CN1700626A - Multi-wavelength CD and PMD compensator - Google Patents

Abstract

The present invention concerns a method for performing optical dispersion compensation of wavelength division multiplexed (WDM) optical signals. The method comprises the steps of providing a compensation stage with an optical transmission medium to be used as parallel operating cascade of polarization controllers and optical birefringent elements to compensate dispersion of the WDM optical signals when being transmitted spectrally dispersed into the different wavelength channels in parallel through the parallel cascades. The method is being characterized by determining the chromatic dispersion of the optical signals at one or few different wavelength channels possibly allowing an extrapolation of the chromatic dispersion to the other wavelength channels for defining accordingly an additional feedback signal to be used for the polarization controller setting.

Description

Multi-wavelength chromatic dispersion (CD) and polarization mode dispersion (PMD) compensator

Technical field

The present invention relates to the wave division multiplexing WDM light signal is carried out the method for optical dispersion compensation, the compensated stage that has optical transmission medium is provided, optical transmission medium is as the parallel running cascade of Polarization Controller and optical birefringence element, thereby when by the wdm optical signal of its spectral dispersion of Polarization Controller parallel transmission in the different wave length channel, the chromatic dispersion of compensation wdm optical signal.In addition, the present invention relates to record on it computer-readable medium of computer program, this computer program comprises and is suitable for carrying out the code of a process with the chromatic dispersion of compensation wavelength division multiplexing wdm optical signal during operation when described computer program on as the control unit of the optical transmission medium of Polarization Controller.

Background technology

Can be considered as the input traffic of optical transmission system to represent a series of light pulses of digital bit.In 10Gbps arrived the scope of 40Gbps, therefore, it is wide that light pulse (or bit period) is respectively 100 to 25 picoseconds (psec) usually for the bit rate of existing optical transmission system.Receiver in the optical transmission system is converted to numeral 1 or numeral 0 by receiving light pulse (numeral 1) in definite each bit period or not receiving light pulse (numeral 0) with each bit period in the data flow.

Polarization mode dispersion (PMD) is a kind of phenomenon, and this phenomenon may make the light pulse distortion of data flow, determines that bit period is converted to 1 still is converted to 0 ability thereby weaken receiver.Therefore, PMD has limited the accuracy of transmission and the capacity of optical transmission system, so PMD is the key parameter in the optical communication.The basic mode of monomode fiber is the separating of wave equation of satisfying the boundary condition of core covering interface.Although this seems and does not intuitively conform to that wave equation has two to separate corresponding with basic mode.Owing to separate and have identical propagation constant for two, so think that optical fiber is monomode fiber.Separate for these two and be called polarization mode.Polarized component is orthogonal.Polarization state means the distribution of luminous energy between two kinds of polarization modes.In fact, because the cross section of optical fiber is not a circular ideal, so the propagation constant of two kinds of polarization modes some difference slightly causes pulse stretching thus.A kind of polarization mode is called " fast mode ", and another kind of polarization mode is called " slow mode ".When with fast mode and slow mode during along spread fiber, two kinds of mode mixture, becoming is difficult to distinguish.Propagation time difference between the polarization mode that causes thus is called Differential Group Delay.

Also to stand to be called the another kind of physical dispersion of chromatic dispersion (CD) by the light signal of Optical Fiber Transmission.The reason of its generation is that the light signal of each wavelength passes through given medium with friction speed, as optical fiber.Because the light of different wave length has friction speed, so the light of setted wavelength arrives this receiver before can arriving the receiver that is arranged on the Transmission Fibers end at the light of another wavelength.Time delay between the light of different wave length causes pulse stretching.By measuring the optical fiber group delay in the time domain, obtain the chromatic dispersion of optical fiber.Chromatic dispersion is a kind of metastable phenomenon.In the 10Gbps system, its scope is 300-500psec before causing the 1dB Power penalty.In the 40Gbps system, this scope drops to 18-25psec.

Because with temperature or STRESS VARIATION, CD can be a time variable, still, the time of CD changes not strong especially usually.On the other hand, PMD is time variable fully, and the variation that therefore compensation should tracking time.PMD describes the statistics broadening of the light pulse in the optical fiber that polarization effect causes.Similar with the pulse stretching that chromatic dispersion causes, this broadening effect finally hinders at the receiver place detection waveform correctly.

WO 03/040777 has described the integrated system that the wavelength channel in WDM or the DWDM transmission (the D representative is intensive) is carried out dispersion compensation.This system comprises an adjustable integrated dispersion compensation module, and this module is carried out chromatic dispersion compensation and polarization mode dispersion compensation to each wavelength channel in the transmission.Utilize feedback to adjust adjustable integrated dispersion compensation module, until the performance of optimizing receiver.The disadvantage of the integrated system that WO 03/040777 describes is that each different wave length channel of light signal all needs a special-purpose dispersion compensation module.Use the dispersion compensation of such scheme to mean high cost, and therefore, be actually very uneconomical.

WO 02/080411 has described the method and apparatus that under the situation of not disturbing lasing light emitter the polarization mode dispersion in the optical transmission system (PMD) is compensated.This type of optics PMD compensator has sizable attraction at 10GHz to the WDM optical transmission system in the 40GHz scope to bit rate.In fact since within the hardware (LCD array that is used for Polarization Controller) of all parallel wavelength channels and a birefringece crystal (polarization maintaining optical fibre) the many channels of parallel processing, so this method cost efficient.Simultaneously, the wavelength (feedback signal) of the unique scanning projection polariscope of all Channel Sharing.In addition, this method is utilized the fast forword feedback adaptive, and the fast forword feedback adaptive can be avoided time-consuming dither technique, that is, the direction of feedback signal is optimized the parameter tuning in the continuous variation of Polarization Controller.

Yet the disadvantage of the scheme that WO 02/080411 recommends is, can not compensate chromatic dispersion (CD) or transmitter and warble, and can not compensate from phase modulated (SPM).

Summary of the invention

In view of the foregoing, the objective of the invention is to, provide the method for not only also wavelength-division-multiplexed optical signal being carried out optical dispersion compensation according to chromatic dispersion according to polarization mode dispersion to have cost-benefit mode.

According to the present invention,, realize above-mentioned purpose by using the method for wavelength division multiplexing (WDM) light signal being carried out optical dispersion compensation.This method may further comprise the steps: the compensated stage that has optical transmission medium is provided, this optical transmission medium is as the parallel running cascade of Polarization Controller and optical birefringence element, thereby when by the wdm optical signal of this its spectral dispersion of parallel cascade parallel transmission in the different wave length channel, compensate the chromatic dispersion of this wdm optical signal.The polarization state (polarization spectrum of each channel) of the different wave length component of each channel of necessary definite whole light signals, thereby when the polarization mode dispersion of the Polarization Controller that utilizes parallel arranged and each different wave length channel of the parallel compensation of birefringence element cascade, the time factor of definition Polarization Controller is provided with the signal (being called the PMD feedback signal herein) of each required channel in a step thus.The method is characterized in that further comprising the steps of: the chromatic dispersion of determining the light signal of or a few different wave length channel in addition.Can use the chromatic dispersion of above measurement to infer the chromatic dispersion that other wavelength channels occur.Then, must define feedback signal (being called feedback signal 2 or CD feedback signal herein) required when walking abreast the chromatic dispersion that compensates each different wave length channel thus.By using from the feedback signal (PMD feedback signal) of polarization mode dispersion and the feedback signal (CD feedback signal) of chromatic dispersion, the Polarization Controller setting of calculating each wavelength channel.And last, parallel setting of using separately on Polarization Controller is to compensate the optical dispersion of the wdm optical signal on each different wave length channel.

According to a further aspect in the invention, utilize the computer-readable medium that records computer program on it to realize purpose of the present invention, this computer program comprises and is suitable for carrying out the code of a process with the chromatic dispersion of compensation wavelength division multiplexing wdm optical signal during operation when described computer program on as the control unit of the optical transmission medium of Polarization Controller.This process is equivalent to carry out the adaptive algorithm of following steps: when the polarization mode dispersion of parallel compensation by each different wave length channel of the wdm optical signal of its spectral dispersion in the different wave length channel of Polarization Controller transmission, collect Polarization Controller required feedback signal (PMD feedback signal) is set.This process is characterised in that, collects feedback signal (CD feedback signal) required when walking abreast the chromatic dispersion that compensates each different wave length channel.Then, by using the feedback signal from polarization mode dispersion and chromatic dispersion, the Polarization Controller setting of calculating each wavelength channel.At last, the parallel setting separately that Polarization Controller is provided of control unit is with the optical dispersion of the wdm optical signal that compensates each different wave length channel.

Useful improvement of the present invention is described in appended claims, following specification and accompanying drawing.

Description of drawings

Further explain illustrative embodiments of the present invention hereinafter with reference to accompanying drawing, wherein accompanying drawing is:

Fig. 1 is the diagram according to compensated stage of the present invention;

Fig. 2 is the diagram of similar compensated stage, represents different may the originating of used feedback signal;

Fig. 3 is the diagram of length scanning projection polariscope; And

Fig. 4 is the diagram of the optional polarization of length scanning projection polariscope.

Embodiment

Fig. 1 represents the structure of multichannel dispersion compensator.It is similar to the multi-channel PMD compensator, just also has at least one that CD is changed responsive feedback signal (FB, FB1, FB2 ...), this feedback signal is used for new control algolithm, even without the PMD of the optimal compensating of determining chromatic dispersion, new control algolithm also can be worked.This type of control algolithm is suitable for determining to alleviate the optimization setting of the distortion that is caused by CD and PMD.At length, the working method of this structure is: wavelength demultiplexing device is distributed each channel in the independent cascade of Polarization Controller PC and birefringence element polarization maintaining optical fibre (PMF).This illustrates two cascades, and cascade also is possible but more.At output, utilize wavelength multiplexer that the signal of all parallel running cascades is focused in the optical fiber once more.At the output of multiplexer, spectropolarimeter is determined the polarization spectrum of each channel; Can utilize the back to follow the scanning optical filter of projection polariscope to realize above-mentioned processing.According to the polarization information that extracts (for example, being used for describing polarization), calculate the optimization setting that makes the minimized PC of distortion (causing worsening), and PC is given in order then with respect to the Stokes vector S of optical frequency ω.

Compare with present technical merit, can add the feedback signal that is used to alleviate CD according to the present invention.Fig. 1 has sketched two kinds of selection schemes of the placement location of feedback signal generation unit.Fig. 2 represents other selection schemes:

● at the public output of signal processing unit, the optical channel filter is selected a channel, and this channel generates the feedback signal FB (PMD feedback signal) that is used for self-adaptive controller.

● extract feedback signal FB2 (CD feedback signal) the single channel path in signal processing (seeing Fig. 1 and Fig. 2).

● selectively, can extract feedback signal in signal processing input, FB4 as shown in Figure 2 (CD feedback signal) perhaps utilizes the FB1 (PMD feedback signal) of optical filter such as Fig. 2, but at public input port.

● also can use feedback signal FB3 (see figure 2) from external equipment.It is directly relevant with forward error correction (FEC) error count, and the FEC error count is to extract in the FEC circuit for a channel from receiver.

Can use two kinds of dissimilar feedback signals and carry out the CD compensation:

I. invariant signal must maximize (or minimizing), for example eye pattern monitoring feedback (" eyes " are opened, and the Q factor is estimated); And must carry out minimized FEC error count and also belong to this grouping.Above-mentioned FB signal is to any signal degradation sensitivity, and is therefore also responsive to the PMD compensation of non-compensation.So can only from the outlet side of compensator extract they (FB1 Fig. 2, FB2, FB3).

Ii. as the signal of the direct measurement of chromatic dispersion.It also is not suitable for all modulation formats.It isolates lower sideband from optical channel, detect them respectively in two receivers, and compares the clock phase difference of two receivers.Difference is the direct measurement of CD.Because it does not pay close attention to the PMD distortion, thus can extract from the two ends of compensator, that is, and FB4 (measure link CD then) or FB2 and FB3 (the compensator CD of measure link+reality setting then).

Adaptive algorithm realizes in control unit, so that carry out PMD and CD compensation simultaneously according to feedback signal.Yet similar with CD self adaptation of carrying out according to the observation of the relevant feedback signal of CD and compensation, the PMD compensation is implemented by the feed-forward adaptive model.Usually, PMD compensation operation (faster) on different time scales.

-measure by length scanning projection polariscope each channel, determine link PMD compensating polarizing transfer function W (following explanation) by control unit.

-then,, determine chromatic dispersion CD based on the CD feedback signal _C, the CD feedback signal is generated by signal processing, and its purpose also is the CD that alleviates link.

-calculate then the signal processing be used for each channel PC p is set _Ik, so that alleviate PMD and CD simultaneously.

At last, being set, order gives PC.

For different feedback signal FB, its CD self adaptation difference:

I. for scalar FB signal, use the shake adaptation mechanism, this pattern-dependent is in a compensator chromatic dispersion CD who generates _CBe demodulated into a new value CD of the channel of corresponding extraction feedback signal _C+ Δ, feedback signal extracts from this channel.

An example of dither algorithm can followingly be represented: if feedback signal FB report has improvement, then make the signal processing (PC+PMF cascade) of (wave band of this shake channel is interior) all channels advance to CD _C+ Δ or a correlation.Otherwise, test another direction, i.e. CD _C-Δ.At last, use new optimal value CD _C=CD _C± Δ (together with correct PMD compensation W).

Ii. for bipolar FB signal, can directly infer and input offset device chromatic dispersion CD as the direct measurement of CD _C, be provided with and PMD compensating polarizing transfer function W so that calculate PC.CD _CShould compensate link dispersion CD _Link, so CD _C=-CD _LinkIf the CD that measures _mFrom outlet side (FB1), extract, then this to be worth be link CD (needs compensation) and compensator CD sum: CD _m=CD _Link+ CD _CWhen measurement is when input side (FB4) carries out, measure link CD:CD then _m=CD _Link

Can utilize real number 3 dimension Stokes vector S (ω) or utilize plural number 2 dimension Jones vector a (ω) to describe the polarization state SOP that is positioned at different spectral positions.

Below explain in detail its mathematical computations:

For the PMD compensation, utilize the length scanning projection polariscope to measure output SOP (polarization state).Fig. 4 represents the implementation of the present art.The subband of an adjustable narrow band optical filter selective light signal spectrum.By adaptive control control wavelength.This output is input in the standard projection polariscope device, and this device utilizes photodiode to determine the relative luminous energy of the different polarization component that the polarizer that has the different polarization transmitter shaft of signal is selected.As shown in Figure 3, I ₁, I ₂And I ₃Be linear 0 degree, the photoelectric current of linear 45 degree and Left-hand circular polarization component.I ₀It is the gross power of this signal.Stokes vector (point on the Poincare sphere) is:

The Stokes vector representation of SOP and Jones vector representation

$a:=\left(\begin{array}{c}\cos \left(\mathrm{\θ}\right)\\ \sin \left(\mathrm{\θ}\right)\·\exp (i\·\mathrm{\φ})\end{array}\right)$

Between the pass be:

$S:=\left[\begin{array}{c}2\·{\left[\right|\stackrel{\‾}{a}\·\left(\begin{array}{c}1\\ 0\end{array}\right)|]}^2-1\\ 2\·{\left[\right|\stackrel{\‾}{a}\·[\frac{1}{\sqrt{2}}\·\left(\begin{array}{c}1\\ 1\end{array}\right)]\left|\right]}^2-1\\ 2\·{\left[\right|\stackrel{\‾}{a}\·[\frac{1}{\sqrt{2}}\·\left(\begin{array}{c}1\\ i\end{array}\right)]\left|\right]}^2-1\end{array}\right]$

Wherein, main vectorial a represents the conjugate complex vector.In a, θ and φ describe the amplitude of two axles respectively and differ each other.

Fig. 4 represents the selectable implementation of length scanning projection polariscope, wherein two projection polariscope parallel runnings.Utilize the preliminary treatment of optical birefringence element to be input to the signal of below in the projection polariscope.This element can improve the chromatic dispersion compensation that is positioned at following position, and light signal processing in these positions Fig. 1 or Fig. 2 and PC and the residing position of PMF cascade can not effectively be converted to the polarization variations in this signal spectrum to the phase distortion of this signal (result from chromatic dispersion).At this moment, known birefringence will be served as discriminating element.

Can calculate PC in the following manner is provided with:

Can utilize Jones vector a _In(ω), scalar amplitude spectrum b (ω) and phase spectrum (ω) describe input (contrast Fig. 1 or Fig. 2), wherein in not having the situation of PMD, and a _In(ω) be wavelength independently.

E _In(ω) :=a _In(ω) e ^{J φ in (ω)}B (ω) E ₀Equation 1

In chromatic dispersion distortion situation, phase spectrum (ω) has secondary and high order component:

$\mathrm{\φ}\left(\mathrm{\ω}\right):={\mathrm{\φ}}_0+\mathrm{\τg}\·(\mathrm{\ω}-{\mathrm{\ω}}_0)-\mathrm{CD}\·\frac{{\mathrm{\λ}}^2}{4\·\mathrm{\π}\·c_0}\·{(\mathrm{\ω}-{\mathrm{\ω}}_0)}^2$ Equation 2

Here, suppose that chromatic dispersion is a constant, the ps/nm that CD describes with link is the CHROMATIC DISPERSION IN FIBER OPTICS of unit.

At the output of compensator, can write down light field with similar type:

E (ω) :=a (ω) e ^{J φ (ω)}B (ω) E ₀Equation 3

According to the polarization spectrum a (ω) that measures, can calculate the PMD (if projection polariscope is positioned at input) of link or PMD (if projection polariscope is positioned at output, as Fig. 1 or shown in Figure 2) that link adds compensator.This is feasible because use p is set _IkUnderstand by the cascade polarized transmission of PC-PMF:

A (ω, p _Ik) e ^{J φ in (ω)+Δ φ (ω)}:=W (p _Ik) a _InE ^{J φ in (ω)}Equation 4

Wherein the Jones matrix W is described polarization variations, and Δ describes the chromatic dispersion that compensator generates. _In+ Δ = is the output phase spectrum.

$w\left(p_{\mathrm{ik}}\right):=e^{j\·\mathrm{\Δ}\·\mathrm{\φ}\left(\mathrm{\ω}\right)}\·\left(\underset{k=N}{\overset{1}{\mathrm{\Π}}}{F}_k{\·M}_k(p_{k,1},p_{k,2}{\·\·p}_{k,m}\right)$ Equation 5

With W factorize Fk and Mk.They describe the matrix of birefringence PMFk and Polarization Controller PCk respectively.Being subjected to the parameter that is provided with of the PC of self-adaptive controller control is p _{K, j}For the specific implementation mode of PC, it can be two parameter p ₁=ξ and p ₂=θ can be written as this matrix:

$M(\mathrm{\θ},\mathrm{\ξ}):=\left(\begin{array}{cc}\cos \mathrm{\ξ}& j\sin \mathrm{\ξ}\\ -j\sin \mathrm{\ξ}& \cos \mathrm{\ξ}\end{array}\right)\·\left(\begin{array}{cc}\exp (j\·\frac{\mathrm{\θ}}{2})& 0\\ 0& \exp (-j\·\frac{\mathrm{\θ}}{2})\end{array}\right)\·\left(\begin{array}{cc}\cos \mathrm{\ξ}& -j\sin \mathrm{\ξ}\\ j\sin \mathrm{\ξ}& \cos \mathrm{\ξ}\end{array}\right)$ Equation 6

Having different Jones matrixes also is feasible with different other implementations that the PC of parameter p i is set.This selection is mainly determined by technology.Therefore, for PC, the different directed wave plates and the cascade of adjustable Faraday rotator may be suitable for favourable implementation.

Have Differential Group Delay Δ τ (for example, for the 10Gbps signal, time delay is 50ps) and the birefringence PMF of main polarization state angle θ and have the Jones matrix:

F:=R ^-1DR equation 7

Wherein

$R\left(\mathrm{\θ}\right):=\left[\begin{array}{cc}\cos \mathrm{\θ}& j\sin \mathrm{\θ}\\ -\left(j\sin \mathrm{\θ}\right)& \cos \mathrm{\θ}\end{array}\right]$ Equation 8

$D\left(\mathrm{\Δ\τ}\right):=\left(\begin{array}{cc}\exp (-j\·\mathrm{\Δ\τ}\·\mathrm{\ω})& 0\\ 0& 0\end{array}\right)$ Equation 9

Can search the appropriate PC parameter p ik that makes CD and PMD distortion minimization in two ways:

I) by means of measuring input signal CD and the PMD that calculates, can control the p that is provided with of PC via look-up table in conjunction with feedback signal _Ik, wherein look-up table is by emulation or generation by experiment.This mode may only be applicable to pure CD compensation.

Ii), can utilize numerical iteration to search the p that is provided with of PC by means of measuring input signal CD and the PMD that calculates in conjunction with feedback signal _IkBelow explain an example:

-by means of input PMD and CD, parameter a _In(ω) and _InBe known (ω).

-by means of each PC p is set _IkAbove-mentioned value, can calculate output valve a ' (ω, p) and ' (ω, p) (equation 4 and equation 5).

-in addition, by means of target CD (providing) be positioned at the minimum PMD of the signal of output, can also calculate the desired value of polarization spectrum and phase spectrum: a by the CD adaptive algorithm _C(ω) and _C(ω).

-can in iterative process, change the p that is provided with of compensator _Ik, so that minimize difference simultaneously | a _C(ω)-a ' (ω, p) | and | _C(ω)- ' (ω, p) |.Utilize amplitude spectrum respectively | b (ω) | or power spectrum b ²It can be favourable (ω) difference being weighted.Can measure or the amplitude spectrum and the power spectrum of estimated signal.

Claims (4)

1. one kind is carried out the method for optical dispersion compensation to the wave division multiplexing WDM light signal, said method comprising the steps of:

The compensated stage that has optical transmission medium is provided, described optical transmission medium is as the parallel running cascade of Polarization Controller and optical birefringence element, thereby when the Polarization Controller by described parallel arranged transmits the described wdm optical signal of its spectral dispersion in the different wave length channel, compensate the chromatic dispersion of described wdm optical signal;

Determine the polarization state of different spectral components of each wavelength channel of the described light signal in all described different wave length channels;

Described Polarization Controller is provided with required feedback signal (FB (ch), FB1 (ch)) when being defined in the polarization mode dispersion of parallel each different wave length channel of compensation thus;

Described method is characterised in that further comprising the steps of:

Determine to allow infer the chromatic dispersion of the light signal of of chromatic dispersion of other wavelength channels or a few different wave length channel;

Required feedback signal when being defined in the chromatic dispersion of parallel each different wave length channel of compensation thus (FB2, FB4);

(FB2 FB4), calculates the Polarization Controller setting of each wavelength channel by the feedback signal of using the feedback signal (FB (ch), FB1 (ch)) that obtained by polarization mode dispersion and obtained by chromatic dispersion;

Parallel setting of using separately on described Polarization Controller is to compensate the optical dispersion of the described wdm optical signal on each different wave length channel.

2. according to the method for carrying out optical dispersion compensation of claim 1, it is characterized in that determining described chromatic dispersion by measuring some wdm optical signal quality of a wavelength channel in a few different wave length channel.

3. according to the method for carrying out optical dispersion compensation of claim 2, it is characterized in that when the polarization mode dispersion compensation characteristic that keeps described wdm optical signal is near optimal value, use described chromatic dispersion feedback signal (FB2, FB4) change the described Polarization Controller setting of each wavelength channel, to obtain improved signal quality.

4. computer-readable medium that records computer program on it, computer program comprises and is suitable for carrying out a process code with the chromatic dispersion of compensation wavelength division multiplexing wdm optical signal during operation when described computer program on as the control unit of the optical transmission medium of Polarization Controller that described process is carried out following steps:

When the Polarization Controller of parallel compensation by described parallel arranged transmits the polarization mode dispersion of each different wave length channel of the described wdm optical signal of its spectral dispersion in the different wave length channel, collect described Polarization Controller required feedback signal (FB (ch), FB1 (ch)) is set;

Described process is characterised in that further comprising the steps of:

Collect when parallel feedback signal required when compensating the chromatic dispersion of each different wave length channel (FB2, FB4);

(FB2 FB4), calculates the described Polarization Controller setting of each wavelength channel by the feedback signal of using the feedback signal (FB (ch), FB1 (ch)) that obtained by polarization mode dispersion and obtained by chromatic dispersion;

Parallel setting of using separately on described Polarization Controller is with the optical dispersion of the described wdm optical signal that compensates each different wave length channel.

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