Method and device for reducing degradation of polarization-multiplexed optical signal

FIELD: optical transmitting systems.

SUBSTANCE: modulated optical signals S1, S2 being transmitted are synchronized with respect to one another or shaped so that phase difference for NRZ-modulated signals is at least about 0 deg. and for RZ-modulated ones, at least about 180 deg. They can be also shaped by means of various synchronizing devices.

EFFECT: reduced permanent degradation of polarization-multiplexed signal without limiting throughput transfer capacity.

15 cl, 8 dwg

 

The invention relates to a method and apparatus for reducing the deterioration of the signal according to the generic concept of paragraph 1 of the claims and implements this method device according to the generic concept of paragraph 9 of the claims.

In optical transmission systems to increase the transmission capacity used method of polarizing seal, in which two polarized preferably orthogonal to each other signals on the same wavelength.

The appearance of the dispersion of the modes of polarization (SLE) leads to a coherent cross-interference between the signals. These crosstalk even at low values SLE make it impossible for error-free transmission of signals with polarization multiplexing, while in transmission systems without polarizing seal these values PDM is still valid. Interference appears as if the amplitude and angular (including multi-level) modulation.

In the application EP 1202485 A1 describes a method of transmitting signals with polarization multiplexing, in which the signal is divided into two components, which are then perpendicular to each other by the polarization planes of the newly combined in a signal with a temporary seal. By way of a temporary seal is possible to avoid mutual interference between the signals and the transmission speed of the data in each signal is reduced by half. However, not achieved the required doubling the transmission capacity.

The objective of the invention is to reduce the degradation of the (gradual deterioration of signals with polarization seal without imposing constraints on the throughput of the transfer.

This problem is solved by a method characterized by signs of paragraph 1 of the claims. The appropriate device described in paragraph 11 of the claims.

Preferred options for implementation are presented in the dependent claims.

The basic idea of the invention consists in that due to the signal interference to distribute in time so that they fall in the non-critical area of another signal, which does not impact on the assessment of the logical signal status. Since this interference is due to bit boundaries (in the case of multi-level phase modulation defined boundaries modulation segments), while the amplitude modulation is caused by the fronts of the signals, both the transmitted signal must be synchronized so that their bit boundaries or fronts of the signals are not accounted for critical evaluation, that is, the middle bits or their surroundings. In the case of signals with no return to zero (NRZ signal) bit boundaries should therefore ppsr is to give. In the case of signals with return to zero (RZ-signals) with short pulses is achieved by a phase shift of 180about. The corresponding condition is true for signals with angular modulation.

If both signals originate from different data sources, the need to synchronize or harmonize transfer rate (symbols of the digital signal).

Due to appropriate the invention, a method of tolerance in relation to SLE is increased more than twice, which leads to an increase of the maximum possible transfer section without regeneration in four times.

This allows the implementation of the transfer method in which a polarizing seal combined with multi-level phase modulation. When applying the four-level phase modulation four possible data rate. Similar advantages arise when you double binary encoding.

Examples of carrying out the invention is described below with reference to the drawings showing the following:

figure 1 - transmitter with an electric barrier for synchronization

figure 2 is a transmitting device for generating a synchronous polarized signals

figure 3 - transmitter to convert the data signal into two parallel-polarized transmitted signal,

figure 4 - areause device with adjustable, applying the multiplexed data signal,

5 is a transmitting device with a comparator to generate a regulating signal,

6 is a corresponding timing diagram,

Fig.7 - transmitting device with two phase detectors,

Fig the corresponding timing diagram.

Figure 1 shows the transmitting device to transmit a signal with dispersion modes of polarization (PDM signal). Consider the example comes from a linearly polarized light, and for ease of explanation adopted by amplitude modulation. However, it is also possible to use other (orthogonal) polarizations, and other types of modulation.

Coherent light source (laser) 1 generates a laser signal LS, which is in the optical polarization divider 2 beam is divided into two parts - bearing signals 1 and OT. The received signals are fed to respective modulators, such as modulators of the Mach-Zehnder interferometers 3 and 4. The modulator 3 is controlled by the first electric source 5 data, which produces a first signal DS1 data. The second electric source 6 data generates the second signal DS2 data through the link 7 time delay (phase shifter) is supplied to the second modulator 4. The modulated signals S1 and S2 are combined using a polarizing adder 8 rays (it refers to Liu is the second adder (block associations), which is suitable for combining signals, for example 3 dB-coupler (coupling element)), and the thus obtained signal PMS with polarizing seal available at the output A. Assume that both sources of data synchronous with each other, so you only need synchronization device 7, 70, which provides the optimum phase relationship between the first signal DS1 data and the second signal DS2 data. This is the optimal phase relationship is provided by the phase shifter, which is implemented in the form of an adjustable electrical element 7 time delay. In principle, an adjustable phase shifter can be placed anywhere in the signal path (including the conductor clock signal) of the signal S1 or S2.

The preferred way link 7 time delay is controlled by a control device 70, which moves the measuring signal MS, branched from the signal PMS with polarizing seal. For adjustment may apply any criteria of regulation, such as the frequency of occurrence of errors or the percentage of harmonics in the signal. To obtain symmetric regulation can be added (electric) link 72 time delay, for example, between the first data source 5 and the first modulator 3. In principle, the electric element 7 time delay can be replaced by a controllable optical element 71 time delay. The optical element 71 time delay is introduced, for example, after the second modulator 4.

An equivalent solution is to add a link delay in the conductor clock signal, if the data source it is a clock generator 11.

In the case of NRZ-signals electric element 7 time delay is set so that the boundaries modulation segments, and in the case of amplitude modulation of the fronts of the transmitted signals S1 and S2, were one and the same time (in the case of angular modulation is the time frequency or phase manipulation, for example the bit boundaries), resulting in interference generated to the maximum extent removed from the assessment, most often from the time the assessment or time of sampling of the reference.

Figure 2 shows a device for transmitting communication systems (nearly synchronous) signals. Two plesiochronous signal PS1 and PS2 data are first written to the memory 12 and 14 respectively and are read clock signal TS1 and TS2 respectively, which are produced by one clock generator 11. The agreement between the data rate plesiochronous signals and clock signals TS1 and TS2 is carried out by the blocks 13 and 15 respectively negotiation speed transmission clock signal to compensate for differences in transmission rates of the data through processes of negotiation speed (transmission symbols of the digital signal). In the case of NRZ signals clock signals TS1 and TS2 have the same phase relationship.

Figure 3 shows a device in which the signal DS data using a demultiplexer is divided into two signals DS1 and DS2 data with half the data rate. These signals transmit data modulated orthogonal components 1, AT laser signal LS, and the modulated signals S1 and S2 are again combined in the polarization element of the connection (the unifier) 9 to receive the signal PMS with polarizing seal. In the case of NRZ signals before modulators connected intermediate memory and the modulation is carried out synchronously.

Significant in accordance with the invention method is the extent possible, the optimal phase relationship between the orthogonal polarized with the same data rate transmitted signals to minimize mutual interference.

4 shows the device with the adjustment. Two sources 5 and 6 of the data signals aktiruyte common clock generator 11. The clock signal TS is fed, respectively, through a permanent link 71 time delay and, accordingly, the adjustable link 7 time delay for the corresponding one of the sources of the data signals. The sources of the data signals give respectively the signals DS1 and DS2 data, which is the modulators 3 and 4 modulate the amplitude of the carrier signal, generated by the laser 1. In this embodiment, there are two knobs 17, 18 polarization, which rotate the polarization of the modulated signals into two orthogonal each other, the plane of polarization. The orthogonal signals are combined in the adder 8 and transmitted as a signal PMS with polarizing seal. Using a measuring tap 9 of this signal is branched off the measuring signal MS, which by the photodiode 19 is converted into an electrical signal ES. The last multiplier 20 squares and then served as a quadratic measuring signal ES2in the filter 21, preferably band-pass filter. If the fronts of the bits in the signals S1 and S2 are synchronous, then the power in the frequency range, which corresponds to the data rate of the data signals, for example in the frequency range of 10 GHz with a data rate of 10 GB/s, minimum. Connected to the output of the filter controller 22 changes the delay of the adjustable link 7 time delay until then, until you reach this minimum. Adjustable link 7 time delay can be included anywhere in the lower second signal path 7,6,4,18,8 device. It is shown in Fig. 1 the device may also be equipped with the specified control.

By squaring the electric signal is a small signal ES (the first through the photodiode 19) get the best criterion of regulation. In principle, one can say that the adjustment is performed on the maximum fundamental frequency or the minimum frequency of disturbances leaves that generally gives less flat response.

Figure 5 presents another option adjustment. Again produced two orthogonal polarized amplitude-modulated signal S1 and S2. Figure 5 differs from figure 4 only in that generated by the laser 1, the optical carrier signal is passed through a polarization divider 9, so the combination of polarization may be missing. Both modulated polarized signals S1, S2 by measuring taps 10, 11 branches corresponding measuring signals MS1 and MS2 supplied to optoelectronica converters 12 and 13 (demodulator). The electrical signals are logically compared to each other in logic exclusive-OR or exclusive-NOT-OR. If the signals S1 and S2 synchronous without phase difference, ϕ=0, as shown in the timing chart of Fig. 6, the output signal EX logic "exclusive OR" has a maximum of half the frequency, respectively, the data transfer rate. However, if there is a phase difference, for example ϕ=90aboutbetween the signals S1 and S2, which is also shown in the timing chart of Fig. 6, the output frequency of advive the Xia. Depending on run filter 24, the controller 22 can be adjusted to the maximum of its input signal at half the data rate or the minimum of its input signal with a higher data transmission rate by setting the link 7 time delay.

7 shows another device for synchronization, which contains two phase detector 30,31,32,33 and 35,36,37,38. They are made in the form of a Hogge phase detector, respectively, with two trigger cascades 32,33 and up RUB 35.36 respectively and two logic "exclusive OR" 32,33 and, accordingly, 37,38. The first phase detector, which refers to the first (upper) signal path 5,3,8 and receives its input through the first measuring coupler 10 and the photodiode 12, provides that between the input signal and a clock signal TSH produced a voltage controlled oscillator (UNG) 34, there is a certain phase relationship. Sampling the input signal of the phase detector is taken in the middle of the bit clock signal TSH and the intermediate image is stored in the trigger stage 30. Since the clock signal TS with the same frequency already produced a clock generator 11, instead of the voltage-controlled oscillator can also be used adjustable link time delay, so the scheme can be implemented significant is but more simply.

When the symmetrical construction of the scheme by the second phase detector 35,36,37,38, which receives its input via a second measuring coupler 11 and the photodiode 13, by means of the regulator 39 adjustable link 7 time delay is set so that the sampling of the input signal of the second phase detector are in the middle, that is, both signals S1 and S2 synchronous phase. On Fig this case is shown in the timing diagram.

If instead of amplitude modulation is applied angular modulation, can be used the same scheme, if the signals are first converted to amplitude-modulated signals.

1. A method of reducing degradation of the signal with the polarizing seal (PMS), which is formed by combining the first modulated optical signal (S1) and the second optical signal (S2), modulated with the same data rate with a different polarization, characterized in that when transmitting modulated signals without return to zero (NRZ signal) (S1, S2) of the first optical signal (S1) and the second optical signal (S2) is transmitted with a phase difference of at least approximately equal to zero so that the boundaries modulation segments of the first and second signals (S1, S2) are maximally removed from time to estimate the another signal (S2.S1).

2. Pic is b according to claim 1, characterized in that between the first signal (S1) and second signal (S2) is set or produces a phase difference of at least approximately equal to zero between the two signals (S1, S2).

3. The method according to claim 1 or 2, characterized in that the first signal (S1) is formed by the modulation of the first optical signal carrier (1) first data signal (DS1), the second signal (S2) is formed by the modulation of the second optical carrier signal (AT)having a different polarization, the second data signal (DS2), and both data signal (DS1, DS2) are synchronized with each other.

4. The method according to claim 1 or 2, characterized in that the first signal (S1) is formed by the modulation of the first optical signal carrier (1) first data signal (DS1), the second signal (S2) is formed by the modulation of the second optical carrier signal (AT)having a different polarization, the second data signal (DS2), and the first data signal (DS1) and the second data signal (DS2) remembered intermediate image and using obtained only from a single clock generator (11) of the clock signals (TS1, TS2) synchronously modulate the carrier signals (1, OT).

5. The method according to claim 1 or 2, characterized in that the first data signal (DS1) and the second data signal (DS2) is formed by de-multiplex data signal (DS), the first signal (S1) is formed by the modulation of the first optical signal carrier (1)first data signal (DS1), the second signal (S2) is formed by the modulation of the second optical carrier signal (AT)having a different polarization, the second data signal (DS2).

6. The method according to claim 1 or 2, characterized in that the first signal (S1) is formed by the modulation of the first optical signal carrier (1) first data signal (DS1) through a two-level or multi-level phase modulation, the second signal (S2) is formed by the modulation of the second optical carrier signal (AT)having a different polarization, the second data signal (DS2) through a two-level or multi-level phase modulation.

7. Device for reducing the loss of signal in optical systems with polarization multiplexing, which contains two signal path(5, 3, 8; 6, 4, 8) with the corresponding modulator (3, 4) each, two sources (5, 6) of the data signals with the same data rate, each of which modulates a corresponding optical carrier signal, and the block (8) combination, which combines the modulated signals (S1, S2) to the signal (PMS) with polarization multiplexing, wherein the at least one signal path (1, 6, 8, 20) contains a phase shifter (7) or means (13, 15) approval of transfer speed, which(th) reduces the phase difference between the modulated and transmitted in the format NRZ signals (S1, S2) of at least approximately to zero.

8. The device according to claim 7, characterized in that the signal path for shifting one or both of the modulated signal (S2) relative to another modulated signal (S1) included adjustable element (7, 71) time delay.

9. The device according to claim 8, characterized in that the phase shifter between one of the sources (6) data signal and a clock generator (11), pulsing source (6) of the data signal using the clock signal (TS)included adjustable link (71) the time delay.

10. The device according to claim 8 or 9, characterized in that is provided by a synchronization device (9, 10, 7), which regulates a phase difference between the modulated signals (S1, S2) to install at least approximately to zero.

11. The device according to claim 10, characterized in that the synchronizing device(9, 19, 20, 21, 22, 7) contains the coupler (9), which forks part of the signal (PMS) with polarizing seal as a measuring signal (MS), optoelektronische Converter (19), which converts the measuring signal (MS) in electrical measurement signal (ES), multiplier (20), which converts the electrical measuring signal (ES) in the quadratic measuring signal (ES2), the filter characteristics of the high-pass filter, the lower cutoff frequency which corresponds to a value above half speed lane is giving data or the filter (21) with the properties of the bandpass filter, the average frequency of which corresponds to the data rate, which is served quadratic measuring signal (ES2regulating device (22,7)which is quadratic measuring signal (ES2with filter (21) as a control signal and which minimizes the amplitude of the output signal of the filter by a time offset of one of both of the modulated signal (S2) relative to another modulated signal (S1).

12. The device according to claim 10, characterized in that the synchronizing device(9, 19, 20, 21, 22, 7) contains the coupler (9), which forks part of the signal (PMS) with polarizing seal as a measuring signal (MS), optoelektronische Converter (19), which converts the measuring signal (MS) in electrical measurement signal (ES), multiplier (20), which converts the electrical measuring signal (ES) in the quadratic measuring signal (ES2), the filter (21) with the properties of the lowpass filter, the lower cutoff frequency which corresponds to a value above half-speed data transmission, or the filter (21) with the properties of the bandpass filter, the average frequency of which corresponds to half the data rate, which is served quadratic measuring signal (ES 2regulating device (22,7)which is quadratic measuring signal (ES2with the filter as a control signal and which minimizes the amplitude of the output signal of the filter by a time offset of one of both of the modulated signal (S2) relative to another modulated signal (S1).

13. The device according to claim 10, in which the optical carrier signal is modulated by two sources (5, 6) of the data signal with the same data rate, wherein the synchronization device comprises two measuring tap (10, 11), from which each of the modulating signals (S1, S2) otvetst measuring signal (MS1, MS2), optoelectronica transducers (12, 13), which convert the measuring signal (MS1, MS2) in electrical measurement signals (ES1, ES2), logic "exclusive OR" (23), which served electrical measurement signals (ES1, ES2), the controller (22), to which is fed the output signal of logic "Exclusive OR" (23) through the filter (24) and which regulates the filtered output signal in the case of the filter characteristics of the high-pass filter or a bandpass filter before reaching the minimum value, and in the case of the filter characteristics of the low pass filter before reaching the maximum value.

14. The device according to claim 10, different is seesa fact, what is synchronizing the device contains

the first measuring coupler (10), which is included in the first signal path and forks first measuring signal (MS1) of the first modulated signal (S1), the first phase detector (30, 31, 32, 33) in the first signal path, which is served first measuring signal (MS1) through the first opto-electronic Converter (12) and which controls a clock regenerator (34), which produces the clock signal comparison (TSH), the second measuring coupler (11), which is included in the second signal path and forks second measuring signal (MS2 from the second modulated signal (S2), a second phase detector (35, 36, 37, 38) in the second signal path to which is supplied the clock signal comparison (TSH) and through the second optoelectronic Converter (13) the second measuring signal (MS2), the controller (39), which is controlled by the second phase detector (35, 36, 37, 38) and controls the link (7) time delay in the second signal path or in the path of the clock signal so that both of the modulated signal (S1, S2) have the same phase relationship.

15. The device according to 14, characterized in that as the clock regenerator (34) is provided by an adjustable link time delay which is the clock signal (TS)generated clock GE is oratorum (11).

Priority items:

10.05.2002 - claims 1 to 7, 9, 10;

06.05.2003 - PP, 11-15.



 

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