A distributed polarization crosstalk measurement device for ultra-high-sensitivity polarization beam splitting detection

Abstract

The present invention belongs to the field of optical fiber measurement technology, and specifically relates to a distributed polarization crosstalk measurement device with ultra-high sensitivity polarization beam splitting detection. The device includes a tunable laser source module, a device under test module, a polarization beam splitting interferometer module, an auxiliary interferometer module, and a signal acquisition and analysis module. The device is characterized by: using a polarization beam splitter to separate the transmission mode and coupling mode light passing through the device under test to suppress the main interference peak, thereby improving the sensitivity to ‑130dB; under the condition of equal light source output power, the amplitude of the optical signal output by the polarization beam splitter can be increased by four times compared with traditional solutions; the invention, combined with a tunable laser, gets rid of the dependence on an optical path scanning delay device in polarization crosstalk measurement, realizes an all-fiber structure without moving parts, enhances the reliability and stability of the test system, and greatly improves the speed and sensitivity of polarization crosstalk measurement in principle.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber measurement, and in particular relates to a distributed polarization crosstalk measurement device for ultra-high-sensitivity polarization beam splitting detection. Background Art

[0002] Polarization optical devices are an important component of high-precision optical measurement and sensing systems. With the improvement of polarization device technology and the improvement of precision, the extinction ratio peak of the device is getting lower and lower, and the polarization crosstalk is getting smaller and smaller. The existing technology has seriously hindered the development of high-precision optical measurement and sensing systems.

[0003] As a key technology for measuring polarization crosstalk in distributed optical systems, optical coherence domain polarimetry (OCDP), based on the principle of white-light interferometry, focuses on high-precision analysis of polarization crosstalk location and intensity, enabling the performance testing and evaluation of polarization-based optical devices. OCDP, with its advantages of high measurement sensitivity, high spatial resolution, and wide dynamic range, has found widespread application in areas such as high-precision alignment of polarization-maintaining fibers, device extinction ratio testing, and fiber gyroscope ring testing. Compared to other distributed detection methods and technologies, such as polarization time-domain reflectometry (POTDR), optical frequency domain reflectometry (OFDR), and optical coherence domain reflectometry (OCDR), OCDP offers advantages such as simple structure, high spatial resolution (5-10 cm), large measurement range (several kilometers), ultra-high sensitivity (coupled energy -80 to -100 dB), and extremely large dynamic range. It holds great promise for development into a high-precision, universal testing technology and system.

[0004] As technology continues to advance, existing optical coherence domain polarization measurement (OCDP) solutions need to be continuously improved to adapt to the ever-changing device testing. Some extremely weak polarization mode couplings or extremely high polarization extinction ratios (>80dB) are extremely susceptible to environmental noise. These small signals will be submerged in the noise floor, causing confusion when identifying peak information. In a distributed polarization coupling measurement system, the measurement sensitivity and the system's dynamic range are directly determined by the system's own noise floor. There are three main types of system noise: optical shot noise, interferometric beat noise, and circuit thermal noise. We need to suppress these noises to improve the dynamic range and detection sensitivity of the polarization mode coupling measurement system.

[0005] To further improve the test sensitivity, dynamic range, and device measurement length, Yang Jun et al. proposed using a polarization beam splitter to separate the transmitted light and the coupled light to suppress beat noise and improve sensitivity, and using an attenuator to rationally select the attenuation factor to attenuate the transmitted energy and reduce relative intensity noise (A Noise Suppression Device and Suppression Method for Distributed Polarization Crosstalk Measurement of Optical Polarization Devices, CN201510212810.8).

[0006] In 2016, Li Chuang and others proposed a polarization coupling measurement system and solution based on PBS (Li Chuang. Research on ultra-high sensitivity polarization coupling measurement technology and Y-waveguide test method [D]. Harbin Engineering University, 2016.). By using white light interferometry technology, the coupler in the traditional solution replaced the polarization beam splitter. The transmitted light and coupled light coming out of the device were completely separated, avoiding the influence of interference noise.

[0007] Because the broadband light source used in optical coherence domain polarimetry (OCDP) has low energy, the signal-to-noise ratio is insufficient, preventing significant improvements in dynamic range. Furthermore, the use of a mechanical stage to scan the optical path increases system complexity, reduces reliability, and increases scanning time.

[0008] Compared with the optical coherence domain polarization measurement technology (OCDP), polarization-sensitive optical frequency domain reflectometry (P-OFDR) fully utilizes the characteristics of tunable lasers to significantly improve the measurement speed and signal-to-noise ratio. However, it cannot measure the critical polarization crosstalk parameter.

[0009] Therefore, in order to solve the above problems, a distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shifting interferometry was proposed in 2021 (a fiber-optic distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shifting interferometry, CN202110828166.2). The applicant used a high-coherence tunable laser light source for fast wavelength scanning, eliminating the original mechanical scanning delay structure, and used a Mach-Zehnder interferometer with an optical fiber extension arm as an auxiliary interferometer to correct the fiber nonlinear noise. The structure is stable and reliable, greatly improving the speed and dynamic range of polarization crosstalk measurement. In the same year, the applicant added a Mach-Zehnder interferometer with an optical fiber extension arm as the main interferometer on the original basis to shift the beat frequency signal to a high frequency, avoiding signal aliasing at low frequencies (a distributed polarization crosstalk fast measurement device based on optical frequency domain frequency shifting interferometry, CN202110828088.6).

[0010] Polarization mode coupling measurement systems based on white-light interferometry have achieved a dynamic range of ~90 dB. However, such systems require that every component and device be maintained in optimal condition. Compared to optical coherence domain polarization measurement systems, extremely weak polarization mode coupling or extremely high polarization extinction ratios (>80 dB) are highly susceptible to environmental noise. These small signals are often lost in the noise floor, leading to confusion in peak information identification.

[0011] The present invention is based on the improvement of the existing technology and discloses a distributed polarization crosstalk measurement device for ultra-high-sensitivity polarization beam splitting detection. Compared with the traditional distributed polarization crosstalk rapid measurement device based on optical frequency domain interference, the polarization beam splitter is used to replace the original 50:50 coupler in the polarization beam splitting interferometer part, so that the orthogonal transmission mode and coupling mode from the device under test can be injected into the reference arm and coupling arm of the interferometer respectively. At this time, the original main interference peak is suppressed, and the phase noise introduced by the main interference peak is also suppressed, thereby obtaining higher sensitivity. Compared with the traditional solution, under the condition of the same light source power, the amplitude of the optical signal output by the polarization beam splitter can be increased by four times, which effectively improves the utilization rate of the light source. In addition, the present invention uses an auxiliary interferometer to eliminate the signal aliasing problem introduced by the nonlinearity of the light source frequency sweep, and has the advantages of small size, reliable and stable system, and high sensitivity. Summary of the Invention

[0012] The purpose of the present invention is to provide a distributed polarization crosstalk measurement device with ultra-high sensitivity polarization beam splitting detection, which can improve the dynamic range and sensitivity of polarization crosstalk testing while overcoming the problems of low-frequency flicker noise and signal aliasing.

[0013] The purpose of the present invention is to be achieved through the following measures:

[0014] A distributed polarization crosstalk measurement device for ultra-high-sensitivity polarization beam splitting detection includes a tunable light source module 1, a device under test module 2, a polarization beam splitting interferometer module 3, an auxiliary interferometer module 4, and a signal acquisition and analysis module 5, characterized in that:

[0015] The tunable laser source module 1 sends a linear frequency modulated continuous light 601, which is injected into the first coupler 602 from the first coupler input end 602a and split into two beams of light;

[0016] The light beam in the first coupler 602 is injected into the DUT module 2 after passing through the first output pigtail 602b of the first coupler;

[0017] The light beam output from the DUT module 2 is injected into the polarization beam splitting interferometer module 3, and then injected into the polarization beam splitter 302 through the polarization beam splitter input pigtail 301 and split into two orthogonal beams. One beam is injected into the reference arm 303 as the reference light, and the other beam is injected into the measurement arm 304 as the test light. The light beams output from the reference arm 303 and the measurement arm 304 are combined in the second coupler 308 and then split into two beams. The first balanced detector 309 then performs differential detection.

[0018] The optical path difference of the device under test module (2) is X1;

[0019] The light beam in the first coupler 602 is injected into the auxiliary interferometer module 4 after passing through the second output end 602c of the first coupler;

[0020] The polarization beam splitting interferometer signal 310 output by the polarization beam splitting interferometer module 3 and the auxiliary interferometer signal 407 output by the auxiliary interferometer module 4 are jointly injected into the acquisition unit 501 in the signal acquisition and analysis module 5 for acquisition, and the obtained signal then enters the signal processing unit 502 for data analysis and processing;

[0021] The device under test module 2 is connected in sequence by a device under test input pigtail 204a, a device under test 204, and a device under test output pigtail 204b; the device under test input pigtail 204a is connected to a polarizer; when the measured parameter of the device under test 204 includes a high extinction ratio, the fusion angle of the first connection point 202 is θ1 = 45°; when the measured parameter of the device under test 204 is polarization crosstalk at a general coupling point, the fusion angle of the first connection point 202 is θ1 = 0°±2° or θ1 = 90°±2°; the fusion angle of the fourth connection point 206 is θ2 = 0°±2° or θ2 = 90°±0°;

[0022] The coupling mode and transmission mode of the light beam introduced by the device under test are orthogonal at the polarization beam splitter input pigtail 301. The polarization beam splitter 302 completely separates the light beam and injects it into the reference arm 303 and measurement arm 304 of the interferometer, respectively. The polarization beam splitter 302 and the second coupler 308 are connected to the fifth connection point 305 and the sixth connection point 306 via flanges. The polarization controller 307 adjusts the polarization state of the polarization beam splitting interferometer to obtain a signal with the largest possible amplitude.

[0023] The optical path difference of the auxiliary interferometer module 4 is X2, X2>2X1;

[0024] The light beam injected into the third coupler 401 is split into two beams. One beam is injected into the reference arm 402 as reference light, and the other beam is injected into the measurement arm 403 as test light. The beams pass through the delay fiber 404. The light beams output from the reference arm 402 and the measurement arm 403 are combined in the fourth coupler 405 and then split into two beams. The beams are then differentially detected by the second balanced detector 406.

[0025] The signal acquisition unit 501 converts the received analog signal auxiliary interferometer signal 407 and polarization beam splitting interferometer signal 310 into digital signals, and the two are input together into the signal processing unit 502; the parameter setting of the acquisition software needs to be consistent with the hardware parameter setting to achieve complete data acquisition; the method of processing data by the signal processing unit 502 is to perform nonlinear correction of the light source on the polarization beam splitting interferometer signal 310 according to the collected auxiliary interferometer signal 407, filter out most of the intensity noise inside the swept frequency light source, intercept the corrected signal to extract effective information, and then perform resampling algorithm processing and delay correction on the intercepted signal to finally obtain a signal that meets the test requirements.

[0026] The distributed polarization crosstalk measurement process performed by a polarization beam splitter interferometer with a polarization beam splitter 302 is as follows: After light passes through the device under test 204, the device's pigtail 204b carries the polarization crosstalk information within the device. Most of the energy is transmitted along the device's transmission axis (fast axis), while a small portion of the energy transitions from the transmission mode to the coupling mode due to internal defects or coupling points. As the light propagates through the device under test, the waveforms 204c of the two modes shift positionally, resulting in a delay. The device's output pigtail 204b and the polarization beam splitter 302's input pigtail 301 are welded together at a fourth connection point 206 via a polarization-maintaining welding machine. In the polarization-maintaining welding machine, the two pigtails have an axial angle 300 of 0°-0°. At this point, the transmission mode and coupling mode light introduced by the device under test can be injected by the polarization beam splitter 302 into the reference arm 303 and measurement arm 304 of the Mach-Zehnder interferometer, respectively. Let the light transmitted in the reference arm 303 be 302b, and the light transmitted in the measurement arm be 302c. The two beams are finally combined and interfered at the second coupler 308. After differential detection by the first balanced detector 309, the data is acquired by the acquisition card. After processing, the polarization crosstalk coupling peak inside the device under test can be obtained.

[0027] Polarization coupling measurement methods calibrated with polarization beam splitter 302 are used to obtain relative dynamic range. Conventional polarization crosstalk measurement devices calibrate coupling peaks by normalizing the main peak to obtain dynamic range and sensitivity. Because the main interference peak is suppressed after using polarization beam splitter 302, calibration is performed by aligning the fiber pigtail of the device under test 204 and the fiber pigtail of polarization beam splitter 302 twice. When the alignment angle between the two is between 0° and 45°, polarization beam splitter 302 acts as a 50:50 coupler. After passing through polarization beam splitter 302, information from the device under test is evenly injected into the fast and slow axes of the optical fiber. The fast and slow axes in reference arm 303 and measurement arm 304 interfere with each other, forming the desired main interference peak. A prominent coupling peak appears symmetrically to the left and right of the main interference peak, which is selected as the calibration peak. Align the axis again and use a welding machine to make the axis angle 0°-0°. At this time, since there is only one mode in the reference arm and the measurement arm respectively and the two arms have equal optical path, the main interference peak will disappear and other peaks will replace it accordingly. At this time, we infer from the actual length of the weld that the coupling peak next to the original main interference peak still exists, but the amplitude will change to a certain extent. By comparing the data obtained by using two different welding angles for the pigtail of the device under test and the pigtail of the polarization beam splitter, the coupling peak obtained by the axis angle of 0°-45° is used to calibrate the coupling peak with an axis angle of 0°-0°. Since the original main interference peak is suppressed after using the polarization beam splitter 302, the phase noise introduced by the main interference peak is also suppressed, thereby greatly improving the sensitivity.

[0028] For the Y-waveguide 204Y device under test, the method for determining the polarization crosstalk position is as follows:

[0029] 1) The delay fiber 404 is a single-mode fiber with a geometric length of Δl. The corresponding optical path is calculated as X2 = Δl × n, where n is the refractive index of the single-mode fiber.

[0030] 2) The polarizer output pigtail 201b is a Panda-type polarization-maintaining fiber with a geometric length of l1. The corresponding optical path is calculated as S1 = l1 × Δn. The polarization beam splitter input pigtail 301 is a Panda-type polarization-maintaining fiber with a geometric length of l1. The corresponding optical path is calculated as S2 = l2 × Δn, where Δn is the linear birefringence of the polarization-maintaining fiber.

[0031] 3) The geometric length of the Y-waveguide input pigtail 204a is l3, the geometric length of the Y-waveguide output pigtail 204b is l4, and the chip length of the Y-waveguide 204Y device under test is l5. The corresponding optical path is calculated as S = S3 + S4 + S5 = l3 × Δn + l4 × Δn + l5 × Δn Y , where Δn is the linear birefringence of the polarization-maintaining fiber, Δn Y is the linear birefringence of the Y-waveguide chip;

[0032] 4) If the Y-waveguide chip operates in the fast-axis mode, the position corresponding to the characteristic peak of the extinction ratio interference is |S1+S2+S3+S4+S5|; if the Y-waveguide chip operates in the slow-axis mode, the position corresponding to the characteristic peak of the extinction ratio interference is |S1+S2+S3+S4-S5|;

[0033] 5) After the light beams are coupled once through the first connection point 202, the second connection point 203, the third connection point 205, and the fourth connection point 206, the positions of the corresponding four first-order polarization crosstalk interference characteristic peaks are |S1|, |S1+S3|, |S2+S4|, and |S2|, respectively. If l1<l2, the positions of the interference peaks are distributed as |S1|<|S1+S3|<|S2|<|S2+S4|; if l1>l2, the positions of the interference peaks are distributed as |S2|<|S2+S4|<|S1|<|S1+S3|;

[0034] 6) When the light beams are coupled twice respectively through the first connection point 202 and the third connection point 207, the first connection point 202 and the fourth connection point 206, the second connection point 205 and the third connection point 207, and the second connection point 205 and the fourth connection point 206, the positions of the corresponding four second-order polarization crosstalk interference characteristic peaks are |S1+S2+S4|, |S1+S2|, |S1+S2+S3+S4|, and |S1+S2+S3|;

[0035] Compared with the prior art, the advantages of the present invention are:

[0036] 1. The present invention is a device for rapid measurement of low-noise distributed polarization crosstalk in the optical frequency domain. It uses an auxiliary interferometer to correct the signal aliasing problem introduced by the nonlinearity of the light source frequency sweep, effectively improving the system signal-to-noise ratio.

[0037] 2. This invention uses the transmission information obtained after light passes through the device under test to measure polarization crosstalk parameters in the frequency domain. A highly coherent, tunable laser source is used to rapidly scan the wavelength of the optical signal, thereby obtaining polarization crosstalk intensity and position information after frequency domain demodulation. Due to the high signal-to-noise ratio provided by the high energy source, this device achieves a wider dynamic range and significantly improves test speed.

[0038] 3. The present invention uses a polarization beam splitter to inject the orthogonal transmission mode and coupling mode from the device under test into the reference arm and measurement arm of the interferometer, respectively. The phase noise introduced by the main interference peak is suppressed along with the main interference peak, and only the interference peak of the two mode coupling exists. By using the coupling peak for calibration, the sensitivity of the device can be increased to -130dB, providing more options for testing devices with high extinction ratios.

[0039] 4. The present invention utilizes a distributed polarization crosstalk rapid measurement device with a polarization beam splitter to overcome the limitations brought by phase noise. Under the premise of sufficiently high light intensity, the sensitivity can be continuously improved as the detector detection light intensity increases. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a schematic diagram of a distributed polarization crosstalk measurement device for ultra-high-sensitivity polarization beam splitting detection.

[0041] Figure 2 This is a schematic diagram of the working principle of the polarization beam splitting interferometer part of a distributed polarization crosstalk measurement device for ultra-high sensitive polarization beam splitting detection.

[0042] Figure 3 This is a schematic diagram of a distributed polarization crosstalk measurement device for ultra-high-sensitivity polarization beam splitting detection of Y-waveguide.

[0043] Figure 4 This is the expected test result using the traditional solution - the Y-waveguide output fiber and the polarization beam splitter input fiber are at an axial angle of 0°-45°.

[0044] Figure 5 This is the expected test result using the polarization beam splitter solution - the Y-waveguide output fiber and the polarization beam splitter input fiber are at an axis angle of 0-0°. DETAILED DESCRIPTION

[0045] In order to more clearly illustrate the distributed polarization crosstalk measurement device and method for ultra-high-sensitivity polarization beam splitting detection proposed by the present invention, the present invention is further described below in conjunction with embodiments and drawings, but this should not limit the scope of protection of the present invention.

[0046] Specific implementation method: A distributed polarization crosstalk measurement device for performing ultra-high sensitive polarization beam splitting detection on a Y-waveguide in the optical frequency domain, as shown in the attached Figure 3 As shown:

[0047] It consists of four parts: tunable laser source module 1, device under test module 2, polarization beam splitting interferometer module 3, auxiliary interferometer module 4 and signal acquisition and analysis module 5, among which:

[0048] 1) The tunable laser source module 1 outputs linear frequency modulated continuous light 601, which is injected into the first coupler 602 from the first coupler input end 602a and split into two beams of light;

[0049] 2) The light beam outputted from the first output terminal 602b of the first coupler is injected into the device under test module 2;

[0050] 3) The light beam output from the DUT module 2 is injected into the polarization beam splitter interferometer module 3, injected into the polarization beam splitter 302 from the polarization beam splitter input end 301, and split into two orthogonal beams. One beam is injected into the reference arm 303 as the reference light, and the other beam is injected into the measurement arm 304 as the test light. The light beams output from the reference arm 303 and the measurement arm 304 are combined in the second coupler 308 and then split into two beams, which are then differentially detected by the first balanced detector 309.

[0051] The light beam outputted from the second output end 602c of the first coupler is injected into the auxiliary interferometer module 4;

[0052] Polarization beam splitting interferometer signal 310 output by polarization beam splitting interferometer module 3 and auxiliary interferometer signal 407 output by auxiliary interferometer module 4 are jointly injected into signal acquisition and analysis module 5 for data acquisition, processing, and demodulation. Signal acquisition and analysis module 5 comprises an acquisition unit 501 and a computer PC 502. Acquisition unit 501, controlled by sbench software, synchronously acquires and stores analog voltage signals obtained from the detector's photoelectric conversion, converting them into digital signals. Computer PC 502 is used to control signal generation and reception, as well as process and demodulate the digital signals.

[0053] The selection and parameters of the main optoelectronic devices in this solution are as follows:

[0054] 1) The tunable laser source module 1 is a narrow-linewidth tunable laser capable of continuous wavelength scanning. The wavelength tuning range is 1480-1640 nm. In coherent mode, the wavelength scanning rate is 40 nm / s and the wavelength scanning time is 4 s.

[0055] 2) The photosensitive material of the first balanced detector 309 and the second balanced detector 406 are both InGaAs, with a common mode rejection ratio of 25 dB, a light detection range of 900 to 1700 nm, a maximum detection bandwidth of 80 MHz, a saturated differential detection power of 55 uW, a transimpedance gain of 50,000 V / A, a minimum noise equivalent power of 1 A / W, and a peak responsivity. For example, a NewFocus 1817 balanced detector is used;

[0056] 3) The 16-bit sampling rate of the acquisition unit 501 is 45 MHz, and the sampling time is about 4 seconds. Combined with the laser for external triggering, the sampling time is longer than the wavelength scanning time, and the parameter design is reasonable;

[0057] 4) The splitting ratio of the first coupler 602 is 5:95, the splitting ratios of the second coupler 308, the third coupler 401, and the fourth coupler 405 are 50:50, the extinction ratios are all greater than 20 dB, the insertion losses are all less than 0.5 dB, and the operating wavelength covers the 1550 nm band;

[0058] 5) The operating wavelength of the polarizer 201 covers the 1550nm band, the polarization angle is 45°, the insertion loss is less than 1dB, the extinction ratio is greater than 30dB, the polarizer input pigtail 201a is a single-mode fiber, and the polarizer output pigtail 201b is a panda-type polarization-maintaining fiber with a diameter of 125um. The length of the polarization-maintaining pigtail is l1=15m. The corresponding optical path S1 is calculated as l1×Δn≈7.5×10 3 um, where the linear birefringence Δn of the polarization-maintaining fiber is 5×10 -4 ;

[0059] 6) The operating wavelength of the polarization beam splitter 302 covers the 1550nm band. The polarization beam splitter input pigtail 301 is a Panda-type polarization-maintaining fiber with a diameter of 125μm. The length of the polarization-maintaining pigtail is l2=10m. The corresponding optical path S2=l2×Δn≈5×10 3 um, where the linear birefringence Δn of the polarization-maintaining fiber is 5×10 -4 ;

[0060] 7) The welding angle of the first connection point 202 is θ1 = 0°, and the welding angle of the fourth connection point 206 is θ2 = 0°;

[0061] 8) The Y-waveguide 204Y device under test uses lithium niobate crystal as the chip substrate, has an extinction ratio of 60 dB, and operates in the fast axis. The geometric length of the Y-waveguide input pigtail 204a is l3 = 1m, and the diameter is 125um. The corresponding optical path S3 is calculated as l3 × Δn = 5 × 10 2 um, the geometric length of the Y-waveguide output pigtail 204b is l4 = 2m, and the diameter is 125um. The corresponding optical path S4 = l4 × Δn = 1 × 10 3 um, the length of the Y waveguide chip is l5 = 0.04m, and the corresponding optical path S = S3 + S4 + S5 = l3 × Δn + l4 × Δn + l5 × Δn Y ≈5236um, where the linear birefringence Δn of the polarization-maintaining fiber is 5×10 -4 , linear birefringence Δn of the Y waveguide chip Y Take 9.34×10 -2 ;

[0062] 9) The auxiliary interferometer module 4 uses the auxiliary delay fiber 404 as l ref = 20m Mach-Zehnder fiber interferometer structure, calculate the optical path difference X2 corresponding to the auxiliary interferometer module 4 in this embodiment = l ref ×Δn=29.12×10 6 um, where the refractive index n of the single-mode optical fiber is 1.456; in this embodiment, the optical path S of the Y waveguide 204Y of the device under test is ≈ 5236 μm, so X2>2X1 is satisfied, and the parameter design is reasonable;

[0063] 10) The position corresponding to the characteristic peak of the extinction ratio interference is |S1+S2+S3+S4+S5|=17730 μm;

[0064] 11) After the light beams are coupled once through the first connection point 202, the second connection point 203, the third connection point 205, and the fourth connection point 206, the positions of the corresponding four first-order polarization crosstalk interference characteristic peaks are |S1|=7500 μm, |S1+S3|=8000 μm, |S2+S4|=6000 μm, and |S2|=5000 μm, respectively. Since l1>l2, the positions of the interference peaks are distributed as |S2|<|S2+S4|<|S1|<|S1+S3|;

[0065] 12) When the light beam is coupled twice through the first connection point 202 and the third connection point 205, the first connection point 202 and the fourth connection point 206, the second connection point 203 and the third connection point 205, and the second connection point 203 and the fourth connection point 206 respectively, the positions of the corresponding four second-order polarization crosstalk interference characteristic peaks are |S1+S2+S4|=13500um, |S1+S2|=12500um, |S1+S2+S3+S4|=14000um, and |S1+S2+S3|=13000um respectively.

[0066] When testing the Y-guide, a conventional distributed polarization test device uses a calibration method that normalizes the amplitude of the main peak 701 to achieve a sensitivity of -100 dB. After replacing the 50:50 coupler in the conventional solution with a polarization beam splitter, the Y-waveguide output pigtail 206b of the device under test and the polarization beam splitter input pigtail 301 are aligned at 0°-0° using a welding machine to obtain a set of results. The main peak 701 is suppressed in the results, so the main peak normalization method cannot be used to calibrate the sensitivity. Therefore, a more prominent welding peak is selected for sensitivity calibration, such as location 205 where the Y-waveguide chip and the Y-waveguide output pigtail are connected. The coupling peak 702 obtained using the conventional method is used to calibrate the solder point peak obtained by the polarization beam splitter solution, thereby improving the sensitivity 703 from -100 dB to -130 dB 704.

[0067] Excluding the time spent changing the axial angle of the solder joint between the output fiber pigtail of the device under test and the input fiber pigtail of the polarization beam splitter, the duration of a single test is approximately 3.95 seconds.

[0068] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A distributed polarization crosstalk measurement device for ultra-high-sensitivity polarization beam splitting detection, comprising a tunable laser source module (1), a device to be measured module (2), a polarization beam splitting interferometer module (3), an auxiliary interferometer module (4) and a signal acquisition and analysis module (5), characterized in that: The tunable laser source module (1) is connected to a first coupler (602); a first output pigtail (602b) of the first coupler is connected to a polarizer input pigtail (201a) of a device under test module (2); and a second output pigtail (602c) of the first coupler is connected to a third coupler input pigtail (401a) of an auxiliary interferometer module (4); The polarization beam splitter input pigtail (301) of the polarization beam splitting interferometer module (3) is connected to the device under test output pigtail (204b) of the device under test module (2) at a fourth connection point (206); the polarization beam splitter (302) and the second coupler (308) are connected to a fifth connection point (305) and a sixth connection point (306) via a flange plate and are combined into a Mach-Zehnder interferometer including a main interferometer reference arm (303) and a main interferometer measurement arm (304); the second coupler (308) is connected to a first balanced detector (309), and differential detection is performed by the first balanced detector (309); The tunable laser source module (1) emits a frequency sweep light (601) whose wavelength changes linearly with time, which is divided into two paths through a first coupler (602), one of which passes through a device-to-be-tested module (2) and a polarization beam splitting interferometer module (3), and the other passes through an auxiliary interferometer module (4) and is detected by a first balanced detector (309) and a second balanced detector (406) to obtain a polarization beam splitting interferometer signal (310) and an auxiliary interferometer signal (407), both of which are injected into a collection unit (501) in a signal collection and analysis module (5) for collection, and the obtained signal then enters a signal processing unit (502) for data analysis and processing; The device under test module (2) is connected in sequence by a polarizer input pigtail (201a), a polarizer (201), a polarizer output pigtail (201b), a first connection point (202), a device under test input pigtail (204a), a second connection point (203), a device under test (204), a third connection point (205), a device under test output pigtail (204b), and a fourth connection point (206); the device under test input pigtail (204a) is connected to the polarizer; when the measured parameter of the device under test (204) includes a high extinction ratio, the fusion angle of the first connection point (202) is θ1=45°; when the measured parameter of the device under test (204) is a polarization crosstalk of a general coupling point, the fusion angle of the first connection point (202) is θ1=0°±2° or θ1=90°±2°, and the measured parameter is θ1=0°±2°. When testing a polarization-maintaining optical fiber with a welding point, the fusion angle of the first connection point (202) should be 0° or 90°. At this time, the light injected into the device to be tested is transmitted in only one mode. When the light passes through the internal defect point of the polarization-maintaining optical fiber, a small part of the energy jumps from the transmission mode to the coupling mode. The coupling peak calibration effect can be achieved by changing the axis angle of the fourth connection point where the polarization beam splitter input pigtail is connected to the device to be tested pigtail using a fusion splicer. When the function of the polarization beam splitter is to separate the orthogonal polarized light from the device to be tested, the fusion angle of the fourth connection point (206) is θ2=0°±2° or θ2=90°±2°. When the function of the polarization beam splitter is to act as a 50:50 coupler to uniformly inject light into the fast and slow axes of the polarization-maintaining optical fiber, the fusion angle of the fourth connection point (206) is θ2=45°.

2. The distributed polarization crosstalk measurement device for ultra-high-sensitivity polarization beam splitting detection according to claim 1, characterized in that: The coupling mode and transmission mode of the light beam introduced by the device to be tested are in an orthogonal state at the polarization beam splitter input pigtail (301), and the polarization beam splitter (302) completely separates them and injects them into the main interferometer reference arm (303) and the main interferometer measurement arm (304) of the interferometer respectively. At this time, since there is no other mode interference in the single arm, only the two modes of the main interferometer reference arm (303) and the main interferometer measurement arm (304) interfere with each other when passing through the second coupler (308). By adjusting the polarization controller (307), a signal with the largest possible amplitude is obtained.

3. The distributed polarization crosstalk measurement device for ultra-high-sensitivity polarization beam splitting detection according to claim 1, characterized in that: The light beam injected into the third coupler (401) is split into two beams, one of which is injected into the auxiliary interferometer reference arm (402) as reference light, and the other is injected into the auxiliary interferometer measurement arm (403) as test light, and passes through the delay optical fiber (404). The light beams output by the auxiliary interferometer reference arm (402) and the auxiliary interferometer measurement arm (403) are combined in the fourth coupler (405) and then split into two beams, and then differentially detected by the second balanced detector (406); the optical path difference of the auxiliary interferometer module (4) is X2, X2>2X1, and X1 is the optical path difference of the device under test module (2).

4. The distributed polarization crosstalk measurement device for ultra-high-sensitivity polarization beam splitting detection according to claim 1, characterized in that: The acquisition unit (501) converts the received analog signal auxiliary interferometer signal (407) and polarization beam splitting interferometer signal (310) into digital signals, and the two are input into the signal processing unit (502); the parameter setting of the acquisition software needs to be consistent with the hardware parameter setting to achieve complete data acquisition; the method of processing data by the signal processing unit (502) is to perform nonlinear phase correction of the light source on the polarization beam splitting interferometer signal (310) according to the acquired auxiliary interferometer signal (407), filter out most of the intensity noise inside the swept frequency light source, intercept the corrected signal to extract effective information, and then use Fourier transform to demodulate the position and intensity information of polarization crosstalk in the frequency domain signal, and finally obtain a signal that meets the test requirements.

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