CN112082651B - Polarization characteristic measurement method for assembling full polarization-maintaining Sagnac closed light path - Google Patents

Abstract

The invention provides a polarization characteristic measurement method for assembly of a full-polarization-maintaining Sagnac closed light path, which comprises the following steps: the Y waveguide and the polarization-maintaining optical fiber ring which are used for assembling the full-polarization-maintaining Sagnac closed optical path are connected into a non-closed optical path and a closed optical path in sequence, and are respectively connected into an optical coherence domain polarization measuring instrument for measurement, and by combining a second-order polarization crosstalk effect, all polarization characteristic information such as first-order polarization crosstalk of all connection points in the closed optical path, distributed polarization crosstalk of the full length of the polarization-maintaining optical fiber ring, extinction ratio of a Y waveguide chip and the like can be obtained. The method realizes the measurement of the polarization characteristic in the assembly process of the full polarization-preserving Sagnac closed optical path, can be widely used for monitoring and evaluating the distributed polarization crosstalk of all optical devices and connecting points in the closed optical path, and has important significance for the development of high-performance interference optical sensors.

Description

A polarization property measurement method for fully polarization-maintaining Sagnac closed optical path assembly

Technical Field

The invention relates to a polarization characteristic measurement method for full polarization-maintaining Sagnac closed optical path assembly, belonging to the technical field of measurement of optical devices and optical path components.

Background Art

The fiber-optic Sagnac interferometer was proposed and demonstrated more than 40 years ago. At first, it was just a closed optical path formed by connecting an ordinary coupler and an optical fiber, and was used to measure the rotational angular velocity in the relative inertial space, which was the optical path structure used by the original fiber-optic gyroscope. After years of research and development, the Sagnac closed optical path structure currently generally adopts a full polarization-maintaining solution formed by connecting a Y-waveguide and a polarization-maintaining fiber ring. The Y-waveguide is a highly integrated optical chip with optical beam splitting, beam combining, polarization and modulation functions, while the polarization-maintaining fiber ring is made of hundreds or even thousands of meters of polarization-maintaining optical fiber wound according to a certain process, which is used to sense changes in the external environment. The proposal of the full polarization-maintaining Sagnac closed optical path has made its application field not limited to fiber-optic gyroscopes, but has also been widely used in the fields of fiber-optic hydrophones, fiber-optic current transformers and detectors. The advantage of this solution is that it can suppress the polarization fading effect and further improve the measurement sensitivity of various interferometric optical sensors. The problem of this solution is usually manifested in that the polarization characteristic defects of optical devices and connection points in the closed optical path will introduce polarization phase errors, which directly affect the measurement accuracy of the sensor. Therefore, during the assembly of the fully polarization-maintaining Sagnac closed optical path, it is very necessary to fully monitor its polarization characteristics in order to ensure that the polarization characteristics of the optical devices and connection points in the closed optical path meet the requirements of high-performance optical sensors.

Fiber optic gyroscope is the most mature type of optical sensor in Sagnac interferometer application. Taking the assembly of fiber optic gyroscope as an example, the core components such as Y waveguide and polarization-maintaining fiber ring are usually tested and screened separately, and the testing method is very mature. For example: in 2008, Yao Xiaotian and others of Suzhou Guanghuan Technology Co., Ltd. disclosed a method and device for measuring the quality of fiber ring for fiber optic gyroscope (Chinese patent application number: CN 200810119075.6), and evaluated the quality of fiber sensitive ring from the perspective of temperature conduction characteristics; in 2013, the inventor of Harbin Engineering University disclosed a dual-channel optical performance test device of integrated waveguide modulator and its polarization crosstalk identification and processing method (Chinese patent application number: CN201310744466.8), and used white light interferometer to realize the test of Y waveguide dual-channel distributed polarization crosstalk. The qualified devices screened out are directly placed in the fiber optic gyroscope system for testing after fusion, and the test performance of the fiber optic gyroscope is evaluated. For example: In 2017, Yao Xiaotian of Suzhou Guanghuan Technology Co., Ltd. disclosed a test method, device, storage medium and computer equipment for fiber optic gyroscopes (Chinese patent application number: CN201710867702.3), bringing the polarization characteristic test results of the core device into the calculation model to obtain the phase error caused by polarization crosstalk in the fiber optic gyroscope system, as well as gyroscope quality parameters such as zero bias stability. This method goes directly from single device testing to system whole machine testing, ignoring the polarization characteristics of the connection points between devices. In order to fully obtain the connection status of the optical path, it is necessary to add an optical path level test between the device level test and the system level test. For the connection status test of sensitive closed optical paths, it is mainly divided into connection loss test and polarization characteristic test. Among them, in 2018, Xu Baoxiang and others of Beijing Aerospace Times Optoelectronics Technology Co., Ltd. disclosed an optical path performance test system for fiber optic gyroscopes (Chinese patent application number: CN201810996752.6), which can realize the test of the connection loss of optical devices in closed optical paths and the total loss of optical paths. However, there is still a lack of effective methods for testing the polarization characteristics of closed optical paths.

With the rapid development of high-precision fiber optic gyroscopes, many researchers have proposed a solution for direct coupling of Y-waveguides and polarization-maintaining fiber rings, with the aim of reducing the number of connection points in a closed optical path and reducing the gyro phase error introduced by polarization crosstalk. For example: in 2013, Liu Yin and others from Beijing Shiweitong Technology Co., Ltd. disclosed a fiber optic gyroscope without fiber fusion points and a method for making the fiber optic gyroscope (Chinese patent application number: CN201310410382.0); in the same year, Huo Guang and others from the 618th Institute of China Aviation Industry disclosed a method for making a high-precision fiber optic gyroscope without fusion (Chinese patent application number: CN201310676683.8), which mentioned the holding device and bonding method for direct coupling of Y-waveguides and polarization-maintaining fiber rings, and could only be adjusted and aligned and fixed by glue by imaging positioning calibration. It is not possible to intuitively monitor the alignment through the polarization crosstalk information of the connection point, which is prone to large alignment deviations. Later in 2016, Yuan Yonggui and others from Harbin Engineering University disclosed a method for assembling the core sensitive optical path of a fiber optic gyroscope (Chinese patent application number: CN201610265230.X). This method monitors the axis alignment of the connection point by measuring polarization crosstalk, but this method can only measure the polarization crosstalk of the connection point in a non-closed state, and cannot be measured when the optical path is in a closed state, and this method is only applicable to the assembly of a fiber optic gyroscope. In summary, there is still a lack of an effective method for measuring the polarization characteristics of all optical devices and connection points in a fully polarization-maintaining Sagnac closed optical path.

In view of the above problems, the present invention proposes a polarization characteristic measurement method for full polarization-maintaining Sagnac closed optical path assembly, which is characterized by: connecting the Y waveguide and the polarization-maintaining fiber ring used for full polarization-maintaining Sagnac closed optical path assembly into a non-closed optical path and a closed optical path in turn, and respectively connecting them to an optical coherence domain polarization measuring instrument for measurement, and combining the second-order polarization crosstalk effect, all polarization characteristic information such as the first-order polarization crosstalk of all connection points in the closed optical path, the distributed polarization crosstalk of the full length of the polarization-maintaining fiber ring, and the extinction ratio of the Y waveguide chip can be obtained. The method realizes the measurement of polarization characteristics during the full polarization-maintaining Sagnac closed optical path assembly process, can be widely used for the monitoring and evaluation of the distributed polarization crosstalk of all optical devices and connection points in the closed optical path, and is of great significance for the development of high-performance interferometric optical sensors.

Summary of the invention

The purpose of the present invention is to provide a polarization characteristic measurement method for a fully polarization-maintaining Sagnac closed optical path assembly, which is used for monitoring and evaluating the distributed polarization crosstalk of all optical devices and connection points in the closed optical path, and can comprehensively improve the polarization performance of the closed optical path.

The object of the present invention is achieved in this way: the steps are as follows:

(1) Select a Y-waveguide 206 for full polarization-maintaining Sagnac closed optical path assembly, measure the lengths of the Y-waveguide input polarization-maintaining pigtail 204, the Y-waveguide first output polarization-maintaining pigtail 208, and the Y-waveguide second output polarization-maintaining pigtail 217, which are l ₁ , l ₂ , and l ₃ , respectively, and require that |l ₃ -l ₂ |＞10 cm, form a connection point B205 between the Y-waveguide 206 and the Y-waveguide input polarization-maintaining pigtail 204, form a connection point C207 between the Y-waveguide 206 and the Y-waveguide first output polarization-maintaining pigtail 208, and form a connection point F216 between the Y-waveguide 206 and the Y-waveguide second output polarization-maintaining pigtail 217;

(2) Selecting a polarization-maintaining fiber ring 211 for full polarization-maintaining Sagnac closed optical path assembly, performing 0° axial welding between the first port 210 of the polarization-maintaining fiber ring and the first output polarization-maintaining pigtail 208 of the Y-waveguide, to form a connection point D209;

(3) Select a 45° polarizer 201 and measure the length of the 45° polarizer polarization-maintaining pigtail 202 as l _p ; select a 45° analyzer 215 and measure the length of the 45° analyzer polarization-maintaining pigtail 214 as l _a ;

(4) The 45° polarizer polarization-maintaining pigtail 202 and the Y-waveguide input polarization-maintaining pigtail 204 are axially welded at 0° to form a connection point A203, and the 45° polarizer single-mode pigtail 218 is connected to the SLD broadband light source 220;

(5) performing 0° axial welding of the 45° analyzer polarization-maintaining pigtail (214) and the second port (212) of the polarization-maintaining optical fiber ring to form a connection point E (213), and connecting the 45° analyzer single-mode pigtail (219) to an optical coherence domain polarization meter (221);

(6) Distributed polarization crosstalk measurement is performed on the non-closed optical path. The first-order polarization crosstalk of the connection points A, B, C, D, and E, the distributed polarization crosstalk of the full length of the polarization-maintaining fiber ring, and the extinction ratio measurement information of the Y waveguide chip are extracted from the measurement spectrum at one time. The position of each interference signal peak can be calculated based on the length of each section of the polarization-maintaining pigtail. Assuming that the birefringence of the polarization-maintaining fiber in the optical path is Δn _f , the birefringence of the Y waveguide chip is Δn _Y , the length of the Y waveguide 206 is l _Y , and the length of the polarization-maintaining fiber ring 211 is l _f , then the positions of the interference signal peaks representing the first-order polarization crosstalk of the connection points A, B, _{C, D, and E are: SA = Δn f ·l p , SB = Δn f · (} l _p +l ₁ ), _SC = Δn _f ·(l ₂ +l _f +l _a ), SD = Δn _f ·(l _f +l _a ), _SE = Δn _f ·l _a , the first-order polarization crosstalk intensities are CT _A , CT _B , CT _C , CT _D , and CT _E respectively, the interference signal peak position representing the extinction ratio of the Y-waveguide chip is S _Y =Δn _f ·(l _p +l ₁ +l ₂ +l _f +l _a )+Δn _Y ·l _Y , the extinction ratio intensity is CT _Y , the distributed polarization crosstalk information of the full length of the polarization-maintaining fiber ring 211 appears between the first-order polarization crosstalk interference signal peaks between the connection point D209 and the connection point E213, and the lumped extinction ratio of the polarization-maintaining fiber ring 211 is calculated as CT _coil through the distributed polarization crosstalk information of the full length;

(7) Determine whether CT _B and CT _C are better than -40 dB, and whether CT _Y is better than 50 dB. If not, return to step (1) and replace the Y waveguide 206. If satisfied, proceed to the next step.

(8) Determine whether the distributed polarization crosstalk of the entire length of the polarization-maintaining fiber ring 211 is better than -50 dB. If not, return to step (2) and replace the polarization-maintaining fiber ring 211. If satisfied, proceed to the next step.

(9) Determine whether the CT _D is better than -40 dB. If not, return to step (2) and re-weld the connection point D209. If satisfied, proceed to the next step.

(10) disconnect the connection point E213, remove the 45° analyzer 215, and perform 0° axial welding between the second port 212 of the polarization-maintaining fiber ring and the second output polarization-maintaining pigtail 217 of the Y-waveguide to form a connection point G301;

(11) Select a 1×2 single-mode coupler 302, connect the first port 303 of the 1×2 single-mode coupler to the 45° polarizer single-mode pigtail 218, connect the second port 304 of the 1×2 single-mode coupler to the SLD broadband light source 220, and connect the third port 305 of the 1×2 single-mode coupler to the optical coherence domain polarization meter 221;

(12) Distributed polarization crosstalk measurement is performed on the closed optical path. The second-order polarization crosstalk measurement information between the connection point D209 and the connection point F216 and the second-order polarization crosstalk measurement information between the connection point D209 and the connection point G301 are extracted from the measurement spectrum. The positions where the interference signal peaks appear are S _DF = Δn _f ·(l _f +l ₃ ) and S _DG = Δn _f ·l _f , respectively. The second-order polarization crosstalk intensities are CT _DF and CT _DG , respectively. The first-order polarization crosstalk intensity of the connection point F216 is inferred to be CT _F = CT _DF - CT _D , and the first-order polarization crosstalk intensity of the connection point G301 is CT _G = CT _DG - CT _D ;

(13) Determine whether the _CTF is better than -40 dB. If not, return to step (1) and replace the Y waveguide 206. If satisfied, proceed to the next step.

(14) Determine whether CT _G is better than -40dB. If not, return to step (10) and re-weld the connection point G301. If satisfied, the measurement ends.

The present invention also includes such structural features:

1. Step (2) requires that l _a -l _p -l ₁ > 10 cm.

2. The 45° polarizer 201 and the 45° analyzer 215 used in the optical path are both dual-port optical devices, one end of which is a polarization-maintaining fiber pigtail and the other end is a single-mode fiber pigtail. The Y waveguide 206 is a three-port optical device, and its input and output pigtails are both polarization-maintaining fibers. All polarization-maintaining fibers in the optical path are ordinary panda-type polarization-maintaining fibers.

3. The 1×2 single-mode coupler 302 in step (9) can also be replaced by a three-port single-mode fiber circulator or a 2×2 single-mode coupler.

4. Step (10) can also extract the second-order polarization crosstalk measurement information between the connection point C207 and the connection point F216, and the second-order polarization crosstalk measurement information between the connection point C207 and the connection point G301 from the measurement spectrum. The positions where the interference signal peaks appear are S _CF = Δn _f ·(l ₂ +l _f +l ₃ ) and S _CG = Δn _f ·(l ₂ +l _f ), respectively. The second-order polarization crosstalk intensities are CT _CF and CT _CG , respectively. Therefore, it is inferred that the first-order polarization crosstalk intensity of the connection point F216 is CT _F = CT _CF - CT _C , and the first-order polarization crosstalk intensity of the connection point G301 is CT _G = CT _CG - CT _C.

Compared with the prior art, the present invention has the following beneficial effects: the present invention provides a polarization characteristic measurement method for fully polarization-maintaining Sagnac closed optical path assembly, which can realize the monitoring and evaluation of distributed polarization crosstalk of all optical devices and connection points in the closed optical path. Compared with the prior art, the present invention has the following advantages:

(1) By measuring the open optical path and the closed optical path once, the distributed polarization crosstalk information of all optical devices and connection points can be obtained, eliminating the need for separate testing and screening of optical devices. The test method is simple and the test efficiency is high.

(2) When the optical path is in a closed state, the first-order polarization crosstalk of all connection points can be calculated by measuring the second-order polarization crosstalk between the connection points inside the closed optical path and combining it with the test results of the non-closed optical path, which effectively solves the problem that the closed optical path cannot be measured;

(3) This method is also applicable to the polarization crosstalk measurement of the connection point when the Y-waveguide and the polarization-maintaining fiber ring are directly coupled. Compared with the traditional imaging positioning method, this method can obtain the polarization axis alignment information at the connection point more intuitively and with higher accuracy, providing an effective monitoring method for direct coupling technology.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a flow chart of polarization characteristic measurement for a fully polarization-maintaining Sagnac closed optical path assembly;

FIG2 is a diagram of a distributed polarization crosstalk measurement device for a non-closed optical path;

FIG3 is a transmission path diagram of an optical signal in a non-closed optical path;

FIG4 is a diagram of a distributed polarization crosstalk measurement device for a closed optical path;

FIG. 5 is a diagram showing the transmission path of an optical signal in a closed optical path.

DETAILED DESCRIPTION

The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

The present invention proposes a method for measuring polarization characteristics of a fully polarization-maintaining Sagnac closed optical path assembly, the steps of which are as follows:

1) Selecting a Y waveguide (206) for full polarization-maintaining Sagnac closed optical path assembly, measuring the lengths of the Y waveguide input polarization-maintaining pigtail (204), the Y waveguide first output polarization-maintaining pigtail (208), and the Y waveguide second output polarization-maintaining pigtail (217), which are l ₁ , l ₂ , and l ₃ , respectively, and requiring that |l ₃ -l ₂ |>10 cm, forming a connection point B (205) between the Y waveguide (206) and the Y waveguide input polarization-maintaining pigtail (204), forming a connection point C (207) between the Y waveguide (206) and the Y waveguide first output polarization-maintaining pigtail (208), and forming a connection point F (216) between the Y waveguide (206) and the Y waveguide second output polarization-maintaining pigtail (217);

2) Selecting a polarization-maintaining fiber ring (211) for assembling a fully polarization-maintaining Sagnac closed optical path, performing 0° axial welding on the first port (210) of the polarization-maintaining fiber ring and the first output polarization-maintaining pigtail (208) of the Y-waveguide to form a connection point D (209). This optical path is the first step in the assembly process of the fully polarization-maintaining Sagnac closed optical path and is referred to as a non-closed optical path;

3) Select a 45° polarizer (201) and measure the length of the 45° polarizer polarization-maintaining pigtail (202) as l _p . Select a 45° analyzer (215) and measure the length of the 45° analyzer polarization-maintaining pigtail (214) as l _a , and require that l _a -l _p -l ₁ >10 cm;

4) performing 0° axial welding of the 45° polarizer polarization-maintaining pigtail (202) and the Y-waveguide input polarization-maintaining pigtail (204) to form a connection point A (203), and connecting the 45° polarizer single-mode pigtail (218) to the SLD wide-spectrum light source (220);

5) performing 0° axial welding of the 45° analyzer polarization-maintaining pigtail (214) and the second port (212) of the polarization-maintaining optical fiber ring to form a connection point E (213), and connecting the 45° analyzer single-mode pigtail (219) to an optical coherence domain polarization measuring instrument (221);

6) Distributed polarization crosstalk is measured on the non-closed optical path, and the first-order polarization crosstalk of the connection points A, B, C, D, and E, the distributed polarization crosstalk of the full length of the polarization-maintaining fiber ring, and the extinction ratio measurement information of the Y waveguide chip are extracted from the measurement spectrum at one time. The position where each interference signal peak appears can be calculated based on the length of each section of the polarization-maintaining pigtail, assuming that the birefringence of the polarization-maintaining fiber in the optical path is Δn _f , the birefringence of the Y waveguide chip is Δn _Y , the length of the Y waveguide (206) is l _Y , and the length of the polarization-maintaining fiber ring (211) is l _f . Then the peak positions of the interference signals representing the first-order polarization crosstalk of the connection points A, B, _C , D, and E are: SA = _Δnf · _lp , _SB = _Δnf ·( _lp + _l1 ), _SC = _Δnf ·( _l2 + _lf +l _a ), SD = _Δnf ·( _lf +l _a ), _SE = _Δnf ·l _a , and the first-order polarization crosstalk intensities are CT _A , CT _B , CT _C , CT _D , and CT _E. The peak position of the interference signal representing the extinction ratio of the Y-waveguide chip is _SY = _Δnf ·( _lp + _l1 + _l2 + _lf +l _a )+ _ΔnY · _lY , and the extinction ratio intensity is CT _Y. The distributed polarization crosstalk information of the entire length of the polarization-maintaining fiber ring (211) appears between the first-order polarization crosstalk interference signal peaks at the connection point D (209) and the connection point E (213), and the lumped extinction ratio of the polarization-maintaining fiber ring (211) is calculated as CT _coil according to the distributed polarization crosstalk information of the entire length;

7) Determine whether CT _B and CT _C are better than -40dB, and whether CT _Y is better than 50dB. If not, return to step 1) and replace the Y waveguide (206). If satisfied, proceed to the next step.

8) judging whether the distributed polarization crosstalk of the entire length of the polarization-maintaining optical fiber ring (211) is better than -50 dB; if not, returning to step 2) and replacing the polarization-maintaining optical fiber ring (211); if satisfied, proceeding to the next step;

9) Determine whether the CT _D is better than -40dB. If not, return to step 2) and re-weld the connection point D (209). If satisfied, proceed to the next step;

10) disconnect the connection point E (213), remove the 45° analyzer (215), perform 0° axial welding on the second port (212) of the polarization-maintaining fiber ring and the second output polarization-maintaining pigtail (217) of the Y-waveguide to form a connection point G (301). This optical path is the second step in the assembly process of the full polarization-maintaining Sagnac closed optical path, and is called a closed optical path;

11) selecting a 1×2 single-mode coupler (302), connecting the first port (303) of the 1×2 single-mode coupler to a 45° polarizer single-mode pigtail (218), connecting the second port (304) of the 1×2 single-mode coupler to an SLD broadband light source (220), and connecting the third port (305) of the 1×2 single-mode coupler to an optical coherence domain polarization meter (221);

12) Perform distributed polarization crosstalk measurement on the closed optical path, and extract the second-order polarization crosstalk measurement information between the connection point D (209) and the connection point F (216), and the second-order polarization crosstalk measurement information between the connection point D (209) and the connection point G (301) from the measurement spectrum, respectively. The positions where the interference signal peaks appear are S _DF = Δn _f · (l _f + l ₃ ) and S _DG = Δn _f ·l _f , respectively, and the second-order polarization crosstalk intensities are CT _DF and CT _DG, respectively. Therefore, it is inferred that the first-order polarization crosstalk intensity of the connection point F (216) is CT _F = CT _DF - CT _D , and the first-order polarization crosstalk intensity of the connection point G (301) is CT _G = CT _DG - CT _D ;

13) Determine whether the _CTF is better than -40dB. If not, return to step 1) and replace the Y waveguide (206). If satisfied, proceed to the next step.

14) Determine whether CT _G is better than -40dB. If not, return to step 10) and re-weld the connection point G (301). If satisfied, the measurement ends.

The polarization characteristic measurement method for a fully polarization-maintaining Sagnac closed optical path assembly is characterized in that: the 45° polarizer (201) and the 45° analyzer (215) used in the optical path are both dual-port optical devices, one end of which is a polarization-maintaining pigtail fiber, and the other end is a single-mode pigtail fiber. The Y waveguide (206) is a three-port optical device, and its input and output pigtail fibers are both polarization-maintaining optical fibers. All polarization-maintaining optical fibers in the optical path are ordinary panda-type polarization-maintaining optical fibers.

The polarization characteristic measurement method for fully polarization-maintaining Sagnac closed optical path assembly is characterized in that the 1×2 single-mode coupler (302) in step 9) can also be replaced by a three-port single-mode fiber circulator or a 2×2 single-mode coupler.

The polarization characteristic measurement method for fully polarization-maintaining Sagnac closed optical path assembly is characterized in that: step 10) can also extract the second-order polarization crosstalk measurement information between the connection point C (207) and the connection point F (216), and the second-order polarization crosstalk measurement information between the connection point C (207) and the connection point G (301) from the measurement spectrum, and the positions where the interference signal peaks appear are S _CF = Δn _f · (l ₂ +l _f +l ₃ ) and S _CG = Δn _f · (l ₂ +l _f ), and the second-order polarization crosstalk intensities are CT _CF and CT _CG , respectively. Therefore, the first-order polarization crosstalk intensity of the connection point F (216) is calculated to be CT _F = CT _CF - CT _C , and the first-order polarization crosstalk intensity of the connection point G (301) is calculated to be CT _G = CT _CG - CT _C.

The distributed polarization crosstalk measurement device of the non-closed optical path is shown in Figure 2, and the part in the dotted frame is the non-closed optical path structure. When measuring the non-closed optical path, it is necessary to use an additional 45° polarizer (201) and a 45° analyzer (215), wherein the function of the 45° polarizer (201) is to realize the transmission of the optical signal in the orthogonal polarization axes of the 45° polarizer polarization-maintaining pigtail (202) in an equal-energy linear polarization state, and the 45° analyzer (215) maps the optical signal transmitted in the orthogonal polarization axes of the 45° analyzer polarization-maintaining pigtail (214) to the same polarization direction, so as to facilitate interference between the signals. The SLD wide-spectrum light source (220) usually uses a Gaussian low-polarization light source, and the central wavelength must be consistent with the working wavelength of the optical device in the closed optical path. The spectrum width is usually greater than 40nm, which is conducive to improving the spatial resolution of the distributed measurement. The optical coherence domain polarimeter (221) is an instrument developed by Harbin Engineering University. It integrates a scanning Michelson interferometer and has an ultra-large spatial optical path scanning range of 6.4m. It can measure polarization-maintaining optical fibers over 5km long. The measurement spatial resolution is better than 10cm, and the polarization crosstalk measurement sensitivity can reach up to -100dB, while the measurement dynamic range is maintained at 100dB.

As shown in FIG3 , all optical signal transmission paths when the first-order polarization crosstalk occurs in a non-closed optical path are enumerated. In order to simplify the analysis, it is assumed that the polarization angles of the polarizer and analyzer used in the test are both ideally 45°, and the splitting ratio of the Y waveguide (206) is 50:50. Taking the first-order polarization crosstalk at the connection point D (209) as an example, assuming that its polarization coupling coefficient is ρ _D , and the optical field amplitude of the optical signal input from the 45° polarizer (201) is E _in , then the optical field amplitude E _ref of the reference optical signal when path 1 is output from the 45° analyzer (215) is expressed as:

The optical field amplitude E _cou-D of the first-order polarization crosstalk optical signal of path 5 when output from the 45° analyzer (215) is expressed as:

表示路径5与路径1之间的相位差，且有：in,

represents the phase difference between path 5 and path 1, and:

Wherein, _SD represents the optical path difference between path 5 and path 1, that is, the position where the interference signal peak of the first-order polarization crosstalk at the connection point D (209) appears in the interference spectrum.

Assuming that the coupler splitting ratio used in the optical coherence domain polarimeter (221) is 50:50 and the responsivity of the photodetector is R, then the photocurrent intensity I _cou-D of the interference between path 5 and path 1 is expressed as:

The photocurrent intensity I _ref of the self-interference of path 1 is expressed as:

Therefore, the normalized logarithm calculation formula representing the first-order polarization crosstalk intensity _CTD at the connection point D (209) is as follows, where the DC term and phase term of the interference signal are ignored:

Similarly, the first-order polarization crosstalk intensities CT _A , CT _B , CT _C , CT _E of connection point A (203), connection point B (205), connection point C (207), and connection point E (213), as well as the chip extinction ratio CT _Y of the Y waveguide (206) can also be obtained according to the above analysis and calculation process. In fact, there are countless energy coupling defect points in the polarization-maintaining fiber ring (211). When the optical signal with energy coupling at any point interferes with the reference optical signal, the first-order polarization crosstalk intensity of the point can be measured, so the above calculation formula is also applicable to it.

The diagram of the distributed polarization crosstalk measurement device for a closed optical path is shown in Figure 4, and the portion in the dotted box is the closed optical path structure. When measuring a closed optical path, it is not necessary to use a 45° analyzer (215), but it is necessary to add a 1×2 single-mode coupler (302). The function of the 1×2 single-mode coupler (302) is, on the one hand, to inject the optical signal of the SLD wide-spectrum light source (220) into the closed optical path, and on the other hand, to inject the polarization crosstalk signal generated in the closed optical path into the optical coherence domain polarimeter (221). In the closed optical path, the optical signal needs to pass through the Y waveguide (206) twice, and because the Y waveguide only allows fast-axis light to pass (slow-axis extinction), only second-order polarization crosstalk signals exist in the closed optical path (higher-order polarization crosstalk signals are extremely weak and can be ignored).

As shown in FIG5, all optical signal transmission paths when the second-order polarization crosstalk occurs in the closed optical path are enumerated. Here, only the path of the optical signal when it is transmitted clockwise after being split by the Y waveguide (206) is given, and the path of the optical signal when it is transmitted counterclockwise is similar. Taking the second-order polarization crosstalk between the connection point D (209) and the connection point G (301) as an example, assuming that the optical field amplitude of the optical signal input from the 45° polarizer (201) is _Ein , then the optical field amplitude E′ _ref of the reference optical signal when the path 8 is output from the 45° polarizer (201) again is expressed as:

Assuming that the polarization coupling coefficient of the connection point G (301) is ρ _G , the optical field amplitude E′ _cou-DG of the second-order polarization crosstalk optical signal when the path 9 is output from the 45° polarizer (201) again is expressed as:

表示路径9与路径8之间的相位差，且有：in,

represents the phase difference between path 9 and path 8, and:

The photocurrent intensity I′ _cou-DG of the interference between path 9 and path 8 and the photocurrent intensity I′ _ref of the self-interference of path 8 are respectively expressed as:

Therefore, the normalized logarithm calculation formula representing the second-order polarization crosstalk intensity CT _DG between the connection point D (209) and the connection point G (301) is as follows, where the DC term and the phase term of the interference signal are ignored:

At this time, the first-order polarization crosstalk intensity CT _G at the connection point G (301) can be calculated by the following formula:

Similarly, following the same calculation steps, assuming that the polarization coupling coefficient of the connection point F (216) is ρ _F , the second-order polarization crosstalk intensity between the connection point D (209) and the connection point F (216) can be measured by interference between the path 10 and the path 8: CT _DF = 10·lg(ρ _D ·ρ _F ) ² , and the optical path difference is: S _DF = Δn _f ·(l _f +l ₃ ). At this time, the first-order polarization crosstalk intensity CT _F of the connection point F (216) can be calculated by the following formula:

As shown in FIG5 , the second-order polarization crosstalk intensity between the connection point C (207) and the connection point G (301) can be measured by interfering the path 11 with the path 8, and the second-order polarization crosstalk intensity between the connection point C (207) and the connection point F (216) can be measured by interfering the path 12 with the path 8. At this time, CT _G and CT _F can also be calculated respectively, as follows:

CT _G = CT _CG - CT _C

CT _F = CT _CF - CT _C

Based on the above analysis and calculation, by measuring the distributed polarization crosstalk of a non-closed optical path and a closed optical path respectively, all polarization characteristic information such as the first-order polarization crosstalk of all connection points in the optical path, the distributed polarization crosstalk of the full length of the polarization-maintaining optical fiber ring (211), and the chip extinction ratio of the Y waveguide (206) can be obtained.

In order to clearly illustrate the polarization characteristic measurement method for a fully polarization-maintaining Sagnac closed optical path assembly proposed in the present invention, the present invention is further described in conjunction with the embodiments and drawings, but the protection scope of the present invention should not be limited thereto. The measurement is performed in conjunction with the parameters and according to the polarization characteristic measurement flow chart for a fully polarization-maintaining Sagnac closed optical path assembly shown in FIG1:

1) According to step 101, a Y waveguide (206) for full polarization-maintaining Sagnac closed optical path assembly is selected, and the lengths of the Y waveguide input polarization-maintaining pigtail (204), the Y waveguide first output polarization-maintaining pigtail (208), and the Y waveguide second output polarization-maintaining pigtail (217) are measured to be l ₁ , l ₂ , and l ₃ , respectively, and it is required that |l ₃ -l ₂ |>10 cm, a connection point B (205) is formed between the Y waveguide (206) and the Y waveguide input polarization-maintaining pigtail (204), a connection point C (207) is formed between the Y waveguide (206) and the Y waveguide first output polarization-maintaining pigtail (208), and a connection point F (216) is formed between the Y waveguide (206) and the Y waveguide second output polarization-maintaining pigtail (217); the operating wavelength of the Y waveguide (206) is 1550 nm, and the fast axis transmits light;

2) According to step 102, a polarization-maintaining fiber ring (211) for full polarization-maintaining Sagnac closed optical path assembly is selected, and the first port (210) of the polarization-maintaining fiber ring is 0° axially welded with the first output polarization-maintaining pigtail (208) of the Y-waveguide to form a connection point D (209). This optical path is the first step in the full polarization-maintaining Sagnac closed optical path assembly process and is referred to as a non-closed optical path. The operating wavelength of the polarization-maintaining fiber ring (211) is 1550 nm and the length is less than 2 km.

3) According to step 103, a 45° polarizer (201) is selected, and the length of the 45° polarizer polarization-maintaining pigtail (202) is measured as l _p . A 45° analyzer (215) is selected, and the length of the 45° analyzer polarization-maintaining pigtail (214) is measured as l _a , and it is required that l _a -l _p -l ₁ >10 cm; the operating wavelength of the 45° polarizer (201) and the 45° analyzer (215) is 1550 nm, the polarization extinction ratio is less than 0.2 dB, and the insertion loss is less than 1 dB;

4) According to step 104, the 45° polarizer polarization-maintaining pigtail (202) and the Y-waveguide input polarization-maintaining pigtail (204) are 0° axially welded to form a connection point A (203), and the 45° polarizer single-mode pigtail (218) is connected to the SLD broadband light source (220); the central wavelength of the SLD broadband light source (220) is 1550 nm, the half-spectrum width is greater than 40 nm, the fiber output power is greater than 6 mW, and the polarization extinction ratio is less than 0.2 dB;

5) According to step 105, the 45° analyzer polarization-maintaining pigtail (214) and the second port (212) of the polarization-maintaining optical fiber ring are 0° axially welded to form a connection point E (213), and the 45° analyzer single-mode pigtail (219) is connected to an optical coherence domain polarization meter (221); the optical coherence domain polarization meter (221) has a scanning Michelson interferometer integrated therein, and has an ultra-large spatial optical path scanning range of 6.4 m. The polarization crosstalk measurement sensitivity can reach -100 dB, and the measurement dynamic range is maintained at 100 dB;

6) According to step 106, the non-closed optical path is measured for distributed polarization crosstalk, and the first-order polarization crosstalk of the connection points A, B, C, D, and E, the distributed polarization crosstalk of the full length of the polarization-maintaining fiber ring, and the extinction ratio measurement information of the Y waveguide chip are extracted from the measurement spectrum at one time. The position where each interference signal peak appears can be calculated based on the length of each section of the polarization-maintaining pigtail, assuming that the birefringence of the polarization-maintaining fiber in the optical path is Δn _f , the birefringence of the Y waveguide chip is Δn _Y , the length of the Y waveguide (206) is l _Y , and the length of the polarization-maintaining fiber ring (211) is l _f . Then the peak positions of the interference signals representing the first-order polarization crosstalk of the connection points A, B, C, _D , and E are respectively: _SA = _Δnf · _lp , _SB = _Δnf ·( _lp + _{l1 )} , _SC = _Δnf ·( _l2 + _lf +l _a ), SD = Δnf·( _lf +l _a ), _SE = _Δnf ·l _a , and the first-order polarization crosstalk intensities are CT _A , CT _B , CT _C , CT _D , and CT _E. The peak position of the interference signal representing the extinction ratio of the Y-waveguide chip is _SY = _Δnf ·( _lp + _l1 + _l2 + _lf +l _a )+ _ΔnY · _lY , and the extinction ratio intensity is CT _Y. The distributed polarization crosstalk information of the entire length of the polarization-maintaining fiber ring (211) appears between the first-order polarization crosstalk interference signal peaks at the connection point D (209) and the connection point E (213), and the lumped extinction ratio of the polarization-maintaining fiber ring (211) is calculated as CT _coil according to the distributed polarization crosstalk information of the entire length;

7) According to step 107, determine whether CT _B and CT _C are better than -40dB, and whether CT _Y is better than 50dB. If not, return to step 101 and replace the Y waveguide (206). If satisfied, proceed to the next step;

8) According to step 108, determine whether the distributed polarization crosstalk of the entire length of the polarization-maintaining optical fiber ring (211) is better than -50 dB. If not, return to step 102 and replace the polarization-maintaining optical fiber ring (211). If satisfied, proceed to the next step;

9) According to step 109, determine whether the CT _D is better than -40dB. If not, return to step 102 and re-weld the connection point D (209). If satisfied, proceed to the next step;

10) According to step 110, disconnect the connection point E (213), remove the 45° analyzer (215), perform 0° axial welding on the second port (212) of the polarization-maintaining fiber ring and the second output polarization-maintaining pigtail (217) of the Y-waveguide to form a connection point G (301). This optical path is the second step in the assembly process of the full polarization-maintaining Sagnac closed optical path, which is called a closed optical path;

11) According to step 111, a 1×2 single-mode coupler (302) is selected, the first port (303) of the 1×2 single-mode coupler is connected to the 45° polarizer single-mode pigtail (218), the second port (304) of the 1×2 single-mode coupler is connected to the SLD wide-spectrum light source (220), and the third port (305) of the 1×2 single-mode coupler is connected to the optical coherence domain polarization meter (221); the operating wavelength of the 1×2 single-mode coupler (302) is 1550 nm, the insertion loss is less than 1 dB, and the splitting ratio is 50:50;

12) According to step 112, the closed optical path is measured for distributed polarization crosstalk, and the second-order polarization crosstalk between the connection point D (209) and the connection point F (216) and the second-order polarization crosstalk between the connection point D (209) and the connection point G (301) are respectively extracted from the measurement spectrum, and the positions where the interference signal peaks appear are S _DF = Δn _f · (l _f + l ₃ ) and S _DG = Δn _f ·l _f , respectively, and the second-order polarization crosstalk intensities are CT _DF and CT _DG , respectively. Therefore, it is inferred that the first-order polarization crosstalk intensity of the connection point F (216) is CT _F = CT _DF - CT _D , and the first-order polarization crosstalk intensity of the connection point G (301) is CT _G = CT _DG - CT _D ;

13) According to step 113, determine whether the _CTF is better than -40dB. If not, return to step 101 and replace the Y waveguide (206). If yes, proceed to the next step;

14) According to step 114, determine whether CT _G is better than -40dB. If not, return to step 110 and re-weld the connection point G (301). If satisfied, the measurement ends.

In summary, the present invention provides a polarization characteristic measurement method for full polarization-maintaining Sagnac closed optical path assembly, which is characterized by: connecting the Y waveguide and the polarization-maintaining fiber ring used for full polarization-maintaining Sagnac closed optical path assembly into a non-closed optical path and a closed optical path in turn, and respectively connecting them to an optical coherence domain polarization measuring instrument for measurement, and combining the second-order polarization crosstalk effect, all polarization characteristic information such as the first-order polarization crosstalk of all connection points in the closed optical path, the distributed polarization crosstalk of the full length of the polarization-maintaining fiber ring, and the extinction ratio of the Y waveguide chip can be obtained. This method realizes the measurement of polarization characteristics during the full polarization-maintaining Sagnac closed optical path assembly process, and can be widely used for the monitoring and evaluation of the distributed polarization crosstalk of all optical devices and connection points in the closed optical path, which is of great significance for the development of high-performance interferometric optical sensors.

Claims (5)

1. A polarization characteristic measurement method for full polarization-preserving Sagnac closed optical path assembly is characterized by comprising the following steps of: the method comprises the following steps:

(1) Selecting a Y waveguide (206) for assembling a full polarization-maintaining Sagnac closed optical path, and measuring the lengths of an input polarization-maintaining tail fiber (204) of the Y waveguide, a first output polarization-maintaining tail fiber (208) of the Y waveguide and a second output polarization-maintaining tail fiber (217) of the Y waveguide to be l respectively ₁ ，l ₂ ，l ₃ And require | l ₃ -l ₂ A connecting point B (205) is formed between a Y waveguide (206) and a Y waveguide input polarization-maintaining tail fiber (204), a connecting point C (207) is formed between the Y waveguide first output polarization-maintaining tail fiber (208), and a connecting point F (216) is formed between the Y waveguide second output polarization-maintaining tail fiber (217);

(2) Selecting a polarization-maintaining optical fiber ring (211) for assembling a full-polarization-maintaining Sagnac closed optical path, and carrying out 0-degree axial welding on a first port (210) of the polarization-maintaining optical fiber ring and a first output polarization-maintaining tail fiber (208) of a Y waveguide to form a connecting point D (209);

(3) Selecting a 45-degree polarizer (201), and measuring the length l of the polarization-maintaining tail fiber (202) of the 45-degree polarizer _p Selecting a 45-degree polarization analyzer (215), and measuring the length l of the polarization-maintaining pigtail (214) of the 45-degree polarization analyzer _a ；

(4) Carrying out 0-degree in-axis welding on a 45-degree polarizer polarization-maintaining tail fiber (202) and a Y waveguide input polarization-maintaining tail fiber (204) to form a connection point A (203), and connecting a 45-degree polarizer single-mode tail fiber (218) with an SLD (narrow-band light source) 220;

(5) Carrying out 0-degree axial welding on a polarization maintaining tail fiber (214) of the 45-degree polarization analyzer and a second port (212) of a polarization maintaining optical fiber ring to form a connection point E (213), and connecting a single-mode tail fiber (219) of the 45-degree polarization analyzer with an optical coherence domain polarization measuring instrument (221);

(6) Carrying out distributed polarization crosstalk measurement on the non-closed optical path, wherein the abscissa of the obtained measurement map is the spatial optical path of optical signal transmission, and the ordinate is the polarization crosstalk intensity of an interference signal peak; extracting the first-order polarization crosstalk of the connection points A, B, C, D and E, the distributed polarization crosstalk of the full length of the polarization-maintaining optical fiber and the extinction ratio measurement information of the Y waveguide chip at one time from the measurement map, calculating the position of each interference signal peak based on the length of each section of polarization-maintaining tail fiber, and assuming that the birefringence of the polarization-maintaining optical fiber in the optical path is delta n _f Birefringence of the Y waveguide chip is Deltan _Y The Y-waveguide (206) has a length l _Y The length of the polarization-maintaining fiber ring (211) is l _f Then, the peak positions of the interference signals representing the first-order polarization crosstalk of the connection points a, B, C, D, E are respectively: s _A ＝Δn _f ·l _p ，S _B ＝Δn _f ·(l _p +l ₁ )，S _C ＝Δn _f ·(l ₂ +l _f +l _a )，S _D ＝Δn _f ·(l _f +l _a )，S _E ＝Δn _f ·l _a The first-order polarization crosstalk intensity is CT _A ，CT _B ，CT _C ，CT _D ，CT _E The peak position of the interference signal representing the extinction ratio of the Y waveguide chip is S _Y ＝Δn _f ·(l _p +l ₁ +l ₂ +l _f +l _a )+Δn _Y ·l _Y Intensity of extinction ratio of CT _Y The full-length distributed polarization crosstalk information of the polarization-maintaining optical fiber ring (211) appears between the first-order polarization crosstalk interference signal peaks of the connection point D (209) and the connection point E (213), and the lumped extinction ratio of the polarization-maintaining optical fiber ring (211) is calculated to be CT through the full-length distributed polarization crosstalk information _coil ；

(7) Judging CT _B And CT _C Whether it is better than-40 dB, CT _Y Whether the power is better than 50dB or not is judged, if not, the step (1) is returned and the Y waveguide (206) is replaced, and if yes, the next step is carried out;

(8) Judging whether the distributed polarization crosstalk of the full length of the polarization maintaining optical fiber ring (211) meets the requirement, wherein due to the fact that a plurality of interference signal peaks are contained in the distributed polarization crosstalk result of the full length of the polarization maintaining optical fiber ring (211), the vertical coordinates of all interference signal peaks read from a measurement map, namely the polarization crosstalk strength, are superior to-50 dB, if not, returning to the step (2) and replacing the polarization maintaining optical fiber ring (211), and if so, performing the next step;

(9) Judging CT _D Whether the current time is better than-40 dB or not is judged, if not, the step (2) is returned to and the connecting point D (209) is welded again, and if so, the next step is carried out;

(10) Disconnecting the connection point E (213), removing the 45-degree polarization analyzer (215), and carrying out 0-degree axial fusion on the polarization-maintaining optical fiber ring second port (212) and the Y waveguide second output polarization-maintaining tail fiber (217) to form a connection point G (301);

(11) Selecting a 1 × 2 single-mode coupler (302), connecting a first port (303) of the 1 × 2 single-mode coupler with a 45-degree polarizer single-mode pigtail (218), connecting a second port (304) of the 1 × 2 single-mode coupler with an SLD (super-low-power line) wide-spectrum light source (220), and connecting a third port (305) of the 1 × 2 single-mode coupler with an optical coherent domain polarization measuring instrument (221);

(12) Distributed polarization crosstalk measurement is carried out on the closed optical path, and second-order polarization crosstalk between a connecting point D (209) and a connecting point F (216) and second polarization crosstalk between the connecting point D (209) and a connecting point G (301) are respectively extracted from a measurement map spectrumThe interference signal peak of the order polarization crosstalk measurement information is S _DF ＝Δn _f ·(l _f +l ₃ ) And S _DG ＝Δn _f ·l _f The second order polarization crosstalk intensity is CT _DF And CT _DG Calculating the first-order polarization crosstalk intensity of the connection point F (216) as CT _F ＝CT _DF -CT _D The first-order polarization crosstalk intensity of the connection point G (301) is CT _G ＝CT _DG -CT _D ；

(13) Judging CT _F Whether the power is better than-40 dB or not is judged, if not, the step (1) is returned and the Y waveguide (206) is replaced, and if so, the next step is carried out;

(14) Judging CT _G If the current value is better than-40 dB, the step (10) is returned to and the connection point G is welded again (301) if the current value is not better than-40 dB, and the measurement is finished if the current value is not better than-40 dB.

2. The polarization characteristic measurement method for the fully-polarization-preserving Sagnac closed optical path assembly according to claim 1, wherein: step (2) requires l _a -l _p -l ₁ ＞10cm。

3. The polarization characteristic measurement method for a fully-polarization-preserving Sagnac closed-path assembly according to claim 1 or 2, characterized in that: a45-degree polarizer (201) and a 45-degree polarization analyzer (215) used in an optical path are both dual-port optical devices, one end of each dual-port optical device is a polarization-maintaining pigtail, the other end of each dual-port optical device is a single-mode pigtail, a Y waveguide (206) is a three-port optical device, input and output pigtails of each dual-port optical device are polarization-maintaining optical fibers, and all polarization-maintaining optical fibers in the optical path are ordinary panda type polarization-maintaining optical fibers.

4. The polarization characteristic measurement method for the fully-polarization-preserving Sagnac closed optical path assembly according to claim 3, wherein: the 1 × 2 single-mode coupler (302) in the step (11) is replaced by a three-port single-mode fiber circulator or a2 × 2 single-mode coupler.

5. A method according to claim 1 or 4 for fully polarization-preserving Sagnac closed path assemblyThe polarization characteristic measuring method of (1), characterized in that: step (12) may also extract, from the measurement map spectrum, second-order polarization crosstalk between the connection point C (207) and the connection point F (216), and second-order polarization crosstalk measurement information between the connection point C (207) and the connection point G (301), where interference signal peaks appear at positions S _CF ＝Δn _f ·(l ₂ +l _f +l ₃ ) And S _CG ＝Δn _f ·(l ₂ +l _f ) The second order polarization crosstalk intensity is CT _CF And CT _CG Therefore, the first-order polarization crosstalk intensity at the connection point F (216) is estimated to be CT _F ＝CT _CF -CT _C The first-order polarization crosstalk intensity of the connection point G (301) is CT _G ＝CT _CG -CT _C 。

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