively. Conversion into two orthogonal optical data signals OS 1  and OS 2  is accomplished by two polarization controllers  14  and  15  which are disposed preceding the polarization combiner  8  and naturally also then convert the carrier signals CW 1  and CW 2  into the orthogonal carrier signals CW x  and CW Y . 
   The phase shift between the carrier signals CW 1  and CW 2  is created by a controlled phase shifter  10  (phase modulator, delay element) which is controlled by a control device  11 . Said control device  11  receives, via a tap  12 , a lower-power measurement signal MS corresponding to the PolMUX signal PMS and monitors the phase shift between the carriers of the orthogonal data signals OS 1  and OS 2 . The time constant of the control device is selected very large so that the controlled phase shifter  10  has a virtually constant value. The phase shifter  10  can likewise be connected following the polarization controller  15 . The carrier signals can therefore be phase shifted by adjusting the carrier signals CW X  and CW Y  or CW 1  and CW 2  or the orthogonal data signals OS 1  and OS 2 . 
   A control criterion for the carrier phases can always be obtained without great complexity if the two PolMUX channels simultaneously transmit a signal, e.g. if the two signals correspond to a logical one. 
     FIG. 3  shows a circuit diagram of the control device for obtaining a control criterion. The measurement principle is based on the fact that the state of polarization depends on the phase between the two polarized signals OS 1  and OS 2  and the phase difference can therefore be determined by measuring the state of polarization. It is only necessary to measure the circular polarization component. To measure same, the measurement signal MS, which like the PolMUX signal has a particular polarization, is split into two sub-signals, one of which is fed via a λ/4 plate and a 45° polarizer (polarization filter). At precisely 90° phase displacement of the carrier signals relative to one another the amplitudes of the two sub-signals OA and OB are of equal size. The optical sub-signals OA and OB are converted by photodiodes  18  and  19  into electrical sub-signals EA and EB and fed to a controller  20  which measures the amplitude difference and adjusts the phase difference of the carrier signals accordingly. 
     FIG. 4  shows another possibility for determining the phase difference by using what is known as a DGD (differential group delay) element such as a polarization-maintaining fiber or birefringent crystal which reverses the 90° phase shift of the carrier signals so that their superimposition produces maximum power (or, in the case of opposite phase displacement, minimum power) in the output signal RS. The polarization planes of the orthogonal signals OS 1  and OS 2  must be at 45° to the main axes of the DGD element. After conversion of the optical superimposition signal OTS into an electrical superimposition signal ETS in a photodiode  22 , the effective power is determined in a control device  23  and adjusted to a maximum (or minimum). 
     FIG. 5  shows another arrangement for controlling the phase. The requirement is again that the PolMUX signal PMS or rather the corresponding measurement signal MS has a particular polarization, as is the case anyway, however, for the transmitter. The PolMUX signal or rather the measurement signal here has two (at least virtually) orthogonal signals OS 1  and OS 2  polarized +45° and −45° relative to a polarization plane of the polarization splitter  24 . The measurement signal MS representing the two orthogonal signals OS 1  and OS 2  is decomposed by the polarization splitter  24  into two polarized signal components OS X  and OS Y  which therefore contain signal components of the two orthogonal signals OS 1  and OS 2 . The signal components MS x  and MS Y  are separately converted into electrical signal components E X  and E Y  in photodiodes  18  and  19 . Only when there is a particular phase between the orthogonal signals OS 1  and OS 2  will the two signal components MS X  and MS Y  be of equal magnitude. A corresponding criterion EA-EB can be used for control. The sensitivity of the control system can be increased by special signal processing in the control device  25 , e.g. by multiplication of the signal components.