CA2340710A1

CA2340710A1 - Device for detecting polarization mode dispersions

Info

Publication number: CA2340710A1
Application number: CA002340710A
Authority: CA
Inventors: Johannes Wollenweber; Rudolf Stelzl
Original assignee: Individual
Current assignee: Siemens AG
Priority date: 1998-08-17
Filing date: 1999-08-17
Publication date: 2000-03-02
Also published as: WO2000011829A3; DE19837216C2; EP1106032A2; DE19837216A1; WO2000011829A2; CN1314066A

Abstract

According to this invention and after the occurrence of an error, the link data redundantly saved elsewhere are used for maintaining the communication link. The present invention also relates to a switching device (EXC) having a central unit (MP) that comprises a data memory for storing the communication-link data which is signaled and switched by the allocated peripheral structural components (LIC1, LIC2)._________________________________________________________________________ _______________________________________________________________________________ _______________________

Description

DEVICE FOR DETECTING POLARIZATION MODE DISPERSIONS
The invention is directed to a means for detecting polarization mode dispersion of an optical data signal according to the preamble of patent claim 1.
Long light waveguide transmission links are utilized in optical transmission technology. The light waveguides, as a result of manufacture, are not completely isotropic but slightly birefringent. Due to the long transmission link, a frequency-dependent polarization transformation occurs - called polarization mode dispersion or polarization dispersion, abbreviated as PMD. This, due to modification of the polarization of the optical signal as function of the optical frequency and -1 o connected therewith - different, frequency-dependent running times, leads to the spread of the transmitted pulses, as a result whereof the recognizability thereof at the reception is reduced and the transmitted data rate is limited as a result thereof.
Added thereto as a complicating factor is that the transmission behavior of the link and, thus, the PMD as well changes due to temperature modification or mechanical stressing. Adaptive PMD compensators are therefore utilized that are inserted into the transmission path. PMD distortions must be detected in the optical receiver for the drive of these compensators. The compensator, for example, can then be optimally set with a gradient algorithm.
Electronic Letters, 17 February 1994, Vol. 30, No. 4, pages 348 through 2 0 349, utilizes a band-pass filter for filtering a data signal whose PMD is to be detected.
A power detector at the filter outputs supplies a signal that is all the higher the lower the PMD distortions are.
It is disadvantageous that, given the presence of high PMD of the first order, this signal does not vary monotonously as a function of the differential group 2 5 delay DGD and unambiguous signals can therefore not be acquired.
Proceedings OEC 94, 14e-12, pages 258 through 259, Makuhari Fare, Japan 1994, employs a different method wherein the power of the difference signal between decision unit output and decision input is interpreted. This signal, however, has a lower sensitivity to PMD distortions than a suitable band filter.
Particularly given great PMD distortions wherein the DGD exceeds the bit duration, moreover, false decisions can occur, so that the acquired signal is an unsuitable criterion in such instances for the presence of PMD distortions.
The object of the invention is comprised in specifying a reliable detector for higher values of the differential group delay as well. Further, a suitable arrangement for the compensation of the polarization mode dispersion and for optimum setting of this detector is to be specified.
The object is achieved by a means for detecting polarization mode l0 dispersion according to claim 1.
A version of the solution is described in independent claim 7.
Advantageous developments of the invention are recited into subclaims.
The particular advantage of the invention is comprised in the combination of output voltages of a plurality of filters proceeding monotonously in the principle ranges employed and their great steepness, which is not possible with a single band-pass filter or a single low-pass filter. As a result thereof, a significantly more exact compensation is possible.
Compared to the employment of low-pass filters, the employment of band-pass filters has the advantage of greater steepness of the filter output voltages as a 2 0 function of existing differential group delay. As a result thereof, an even more exact/faster compensation can be implemented.
A switchable/controllable band-pass filter or a switchable/controllable low-pass filter can also be employed instead of a plurality of band-pass filters/low-pass filters.
2 5 The detection means can be supplemented by further control criteria.
Devices that evaluate intentionally generated error rates of an auxiliary data signal that is acquired from the received, optical signal are thereby especially advantageous.
An especially simple circuit can be realized by a controllable sampling threshold in the evaluation of the data signal.

Exemplary embodiments of the invention are described with reference to Figures.
Shown are:
Figure 1 the normed curve of the filter output voltages;
Figure 2 an exemplary embodiment of the invention with three band-pass filters;
Figure 3 a further exemplary embodiment with a controllable band-pass filter;
Figure 4 a further exemplary embodiment with additional interpretation of an auxiliary data signal; and Figure 5 a fiirther version of this exemplary embodiment.
Figure 1 shows the normed curve of the filter output voltages U1 through U3 of three band-pass filters whose center frequency [sicJ amount to 0.125/T, 0.25/T
and 0.5/T, whereby T is the bit duration of the transmitted data signal.
Moreover, the output voltage U (LPF) of a low-pass filter having the limit frequency 0.125/T
is entered dependent on the normed differential group delay DGD/T given equally pronounced excitation of both principal polarizations. (Those two mutually orthogonal polarizations that do not change in a first approximation given change of the optical frequency are referred to below as principal polarizations or "principal states of polarization", referenced below as PSP. The principal polarizations coincide with the principal axes in polarization-preserving light waveguides, i.e. are horizontal 2 o and vertical. In general, the principal polarizations, however, are arbitrary, orthogonal pairs of elliptical polarizations. The principal polarizations have different group delays whose difference is referred to as "differential group delay", referred to below as DGD or differential group running time. When an optical signal having a principal polarization is transmitted, then no pulse spread occurs in approximation of the first 2 5 order. When it is transmitted with a polarization that, given division according to the principal polarizations, corresponds to equal power parts thereat, maximum pulse spread occurs because two equally intense pulses having delay differences of the magnitude DGD are superimposed.

When the principal polarizations change as a function of the optical frequency, then, given input-side employment of a principal polarization that corresponds to a specific frequency, the output polarization [sic] nonetheless change as function of the frequency, but only in a higher order than the first order. This is referred to as PMD
of a higher order. In general, PMD of a higher order occurs, whereby, however, PMD
of the first order dominates due to its effects and must therefore be compensated with priority.) As can be seen, the output signal U3 enables an error-free detection of the PMD only up to a value of the DGD of 1T because the slope of the function changes the operational sign for values between 1T and 2T. The analogous case applies to the output voltages of the other band-pass filters and also to those of the low-pass filter to a lesser extent.
Figure 2 shows the employment of the means for detecting PMD in a compensator. An optical transmitter TR sends an optical signal OS via a light waveguide LWL to an optical receiver RX. The latter has a photodiode PD in order [...] conversion of the optical signal into an electrical signal. A following decision unit DFF outputs the transmitted data signal DS at the output OD.
The photodiode is preceded by a polarization mode transformer C for compensation of the polarization mode dispersion, the input IN thereof being identical 2 0 to the receiver input.
The control criterion for the polarization mode transformer C is acquired from the baseband signal BB output by the photodiode. This is supplied to a plurality of filters FE1 through FE3 whose outputs are respectively followed by a power measuring means DET 1 through DET3. As a result of smoothing capacitors or 2 5 similar devices, these power measuring means also have a smoothing or low-pass function. The band-pass filters advantageously comprise center frequencies of 0.125/T, 0.25/T and 0.5/T. The bandwidths amount to approximately 0.0001 times through 0.2 times the respective center frequency. Given a low bandwidth of a band-pass filter, smoothing can be largely foregone in the power measuring means through DET3 during the course of the power measurement.
Details such as amplifiers have not been shown for reasons of clarity.
In order to graphically explain the setting of the compensator, the initial 5 presence of a great differential delay is best assumed. First, the output voltage U1 of the band-pass filter FI1 (that is measured by the power measuring means) having the lowest center frequency 0.125/T is employed for optimizing the compensator setting, being employed by a microprocessor (with A/D and D/A converter) employed as regulator MP. As soon as this signal upwardly transgresses a threshold SO (an upper threshold in the Figure), the output signal of the band-pass filter FI2 having the next-higher center frequency 0.25/T is employed for optimizing the output signal.
When this also supplies too strong an output signal that exceeds the threshold (or a different threshold selected in conformity with the embodiment), a switch is made to the band-pass filter having the highest center frequency 0.5/T. Although this has the smallest monotony range of the output signal, the co-evaluation of the output signals of the other band-pass filters assures that it supplies output signals in the first monotony range 0 <_ DGD s T. Its high sensitivity can therefore be especially advantageously utilized for the compensation of the PMD distortions. The monotony ranges that are employed are entered with solid lines in Figure 1 as principal again values.
2 o In order to achieve an optimum bit error rate, a non-linear or linear combination of the band-pass filter output signals or, respectively, of the output signals of the following power detectors can also be undertaken. To this end, the output signal or signals of the lower-frequency signals are simply co-employed instead of the filter output signals selected as function of the output signals of the 2 5 lower-frequency band-pass filters: insofar as the output signal of DET 1 has not exceeded its threshold, only this is employed. When the threshold has been exceeded, then the output signal of DET2 is also added. When, finally, this threshold has also been exceeded, the output signal of DET3 is added.

For measuring purposes, measuring devices can be directly connected to the outputs of the detectors DETI through DET3, one measuring device, MG3 thereof being shown in Figure 2.
Figure 3 shows a version of the detection means wherein the three band-s pass filters are replaced by a single switchable/controllable band-pass filter FIU. The procedure in the compensation remains the same. The microprocessor MP employed as regulator respectively takes note of the preceding output voltages, so that an allocation of the principal values (monotony ranges) of the filters with higher center frequencies is unambiguously possible. The setting of the filter ensues with a control l0 signal ST.
Figure 4 shows a further version wherein a second decision unit DFF2 is employed, this being likewise supplied with the baseband signal BB. In this exemplary embodiment, the threshold of the decision unit is adjustable to such an extent via a setting means EG that this already supplies an error-effected auxiliary 15 data signal DH when the first decision unit DFF still outputs an essentially error-free data signal DS. The output signals are compared to one another in an exclusive-OR
gate EXOR, and the error signal FS acquired in this way is likewise employed by the microprocessor MP for controlling the polarization mode transformer C. By shifting the threshold of the second decision unit, a criterion is constantly developed for how 2 0 good the signal quality in view of an obtainable bit error rate. The signal quality is all the better the lower the error rate of the auxiliary data signal is given a shift of the threshold from the optimum. A maximum output of the switchable/controllable filter FILJ and a minimum error rate will roughly coincide. A more exact evaluation, which leads to a lower bit error rate of the decision unit DFF, in contrast, derives given 2 5 employment of the error signal FS. Since deviations of the auxiliary data signal DH
from the data signal DS, however, occur stochastically, a relatively long measuring or averaging time for the error signal FS is required in order to acquire an especially good signal-to-noise ratio and, thus, an optimum compensation. The additional information acquired with the assistance of the second decision unit is utilized for optimizing the filter FILJ, i.e. for modifying its transfer function. This adaptive operation seems especially beneficial in order to make it possible to compensate unit scatters, temperature fluctuations, the occurrence of non-linear effects, etc.
The great advantage of these embodiments is comprised therein that a fast compensation is already possible on the basis of the filter output signal, and adequate time is available for the fine adjustment and the setting of the transfer function of the filter.
Particularly in instances wherein a fast setting of the polarization mode transformer C is not a matter of concern, however, the employment of only one error signal FS is also possible, so that the filter FIL1 and the power detector DET1 in Figure 4 can be eliminated.
Given employment of a plurality of band-pass filters, as shown in Figure 5, the transfer functions of the filters or the weightings of the individual filter output signals can be modified such that the lowest PMD distortions occur. Since this can ensue slowly, whereas the filter output signals and their combination are quickly available, the same advantages as in the exemplary embodiment of Figure 4 derive as a result of this adaptive operation.
Fundamentally, the control of the polarization mode transformer can also ensue with the error signal.

Claims

1. A method for reversion of a fault in an active peripheral assembly (LIC1, LIC2) of a switching device (EXC) in a communications system, in particular in an ATM
(Asynchronous Transfer Mode) communications system, in which at least one signaled communications link is switched via the active peripheral assembly (LIC1, LIC2), and in which connection data for the communications link are stored in the active peripheral assembly (LIC1, LIC2) in order to handle the communications link, and in which case, after the occurrence of the fault, the connection data which are stored in a redundant manner in memory devices (RHS) which are central for a number of peripheral assemblies (LIC1, LIC2) are transmitted to the active peripheral assembly (LIC1, LIC2), characterized in that the transmission of the connection data is interrupted or is started at a later time, in order to allow the setting up of new communications links.

2. The method as claimed in claim 1, characterized in that the connection data are stored in the memory device (RHS), which is central for a number of peripheral assemblies (LIC1, LIC2), before the occurrence of the fault.

3. The method as claimed in claim 1 or 2, characterized in that a redundant passive peripheral assembly (LIC2), in which the connection data are stored in a redundant manner, is provided for the active peripheral assembly (LIC1).

4. The method as claimed in one of claims 1 to 3, in which a fault occurs in the software of the active peripheral assembly (LIC2), characterized in that the active peripheral assembly (LIC2) is still active after the occurrence of the fault.

5. The method as claimed in one of claims 1 to 3, characterized in that, after the occurrence of the fault, the previously active peripheral assembly (LIC1) becomes passive, and a redundant assembly is used as the active peripheral assembly (LIC2), to which the connection data which are stored in a redundant manner elsewhere are transmitted.

6. The method as claimed in claim 4 or 5, characterized in that the connection data to be transmitted remain stored at the other location.

7. The method as claimed in one of claims 1 to 6, characterized in that the connection data to be transmitted are transmitted in blocks to the active peripheral assembly (LIC2).

8. The method as claimed in one of claims 1 to 7, characterized in that, after the at least partial transmission of the connection data, hardware settings which already exist in the active peripheral assembly (LIC2) are checked on the basis of the received connection data, and are corrected if necessary.

9. A switching device (EXC) for a communications system, in particular for an ATM communications system, having a central control unit (MP) for controlling a number of associated peripheral assemblies (LIC1, LIC2) via which communications links can be switched, in which case the central control unit (MP) has a data memory in which connection data for signaled communications links which are switched via the associated peripheral assemblies (LIC1, LTC2) can be stored, and wherein a transmission unit (RHS) is provided for reading and transmitting the connection data to the associated peripheral assemblies (LIC, LIC2), characterized in that a connection manager (COH) in the associated peripheral assemblies (LIC1, LIC2) interrupts the transmission of the connection data, or starts such transmission at a later time, in order to allow new communications links to be set up.