Abstract:
Method for reducing signal degradation in an optical polarisation-multiplex system. The modulated optical signals to be transmitted are synchronised or generated such that the phase difference for NRZ-modulated signals is at least approximately 0° and the phase difference for RZ-modulated signals is at least approximately 180°. They can also be achieved by means of different synchronising devices.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/DE03/01446, filed May 6, 2003 and claims the benefit thereof. The International Application claims the benefits of German application No. 10220929.4 DE filed May 10, 2002 and German application No. 10242915.4 DE filed Sep. 16, 2002, all of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The invention relates to a method and a system for reducing the signal degradation according to the Claims. 
     BACKGROUND OF INVENTION 
     In order to increase the transmission capacity in optical transmission systems the polarisation-multiplex method is used, whereby two signals are polarised orthogonally to one another by an advantageous means and transmitted on the same wavelength. 
     If polarisation mode dispersion (PMD) occurs, this leads to coherent crosstalk between the signals. Even at low PMD values this crosstalk makes error-free transmission of polarisation-multiplex signals impossible, whereas in transmission systems without polarisation-multiplex such PMD values are still tolerable. Interference makes itself felt both in amplitude modulation (including multistage) and in angle modulation. 
     European patent application EP 1 202 485 AI discloses a method for transmitting polarisation-multiplex signals in which a signal is divided into two part-signals which are then reassembled into a time-multiplex signal with mutually perpendicular polarisation levels. The time-multiplex method avoids mutual signal interference and halves the data transfer rate in each signal. However the desired doubling of the transmission capacity is not achieved. 
     SUMMARY OF INVENTION 
     The object of the invention is to reduce the signal degradation in polarisation-multiplex signals without restricting the transmission capacity. 
     This object is achieved by means of a method and a system with features which will emerge from the claims. 
     Advantageous embodiments are specified in the individual claims. 
     The core of the invention consists in a temporal arrangement of the interference caused by a signal to ensure that it falls in the non-critical area of the other signal, where it has no effect on the evaluation of the logical state. Since this interference originates from the bit boundaries (in the case of multiphase modulation this refers to the modulation segment boundaries) and in the case of amplitude modulation is caused by the signal edges, the two signals should be synchronised on transmission in such a way that their bit boundaries or signal edges do not fall in the critical evaluation areas, that is, not in and around bit centres. In the case of NRZ (non-return-to-zero duty cycle&gt;50%) signals, therefore, the bit boundaries have to coincide. In short-pulse RZ signals a 180 degree is applied. phase shift. The same applies to angle-modulated signals. 
     Two signals from different data sources must be synchronised or their clock pulses must be adapted as necessary. 
     The method to which the invention relates more than doubles the tolerance to PMD, enabling the maximum possible number of regenerator-free transmission links to be increased by a factor of 4. 
     This also makes it possible to have a transmission method which combines polarisation-multiplex with multistage phase modulation. If four-phase modulation is used a fourfold data transfer rate is possible. Similar advantages are obtained in the case of double-binary encoding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Typical embodiments of the invention will be explained in greater detail with the aid of figures. 
       These show the following: 
         FIG. 1 : a transmitter arrangement with an electrical phase-shifter for the purpose of synchronisation, 
         FIG. 2 : a transmission arrangement for generating synchronous polarised signals, 
         FIG. 3 : a transmission arrangement for converting a data signal into two transmission signals polarised in parallel, 
         FIG. 4 : a transmission arrangement with a controller making use of the data multiplex signal, 
         FIG. 5 : a transmission arrangement with a comparator for generating the control signal, 
         FIG. 6 : an associated time diagram, 
         FIG. 7 : a transmission arrangement with two phase detectors, and 
         FIG. 8 : an associated time diagram. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  shows a transmission arrangement for transmitting a PMD signal. The typical embodiment assumes that the light has been polarised in linear fashion and amplitude modulation has been assumed for ease of understanding. However, other (orthogonal) polarisation types and other modulation types are also possible. 
     A coherent light source (laser)  1  generates a laser signal LS that is divided in an optical polarisation beam splitter  2  into two orthogonal components, carrier signals OT 1  and OT 2 . Each of these is fed to a modulator of its own, such as a Mach-Zehnder modulator  3  and  4 . The modulator  3  is controlled by a first electrical data source  5 , which generates a first data signal DS 1 . A second electrical data source  6  generates a second data signal DS 2 , which is fed via an electrical delay element (phase shifter)  7  to the second modulator  4 . The modulated signals S 1  and S 2  are fed together via a polarisation-beam combiner  8  (meaning any combiner that is suitable for combining signals, such as a 3 dB coupler) and the polarisation-multiplex signal PMS so obtained is delivered at output A. It is assumed that the two data sources are synchronised with one another so that only one synchronisation device  7 ,  70  is needed to provide the optimum phase position between the first data signal DS 1  and the second data signal DS 2 . This optimum phase position is produced by a phase shifter which takes the form of an adjustable, electrical delay element  7 . In principle the adjustable phase shifter can be arranged at any point in the signal path of signal S 1  or S 2  (including the clock pulse feed). 
     Advantageously the delay element  7  is regulated by a control system  70  which is fed with a measurement signal MS tapped from the polarisation-multiplex signal PMS. Any criteria can be used for control purposes, including the error rate or a harmonic component of the signal. In order to obtain a symmetrical control range, a further (electrical) delay element  72  can be inserted between for example the first data source  5  and the first modulator  3 . In principle the electrical delay element  7  could be replaced by a controllable optical delay element  71 . The optical delay element  71  is then inserted after the second modulator  4 , for example. 
     A solution of equal value consists in inserting the delay element in a clock signal feed line, if a data source is triggered by a clock pulse generator  11 . 
     In the case of NRZ signals which shall include all signals with a duty cycle &gt;50% the electrical delay element  7  is set so that modulation segment boundaries, or in the case of amplitude modulation the edges of the signals S 1  and S 2  being transmitted, occur at the same instants (in the case of angle modulation, the instants in which the frequency or phase are rekeyed, e.g. bit boundaries), so that the generated interference is as far removed as possible from the evaluation area, which is usually the evaluation or sampling point in the bit centre. 
       FIG. 2  shows an arrangement for transmitting plesiochronous signals. Two plesiochronous data signals PS 1  and PS 2  are first written to memory  12  or  14  and then retrieved from these with the aid of a clock signal TS 1  or TS 2 , both being generated by a single clock pulse generator  11 . Adaptation between the data transfer rate of the plesiochronous signals and the clock signals TS 1  and TS 2  is effected by pulse adapters  13  or  15 , which use padding routines to compensate for differences in the data transfer rates. In the case of NRZ signals the clock signals TS 1  and TS 2  have the same phase position. 
       FIG. 3  shows an arrangement in which a data signal DS is divided by a demultiplexer into two data signals DS 1  and DS 2  at half the data transfer rate. These data signals are used to modulate the orthogonal components OT 1 , OT 2  of the laser signal LS and the modulated signals S 1 , S 2  are assembled in the polarisation coupler  9  into the polarisation-multiplex signal PMS. In the case of NRZ signals, buffer stores are connected before the modulators and modulation proceeds in synchronous mode. 
     An important aspect of the method to which the invention relates is a phase position which is as far as possible optimum between the orthogonal polarised transmission signals with the same data transfer rate, in order to minimise mutual interference. 
       FIG. 4  describes an arrangement involving a control device. Two data signal sources  5  and  6  receive their clock pulses from a common clock generator  11 . The clock signal TS is fed via a fixed delay element  71  and an adjustable delay element  7  to each of the data signal sources. The data signal sources deliver a data signal each DS 1  and DS 2 , which the modulators  3  and  4  use to modulate the amplitude of a carrier signal generated by the laser  1 . This embodiment envisages two polarisation controllers  17  and  18  to rotate the modulated signals in two mutually orthogonal polarisation planes. The orthogonal signals are combined in an adder  8  and are then output as a polarisation-multiplex signal PMS. A measurement signal MS is tapped from this signal with the aid of a measuring coupler  9  and converted in a photodiode  19  into an electrical signal ES. This is squared in a multiplier  20  and then fed as a squared measurement signal ES 2  to a filter  21 , advantageously a bandpass filter. If the bit edges of the signals S 1  and S 2  are synchronous, the power is in a frequency range corresponding to the data transfer rate of the data signals, for example in the 10 GHz frequency range at a data transfer rate of 10 Gbit/s minimum. A controller  22  connected to the output from the filter varies the adjustable delay element  7  until this minimum is reached. The adjustable delay element  7  can be connected in at any point in the lower second signal path  7 ,  6 ,  4 ,  18 ,  8  of the arrangement. It goes without saying that the arrangement shown in  FIG. 1  can also be fitted with this controller. 
     Further squaring of the electrical measurement signal ES (the first takes place with the aid of the photodiode  19 ) provides an improved control criterion. In principle it can be said that either the fundamental frequency is controlled to achieve a maximum or the interfering frequency components are controlled to achieve a minimum, which in general produces a slightly flatter trend line. 
       FIG. 5  shows a further variant of the control system. Again two orthogonally polarised, amplitude-modulated signals S 1  and S 2  are generated.  FIG. 5  differs from  FIG. 4  only in that the optical carrier generated by the laser  1  is fed via a polarisation splitter  9 , which means that the polarisation controllers can be omitted. From each of the two modulated, polarised signals S 1  and S 2  a measurement signal MS 1  and MS 2  is tapped with the aid of measuring couplers  10  and  25  and fed to opto-electrical converters  12  and  13  (demodulators). The electrical signals are logically compared with one another in an exclusive OR gate or an exclusive NOR gate. If the signals S 1  and S 2  are synchronous and without any phase difference, .phi.=0, as shown in the time diagram  FIG. 6 , the output signal EX from the exclusive OR gate has no more than half the frequency of the data transfer rate. However if a phase difference exists, e.g. .phi.=90 degree. between the signals S 1  and S 2 , as also shown in the time diagram  FIG. 6  in one case, the output frequency is doubled. Depending on the version of the filter  24  the controller  22  can control to achieve a maximum of its input signal of half the data transfer rate or to achieve a minimum of its input signal with a higher data transfer rate by adjusting the delay element  7 . 
       FIG. 7  shows a further arrangement for synchronisation which contains two phase detectors  30 ,  31 ,  32 ,  33  and  35 ,  36 ,  37 ,  38 . These are configured as Hogge phase detectors, each with two flip-flops  32 ,  33  or  35 ,  36  and two exclusive OR gates  32 ,  33  or  37 ,  38 . The first phase detector, which is allocated to the first (upper) signal path  5 ,  3 ,  8  and receives its input signal via a first measuring coupler  10  and the photodiode  12 , ensures that a defined phase relationship exists between the input signal and the clock signal TSH generated by a controllable oscillator (VCO)  34 . For this the input signal to the phase detector is sampled in the bit centre of the clock signal TSH and buffered in the flip-flop  30 . Since a clock signal TS with the same frequency is already being generated by the clock generator  11 , an adjustable delay element can be used instead of the oscillator, making the circuit considerably simpler to produce. 
     In a symmetrical configuration the second phase detector  35 ,  36 ,  37 ,  38 , which receives its input signal via the second measuring coupler  25  and the photodiode  13 , uses the controller  39  to set the adjustable delay element  7  in such a way that the input signal to the second phase detector is also sampled in the centre, i.e. both signals S 1  and S 2  are phase-synchronous.  FIG. 8  shows this case in a time diagram. 
     If angle modulation is used instead of amplitude modulation, the same circuits can be used if the signals are first converted into amplitude modulated signals.