CN1238750C - Variable birefringence cell and dispersion compensator in polarization mode - Google Patents

Abstract

The present invention relates to a variable birefringence unit and a PMD (polarization mode dispersion) compensator adopting the unit. The variable birefringence unit comprises a chirp optical fiber grating and a pressure generator, wherein the chirp optical fiber grating is positioned in the extrusion direction of the pressure generator. The PMD compensator adopting the variable birefringence unit comprises a polarization controller, a detector for the degree of polarization, a central control unit and the variable birefringence unit, wherein the polarization controller receives incident light and adjusts the polarization state of optical signals. The use of the variable birefringence unit and the PMD compensator in optical communication can realize continuous adjustment of birefringence, and accordingly, the present invention has the advantages of continuously adjusting the delay time of difference groups among polarization states, reducing the difficulty degree and the cost of a software algorithm and enhancing compensating precision.

Description

A kind of polarization mode dispersion compensator

Technical field

The present invention relates to a kind of polarization mode dispersion compensator, particularly utilize the unit of stress changes linear chrip grating refractive index in the optical communication system and adopt the polarization mode dispersion compensator of this variable birefringent elements.

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.At fiber optic collimator, xsect is the circle of standard, 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 merger, 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, 1 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 1＞＞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 reduce signal to noise ratio (S/N ratio) (Signal tonoise 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:

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

Wherein, X _MBe the PMD value of polyphone optical fiber link, X _iBe the PMD of simple optical fiber, L _iBe the length of polyphone light section, 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, after one road light signal is by the transmission of certain distance of optical fiber process, two principal state of polarization (psp) (Principal State ofPolarization of light pulse, 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, the delay that we need make a principal state of polarization (psp) walking fast produce 10ps later on by the PMD compensator, and another principal state of polarization (psp) does not postpone, two polarization states are coupled, have offset the time delay between two principal state of polarization (psp) like this, finally reach the effect of compensation PMD.

Laid-open U.S. Patents 981194.0 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 that Polarization Controller adds the PMD bucking-out system schematic diagram that polarization maintaining optical fibre is made the PMD compensator.Fig. 4 is a PMD bucking-out system schematic diagram of making the PMD compensator with the Variable delay line, from two different schemes as can be seen, no matter is any scheme, and whole PMD bucking-out system generally all comprises four major parts, the one, and Polarization Controller is used for the aligning of main shaft; The 2nd, 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. 3 and Fig. 4 provide.

From above-mentioned two bucking-out systems as can be seen, the advantage of the described technical scheme of Fig. 4 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 3 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 compensator, 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 compensator that adopts described variable birefringent elements, by birefringent adjustable continuously, thereby realize adjustable continuously to the differential group delay of incident light, increase governing speed and precision, reduce the difficulty and the cost of software algorithm simultaneously.

For realizing purpose of the present invention, we provide a kind of polarization mode dispersion compensator, comprise Polarization Controller, the degree of polarization detecting device, central control unit, wherein said Polarization Controller receives incident light, and the polarization state of adjustment light signal, it is characterized in that this polarization mode dispersion compensator also comprises: variable birefringent elements, described variable birefringent elements comprises chirped fiber grating and Pressure generator, described chirped fiber grating is positioned on the direction of extrusion of Pressure generator.

Described polarization mode dispersion compensator, wherein said variable birefringent elements also comprises three port optical loop devices, wherein first port of three port optical loop devices links to each other with the output of Polarization Controller, second port links to each other with chirped fiber grating, the 3rd port links to each other by beam splitter output and with the degree of polarization detecting device, the principal state of polarization (psp) of two quadratures of described Polarization Controller control, so that leading in time principal state of polarization (psp) enters in the slow axis that fiber grating produces because of extruding, the principal state of polarization (psp) of time lag enters in the fast axle that fiber grating produces because of extruding.

Described polarization mode dispersion compensator, wherein said Pressure generator are piezoelectric ceramics, and described chirped fiber grating is wrapped on the Pressure generator, and the periphery puts a smooth metal-coating again, and this metal-coating is close on the fiber grating.

Described polarization mode dispersion compensator, wherein said central control unit is according to the size of the degree of polarization of degree of polarization detecting device institute output signal, generate the two-way feedback signal, one tunnel feedback signal links to each other with Polarization Controller, be used to control the output polarization attitude of Polarization Controller, another road feedback signal links to each other with the input end of piezoelectric ceramics, the size of the voltage that is applied on the control piezoelectric ceramics.

Compared with prior art, major advantage of the present invention is, compared with prior art, major advantage of the present invention is, the first, with the PMD compensator of the polarization maintaining optical fibre that uses regular length as the time delay unit, the present invention can realize birefringent adjustable continuously, thereby realize the adjustable continuously of DGD, compensation effect can be better.Second, adding a Polarization Controller with two sections polarization maintaining optical fibres centres of employing compares as the PMD compensator of time delay unit, feedback control signal way of the present invention is less, thereby reduced the difficulty and the cost of software algorithm, the 3rd, compare with the method that adopts the variable time delay line, it is the adjusting of voltage and do not have mechanical part that DGD of the present invention regulates what use, so governing speed is faster, can better follow the tracks of the variation of single order PMD, the precision of compensation also will be higher.

Description of drawings

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

Fig. 2 is the formation and the compensation synoptic diagram of polarization mode dispersion;

Fig. 3 is that Polarization Controller adds the PMD bucking-out system schematic diagram that polarization maintaining optical fibre is made the PMD compensator;

Fig. 4 is a PMD bucking-out system schematic diagram of making the PMD compensator with the Variable delay line;

Fig. 5 is the structural drawing of PMD compensator of the present invention;

Fig. 6 is the synoptic diagram of the delay inequality of birefringence chirped fiber grating generation;

Fig. 7 is the schematic diagram of lateral compression chirped fiber grating;

Fig. 8 is the application synoptic diagram of PMD compensator of the present invention in optical communication system.

Embodiment

With reference to figure 5, as can be seen from Figure 5, the structure of PMD compensator of the present invention comprises following four parts: 1, Polarization Controller; 2, variable birefringent elements; 3, DOP (degree of polarization) monitoring means; 4, central control unit.With regard to the feedback control structure that the present invention adopts, introduce as our front, be a kind of structure commonly used by the industry, and second portion wherein (being variable birefringent elements) is a core content of the present invention.

Described variable birefringent elements mainly comprises chirped fiber grating (being chirp FBG) and Pressure generator, and wherein chirped fiber grating is positioned on the direction of extrusion of pressure extrusion device.This variable birefringent elements also comprises one three port optical loop device (Opticalcirculator) in addition, wherein first port of three port optical loop devices links to each other with the output of Polarization Controller, second port links to each other with chirped fiber grating, and the 3rd port links to each other by beam splitter output and with the degree of polarization detecting device.

Described Pressure generator can have multiple different device, the present invention mainly provides a kind of piezoelectric ceramics, by powering up for this piezoelectric ceramics, make it produce change in size diametrically, wherein, chirped fiber grating is wrapped on the piezoelectric ceramics, the periphery is overlapped a smooth metal-coating again, metal-coating is close on the fiber grating, like this, when piezoelectric ceramics radially changes, because metal shell is indeclinable, so just chirped fiber grating is formed extruding, thereby change its birefraction.

Certainly, the method for extruding and Pressure generator have a variety of, though among the present invention only itemizing wherein a kind of, do not get rid of other Pressure generator.For example, can also with fibre clip between two level and smooth straight plates, by two planks of extruding, realize extruding, then to change its birefringence to optical fiber.As long as pressure is even, make the lateral compression power equal and opposite in direction of fiber grating, just can make the variation uniformity of birefraction, if push with straight plate, though no problem in theory, but the actual inconvenience that operates.Below be the course of work that the preferred embodiments of the present invention of Pressure generator illustrate the PMD compensator to adopt piezoelectric ceramics, and the principle of work of variable birefringent elements.

The course of work of PMD compensator of the present invention is: from the light signal that has PMD of input end input, carry out entering after the adjustment of polarization state 1 port of three port photocirculators through Polarization Controller, light reflects into port 3 from port one entry port 2 through chirped fiber grating, enter photoreceiver from the light signal one tunnel of port 3 outputs, divide one the tunnel in addition as the PMD detection signal, it is the size of the degree of polarization (DOP) of monitoring output signal, central control unit generates the two-way feedback signal by the monitoring to DOP, one tunnel feedback signal is used for controlling the output polarization attitude of Polarization Controller, leading in time in the Principal State of Polarization of an input signal principal state of polarization (psp) is entered in the slow axis that fiber grating produces because of extruding, and the principal state of polarization (psp) of time lag enters in the fast axle that fiber grating produces because of extruding; Another road feedback signal is used for controlling alive size on the piezoelectric ceramics, control piezoelectric ceramics at radially stroke, finally change the size of DGD (Differential Group Delay), make it be numerically equal to the size of input optical signal PMD by the size that changes lateral compression power.Like this, by the self-adaptation adjustment of whole PMD compensator, make the DOP value reach maximum (between 0.9 to 1) final effect that realizes compensation single order PMD.

Below we introduce the principle of work of core content of the present invention " variable birefringent elements ".It is a refractive index of having utilized the lateral pressure change chirped fiber grating side direction of piezoelectric ceramics generation, thereby make two orthogonal polarisation state components of a certain frequency incident light, reflection position in fiber grating produces difference, finally forms two Differential Group Delays (DGD) between the polarization state.

The phenomenon that produces differential group delay because of birefringence of chirped fiber grating because the refractive index difference of pairwise orthogonal direction makes the position of the reflection spot of same wavelength differ Δ L, thereby produces differential group delay as shown in Figure 6:

$\mathrm{DGD}\left(\mathrm{\λ}\right)=|D_x\left(\mathrm{\λ}\right)-D_y\left(\mathrm{\λ}\right)|=\left|C\right|{\mathrm{\Δ\λ}}_B=\left|C\right|\mathrm{\Δ}2\mathrm{n\Λ}=\left|C\right|\frac{\mathrm{\Δn}}{n}\mathrm{\λ}---------\left(7\right)$

In the following formula, C is the abbe number of fiber grating, and Δ n is birefringent size, and Λ is a grating space, from (7) formula we as can be known, under the situation of setted wavelength, DGD and birefringence are proportional.Promptly just can regulate the size of DGD by regulating birefringent size.In fact, all can change its refractive index and cause birefringence for any optical fiber stress application, we have used the big dispersion values of chirped fiber grating and to the reflex of specific wavelength at this.

The canonical parameter of linear chirp optical fiber grating is: | C|=1000ps/nm, λ=1550nm, n=1.447.We can obtain following relational expression:

$\mathrm{DGD}=1000\×\frac{\mathrm{\Δn}}{n}\×\mathrm{\λ}=1.07118\×{10}^6\·\mathrm{\Δn}----------\left(8\right)$

By (8) formula as can be known, when the variation range of Δ n 10 ^-6To 10 ^-4Between when changing, the variation range of DGD between 1～100ps, that is to say that DGD can be adjustable continuously by birefringent adjusting is realized greatly, this is very important for single order PMD compensation.So how the variation by lateral pressure realizes the variation of birefringence in above scope, and next we explain this problem.

Can cause that the birefringent factor of ordinary optic fibre comprises stress, bending and distortion etc.Here we only consider bending and stress, because these two kinds of influences are the most tangible two factors in the present invention.At first let us is had a look the birefringence that bending causes, we know that bend birefringence can be expressed as:

$B_b=\frac{\mathrm{\δ}{\mathrm{\β}}_b}{\mathrm{\β}}=\frac{1}{4}{n}^2(1+v)(p_{12}-p_{11}){\left(\frac{A}{R}\right)}^2------------\left(9\right)$

In the formula, n is a fiber core refractive index, and ν is a Poisson ratio; P11, p12 are photoelastic tensor, and A is the optical fiber external diameter, and R is a bending radius.Can get for silica fibre

$B_b=0.093{\left(\frac{A}{R}\right)}^2------------------\left(10\right)$

When A=2.5 * 10 ^-6During m, $B_b=5.8125\×{10}^{-11}\×{\left(\frac{1}{R}\right)}^2---------------\left(11\right)$

We can extrapolate thus, and when bending radius during in centimetre-sized, the birefringence that causes is in the 1E-8 magnitude, and according to (8) formula as can be known, the DGD that is caused by bending is 10 ^-2Picosecond magnitude is so we can ignore the birefringence that is caused by bending.

For stress birefrin, be Δ σ=σ as if the stress difference between two orthogonal directionss _x-σ _yThe time, as shown in Figure 7, then the difference of the refractive index on this both direction is

$\mathrm{\&Delta;n}=\frac{{\mathrm{<figure-callout\; id="3"\; label="n"\; filenames="C0214835900161.png"\; state="\{\{state\}\}">n</figure-callout>}}^3}{2E}(1+v)(p_{12}-p_{11})\mathrm{\&Delta;\&sigma;}---------------\left(12\right)$

In the following formula, E is a Young modulus, for silica fibre E=7.0 * 10 ¹⁰Pa, ν=0.17, p ₁₁=0.121, p ₁₂=0.270, if n=1.447 then has

Δn＝3.7727×10 ^-12×Δσ----------------(13)

Again because $\mathrm{\&Delta;\&sigma;}={\mathrm{\&sigma;}}_x-{\mathrm{\&sigma;}}_y=\frac{2F}{\mathrm{\&pi;L}D}-(-\frac{6F}{\mathrm{\&pi;LD}})=\frac{8F}{\mathrm{\&pi;LD}},$ Wherein L is extruded fiber grating length, and D is a fibre diameter, and F is suffered extruding force.If L=12cm, the D=125 micron is so have

Δn＝6.4047×10 ^-7F-----------------(14)

As seen, when pressure when 1 newton changes in 100 Newton range, approximately can realize the DGD variation range of 1 psec to 70 psec, this pressure is to realize very easily in the application of actual piezoelectric ceramics.

In sum, the birefringence that bending causes can be ignored on the order of magnitude, and because the birefringence that lateral compression produces can realize the continuous adjusting of DGD in 100 picosecond range, thereby realize the compensation to single order PMD of the present invention, and this gordian technique of the present invention just place.

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.

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 (4)

1. polarization mode dispersion compensator, comprise Polarization Controller, the degree of polarization detecting device, central control unit, wherein said Polarization Controller receives incident light, and adjusts the polarization state of light signal, it is characterized in that this polarization mode dispersion compensator also comprises: variable birefringent elements, described variable birefringent elements comprises chirped fiber grating and Pressure generator, and described chirped fiber grating is positioned on the direction of extrusion of Pressure generator.

2. polarization mode dispersion compensator as claimed in claim 1, it is characterized in that described variable birefringent elements also comprises three port optical loop devices, wherein first port (1) of three port optical loop devices links to each other with the output of Polarization Controller, second port (2) links to each other with chirped fiber grating, the 3rd port (3) links to each other by beam splitter output and with the degree of polarization detecting device, the principal state of polarization (psp) of two quadratures of described Polarization Controller control, so that leading in time principal state of polarization (psp) enters in the slow axis that fiber grating produces because of extruding, the principal state of polarization (psp) of time lag enters in the fast axle that fiber grating produces because of extruding.

3. polarization mode dispersion compensator as claimed in claim 1 or 2, it is characterized in that described Pressure generator is a piezoelectric ceramics, described chirped fiber grating is wrapped on the Pressure generator, and the periphery puts a smooth metal-coating again, and this metal-coating is close on the fiber grating.

4. polarization mode dispersion compensator as claimed in claim 3, it is characterized in that the size of described central control unit according to the degree of polarization of degree of polarization detecting device institute output signal, generate the two-way feedback signal, one tunnel feedback signal links to each other with Polarization Controller, be used to control the output polarization attitude of Polarization Controller, another road feedback signal links to each other with the input end of piezoelectric ceramics, the size of the voltage that is applied on the control piezoelectric ceramics.

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