CN103245490B - The measuring method of polarization modes coupling distribution in polarization maintaining optical fibre - Google Patents

Abstract

The invention discloses the measuring method of polarization modes coupling distribution in a kind of polarization maintaining optical fibre, relate to polarization maintaining optical fibre field, this measuring method comprises the following steps: the radio frequency range and the scanning step that arrange vector network analyzer, computer memory resolution, vector network analyzer launches modulated radio signal to intensity modulator, the light signal that intensity modulator exports is coupled in polarization maintaining optical fibre through the first polarizer, the light signal that second polarizer exports is amplified into photodetector through image intensifer, light signal is converted to electric signal by photodetector, output to vector network analyzer, vector network analyzer receives, display, store electricity signal, a square formula for radio frequency filter response gets inverse Fourier transform, calculates position and the intensity of polarization modes coupling generation.The present invention can measure the polarization modes coupling distribution of the polarization maintaining optical fibre of 50 kilometers long, and the minimum detectable polarization modes coupling intensity measured is 10 ^-6~ 10 ^-5, signal to noise ratio (S/N ratio) improves more than 10 decibels than existing scheme.

Description

The measuring method of polarization modes coupling distribution in polarization maintaining optical fibre

Technical field

The present invention relates to polarization maintaining optical fibre field, particularly relate to the measuring method of polarization modes coupling distribution in a kind of polarization maintaining optical fibre.

Background technology

Light signal is at PMF (Polarization Maintaining Fiber, polarization maintaining optical fibre) in propagate time, if input polarisation of light state is along the fast axle of polarization maintaining optical fibre or slow-axis direction, then polarization state remains unchanged, and polarization maintaining optical fibre obtains this character by the birefringence introduced between very large two polarization modes.Due to this special character, polarization maintaining optical fibre has very important application in coherent fiber communication system, optical fiber sensing system, such as: well-known fibre optic gyroscope; In addition, polarization maintaining optical fibre is also applied in various optical device, such as: PMD (Polarization Mode Dispersion, polarization mode dispersion) emulator, PMD compensator, the optical fiber polarizer, fiber optics interferometer etc.

PMC (Polarization Mode Coupling, polarization modes coupling) be some specific positions in polarization maintaining optical fibre two polarization modes between energy exchange, this energy exchange is caused by the disturbance of the inside or outside that act on polarization maintaining optical fibre, may be distributed on whole fiber lengths.Polarization modes coupling limits in polarization maintaining optical fibre protects inclined effect, reduces the performance of coherent optical communication system or optical fiber sensing system.In order to eliminate polarization modes coupling, the position that polarization modes coupling occurs must be found out in polarization maintaining optical fibre.

By search patent and paper database, find the two schemes being completed the distribution measuring of polarization modes coupling in polarization maintaining optical fibre by harmless mode.

The first is based on white light source and MI (Michelson Interferometer, Michelson interferometer) scheme, the measurement mechanism of this scheme is with SLD (super luminescentdiode, super-radiance light emitting diode) as wideband light source, measurement mechanism comprises polarization adjustment member and scanning Michelson interferometer, in order to implementation space distribution measuring, simultaneously also in order to improve degree of accuracy, PZT (PiezoelectricTransducer, piezoelectric transducer) is employed in scanning Michelson interferometer.The maximum length of the tested polarization maintaining optical fibre reported is 1km, and the spatial resolution of this scheme follows relational expression: L _d=L _c/ D n, wherein: L _dspatial resolution, L _cbe the coherent length of light source, Dn is the refringence between two kinds of polarization modes.Minimum detectable polarization modes coupling intensity depends on the performance of system, and the value of the minimum polarization modes coupling intensity reported is 10 ^-4.Owing to make use of PZT and the MI with adjustable platform, this measurement mechanism is more responsive to environmental perturbation, and may lose efficacy in long-time test.

The second is the scheme based on P-OTDR (Polarimetric-Optical Time-DomainReflectometry, polarization-optical time domain reflectometry), and P-OTDR can measure the space distribution of polarization state along fiber lengths.If there is polarization modes coupling in polarization maintaining optical fibre, polarization state will change where at polarization modes coupling, even if input is only injected along the fast axle of polarization maintaining optical fibre or slow axis, therefore POTDR can be used for the polarization modes coupling distribution of to measure in polarization maintaining optical fibre.Its spatial resolution is determined by direct impulse width, and representative value is the pulse width of the corresponding 10ns of 1m.Because P-OTDR utilizes rayleigh backscattering light as detectable signal, so SNR (Signal to Noise Ratio, signal to noise ratio (S/N ratio)) is lower.In order to increase SNR, wider pulse width can be adopted, but easily cause reducing spatial resolution; Or, repetitive measurement can be utilized to be averaged to improve SNR, but easily to cause Measuring Time to extend to a few minutes.

Above-mentioned two schemes can be used for realizing utilizing the distributing optical fiber sensing of polarization maintaining optical fibre, when external disturbance acts on some specific positions on polarization maintaining optical fibre, polarization modes coupling will there will be, this two schemes all can measure polarization modes coupling, get final product the parameters such as sensing such as stress according to this, but all there is following shortcoming in this two schemes: the polarization modes coupling that (1) is only only applicable to the shorter polarization maintaining optical fibre being less than 1 kilometer is measured; (2) because measured signal is very faint, signal to noise ratio (S/N ratio) is very little.

Summary of the invention

The object of the invention is the deficiency in order to overcome above-mentioned background technology, the measuring method of polarization modes coupling distribution in a kind of polarization maintaining optical fibre is provided, can measure the polarization modes coupling distribution situation of the polarization maintaining optical fibre of 50 kilometers long, the minimum detectable polarization modes coupling intensity measured is 10 ^-6~ 10 ^-5, can also signal to noise ratio (S/N ratio) be improved, improve more than 10 decibels than the signal to noise ratio (S/N ratio) of existing scheme.

The measuring method of polarization modes coupling distribution in polarization maintaining optical fibre provided by the invention, comprises the following steps:

S1, build the measurement mechanism of the distribution of polarization modes coupling in polarization maintaining optical fibre: this measurement mechanism comprises wideband light source, intensity modulator, the first polarizer, the second polarizer, image intensifer, photodetector and vector network analyzer, wideband light source is connected with intensity modulator by optical patchcord, intensity modulator is connected with the first polarizer by optical patchcord, measure the total length L of tested polarization maintaining optical fibre, calculate the time delay T of whole tested polarization maintaining optical fibre ₀: T ₀=L/ the light velocity, spacing between first polarizer and the second polarizer is the total length L of tested polarization maintaining optical fibre, second polarizer is connected with image intensifer by optical patchcord, image intensifer is connected with photodetector by optical patchcord, photodetector is connected with vector network analyzer by RF cable, and vector network analyzer is connected with intensity modulator by RF cable;

The light signal that S2, wideband light source export enters intensity modulator through optical patchcord, rotate the first polarizer, the light signal that intensity modulator exports enters the first polarizer through optical patchcord, the output terminal of the first polarizer is connected with one end of tested polarization maintaining optical fibre, the other end of tested polarization maintaining optical fibre is connected with the input end of the second polarizer, measures the angle theta between the optical axis of the first polarizer and the fast axle of polarization maintaining optical fibre ₀, the first polarizer is with θ ₀angle is coupled in polarization maintaining optical fibre, calculates the polarization modes coupling intensity η of polarization maintaining optical fibre incidence end ₀: η ₀=sin ²θ ₀; Rotating the second polarizer P2, measure the angle α between the optical axis of the second polarizer P2 and the fast axle of polarization maintaining optical fibre, adjusting extinction ratio between two polarization modes by regulating the angle of deflection of the second polarizer P2; Have n to put in polarization maintaining optical fibre and polarization modes coupling occurs, n is natural number, and T is variable time delay, θ _ibe the angle of fast axle between adjacent two sections in polarization maintaining optical fibre, i=1,2..., n, i are the Position Numbers of polarization modes coupling point, L _ibe the position of i-th polarization modes coupling point, the polarization modes coupling intensity of i-th polarization modes coupling point is η _i;

S3, the radio frequency range F of vector network analyzer is set ₀with scanning step △ f, according to formula: calculate the spatial resolution △ L of the measurement mechanism of polarization modes coupling distribution in polarization maintaining optical fibre; Vector network analyzer launches modulated radio signal S (t) to intensity modulator, and t is time variable, to light signal load-modulate RF signal S (t) of input intensity modulator;

The electric field strength E of the second polarizer P2 rear end is:

$\begin{array}{c}E\∝\cos \mathrm{\α}\sqrt{S\left(t\right)}\exp \left[j({\mathrm{\ω}}_{0}t-{\mathrm{\β}}_F{L}_0)\right]+\\ \sin \mathrm{\α}\left\{\begin{array}{c}\sin {\mathrm{\θ}}_0\sqrt{S(t-T_0)}\exp \left[j({\mathrm{\ω}}_{0}t-{\mathrm{\β}}_S{L}_0)\right]+...+\\ \sin {\mathrm{\θ}}_n\sqrt{S(t-T_n)}\exp \left[j({\mathrm{\ω}}_{0}t-{\mathrm{\β}}_F(L_0-L_n)-{\mathrm{\β}}_S{L}_n)\right]\end{array}\right\}\end{array}---\left(1\right)$

Wherein, j is imaginary unit, ω ₀the centre frequency of light wave, β _fthe propagation constant of the fast axle of polarization maintaining optical fibre, L ₀the position of incidence end, β _sthe propagation constant of the slow axis of polarization maintaining optical fibre, T _nthe time delay of the n-th polarization modes coupling point, L _nit is the position of the n-th polarization modes coupling point;

Square being directly proportional of the luminous power P of light signal and the electric field strength E of the second polarizer P2 rear end that second polarizer P2 exports:

$P\∝\mathrm{co}{s}^2\mathrm{\αS}\left(t\right)+\mathrm{si}{n}^2\α\underset{i=0}{\overset{n}{\mathrm{\∑}}}{\mathrm{\η}}_{i}S(t-T_i)---\left(2\right)$

Wherein, T _ithe time delay of i-th polarization modes coupling point, S (t-T _i) be the radiofrequency signal expression formula of i-th polarization modes coupling point, the stiffness of coupling of all polarization modes couplings point and the sum of products of radiofrequency signal;

The light signal that second polarizer P2 exports amplifies through image intensifer, and the radio-frequency filter response that image intensifer produces is H (ω _m):

$H\left({\mathrm{\ω}}_m\right)={\cos }^2\mathrm{\α}+{\sin }^2\mathrm{\α}\underset{i=0}{\overset{n}{\mathrm{\Σ}}}{\mathrm{\η}}_i\exp \left(j{\mathrm{\ω}}_m{T}_i\right)---\left(3\right)$

Wherein, ω _mthe signal frequency that time-domain signal transforms in frequency domain, stiffness of coupling and the light signal expression formula sum of products of all polarization modes couplings point;

The light signal that image intensifer exports enters photodetector, and photodetector detects the light signal received, and the light intensity expression detecting light signal is:

$\begin{array}{c}{\left|H\left({\mathrm{\ω}}_m\right)\right|}^2={\cos }^4\mathrm{\α}+{\sin }^4\mathrm{\α}\underset{i=0}{\overset{n}{\mathrm{\Σ}}}{{\mathrm{\η}}_i}^2+2{\sin }^2\mathrm{\α}{\cos }^2\mathrm{\α}\underset{i=0}{\overset{n}{\mathrm{\Σ}}}{\mathrm{\η}}_i\cos {\mathrm{\ω}}_m{T}_i+\\ 2{\sin }^4\mathrm{\α}\underset{i=0}{\overset{n}{\mathrm{\Σ}}}\underset{k=i+1}{\overset{n}{\mathrm{\Σ}}}{\mathrm{\η}}_i{\mathrm{\η}}_k\cos {\mathrm{\ω}}_m(T_i-T_k)\end{array}---\left(4\right)$

Wherein, i, k are all Position Numbers, η _ithe polarization modes coupling intensity of i-th polarization modes coupling point, η _kthe polarization modes coupling intensity of a kth polarization modes coupling point, T _ithe time delay of i-th polarization modes coupling point, T _kthe time delay of a kth polarization modes coupling point, it is the quadratic sum of the polarization modes coupling intensity of all polarization modes couplings point;

The light signal detected is converted to electric signal by photodetector, exports electric signal to vector network analyzer, the electric signal that vector network analyzer reception, display, storage photodetector are sent; Vector network analyzer does inverse Fourier transform to equation (4), finally calculates the position L that polarization modes coupling occurs _iwith intensity η _i.

On the basis of technique scheme, angle α=π/4 between the optical axis of described second polarizer and the fast axle of tested polarization maintaining optical fibre.

On the basis of technique scheme, during described α=π/4, η _i<<1, equation (4) is rewritten as:

$\begin{array}{c}{\left|H\left({\mathrm{\&omega;}}_m\right)\right|}^2=\frac{1}{4}[1+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2+2\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i\cos {\mathrm{\&omega;}}_m{T}_i+2\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}\underset{j=i+1}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i{\mathrm{\&eta;}}_j\cos {\mathrm{\&omega;}}_m(T_i-T_j)]\\ \&ap;\frac{1}{4}[1+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2+2\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i\cos {\mathrm{\&omega;}}_m{T}_i]\end{array}---\left(5\right)$

Inverse Fourier transform is carried out to equation (5):

$F\left({\left|H\left({\mathrm{\&omega;}}_m\right)\right|}^2\right)=k\{(1+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2)\mathrm{\&delta;}\left(T\right)+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i[\mathrm{\&delta;}(T-T_i)+\mathrm{\&delta;}(T+T_i)\left]\right\}---\left(6\right)$

Wherein, k is the constant factor that Fourier transform produces, and δ (T) is impulse function, δ (T-T _i) be that position is at T _ithe impulse function at place, δ (T+T _i) be that position is at-T _ithe impulse function at place, vector network analyzer draws the time domain response figure of equation (6), from the time domain response figure of equation (6), read T _iand η _i/ η ₀ratio, whole measurement mechanism produces microwave photon filter effect, the position L of i-th polarization modes coupling point _ifor:

$L_i=\frac{T_i}{T_0}L---\left(7\right)$

The polarization modes coupling intensity η of known measurement polarization maintaining optical fibre incidence end ₀and η _i/ η ₀ratio, according to formula: η _i=(η _i/ η ₀) × η ₀, calculate the intensity η of polarization modes coupling _i, i=1,2 ..., n.

On the basis of technique scheme, the angle α=pi/2 between the optical axis of described second polarizer and the fast axle of tested polarization maintaining optical fibre.

On the basis of technique scheme, during described α=pi/2, equation (4) is reduced to:

${\left|H\left({\mathrm{\&omega;}}_m\right)\right|}^2=\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2+2\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}\underset{j=i+1}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i{\mathrm{\&eta;}}_j\cos {\mathrm{\&omega;}}_m(T_i-T_j).---\left(8\right)$

Inverse Fourier transform is carried out to equation (8):

$F\left({\left|H\left({\mathrm{\&omega;}}_m\right)\right|}^2\right)=k\{\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2\mathrm{\&delta;}\left(T\right)+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}\underset{j=i+1}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i{\mathrm{\&eta;}}_j[\mathrm{\&delta;}(T-(T_i-T_j))+\mathrm{\&delta;}(T+(T_i-T_j))\left]\right\}---\left(9\right)$

Vector network analyzer draws the time domain response figure of equation (9), from the time domain response figure of equation (9), read T _iand η ₀η _ivalue, whole measurement mechanism produces microwave photon filter effect, according to formula: calculate the polarization modes coupling position L of i-th polarization modes coupling point _i, according to formula: η _i=η ₀η _i/ η ₀, calculate the polarization modes coupling intensity η of i-th polarization modes coupling point _i.

On the basis of technique scheme, described image intensifer adopts Erbium-Doped Fiber Amplifier (EDFA).

On the basis of technique scheme, the spacing between described first polarizer and the second polarizer is 50 kilometers.

On the basis of technique scheme, the minimum detectable polarization modes coupling intensity that described measurement mechanism measures is 10 ^-6~ 10 ^-5.

Compared with prior art, advantage of the present invention is as follows:

(1) the present invention can measure the polarization modes coupling distribution situation of the polarization maintaining optical fibre of 50 kilometers long, and the minimum detectable polarization modes coupling intensity measured is 10 ^-6~ 10 ^-5.

(2) use image intensifer in proving installation of the present invention, can signal to noise ratio (S/N ratio) have been improved, improve more than 10 decibels than the signal to noise ratio (S/N ratio) of existing scheme.

(3) the present invention carrys out implementation space measurement by introducing rf modulations, and do not move any parts in whole test process, compared with the method based on Michelson interferometer, proving installation of the present invention is more firm, stable.

(4) along with the increase of polarization maintaining optical fibre length to be measured, the Free Spectral Range of radiofrequency signal reduces thereupon, can select the radio-frequency signal source of low-frequency range, radio-frequency probe and modulator, can reduce costs.

Accompanying drawing explanation

Fig. 1 is the structured flowchart of polarization modes coupling apparatus for measuring distribution in the embodiment of the present invention.

Fig. 2 is the structural representation of the polarization maintaining optical fibre of n some generation polarization modes coupling in the embodiment of the present invention.

When Fig. 3 is α=π/4 in the embodiment of the present invention, the time domain response figure of the microwave photon filter effect that the polarization maintaining optical fibre containing n PMC produces.

When Fig. 4 is α=pi/2 in the embodiment of the present invention, the time domain response figure of the microwave photon filter effect that the polarization maintaining optical fibre containing n PMC produces.

Embodiment

Below in conjunction with drawings and the specific embodiments, the present invention is described in further detail.

The embodiment of the present invention provides the measuring method of polarization modes coupling distribution in a kind of polarization maintaining optical fibre, comprises the following steps:

S1, build the measurement mechanism of the distribution of polarization modes coupling in polarization maintaining optical fibre: shown in Figure 1, this measurement mechanism comprises wideband light source, intensity modulator, the first polarizer P1, the second polarizer P2, image intensifer, photodetector and vector network analyzer, wideband light source is connected with intensity modulator by optical patchcord, intensity modulator is connected with the first polarizer P1 by optical patchcord, measure the total length L of tested polarization maintaining optical fibre, calculate the time delay T of whole tested polarization maintaining optical fibre ₀: T ₀=L/ the light velocity, spacing between first polarizer P1 and the second polarizer P2 is the total length L of tested polarization maintaining optical fibre, the total length of tested polarization maintaining optical fibre can reach 50 kilometers, and the spacing namely between the first polarizer P1 and the second polarizer P2 also can reach 50 kilometers; Second polarizer P2 is connected with image intensifer by optical patchcord, image intensifer is connected with photodetector by optical patchcord, photodetector is connected with vector network analyzer by RF cable, and vector network analyzer is connected with intensity modulator by RF cable.Image intensifer generally adopts EDFA (Erbium-Doped Optical Fiber Amplifier, Erbium-Doped Fiber Amplifier (EDFA)).

The light signal that S2, wideband light source export enters intensity modulator through optical patchcord, rotate the first polarizer P1, the light signal that intensity modulator exports enters the first polarizer P1 through optical patchcord, the output terminal of the first polarizer P1 is connected with one end of tested polarization maintaining optical fibre, the other end of tested polarization maintaining optical fibre is connected with the input end of the second polarizer P2, measures the angle theta between the optical axis of the first polarizer P1 and the fast axle of polarization maintaining optical fibre ₀, the first polarizer P1 is with θ ₀angle is coupled in polarization maintaining optical fibre, calculates the polarization modes coupling intensity η of polarization maintaining optical fibre incidence end ₀: η ₀=sin ²θ ₀; Rotating the second polarizer P2, measure the angle α between the optical axis of the second polarizer P2 and the fast axle of polarization maintaining optical fibre, adjusting extinction ratio between two polarization modes by regulating the angle of deflection of the second polarizer P2.

Shown in Figure 2, have n to put in polarization maintaining optical fibre and polarization modes coupling occurs, n is natural number.When light beam is incident from one end of optical fiber, the energy exported from the optical fiber other end comprises a lot of part, every part has different time delays, if incident optical signal is by rf-signal modulation mistake, so the radio-frequency responsive of output terminal is just equivalent to a wave filter, therefore serves the effect of microwave photon filter.The direction that in Fig. 2, L points to is change in coordinate axis direction, and T is variable time delay, θ _i(i=1,2..., n) is the angle of fast axle between adjacent two sections in polarization maintaining optical fibre, and i is the Position Number of polarization modes coupling point, L _iit is the position of i-th polarization modes coupling point.During n=0, in polarization maintaining optical fibre, not there is polarization modes coupling, θ _i=0; If polarization maintaining optical fibre has the disturbance of inside or outside, then θ _i≠ 0, show to there is polarization modes coupling in polarization maintaining optical fibre, the polarization modes coupling intensity of i-th polarization modes coupling point is η _i, η in theory _i=sin ²θ _ibut, generally, θ _iand η _iall very little, be difficult to measure.

S3, RF (Radio Frequency, radio frequency) the frequency range F of vector network analyzer is set ₀with scanning step △ f, according to formula: calculate the spatial resolution △ L of the measurement mechanism of polarization modes coupling distribution in polarization maintaining optical fibre, spatial resolution △ L represents the minimum orientable spatial accuracy of the measurement mechanism of polarization modes coupling distribution in polarisation fibre.Vector network analyzer launches modulated radio signal S (t) to intensity modulator, and t is time variable, to light signal load-modulate RF signal S (t) of input intensity modulator;

Due to θ _i(i=1,2..., n) is very little, supposes cos θ _i=1, then the electric field strength E of the second polarizer P2 rear end is:

$\begin{array}{c}E\&Proportional;\cos \mathrm{\&alpha;}\sqrt{S\left(t\right)}\exp \left[j({\mathrm{\&omega;}}_{0}t-{\mathrm{\&beta;}}_F{L}_0)\right]+\\ \sin \mathrm{\&alpha;}\left\{\begin{array}{c}\sin {\mathrm{\&theta;}}_0\sqrt{S(t-T_0)}\exp \left[j({\mathrm{\&omega;}}_{0}t-{\mathrm{\&beta;}}_S{L}_0)\right]+...+\\ \sin {\mathrm{\&theta;}}_n\sqrt{S(t-T_n)}\exp \left[j({\mathrm{\&omega;}}_{0}t-{\mathrm{\&beta;}}_F(L_0-L_n)-{\mathrm{\&beta;}}_S{L}_n)\right]\end{array}\right\}\end{array}---\left(1\right)$

Wherein, j is imaginary unit, ω ₀the centre frequency of light wave, β _fthe propagation constant of the fast axle of polarization maintaining optical fibre, L ₀the position of incidence end, β _sthe propagation constant of the slow axis of polarization maintaining optical fibre, T _nthe time delay of the n-th polarization modes coupling point, L _nit is the position of the n-th polarization modes coupling point.Usually the higher-order shear deformation item that light field is caused to fast axle by slow axis reverse coupled can be ignored, square being directly proportional of the luminous power P of light signal and the electric field strength E of the second polarizer P2 rear end that the second polarizer P2 exports:

$P\&Proportional;\mathrm{co}{s}^2\mathrm{\&alpha;S}\left(t\right)+\mathrm{si}{n}^2\mathrm{\&alpha;}\underset{i=o}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_{i}S(t-T_i)...\left(2\right)$

Wherein, T _ithe time delay of i-th polarization modes coupling point, S (t-T _i) be the radiofrequency signal expression formula of i-th polarization modes coupling point, the stiffness of coupling of all polarization modes couplings point and the sum of products of radiofrequency signal.

The light signal that second polarizer P2 exports amplifies through Erbium-Doped Fiber Amplifier (EDFA), and the radio-frequency filter response that Erbium-Doped Fiber Amplifier (EDFA) produces is H (ω _m):

$H\left({\mathrm{\&omega;}}_m\right)=\mathrm{co}{s}^2\mathrm{\&alpha;}+\mathrm{si}{n}^2\mathrm{\&alpha;}\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i\exp \left(j{\mathrm{\&omega;}}_m{T}_i\right)---\left(3\right)$

Wherein, ω _mthe signal frequency that time-domain signal transforms in frequency domain, stiffness of coupling and the light signal expression formula sum of products of all polarization modes couplings point.

The light signal that Erbium-Doped Fiber Amplifier (EDFA) exports enters photodetector, and photodetector detects the light signal received, and the light intensity expression detecting light signal is:

$\begin{array}{c}{\left|H\left({\mathrm{\&omega;}}_m\right)\right|}^2={\cos }^4\mathrm{\&alpha;}+{\sin }^4\mathrm{\&alpha;}\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2+2{\sin }^2\mathrm{\&alpha;}{\cos }^2\mathrm{\&alpha;}\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i\cos {\mathrm{\&omega;}}_m{T}_i+\\ 2{\sin }^4\mathrm{\&alpha;}\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}\underset{k=i+1}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i{\mathrm{\&eta;}}_k\cos {\mathrm{\&omega;}}_m(T_i-T_k)\end{array}---\left(4\right)$

Wherein, i, k are all Position Numbers, η _ithe polarization modes coupling intensity of i-th polarization modes coupling point, η _kthe polarization modes coupling intensity of a kth polarization modes coupling point, T _ithe time delay of i-th polarization modes coupling point, T _kthe time delay of a kth polarization modes coupling point, it is the quadratic sum of the polarization modes coupling intensity of all polarization modes couplings point.

The light signal detected is converted to electric signal by photodetector, export electric signal to vector network analyzer, the electric signal that vector network analyzer reception, display, storage photodetector are sent, vector network analyzer does inverse Fourier transform to equation (4), finally calculates position and T corresponding to intensity that polarization modes coupling occurs _iand η _ivalue.

In actual applications, α can get arbitrary value, and according to the difference selected α angle, the concrete computational details can be slightly different, get π/4 below, pi/2 carries out labor for α.

(1) when α is away from pi/2, such as: α=π/4, η _i<<1, equation (4) is rewritten as:

$\begin{array}{c}{\left|H\left({\mathrm{\&omega;}}_m\right)\right|}^2=\frac{1}{4}[1+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2+2\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i\cos {\mathrm{\&omega;}}_m{T}_i+2\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}\underset{j=i+1}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i{\mathrm{\&eta;}}_j\cos {\mathrm{\&omega;}}_m(T_i-T_j)]\\ \&ap;\frac{1}{4}[1+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2+2\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i\cos {\mathrm{\&omega;}}_m{T}_i]\end{array}---\left(5\right)$

Inverse Fourier transform is carried out to equation (5):

$F\left({\left|H\left({\mathrm{\&omega;}}_m\right)\right|}^2\right)=k\{(1+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2)\mathrm{\&delta;}\left(T\right)+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i[\mathrm{\&delta;}(T-T_i)+\mathrm{\&delta;}(T+T_i)\left]\right\}---\left(6\right)$

Wherein, k is the constant factor that Fourier transform produces, and do not affect last result, δ (T) is impulse function, δ (T-T _i) be that position is at T _ithe impulse function at place, δ (T+T _i) be that position is at-T _ithe impulse function at place, vector network analyzer draws the time domain response figure of equation (6), from the time domain response figure of equation (6), read T _iand η _i/ η ₀ratio.Iff the positive axis considering T, the time domain response of equation (6) is shown in Figure 3, due to T ₀the DGD (Differential Group Delay, Differential Group Delay) of whole polarization maintaining optical fibre, η ₀=sin ²θ ₀, wherein, θ ₀for known quantity, whole measurement mechanism produces MPF (Microwave Photonic Filter, microwave photon filter) effect, the position L of i-th polarization modes coupling point _ifor:

$L_i=\frac{T_i}{T_0}L---\left(7\right)$

The polarization modes coupling intensity η of known measurement polarization maintaining optical fibre incidence end ₀and η _i/ η ₀ratio, according to formula: η _i=(η _i/ η ₀) × η ₀, calculate the intensity η of polarization modes coupling _i, i=1,2 ..., n.

In this case signal processing is very clear, calculates simple, but owing to there is larger flip-flop, the extinction ratio of signal is smaller.

(2), during α=pi/2, the extinction ratio of signal is best, and equation (4) is reduced to:

${\left|H\left({\mathrm{\&omega;}}_m\right)\right|}^2=\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2+2\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}\underset{j=i+1}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i{\mathrm{\&eta;}}_j\cos {\mathrm{\&omega;}}_m(T_i-T_j).---\left(8\right)$

Inverse Fourier transform is carried out to equation (8):

$F\left({\left|H\left({\mathrm{\&omega;}}_m\right)\right|}^2\right)=k\{\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2\mathrm{\&delta;}\left(T\right)+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}\underset{j=i+1}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i{\mathrm{\&eta;}}_j[\mathrm{\&delta;}(T-(T_i-T_j))+\mathrm{\&delta;}(T+(T_i-T_j))\left]\right\}---\left(9\right)$

Vector network analyzer draws the time domain response figure of equation (9), from the time domain response figure of equation (9), read T _iand η ₀η _ivalue.Iff the positive axis considering T axle, the time domain response that equation (9) is corresponding is shown in Figure 4, whole measurement mechanism produces microwave photon filter effect, different from Fig. 3, clearly can not demarcate the time of all time domain responses in the diagram, a confirmable response is rightmost one maximum time delay T ₀-T _npoint.Because T ₀known, T _ncan obtain at first.In fact, counting of time domain response is a combination do not comprise zero time delay point, these response point write as the form of matrix:

Respectively-T is added to each element of matrix now ₀and T _n, obtain

With

Contrast this two groups of results, find two sequences with n-1 element, a sequence is the opposite number of another sequence just, and these n-1 element is { T ₁t ₂t _n-1, but be random alignment.On the other hand, according to T in Fig. 2 _idefinition, T ₀>T ₁> ... >T _n-1>T _n, therefore can determine all T _i.According to formula: calculate the polarization modes coupling position L of i-th polarization modes coupling point _i.Because η ₀known, η ₀η _iread, according to formula: η by the ordinate in Fig. 4 _i=η ₀η _i/ η ₀, calculate the polarization modes coupling intensity η of i-th polarization modes coupling point _i.

Those skilled in the art can carry out various modifications and variations to the embodiment of the present invention, if these amendments and modification belong within the scope of the claims in the present invention and equivalent technologies thereof, then these revise and modification also within protection scope of the present invention.

The prior art that the content do not described in detail in instructions is known to the skilled person.

Claims (8)

1. a measuring method for polarization modes coupling distribution in polarization maintaining optical fibre, is characterized in that, comprise the following steps:

S1, build the measurement mechanism of the distribution of polarization modes coupling in polarization maintaining optical fibre: this measurement mechanism comprises wideband light source, intensity modulator, the first polarizer, the second polarizer, image intensifer, photodetector and vector network analyzer, wideband light source is connected with intensity modulator by optical patchcord, intensity modulator is connected with the first polarizer by optical patchcord, measure the total length L of tested polarization maintaining optical fibre, calculate the time delay T of whole tested polarization maintaining optical fibre ₀: T ₀=L/ the light velocity, spacing between first polarizer and the second polarizer is the total length L of tested polarization maintaining optical fibre, second polarizer is connected with image intensifer by optical patchcord, image intensifer is connected with photodetector by optical patchcord, photodetector is connected with vector network analyzer by RF cable, and vector network analyzer is connected with intensity modulator by RF cable;

The light signal that S2, wideband light source export enters intensity modulator through optical patchcord, rotate the first polarizer, the light signal that intensity modulator exports enters the first polarizer through optical patchcord, the output terminal of the first polarizer is connected with one end of tested polarization maintaining optical fibre, the other end of tested polarization maintaining optical fibre is connected with the input end of the second polarizer, measures the angle theta between the optical axis of the first polarizer and the fast axle of polarization maintaining optical fibre ₀, the first polarizer is with θ ₀angle is coupled in polarization maintaining optical fibre, calculates the polarization modes coupling intensity η of polarization maintaining optical fibre incidence end ₀: η ₀=sin ²θ ₀; Rotating the second polarizer P2, measure the angle α between the optical axis of the second polarizer P2 and the fast axle of polarization maintaining optical fibre, adjusting extinction ratio between two polarization modes by regulating the angle of deflection of the second polarizer P2; Have n to put in polarization maintaining optical fibre and polarization modes coupling occurs, n is natural number, and T is variable time delay, θ _ibe the angle of fast axle between adjacent two sections in polarization maintaining optical fibre, i=1,2..., n, i are the Position Numbers of polarization modes coupling point, L _ibe the position of i-th polarization modes coupling point, the polarization modes coupling intensity of i-th polarization modes coupling point is η _i;

S3, the radio frequency range F of vector network analyzer is set ₀with scanning step Δ f, according to formula: calculate the spatial resolution Δ L of the measurement mechanism of polarization modes coupling distribution in polarization maintaining optical fibre; Vector network analyzer launches modulated radio signal S (t) to intensity modulator, and t is time variable, to light signal load-modulate RF signal S (t) of input intensity modulator;

The electric field strength E of the second polarizer P2 rear end is:

$\begin{array}{c}E\&Proportional;\cos \mathrm{\&alpha;}\sqrt{S\left(t\right)}\exp \left[j({\mathrm{\&omega;}}_{0}t-{\mathrm{\&beta;}}_F{L}_0)\right]+\\ \sin \mathrm{\&alpha;}\left\{\begin{array}{c}\sin {\mathrm{\&theta;}}_0\sqrt{S(t-T_0)}\exp \left[j({\mathrm{\&omega;}}_{0}t-{\mathrm{\&beta;}}_S{L}_0)\right]+...+\\ \sin {\mathrm{\&theta;}}_0\sqrt{S(t-T_n)}\exp \left[j({\mathrm{\&omega;}}_{0}t-{\mathrm{\&beta;}}_F(L_0-L_n)-{\mathrm{\&beta;}}_S{L}_n)\right]\end{array}\right\}\end{array}---\left(1\right)$

Wherein, j is imaginary unit, ω ₀the centre frequency of light wave, β _fthe propagation constant of the fast axle of polarization maintaining optical fibre, L ₀the position of incidence end, β _sthe propagation constant of the slow axis of polarization maintaining optical fibre, T _nthe time delay of the n-th polarization modes coupling point, L _nit is the position of the n-th polarization modes coupling point;

Square being directly proportional of the luminous power P of light signal and the electric field strength E of the second polarizer P2 rear end that second polarizer P2 exports:

$P\&Proportional;{\cos }^2\mathrm{\&alpha;S}\left(t\right)+{\sin }^2\mathrm{\&alpha;}\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_{i}S(t-T_i)---\left(2\right)$

Wherein, T _ithe time delay of i-th polarization modes coupling point, S (t-T _i) be the radiofrequency signal expression formula of i-th polarization modes coupling point, the stiffness of coupling of all polarization modes couplings point and the sum of products of radiofrequency signal;

The light signal that second polarizer P2 exports amplifies through image intensifer, and the radio-frequency filter response that image intensifer produces is H (ω _m):

$H\left({\mathrm{\&omega;}}_m\right)={\cos }^2\mathrm{\&alpha;}+{\sin }^2\mathrm{\&alpha;}\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i\exp \left({\mathrm{j\&omega;}}_m{T}_i\right)---\left(3\right)$

Wherein, ω _mthe signal frequency that time-domain signal transforms in frequency domain, stiffness of coupling and the light signal expression formula sum of products of all polarization modes couplings point;

The light signal that image intensifer exports enters photodetector, and photodetector detects the light signal received, and the light intensity expression detecting light signal is:

$\begin{array}{c}{\left|H\left({\mathrm{\&omega;}}_m\right)\right|}^2={\cos }^4\mathrm{\&alpha;}+{\sin }^4\mathrm{\&alpha;}\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2+{2\sin }^2\mathrm{\&alpha;}{\cos }^2\mathrm{\&alpha;}\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i{\cos \mathrm{\&omega;}}_m{T}_i+\\ {2\sin }^4\mathrm{\&alpha;}\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}\underset{k=i+1}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i{\mathrm{\&eta;}}_k\cos {\mathrm{\&omega;}}_m(T_i-T_k)\end{array}---\left(4\right)$

Wherein, i, k are all Position Numbers, η _ithe polarization modes coupling intensity of i-th polarization modes coupling point, η _kthe polarization modes coupling intensity of a kth polarization modes coupling point, T _ithe time delay of i-th polarization modes coupling point, T _kthe time delay of a kth polarization modes coupling point, it is the quadratic sum of the polarization modes coupling intensity of all polarization modes couplings point;

The light signal detected is converted to electric signal by photodetector, exports electric signal to vector network analyzer, the electric signal that vector network analyzer reception, display, storage photodetector are sent; Vector network analyzer does inverse Fourier transform to equation (4), finally calculates the position L that polarization modes coupling occurs _iwith intensity η _i.

2. the measuring method of polarization modes coupling distribution in polarization maintaining optical fibre as claimed in claim 1, is characterized in that: angle α=π/4 between the optical axis of described second polarizer and the fast axle of tested polarization maintaining optical fibre.

3. the measuring method of polarization modes coupling distribution in polarization maintaining optical fibre as claimed in claim 2, is characterized in that: during described α=π/4, η _i< < 1, equation (4) is rewritten as:

$\begin{array}{c}{\left|H\left({\mathrm{\&omega;}}_m\right)\right|}^2=\frac{1}{4}[1+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2+2\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i\cos {\mathrm{\&omega;}}_m{T}_i+2\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}\underset{j=i+1}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i{\mathrm{\&eta;}}_j\cos {\mathrm{\&omega;}}_m(T_i-T_j)]\\ \&ap;\frac{1}{4}[1+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2+2\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i\cos {\mathrm{\&omega;}}_m{T}_i]\end{array}---\left(5\right)$

Inverse Fourier transform is carried out to equation (5):

$F\left({\left|H\left({\mathrm{\&omega;}}_m\right)\right|}^2\right)=k\{(1+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2)\mathrm{\&delta;}\left(T\right)+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i[\mathrm{\&delta;}(T-T_i)+\mathrm{\&delta;}(T+T_i)\left]\right\}---\left(6\right)$

Wherein, k is the constant factor that Fourier transform produces, and δ (T) is impulse function, δ (T-T _i) be that position is at T _ithe impulse function at place, δ (T+T _i) be that position is at-T _ithe impulse function at place, vector network analyzer draws the time domain response figure of equation (6), from the time domain response figure of equation (6), read T _iand η _i/ η ₀ratio, whole measurement mechanism produces microwave photon filter effect, the position L of i-th polarization modes coupling point _ifor:

$L=\frac{T_i}{T_0}L---\left(7\right)$

The polarization modes coupling intensity η of known measurement polarization maintaining optical fibre incidence end ₀and η _i/ η ₀ratio, according to formula: η _i=(η _i/ η ₀) × η ₀, calculate the intensity η of polarization modes coupling _i, i=1,2 ..., n.

4. the measuring method of polarization modes coupling distribution in polarization maintaining optical fibre as claimed in claim 1, is characterized in that: the angle α=pi/2 between the optical axis of described second polarizer and the fast axle of tested polarization maintaining optical fibre.

5. the measuring method of polarization modes coupling distribution in polarization maintaining optical fibre as claimed in claim 4, it is characterized in that: during described α=pi/2, equation (4) is reduced to:

${\left|H\left({\mathrm{\&omega;}}_m\right)\right|}^2=\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2+2\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}\underset{j=i+1}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i{\mathrm{\&eta;}}_j\cos {\mathrm{\&omega;}}_m(T_i-T_j).---\left(8\right)$

Inverse Fourier transform is carried out to equation (8):

$F\left({\left|H\left({\mathrm{\&omega;}}_m\right)\right|}^2\right)=k\{\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}{{\mathrm{\&eta;}}_i}^2\mathrm{\&delta;}\left(T\right)+\underset{i=0}{\overset{n}{\mathrm{\&Sigma;}}}\underset{j=i+1}{\overset{n}{\mathrm{\&Sigma;}}}{\mathrm{\&eta;}}_i{\mathrm{\&eta;}}_j[\mathrm{\&delta;}(T-(T_i-T_j))+\mathrm{\&delta;}(T+(T_i-T_j))\left]\right\}---\left(9\right)$

Wherein, k is the constant factor that Fourier transform produces, and δ (T) is impulse function;

Vector network analyzer draws the time domain response figure of equation (9), from the time domain response figure of equation (9), read T _iand η ₀η _ivalue, whole measurement mechanism produces microwave photon filter effect, according to formula: calculate the polarization modes coupling position L of i-th polarization modes coupling point _i, according to formula: η _i=η ₀η _i/ η ₀, calculate the polarization modes coupling intensity η of i-th polarization modes coupling point _i.

6. the measuring method of polarization modes coupling distribution in the polarization maintaining optical fibre according to any one of claim 1 to 5, is characterized in that: described image intensifer adopts Erbium-Doped Fiber Amplifier (EDFA).

7. the measuring method of polarization modes coupling distribution in the polarization maintaining optical fibre according to any one of claim 1 to 5, is characterized in that: the spacing between described first polarizer and the second polarizer is 50 kilometers.

8. the measuring method of polarization modes coupling distribution in the polarization maintaining optical fibre according to any one of claim 1 to 5, is characterized in that: the minimum detectable polarization modes coupling intensity that described measurement mechanism measures is 10 ^-6~ 10 ^-5.