CN1322690C - A method of polarization mode dispersion compensation - Google Patents
A method of polarization mode dispersion compensation Download PDFInfo
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- CN1322690C CN1322690C CNB200410061369XA CN200410061369A CN1322690C CN 1322690 C CN1322690 C CN 1322690C CN B200410061369X A CNB200410061369X A CN B200410061369XA CN 200410061369 A CN200410061369 A CN 200410061369A CN 1322690 C CN1322690 C CN 1322690C
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Abstract
The present invention relates to a polarization mode dispersion compensation method. The present invention can be applied to a high-speed long range DWDM optical fiber communication system with 10 Gb/s and more than 10 Gb/s of transmission rate, can quickly and accurately compensate PMD of transmission optical signals, and can be especially suitable for multiple channels. The present invention is characterized in that the present invention adopts a high-speed depolarizing device to depolarize modulation optical signals into natural light at the time domain, the optical signals enter long range transmission optical fibers, and are divided into two components with even energy by two orthogonal polarization main shafts along the optical fibers, and the differential group delay of the components still exists; a polarization device is added, and therefore, the components along any one orthogonal polarization shaft are removed before the optical signals enter a receiving machine. The same set of devices can simultaneously compensate polarization mode dispersion PMD of the channels, and therefore, the present invention greatly reduces the compensating cost.
Description
Technical field
The present invention relates to a kind of method that realizes the polarization mode dispersion dynamic compensation, can be used for transmission rate 10Gb/s and above high speed, long, be particularly suitable for realizing accurate compensation the polarization mode dispersion of multichannel light signal apart from dense wave division multipurpose (DWDM) optical fiber telecommunications system.
Background technology
For dense wave division multipurpose (DWDM) system of transmission rate 10Gb/s and above high speed, long distance, a polarization mode dispersion that key issue is each passage of appearance.Because the influence of manufacture process and external environment, the cross section of actual monomode fiber is desirable circle no longer, causes the propagation constant of two orthogonal polarization modes in the optical fiber also different thereupon, and the pulse stretching that causes thus just is called polarization mode dispersion (PMD).The same with nonlinear fiber and chromatic dispersion, PMD can damage the transmission performance of system, the transmission rate of restriction system and distance, and be considered to limit the final factor of speed fiber optic communication systems transmission capacity and distance.
List of references
【1】Iannone，E.F.Matera，A.Mecozzi，and?M.Settembre，Nonlinear?Optical?CommunicationNetworks，New?York：John?Wiley&Sons，Inc.，pp.30-35，1998.
【2】Christopher?Allen，Pradeep?Kumar?Kondamuri，Douglas?L.Richards，Douglas?C.Hague，“Measured?Temporal?and?Spectral?PMD?Characteristics?and?Their?Implications?forNetwork-Level?Mitigation?Approaches”.Journal?of?Lightwave?Technology，21(1)，pp.79-86，2003.
【3】Betti，S.F.Curti，B.Daino，G?De?Marchis，E.Iannone，and?F.Matera，“Evaluation?of?thebandwidth?of?the?principle?states?of?polarization?in?single-mode?fibers”，Optics?Letters，16(7)，pp.467-469，1991.
【4】R.Khosravani，S.A.Havstad，Y.W.Song，P.Ebrahimi，and?A.E.Willner.“Polarization-mode?dispersion?compensation?in?WDM?systems”，IEEE?Photonics?TechnologyLetters，13(12)，pp.1370-1372，2001.
Can think in actual long-distance optical fiber exist by input optical pulse resolve into along pairwise orthogonal direction polarization and with defeated Going out polarization state has the light pulse of minimal frequency dependence, and these two polarized optical pulses are principal state of polarization (psp) (PSP). In the real system PSP always has certain bandwidth, and PSP can be considered constant [1], [2] in this bandwidth. To standard single-mode fiber, its PSP Bandwidth will be at the order of magnitude of 100GHZ.
At fiber-optic output, be different the time of advent of two PSP, and first pulse signal is propagated with preferential polarization state And at first arrive; Second pulse signal arrives the time between the two to postpone the polarization state propagation and to postpone a time delay value Difference is exactly the Differential Group Delay (DGD) of PMD. In actual high-speed optical transmission system, the variation of DGD is exactly basically The transient change of PMD, and DGD mean value and PSP bandwidth relation [1]～[3] that seemingly are inversely proportional to. For example, to 3.2ps Average DGD value, its PSP bandwidth will be about 300GHZ; To the average DGD value of 10ps, its PSP bandwidth exists About 100GHZ. To actual 40Gb/s system, before carrying out the PMD compensation the average DGD value maximum of system generally 5～6 ps[2], the 200GHZ so the PSP bandwidth of 40Gb/s system is generally always had an appointment. Therefore, as long as can make in compensation place The PMD vector that gets the multichannel light signal in the PSP bandwidth range is identical, and the channel spacing relative narrower of dwdm system (such as the dwdm system of 50GHZ or 100GHZ) just can be realized compensating simultaneously these a plurality of passages with same set of device The dream of PMD.
But, because the difference of each passage light signal input polarization in the actual dwdm system, make each passage differ widely through the PMD vector after the long-distance optical fiber transmission, promptly the size of DGD average and the Energy distribution of light signal on two PSP differ widely in these passages.So in existing PMD compensation scheme, can only carry out single pass PMD compensation in fact based on the vector superposed principle of PMD.And because in the actual light transmission system, uncertain factors such as temperature, stress cause the PMD change at random in time in the fibre circuit, also must select real-time and dynamic single channel compensation way.For United States Patents 6, multichannel PMD compensation scheme described in 603,890, its essence also are on the basis of existing single channel PMD collocation structure, passage to limited number (≤4) is monitored PMD respectively, and just therefrom selects the worst passage of PMD to compensate.It is extremely low that this scheme not only requires each passage to experience the probability of the worst PMD simultaneously, and require the DGD average of system must be enough (more than the 20ps) [4] greatly.So not only there are many restrictions in this multichannel scheme in the application of actual 10Gb/s system, and almost can not be used for the 40Gb/s system.
Fig. 1 shows and adopts existing single channel compensation scheme to carry out the principle of PMD dynamic compensation in the optical fiber telecommunications system.Existing single channel PMD compensation scheme generally speaking has electric territory and two kinds of schemes of light domain compensation, and both essence all is to utilize the delay inequality between two polarization modes that delay line certain light or electricity causes PMD to compensate.But the electrical domain compensation mode is subjected to the restriction of electronic bottleneck, and using in the system more than 10Gb/s has great difficulty, and then there is not this problem in the light domain compensation, so the PMD compensation scheme is commonly light territory scheme in the High Speed System of present indication.The canonical system composition of light territory scheme can be summarized as United States Patents 6 shown in Figure 2,748, the method that proposes in 126 is promptly carried out single pass PMD compensation according to feedback signal by control unit real-time regulated Polarization Controller and polarization compensation time delay device.Adopt this compensation scheme not only to need to formulate hard to tackle feedback control algorithm, select colory high speed Polarization Controller and polarization compensation time delay device, and for the wdm system of reality, because the price of this single channel PMD compensator commercial product is between 1～20,000 dollar, this scheme can can't be applied because of cost is too high rather.
Summary of the invention
The purpose of this invention is to provide a kind of polarization mode dispersion (PMD) compensation method, be particularly suitable for being applied in transmission rate 10Gb/s and above high speed, the long distance D WDM optical fiber telecommunications system, quick and precisely compensate the polarization mode dispersion PMD of transmitting optical signal.Further aim of the present invention is the polarization mode dispersion PMD that compensates a plurality of passages with same set of device simultaneously.
Technical scheme of the present invention is: a kind of polarization mode dispersion (PMD) compensation method, it is characterized in that adopting a high speed depolarizer is natural daylight with modulated light signal in the time domain depolarization, light signal enters long Distance Transmission optical fiber subsequently, is decomposed into the energy equalization along the pairwise orthogonal polarization principal axis of optical fiber but still has two components of differential group delay; At last, add an analyzer, make light signal before entering receiver, be removed and get final product along the component of any one orthogonal polarization axes.
Aforesaid polarization mode dispersion (PMD) compensation method is characterized in that polarization axle variation according to the optical fiber polarisation main shaft in perpendicular to the plane of optical signal transmission direction of described analyzer is carried out dynamic adjustments with the speed of millisecond magnitude.
Aforesaid polarization mode dispersion (PMD) compensation method is characterized in that described modulated light signal is the multichannel modulated light signal behind the wavelength division multiplexing, is natural daylight with the multichannel modulated light signal in the whole depolarizations of time domain with described high speed depolarizer; Behind the wavelength-division demultiplexing, in each passage, add an analyzer respectively, make light signal before entering receiver, be removed and get final product along the component of any one orthogonal polarization axes.
Aforesaid polarization mode dispersion (PMD) compensation method is characterized in that described modulated light signal is the multichannel modulated light signal behind the wavelength division multiplexing, is natural daylight with the multichannel modulated light signal in the whole depolarizations of time domain with described high speed depolarizer; Behind the wavelength-division demultiplexing, bandwidth according to principal state of polarization (psp) in the real system, selection can be carried out the port number of PMD compensation simultaneously, adds a public analyzer in every group of passage respectively, makes light signal be removed before entering receiver along the component of any one orthogonal polarization axes and gets final product.
Use the advantage of this method to be:
1. the control that light energy is constant is very simple.Modulated light signal with random polarization state by behind the high speed depolarizer on optical fiber pairwise orthogonal polarization principal axis the equipartition of energy; Though light signal more still can be poor relative time delay along the generation of pairwise orthogonal polarization principal axis behind long Distance Transmission optical fiber, but because of its equipartition of energy on two polarization principal axis, through dynamic analyzer and removing behind the light component of a polarization principal axis of optical fiber, can not produce the fluctuation of light energy so can guarantee it; Adopt the method for removing, control becomes very simple.Can remove the influence of Polarization Dependent Loss (PDL), Polarization-Dependent Gain polarization effects such as (PDG) simultaneously.
2. can compensate the PMD of a plurality of passages simultaneously with same set of device.Though each channel modulation light signal input polarization difference, each channel modulation light signal becomes natural daylight in the whole already depolarizations of time domain, make it identical through the PMD vector after the long-distance optical fiber transmission, thereby available same set of device compensates the PMD of a plurality of passages simultaneously.Because adopt the method for removing, make the big (factors such as Insertion Loss that the consideration device is introduced of transmitting optical signal energy attenuation, about the about altogether 7dB of total losses), but this can solve by add several image intensifers in optical fiber link fully, and the cost of image intensifer is far below the cost of traditional single channel PMD compensator.By contrast, this scheme has just greatly reduced the compensation cost.
3. the method and speed are irrelevant.
Description of drawings
Fig. 1 be PMD causes in the optical fiber telecommunications system signal distortion and adopt the existing dynamic PMD compensation scheme of single channel after the effect schematic diagram.
Fig. 2 is the structure chart of traditional single channel PMD dynamic compensation.
Fig. 3 (a) and Fig. 3 (b) are two kinds of specific embodiments of the multichannel PMD compensation that the present invention relates to.Wherein, Txi and Rxi be respectively i optical transmitter and receiver (i=1,2 ..., n).
Fig. 4 is the transmitting optical signal of arbitrary passage in the optical fiber telecommunications system variation schematic diagram after by the multichannel PMD compensation arrangement that the present invention relates to.
Fig. 5 is two kinds of specific implementations of each passage analyzer in the PMD compensation arrangement that the present invention relates to.Wherein Fig. 5 (a) adopts magneto-optical crystal and the fashionable analyzer structure chart of static polarizer group; Fig. 5 (b) is the analyzer structure chart when adopting mechanical system rotatory polarization sheet mode.
Fig. 6 (a) and Fig. 6 (b) are respectively the control principle figure and the algorithm flow chart of each passage analyzer in the PMD compensation arrangement that the present invention relates to.
Fig. 7 is traditional high speed depolarizer structure.
Embodiment
Further specify operation principle of the present invention below in conjunction with the drawings and specific embodiments:
If the Stokes form of certain passage input polarization light is in the dwdm system:
Wherein, θ is the azimuth of elliptically polarized light, promptly elliptically polarized light on the Poincere ball with S
1S
2The angle on plane, and-pi/2≤θ≤pi/2; β is the ellipticity angle ,-π/4≤β≤π/4.The degree of polarization of light signal then
If make three Stokes components of single wavelength light signal of each passage be divided into 0, promptly a bit period T inner product
Then have DOP=0, the equipartition of energy of light signal on two PSP arranged simultaneously.Based on this, shown in Fig. 3 (a), the multichannel modulated light signal after adopting a light time territory high speed depolarizer with wavelength division multiplexing is a natural daylight in the time domain depolarization, make its behind the transmission long-distance optical fiber along the optical fiber pairwise orthogonal polarization principal axis equipartition of energy; On each passage behind the demultiplexing, add an analyzer then, make signal before entering receiver, be removed light component along arbitrary polarization principal axis, just can realize compensation (just before receiver, add an analyzer if do not add depolarizer, will cause the luminous intensity change at random of coming out) because of transmitting optical signal Energy distribution change at random on the pairwise orthogonal polarization axle from analyzer to multichannel PMD.
For the optical fiber telecommunications system of reality, because the influence that two polarization principal axis of optical fiber are subjected to external environment change at random in time, and its transformation period be a millisecond magnitude.So the polarization axle of analyzer will the variation according to the optical fiber polarisation main shaft carry out dynamic adjustments with the speed of millisecond magnitude in perpendicular to the plane of optical signal transmission direction.Analyzer can adopt magneto-optical crystal that traditional mechanical rotation polarizer or crystallographic axis can change etc. to add modes such as static polarizer and realize, shown in Fig. 5 (a) and Fig. 5 (b).Simultaneously, because the rate request of light time territory high speed depolarizer reaches and can make three Stokes change component of light signal be divided into 0 a bit period inner product, so another key of the method is the realization of high speed depolarizer, this can adopt realizations such as traditional LiNbO3 waveguide or electrooptic crystal, as Fred Heismann at article " Compact Electro-Optic PolarizationScramblers for Optically Amplified Lightwave Systems " (J.Lightwave Technol., vol.14, pp.1801-1813,1996) the middle high speed depolarizer of introducing.As shown in Figure 7.Feedback signal can adopt electric radio-frequency spectrum (RF) or the like.
Because the PSP bandwidth of standard single-mode fiber will be at the order of magnitude [1] [2] of 100GHZ, and in the actual 40Gb/s system, before carrying out the PMD compensation on average the DGD value generally at 5～6 ps[2], so the PSP bandwidth of 40Gb/s system always has 200GHZ.Therefore, the bandwidth of principal state of polarization (psp) in the operation cost of taking into account system and the real system, the embodiment of the present invention shown in Fig. 3 (a) also has a kind of work-around solution available, and is concrete shown in Fig. 3 (b).The difference of scheme is shown in this scheme and Fig. 3 (a): after multichannel modulated light signal time domain depolarization, at first according to the number of active lanes i that can carry out the PMD compensation simultaneously, adopt the wavelength-division demultiplexing of lower frequency (for example 50GHZ) that the multichannel multiplexed signals is divided into some passages (each passage all comprises i the passage that frequency is higher), respectively each passage is carried out analyzing; And then each channel signal is carried out the wavelength-division demultiplexing of higher frequency (for example 100GHZ or 200GHz); At last each passage behind the demultiplexing is detected reception respectively.
Fig. 4 is the variation schematic diagram of transmitting optical signal after by the multichannel PMD compensation arrangement that the present invention relates to that has random polarization state in arbitrary passage in the optical fiber telecommunications system.As Fig. 4, the modulated light signal that has random polarization state in arbitrary passage of adjacent wavelength by behind the high speed depolarizer on optical fiber pairwise orthogonal polarization principal axis the equipartition of energy; Though light signal more still can be poor relative time delay along the generation of pairwise orthogonal polarization principal axis behind long Distance Transmission optical fiber, but because of its equipartition of energy on two polarization principal axis, through analyzer and removing behind the light component of a polarization principal axis of optical fiber, can not produce the fluctuation of light energy so can guarantee it.
Fig. 6 has provided the schematic diagram and the algorithm flow chart of FEEDBACK CONTROL.Shown in Fig. 6 (a), at first, reflection PMD influences the real-time feedback signal of situation to transmitting optical signal in the acquisition system, sends into control unit; Then, send a drive signal by control unit, the polarization axle that makes analyzer is to changing an angle, (the concrete size of stepping can be decided by the response minimum voltage gradient of control section in the analyzer in i.e. stepping, it also can be the integral multiple of stepping, specifically to decide) according to the effect of experiment compensation and the time of compensation, after the analyzer response, a new feedback signal is sent to control unit at once, with the feedback signal value of former storage relatively when each initial launch (system earlier be made as optimal value with the feedback signal default value), if improve, then continue to send an equidirectional drive signal; If variation then sends the instruction of a reciprocal twice stepping.So move in circles and get final product.The specific algorithm flow process is shown in Fig. 6 (b).As seen, compare with control algolithm loaded down with trivial details in the traditional single channel PMD compensation scheme, control algolithm to analyzer among the present invention is extremely simple, this has not only saved the expense of buying high speed Polarization Controller shown in Figure 2, and for the about 10,000 dollars traditional PMD compensation Control Software of present price, just greatly reduced software development cost.
We have realized the compensation of multichannel PMD in high speed, the long distance D wdm system by the present invention.Though owing to adopt analyzer in the PMD compensation method of the present invention, thereby cause the transmitting optical signal energy to decay and (consider the factors such as Insertion Loss that device is introduced greatly, about the about altogether 7dB of total losses), but this can solve by add several image intensifers in optical fiber link fully, and the cost of image intensifer is far below the cost of single channel PMD compensator.By contrast, this scheme has just greatly reduced the cost of PMD compensation.
The present invention not only is applicable to the PMD compensation of multichannel transmitting optical signal in the dwdm system, equally also is applicable to the compensation of polarization such as Polarization Dependent Loss, Polarization-Dependent Gain correlation effect in the system.
Claims (4)
1, a kind of polarization mode dispersion (PMD) compensation method, it is characterized in that adopting a high speed depolarizer is natural daylight with modulated light signal in the time domain depolarization, light signal enters long Distance Transmission optical fiber subsequently, is decomposed into the energy equalization along the pairwise orthogonal polarization principal axis of optical fiber but still has two components of differential group delay; At last, before the receiver of long Distance Transmission optical fiber receiving terminal, add an analyzer, make light signal before entering receiver, be removed and get final product along the component of any one orthogonal polarization axes.
2, polarization mode dispersion (PMD) compensation method as claimed in claim 1 is characterized in that polarization axle variation according to the optical fiber polarisation main shaft in perpendicular to the plane of optical signal transmission direction of described analyzer is carried out dynamic adjustments with the speed of millisecond magnitude.
3, polarization mode dispersion (PMD) compensation method as claimed in claim 1 or 2 is characterized in that described modulated light signal is the multichannel modulated light signal behind the wavelength division multiplexing, is natural daylight with the multichannel modulated light signal in the whole depolarizations of time domain with described high speed depolarizer; Behind the wavelength-division demultiplexing, in each passage, add an analyzer respectively, make light signal before entering receiver, be removed and get final product along the component of any one orthogonal polarization axes.
4, polarization mode dispersion (PMD) compensation method as claimed in claim 1 or 2 is characterized in that described modulated light signal is the multichannel modulated light signal behind the wavelength division multiplexing, is natural daylight with the multichannel modulated light signal in the whole depolarizations of time domain with described high speed depolarizer; Behind the wavelength-division demultiplexing, bandwidth according to principal state of polarization (psp) in the real system, selection can be carried out the port number of PMD compensation simultaneously, adds a public analyzer in every group of passage respectively, makes light signal be removed before entering receiver along the component of any one orthogonal polarization axes and gets final product.
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| US20030223759A1 (en) * | 2002-05-29 | 2003-12-04 | Alcatel | Device and method for compensation of polarization mode dispersion |
| EP1421716A1 (en) * | 2001-08-31 | 2004-05-26 | Fujitsu Limited | Polarisation mode dispersion compensator |
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