CA2338343A1 - System for minimising or compensating pmd-induced distortions in optical transmission systems and transmision fibres in particular - Google Patents
System for minimising or compensating pmd-induced distortions in optical transmission systems and transmision fibres in particular Download PDFInfo
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- CA2338343A1 CA2338343A1 CA002338343A CA2338343A CA2338343A1 CA 2338343 A1 CA2338343 A1 CA 2338343A1 CA 002338343 A CA002338343 A CA 002338343A CA 2338343 A CA2338343 A CA 2338343A CA 2338343 A1 CA2338343 A1 CA 2338343A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/25—Arrangements specific to fibre transmission
- H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
- H04B10/2569—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to polarisation mode dispersion [PMD]
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
- G01M11/30—Testing of optical devices, constituted by fibre optics or optical waveguides
- G01M11/33—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face
- G01M11/336—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face by measuring polarization mode dispersion [PMD]
Abstract
What is described here is a system for compensating distortions induced by polari-sation modulation dispersion (PMD) in optical transmission systems and in transmis-sion fibres in particular, comprising - a means for measuring PMD-induced distortions, - an emulator unit for adjustable PMD levels, and - a controller which the output signal of said measuring means is applied to and which serves to control said emulator unit.
In accordance with the invention both the emulator unit and the measuring means are improved for the PMD-induced distortions as well as the controller and the ap-plied control criterion (alone or in combination).
In accordance with the invention both the emulator unit and the measuring means are improved for the PMD-induced distortions as well as the controller and the ap-plied control criterion (alone or in combination).
Description
PCTlDE 00/03809 System for Minimising or Compensating PMD-induced Distortions in Optical Trans mission Systems and Transmission Fibres in particular DESCRIPTION
Field of the invention The invention relates to a system for minimising or compensating distortions due to polarisation modulation dispersion (PMD) in optical transmission systems and trans-mission fibres in particular.
Prior Art As any glass fibre is unintentionally birefringent to a slight extent light signals of dif ferent polarisations are passed through the glass fibre with different group rates. The light fractions of different polarisations therefore arrive at the receiver with transit times relative to each other; this transit time effect results in a widening of the re-ceived signal and hence in an impairment of the quality in transmission. This may lead to an increase of the bit error rate in particular. The useful transmission rate in optical fibre communication networks is hence restricted by PMD-induced distortions.
Due to the PMD of the transmission path, which varies in the course of time, high bit error rates and temporary breakdown of the transmission may occur. The existing PMD of the path is a restricting factor especially for the improvement of fibre paths already installed.
The polarisation mode dispersion encompasses all polarisation-dependent transit time effects where the signal propagation can be described completely by the propa-gating characteristics of two polarisation modes which are independent from each other and orthogonal relative to each other. With birefringence being permanently varied by outside influences such as temperature and mechanical load, and being moreover dependent on the w<~velength, both the position of the "principal states of - 2 - PCTlDE 00/03809 polarisation" (which will be briefly referred to as PSP in the following) and the differ-ence in transit time between the PSPs undergo a permanent variation. This is also referred to as second-order polarisation mode dispersion (PMD).
A wavelength-dependent PMD behaviour with time constants in the ms range up to the range of minutes results fram the aforementioned effects, which fluctuates versus time.
Distortions in transmission systems, which are created by polarisation mode disper-sion (PMD), must be compensated for high-rate data transmission applications in or-der to maintain the signal quality.
Prior Art The influence of polarisation mode dispersion along long high-rate transmission paths have been intensively studied and measured in the past few years.
In this respect reference is made to the following articles which - like the other arti-cles additionally mentioned in the following, too - are explicitly referred to for expla-nation of all the particulars not explained here in more details:
- Poole, C.D.; Tkach, R.WV.; Chraplyvy, A. R.;
Fishman, D.A.:
Fading in lightwave systems due to polarization-mode dispersion IEEE Photoonics Technology Letters, vol. 3, No. 1, 1991, pp. 68 -70 - Clesca, B; Thiery, J.-P.; Pierre, V.; Havard, V.; Bruy~re, F.:
Impact of polarisation mode dispersion on 10 Gbit/s terrestrial systems over non-dispersion-shifted fibre Electronics Letters, vol. 31, NO. 18, 1995, pp. 1594-1596 Moreover, the effects of second-order PMD and also of polarisation-dependent loss (PDL) have been analysed:
- Bruyere, F.:
Impact of First- and Second-Order PMD in Optical Digital Transmission Sys-tems Optical Fiber Technology 2 (1996), Article 33, pp. 269 - 280 - Gisin, N.; Huttner, B.:
Combined effects of polarization mode dispersion dependent losses in optical - 3 - PCTlDE 00/03809 fibers Optics Communications 142 (1997), pp. 119 -125 Fairly old fibres in particular, which had been installed in the first years of optical fibre transmission present a high F'MD level. For paths to be installed in the future an up-per limit of 0.5 ps~lkm applies. Even though the fibre manufacturers take any effort to offer values lower than this maximum, the influence of such comparatively small PMDs is troublesome in the case of high transmission rates and long distances.
The effects of other dispersive phenomena such as chromatic dispersion may be pushed into the background by a suitable selection of the wave length or by means of fibres compensated in terms of dispersion.
The only factor which involves a restriction of the band width and the length of the distance is hence PMD.
On account of the occurrence of PMD, which is invariant in terms of time, compen-sation is not possible by the use of a constant-PMD fibre. Various simulations have become known - cf. in this respect Ozeki, T.; Kudo, T.:
Adaptive equalization of polarization-mode dispersion OFC/IOOC 1993, Technical Digest, pp. 143 - 144 and laboratory experiments - cf. in this context Hakki, B. W.:
Polarization Mode Dispersion Compensation by Phase Diversity Detection IEEE Photoonics Technology Letters, vol. 9, No. 1, 1997, pp. 121 - 123 have become known in relation to the wide-band and flexible design of a PMD
com-pensator. These publications, however, refer to laboratory set-ups which are not suitable for application in pracaice.
From prior art literature various approaches have become known for PMD compen-sation, with provisions on the receiver side being promising only in view of their im-plementation. These approaches include:
Field of the invention The invention relates to a system for minimising or compensating distortions due to polarisation modulation dispersion (PMD) in optical transmission systems and trans-mission fibres in particular.
Prior Art As any glass fibre is unintentionally birefringent to a slight extent light signals of dif ferent polarisations are passed through the glass fibre with different group rates. The light fractions of different polarisations therefore arrive at the receiver with transit times relative to each other; this transit time effect results in a widening of the re-ceived signal and hence in an impairment of the quality in transmission. This may lead to an increase of the bit error rate in particular. The useful transmission rate in optical fibre communication networks is hence restricted by PMD-induced distortions.
Due to the PMD of the transmission path, which varies in the course of time, high bit error rates and temporary breakdown of the transmission may occur. The existing PMD of the path is a restricting factor especially for the improvement of fibre paths already installed.
The polarisation mode dispersion encompasses all polarisation-dependent transit time effects where the signal propagation can be described completely by the propa-gating characteristics of two polarisation modes which are independent from each other and orthogonal relative to each other. With birefringence being permanently varied by outside influences such as temperature and mechanical load, and being moreover dependent on the w<~velength, both the position of the "principal states of - 2 - PCTlDE 00/03809 polarisation" (which will be briefly referred to as PSP in the following) and the differ-ence in transit time between the PSPs undergo a permanent variation. This is also referred to as second-order polarisation mode dispersion (PMD).
A wavelength-dependent PMD behaviour with time constants in the ms range up to the range of minutes results fram the aforementioned effects, which fluctuates versus time.
Distortions in transmission systems, which are created by polarisation mode disper-sion (PMD), must be compensated for high-rate data transmission applications in or-der to maintain the signal quality.
Prior Art The influence of polarisation mode dispersion along long high-rate transmission paths have been intensively studied and measured in the past few years.
In this respect reference is made to the following articles which - like the other arti-cles additionally mentioned in the following, too - are explicitly referred to for expla-nation of all the particulars not explained here in more details:
- Poole, C.D.; Tkach, R.WV.; Chraplyvy, A. R.;
Fishman, D.A.:
Fading in lightwave systems due to polarization-mode dispersion IEEE Photoonics Technology Letters, vol. 3, No. 1, 1991, pp. 68 -70 - Clesca, B; Thiery, J.-P.; Pierre, V.; Havard, V.; Bruy~re, F.:
Impact of polarisation mode dispersion on 10 Gbit/s terrestrial systems over non-dispersion-shifted fibre Electronics Letters, vol. 31, NO. 18, 1995, pp. 1594-1596 Moreover, the effects of second-order PMD and also of polarisation-dependent loss (PDL) have been analysed:
- Bruyere, F.:
Impact of First- and Second-Order PMD in Optical Digital Transmission Sys-tems Optical Fiber Technology 2 (1996), Article 33, pp. 269 - 280 - Gisin, N.; Huttner, B.:
Combined effects of polarization mode dispersion dependent losses in optical - 3 - PCTlDE 00/03809 fibers Optics Communications 142 (1997), pp. 119 -125 Fairly old fibres in particular, which had been installed in the first years of optical fibre transmission present a high F'MD level. For paths to be installed in the future an up-per limit of 0.5 ps~lkm applies. Even though the fibre manufacturers take any effort to offer values lower than this maximum, the influence of such comparatively small PMDs is troublesome in the case of high transmission rates and long distances.
The effects of other dispersive phenomena such as chromatic dispersion may be pushed into the background by a suitable selection of the wave length or by means of fibres compensated in terms of dispersion.
The only factor which involves a restriction of the band width and the length of the distance is hence PMD.
On account of the occurrence of PMD, which is invariant in terms of time, compen-sation is not possible by the use of a constant-PMD fibre. Various simulations have become known - cf. in this respect Ozeki, T.; Kudo, T.:
Adaptive equalization of polarization-mode dispersion OFC/IOOC 1993, Technical Digest, pp. 143 - 144 and laboratory experiments - cf. in this context Hakki, B. W.:
Polarization Mode Dispersion Compensation by Phase Diversity Detection IEEE Photoonics Technology Letters, vol. 9, No. 1, 1997, pp. 121 - 123 have become known in relation to the wide-band and flexible design of a PMD
com-pensator. These publications, however, refer to laboratory set-ups which are not suitable for application in pracaice.
From prior art literature various approaches have become known for PMD compen-sation, with provisions on the receiver side being promising only in view of their im-plementation. These approaches include:
4 ' PCTlDE 00/03809 the variation of PSP of the fibre path by a polarisation regulator on the re-ceiver side in such a way that the polarisation of the transmitter laser will coin-cide with a PSP, the application of a polarisation diversity receiver with a series-connected polarisation regulator that separates the signals of high-speed and low-speed PSP from each other and joins them again at the output of an electrical time-lag line, the application of a birefringent fibre of constant PMD and a series-connected polarisation regulator.
It is moreover known to use a high-speed electronic system for implementation of electronic PMD equalisation and a mechanical adjustable time-lag device for PMD
compensation.
The aforementioned proposal;> are either incomplete because the manner of selec-tive control is not clarified, or they involve a high expenditure in terms of optical and electrical devices, or they do not function properly. Products developed to be mar-ketable have so far not become known worldwide.
One reason for this resides firstly in the aspect that in the past means have not been available for measuring PMD-induced distortions, which are sufficiently rapid and present a sufficiently simple design.
Another reason for this is the fact that an emulator unit has not been available which is capable of emulating the PMD of a real transmission fibre as precisely as possible.
Brief outline of the problem ~of the invention Typical demands on a PMD compensator for optical transmission paths are as fol-lows:
a wide range suitable far compensation: e.g. 0 to 100 ps, - thorough control down to the lowest possible residual PMD, - high-speed thorough control in the case of variations along the fibre path, - reliable control characteristics for any kind of PMD and for PMD with different PSP levels in particular, - 5 - PCTlDE 00/03809 no persistence of control in local minimums, low insertion attenuation, low variance of the insertion attenuation.
Brief description of the invention The present invention is based on the problem of providing a system for minimising or compensation PMD-induced distortions in optical transmission systems and in transmission fibres in particular, that permits a high-speed compensation of PMD-in-duced distortions in a form appropriate for practical application -particularly in view of the afore-defined demands..
Inventive solutions to this problem are defined in the independent Patent Claims. Im-provements of these solutions are the subject matters of the dependent Claims.
A system suitable to minimise or compensate PMD-induced distortions must include a means for measuring the PMD-induced distortions. Moreover, (at least) one emu-lator unit must be provided for adjustable PMD-values, as well as at least one matching element or a polarisation transformer element, respectively, if necessary, which matches the PSPs of the signals leaving a transmission system with the PSPs of the PMD emulator unit.
In accordance with the present invention both the emulator unit and the means for measuring the PMD-induced distortions, as well as the controller and the employed control criterion (alone or in combination) are improved.
In the emulator unit presenting the inventive design an optically operating variable PMD time-lag element is used. This element is preferably composed of two disper-sive constant-PMD elements which are connected via a polarisation regulator to a variable PMD element.
By insertion of a variable automatic PMD compensator between the transmission path and the optical receiver an optical compensation of the PMD-induced distortions is successfully achieved so that the bit error rate will be minimised. The transmission - 6 - PCTlDE 00/03809 capacity of the path and the maximum distance that can be covered can hence be multiplied by application of this PMD compensator.
In a preferred solution of the invention, which relates to the emulator unit, this emu-lating unit comprises a PMD emulator which is capable of emulating also the second-order PMD and of emulating the PMD of a real transmission fibre as precisely as possible. The emulator unit of the inventive design presents the particular advantage that a series-connected polarisation transformation element is not required.
It is, of course, also possible - even though not necessary - to connect the fibre path to be compensated to the adjustable PMD time-lag element via a further polarisation regulator that operates continuously and causes the principal states of polarization (PSP) of both PMD elements to coincide.
The aforementioned preferred improvement of the invention starts out from a system for compensating distortions induced in optical transmission systems, and transmis-sion fibres in particular, by polarisation modulation dispersion (PMD), which system comprises a means for measuring PMD-induced distortions, an emulator unit for ad-justable PMD values and a controller which the output signal of the measuring means is applied to and which controls the emulator unit.
In accordance with the present invention, this emulator unit comprises at least one basic emulator unit composecl of two differential group delay elements (DGD
ele-ments) having each a defined invariable time-lag period for the incoming signal, which elements are interconnected via a connecting element producing the effect of a transformation element, with all three elements forming a defined angle of the bire-fringence axes relative to each other.
The birefringence axes of the connecting element are distinguished from the birefrin-gence axes of the two DGD elements in terms of their angular position.
Moreover, at least one regulator element is provided for each basic emulator unit, which produces its effects on one of the elements of this basic emulator unit and preferably on the connecting element in such a way that the overall system can be completely ad-PCTlDE 00/03809 justed by a slight variation of the time-lag generated by the influenced element of the DGD.
The most different elements known from prior art may be used as dispersive ele-ments and specifically as DGD elements, which may be employed in the inventive system.
A multitude of polarisation regulator variants is available for use as polarisation regulators determining the essential parameters of the overall system, such as the response time, the insertion attenuation and the long service life:
- rotatable hI2 and h/4 wave plates in the free path of the rays, - fibre squeezers, force E>roduced on highly birefringent fibres, lithium niobate or other electrically controllable birefringent crystals, - magneto-optical YIG crystals, - nematic or ferroelectric liquid crystals.
The aforementioned elements may be integrated into fibre-optical systems by appro-priate fibre coupling systems.
The elements may be PM fibres in particular. In such a case the regulator element may produce mechanical effects on at least one of the DGD elements, expediently the connecting element, for variation of the time-lag interval and hence the polarisa-tion. In particular, the regulator element or elements, which produce a mechanical action, may be fibre squeezers or stretchers with electrically controllable elements such as piezo elements creating a mechanical action on the PM fibre.
The implementation of the different angles of the birefringence axes may be expedi-ently realised by splicing of the individual PM fibres at the desired angle in the case of PM fibres.
It is particularly preferred in such a case that at least one of the regulator elements comprises a ring for distributing the mechanical effect over the longest fibre length possible, on which ring the PM fibre is wound without being twisted. It is moreover expedient that at least one pressurising element creates a pressure on a plurality of 8 ' PCTlDE 00/03809 fibre segments of the wound fibre at least at one site. This pressurising element may be an elongating element such as a piezo element or a magnetostrictive element which acts upon at least one segment of a circle that bears against the ring.
In such a configuration it is preferable to provide counter-segments relative to at least one part of the circle segments, which bear against the fibre segments and create a pressure on the fibre.
As an alternative and/or additionally to the application of PM fibres it is possible that the elements are birefringent crystals adapted to be electronically influenced in terms of birefringence or that they are one of the other aforementioned elements.
In any case it is preferred that the time-lag interval created by the two DGD
elements of each basic emulator unit is equal to and distinctly longer than the delay created by the associated connecting element.
It is furthermore advantageous to select the angles of the birefringence axes of the first DGD elements to be 0° and of the second DGD element to be 90° and that of the connecting element to be ~45°, which means a 0°, 45=, 90° system, or alterna-tively a 0°, 45°, 0° or a 90°, 45°, 0° system, or in any other appropriate manner.
In one embodiment of the invention another element is provided in series at the input side of the two DGD elements and the connecting element for adjusting an optional input PSP level, which may comprise a further birefringent element such as a PM fi-bre in particular. The angles of the birefringence axes of the series-connected ele-ment and the first DGD element are necessarily different from each other. The an-gular difference corresponds preferably to 45°. In the case of a PM
fibre input PSP
may be adjusted particularly by creating a mechanical action on the series-con-nected element or on the series-connected element and the first DGD element.
The series-connected element and/or the connecting element may consist of two PM
fibres or two birefringent crystals presenting different angular positions of their bire-fringence axes, preferably different by 90 ° relative to each other, with the regulator element acting upon one of the two fibres or on one of the crystals in particular.
9 - PCTlDE 00/03809 In order to be able to compensate also higher-order PMD levels it is preferred that at least two systems be connected in tandem for adjustment of a variable DGD, whereof at least one comprises a basic emulator unit, if necessary in combination with a PSP adjusting element. In such a system it is advantageous to provide the in-dividual systems of higher-order PMD compensation in a way that they are com-posed of basic emulator units including DGD elements providing different time lags.
In accordance with the present invention the measuring means is so configured that for detection of the PMD distortion it detects the polarisation of all spectral fractions contained in the signal output by the emulator unit. To this end the polarisation measuring means may consist of any polarimeter; for example it is possible to em-ploy a system consisting of at least three photodiodes for detecting the Stokes pa-rameters.
Within the scope of the present invention the simplest possible system is preferred which consists, for instance, of a polarizes and an opto-electronic converter such as a photo receiver, that is series-connected to the output side of the polarizes.
As an alternative it is possible that the measuring means includes a polarisation beam splitter with opto-electrical converters such as photo receivers connected to the output terminals of the beam splitter, which output signals are subjected to quo-tient formation for generating an actual signal for the controller.
Ahead of the polarisation measuring means, a polarisation matching unit may be provided which matches the output polarisation of the emulator unit to that of the polarizes and sets the polarisation for instance in a way that control may aim at a power minimum at the output side of the polarizes.
The polarisation matching unit may be arranged optionally either directly on the po-larisation measuring means or directly downstream of the PMD emulator unit and still ahead of the branching coupler leading to the polarisation measuring unit.
The polarisation matching unit: may, for instance, comprise two birefringent elements having birefringence axes forming an angle different from 0°, preferably 45°; for ad-- 10 - PCTlDE 00/03809 justment of the output polarisation at least one regulator element may be provided which acts upon at least one of the birefringent elements. These elements may be birefringent crystals or PM fibres.
The signal for readjustment of the PMD compensator may be derived from the de-tected signal of the optical receiver directly via electrical filters. Two different pass characteristics of the filters enable a valuation of the detected signal in terms of oc-currence of distortions independently of the signal power. A control algorithm opti-mises the polarisation elements of the PMD compensator so that the detected signal of the receiver presents the lowest PMD distortions.
It is particularly preferred that a system for distortions induced by polarisation modu-lation dispersion (PMD) in optical transmission systems and in transmission fibres in particular is so improved that the controller includes several automatic-control loops in which it modulates regulator elements of the emulator unit with different frequen-cies - in a form resembling the dither technique - such that the controller detects in-formation about the amount and the phase position of the signal output from the emulator unit on the basis of the output signal of the measuring unit, and uses this information for performing a rapid and direct control, and that the controller sets the individual control loops in such a manner that the polarisation will be constant for all spectral fractions contained in the signal.
In such a configuration it is preferred that the controller uses a minimum photo cur-rent of the opto-electrical converters) as control criterion for setting a constant po-larisation for all spectral fractions contained in the signal. In this context the controller is capable of evaluating the output signal of the opto-electrical converters) selec-tively in terms of frequency and phase.
To achieve a particularly high control speed it is expedient that the controller com-prises analog control circuits for the regulator elements to which the frequency-se-lective and phase-selective signals are applied by application of the dither technique.
- 11 - PCTlDE 00/03809 Moreover, the controller may also control the regulator elements of the polarisation matching unit, particularly with the same control algorithm as that employed for the emulator unit.
It is furthermore possible that the controller comprises a CPU or at least one DSP
switching circuit for performing various functions such as for frequency-selective and phase-selective evaluation or for control of the sequence of operations within the system.
In any case, however, the regulated values are set or controlled in a manner that they are defined on the basis of the employed principle of measurement, so that control based on the trial-and-error principle rnay be omitted.
Due to this inventive configuration it is possible, inter alia, to desist from the applica-tion of reset algorithms.
It is particularly expedient within the scope of the present invention - also in the sense of an independent solution - to use elements producing a mechanical effect.
These elements may be fibre squeezers or stretchers with electrically controllable elements, such as piezo elements, in particular, which produce a mechanical action on the fibre.
When elements producing a rnechanical action are employed it is particularly expe-dient to provide elements having a ring for distribution of the mechanical action over the longest fibre length possible, onto which ring the fibre is wound without being twisted. With this provision, due to the long effective fibre distance, it is possible to operate with comparatively low pressures. Hence fibres may be used which present a standard coating, without a reduction of the service life of the fibre in practical ap-plication. In all other cases it would be necessary to use a particularly hard coating so as to avoid a reduction of the service life beyond a reasonable measure.
In another preferred embodiment at least one pressurising element is provided which exerts pressure on a plurality of fibre segments of the wound fibre at least at one site. This pressurising element may be an elongating element in particular, such as a PCTlDE 00/03809 piezo element that acts upon at least one circle segment of the wound fibres and that bears against the ring. In correspondence with the segments of the circle counter-segments are provided which bear against the fibre segments and exert a pressure on the fibre. This configuration presents the advantage that pressurisation of the fi-bre is achieved without "stretching" the fibre. It is expedient in this configuration to design it in a way that thermal influence will not be produced on the DGD
element.
As the control criterion is preferably derived in an optical manner according to the in-vention, i.e. not after opto-electronic conversion, the following advantages are achieved in this case:
(a) The PMD compensator system is independent of the bit rate of the data signal (10 GBit or higher).
(b) The PMD compensator system is independent of signal coding (RZ, NRZ, etc.).
(c) The maximum DGD level to be compensated is not limited, as is the case in conventional systems where the limit ranges at 100 ps for 10 Gbit or 25 ps at 40 Gbit, respectively.
(d) Due to the optical signal processing it is possible to employ low-cost opto-electronic converters with a low limiting frequency (in the kHz range rather than in the GHz range as is common in prior art).
Independently of the derivation of the control criterion the following further advan-tages are achieved:
(a) high-speed compensation (b) low insertion attenuation (c) simple and low-cost structure (d) a rugged structure (e) trial-and-error control is not required.
When, in accordance with the present invention, the modulation of the regulator ele-ments is performed with differE~nt frequencies, the further advantages are also achieved:
- 13 - PCTlDE 00/03809 (a) a reset algorithm is not necessary (b) trial-and-error control is not required, and (c) expensive signal processors are not necessary.
Brief description of the drawing The invention will now be described in more details by exemplary embodiments with reference to the drawing wherein:
Fig. 1 shows the principle of the structure of a basic emulator unit designed in accordance with the invention;
Fig. 2 illustrates an improvement of the emulator unit shown in Fig. 1;
Fig. 3 illustrates a first embodiment, and Fig. 4 shows a second embodiment of an inventive system for minimising or compensation of distortions induced by polarisation modulation dispersion (PMD);
Fig. 5 illustrates one example of a rotator used as polarisation regulator;
Fig. 6 shows an example of a polarisation regulator for PSP matching, and Fig. 7 is a view of an example of a fibre squeezer.
Description of embodiments Fig. 1 shows the structure of a.n inventive basic emulator unit. This unit comprises two DGD elements (differential group delay elements) DGD-1 and DGD-2 which pre-sent each a defined invariable time lag for the incoming signal, which amounts to 50 ps in the illustrated embodiment, without any restriction of the possible values. The two DGD elements DGD-1 and DGD-2 are interconnected via a connecting element T-DGD having a time lag of 1 ps in the illustrated embodiment.
All three elements present a defined angle of their birefringence axes, with the bire-fringence axis of the connecting element T-DGD being different in terms of its angu-lar position from the birefringence axes of the two DGD elements DGD-1 and DGD-2.
- 14 - PCTlDE 00/03809 In the illustrated embodiment the (absolute) angles amount to 0°, 45° (in the initial setting) and 90°.
In the illustrated embodiment moreover a regulator element is provided which is not shown in Fig. 1 and which acts upon the connecting element T-DGD in such a way that the DGD level of the system can be completely set by a slight variation of the time lag of this element.
It is preferable that the elements DGD-1, DGD-2 and T-DGD are PM fibres in the embodiment shown in Fig. 1. 'The angles may then be set by splicing. The regulator element may create a mechanical action upon at least one of the PM fibres for modifying the time lag and hence the polarisation; for instance it may be a fibre squeezer or stretcher with electrically controllable elements such as piezo elements.
With this arrangement it is possible to set an overall DGD level from 0 ps up to a total of the individual DGD levels (100 ps), to which end merely the DGD level of the transformation element T-DGD by 0.0025 ps is sufficient.
Fig. 2 shows a modification of the embodiment according to Fig. 1 wherein the same elements as those of Fig. 1 are identified by the same reference numerals.
In this embodiment a further element A-DGD is series-connected at the input side of the system consisting of the elements DGD-1, T-DGD and DGD-2, which further element presents an angle of ~~5° and a time lag of 1 ps in the embodiment shown here. In the illustrated embodiment the time lag of the elements DGD-1 and DGD-corresponds to 30 ps in each case, without any restriction of the general applicability.
Moreover, regulators are also provided for the element A-DGD and the element DGD-1. These regulator elements permit the matching of the PSP of the system to the respective application. The regulator element for the transformation element T-DGD serves - like in the embodiment according to Fig. 1 - to set the DGD. In distinc-tion from the system shown in Fig. 1, the system according to Fig. 2 presents the ad-vantage that the dependence of the PSP on the wave length can be compensated.
15 - PCTlDE 00/03809 Fig. 3 illustrates a system for compensating distortions which are induced by polari-sation modulation dispersion (PMD) in optical transmission systems and particularly in transmission fibres, wherein two basic emulator units 1 and 2 are employed which are connected in tandem and whereof each presents a structure corresponding to Fig. 2; these two units serve to set the PSP and DGD levels of the signal IN
which arrives from the transmission system, for instance a transmission fibre. The signal output from the second basic emulator unit 2 enters a beam splitter 3 that branches off a small fraction of the signal (1 to 5% into a means for measuring PMD-induced distortion.
This measuring means includes a polarisation controller 4 consisting of two fibre segments having each a time lag of 1 ps (in the illustrated embodiment), which seg-ments are connected to each other at an angle of 45°. These two fibre segments are pressurised for setting the polarisation in the manner to be described in the following.
The signal output from the second fibre segment enters a polarizer 4' having an am-plifier 6 with low-pass effect connected in series at the output side. The output signal of the amplifier 6 serves as input or ACTUAL signal for the controller that is used to set the time lag of the various fibre segments and which will be described in the fol-lowing.
The controller comprises a phase-sensitive amplifier 7 for each of the regulator ele-ments - which are not illustrated either in Fig. 3 - having a configuration illustrated in the partial view in Fig. 3. Each of the amplifiers 7 presents a comparatively narrow bandwidth of 2 kHz, for example, with the frequency typically ranging between and 90 kHz. The output signal of the phase-sensitive amplifier 7 is applied to the power amplifiers 8 producing an output signal for controlling the regulator elements, which may include piezo elements, for instance, as is shown in Fig. 7 in particular.
The emulator unit presenting the inventive configuration operates as follows:
The PDMC controller is composed of analog automatic-control loops independent of each other, which operate on the principle of modulated regulator elements.
The - 16 - PCTlDE 00/03809 regulator elements are controlled by an appropriate selection of the frequency (e.g.
50, 55, ... 90 kHz) for the modulation of the individual regulator elements.
The control criterion is the constancy of polarisation for all spectral fractions carried in the signal (DOP = 100 % and polarisation = constant). The polarisation at the input side of the polarizer is so set that a minimum of power will be transmitted.
This fur-nishes a very precise criterion for DOP and SOP. The modulation frequencies arrive at the photo receiver 5 with a corresponding amplitude and phase position and are available for frequency-selective evaluation in correct phase. Hence also the control circuits for the individual regulator elements may be optimised simultaneously and independently of each other.
Fig. 4 shows a second embodiment of an inventive system for minimising or com-pensating distortions induced in optical transmission systems, and specifically in a transmission fibre IN used as transmission path, which are induced by polarisation modulation dispersion (PMD); this embodiment, too, is based on the fundamental idea to compensate the PMD level of the transmission path by counter-connecting a variable PMD delay element 1. The PMD delay element 1 is connected via a variable polarising regulator 1' to the output of the fibre IN to be compensated. An optical re-ceiver 5 with an amplifier 6 is connected at the output side of the delay element 1, which is followed by a power distributor 51 that distributes the detected data signal 52 from the optical receiver 5 to filters 53 and 54 joined by detectors 55.
The output signals 55' and 55" of the detectors 55 are applied to a controller 56 that applies a control algorithm to obtain a control signal which involves a dependence on the de-gree of distortion of the data signal 52. The control signal is used to readjust the pa-rameters of the variable PMD delay element 1 and the polarisation regulator 1' in such a way that the signal distortion will be reduced to a minimum.
To this end the variable PMD delay element 1 consists of two dispersive elements 11 of the same type, which are connected, for instance, via polarisation regulator 12.
Depending on the polarisation transformation, hence the resulting PMD of this PMD
delay element 1 can be infinitely set to a value from 0 up to the total of the individual dispersion levels.
PCTlDE 00/03809 As an example, the dispersive elements 11 may be two elements with linear birefrin-gence and consist of highly birefringent fibres (= polarisation-maintaining fibres). The resulting PMD then amounts to:
(PMD 1 + PMD 2) * cos(angle of polarisation rotation).
A simple rotator such as a J~/2'. wave plate or a Faraday rotator is suitable for use as polarisation regulator. As an alternative, the same effect may be achieved by rotating the two dispersive elements relative to each other at the site of their coupling.
Fig. 5 shows an example of a rotator based on a lU2 wave plate. The light from the polarisation-maintaining fibre ~PMF 20 is subjected to collimation by a lens 21, passes through the A/2 wave plate, and is then focussed into the PMF output fibre 24 by means of a further lens 23.
The variable polarisation regulator 1 has the function of imaging the two principal states of polarisation (PSP) of the fibre to be compensated onto the PSP of the vari-able PMD delay element 1 so that the "high-speed" PSP of the fibre will coincide with the "low-speed" PSP of the delay element and the "low-speed PSP" of the fibre will coincide with the "high-speed" PSP of the delay element.
The variable polarisation regulator 1' operates continuously, which means that it does not present any direction in which there is a mechanical or polarisation-optical limitation. For this function it is not sufficient that the polarisation regulator 1' is capa-ble of converting any input polarisation into any output polarisation. The polarisation regulator 1' must therefore have sufficient degrees of freedom in order to be able to ensure a global minimisation o~f the overall PMD in all cases. When too little degrees of freedom are available there is the risk of control persisting too long in a local PMD
minimum, rather than finding the global minimum.
As an example, the variable polarisation regulator 1' according to Fig. 6 may be composed of four h/4 wave plates 32 - 35 disposed in tandem, which are freely ro-tatable. All polarisation transformation operations are infinite, which means that it is possible to realise them without a limit which were complex to circumvent. For cou-PCTlDE 00/03809 pling the light out of the. single-mode input fibre a lens 31 or a fibre collimator is re-quired, and the light is coupled into the output fibre 37 again via a lens 36 after it has passed through the four A/4 wave plates 32 - 35.
A control signal reflecting the degree of distortion of the detected data signal 52 is obtained by filtering high-frequency spectral fractions out. To this end the data signal 52 is subdivided by means of 'the power distributor 51 and supplied to different filters 53 and 54. The basic frequency amounts to 5 GHz, for example, for the transmission of a 10 Gbit/s signal.
This frequency is always present and contributes mainly to the amplitude of the sig-nal. The frequencies responsible for a high edge steepness range at multiples of the basic frequency, i.e. at 10, 15, 20 GHz or at odd-numbered multiples of the basic frequencies.
For instance, two different filters (53 + 54) are employed. Filter 53 is a band-pass fil-ter that selects the basic frequency at 5 GHz whilst filter 54 may be designed as high-pass filter for filtering out frequencies beyond 15 GHz approximately.
The two detectors 55 connected on the output side convert the signal amplitudes into two analog signals 55' and 55". The ratio between these two analog values then fur-nishes, when used as control signal, the degree of distortion of the data signal inde-pendently of the signal power. The control algorithm of the controller 56 tends to minimise the control signal, e.g. by performing slight modifications in alternation on all elements taking an influence on the polarisation.
This is possible at a very high rate so that the PMD compensation may be performed in real-time. When the modification results in a reduction of the control signal it per-sists, or else it is rejected and the next polarisation element is subjected to a varia-tion.
Fig. 7 illustrates a preferred embodiment for an element producing a mechanical ef fect on a fibre 100 for influencing the polarisation; this element may be a component of the elements A-DGD, T-DGD, DGD or 1' or 12, respectively, for instance. A
ring 121 is provided in the housing 121' for distributing the mechanical action on the PCTlDE 00/03809 longest fibre length possible, onto which ring the fibre is wound without being twisted.
What is not represented is the way in which the fibre is introduced into the ring and passed out of the ring or the housing, respectively. The ring 121 consists, for exam-ple, of a thin deformable special-steel part. A pressurising element 122, e.g.
a piezo element, is disposed in the ring (121), which is supported on two segments 123 of a circle - on one side via an equalising element 122' - which segments in their turn bear against the ring 121. On the side opposite to the circle segments 123 counter-segments 124 are provided which are supported on the housing 121' and bear against the fibre segments so that they pressurise the fibre 100 when the element 122 undergoes a correspondiing elongation. Due to the elongation of the piezo ele-ment 122 the fibre 100 can hence be selectively subjected to a mechanical load.
Even though the invention has been described in the foregoing by embodiments, without any restriction of the general concept, the most different modifications are conceivable, of course; moreover, it is not only possible to combine the various fea tures of the individual elements in the aforedescribed embodiment with each other, which are claimed as independent inventions in the claims, but it is also possible to combine individual features with embodiments for other elements such as those known from prior art.
The emulator unit provided in .correspondence with the invention may, of course, also be employed in other devices 'which are not envisaged for compensating distortions induced by polarisation modulation dispersion (PMD) in optical transmission systems and transmission fibres, in particularly, but serve merely to generate PMD-induced distortions, e.g. for test applications.
It is moreover known to use a high-speed electronic system for implementation of electronic PMD equalisation and a mechanical adjustable time-lag device for PMD
compensation.
The aforementioned proposal;> are either incomplete because the manner of selec-tive control is not clarified, or they involve a high expenditure in terms of optical and electrical devices, or they do not function properly. Products developed to be mar-ketable have so far not become known worldwide.
One reason for this resides firstly in the aspect that in the past means have not been available for measuring PMD-induced distortions, which are sufficiently rapid and present a sufficiently simple design.
Another reason for this is the fact that an emulator unit has not been available which is capable of emulating the PMD of a real transmission fibre as precisely as possible.
Brief outline of the problem ~of the invention Typical demands on a PMD compensator for optical transmission paths are as fol-lows:
a wide range suitable far compensation: e.g. 0 to 100 ps, - thorough control down to the lowest possible residual PMD, - high-speed thorough control in the case of variations along the fibre path, - reliable control characteristics for any kind of PMD and for PMD with different PSP levels in particular, - 5 - PCTlDE 00/03809 no persistence of control in local minimums, low insertion attenuation, low variance of the insertion attenuation.
Brief description of the invention The present invention is based on the problem of providing a system for minimising or compensation PMD-induced distortions in optical transmission systems and in transmission fibres in particular, that permits a high-speed compensation of PMD-in-duced distortions in a form appropriate for practical application -particularly in view of the afore-defined demands..
Inventive solutions to this problem are defined in the independent Patent Claims. Im-provements of these solutions are the subject matters of the dependent Claims.
A system suitable to minimise or compensate PMD-induced distortions must include a means for measuring the PMD-induced distortions. Moreover, (at least) one emu-lator unit must be provided for adjustable PMD-values, as well as at least one matching element or a polarisation transformer element, respectively, if necessary, which matches the PSPs of the signals leaving a transmission system with the PSPs of the PMD emulator unit.
In accordance with the present invention both the emulator unit and the means for measuring the PMD-induced distortions, as well as the controller and the employed control criterion (alone or in combination) are improved.
In the emulator unit presenting the inventive design an optically operating variable PMD time-lag element is used. This element is preferably composed of two disper-sive constant-PMD elements which are connected via a polarisation regulator to a variable PMD element.
By insertion of a variable automatic PMD compensator between the transmission path and the optical receiver an optical compensation of the PMD-induced distortions is successfully achieved so that the bit error rate will be minimised. The transmission - 6 - PCTlDE 00/03809 capacity of the path and the maximum distance that can be covered can hence be multiplied by application of this PMD compensator.
In a preferred solution of the invention, which relates to the emulator unit, this emu-lating unit comprises a PMD emulator which is capable of emulating also the second-order PMD and of emulating the PMD of a real transmission fibre as precisely as possible. The emulator unit of the inventive design presents the particular advantage that a series-connected polarisation transformation element is not required.
It is, of course, also possible - even though not necessary - to connect the fibre path to be compensated to the adjustable PMD time-lag element via a further polarisation regulator that operates continuously and causes the principal states of polarization (PSP) of both PMD elements to coincide.
The aforementioned preferred improvement of the invention starts out from a system for compensating distortions induced in optical transmission systems, and transmis-sion fibres in particular, by polarisation modulation dispersion (PMD), which system comprises a means for measuring PMD-induced distortions, an emulator unit for ad-justable PMD values and a controller which the output signal of the measuring means is applied to and which controls the emulator unit.
In accordance with the present invention, this emulator unit comprises at least one basic emulator unit composecl of two differential group delay elements (DGD
ele-ments) having each a defined invariable time-lag period for the incoming signal, which elements are interconnected via a connecting element producing the effect of a transformation element, with all three elements forming a defined angle of the bire-fringence axes relative to each other.
The birefringence axes of the connecting element are distinguished from the birefrin-gence axes of the two DGD elements in terms of their angular position.
Moreover, at least one regulator element is provided for each basic emulator unit, which produces its effects on one of the elements of this basic emulator unit and preferably on the connecting element in such a way that the overall system can be completely ad-PCTlDE 00/03809 justed by a slight variation of the time-lag generated by the influenced element of the DGD.
The most different elements known from prior art may be used as dispersive ele-ments and specifically as DGD elements, which may be employed in the inventive system.
A multitude of polarisation regulator variants is available for use as polarisation regulators determining the essential parameters of the overall system, such as the response time, the insertion attenuation and the long service life:
- rotatable hI2 and h/4 wave plates in the free path of the rays, - fibre squeezers, force E>roduced on highly birefringent fibres, lithium niobate or other electrically controllable birefringent crystals, - magneto-optical YIG crystals, - nematic or ferroelectric liquid crystals.
The aforementioned elements may be integrated into fibre-optical systems by appro-priate fibre coupling systems.
The elements may be PM fibres in particular. In such a case the regulator element may produce mechanical effects on at least one of the DGD elements, expediently the connecting element, for variation of the time-lag interval and hence the polarisa-tion. In particular, the regulator element or elements, which produce a mechanical action, may be fibre squeezers or stretchers with electrically controllable elements such as piezo elements creating a mechanical action on the PM fibre.
The implementation of the different angles of the birefringence axes may be expedi-ently realised by splicing of the individual PM fibres at the desired angle in the case of PM fibres.
It is particularly preferred in such a case that at least one of the regulator elements comprises a ring for distributing the mechanical effect over the longest fibre length possible, on which ring the PM fibre is wound without being twisted. It is moreover expedient that at least one pressurising element creates a pressure on a plurality of 8 ' PCTlDE 00/03809 fibre segments of the wound fibre at least at one site. This pressurising element may be an elongating element such as a piezo element or a magnetostrictive element which acts upon at least one segment of a circle that bears against the ring.
In such a configuration it is preferable to provide counter-segments relative to at least one part of the circle segments, which bear against the fibre segments and create a pressure on the fibre.
As an alternative and/or additionally to the application of PM fibres it is possible that the elements are birefringent crystals adapted to be electronically influenced in terms of birefringence or that they are one of the other aforementioned elements.
In any case it is preferred that the time-lag interval created by the two DGD
elements of each basic emulator unit is equal to and distinctly longer than the delay created by the associated connecting element.
It is furthermore advantageous to select the angles of the birefringence axes of the first DGD elements to be 0° and of the second DGD element to be 90° and that of the connecting element to be ~45°, which means a 0°, 45=, 90° system, or alterna-tively a 0°, 45°, 0° or a 90°, 45°, 0° system, or in any other appropriate manner.
In one embodiment of the invention another element is provided in series at the input side of the two DGD elements and the connecting element for adjusting an optional input PSP level, which may comprise a further birefringent element such as a PM fi-bre in particular. The angles of the birefringence axes of the series-connected ele-ment and the first DGD element are necessarily different from each other. The an-gular difference corresponds preferably to 45°. In the case of a PM
fibre input PSP
may be adjusted particularly by creating a mechanical action on the series-con-nected element or on the series-connected element and the first DGD element.
The series-connected element and/or the connecting element may consist of two PM
fibres or two birefringent crystals presenting different angular positions of their bire-fringence axes, preferably different by 90 ° relative to each other, with the regulator element acting upon one of the two fibres or on one of the crystals in particular.
9 - PCTlDE 00/03809 In order to be able to compensate also higher-order PMD levels it is preferred that at least two systems be connected in tandem for adjustment of a variable DGD, whereof at least one comprises a basic emulator unit, if necessary in combination with a PSP adjusting element. In such a system it is advantageous to provide the in-dividual systems of higher-order PMD compensation in a way that they are com-posed of basic emulator units including DGD elements providing different time lags.
In accordance with the present invention the measuring means is so configured that for detection of the PMD distortion it detects the polarisation of all spectral fractions contained in the signal output by the emulator unit. To this end the polarisation measuring means may consist of any polarimeter; for example it is possible to em-ploy a system consisting of at least three photodiodes for detecting the Stokes pa-rameters.
Within the scope of the present invention the simplest possible system is preferred which consists, for instance, of a polarizes and an opto-electronic converter such as a photo receiver, that is series-connected to the output side of the polarizes.
As an alternative it is possible that the measuring means includes a polarisation beam splitter with opto-electrical converters such as photo receivers connected to the output terminals of the beam splitter, which output signals are subjected to quo-tient formation for generating an actual signal for the controller.
Ahead of the polarisation measuring means, a polarisation matching unit may be provided which matches the output polarisation of the emulator unit to that of the polarizes and sets the polarisation for instance in a way that control may aim at a power minimum at the output side of the polarizes.
The polarisation matching unit may be arranged optionally either directly on the po-larisation measuring means or directly downstream of the PMD emulator unit and still ahead of the branching coupler leading to the polarisation measuring unit.
The polarisation matching unit: may, for instance, comprise two birefringent elements having birefringence axes forming an angle different from 0°, preferably 45°; for ad-- 10 - PCTlDE 00/03809 justment of the output polarisation at least one regulator element may be provided which acts upon at least one of the birefringent elements. These elements may be birefringent crystals or PM fibres.
The signal for readjustment of the PMD compensator may be derived from the de-tected signal of the optical receiver directly via electrical filters. Two different pass characteristics of the filters enable a valuation of the detected signal in terms of oc-currence of distortions independently of the signal power. A control algorithm opti-mises the polarisation elements of the PMD compensator so that the detected signal of the receiver presents the lowest PMD distortions.
It is particularly preferred that a system for distortions induced by polarisation modu-lation dispersion (PMD) in optical transmission systems and in transmission fibres in particular is so improved that the controller includes several automatic-control loops in which it modulates regulator elements of the emulator unit with different frequen-cies - in a form resembling the dither technique - such that the controller detects in-formation about the amount and the phase position of the signal output from the emulator unit on the basis of the output signal of the measuring unit, and uses this information for performing a rapid and direct control, and that the controller sets the individual control loops in such a manner that the polarisation will be constant for all spectral fractions contained in the signal.
In such a configuration it is preferred that the controller uses a minimum photo cur-rent of the opto-electrical converters) as control criterion for setting a constant po-larisation for all spectral fractions contained in the signal. In this context the controller is capable of evaluating the output signal of the opto-electrical converters) selec-tively in terms of frequency and phase.
To achieve a particularly high control speed it is expedient that the controller com-prises analog control circuits for the regulator elements to which the frequency-se-lective and phase-selective signals are applied by application of the dither technique.
- 11 - PCTlDE 00/03809 Moreover, the controller may also control the regulator elements of the polarisation matching unit, particularly with the same control algorithm as that employed for the emulator unit.
It is furthermore possible that the controller comprises a CPU or at least one DSP
switching circuit for performing various functions such as for frequency-selective and phase-selective evaluation or for control of the sequence of operations within the system.
In any case, however, the regulated values are set or controlled in a manner that they are defined on the basis of the employed principle of measurement, so that control based on the trial-and-error principle rnay be omitted.
Due to this inventive configuration it is possible, inter alia, to desist from the applica-tion of reset algorithms.
It is particularly expedient within the scope of the present invention - also in the sense of an independent solution - to use elements producing a mechanical effect.
These elements may be fibre squeezers or stretchers with electrically controllable elements, such as piezo elements, in particular, which produce a mechanical action on the fibre.
When elements producing a rnechanical action are employed it is particularly expe-dient to provide elements having a ring for distribution of the mechanical action over the longest fibre length possible, onto which ring the fibre is wound without being twisted. With this provision, due to the long effective fibre distance, it is possible to operate with comparatively low pressures. Hence fibres may be used which present a standard coating, without a reduction of the service life of the fibre in practical ap-plication. In all other cases it would be necessary to use a particularly hard coating so as to avoid a reduction of the service life beyond a reasonable measure.
In another preferred embodiment at least one pressurising element is provided which exerts pressure on a plurality of fibre segments of the wound fibre at least at one site. This pressurising element may be an elongating element in particular, such as a PCTlDE 00/03809 piezo element that acts upon at least one circle segment of the wound fibres and that bears against the ring. In correspondence with the segments of the circle counter-segments are provided which bear against the fibre segments and exert a pressure on the fibre. This configuration presents the advantage that pressurisation of the fi-bre is achieved without "stretching" the fibre. It is expedient in this configuration to design it in a way that thermal influence will not be produced on the DGD
element.
As the control criterion is preferably derived in an optical manner according to the in-vention, i.e. not after opto-electronic conversion, the following advantages are achieved in this case:
(a) The PMD compensator system is independent of the bit rate of the data signal (10 GBit or higher).
(b) The PMD compensator system is independent of signal coding (RZ, NRZ, etc.).
(c) The maximum DGD level to be compensated is not limited, as is the case in conventional systems where the limit ranges at 100 ps for 10 Gbit or 25 ps at 40 Gbit, respectively.
(d) Due to the optical signal processing it is possible to employ low-cost opto-electronic converters with a low limiting frequency (in the kHz range rather than in the GHz range as is common in prior art).
Independently of the derivation of the control criterion the following further advan-tages are achieved:
(a) high-speed compensation (b) low insertion attenuation (c) simple and low-cost structure (d) a rugged structure (e) trial-and-error control is not required.
When, in accordance with the present invention, the modulation of the regulator ele-ments is performed with differE~nt frequencies, the further advantages are also achieved:
- 13 - PCTlDE 00/03809 (a) a reset algorithm is not necessary (b) trial-and-error control is not required, and (c) expensive signal processors are not necessary.
Brief description of the drawing The invention will now be described in more details by exemplary embodiments with reference to the drawing wherein:
Fig. 1 shows the principle of the structure of a basic emulator unit designed in accordance with the invention;
Fig. 2 illustrates an improvement of the emulator unit shown in Fig. 1;
Fig. 3 illustrates a first embodiment, and Fig. 4 shows a second embodiment of an inventive system for minimising or compensation of distortions induced by polarisation modulation dispersion (PMD);
Fig. 5 illustrates one example of a rotator used as polarisation regulator;
Fig. 6 shows an example of a polarisation regulator for PSP matching, and Fig. 7 is a view of an example of a fibre squeezer.
Description of embodiments Fig. 1 shows the structure of a.n inventive basic emulator unit. This unit comprises two DGD elements (differential group delay elements) DGD-1 and DGD-2 which pre-sent each a defined invariable time lag for the incoming signal, which amounts to 50 ps in the illustrated embodiment, without any restriction of the possible values. The two DGD elements DGD-1 and DGD-2 are interconnected via a connecting element T-DGD having a time lag of 1 ps in the illustrated embodiment.
All three elements present a defined angle of their birefringence axes, with the bire-fringence axis of the connecting element T-DGD being different in terms of its angu-lar position from the birefringence axes of the two DGD elements DGD-1 and DGD-2.
- 14 - PCTlDE 00/03809 In the illustrated embodiment the (absolute) angles amount to 0°, 45° (in the initial setting) and 90°.
In the illustrated embodiment moreover a regulator element is provided which is not shown in Fig. 1 and which acts upon the connecting element T-DGD in such a way that the DGD level of the system can be completely set by a slight variation of the time lag of this element.
It is preferable that the elements DGD-1, DGD-2 and T-DGD are PM fibres in the embodiment shown in Fig. 1. 'The angles may then be set by splicing. The regulator element may create a mechanical action upon at least one of the PM fibres for modifying the time lag and hence the polarisation; for instance it may be a fibre squeezer or stretcher with electrically controllable elements such as piezo elements.
With this arrangement it is possible to set an overall DGD level from 0 ps up to a total of the individual DGD levels (100 ps), to which end merely the DGD level of the transformation element T-DGD by 0.0025 ps is sufficient.
Fig. 2 shows a modification of the embodiment according to Fig. 1 wherein the same elements as those of Fig. 1 are identified by the same reference numerals.
In this embodiment a further element A-DGD is series-connected at the input side of the system consisting of the elements DGD-1, T-DGD and DGD-2, which further element presents an angle of ~~5° and a time lag of 1 ps in the embodiment shown here. In the illustrated embodiment the time lag of the elements DGD-1 and DGD-corresponds to 30 ps in each case, without any restriction of the general applicability.
Moreover, regulators are also provided for the element A-DGD and the element DGD-1. These regulator elements permit the matching of the PSP of the system to the respective application. The regulator element for the transformation element T-DGD serves - like in the embodiment according to Fig. 1 - to set the DGD. In distinc-tion from the system shown in Fig. 1, the system according to Fig. 2 presents the ad-vantage that the dependence of the PSP on the wave length can be compensated.
15 - PCTlDE 00/03809 Fig. 3 illustrates a system for compensating distortions which are induced by polari-sation modulation dispersion (PMD) in optical transmission systems and particularly in transmission fibres, wherein two basic emulator units 1 and 2 are employed which are connected in tandem and whereof each presents a structure corresponding to Fig. 2; these two units serve to set the PSP and DGD levels of the signal IN
which arrives from the transmission system, for instance a transmission fibre. The signal output from the second basic emulator unit 2 enters a beam splitter 3 that branches off a small fraction of the signal (1 to 5% into a means for measuring PMD-induced distortion.
This measuring means includes a polarisation controller 4 consisting of two fibre segments having each a time lag of 1 ps (in the illustrated embodiment), which seg-ments are connected to each other at an angle of 45°. These two fibre segments are pressurised for setting the polarisation in the manner to be described in the following.
The signal output from the second fibre segment enters a polarizer 4' having an am-plifier 6 with low-pass effect connected in series at the output side. The output signal of the amplifier 6 serves as input or ACTUAL signal for the controller that is used to set the time lag of the various fibre segments and which will be described in the fol-lowing.
The controller comprises a phase-sensitive amplifier 7 for each of the regulator ele-ments - which are not illustrated either in Fig. 3 - having a configuration illustrated in the partial view in Fig. 3. Each of the amplifiers 7 presents a comparatively narrow bandwidth of 2 kHz, for example, with the frequency typically ranging between and 90 kHz. The output signal of the phase-sensitive amplifier 7 is applied to the power amplifiers 8 producing an output signal for controlling the regulator elements, which may include piezo elements, for instance, as is shown in Fig. 7 in particular.
The emulator unit presenting the inventive configuration operates as follows:
The PDMC controller is composed of analog automatic-control loops independent of each other, which operate on the principle of modulated regulator elements.
The - 16 - PCTlDE 00/03809 regulator elements are controlled by an appropriate selection of the frequency (e.g.
50, 55, ... 90 kHz) for the modulation of the individual regulator elements.
The control criterion is the constancy of polarisation for all spectral fractions carried in the signal (DOP = 100 % and polarisation = constant). The polarisation at the input side of the polarizer is so set that a minimum of power will be transmitted.
This fur-nishes a very precise criterion for DOP and SOP. The modulation frequencies arrive at the photo receiver 5 with a corresponding amplitude and phase position and are available for frequency-selective evaluation in correct phase. Hence also the control circuits for the individual regulator elements may be optimised simultaneously and independently of each other.
Fig. 4 shows a second embodiment of an inventive system for minimising or com-pensating distortions induced in optical transmission systems, and specifically in a transmission fibre IN used as transmission path, which are induced by polarisation modulation dispersion (PMD); this embodiment, too, is based on the fundamental idea to compensate the PMD level of the transmission path by counter-connecting a variable PMD delay element 1. The PMD delay element 1 is connected via a variable polarising regulator 1' to the output of the fibre IN to be compensated. An optical re-ceiver 5 with an amplifier 6 is connected at the output side of the delay element 1, which is followed by a power distributor 51 that distributes the detected data signal 52 from the optical receiver 5 to filters 53 and 54 joined by detectors 55.
The output signals 55' and 55" of the detectors 55 are applied to a controller 56 that applies a control algorithm to obtain a control signal which involves a dependence on the de-gree of distortion of the data signal 52. The control signal is used to readjust the pa-rameters of the variable PMD delay element 1 and the polarisation regulator 1' in such a way that the signal distortion will be reduced to a minimum.
To this end the variable PMD delay element 1 consists of two dispersive elements 11 of the same type, which are connected, for instance, via polarisation regulator 12.
Depending on the polarisation transformation, hence the resulting PMD of this PMD
delay element 1 can be infinitely set to a value from 0 up to the total of the individual dispersion levels.
PCTlDE 00/03809 As an example, the dispersive elements 11 may be two elements with linear birefrin-gence and consist of highly birefringent fibres (= polarisation-maintaining fibres). The resulting PMD then amounts to:
(PMD 1 + PMD 2) * cos(angle of polarisation rotation).
A simple rotator such as a J~/2'. wave plate or a Faraday rotator is suitable for use as polarisation regulator. As an alternative, the same effect may be achieved by rotating the two dispersive elements relative to each other at the site of their coupling.
Fig. 5 shows an example of a rotator based on a lU2 wave plate. The light from the polarisation-maintaining fibre ~PMF 20 is subjected to collimation by a lens 21, passes through the A/2 wave plate, and is then focussed into the PMF output fibre 24 by means of a further lens 23.
The variable polarisation regulator 1 has the function of imaging the two principal states of polarisation (PSP) of the fibre to be compensated onto the PSP of the vari-able PMD delay element 1 so that the "high-speed" PSP of the fibre will coincide with the "low-speed" PSP of the delay element and the "low-speed PSP" of the fibre will coincide with the "high-speed" PSP of the delay element.
The variable polarisation regulator 1' operates continuously, which means that it does not present any direction in which there is a mechanical or polarisation-optical limitation. For this function it is not sufficient that the polarisation regulator 1' is capa-ble of converting any input polarisation into any output polarisation. The polarisation regulator 1' must therefore have sufficient degrees of freedom in order to be able to ensure a global minimisation o~f the overall PMD in all cases. When too little degrees of freedom are available there is the risk of control persisting too long in a local PMD
minimum, rather than finding the global minimum.
As an example, the variable polarisation regulator 1' according to Fig. 6 may be composed of four h/4 wave plates 32 - 35 disposed in tandem, which are freely ro-tatable. All polarisation transformation operations are infinite, which means that it is possible to realise them without a limit which were complex to circumvent. For cou-PCTlDE 00/03809 pling the light out of the. single-mode input fibre a lens 31 or a fibre collimator is re-quired, and the light is coupled into the output fibre 37 again via a lens 36 after it has passed through the four A/4 wave plates 32 - 35.
A control signal reflecting the degree of distortion of the detected data signal 52 is obtained by filtering high-frequency spectral fractions out. To this end the data signal 52 is subdivided by means of 'the power distributor 51 and supplied to different filters 53 and 54. The basic frequency amounts to 5 GHz, for example, for the transmission of a 10 Gbit/s signal.
This frequency is always present and contributes mainly to the amplitude of the sig-nal. The frequencies responsible for a high edge steepness range at multiples of the basic frequency, i.e. at 10, 15, 20 GHz or at odd-numbered multiples of the basic frequencies.
For instance, two different filters (53 + 54) are employed. Filter 53 is a band-pass fil-ter that selects the basic frequency at 5 GHz whilst filter 54 may be designed as high-pass filter for filtering out frequencies beyond 15 GHz approximately.
The two detectors 55 connected on the output side convert the signal amplitudes into two analog signals 55' and 55". The ratio between these two analog values then fur-nishes, when used as control signal, the degree of distortion of the data signal inde-pendently of the signal power. The control algorithm of the controller 56 tends to minimise the control signal, e.g. by performing slight modifications in alternation on all elements taking an influence on the polarisation.
This is possible at a very high rate so that the PMD compensation may be performed in real-time. When the modification results in a reduction of the control signal it per-sists, or else it is rejected and the next polarisation element is subjected to a varia-tion.
Fig. 7 illustrates a preferred embodiment for an element producing a mechanical ef fect on a fibre 100 for influencing the polarisation; this element may be a component of the elements A-DGD, T-DGD, DGD or 1' or 12, respectively, for instance. A
ring 121 is provided in the housing 121' for distributing the mechanical action on the PCTlDE 00/03809 longest fibre length possible, onto which ring the fibre is wound without being twisted.
What is not represented is the way in which the fibre is introduced into the ring and passed out of the ring or the housing, respectively. The ring 121 consists, for exam-ple, of a thin deformable special-steel part. A pressurising element 122, e.g.
a piezo element, is disposed in the ring (121), which is supported on two segments 123 of a circle - on one side via an equalising element 122' - which segments in their turn bear against the ring 121. On the side opposite to the circle segments 123 counter-segments 124 are provided which are supported on the housing 121' and bear against the fibre segments so that they pressurise the fibre 100 when the element 122 undergoes a correspondiing elongation. Due to the elongation of the piezo ele-ment 122 the fibre 100 can hence be selectively subjected to a mechanical load.
Even though the invention has been described in the foregoing by embodiments, without any restriction of the general concept, the most different modifications are conceivable, of course; moreover, it is not only possible to combine the various fea tures of the individual elements in the aforedescribed embodiment with each other, which are claimed as independent inventions in the claims, but it is also possible to combine individual features with embodiments for other elements such as those known from prior art.
The emulator unit provided in .correspondence with the invention may, of course, also be employed in other devices 'which are not envisaged for compensating distortions induced by polarisation modulation dispersion (PMD) in optical transmission systems and transmission fibres, in particularly, but serve merely to generate PMD-induced distortions, e.g. for test applications.
Claims (46)
1. System for compensating distortions induced by polarisation modulation disper-sion (PMD) in optical transmission systems and in transmission fibres in par-ticular, comprising a means for measuring PMD-induced distortions, an emulator unit for adjustable PMD levels, and a controller which the output signal of said measuring means is applied to and which serves to control said emulator unit, characterised in that said controller controls said PMD emulator unit in such that continuous compensation of the PMD-induced signal distortion will be per-formed.
2. System according to Claim 1, characterised in that said PMD emulator unit includes a variable PMD delay unit which consists of two PMD-involving elements with a polarisation regulator disposed therebetween.
3. System according to Claim 2, characterised in that said PMD-involving elements are dispersive elements.
4. System according to Claim 3, characterised in that said PMD-involving elements of said variable PMD delay element are polarisation-maintaining fibres.
5. System according to Claim 2, characterised in that said polarisation regulator of said variable PMD delay element comprises a .lambda./2 wave plate or a Faraday rotator.
6. System according to Claim 2, characterised in that said polarisation regulator is implemented by a rotatable connection of the coupling site of the two PMD-involving elements.
7. System for compensating distortions induced by polarisation modulation disper-sion (PMD) in optical transmission systems and in transmission fibres in par-ticular, comprising - a means for measuring PMD-induced distortions, - an emulator unit for adjustable PMD levels, and - a controller which the output signal of said measuring means is applied to and which serves to control said emulator unit, characterised in that said emulator unit comprises at least one basic emulator unit consisting of two DGD (differential group delay elements) elements having each a defined invariable time lag for the incoming signal, which elements are connected to each other via a connecting element producing the effect of a transformation element, with all the three elements having a defined angle of the birefringence axes such that the birefringence axes of said connecting ele-ment will be distinguished in terms of their angular position from the birefrin-gence axes of said two DGD elements, and that at least one regulator element is provided for each basic emulation unit, which acts upon one of said elements of this basic emulator unit in such a way that the DGD level of the system can be completely adjusted by a slight varia-tion of the time lag of the influenced element.
8. System according to Claim 7, characterised in that said elements are PM fibres, and that said regulator element exerts a mechanical effect upon at least one of said DGD elements for varying the time lag and hence polarisation.
9. System according to Claim 8, characterised in that said regulator element or elements, respectively, which produce a mechanical action, are fibre squeezers or stretchers with electrically controllable elements such as piezo elements creating a mechanical action upon the PM fibre.
10. System according to Claim 9, characterised in that at least one of said regulator elements comprises a ring onto which said PM fibre is wound without being twisted, for distributing the mechanical action over the longest fibre length possible.
11. System according to Claim 4 or 10, characterised in that at least one pressurising element creates a pressure on a plurality of fibre segments of said wound fibre at least at one site.
12. System according to Claim 11, characterised in that said pressurising element is an elongating element such as a piezo element that acts upon at least one circle segment bearing against said ring, and that counter-segments are provided for at least one part of said circle elements, which bear against said fibre segments and create pressure on said fibre.
13. System according to Claim 1 or 7, characterised in that said elements are birefringent crystals having a birefrin-gence adapted to be electronically influenced.
14. System according to Claim 1, characterised in that the time lag of said two DGD elements of each basic emulator unit is equal to and distinctly greater than that of the associated con-necting element.
15. System according to Claim 7, characterised in that the angle of the birefringence axis of said first DGD
ele-ment is selected to be 0À and that of the second DGD element to be 90°
and that of said connecting element to be 45°, or to be 0°, 45°, 0°, or 90°, 45°, 0°.
ele-ment is selected to be 0À and that of the second DGD element to be 90°
and that of said connecting element to be 45°, or to be 0°, 45°, 0°, or 90°, 45°, 0°.
16. System according to Claim 7, characterised in that a further element is series-connected to said two DGD
elements and said connecting element for setting an optional input PSP.
elements and said connecting element for setting an optional input PSP.
17. System according to Claim 16, characterised in that said series-connected element comprises a further bire-fringent element such as a PM fibre, and that the angle of said birefringence axes of said series-connected element and of said first DGD element are different from each other.
18. System according to Claim 17, characterised in that said angular difference amounts to 45°.
19. System according to Claim 16, characterised in that said series-connected element comprises a further bire-fringent element such as a PM fibre, and that a regulator element produces an effect on said first DGD element for vary-ing the time lag and hence the polarisation.
20. System according to Claim 19, characterised in that said series-connected element and/or said connecting element consists of two PM fibres or two birefringent crystals having each a different angular position of the birefringence axes.
21. System according to Claim 19, characterised in that said regulator element acts upon one of said two fibres or on one of said crystals.
22. System according to Claim 7, characterised in that at least two systems for setting a variable DGD are con-nected in tandem, whereof at least one comprises a basic emulator unit, if nec-essary with a PSP setting element.
23. System according to Claim 22, characterised in that the individual systems for higher-order PMD compensa-tion consist of basic emulator units with DGD elements having different individ-ual time lags.
24. System for compensating distortions induced by polarisation modulation disper-sion (PMD) in optical transmission systems and in transmission fibres in par-ticular, comprising - a means for measuring PMD-induced distortions, - an emulator unit for adjustable PMD levels, and - a controller which the output signal of said measuring means is applied to and which serves to control said emulator unit, characterised in that said measuring means detects the polarisation of all spectral fractions contained in the signal output from said emulator unit, for de-tecting the PMD.
25. System according to Claim 24, characterised in that said measuring means comprises a polarizer and an opto-electrical converter such as a photo receiver that is disposed to join said polarizer, and that a polarisation matching unit is provided that matches the output polarisa-tion of said emulator unit to that of said polarizer.
26. System according to Claim 24, characterised in that said measuring means comprises a polarisation beam splitter, with opto-electrical converters such as photo receivers being provided on the output terminals of said splitter and issuing signals for generating an ACTUAL signal for said controller, which are subjected to quotient formation.
27. System according to Claim 26, characterised in that for detection of the polarisation a polarimeter array known per se is provided.
28. System according to Claim 24, characterised in that said polarisation matching unit comprises two birefringent elements having birefringence axes forming an angle different from 0°, prefera-bly 45°, and that at least one regulator element is provided for setting the output polarisa-tion, which acts upon at least one of said birefringent elements.
29. System according to Claim 28, characterised in that said birefringent elements are birefringent crystals or PM
fibres.
fibres.
30. System according to Claim 1, 7 or 18, characterised in that a polarisation matching unit is disposed directly upstream of said polarisation measuring unit or directly downstream of said emulator.
31. System according to Claim 1, 7 or 18, characterised in that a polarisation matching unit is integrated as additional element into said emulator.
32. System according to Claim 25, characterised in that said polarisation matching unit is a series-connected up-stream or downstream DGD element having an angle of 45°, with a regulator element acting upon this series-connected element and upon the DGD con-nected upstream or downstream thereof.
33. System according to Claim 1 or 31, characterised in that said PMD emulator unit is a variable infinite polarisation regulator having sufficient degrees of freedom, which projects said two PSP of the fibre to be compensated onto the PSP of said variable PMD delay element, without thoroughly controlling a local minimum of the overall PMD.
34. System according to Claim 33, characterised in that said variable polarisation regulator comprises an array of four ~4 wave plates disposed in tandem.
35. System for compensating distortions induced by polarisation modulation disper-sion (PMD) in optical transmission systems and in transmission fibres in par-ticular, comprising - a means for measuring PMD-induced distortions, an emulator unit for adjustable PMD levels, and - a controller which the output signal of said measuring means is applied to and which serves to control said emulator unit, or according to Claim 1, 7 or 18, characterised in that said controller comprises several control loops in which it modulates regulator elements of said emulator unit with different frequencies, that said controller derives from the output signal of said measuring means in-formation about the amount and the phase position of the signal output from said emulator unit, and uses this information to perform a high-speed and direct control function.
36. System according to Claim 35, characterised in that the bandwidth or limit frequency of said opto-electrical converter is matched with the modulation frequency, and that said controller sets the individual control loops in such a way that the po-larisation will be constant for all spectral fractions contained in the signal.
37. System according to Claim 36, characterised in that said controller uses a minimum photo current of said opto-electrical converter or converters as a control criterion for setting a con-stant polarisation for all spectral fractions contained in the signal.
38. System according to Claim 36, characterised in that said controller evaluates the output signal from said opto-electrical converter or converters selectively in terms of frequency and phase.
39. System according to Claim 38, characterised in that said controller comprises analog automatic-control cir-cults for said regulator elements to which said frequency-selective and phase-selective signals are applied.
40. System according to Claim 35, characterised in that said controller also controls the regulator elements of said polarisation matching unit.
41. System according to Claim 40, characterised in that said controller controls the regulator elements of said polarisation matching unit with the same control algorithm as that used in said emulator unit.
42. System according to Claim 35, characterised in that said controller comprises at least one CPU or at least one DSP circuit for performing various functions such as for frequency-selective and phase-selective evaluation or for controlling the operational sequence within the system.
43. System according to Claim 35, characterised in that said controller performs essential parts of said control al-gorithm by using analog circuits.
44. System according to Claim 1, characterised in that said controller comprises filters for generating a control signal, which filter out high-frequency spectral fractions of the data signal so that the filtered signal reflects the degree of distortion of said detected data sig-nal.
45. System according to Claim 44, characterised in that said controller comprises two different filters with respec-tively series-connected detectors on the output side, which generate two analog signals on the basis of said data signal, whose ratio reflects the degree of dis-tortion of said data signal independently of the signal power.
46. System according to Claim 45, characterised in that said controller minimises the PMD-induced signal distor-tion by readjustment, in alternation, at the polarisation-influencing elements of said variable polarisation regulator and said variable PMD delay element.
Applications Claiming Priority (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE10035083.6 | 2000-07-17 | ||
| DE10035083A DE10035083A1 (en) | 2000-03-04 | 2000-07-17 | PMD-compensator for compensating polarization mode conditioned distortions in optical transmission systems, esp. transmission fibers, uses polarization transformation element |
| DE10049784.5 | 2000-10-09 | ||
| DE10049784A DE10049784A1 (en) | 2000-10-09 | 2000-10-09 | Polarization mode dispersion emulation device for optical information transmission system uses controllable double refraction polarization transformation element and double refraction elements |
| PCT/DE2000/003809 WO2002007351A1 (en) | 2000-07-17 | 2000-10-28 | Arrangement for the minimisation or compensation of pmd induced distortion in optical transmission systems and in particular transmission fibres |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2338343A1 true CA2338343A1 (en) | 2002-01-17 |
Family
ID=7659046
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002338343A Abandoned CA2338343A1 (en) | 2000-07-17 | 2000-10-28 | System for minimising or compensating pmd-induced distortions in optical transmission systems and transmision fibres in particular |
Country Status (7)
| Country | Link |
|---|---|
| US (2) | US6996297B2 (en) |
| EP (1) | EP1325574B1 (en) |
| JP (1) | JP2004511789A (en) |
| AU (1) | AU2002221607A1 (en) |
| CA (1) | CA2338343A1 (en) |
| DE (1) | DE10049784A1 (en) |
| WO (2) | WO2002007351A1 (en) |
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| DE19816178A1 (en) | 1998-04-14 | 1999-10-21 | Siemens Ag | Emulator and compensator for polarization mode dispersion |
| US6654105B2 (en) | 2000-03-06 | 2003-11-25 | Corning Applied Technologies Corporation | Cross-correlating PMD detector |
| DE10049784A1 (en) * | 2000-10-09 | 2002-05-16 | Adalbert Bandemer | Polarization mode dispersion emulation device for optical information transmission system uses controllable double refraction polarization transformation element and double refraction elements |
| US7495765B2 (en) * | 2001-05-17 | 2009-02-24 | Thorlabs Gmbh | Fiber polarimeter, the use thereof, as well as polarimetric method |
| EP1262752B1 (en) * | 2001-05-17 | 2005-08-03 | THORLABS GmbH | Fiber polarimeter, its use, and polarimetric method |
| DE10238993A1 (en) * | 2002-08-20 | 2004-02-26 | Deutsche Telekom Ag | Method for adaptive compensation of polarization mode dispersion (PMD) e.g. for optical signals in glass fibers, requires decoupling part of optical output signal of PMD compensator to derive signal quality |
| US6856711B1 (en) * | 2003-04-09 | 2005-02-15 | At&T Corp. | Technique for mitigation of polarization mode dispersion in fiber optic transmission links |
| US7768630B2 (en) | 2003-09-19 | 2010-08-03 | Prysmian Cavi E Sistemi Energia S.R.L. | Method and apparatus for the evaluation of polarization mode dispersion in optical fibers |
| US7203385B1 (en) * | 2004-12-07 | 2007-04-10 | Sprint Communications Company L.P. | Optimizing PMD measurements based on temperature for installed fibers |
| EP1746754A1 (en) * | 2005-07-19 | 2007-01-24 | Alcatel | Method for operating a switched optical network |
| US7454092B2 (en) * | 2006-10-24 | 2008-11-18 | Kailight Photonics, Inc. | Systems and methods for polarization mode dispersion mitigation |
| US20080205814A1 (en) * | 2007-02-22 | 2008-08-28 | Lijie Qiao | Method and Apparatus for Dynamic Polarization Mode Dispersion Compensation |
| US7693357B2 (en) * | 2008-07-14 | 2010-04-06 | Ciena Corporation | Methods and systems for eliminating deleterious polarization effects in an optical fiber dispersion compensation module |
| JP2012090259A (en) * | 2010-09-21 | 2012-05-10 | Panasonic Corp | Imaging apparatus |
| EP2538587A1 (en) * | 2011-06-21 | 2012-12-26 | Nokia Siemens Networks Oy | Optical communication system and method of upgrading an optical network |
| EP2568627A1 (en) * | 2011-08-30 | 2013-03-13 | Telefonaktiebolaget LM Ericsson (PUBL) | Optical signal monitoring method and apparatus |
| CN106330810B (en) * | 2016-07-12 | 2019-02-22 | 北京邮电大学 | A kind of XPD compensation method promoting polarization modulation bit error rate performance |
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| FR2659754B1 (en) * | 1990-03-16 | 1994-03-25 | Thomson Csf | DEVICE FOR CREATING OPTICAL DELAYS AND APPLICATION TO AN OPTICAL CONTROL SYSTEM OF A SCANNING ANTENNA. |
| EP0477699A3 (en) * | 1990-09-14 | 1993-09-01 | Fujitsu Limited | Optical communication system |
| JP2739813B2 (en) * | 1993-12-20 | 1998-04-15 | 日本電気株式会社 | Polarization dispersion compensation method |
| TW312744B (en) * | 1994-10-11 | 1997-08-11 | Adoban Tesuto Kk | |
| DE69739862D1 (en) * | 1996-08-22 | 2010-06-10 | Fujitsu Ltd | Fiber optic transmission system with optical phase conjugation |
| JPH10206661A (en) * | 1997-01-28 | 1998-08-07 | Fujitsu Ltd | Polarization scrambler, and optical integrated circuit using it |
| US5859939A (en) * | 1997-02-25 | 1999-01-12 | Mci Communications Corporation | Method and system for equalizing PMD using incremental delay switching |
| US5930414A (en) * | 1997-09-16 | 1999-07-27 | Lucent Technologies Inc. | Method and apparatus for automatic compensation of first-order polarization mode dispersion (PMD) |
| US5960414A (en) * | 1997-11-25 | 1999-09-28 | Hewlett-Packard Company | Method for monitoring excess inventory |
| DE19816178A1 (en) * | 1998-04-14 | 1999-10-21 | Siemens Ag | Emulator and compensator for polarization mode dispersion |
| DE19818699A1 (en) * | 1998-04-25 | 1999-10-28 | Bandemer Adalbert | Fast-acting compensation system minimizing signal distortion caused by polarization mode dispersion in optical fiber propagation |
| DE19827638A1 (en) * | 1998-06-20 | 1999-12-23 | Alcatel Sa | Method for measuring interference effects on fiber optic transmission links and transmission system |
| JP2000031903A (en) * | 1998-07-07 | 2000-01-28 | Hitachi Ltd | Polarized wave dispersion compensation system and polarized wave dispersion compensation method |
| DE19830990A1 (en) * | 1998-07-10 | 2000-01-20 | Siemens Ag | Polarization transformer |
| DE19841755A1 (en) * | 1998-09-11 | 2000-03-23 | Siemens Ag | Polarisation transformer for optical transmission engineering |
| GB9818941D0 (en) * | 1998-08-28 | 1998-10-21 | Northern Telecom Ltd | Polarisation mode dispersion compensation |
| US6342945B1 (en) * | 1999-03-31 | 2002-01-29 | Corning Incorporated | System and method for measuring polarization mode dispersion suitable for a production environment |
| US6144450A (en) * | 1999-09-13 | 2000-11-07 | Lucent Technologies | Apparatus and method for improving the accuracy of polarization mode dispersion measurements |
| US6538787B1 (en) * | 1999-09-24 | 2003-03-25 | Lucent Technologies Inc. | Apparatus and method for polarization mode dispersion emulation and compensation |
| US6542650B2 (en) * | 1999-11-30 | 2003-04-01 | University Of Southern California | Polarization-mode dispersion emulator |
| US6330375B1 (en) | 1999-12-16 | 2001-12-11 | Lucent Technologies Inc. | Distortion analyzer for compensation apparatus of first order polarization mode dispersion (PMD) |
| US6381385B1 (en) * | 1999-12-22 | 2002-04-30 | Nortel Networks Limited | Polarization mode dispersion emulation |
| US20010028760A1 (en) * | 2000-03-03 | 2001-10-11 | Yaffe Henry H. | Methods and apparatus for compensating chromatic and polarization mode dispersion |
| US6483958B2 (en) * | 2000-05-06 | 2002-11-19 | Tektronix Munich | PMD compensator |
| EP1282835B1 (en) * | 2000-05-06 | 2006-07-26 | THORLABS GmbH | Emulator for second order polarization mode dispersion (pmd) |
| DE10049784A1 (en) * | 2000-10-09 | 2002-05-16 | Adalbert Bandemer | Polarization mode dispersion emulation device for optical information transmission system uses controllable double refraction polarization transformation element and double refraction elements |
| US6654103B2 (en) * | 2000-09-01 | 2003-11-25 | University Of Southern California | Compensation and control of both first-order and higher-order polarization-mode dispersion |
| US6563590B2 (en) * | 2001-02-16 | 2003-05-13 | Corning Incorporated | System and method for measurement of the state of polarization over wavelength |
-
2000
- 2000-10-09 DE DE10049784A patent/DE10049784A1/en not_active Ceased
- 2000-10-28 CA CA002338343A patent/CA2338343A1/en not_active Abandoned
- 2000-10-28 WO PCT/DE2000/003809 patent/WO2002007351A1/en active Application Filing
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2001
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- 2001-09-18 EP EP01986813A patent/EP1325574B1/en not_active Expired - Lifetime
- 2001-09-18 WO PCT/EP2001/010794 patent/WO2002032023A1/en active IP Right Grant
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2005
- 2005-12-12 US US11/302,015 patent/US20060098989A1/en not_active Abandoned
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| DE10049784A1 (en) | 2002-05-16 |
| EP1325574B1 (en) | 2006-11-15 |
| US20060098989A1 (en) | 2006-05-11 |
| EP1325574A1 (en) | 2003-07-09 |
| US20040028309A1 (en) | 2004-02-12 |
| US6996297B2 (en) | 2006-02-07 |
| JP2004511789A (en) | 2004-04-15 |
| WO2002007351A1 (en) | 2002-01-24 |
| WO2002032023A1 (en) | 2002-04-18 |
| AU2002221607A1 (en) | 2002-04-22 |
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| FZDE | Discontinued |