CN111964873A - High-precision distributed extinction ratio measuring method for polarization maintaining optical fiber - Google Patents

High-precision distributed extinction ratio measuring method for polarization maintaining optical fiber Download PDF

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CN111964873A
CN111964873A CN202010740511.2A CN202010740511A CN111964873A CN 111964873 A CN111964873 A CN 111964873A CN 202010740511 A CN202010740511 A CN 202010740511A CN 111964873 A CN111964873 A CN 111964873A
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polarization
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optical fiber
crosstalk
polarization maintaining
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CN111964873B (en
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喻张俊
杨军
侯成城
蒋泽军
王云才
秦玉文
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Guangdong University of Technology
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Abstract

The invention discloses a high-precision distributed extinction ratio measuring method for a polarization maintaining optical fiber, which comprises the following steps: s1: measuring the polarization crosstalk of the polarization maintaining optical fiber to be measured; s2: calculating the measurement error of the polarization crosstalk; s3: according to the measurement error, correcting the polarization crosstalk of the polarization-maintaining optical fiber to be measured; s4: and measuring the distributed extinction ratio of the polarization maintaining fiber to be measured according to the polarization crosstalk corresponding to the corrected polarization maintaining fiber. The distributed extinction ratio of the optical fiber is measured based on the white light interference principle on the basis of obtaining the distributed polarization crosstalk of the polarization maintaining optical fiber. The method has the advantages of high measurement precision, large dynamic range measurement and the like, and can be widely applied to the analysis of the optical fiber polarization maintaining device with high measurement requirement on the polarization parameters.

Description

High-precision distributed extinction ratio measuring method for polarization maintaining optical fiber
Technical Field
The invention relates to the technical field of optical device measurement, in particular to a high-precision distributed extinction ratio measurement method for a polarization maintaining optical fiber.
Background
The polarization maintaining optical fiber is used as a special optical fiber capable of maintaining the light polarization state, and is widely applied to the fields of daily civilian use, aerospace, national defense and the like. However, polarization coupling inevitably exists inside the optical fiber device due to defects of the manufacturing process or disturbance of the external environment. The coupling phenomenon can reduce the signal-to-noise ratio of coherent signals of a sensing system and reduce the sensing precision. Therefore, how to suppress the coupling effect generated in the optical fiber device has become a hot research topic. By measuring the extinction ratio of the polarization parameter, the method is helpful for knowing the quality of the device preparation process level and the welding level of each device welding point. Therefore, accurately measuring the distributed extinction ratio of the polarizer device would provide a great help to improve the overall performance of the sensing system.
The current technical means for testing the extinction ratio mainly comprise: a rotary analyzer method, a polarization type optical time domain reflectometer, an optical correlation domain polarization measuring instrument, and the like. The rotating analyzer method measures extinction ratio and polarization crosstalk by measuring the light intensity at the output end of the device, and has the advantages of simple measuring light path and short measuring time. However, the dynamic range measured by the method is small, and meanwhile, the method cannot measure the distributed extinction ratio of the optical fiber device. The optical time domain reflectometer obtains the distributed polarization crosstalk by measuring three Stokes vectors of scattered light, and the distributed extinction ratio of the optical fiber can be accurately calculated by utilizing the polarization crosstalk. However, the spatial resolution of the method is low, and the resolution is only in the meter level.
The white light interferometer based on the optical coherence domain polarization measurement technology has the advantages of high resolution, long measurement distance, high sensitivity and the like, and is selected as the optimal measurement method of the distributed extinction ratio. The distributed extinction ratio of the device to be tested can be calculated by utilizing the polarization crosstalk measured by the white light interferometer, and the white light interferometer can not only diagnose the performance of a single device, but also evaluate the performance of an assembly formed by a plurality of devices. Although the white light interferometer provides a technical support for the measurement of the extinction ratio of the device, the measurement accuracy of the white light interferometer is still interfered by various factors in the actual measurement process. In order to better improve the accuracy of extinction ratio measurement, a main error source of a white light interferometer needs to be analyzed and suppressed, and the level of a manufacturing process of a polarizing device can be effectively improved only by continuously improving the accuracy of polarization parameter measurement.
In 2015, clever et al proposed a real-time testing device (CN201510224794.4) for extinction ratio of polarization-maintaining fiber, which improves the photoelectric detector and related devices for signal processing, more accurately obtains the optical power at the output end of the polarization-maintaining fiber, and obtains the lumped extinction ratio of the measured optical fiber by using the optical power. The method improves the accuracy of the lumped extinction ratio measurement, but cannot measure the distributed extinction ratio of the optical fiber.
In 2014, vitally mikhailalov et al (US 20140218733) of the company OFS in usa discloses a polarization crosstalk measurement detection device, which converts energy of a coupling axis into electric signals in real time and obtains distributed polarization crosstalk by using a ratio between the electric signals. The method can obtain the distributed extinction ratio of the polarization maintaining fiber to be measured by utilizing the polarization crosstalk, can eliminate the influence of stray light in the measurement process, and can obtain the distributed extinction ratio by measurement.
In 2017, the Yangjun et al proposed a method (CN201710284514.8) for measuring the extinction ratio of polarization maintaining fiber, which utilizes the energy of white light interference signals obtained by instrument measurement to obtain the extinction ratio, and designs a compensation algorithm to suppress the influence of dispersion and noise on the extinction ratio measurement, so that the distributed extinction ratio measurement is more accurate, but the method still suffers from the influence of the system error of the white light interferometer, so that the result has some deviation. The polarization maintaining optical fiber is used as a special optical fiber capable of maintaining the light polarization state, and is widely applied to the fields of daily civilian use, aerospace, national defense and the like. However, polarization coupling inevitably exists inside the optical fiber device due to defects of the manufacturing process or disturbance of the external environment. The coupling phenomenon can reduce the signal-to-noise ratio of coherent signals of a sensing system and reduce the sensing precision.
Disclosure of Invention
The invention aims to accurately measure the distributed extinction ratio of the polarization maintaining optical fiber, and the measurement result eliminates the influence of system errors caused by optical fiber loss, thereby improving the accuracy of the test; a high-precision distributed extinction ratio measurement method for a polarization maintaining optical fiber is provided.
The method comprises the following steps:
s1: measuring the polarization crosstalk of the polarization maintaining optical fiber to be measured;
s2: calculating the measurement error of the polarization crosstalk;
s3: according to the measurement error, correcting the polarization crosstalk of the polarization-maintaining optical fiber to be measured;
s4: and measuring the distributed extinction ratio of the polarization maintaining fiber to be measured according to the polarization crosstalk corresponding to the corrected polarization maintaining fiber.
The distributed extinction ratio of the optical fiber is measured based on the white light interference principle on the basis of obtaining the distributed polarization crosstalk of the polarization maintaining optical fiber. The method has the advantages of high measurement precision, large dynamic range measurement and the like, and can be widely applied to the analysis of the optical fiber polarization maintaining device with high measurement requirement on the polarization parameters.
Preferably, S1 includes the steps of:
s1.1: measuring polarization crosstalk; firstly, the length L of the polarization maintaining optical fiber to be measured is measuredfLength L of polarizer output tail fiberinAnd length L of input tail fiber of analyzerout；
S1.2: welding the polarizer, analyzer and polarization maintaining fiber at 0-0 deg, connecting the polarizer and analyzer with optical relative domain polarizer, and opening the instrument to obtain the polarization crosstalk CT x of the measured optical path.
Preferably, S2 includes the steps of:
s2.1: determining the positions of welding points and signal main peaks in the polarization crosstalk, and extracting the polarization crosstalk CT of the measured optical circuit from the polarization crosstalkDevice under test[x]Polarization crosstalk CT corresponding to each main peakMain peak[x]；
S2.2: cumulative CTDevice under test[x]Obtaining a lumped value, and calculating to obtain an error factor alpha introduced by power loss by using the lumped valueXAnd calculating according to the error factor to obtain the measurement error.
Preferably, S2.2 comprises the steps of:
s2.2.1: CT according to the intercepted patternMain peak[x]Using the formula:
Figure BDA0002606563240000041
adding up to obtain a numerical value
Figure BDA0002606563240000042
S2.2.2: CT according to the intercepted patternDevice under test[x]Using the formula:
Figure BDA0002606563240000051
accumulating the polarization crosstalk of the polarization maintaining device to be tested;
s2.2.3: after obtaining the accumulated value of the polarization crosstalk, the following formula is used:
Figure BDA0002606563240000052
calculating to obtain an error factor alpha introduced by power lossX。
S2.2.4: according to the following formula:
Figure BDA0002606563240000053
calculating to obtain a loss factor alphaXResulting in measurement errors.
Preferably, S3 includes:
s3.1: by polarization crosstalk measurements CTDevice under test[x]The polarization crosstalk is corrected by subtracting the Error value Error to obtain the corrected polarization crosstalk CTAfter correction[x]；
S3.2: and intercepting the polarization crosstalk corresponding to the polarization maintaining optical fiber to be measured from the corrected polarization crosstalk.
Preferably, S4 includes the steps of:
s4.1: according to the corrected polarization crosstalk CTAfter correction[x]Calculating to obtain the ratio of the transmission optical power of the polarization maintaining optical fiber transmission shaft to be tested and the transmission optical power of the coupling shaft distributed along the length of the optical fiber, wherein the power ratio is the distributed extinction ratio;
s4.2: and calculating the extinction ratio value of the random position of the polarization maintaining fiber to be measured by utilizing the distributed extinction ratio.
Preferably, S4.1 comprises the steps of:
s4.1.1: corrected polarization crosstalk CTAfter correction[x]Intercepting interference data distributed along the length of the optical fiber by the transmission axis and the coupling axis of the polarization maintaining optical fiber to be tested to obtain polarization crosstalk CTMeasured optical fiber[x]Calculating the measured amplitude I distributed along the optical path x2[x]；
S4.1.2: according to the measured amplitude I2[x]Calculating the change condition of the transmission power ratio r (x) of the transmission shaft and the coupling shaft in the polarization maintaining fiber to be tested along with the position x;
s4.1.3: according to the following relation:
PER[x]＝10log10(r[x])
and obtaining the distributed extinction ratio of the polarization maintaining fiber to be measured along the change of the optical path x, and measuring according to PER [ x ] to obtain the extinction ratio of the polarization maintaining fiber to be measured at any interval.
Preferably, the amplitude I is measured2[x]The calculation formula of (2) is as follows:
I2[x]＝10^(CTmeasured optical fiber[x]/10)。
Preferably, the calculation formula of the ratio of transmission powers r (x) is:
Figure BDA0002606563240000061
preferably, S4.2 comprises the steps of:
s4.2.1: calculating the x of the polarization maintaining optical fiber to be measured at any two points1，x2The ratio of the transmission power of the coupling shaft and the transmission shaft is calculated by the following formula:
Figure BDA0002606563240000062
s4.2.1: calculating x1，x2The extinction ratio between the two points is calculated by the following formula:
PER[x1,x2]＝-10log10(r[x1,x2])。
according to the polarization crosstalk of the polarization maintaining device, the power loss generated when polarized light is transmitted in the polarization maintaining device is calculated, the influence of the power loss on polarization crosstalk measurement is eliminated, and the corrected polarization crosstalk is used for calculating the distributed extinction ratio; accumulating the polarization crosstalk of the polarization maintaining optical fiber, the polarizer and the analyzer to obtain a lumped value, calculating the power loss generated when light is transmitted in the device according to the lumped value, calculating by using the power loss to obtain a measurement error value, eliminating the error in a mode of subtracting the measurement error value from a crosstalk measurement value, calculating by using the corrected crosstalk to obtain the ratio of the transmission optical power of the polarization maintaining optical fiber along the length distribution of the optical fiber between the transmission axis and the coupling axis, wherein the power ratio is the distributed extinction ratio; the method has high measurement precision and large dynamic range, and can be applied to the analysis of optical devices with high requirement on the measurement precision of the extinction ratio.
Compared with the prior art, the technical scheme of the invention has the beneficial effects that:
the method can eliminate the influence of power loss on the polarization crosstalk measurement, and the extinction ratio is measured by using the corrected polarization crosstalk, so that the measurement precision is higher than that of the traditional measurement method, and the requirement of high-precision measurement is met.
The method calculates the extinction ratio by measuring the ratio of the transmission optical power of the polarization maintaining optical fiber to the transmission optical power of the coupling shaft, and considers the second-order coupling energy in the device to be measured, so that the extinction ratio of the polarization maintaining optical fiber can be accurately measured.
The method can accurately measure the extinction ratio of any position in the polarization maintaining fiber, and has important significance for analyzing the performance of devices.
Drawings
FIG. 1 is a diagram of an experimental apparatus for measuring polarization maintaining fiber based on an optical coherence domain polarization measuring apparatus;
FIG. 2 is a diagram illustrating the relationship between the amplitude and the optical path of an interference signal formed by polarization crosstalk;
FIG. 3 is a flow chart of a method of measuring a distributed extinction ratio of a polarization maintaining fiber;
FIG. 4 is a distributed polarization cross-talk chart of a white light interferometer measurement polarization maintaining fiber;
FIG. 5 is a cross-talk plot of the distributed polarization after correction;
FIG. 6 is a diagram of the distributed extinction ratio of the polarization maintaining fiber under test;
FIG. 7 is a diagram of distributed extinction ratios of polarization maintaining fiber to be tested in an optical path interval of 500mm to 2000 mm.
In the figure: 1-wide spectrum light source, 2-first coupler, 3-first detector, 4-isolator, 5-polarizer input tail fiber, 6-polarizer, 7-polarizer output tail fiber, 8-first welding point, 9-input end of polarization maintaining fiber to be tested, 10-polarization maintaining fiber to be tested, 11-output end of polarization maintaining fiber to be tested, 12-second welding point, 13-input tail fiber of analyzer, 14-analyzer, 15-output tail fiber of analyzer, 16-second coupler, 17-three-port circulator, 18-self-focusing lens, 19-drive optical path scanner, 20-third coupler, 21-second detector, 22-third detector, 23-acquisition card and 24-computer.
Detailed Description
The drawings are for illustrative purposes only and are not to be construed as limiting the patent;
for the purpose of better illustrating the embodiments, certain features of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product;
it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.
The technical solution of the present invention is further described below with reference to the accompanying drawings and examples.
Example 1
The embodiment provides a high-precision distributed extinction ratio measuring method for a polarization maintaining optical fiber, which is realized based on an experimental device for measuring the polarization maintaining optical fiber by an optical coherence domain polarization measuring device.
The experimental device comprises a wide-spectrum light source 1, a first coupler 2, a detector 3, an isolator 4, a polarizer input tail fiber 5, a polarizer 6, a polarizer output tail fiber 7, a first welding point 8, a polarization maintaining fiber input end 9 to be detected, a polarization maintaining fiber 10 to be detected, a polarization maintaining fiber output end 11 to be detected, a second welding point 12, a polarization analyzer input tail fiber 13, a polarization analyzer 14, a polarization analyzer output tail fiber 15, a second coupler 16, a three-port circulator 17, a self-focusing lens 18, a driving optical path scanner 19, a third coupler 20, a first detector 21, a second detector 22, an acquisition card 23 and a computer 24.
The polarizer 6 is a 0-degree polarizer, and the analyzer 14 is a 45-degree analyzer.
The input end of the first coupler 2 is connected with the wide-spectrum light source 1, the output end of the first coupler is respectively connected with the input ends of the first detector 3 and the isolator 4, the output end of the isolator 4 is connected with the polarizer input tail fiber 5, the polarizer output tail fiber 7 is connected with the input end 9 end of the polarization maintaining optical fiber to be tested in a welding mode, and the welding position is a first welding spot 8;
the output end 11 of the polarization maintaining optical fiber to be tested is connected with the input tail fiber 13 of the polarization analyzer in a welding mode, and the welding position is a second welding spot 12;
the output tail fiber of the analyzer is connected with the output end of the second coupler 16; the output end of the second coupler 16 is respectively connected with one input end of the third coupler 20 and the input end of the three-port circulator 17; the output end of the three-port circulator 17 is connected with the other input end of the third coupler 20; the control end of the three-port circulator 17 is connected with the self-focusing lens 18, and the output end of the third coupler 20 is respectively connected with the second detector 21 and the third detector 22; driving the optical path scanner 19 to scan the data collected by the self-focusing lens 18; the acquisition card 23 acquires the data detected by the second detector 21 and the third detector 22 and the data scanned by the driving optical path scanner 19, and sends the data to the computer 24 in a summary manner.
The method comprises the following specific steps:
s1: the polarization crosstalk is measured.
S1.1: measuring the lengths of the polarization maintaining fiber 10 to be measured, the polarizer output tail fiber 7 and the analyzer input tail fiber 13 to obtain data Lf、Lin、Lout；
S1.2: then, the polarizer output tail fiber 7 and the optical fiber input end 9, the analyzer input tail fiber 13 and the polarization maintaining optical fiber output end 11 to be tested are all welded in an alignment way of 0-0 degrees, the polarizer and the analyzer are respectively connected with the input and output ends of the light related domain polarization instrument, the wide spectrum light source 1 is turned on, the optical path scanner 19 is driven, and the polarization crosstalk CT (computed tomography) which changes along with the optical path x is obtained;
s2: the measurement error of the polarization crosstalk, which is introduced by the power loss generated when light is transmitted in the polarization maintaining device, is calculated.
S2.1: using the measured fiber length, the first and second welding points 8 and 12 and the polarizer 6 are estimated at CT x by the formula x ═ Δ n × L (Δ n is the birefringence of the polarization maintaining device to be measured)]And the interference data between the two interference peaks (including the two interference peaks) of the polarizer 6 and the second welding point 12 is measured from CT x]Is intercepted to obtain CTDevice under test[x]From the interference pattern CT [ x ]]The main peak of the signal at the position of zero optical path is taken out by middle section to obtain CTMain peak[x]；
S2.2: cumulative CTDevice under test[x]Obtaining a lumped value, and calculating to obtain an error factor alpha introduced by power loss by using the lumped valueXAnd calculating according to the error factor to obtain the measurement error.
S2.2 specifically comprises the following steps:
s2.2.1 CT according to the intercepted patternMain peak[x]Using the formula:
Figure BDA0002606563240000111
adding up to obtain a numerical value
Figure BDA0002606563240000112
S2.2.2 CT according to the intercepted patternDevice under test[x]Using the formula:
Figure BDA0002606563240000113
accumulating the polarization crosstalk of the polarization maintaining device to be tested;
s2.2.3 after obtaining the accumulated value of the polarization crosstalk, according to the following formula:
Figure BDA0002606563240000114
calculating to obtain an error factor alpha introduced by power lossX。
S2.2.4 according to the following formula:
Figure BDA0002606563240000115
calculating to obtain a loss factor alphaXResulting in measurement errors.
S3: the polarization crosstalk is corrected.
S3.1: method for correcting polarization crosstalk CT by subtracting measurement error value from crosstalk measurement valueAfter correction[x](ii) a The calculation formula is as follows:
CTafter correction[x]＝CTDevice under test[x]-Error (5)
S3.2: intercepting the polarization crosstalk corresponding to the polarization maintaining optical fiber to be measured from the corrected polarization crosstalk;
s4: and measuring the distributed extinction ratio of the polarization maintaining fiber.
S4.1: and measuring to obtain the distributed extinction ratio of the polarization maintaining optical fiber to be measured according to the polarization crosstalk corresponding to the corrected polarization maintaining optical fiber.
S4.1.1: from CTAfter correction[x]The polarization crosstalk CT is obtained by intercepting the interference data between the two interference peaks (not including the two interference peaks) of the polarizer 6 and the second welding point 12Measured optical fiber[x]And using the following formula:
I2[x]＝10^(CTmeasured optical fiber[x]/10) (6)
Calculating to obtain the measurement amplitude I distributed along the optical path x2[x]；
S4.1.2: using measured amplitude I2[x]According to the following formula:
Figure BDA0002606563240000121
calculating the change condition of the transmission power ratio r (x) of the transmission shaft and the coupling shaft in the polarization maintaining fiber to be tested along with the position x;
s4.1.3: according to the following relation:
PER[x]＝10log10(r[x]) (8)
and obtaining the distributed extinction ratio of the polarization maintaining fiber to be measured along the change of the optical path x, and measuring the extinction ratio of the polarization maintaining fiber to be measured at any interval according to PER [ x ].
S4.2: and measuring by using the distributed extinction ratio to obtain the extinction ratio value of the to-be-measured polarization maintaining fiber at any position.
S4.2.1: using the formula:
Figure BDA0002606563240000122
obtaining the polarization maintaining optical fiber to be measured in x1，x2The ratio of the transmission power of the coupling axis and the transmission axis between the two points;
s4.2.2: using the formula:
PER[x1,x2]＝-10log10(r[x1,x2]) (10)
calculating to obtain x1，x2The extinction ratio between the two points.
The principle of the method of the invention is as follows: as shown in fig. 1, after a signal emitted by a broad spectrum light source 1 passes through a coupler 2, light with 98% power passes through an isolator 4 and is changed into polarized light by a polarizer 6, and the rest of light is detected by a detector 3 and is used for detecting the power of the light source, the polarized light passes through a polarizer output tail fiber 7, passes through a welding spot 8, passes through a polarization maintaining device input optical fiber 9 and enters a polarization maintaining optical fiber device to be detected 10; the light meets a plurality of crosstalk points in the transmission process, the crosstalk points are caused by optical fiber welding spots, defects in the optical fibers and the like, the crosstalk points can enable the light transmitted on one characteristic axis to be coupled to the other axis, the coupled light and the residual transmission light are output to a white light measuring device after passing through an analyzer 14, optical path compensation is carried out by driving an optical path scanner 19, the coupled light and the transmission light interfere, and the intensity of interference peaks and the optical path positions correspond to the optical performance of an optical device one by one.
Assuming a beam of linearly polarized light is injected from the slow axis of the polarization maintaining fiber, the polarized light will generate power loss due to polarization crosstalk during transmission. The polarized light is linearly polarized light injected into a characteristic axis (such as a fast axis) of a polarization-maintaining fiber or a device, and meanwhile, when loss such as scattering of the polarized light at a crosstalk point position is considered, at a position x, the optical power of a transmission mode has the following relation with input power:
Figure BDA0002606563240000131
in the formula (I), the compound is shown in the specification,
Figure BDA0002606563240000132
is the optical power injected in the slow axis; ps(z) is the optical power of the transmission mode in the slow axis at position z; alpha is alphas(z) is the total loss factor in the slow axis from the incident end to position z,
Figure BDA0002606563240000133
Figure BDA0002606563240000134
is polarization in the slow axis from the forward directionOptical power loss coefficient introduced by crosstalk;
Figure BDA0002606563240000135
is the optical power loss coefficient in the slow axis introduced by backward rayleigh scattering; finally, at the output of the device, the optical power of the transmission mode in the slow axis has the following relationship to the input power:
Figure BDA0002606563240000141
in the formula
Figure BDA0002606563240000142
Is the optical power at the output of the device for the transmission mode in the slow axis, and L is the length of the device.
For a polarization crosstalk occurring at position z, its crosstalk strength can be expressed as:
Figure BDA0002606563240000143
in the formula, xis(z) is the polarization crosstalk strength coupled by the slow axis to the fast axis at position z; ps(z) is the optical power of the transmitted light in the slow axis at position z after the occurrence of polarization crosstalk;
Figure BDA0002606563240000148
is the optical power of the light coupled by the slow axis to the fast axis at position z.
For a small segment of fiber infinitesimal of length Δ z at position z, we can construct the following relationship for polarization crosstalk loss:
Figure BDA0002606563240000144
in the formula, xis(z) is the polarization crosstalk coupled by the slow axis to the fast axis at position z. If can measure
Figure BDA0002606563240000145
Then the polarization crosstalk ξ can be obtaineds(z). Solving equation (14) also yields the following relationship:
Figure BDA0002606563240000146
i.e. from the polarization crosstalk ξs(z) inverting crosstalk loss
Figure BDA0002606563240000147
As shown in fig. 2, when the optical path difference is equal to Δ nl, the optical paths of the coupling light 204 in the scanning beam and the transmission light 201 in the reference beam are matched, so as to generate a white light interference signal with a peak amplitude of,
Figure BDA0002606563240000151
it is proportional to the coupling amplitude factor of the defect point and the light source intensity, when the optical path difference is equal to-delta nl, the optical paths of the transmission light 207 in the scanning beam and the coupling light 202 in the reference beam are matched, and then a white light interference signal is generated, the peak amplitude of which is
Figure BDA0002606563240000152
It is the same as when the optical path difference is Δ nl. As can be seen, the white light interference signal is symmetrical in optical path and identical in amplitude compared to when the optical path difference is Δ nl.
When power loss due to forward polarization crosstalk and backward rayleigh scattering is neglected, the square of the ratio of the coupling interference peak and main peak light intensity can be used as an approximate measurement of the polarization crosstalk intensity, as follows:
Figure BDA0002606563240000153
in the formula, xis' (z) is the measured polarization crosstalk intensity coupled by the slow axis to the fast axis at position z;
Figure BDA0002606563240000154
the optical power of the slow-axis fast-axis coupled light at the output end of the device at the position z;
Figure BDA0002606563240000155
is the optical power in the optical path corresponding to the main peak of interference light intensity. In the equation (16), the sum of the self-coherent terms of all the orders of the coupled mode and the transmission mode is equal to the incident power without considering the loss, so that there is a relationship,
Figure BDA0002606563240000156
and
Figure BDA0002606563240000157
in this case, the measurement value can be used as an approximation of the polarization crosstalk when the crosstalk loss and the scattering loss are negligible.
However, this approximate measurement introduces large errors when the crosstalk loss and scattering loss cannot be neglected, and the measurement errors will be analyzed in detail below. Order to
Figure BDA0002606563240000158
Equation (13) can be expressed as:
Figure BDA0002606563240000159
the optical power of the coupled light in the fast axis at the coupling point and the output end of the device has the following relationship:
Figure BDA0002606563240000161
in the formula, alphaf(z) is the total loss in the fast axis from the incident end to position z. Finally, the actual intensity of the polarization crosstalk is expressed in terms of the measured intensity as follows:
Figure BDA0002606563240000162
when the scattering loss difference between the fast and slow axes can be ignored, let
Figure BDA0002606563240000163
The following relationship of the measured crosstalk value to the crosstalk loss can be constructed:
Figure BDA0002606563240000164
by changing the injection axis of light from the slow axis to the fast axis, a result similar to equation (22) can be obtained, as follows:
Figure BDA0002606563240000165
in the formula, xif(z) is the measured polarization crosstalk intensity coupled by the fast axis to the slow axis at position z. Using measured polarization crosstalk xis' (z) and xif' (z) the crosstalk loss can be solved numerically
Figure BDA0002606563240000166
And
Figure BDA0002606563240000167
finally, the relationship that can be derived
Figure BDA0002606563240000168
And solving the true value of the crosstalk.
Furthermore, when the difference between the polarization crosstalk loss of the fast and slow axes can be ignored, let α (z) be αs(z)＝αf(z). Accordingly, the following relationship κ also existss＝κf＝κ，ξs(z)＝ξf(z) ═ ξ (z), and ξs′(z)＝ξf'(z) ═ ξ' (z). Equation (10) can be simplified as:
Figure BDA0002606563240000169
that is, at any position, the measurement error of the polarization crosstalk can be expressed as:
Figure BDA0002606563240000171
the slow axis injects linearly polarized light, and the coupling axis output optical power is expressed as:
Figure BDA0002606563240000172
and the output optical power of the transmission axis is:
Figure BDA0002606563240000173
the ratio of the transmission axis to the coupling axis output optical power at position z can be expressed as:
Figure BDA0002606563240000174
the ratio of the powers is the extinction ratio. The above measurement process is only the principle of measuring the distributed extinction ratio, and only discrete measurement data can be obtained in the actual measurement process, but a function expression changing along with the optical path is not obtained by measurement. The measured polarization crosstalk is represented by the peak of the signal, which can be re-expressed as the first term in the amplitude calculation (26):
Figure BDA0002606563240000175
wherein
Figure BDA0002606563240000176
Represents the accumulation of the square of the amplitude of the entire main peak signal, where Ik]Is whiteThe amplitude of the k-th discrete point measured by the optical interference signal, the second term of equation (26) can be re-expressed as:
Figure BDA0002606563240000177
the distributed extinction ratio can be expressed as
Figure BDA0002606563240000181
The distributed extinction ratio is calculated by the equation (26) in an integral mode, the measurement principle can be well understood, but the algorithm is complex, and the calculation time is too long. In the actual measurement process, the discrete points of the white light interference pattern are obtained by measurement and then calculated by using the formula (29), the calculation is fast, but the understanding is not convenient, but the principle is consistent with the formula (26).
The following describes the present embodiment in detail with reference to specific data:
(1) device measurement apparatus as shown in fig. 1, the light source SLD1 used in the white light interferometry apparatus has a center wavelength of 1550nm and a spectral width of about 50nm, so that the effective width of one interference peak is about 100 μm.
(2) According to the step S1, the length of the polarizer output tail fiber is 12.4m, the length of the analyzer input tail fiber is 10m, and the length of the polarization maintaining optical fiber to be measured is 3000 m. The polarizer, the analyzer and the polarization maintaining fiber to be measured are welded at 0-0 degree, the measuring instrument is opened, and the distributed polarization crosstalk result CT x of the light path to be measured is obtained through measurement, wherein CT x is shown in figure 4.
(3) Using the measured fiber length, the formula x ═ Δ n × L (Δ n ≈ 7 × 10 according to the parameter Δ n provided by the polarization maintaining fiber manufacturer) is used according to step S2-4) Roughly defining the first welding point 8, the second welding point 12 and the polarizer 6 at CT x]In 2108mm, 7mm, 2115 mm. Such as 402,403,404 in figure 4. And interference data between the two interference peaks (including the two interference peaks with effective width of 100 μm) 402 and 404 is extracted from CT x]Is cut off to obtainInterference data CT corresponding to polarization analyzer, polarization analyzer and optical fiber to be measuredDevice under test[x]. Determining an interference pattern CT x]The position 401 of the interference peak with the highest medium amplitude, and the main peak is selected from CT x]Is intercepted to obtain CTMain peak[x]Using the formula:
Figure BDA0002606563240000191
accumulating the polarization crosstalk of the optical path to be measured to obtain a lumped value
Figure BDA0002606563240000192
And using the formula:
Figure BDA0002606563240000193
calculating to obtain an error factor alpha introduced by transmission lossX0.4903, and then according to the formula
Figure BDA0002606563240000194
The measurement Error introduced by the transmission loss is calculated to be-6 dB.
(4) According to step S3, the following formula is utilized:
CTafter correction[x]＝CTDevice under test[x]-Error
The corrected distributed polarization crosstalk is obtained, as shown in FIG. 5, from CTAfter correction[x]Intercepting the interference data between the two interference peaks 501 and 502 (not including the two interference peaks) to obtain CTMeasured optical fiber[x]. By comparing fig. 5 with fig. 4, it can be found that the polarization crosstalk value after correction is raised as a whole.
(5) According to step S4, the following formula is utilized:
Figure BDA0002606563240000195
calculating to obtain the transmission light power ratio of the transmission axis and the coupling axis of the polarization maintaining optical fiber to be measured distributed along the optical path x, and utilizing a formula:
PER[x]＝10log10(r[x])
and calculating a distributed extinction ratio curve of the polarization maintaining fiber to be tested, as shown in fig. 6. Using the formula:
Figure BDA0002606563240000201
obtaining an optical path of 500mm to 500mm<x2Energy between two points less than or equal to 2000 is obtained by using the formula:
PER(500,x2)＝-10log10[r(500,x2)]
the distributed extinction ratio curve is calculated for optical lengths from 500mm to 2000mm as shown in FIG. 7.
The terms describing positional relationships in the drawings are for illustrative purposes only and are not to be construed as limiting the patent;
it should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.

Claims (10)

1. A high precision distributed extinction ratio measurement method for a polarization maintaining optical fiber, the method comprising the steps of:
s1: measuring the polarization crosstalk of the polarization maintaining optical fiber to be measured;
s2: calculating the measurement error of the polarization crosstalk;
s3: according to the measurement error, correcting the polarization crosstalk of the polarization-maintaining optical fiber to be measured;
s4: and measuring the distributed extinction ratio of the polarization maintaining fiber to be measured according to the polarization crosstalk corresponding to the corrected polarization maintaining fiber.
2. The method of claim 1, wherein S1 comprises the following steps:
s1.1: measuring polarization crosstalk; firstly, the length L of the polarization maintaining optical fiber to be measured is measuredfLength L of polarizer output tail fiberinAnd length L of input tail fiber of analyzerout；
S1.2: welding the polarizer, analyzer and polarization maintaining fiber at 0-0 deg, connecting the polarizer and analyzer with optical relative domain polarizer, and opening the instrument to obtain the polarization crosstalk CT x of the measured optical path.
3. The method of claim 2, wherein S2 comprises the following steps:
s2.1: determining the positions of welding points and signal main peaks in the polarization crosstalk, and extracting the polarization crosstalk CT of the measured optical circuit from the polarization crosstalkDevice under test[x]Polarization crosstalk CT corresponding to each main peakMain peak[x]；
S2.2: cumulative CTDevice under test[x]Obtaining a lumped value, and calculating to obtain an error factor alpha introduced by power loss by using the lumped valueXAnd calculating according to the error factor to obtain the measurement error.
4. A high precision distributed extinction ratio measurement method for a polarization maintaining optical fiber according to claim 3, wherein S2.2 includes the steps of:
s2.2.1: CT according to the intercepted patternMain peak[x]Using the formula:
Figure FDA0002606563230000021
adding up to obtain a numerical value
Figure FDA0002606563230000022
S2.2.2: CT according to the intercepted patternDevice under test[x]Using the formula:
Figure FDA0002606563230000023
accumulating the polarization crosstalk of the polarization maintaining device to be tested;
s2.2.3: after obtaining the accumulated value of the polarization crosstalk, the following formula is used:
Figure FDA0002606563230000024
calculating to obtain an error factor alpha introduced by power lossX；
S2.2.4: according to the following formula:
Figure FDA0002606563230000025
calculating to obtain a loss factor alphaXResulting in measurement errors.
5. The method according to claim 4, wherein S3 includes:
s3.1: by polarization crosstalk measurements CTDevice under test[x]The polarization crosstalk is corrected by subtracting the Error value Error to obtain the corrected polarization crosstalk CTAfter correction[x]；
S3.2: and intercepting the polarization crosstalk corresponding to the polarization maintaining optical fiber to be measured from the corrected polarization crosstalk.
6. The method according to claim 4, wherein S4 comprises the following steps:
s4.1: according to the corrected polarization crosstalk CTAfter correction[x]Calculating to obtain the ratio of the transmission optical power of the polarization maintaining optical fiber transmission shaft to be tested and the transmission optical power of the coupling shaft distributed along the length of the optical fiber, wherein the power ratio is the distributed extinction ratio;
s4.2: and calculating the extinction ratio value of the random position of the polarization maintaining fiber to be measured by utilizing the distributed extinction ratio.
7. The method of claim 4, wherein S4.1 comprises the steps of:
s4.1.1: corrected polarization crosstalk CTAfter correction[x]Intercepting interference data distributed along the length of the optical fiber by the transmission axis and the coupling axis of the polarization maintaining optical fiber to be tested to obtain polarization crosstalk CTMeasured optical fiber[x]Calculating the measured amplitude I distributed along the optical path x2[x]；
S4.1.2: according to the measured amplitude I2[x]Calculating the change condition of the transmission power ratio r (x) of the transmission shaft and the coupling shaft in the polarization maintaining fiber to be tested along with the position x;
s4.1.3: according to the following relation:
PER[x]＝10log10(r[x])
and obtaining the distributed extinction ratio of the polarization maintaining fiber to be measured along the change of the optical path x, and measuring according to PER [ x ] to obtain the extinction ratio of the polarization maintaining fiber to be measured at any interval.
8. A method for measuring a high precision distributed extinction ratio for a polarization maintaining optical fiber according to claim 4, wherein the magnitude I is measured2[x]The calculation formula of (2) is as follows:
I2[x]＝10^(CTmeasured optical fiber[x]/10)。
9. The method of claim 8, wherein the ratio of transmission powers r (x) is calculated by the formula:
Figure FDA0002606563230000041
10. the method of claim 8, wherein S4.2 comprises the steps of:
s4.2.1: calculating the x of the polarization maintaining optical fiber to be measured at any two points1，x2The ratio of the transmission power of the coupling shaft and the transmission shaft is calculated by the following formula:
Figure FDA0002606563230000042
s4.2.1: calculating x1，x2The extinction ratio between the two points is calculated by the following formula:
PER[x1,x2]＝-10log10(r[x1,x2])。
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