CN1222796C - Polarization mode dispersion compensation process and compensator thereof - Google Patents

Abstract

The present invention relates to a polarization mode dispersion compensation method and a compensator. The method is used for solving the polarization mode dispersion compensation problem in optical communication systems. The polarization mode dispersion compensator comprises a polarization controller, a detector and a controller, wherein the polarization controller receives incident light, adjusts the polarization state of the incident light and outputs an optical signal. The polarization mode dispersion compensator also comprises a polarization beam splitter which directly receives the optical signal from the polarization controller, separates two orthogonal polarization main states of the input light and then respectively outputs two orthogonal polarization components. The detector receives and detects one orthogonal polarization component from one output end of the polarization beam splitter and outputs a detection signal. The controller generates a control signal according to the detection signal for controlling the polarization controller. Because a time delay circuit does not exist, the hardware cost of the PMD compensator is greatly reduced; monitoring signals are easily obtained, and the path number of feedback control signals is greatly decreased.

Description

Polarization mode dispersion (PMD) compensation method and compensator thereof

Technical field

The present invention relates to a kind of polarization mode dispersion (PMD) compensation method and compensator thereof, have the polarization mode dispersion (PMD) compensation method and the compensator thereof of single order and second order polarization mode dispersion (PMD) compensation function in particularly a kind of optical communication system.

Background technology

PMD (polarization mode dispersion, polarization mode dispersion) is meant two mutually orthogonal polarization mode LP that transmit in single-mode fiber ₀₁ ^XAnd LP ₀₁ ^Y, poor through the time of arrival after the transmission of certain distance in optical fiber.The unit of PMD is ps.In the ideal case, be the circle of standard at fiber optic collimator, xsect, index distribution everywhere symmetrically and ideally isotropic, LP ₀₁ ^XAnd LP ₀₁ ^YTransmission β x and β y equate that two polarization modes are that complete two degree annex (propagation constant is identical), are independent of each other in transmission course.

But, under actual conditions, at production, the stranding of optical fiber, lay, with and process such as surrounding environment change in, all can inevitably make the refractive index of optical fiber produce different variations along different directions, promptly present birefringence effect.Fig. 1 is the formation figure of polarization mode dispersion, as can be seen from Figure 1, through the transmission of certain hour, in transmission direction because LP ₀₁ ^XAnd LP ₀₁ ^YTransmission β x and β y different and produce certain PMD.In addition, when light signal during by some optic communication devices such as isolator, coupling mechanism, wave filter, because the imperfection of device architecture and material itself also can cause birefringence.And this birefringence effect can directly cause the polarization mode of pairwise orthogonal to have different phase velocities and group velocity, thereby has lost degeneracy, produces PMD.

Birefringence is the root that produces polarization mode dispersion.Birefringence comprises intrinsic birefringence and induced birefringence, wherein, intrinsic birefringence mainly is meant the birefringence that is caused by aspects such as material and manufacturing process in the manufacture process of optical fiber, induced birefringence is meant the birefringence that is caused by the photoelastic effect of external force by fiber medium, compare with the former, induced birefringence has the feature of randomness more.

Take all factors into consideration intrinsic birefringence (comprising how much birefringences, stress birefrin) and induced birefringence (comprising bending, side force, turn-knob extra electric field and externally-applied magnetic field), ignore the interference between them, think that they are incoherent, then total birefringence can be expressed as:

Δβ＝Δβ _G+Δβ _S+Δβ _bF+Δβ _f+Δβ _C+Δβ _E+Δβ _h ----(1)

Do not having under the situation of Mode Coupling, the PMD on the corresponding unit length can simply be expressed as:

And when the propagation speed differential between two polarization modes very hour, externalities is easy to make energy exchange takes place between two polarization modes, promptly produces Mode Coupling.In general external action has randomness, and therefore, this Mode Coupling also just has randomicity characteristics, and it has very significant effects effect to the performance of PMD.

The average total birefringence τ and the average polarization mode coupling length h of PMD and optical fiber have following relation:

In the following formula, l is the length of optical fiber.

When l＜＜h, the coupling between two polarization modes can be ignored, then:

In time domain, the PMD effect is presented as respectively along the delay inequality between the light pulse component of fast, slow axis transmission, and after this delay inequality made light through one section transmission, total light pulse was broadening, thereby limited the transfer rate of optical communication system.For short fiber, the value of PMD increases along with transmission length is linear, and unit is ps/km ^1/2

When l＞＞during h, the value in the bracket of (3) formula the right is about 2l/h, has:

When light pulse when long optical fibers transmits because the variation of external factor,, can cause Mode Coupling, the energy exchange between promptly fast, the slow mode as variation of temperature etc.Because the extraneous randomness that changes, Mode Coupling also takes place at random.From following formula we as can be seen, for long optical fibers, PMD is that the square root along with transmission length increases.Unit is

Mode Coupling has not only determined the relation of PMD and fiber lengths simply, and is factors such as the slight jitter all very sensitive reason of PMD to temperature, vibration, optical source wavelength.Under equal conditions, stronger Mode Coupling correspondence less polarization mode dispersion.Proved the susceptibility of PMD to temperature variation as C.D.Poole in experiment in 1991, not only the value of PMD changes along with variation of temperature, and its rate of change also depends on the speed of temperature variation simultaneously.During temperature constant, PMD does not almost have any obvious variation, and when temperature increased fast, the fluctuation of PMD also significantly increased.

PMD causes pulse strenching in digital display circuit, cause the bit error rate to increase, the bandwidth of restriction system; In simulation system, cause distorted signals, restricting channel quantity.Before several years, the numeral and simulation system in, when data transmission rate lower with apart from relative more in short-term, PMD is insignificant to the influence of single mode fiber system.Along with the growth to bandwidth demand, particularly in the system of 10Gb/s, 40Gb/s and higher rate, PMD begins to become the key factor of restriction system performance.Because it can cause excessive pulse strenching or caused low signal to noise ratio (S/N ratio) (Signalto noise ratio, SNR).

By system's maximum transmission distance of PMD restriction, what promptly ITU-T advised is the maximum transmission distance of reference with the 1dB Power penalty, can be drawn by following formula theoretically:

According to following formula, the maximum transmission distance of PMD restriction can be listed in the table below, this table has provided the relation of transmission range to PMD and bit rate

Because the statistical property of PMD, the PMD index of simple optical fiber (or the optical fiber behind the stranding) is not suitable as the index of power system capacity.Otherwise link value (i.e. the fiber segment of Xiang Lianing) often is used.Because every fiber segment is random quantity, thereby the link value also is a random quantity and since average effect it have littler variance.PMD link value is by following formulae express:

$X_M=\sqrt{\frac{\mathrm{\Σ}{X}_i^2{L}_i}{\mathrm{\Σ}{L}_i}}(I=\mathrm{1,2}\·\·\·M)---\left(6\right)$

Wherein, X _MBe the PMD value of polyphone optical fiber link, Xi is the PMD of simple optical fiber, and Li is the length of polyphone light section, and M is the number of polyphone optical fiber.The link value of PMD is more accurate more effectively to have reflected the PMD of system value, and can make full use of the real potential of optical fiber.

The polarization mode dispersion effect that stands by the Transmission Fibers propagating optical signal, be to utilize the delay compensation device or the birefringent compensator of reception place to be compensated, wherein compensator automatically and a self-adaptation generation retardation derivative time, be substantially equal to delay derivative time that light signal stands, offset undesirable delay basically.For example, with reference to figure 2, as can be seen from Figure 2, after one road light signal is by the transmission of certain distance of optical fiber process, two principal state of polarization (psp) of light pulse (Principal State of Polarization, PSP) produced the delay of 10ps, it is the leading another one principal state of polarization (psp) of one of them principal state of polarization (psp), in order to manage to offset this retardation, we need make the delay that principal state of polarization (psp) produces 10ps later on by the PMD compensator of walking fast, and another principal state of polarization (psp) does not postpone, and two polarization states are coupled, offset the time delay between two principal state of polarization (psp) like this, finally reached the effect of compensation PMD.

Laid-open U.S. Patents 98119194 discloses and utilizes polarization maintaining optical fibre to make the compensation of delay device and do two kinds of schemes of compensator with the variable time delay line at present.Fig. 3 is a PMD bucking-out system schematic diagram of making the PMD compensator with the Variable delay line, and Fig. 4 is that Polarization Controller adds the PMD bucking-out system schematic diagram that polarization maintaining optical fibre is made the PMD compensator.From two different schemes as can be seen, no matter be any scheme, whole PMD bucking-out system generally all comprises four major parts, the one, Polarization Controller is used for the aligning of main shaft; The 2nd, the PMD compensator is used for the PMD amount that bucking-out system produces; The 3rd, the PMD detecting device is used to test the size of PMD value, generates monitor signal; The 4th, feedback controller is used for the generation of feedback signal size and to the control of Polarization Controller or variable time delay line.As can be seen, the difference of two systems is to have adopted different PMD compensator (variable delay unit between polarization mode) from the PMD bucking-out system schematic diagram that Fig. 2 and Fig. 3 provide.

From above-mentioned two bucking-out systems as can be seen, the advantage of the described technical scheme of Fig. 3 is less (three parameters of Polarization Controller of parameter that need control, parameter of variable time delay line), so algorithm is simple, realize than being easier to, but that shortcoming is a feedback speed is slower, mainly be to be that moving horizontally of scioptics realized one tunnel time delay in the variable time delay line, so reaction rate is subjected to certain restriction, and very high to the requirement of the collimation between lens and the optical fiber, otherwise will produce bigger decay.

The advantage of technical scheme shown in Figure 4 is that structure is simple relatively, reaction rate height, but shortcoming is computing complexity (two Polarization Controller totally six parameters), software design cost height, and the complexity of the algorithm speed of response of sacrificial section hardware probably.Bu Chang dynamic range is also received certain restriction simultaneously.

Hence one can see that, in optical fiber telecommunications system, along with the raising of single channel transfer rate and the increase of analog signal transmission bandwidth, except chromatic dispersion, limiting factor such as non-linear, the originally polarization mode dispersion of not too being paid close attention to (Polarization Mode Dispersion, PMD) problem becomes very outstanding recently, and particularly for the long Distance Transmission of the transmission system more than the 40Gbit/s, PMD is considered to final limiting factor.Polarization mode dispersion will cause pulse strenching in digital communication system, increase the bit error rate; In analog communication system, will produce the high-order distortion effect, make the distorted signals distortion.Thus, need provide a kind of bucking-out system, be used for the polarization mode dispersion of compensated high-speed optical communication system.

Summary of the invention

The purpose of this invention is to provide a kind of PMD compensation method, utilize this method can realize the PMD compensation, and reaction velocity is fast, algorithm is simple.

Another object of the present invention provides a kind of PMD compensator, carries out polarization mode dispersion (PMD) compensation according to polarization beam apparatus and chirped fiber grating, and simple in structure, and cost is lower.

For realizing purpose of the present invention, we provide a kind of polarization mode dispersion (PMD) compensation method, and it comprises the steps: 1) adjust the polarization of incident light attitude, two principal state of polarization (psp) importing light are divided into two orthogonal orthogonal polarization components; 2) tell one tunnel orthogonal polarization components and output, then it is detected, and output detection signal; 3) generate control signal according to detection signal, be used for the polarization of incident light attitude of set-up procedure 1, be higher than a threshold value with the luminous power of another output orthogonal polarized component of realizing output; 4) export another output orthogonal polarized component (4) to a reflection-type element, the frequency that makes orthogonal polarization components (4) is exported described light signal then at the different position reflected light signal of reflection-type element.

According to another aspect of the present invention, we provide a kind of polarization mode dispersion compensator, comprise Polarization Controller, detecting device and controller, described Polarization Controller receives incident light, its polarization state is adjusted, and the output light signal, wherein said polarization mode dispersion compensator also comprises, polarization beam apparatus, it directly receives the light signal from Polarization Controller, and these two orthogonal principal state of polarization (psp) importing light are separated, and exports two orthogonal polarization components then respectively; Wherein said detecting device receives an orthogonal polarization components from an output terminal of polarization beam apparatus, it detected, and output detection signal; Described controller generates a control signal according to detection signal, is used for Polarization Controller is controlled; Circulator has three ports at least, receives an orthogonal polarization components of polarization beam apparatus output from first port, and is transferred to second port; Fiber grating receives described orthogonal polarization components from second port of circulator, according to the frequency of described orthogonal polarization components at different position reflected light signals, and from the 3rd port output light signal of circulator, with the polarization mode dispersion of compensation second order.

Compare with prior art, major advantage of the present invention is, the first, removed " delay compensation device " unit, and greatly reduce the hardware cost of PMD compensator; The second, monitor signal obtains easily, and the feedback control signal way significantly reduces, and has reduced the software algorithm cost; The 3rd, owing to the PMD compensation method of directly taking polarization beam splitting to obtain a principal state of polarization (psp), the compensation range to single order PMD can reach needed any value in theory, i.e. the dynamic range of PMD compensation is very big; The 4th, part second order PMD (polarization be correlated with chromatic dispersion) has been carried out effective compensation.

Description of drawings

Fig. 1 is the formation figure of polarization mode dispersion;

Fig. 2 is a PMD compensation process synoptic diagram;

Fig. 3 is a PMD bucking-out system schematic diagram of making the PMD compensator with the Variable delay line;

Fig. 4 is that Polarization Controller adds the PMD bucking-out system schematic diagram that polarization maintaining optical fibre is made the PMD compensator;

Fig. 5 is the structural drawing of PMD bucking-out system of the present invention;

Fig. 6 is the form of expression synoptic diagram of second order PMD;

Fig. 7 is the relevant dispersion compensation synoptic diagram of polarization;

Fig. 8 is that PMD compensator of the present invention is applied to the synoptic diagram in the optical communication system;

Fig. 9 is an experimental verification schematic diagram of the present invention;

Figure 10 is the graph of relation of Q parameter and PMD size when PMD compensation being arranged and not having the PMD compensation.

Embodiment

At first, as can be seen from Figure 5, the present invention includes following components: Polarization Controller (PC, polarization controler) referring to Fig. 5; Polarization beam apparatus (PBS, polarization beamsplitter); Three port optical loop devices (Optical circulator); The chirped fiber grating (being chirp FBG) that is used for dispersion compensation; Optical power detector (OPM, optical power meter) and central control unit (Centre Control Unit).

Principle of work of the present invention and other foreign patent are different.In the argumentation in front, we can find that in the structure of the described PMD bucking-out system of other patents a compensator (time delay device) can be arranged, and the fundamental purpose of this unit is exactly the delay inequality-Δ τ that produces an additional equal and opposite in direction, opposite in sign _cCome compensated optical signal through going up the delay inequality-Δ τ that produces at two principal state of polarization (psp) (PSP) after the Optical Fiber Transmission _f, finally make Δ τ _f--Δ τ _c=0, promptly compensated the PMD influence in the system.But we also can realize the compensation to PMD under the situation of compensator not having.Principal state of polarization (psp) (PSP) theory is thought, in the single-mode fiber transmission, there are two orthogonal polarization states in the basic mode of light wave, the physical dimension of ideal fiber is uniform, and there is not other stress, thereby these two orthogonal polarization states of light wave are propagated with identical speed, at the other end of optical fiber without any delay, yet in the optical fiber of reality, because the defective and the stress of optical fiber itself, make two principal state of polarization (psp) propagate with different speed, thereby it is also inequality to arrive time of the optical fiber other end, this delay inequality is exactly our said PMD.We notice, PMD is meant and is present in two delay inequalities between the principal state of polarization (psp), and with regard to each principal state of polarization (psp) component itself, the waveform of its signal is (under the situation of not considering high-order PMD) that remains unchanged substantially, and under normal conditions, each principal state of polarization (psp) with energy all be approximately half of gross energy.Based on above consideration, we just can take another to give up a principal state of polarization (psp) and the method that receives only another principal state of polarization (psp) signal reaches and eliminates or method that " compensation " PMD influences.In order to overcome the situation of the energy distribution ratio great disparity on two principal state of polarization (psp) that may occur, we can utilize the signal of a principal state of polarization (psp) will giving up to do monitor signal, adjust the polarization state of input end, to realize the making signal intensity of road polarization state to be received maintain stronger level.

More than we have only spoken of the situation of single order PMD, when considering second order PMD, we need add other device to reduce the influence of second order PMD in system.The form of expression of second order PMD as shown in Figure 6.We are as can be seen from figure, single order PMD mainly shows as delay inequality between two principal state of polarization (psp), second order PMD then mainly shows as the broadening of each principal state of polarization (psp) itself, this broadening has embodied wavelength (frequency) dependence of PMD, be similar to dispersion phenomenon, we be referred to as the relevant chromatic dispersion of polarization (Polarization-dependent chromatic dispersion, PCD).Single order PMD and second order PMD are respectively at the description vector of Stokes space

With Wherein,

$\stackrel{\→}{\mathrm{\Ω}}=\mathrm{\Δ\τ}\stackrel{\→}{q};---\left(7\right)$

${\stackrel{\→}{\mathrm{\Ω}}}_{\mathrm{\ω}}={\mathrm{\Δ\τ}}_{\mathrm{\ω}}\stackrel{\→}{q}+\mathrm{\Δ\τ}{\stackrel{\→}{q}}_{\mathrm{\ω}}---\left(8\right)$

From (8) formula we as can be seen, the correlativity of second order PMD and frequency also shows the variation of the direction of polarization principal axis with frequency.So, realize that the full remuneration to second order PMD is more complicated and difficulty, the present invention is to the also just part compensation of compensation of second order PMD.

The present invention is such to the implementation procedure of single order and second order PMD compensation, please refer to Fig. 5, import from input port 1 through the light signal after the system transmissions, by Polarization Controller (PC) its polarization state is adjusted then, the purpose of adjusting is to make the signal that is input to polarization beam apparatus (PBS) on two arms 4 and 5 of its output the certain power distribution ratio be arranged, the effect of PBS is exactly will import two principal state of polarization (psp) of light separately, the orthogonal principal state of polarization (psp) signal of two-way through PBS, one the road from 4 by 6 input ends that enter the light annular device, one the tunnel directly enters optical power detector (Optical power Meter, OPM), optical power detector is delivered to control module with detected luminous power as monitor signal and is generated control signal, control Polarization Controller, it is said that its purpose is exactly our front, make Polarization Controller change the polarization state of input optical signal, realizing being higher than a threshold value, such as half of input signal power from the luminous power of port 4 outputs.Like this, from one road polarization signal of port 4 output in fact " compensation " single order PMD, the purpose of circulator and fiber grating is exactly the compensation of implementing second order PMD, polarization signal from input end 6 input of circulator, through 7 ports to fiber grating, then by fiber grating to the different compensation that realize the second order PMD of frequency dependence of the reflection position of the light component of different frequency, the relevant dispersion compensation synoptic diagram of concrete polarization is with reference to figure 7, as can be seen from Figure 7, described light is imported from chirped fiber grating, its different wavelength signals is from different stop position output, because the distance difference, so the phase place of the signal of its output also has nothing in common with each other.Last light is from port 8 outputs.That is to say, from output terminal 8 outputs the time, realized the compensation of the second order PMD of the compensation of single order PMD and part from the signal that has PMD of input end 1 input.

As mentioned above, we can adopt following method to carry out polarization mode dispersion (PMD) compensation, and it comprises the steps: 1) adjust the polarization of incident light attitude, two principal state of polarization (psp) importing light are divided into two orthogonal orthogonal polarization components; 2) tell one tunnel orthogonal polarization components 5 and output, then it is detected, and output detection signal (9); 3) generate control signals according to detection signal 9, be used for the polarization of incident light attitude of set-up procedure 1, be higher than a threshold value with the luminous power of another output orthogonal polarized component 4 of realizing output; 4) export another orthogonal polarization components 4.

After having carried out single order PMD compensation, we can also adopt following method to carry out described second order polarization mode dispersion (PMD) compensation, and it comprises the steps: 1) receive another orthogonal polarization components 4 of output; 2) according to the frequency of described orthogonal polarization components at different position reflected light signals, export described light signal then.

The application of the present invention in optical transmission system as shown in Figure 8, after the light signal process Optical Fiber Transmission of certain distance, produced polarization mode dispersion (PMD), we have added the PMD compensator at the front end of photoreceiver, to eliminate influences such as pulse strenching that PMD produces and distortion, the final signal that reduces photoreceiver receives the bit error rate, improves the performance of overall optical transmission system.

The present invention has passed through the related experiment checking.Experimental principle figure and test result are respectively as Fig. 9 and shown in Figure 10, and wherein, ATT is for can coordinate attenuator, and CSA8000 is the eye pattern analyser.What we taked is that the manual adjustments Polarization Controller is also observed the variation of luminous power simultaneously and the situation of change of eye pattern determines to reach The optimal compensation point.In experiment, the single-mode fiber that we use has very big PMD value, from 10 figure as can be seen, its horizontal ordinate is the value of average DGD, and ordinate is the Q value, and it is different that the Q value of PMD compensator and no PMD compensator is arranged, receiving optical signals with PMD compensator, the value of its PMD is much smaller.

It can also be seen that in Figure 10 the maximal value of Q value can reach three times of mean value, timely compensation scope promptly of the present invention can reach 180ps, compares with external report, also wants more bigger.

Description of the invention, describe in detail and above-mentioned accompanying drawing be not be used for limiting of the present invention.To those skilled in the art, under instruction of the present invention, can carry out various corresponding modification and can not exceed the spirit and scope of the present invention, but this variation should be included within claim of the present invention and the equivalent scope thereof.

Claims (6)

1. a polarization mode dispersion (PMD) compensation method is characterized in that comprising the steps:

1) adjusts the polarization of incident light attitude, two principal state of polarization (psp) importing light are divided into two orthogonal orthogonal polarization components;

2) tell one tunnel orthogonal polarization components (5) and output, then it is detected, and output detection signal (9);

3) generate control signal according to detection signal (9), be used for the polarization of incident light attitude of set-up procedure 1, be higher than a threshold value with the luminous power of another output orthogonal polarized component (4) of realizing output;

4) export another output orthogonal polarized component (4) to a reflection-type element, the frequency that makes orthogonal polarization components (4) is exported described light signal then at the different position reflected light signal of reflection-type element.

2. a polarization mode dispersion compensator comprises Polarization Controller, detecting device and controller, and described Polarization Controller receives incident light (1), its polarization state adjusted, and output light signal (2), it is characterized in that described polarization mode dispersion compensator also comprises,

Polarization beam apparatus, it directly receives the light signal (3) from Polarization Controller, and these two orthogonal principal state of polarization (psp) importing light are separated, and exports two orthogonal polarization components (4,5) then respectively;

Wherein said detecting device receives the orthogonal polarization components (5) from an output terminal of polarization beam apparatus, it detected, and output detection signal (9);

Described controller generates a control signal (10) according to detection signal, is used for Polarization Controller is controlled;

Circulator has three ports at least, receives an orthogonal polarization components (4) of polarization beam apparatus output from first port (6), and is transferred to second port (7);

Fiber grating, receive described orthogonal polarization components from second port (7) of circulator, according to the frequency of described orthogonal polarization components at different position reflected light signals, and from the 3rd port (8) the output light signal of circulator, with the polarization mode dispersion of compensation second order.

3. polarization mode dispersion compensator as claimed in claim 2 is characterized in that described detecting device is an optical power detector, receives the orthogonal polarization components from an output terminal of polarization beam apparatus, and with detected optical power signals output.

4. polarization mode dispersion compensator as claimed in claim 2 is characterized in that described controller is a central control unit, receives the optical power signals of detecting device output, produces a control signal, is used to control Polarization Controller to change the polarization state of input signal.

5. polarization mode dispersion compensator as claimed in claim 2, it is characterized in that described Polarization Controller exports respectively from two output terminals of polarization beam apparatus than with two orthogonal polarization components with certain power, and make the power that is input to circulator be higher than a threshold value.

6. polarization mode dispersion compensator as claimed in claim 2 is characterized in that described fiber grating is a chirped fiber grating.

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