CN108287056B - System and method for evaluating coupling characteristics of optical fiber sensitive ring polarization mode - Google Patents

Abstract

The invention discloses an evaluation system and an evaluation method for the polarization mode coupling characteristic of an optical fiber sensing ring, which relate to the technical field of optical fiber sensing and comprise a pulse laser, a polarization controller, an optical fiber circulator, an optical fiber sensing ring, an online pulse polarization state receiver, a digital oscilloscope and a calculation module, wherein the pulse laser can emit continuous light and pulsed light, and the testing tail end of the optical fiber sensing ring is connected with a polarization analyzer; when the coupling characteristic is evaluated, firstly, a polarization analyzer is used for carrying out polarization eigenstate alignment, then pulsed light consistent with the polarization eigenstate is input, backscattered light is used for measuring the Stokes vector of reflected light of each point of the optical fiber sensitive ring, and finally, a three-point quaternion method and a birefringence vector projection method are used for calculating the polarization state electric field vector mode coupling coefficient and the extinction ratio of each point. The invention is suitable for high-precision measurement of high-quality optical fiber sensitive ring polarization mode coupling and the change of polarization maintaining optical fiber polarization mode coupling parameters caused by external factors, and realizes multi-parameter distributed sensing.

Description

Optical fiber sensitive ring polarization mode coupling characteristic evaluation system and evaluation method

technical field

The invention relates to the technical field of optical fiber sensing, in particular to an evaluation system and an evaluation method for the polarization mode coupling distribution characteristics of an optical fiber sensitive ring.

Background technique

As a key inertial measurement technology, fiber optic gyroscope (FOG) has attracted the attention of many countries since it was first proposed in 1976. After years of rapid development, FOG has become the mainstream choice in the field of inertial technology and is widely used It is used in aircraft and ship navigation, attitude control of armored vehicles, tanks and pendulum trains, and spacecraft stabilization. The fiber-optic sensing ring is the core component of the fiber optic gyroscope. It is wound from polarization-maintaining fiber (PMF) with good polarization-maintaining performance. The induced polarization mode coupling introduced by the quasi-induced polarization can cause significant polarization state fluctuations, resulting in zero drift. To fundamentally improve the performance of the fiber optic gyroscope, it is necessary to measure the polarization mode coupling coefficient of the fiber sensitive ring more accurately, improve the positioning accuracy of the coupling point, and deeply study the polarization-maintaining performance of the fiber sensitive ring.

In addition to being used in fiber-optic gyroscopes, fiber-optic sensitive loops are also widely used in fiber-optic current sensors. Different from the fiber sensitive ring of the fiber optic gyroscope, it adopts an elliptical polarization state maintaining fiber, and its polarization eigenstates are two orthogonal ellipses.

At present, the method used for polarization-maintaining fiber polarization mode coupling detection is mainly fiber white light interferometer. The principle is to determine the mode coupling point by compensating for the optical path difference of the optical power coupling point. The following problems will occur: First, the broad-spectrum light in the fiber-optic sensitive ring can only resonate at specific wavelengths, and the polarization mode coupling is wavelength-dependent; however, the light source of white light interference is a broad-spectrum light source, and the measured mode coupling strength is The average mode coupling of the broad-spectrum light, therefore, the broad-spectrum light average mode coupling using white light interferometry cannot accurately describe the effect of the polarization mode coupling on the resonant wavelengths of different fiber resonators; second, the polarization mode coupling coefficient can be positive or negative. , because the polarization mode coupling is essentially caused by the misalignment of the birefringence axes (also known as the polarization principal axes) of the two adjacent optical fibers, and the misalignment angle of the principal axes can be positive or negative; the mode coupling coefficient measured by the white light interferometer is the intensity coupling coefficient, and its values are all positive, which cannot reflect the coupling coefficient of the real polarization mode electric field vector; thirdly, due to the use of a broad-spectrum light source, it will lead to a large polarization mode dispersion, and its spatial resolution will vary with the measurement length. Fourth, the white light interferometer changes the optical path difference between the two arms of the interferometer by mechanical scanning, and the mirror will change the polarization state of the reflected light during the mechanical movement process, thereby affecting the measurement accuracy. Fifth, the white light interferometer uses a polarization beam splitter in the measurement process, so it can only detect the polarization mode coupling of the linear polarization state maintaining fiber, and cannot detect the mode coupling between two orthogonal elliptical polarization states in the polarization elliptical fiber. Although fiber optic white light interferometer technology has reached a fairly high level over the years, it has been practical and commercialized. However, for higher-quality fiber-optic sensitive rings, the mode coupling coefficient is smaller, and the measurement requirements are higher, while the white light interferometer is limited by its principle, and the space for further improvement is limited. Therefore, in order to further improve the detection sensitivity, detect the mode coupling coefficient of the electric field vector of the polarization mode, and also realize the measurement of the polarization mode coupling of the optical fiber sensitive ring made of the elliptically polarization state-preserving fiber, it is an important requirement to explore new methods. .

Optical time domain reflectometry is a commonly used technique for measuring fiber distribution parameters. The polarization state of the Rayleigh scattered light at a certain point in the fiber is consistent with the incident light at that point, so the Rayleigh scattered light can be used to detect the polarization state of the light. Variations along the fiber. However, the optical time domain reflectometry technology has the following problems in specific applications: first, the pulse time for detecting the change of polarization state must be less than the transmission time of the optical fiber beat length, and the beat length of the polarization maintaining fiber is very small, usually about 2~3mm. The pulse is required to be in the order of ps or even fs; secondly, the optical time domain reflectometry technology usually used to detect the polarization state uses a linear polarization state in a fixed direction, which may not be able to align with the polarization axis of the polarization maintaining fiber, especially there is no way to ensure Align the elliptical polarization state to maintain the polarization eigenstate of the fiber; thirdly, there are two methods to detect the polarization state change. One is to use an analyzer to measure the optical power in a specific polarization direction, so the polarization state cannot be detected. All Stokes parameters of ; the other method is full polarization state detection, which is the complete polarization state information by obtaining three Stokes parameters [S1, S2, S3] distributed along the length of the fiber, but, There is currently no online device suitable for full polarization state detection of high-speed pulses. Therefore, the current optical time domain reflectometry technology based on full polarization state detection is limited to ordinary single-mode fibers, and has not been used for the measurement of polarization mode coupling performance of polarization-maintaining fibers.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method capable of measuring the mode coupling performance of a higher-quality optical fiber sensitive ring, with higher detection sensitivity, and capable of detecting the coupling coefficient of the electric field vector of the polarization mode, and capable of maintaining the elliptical polarization state in the production of an optical fiber. The optical fiber sensitive ring polarization mode coupling characteristic evaluation system is used to measure the polarization mode coupling of the optical fiber sensitive ring, so as to solve the technical problem that the full polarization state cannot be accurately detected in the above-mentioned background art, and the polarization mode coupling performance measurement of the polarization maintaining fiber is realized.

In order to achieve the above object, the present invention has adopted the following technical solutions:

An optical fiber sensitive ring polarization mode coupling characteristic evaluation system, comprising a sending module, a receiving module, and a computing module,

The sending module includes a pulsed laser, a polarization controller, and an optical fiber circulator connected in sequence, the output end of the pulsed laser is connected to the input end of the polarization controller, and the output end of the polarization controller is connected to the optical fiber circulator The first end of the fiber circulator is connected to the test start end of the fiber sensitive ring to be tested.

The receiving module includes an online pulse polarization state receiver and a digital oscilloscope, the first input end of the online pulse polarization state receiver is connected to the third end of the optical fiber circulator, and the output end of the online pulse polarization state receiver is The input end of the digital oscilloscope is connected, and the output end of the digital oscilloscope is connected to the calculation module.

A polarization analyzer is connected to the test end of the optical fiber sensitive ring to be tested.

Further, the pulsed laser includes a narrowband continuous light laser, a lithium niobate modulator, a programmable pulse code generator, a modulator driver, a fiber amplifier and an optical filter; the narrowband continuous light laser is connected to the lithium niobate modulator. The output end of the lithium niobate modulator is connected to the input end of the fiber amplifier, the output end of the fiber amplifier is connected to the input end of the optical filter, and the output end of the optical filter is connected to the input end of the optical filter. the polarization controller; the output end of the programmable pulse code generator is connected to the input end of the modulator driver, and the output end of the modulator driver is connected to the second input end of the lithium niobate modulator.

Further, the online pulse polarization state receiver includes a polarization beam splitter, a coupler, a 90° optical mixer, and three balanced optical detectors; the output end of the polarization beam splitter is connected with two of the couplings. The first output ends of the two couplers are respectively connected to the two input ends of the 90° optical mixer, and the two output ends of the 90° optical mixer are respectively connected to the first balanced optical detector and a second balanced photodetector, the second output ends of the two couplers are connected to a third balanced photodetector, and the second output ends of the two couplers are respectively connected to the third balanced photodetector The input end of the polarization beam splitter is connected to the third end of the fiber circulator, the output end of the first balanced photodetector, the output end of the second balanced photodetector, The output ends of the third balanced photodetector are all connected to the digital oscilloscope.

Further, the online pulse polarization state receiver includes a coupler, a 0° line analyzer, a 45° line analyzer, a right-handed circular analyzer, and a balanced light detector; the three output ends of the coupler Connect the input ends of the 0° line analyzer, the 45° line analyzer and the right-hand circular analyzer respectively, the 0° line analyzer, the 45° line analyzer and the The output ends of the right-hand circular analyzer are respectively connected with a balanced photodetector, the output ends of the three balanced photodetectors are all connected to the digital oscilloscope, and the input end of the coupler is connected to the optical fiber loop the third end of the device.

A method for evaluating the polarization mode coupling characteristics of an optical fiber sensitive ring by using the above system, comprising the following steps:

Adjust the polarization state of the optical signal input to the optical fiber sensitive ring to be tested to be consistent with the polarization eigenstate of the optical fiber sensitive ring to be tested, and use the optical signal consistent with the polarization eigenstate of the optical fiber sensitive ring to be tested as the test Optical signal input;

The complete polarization state Stokes vector S of the reflected optical signal distributed along the longitudinal direction is collected by the online pulse polarization state receiver and the digital oscilloscope;

According to the Stokes vector, the polarization mode coupling coefficient and extinction ratio distributed along the longitudinal direction of the fiber sensitive ring to be tested are calculated.

Further, the adjustment of the polarization state of the test optical signal input to the optical fiber sensitive ring to be tested is consistent with the polarization eigenstate of the optical fiber sensitive ring to be tested includes adjusting the pulsed laser to a continuous light output state, through the The polarization controller modulates the polarization state, and the polarization analyzer is used to observe the output polarization state of the fiber sensitive ring to be tested, so that the input polarization state of the fiber sensitive ring to be tested is modulated by the polarization controller and the polarization state of the fiber sensitive ring to be tested is the same. Consistent.

Further, the acquisition of the complete polarization state Stokes vector of the reflected optical signal along the longitudinal distribution by the online pulse polarization state receiver and the digital oscilloscope includes: circulating the test optical signal through the optical fiber. After coupling, it enters the optical fiber sensitive ring to be tested, and forms a reflected light signal through back Rayleigh scattering. The reflected light signal returns to the second end of the optical fiber circulator and enters the optical fiber circulator, and circulates through the optical fiber The third end of the device enters the online pulse polarization state receiver.

根据公式

计算出双折射矢量

单位矢量

表示本地偏振主轴的方向，大小|B|是偏振态绕偏振主轴旋转的速率，即在该主轴下双折射的大小；根据公式

计算出模耦合系数k。Further, calculating the polarization mode coupling coefficient and extinction ratio longitudinally distributed along the optical fiber sensitive ring to be tested according to the Stokes vector includes calculating the Stokes vector S according to the complete polarization state Stokes vector. The rate of change of the S vector

According to the formula

Calculate the birefringence vector

unit vector

represents the direction of the local polarization principal axis, and the magnitude |B| is the rate at which the polarization state rotates around the polarization principal axis, that is, the magnitude of birefringence under this principal axis; according to the formula

Calculate the mode coupling coefficient k.

Further, according to the Stokes vector relationship of the three adjacent detection points on the optical fiber sensitive ring to be measured, the birefringence vector B is calculated by a three-point quaternion method.

两轴进行投影计算所述模耦合系数k。Further, according to the birefringence vector B on the Bonga sphere

The two axes are projected to calculate the mode coupling coefficient k.

The principle of realizing the polarization mode coupling distributed measurement of the optical fiber sensitive ring in the present invention is as follows:

与当前的斯托克斯矢量垂直，从而有Because the Stokes vector S rotates on the Bonga sphere, its radius is always 1, so its rate of change

is perpendicular to the current Stokes vector, thus having

其方向(单位矢量

)是本地偏振主轴的方向，大小|B|是偏振态绕偏振主轴旋转的速率，即在该主轴下双折射的大小。当考虑偏振模耦合对偏振态传输的影响时，B应包含偏振模耦合系数，以下我们使用四元数的方法导出B与两个垂直偏振模的传输常数差Δβ＝β₊-β_-和它们的模耦合系数k之间的关系。in

its direction (unit vector

) is the orientation of the local polarization principal axis, and the magnitude |B| is the rate at which the polarization state rotates around the polarization principal axis, that is, the magnitude of birefringence under this principal axis. When considering the effect of polarization mode coupling on the transmission of polarization state, B should contain the polarization mode coupling coefficient. Below we use the quaternion method to derive the transmission constant difference between B and two vertically polarized modes Δβ=β ₊ -β _- and their The relationship between the mode coupling coefficient k of .

Coupling between two orthogonal polarization states (including two orthogonal linear polarization states in linear polarization maintaining fibers, and two orthogonal elliptical polarization states in elliptical polarization maintaining fibers), ignoring the loss of polarization maintaining fibers, theoretical can be written as

分别为两个正交本征态投影到本地邦加球上的电场分量，并且β₊,β-分别为两个正交本征态的传输常数，k为它们之间的耦合系数，z为沿着光纤纵向的长度。

are the electric field components of the two orthogonal eigenstates projected onto the local Bonga sphere, respectively, and β ₊ , β- are the transmission constants of the two orthogonal eigenstates, k is the coupling coefficient between them, z is along the longitudinal length of the fiber.

When formula (2) is expressed in the form of Jones vector, it is

Formula (3) is expressed in the form of quaternion [32], as

(用英文花体Edwardian Script ITC表示，下同)是琼斯矢量

对应的四元数，

是琼斯矩阵

对应的四元数，由于琼斯矩阵可以分解为where quaternion

(expressed in English swash Edwardian Script ITC, the same below) is the Jones vector

The corresponding quaternion,

is the Jones matrix

The corresponding quaternion, since the Jones matrix can be decomposed into

传输常数差Δβ＝β₊-β_-，可得到琼斯矩阵对应的四元数为where the average transfer constant of the fast and slow axes

Transmission constant difference Δβ=β ₊ -β _- , the quaternion corresponding to Jones matrix can be obtained as

therefore

是

的厄米转置。由于斯托克斯四元数

因此in,

Yes

The Hermitian transpose of . Due to Stokes Quaternion

therefore

Substituting (7) into (8), we get

Let Stokes Quaternion where s ₀ is the scalar part and S is the vector part, and substituting in (9), we get

Comparing (11) and (1), we can get

两轴投影，就可以得到两个正交本征态的传输常数差Δβ和偏振模耦合系数k。Therefore, as long as the B pair is measured

Two-axis projection, the transmission constant difference Δβ and the polarization mode coupling coefficient k of the two orthogonal eigenstates can be obtained.

两轴投影，即得到该小段光纤的传输常数差Δβ和偏振模耦合系数k。另外，在本系统中从光纤始端检测到的相邻三点A、B、C的偏振态四元数

之间的旋转角度与在A点检测到的

之间的旋转角度相同，因此在邦加球上S_out(A)、S_out(B)、S_out(C)之间的变化关系与S_A(A)、S_A(B)、S_A(C)之间的变化关系是一致的。因此，在本系统中，通过计算光纤始端获得的A、B、C三个位置对应的斯托克斯矢量S_out(A)、S_out(B)、S_out(C)，根据(12)式计算得到的偏振模耦合系数k，便可描述A、B、C相邻三点之间的偏振模耦合。Based on the three-point quaternion method, the B pair can be calculated only by knowing the Stokes vectors of three adjacent points of a small section of fiber.

Two-axis projection, that is, the transmission constant difference Δβ and the polarization mode coupling coefficient k of the small section of optical fiber are obtained. In addition, the polarization state quaternion of three adjacent points A, B, and C detected from the beginning of the fiber in this system

The rotation angle between the detected at point A and the

_{The rotation angles between the _} (C) The variation relationship between is consistent. Therefore, in this system, the Stokes vectors S _out (A), S _out (B), and S _out (C) corresponding to the three positions of A, B, and C obtained by calculating the beginning of the fiber, according to (12) The polarization mode coupling coefficient k calculated by the formula can describe the polarization mode coupling between the adjacent three points A, B, and C.

The invention has beneficial effects: it is suitable for measuring high-quality optical fiber sensitive ring polarization mode coupling, and makes up for the shortcomings of the existing white light interference technology in measuring optical fiber sensitive ring polarization mode coupling, which is not high in accuracy and is not suitable for elliptical polarization state-preserving fibers. It is used to measure the polarization mode coupling of polarization-maintaining fibers caused by external factors (such as stress, bending, vibration, etc.), and realize distributed measurement of various parameters.

Additional aspects and advantages of the present invention will be set forth in part in the following description, which will be apparent from the following description, or may be learned by practice of the present invention.

Description of drawings

In order to illustrate the technical solutions of the embodiments of the present invention more clearly, the following briefly introduces the accompanying drawings used in the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained from these drawings without any creative effort.

FIG. 1 is a schematic diagram of a system for evaluating the polarization mode coupling characteristics of an optical fiber sensitive ring according to an embodiment of the present invention.

FIG. 2 is a structural block diagram of a pulsed laser in the optical fiber sensitive ring polarization mode coupling characteristic evaluation system according to an embodiment of the present invention.

3 is a structural block diagram of an online pulse polarization state receiver in the optical fiber sensitive ring polarization mode coupling characteristic evaluation system according to Embodiment 1 of the present invention.

FIG. 4 is a structural block diagram of an online pulse polarization state receiver in the optical fiber sensitive ring polarization mode coupling characteristic evaluation system according to Embodiment 2 of the present invention.

FIG. 5 is a longitudinal distribution diagram of the Rayleigh scattered light power corresponding to the three components S1 , S2 and S3 of the Stokes vector according to the embodiment of the present invention along the optical fiber sensitive ring.

6 is a graph of polarization mode coupling along the longitudinal distribution of the optical fiber sensitive ring obtained from the forward and reverse measurements of the optical fiber sensitive ring according to the embodiment of the present invention.

FIG. 7 is a distribution diagram of the polarization states of positions A(a) and E(b) obtained by repeated measurements on the Bonga sphere according to the embodiment of the present invention.

Among them: 100-sending module; 200-receiving module; 300-calculating module; 110-pulse laser; 120-polarization controller; 130-fiber circulator; ;220-digital oscilloscope;500-polarization analyzer;111-narrowband continuous light laser;112-lithium niobate modulator;113-programmable pulse code generator;114-modulator driver;115-fiber amplifier;116-optical 211-polarization beam splitter; 212-coupler; 213-90° optical mixer; 214-first balanced photodetector; 215-second balanced photodetector; 216-third balanced photodetector 211a-coupler; 212a-0° line analyzer; 213a-45° line analyzer; 214a-right circular analyzer; 215a-balanced photodetector.

Detailed ways

Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals refer to the same or similar elements or elements having the same or similar functions throughout. The embodiments described below through the accompanying drawings are exemplary and are only used to explain the present invention, but not to be construed as a limitation of the present invention.

It will be understood by those skilled in the art that the singular forms "a", "an", "the" and "the" as used herein can include the plural forms as well, unless expressly stated otherwise. It should be further understood that the word "comprising" used in the description of the present invention refers to the presence of stated features, integers, steps, operations, elements and/or components, but does not exclude the presence or addition of one or more other features, Integers, steps, operations, elements and/or groups thereof. It should be understood that "connected" or "coupled" as used herein can include wirelessly connected or coupled, and that use of the term "and/or" includes any and all combinations of one or more of the associated listed items.

It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It should also be understood that terms such as those defined in general dictionaries should be understood to have meanings consistent with their meanings in the context of the prior art and, unless defined as herein, are not to be taken in an idealized or overly formal sense. explain.

In order to facilitate the understanding of the present invention, the present invention will be further explained and described below with reference to the accompanying drawings with specific embodiments, and the specific embodiments do not constitute limitations to the embodiments of the present invention.

1 is a schematic diagram of a system for evaluating the polarization mode coupling characteristics of an optical fiber sensitive ring according to an embodiment of the present invention, FIG. 2 is a structural block diagram of a pulsed laser in the system for evaluating the polarization mode coupling characteristics of an optical fiber sensitive ring according to an embodiment of the present invention, and FIG. 3 It is a structural block diagram of the online pulse polarization state receiver in the optical fiber sensitive ring polarization mode coupling characteristic evaluation system according to the first embodiment of the present invention, and FIG. 4 is the optical fiber sensitive ring polarization mode coupling characteristic evaluation system according to the second embodiment of the present invention. The structural block diagram of the online pulse polarization state receiver, Fig. 5 is the longitudinal distribution diagram of the Rayleigh scattered light power corresponding to the three components S1, S2 and S3 of the Stokes vector according to the embodiment of the present invention along the optical fiber sensitive ring, Fig. 6 is the The polarization mode coupling curve along the longitudinal distribution of the optical fiber sensitive ring obtained from the forward and reverse measurements of the optical fiber sensitive ring according to the embodiment of the present invention, FIG. 7 is the position obtained by repeated measurements according to the embodiment of the present invention Distributions of the polarization states of A(a) and E(b) on the Bonga sphere.

Those skilled in the art should understand that the accompanying drawings are only schematic diagrams of the embodiments, and the components in the accompanying drawings are not necessarily necessary to implement the present invention.

As shown in FIG. 1 to FIG. 4 , an embodiment of the present invention provides an optical fiber sensitive ring polarization mode coupling characteristic evaluation system, including a sending module 100, a receiving module 200, and a computing module 300,

The sending module 100 includes a pulsed laser 110, a polarization controller 120, and a fiber circulator 130 connected in sequence. The output end of the pulsed laser 110 is connected to the input end of the polarization controller 120, and the output of the polarization controller 120 is connected. The end is connected to the first end of the optical fiber circulator 130, and the second end of the optical fiber circulator 130 is connected to the test start end of the optical fiber sensitive ring 400 to be tested;

The receiving module 200 includes an online pulse polarization state receiver 210 and a digital oscilloscope 220. The first input end of the online pulse polarization state receiver 210 is connected to the third end of the optical fiber circulator 130, and the online pulse polarization state The output end of the receiver 210 is connected to the input end of the digital oscilloscope 220, and the output end of the digital oscilloscope 220 is connected to the calculation module 300;

A polarization analyzer 500 is connected to the test end of the fiber optic ring 400 to be tested.

In a specific embodiment of the present invention, the pulsed laser 110 includes a narrowband continuous light laser 111, a lithium niobate modulator 112, a programmable pulse code generator 113, a modulator driver 114, a fiber amplifier 115 and an optical filter 116; The narrowband continuous light laser 111 is connected to the first input end of the lithium niobate modulator 112 , the output end of the lithium niobate modulator 112 is connected to the input end of a fiber amplifier 115 , and the output end of the fiber amplifier 115 is connected to The input end of the optical filter 116, the output end of the optical filter 116 is connected to the polarization controller 120; the output end of the programmable pulse code generator 113 is connected to the input end of the modulator driver 114, The output terminal of the modulator driver 114 is connected to the second input terminal of the lithium niobate modulator 112 .

In a specific embodiment of the present invention, the online pulse polarization state receiver 210 includes a polarization beam splitter 211, a coupler 212, a 90° optical mixer 213, and three balanced optical detectors 214, 215, and 216; The output ends of the polarization beam splitter 211 are connected with two couplers 212, and the first output ends of the two couplers 212 are respectively connected with the two input ends of the 90° optical mixer 213, so The two output ends of the 90° optical mixer 213 are respectively connected with a first balanced light detector 214 and a second balanced light detector 215, and the second output ends of the two couplers 212 are connected with a third balanced light detector The detector 216, the second output ends of the two couplers 212 are respectively connected to the two input ends of the third balanced optical detector 216; the input end of the polarization beam splitter 211 is connected to the fiber circulator 130 The output end of the first balanced photodetector 214, the output end of the second balanced photodetector 215, and the output end of the third balanced photodetector 216 are all connected to the digital oscilloscope 220 .

In a specific embodiment of the present invention, the online pulse polarization state receiver 210 includes a coupler 211a, a 0° linear analyzer 212a, a 45° linear analyzer 213a, a right-handed circular analyzer 214a, a balanced light detector 215a; the three output ends of the coupler 211a are respectively connected to the input ends of the 0° line analyzer 212a, the 45° line analyzer 213a and the right-hand circular analyzer 214a, so The output ends of the 0° line analyzer 212a, the 45° line analyzer 213a and the right-hand circular analyzer 214a are respectively connected with a balanced photodetector 215a, and the three balanced photodetectors 215a The output ends of the two are connected to the digital oscilloscope 220 , and the input end of the coupler 211 a is connected to the third end of the optical fiber circulator 130 .

As shown in FIG. 5 to FIG. 7 , an embodiment of the present invention further provides a method for evaluating the polarization mode coupling characteristics of an optical fiber ring by using the above system, including the following steps:

Adjust the polarization state of the optical signal input to the optical fiber sensitive ring 400 to be tested to be consistent with the polarization eigenstate of the optical fiber sensitive ring 400 to be tested, and the light that is consistent with the polarization eigenstate of the optical fiber sensitive ring to be tested The signal is input as a test optical signal;

The complete polarization state Stokes vector S of the reflected optical signal distributed along the longitudinal direction is collected by the online pulse polarization state receiver 210 and the digital oscilloscope 220;

The polarization mode coupling coefficient and extinction ratio distributed along the longitudinal direction of the optical fiber sensing ring 400 to be tested are calculated according to the Stokes vector.

In the method embodiment of the present invention, the adjusting the polarization state of the test optical signal input to the optical fiber sensing ring 400 under test to be consistent with the polarization eigenstate of the optical fiber sensing ring 400 under test includes adjusting the The pulsed laser 110 is in the continuous light output state, the polarization state is modulated by the polarization controller 120, and the polarization analyzer 500 is used to observe the output polarization state of the fiber-optic sensitive ring 400 under test, so that the polarization controller 120 modulates the output polarization state of the fiber under test. The input polarization state of the sensitive ring 400 is consistent with the polarization eigenstate of the optical fiber sensitive ring 400 to be tested.

In the method embodiment of the present invention, the acquisition of the complete polarization state Stokes vector distributed along the longitudinal direction of the reflected optical signal through the online pulse polarization state receiver 210 and the digital oscilloscope 220 includes: The test optical signal is coupled by the optical fiber circulator 130 and then enters the optical fiber sensitive ring 400 to be tested, and forms a reflected optical signal through back-Rayleigh scattering, and the reflected optical signal returns to the first position of the optical fiber circulator 130 . The second end enters the fiber circulator 130 , and the third end of the fiber circulator 130 enters the online pulse polarization state receiver 210 .

根据公式

计算出双折射矢量单位矢量

表示本地偏振主轴的方向，大小|B|是偏振态绕偏振主轴旋转的速率，即在该主轴下双折射的大小；根据公式

计算出模耦合系数k。In the method embodiment of the present invention, the calculating the polarization mode coupling coefficient and the extinction ratio distributed along the longitudinal direction of the optical fiber sensing ring 400 to be tested according to the Stokes vector includes, according to the complete polarization state Stokes Stokes vector S, calculate the rate of change of Stokes vector S

According to the formula

Calculate the birefringence vector unit vector

represents the direction of the local polarization principal axis, and the magnitude |B| is the rate at which the polarization state rotates around the polarization principal axis, that is, the magnitude of birefringence under this principal axis; according to the formula

Calculate the mode coupling coefficient k.

In the method embodiment of the present invention, the birefringence vector B is calculated by the method of three-point quaternion according to the Stokes vector relationship of the three adjacent detection points on the optical fiber sensing ring 400 to be tested. .

两轴进行投影计算所述模耦合系数k。In the method embodiment of the present invention, according to the birefringence vector B on the Bonga sphere

The two axes are projected to calculate the mode coupling coefficient k.

Example 1

An optical fiber sensitive ring polarization mode coupling characteristic evaluation system according to Embodiment 1 of the present invention includes a sending module 100, a receiving module 200, and a computing module 300,

The sending module 100 includes a pulsed laser 110, a polarization controller 120, and a fiber circulator 130 connected in sequence. The output end of the pulsed laser 110 is connected to the input end of the polarization controller 120, and the output of the polarization controller 120 is connected. The end is connected to the first end of the optical fiber circulator 130, and the second end of the optical fiber circulator 130 is connected to the test start end of the optical fiber sensitive ring 400 to be tested;

The receiving module 200 includes an online pulse polarization state receiver 210 and a digital oscilloscope 220. The first input end of the online pulse polarization state receiver 210 is connected to the third end of the optical fiber circulator 130, and the online pulse polarization state The output end of the receiver 210 is connected to the input end of the digital oscilloscope 220, and the output end of the digital oscilloscope 220 is connected to the calculation module 300;

A polarization analyzer 500 is connected to the test end of the fiber optic ring 400 to be tested.

As shown in FIG. 2 , the pulsed laser 110 according to Embodiment 1 of the present invention includes a narrowband continuous light laser 111 , a lithium niobate modulator 112 , a programmable pulse code generator 113 , a modulator driver 114 , a fiber amplifier 115 and an optical filter 116; the narrowband continuous light laser 111 is connected to the first input end of the lithium niobate modulator 112, the output end of the lithium niobate modulator 112 is connected to the input end of a fiber amplifier 115, and the output end of the fiber amplifier 115 The output end of the optical filter 116 is connected to the input end of the optical filter 116, and the output end of the optical filter 116 is connected to the polarization controller 120; the output end of the programmable pulse code generator 113 is connected to the input end of the modulator driver 114. terminal, the output terminal of the modulator driver 114 is connected to the second input terminal of the lithium niobate modulator 112 .

The switching between continuous light and pulsed light can be realized by the pulsed laser 110 as shown in FIG. 2 . When the continuous light is emitted, the polarization state of the continuous light is modulated by the polarization controller 120, and enters the optical fiber sensitive ring 400 to be tested through the second end of the optical fiber ring 130, and the continuous light enters the polarization analyzer 500 from the test end of the optical fiber sensitive ring 400 to be tested, Use the polarization analyzer 500 to observe the polarization state modulated by the polarization controller 120, and use the polarization analyzer 500 to observe the output polarization state of the fiber sensitive ring 400 to be tested, so that the input of the fiber sensitive ring 400 to be tested is modulated by the polarization controller 120 and injected into the input of the fiber sensitive ring 400 to be tested. The polarization state of the continuous light is consistent with the polarization eigenstate of the optical fiber sensitive ring 400 to be tested. The pulsed laser 110 is used to emit pulsed light, and the pulsed light signal is modulated by the adjusted polarization controller 120 as a test signal from the second end of the fiber ring 130 into the fiber optic ring 400 to be tested, and is Rayleigh scattering by the fiber optic ring 400 to be tested. Then, a reflected light signal is formed, and the reflected light signal enters the receiving module 200 from the third end of the optical fiber ring 130 to further complete the coupling characteristic evaluation.

As shown in FIG. 3 , the online pulse polarization state receiver 210 includes a polarization beam splitter 211, a coupler 212, a 90° optical mixer 213, and three balanced optical detectors 214, 215, and 216; The output end of the beamer 211 is connected with two of the couplers 212, the first output ends of the two couplers 212 are respectively connected to the two input ends of the 90° optical mixer 213, and the 90° optical The two output ends of the mixer 213 are respectively connected with a first balanced photodetector 214 and a second balanced photodetector 215, and the second output ends of the two couplers 212 are connected with a third balanced photodetector 216, The second output ends of the two couplers 212 are respectively connected to the two input ends of the third balanced optical detector 216 ; the input end of the polarization beam splitter 211 is connected to the third end of the fiber circulator 130 , the output end of the first balanced photodetector 214 , the output end of the second balanced photodetector 215 , and the output end of the third balanced photodetector 216 are all connected to the digital oscilloscope 220 .

Example 2

An optical fiber sensitive ring polarization mode coupling characteristic evaluation system according to Embodiment 2 of the present invention includes a sending module 100, a receiving module 200, and a computing module 300,

The sending module 100 includes a pulsed laser 110, a polarization controller 120, and a fiber circulator 130 connected in sequence. The output end of the pulsed laser 110 is connected to the input end of the polarization controller 120, and the output of the polarization controller 120 is connected. The end is connected to the first end of the optical fiber circulator 130, and the second end of the optical fiber circulator 130 is connected to the test start end of the optical fiber sensitive ring 400 to be tested;

The receiving module 200 includes an online pulse polarization state receiver 210 and a digital oscilloscope 220. The first input end of the online pulse polarization state receiver 210 is connected to the third end of the optical fiber circulator 130, and the online pulse polarization state The output end of the receiver 210 is connected to the input end of the digital oscilloscope 220, and the output end of the digital oscilloscope 220 is connected to the calculation module 300;

A polarization analyzer 500 is connected to the test end of the fiber optic ring 400 to be tested.

As shown in FIG. 2 , the pulsed laser 110 according to Embodiment 1 of the present invention includes a narrowband continuous light laser 111 , a lithium niobate modulator 112 , a programmable pulse code generator 113 , a modulator driver 114 , a fiber amplifier 115 and an optical filter 116; the narrowband continuous light laser 111 is connected to the first input end of the lithium niobate modulator 112, the output end of the lithium niobate modulator 112 is connected to the input end of a fiber amplifier 115, and the output end of the fiber amplifier 115 The output end of the optical filter 116 is connected to the input end of the optical filter 116, and the output end of the optical filter 116 is connected to the polarization controller 120; the output end of the programmable pulse code generator 113 is connected to the input end of the modulator driver 114. terminal, the output terminal of the modulator driver 114 is connected to the second input terminal of the lithium niobate modulator 112 .

The switching between continuous light and pulsed light can be realized by the pulsed laser 110 as shown in FIG. 2 . When the continuous light is emitted, the polarization state of the continuous light is modulated by the polarization controller 120, and enters the optical fiber sensitive ring 400 to be tested through the second end of the optical fiber ring 130, and the continuous light enters the polarization analyzer 500 from the test end of the optical fiber sensitive ring 400 to be tested, Use the polarization analyzer 500 to observe the polarization state modulated by the polarization controller 120, and use the polarization analyzer 500 to observe the output polarization state of the fiber sensitive ring 400 to be tested, so that the input of the fiber sensitive ring 400 to be tested is modulated by the polarization controller 120 and injected into the input of the fiber sensitive ring 400 to be tested. The polarization state of the continuous light is consistent with the polarization eigenstate of the optical fiber sensitive ring 400 to be tested. The pulsed laser 110 is used to emit pulsed light, and the pulsed light signal is modulated by the adjusted polarization controller 120 as a test signal from the second end of the fiber ring 130 into the fiber optic ring 400 to be tested, and is Rayleigh scattering by the fiber optic ring 400 to be tested. Then, a reflected light signal is formed, and the reflected light signal enters the receiving module 200 from the third end of the optical fiber ring 130 to further complete the coupling characteristic evaluation.

As shown in FIG. 4 , the online pulse polarization state receiver 210 according to Embodiment 2 of the present invention includes a coupler 211a, a 0° linear analyzer 212a, a 45° linear analyzer 213a, a right-handed circular analyzer 214a, Balanced photodetector 215a; the three output ends of the coupler 211a are respectively connected to the input ends of the 0° line analyzer 212a, the 45° line analyzer 213a and the right-hand circular analyzer 214a , the output ends of the 0° line analyzer 212a, the 45° line analyzer 213a and the right-hand circular analyzer 214a are respectively connected with a balanced light detector 215a, the three balanced light detectors The output ends of the coupler 215a are both connected to the digital oscilloscope 220 , and the input end of the coupler 211a is connected to the third end of the fiber optic circulator 130 .

As shown in FIG. 5 , the Rayleigh scattered light power corresponding to the three components S1 , S2 and S3 of the polarization state Stokes vector collected by the digital oscilloscope 220 is distributed along the longitudinal direction of the optical fiber sensitive ring. The complete polarization state information along the longitudinal distribution of the fiber sensitive ring can be obtained by algorithm calculation.

Figure 6 is the polarization mode coupling curve along the longitudinal distribution of the optical fiber sensitive ring calculated by the Matlab algorithm from the forward and reverse measurements of the optical fiber sensitive ring; it can be seen that the two curves are basically the same, and the Relatively large polarization mode coupling is detected at positions A, B, C, and D, while the polarization mode coupling at position E is small.

Figure 7 shows the distribution of polarization states at two different positions on the Bonga sphere obtained by repeated measurements, where (a) and (b) correspond to positions A and E of the fiber sensitive ring shown in Figure 6, respectively. It can be seen that the polarization mode coupling at position A is large, causing the polarization state to deviate from the polarization axis [1,0,0] distribution; while the polarization mode coupling at position B is small, and its polarization state is concentrated in the polarization axis [1,0,0] 0].

The principle of realizing the polarization mode coupling distributed measurement of the optical fiber sensitive ring in the present invention is as follows:

与当前的斯托克斯矢量垂直，从而有Because the Stokes vector S rotates on the Bonga sphere, its radius is always 1, so its rate of change

is perpendicular to the current Stokes vector, thus having

)是本地偏振主轴的方向，大小|B|是偏振态绕偏振主轴旋转的速率，即在该主轴下双折射的大小。当考虑偏振模耦合对偏振态传输的影响时，B应包含偏振模耦合系数，以下我们使用四元数的方法导出B与两个垂直偏振模的传输常数差Δβ＝β₊-β_{_}和它们的模耦合系数k之间的关系。in its direction (unit vector

) is the orientation of the local polarization principal axis, and the magnitude |B| is the rate at which the polarization state rotates around the polarization principal axis, that is, the magnitude of birefringence under this principal axis. When considering the effect of polarization mode coupling on the transmission of polarization state, B should contain the polarization mode coupling coefficient. Below we use the quaternion method to derive the transmission constant difference between B and the two vertically polarized modes Δβ = β ₊ -β _{_} and their The relationship between the mode coupling coefficient k of .

Coupling between two orthogonal polarization states (including two orthogonal linear polarization states in linear polarization maintaining fibers, and two orthogonal elliptical polarization states in elliptical polarization maintaining fibers), ignoring the loss of polarization maintaining fibers, theoretical can be written as

分别为两个正交本征态投影到本地邦加球上的电场分量，并且β₊,β_分别为两个正交本征态的传输常数，k为它们之间的耦合系数，z为沿着光纤纵向的长度。

are the electric field components of the two orthogonal eigenstates projected onto the local Bonga sphere, respectively, and β ₊ , β_ are the transfer constants of the two orthogonal eigenstates, k is the coupling coefficient between them, z is along the longitudinal length of the fiber.

When formula (2) is expressed in the form of Jones vector, it is

Formula (3) is expressed in the form of quaternion [32], as

(用英文花体Edwardian Script ITC表示，下同)是琼斯矢量对应的四元数，

是琼斯矩阵

对应的四元数，由于琼斯矩阵可以分解为where quaternion

(expressed in English swash Edwardian Script ITC, the same below) is the Jones vector The corresponding quaternion,

is the Jones matrix

The corresponding quaternion, since the Jones matrix can be decomposed into

传输常数差Δβ＝β₊-β_{_}，可得到琼斯矩阵对应的四元数为where the average transfer constant of the fast and slow axes

Transmission constant difference Δβ=β ₊ -β _{_} , the quaternion corresponding to Jones matrix can be obtained as

therefore

是

的厄米转置。由于斯托克斯四元数

因此in,

Yes

The Hermitian transpose of . Due to Stokes Quaternion

therefore

Substituting (7) into (8), we get

其中s₀是标量部分，S是矢量部分，代入(9)，可得Let Stokes Quaternion

where s ₀ is the scalar part and S is the vector part, and substituting in (9), we get

Comparing (11) and (1), we can get

两轴投影，就可以得到两个正交本征态的传输常数差Δβ和偏振模耦合系数k。Therefore, as long as the B pair is measured

Two-axis projection, the transmission constant difference Δβ and the polarization mode coupling coefficient k of the two orthogonal eigenstates can be obtained.

之间的旋转角度与在A点检测到的

之间的旋转角度相同，因此在邦加球上S_out(A)、S_out(B)、S_out(C)之间的变化关系与S_A(A)、S_A(B)、S_A(C)之间的变化关系是一致的。因此，在本系统中，通过计算光纤始端获得的A、B、C三个位置对应的斯托克斯矢量S_out(A)、S_out(B)、S_out(C)，根据(12)式计算得到的偏振模耦合系数k，便可描述A,B,C相邻三点之间的偏振模耦合。Based on the three-point quaternion method, the B pair can be calculated only by knowing the Stokes vectors of three adjacent points of a small section of fiber. Two-axis projection, that is, the transmission constant difference Δβ and the polarization mode coupling coefficient k of the small section of optical fiber are obtained. In addition, the polarization state quaternion of three adjacent points A, B, and C detected from the beginning of the fiber in this system

The rotation angle between the detected at point A and the

_{The rotation angles between the _} (C) The variation relationship between is consistent. Therefore, in this system, the Stokes vectors S _out (A), S _out (B), and S _out (C) corresponding to the three positions of A, B, and C obtained by calculating the beginning of the fiber, according to (12) The polarization mode coupling coefficient k calculated by the formula can describe the polarization mode coupling between the three adjacent points A, B, and C.

4000 repeated experiments show that the detection method has high repeatability, and the measurement spatial resolution reaches 1 meter. In addition, experiments show that the detection method can also be used to measure the polarization mode coupling of polarization-maintaining fibers caused by external factors (such as stress, bending, vibration, etc.), and realize distributed measurement of various parameters.

To sum up, the present invention can be used to measure the high-quality optical fiber sensitive ring polarization mode coupling, which makes up for the shortcomings of the existing white light interference technology in measuring the optical fiber sensitive ring polarization mode coupling, which is not high in accuracy and is not suitable for elliptically polarized state-preserving fibers. In addition, it can be used to measure polarization-maintaining fiber polarization mode coupling caused by external factors (such as stress, bending, vibration, etc.), and realize distributed measurement of various parameters.

Those of ordinary skill in the art can understand that: the components of the apparatus in the embodiment of the present invention may be distributed in the apparatus of the embodiment according to the description of the embodiment, and may also be located in one or more apparatuses different from this embodiment by making corresponding changes . The components of the above-mentioned embodiments may be combined into one component, or may be further divided into multiple sub-components.

The above description is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited to this. Substitutions should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims (8)

1. An optical fiber sensitive ring polarization mode coupling characteristic evaluation system, comprising a sending module (100), a receiving module (200), and a computing module (300), characterized in that, The sending module (100) includes a pulsed laser (110), a polarization controller (120), and an optical fiber circulator (130) connected in sequence, and an output end of the pulsed laser (110) is connected to the polarization controller (120) The input end of the polarization controller (120) is connected to the first end of the optical fiber circulator (130), and the second end of the optical fiber circulator (130) is connected to the fiber sensitive ring (400) to be tested The test start end of the fiber optic ring to be tested (400) is connected with a polarization analyzer (500); The pulsed laser (110) includes a narrowband continuous light laser (111), a lithium niobate modulator (112), a programmable pulse code generator (113), a modulator driver (114), a fiber amplifier (115) and an optical filter (116); the narrowband continuous light laser (111) is connected to the first input end of the lithium niobate modulator (112), and the output end of the lithium niobate modulator (112) is connected to the output end of the fiber amplifier (115). an input end, the output end of the optical fiber amplifier (115) is connected to the input end of the optical filter (116), and the output end of the optical filter (116) is connected to the polarization controller (120); The output terminal of the programming pulse code generator (113) is connected to the input terminal of the modulator driver (114), and the output terminal of the modulator driver (114) is connected to the second input of the lithium niobate modulator (112) end; The receiving module (200) includes an online pulse polarization state receiver (210) and a digital oscilloscope (220), and a first input end of the online pulse polarization state receiver (210) is connected to the optical fiber circulator (130). At the third end, the output end of the online pulse polarization state receiver (210) is connected to the input end of the digital oscilloscope (220), and the output end of the digital oscilloscope (220) is connected to the calculation module (300). 2. The optical fiber sensitive ring polarization mode coupling characteristic evaluation system according to claim 1, wherein the online pulse polarization state receiver (210) comprises a polarization beam splitter (211), a coupler (212), a 90 ° optical mixer (213), three balanced photodetectors (214, 215, 216); the output end of the polarization beam splitter (211) is connected with two of the couplers (212), the two The first output ends of the couplers (212) are respectively connected to the two input ends of the 90° optical mixer (213), and the two output ends of the 90° optical mixer (213) are respectively connected to the second A balanced photodetector (214) and a second balanced photodetector (215), the second output ends of the two couplers (212) are connected with a third balanced photodetector (216), the two coupled The second output end of the device (212) is respectively connected to the two input ends of the third balanced optical detector (216); the input end of the polarization beam splitter (211) is connected to the optical fiber circulator (130). The third end, the output end of the first balanced photodetector (214), the output end of the second balanced photodetector (215), and the output end of the third balanced photodetector (216) are all connected The digital oscilloscope (220). 3. The optical fiber sensitive ring polarization mode coupling characteristic evaluation system according to claim 2, wherein the online pulse polarization state receiver (210) comprises a coupler (211a), a 0° linear analyzer (212a) , a 45° line analyzer (213a), a right-handed circular analyzer (214a), and a balanced photodetector (215a); the three output ends of the coupler (211a) are respectively connected to the 0° line analyzer input ends of the 0° line analyzer (212a), the 45° line analyzer (213a) and the right-hand circular analyzer (214a), the 0° line analyzer (212a), the 45° line analyzer The output ends of the polarizer (213a) and the right-handed circular analyzer (214a) are respectively connected with a balanced photodetector (215a), and the output ends of the three balanced photodetectors (215a) are all connected to the A digital oscilloscope (220), the input end of the coupler (211a) is connected to the third end of the fiber optic circulator (130). 4. a method utilizing the system as claimed in any one of claims 1-3 to evaluate the optical fiber sensitive ring polarization mode coupling characteristic, is characterized in that, comprises the steps: Adjust the polarization state of the optical signal input to the optical fiber sensitive ring (400) to be tested to be consistent with the polarization eigenstate of the optical fiber sensitive ring (400) to be tested, and will be the same as the polarization state of the optical fiber sensitive ring (400) to be tested. The optical signal with the same eigenstate is input as the test optical signal; wherein, the pulsed laser (110) is adjusted to a continuous light output state, the polarization state is modulated by the polarization controller (120), and the polarization analyzer (500) is used to modulate the polarization state. Observing the output polarization state of the optical fiber sensitive ring (400) to be tested, so that the input polarization state of the optical fiber sensitive ring (400) to be tested and the polarization intrinsic of the optical fiber sensitive ring (400) to be tested are modulated and injected into the optical fiber sensitive ring (400) to be tested. Consistent; A pulsed laser (110) is used to emit pulsed light, and the pulsed light signal is modulated by the adjusted polarization controller (120) as a test signal from the second end of the optical fiber ring (130) into the fiber-optic sensitive ring (400) to be tested. Detecting optical fiber sensitive ring (400) to form reflected light signal after Rayleigh scattering; The complete polarization state Stokes vector S distributed along the longitudinal direction of the reflected optical signal is collected by the online pulse polarization state receiver (210) and the digital oscilloscope (220); The polarization mode coupling coefficient and extinction ratio distributed along the longitudinal direction of the optical fiber sensitive ring (400) to be tested are calculated according to the Stokes vector. 5. The method according to claim 4, wherein the complete polarization of the reflected optical signal along the longitudinal distribution is collected by the online pulse polarization state receiver (210) and the digital oscilloscope (220) The state Stokes vector includes: coupling the test optical signal through the optical fiber circulator (130) into the optical fiber sensitive ring (400) to be tested, and back-scattering to form a reflected optical signal, the The reflected optical signal returns to the second end of the optical fiber circulator (130) and enters the optical fiber circulator (130), and enters the online pulse polarization state receiver (210) through the third end of the optical fiber circulator (130). 6. The method according to claim 4, wherein calculating the polarization mode coupling coefficient and extinction ratio longitudinally distributed along the optical fiber sensitive ring (400) under test according to the Stokes vector comprises: According to the complete polarization state Stokes vector S, the rate of change of the Stokes vector S is calculated According to the formula

Calculate the birefringence vector

unit vector

represents the direction of the local polarization principal axis, and the magnitude |B| is the rate at which the polarization state rotates around the polarization principal axis, that is, the magnitude of birefringence under this principal axis; According to the formula

Calculate the mode coupling coefficient k. 7. The method according to claim 6, characterized in that, according to the Stokes vector relationship of three adjacent detection points on the optical fiber sensitive ring (400) to be tested, a method of three-point quaternion is used to calculate The birefringence vector B. 8. The method according to claim 7, characterized in that, according to the birefringence vector B on the Bonga sphere

The two axes are projected to calculate the mode coupling coefficient k.

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