An ultra-high sensitivity polarization beam splitting structure frequency shift detection optical frequency domain polarimeter

Abstract

The application belongs to the technical field of optical fiber measurement, and particularly relates to an optical frequency domain polarimeter of polarization beam splitting structure frequency shift detection with ultra-high sensitivity, which comprises a tunable laser source module, a device to be measured module, a polarization beam splitting frequency shift interferometer module, an auxiliary interferometer module and a signal acquisition and analysis module, and is characterized in that: a frequency shifter contained in the polarization beam splitting frequency shift interferometer module is used to move the signal to a high frequency, thereby avoiding the noise aliasing problem from a low frequency; a polarization beam splitter is used to separate the transmission mode and the coupling mode light passing through the device to be measured to suppress the main interference peak, thereby improving the sensitivity to-130 dB, and greatly improving the speed and sensitivity of the polarization crosstalk measurement in principle. Meanwhile, the main extended optical fiber and the acousto-optic modulator are combined to shift the signal, thereby meeting the acquisition of the polarization crosstalk information when the long-distance device is tested.

Description

Technical fields:

[0001] This invention belongs to the field of optical fiber measurement technology, specifically involving an optical frequency domain polarimeter with ultra-high sensitivity polarization beam splitting structure frequency shift detection. Background technology:

[0002] Polarization optical devices are a crucial component of high-precision optical measurement and sensing systems. With advancements in polarization device technology and precision, the extinction ratio peaks and polarization crosstalk of these devices are decreasing. As technology continues to develop, existing Optical Coherence Domain Polarization (OCDP) measurement techniques require constant improvement to adapt to the testing of increasingly sophisticated devices. Extremely weak polarization mode coupling or extremely high polarization extinction ratios (>80dB) are highly susceptible to environmental noise. These small signals are submerged in the noise floor, leading to confusion in peak information identification. In distributed polarization coupling measurement systems, the measurement sensitivity and dynamic range are directly determined by the system's own noise floor. System noise primarily consists of three typical types: optical shot noise, interference beat noise, and circuit thermal noise. We need to suppress these noises to improve the dynamic range and detection sensitivity of polarization mode coupling measurement systems.

[0003] To further improve the sensitivity, dynamic range, and device measurement length of the test, 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 reasonably select the attenuation factor to attenuate the transmitted energy and reduce relative intensity noise (a noise suppression device and method for distributed polarization crosstalk measurement of optical polarization devices, CN20510212810.8).

[0004] In 2021, Li Chuang et al. proposed a polarization coupling measurement system and scheme based on PBS (Li Chuang. Research on ultra-high sensitivity polarization coupling measurement technology and Y waveguide testing method [D]. Harbin Engineering University, 21.). By using white light interferometry, the polarization beam splitter was replaced by the coupler in the traditional scheme. The transmitted light and coupled light from the device were completely separated, avoiding the influence of interference beat noise.

[0005] Because the broadband light source used in Optical Coherence Domain Polarization Measurement (OCDP) has low energy, the signal-to-noise ratio is insufficient, preventing a significant improvement in dynamic range. Furthermore, the use of a mechanical displacement stage to control the optical path scanning increases system complexity, reduces reliability, and lengthens scanning time.

[0006] Polarization-sensitive optical frequency domain reflectometry (P-OFDR), compared to optical coherent domain polarization measurement (OCDP), significantly improves measurement speed and signal-to-noise ratio by fully utilizing the characteristics of tunable lasers. However, it cannot measure critical polarization crosstalk parameters. In contrast to the well-developed OCDP, optical frequency domain polarization (OFDP) has been widely used in high-precision testing applications due to its advantages such as shorter testing time, wider dynamic range, and higher spatial resolution.

[0007] In 2021, a rapid measurement device for distributed polarization crosstalk based on optical frequency-domain frequency-shifting interferometry was proposed (A rapid measurement device for distributed polarization crosstalk based on optical frequency-domain interferometry, CN21110828166.2). This applicant used a highly coherent tunable laser source for rapid wavelength scanning, eliminating the need for the original mechanical scanning tester structure. A Mach-Zehnder interferometer with an optical fiber extension arm was used as an auxiliary interferometer to correct optical 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 to shift the beat frequency signal to a higher frequency, avoiding signal aliasing at low frequencies (A rapid measurement device for distributed polarization crosstalk based on optical frequency-domain frequency-shifting interferometry, CN21110828088.6). In 2021, Wang Feng and others from Nanjing University disclosed an acousto-optic modulator frequency shifting method (optical fiber parameter measurement device for correcting nonlinear tuning effects, CN202110135197.X) to shift the frequency of light waves and increase the beat frequency signal frequency.

[0008] As technology continues to develop, optical frequency domain polarimeters are constantly being updated and iterated, but they still have shortcomings. For example, the testing requirements for long-distance devices force us to add an extension fiber to one arm of the interferometer to increase the beat frequency effect to meet the requirements, and the signal background will also be raised due to phase noise. At the same time, testing devices with high extinction ratios requires the highest possible sensitivity.

[0009] To overcome the aforementioned shortcomings, this invention, based on improvements to existing technologies, discloses an ultra-sensitive optical frequency domain polarimeter with a polarization beam-splitting structure for frequency shift detection. Compared to traditional distributed polarization crosstalk rapid measurement devices based on optical frequency domain interferometry, this invention replaces the original 50:50 coupler with a polarization beam splitter in the polarization beam-splitting interferometer section. This allows the orthogonal transmission and coupling modes from the device under test to be injected into the reference and coupling arms of the interferometer, respectively. This suppresses the original main interference peak, resulting in higher sensitivity. Furthermore, this invention uses an auxiliary interferometer to eliminate signal aliasing caused by the nonlinearity of the light source sweep frequency, offering advantages such as small size, reliable and stable system, and high sensitivity. Additionally, this invention utilizes a frequency shifter within the polarization beam-splitting frequency-shifting interferometer module to shift the signal to a higher frequency, avoiding noise aliasing from low frequencies. This meets the requirements for testing polarization crosstalk information of long-distance devices. Summary of the Invention:

[0010] The purpose of this invention is to provide an optical frequency domain polarimeter with ultra-high sensitivity polarization beam splitter frequency shift detection that can improve the sensitivity of polarization crosstalk testing while overcoming low-frequency flicker noise and signal aliasing problems.

[0011] The objective of this invention is achieved through the following measures:

[0012] An ultra-sensitive optical frequency domain polarimeter for frequency shift detection using a polarization beam splitter structure includes a tunable light source module 1, a device under test module 2, a polarization beam splitter interferometer module 3, an auxiliary interferometer module 4, and a signal acquisition and analysis module 5, characterized in that:

[0013] The tunable laser source module 1 emits linearly frequency-modulated continuous light 60, which is injected into the first coupler 61 from the input end 61a and split into two beams. The beam from the first coupler 61 is injected into the device under test module 2 after passing through the first output pigtail 61b of the first coupler. The beam output from the device under test module 2 is injected into the polarization beam splitter interferometer module 3, and then injected into the polarization beam splitter 31 from the input pigtail 30 of the polarization beam splitter, splitting into two orthogonal beams. One beam is injected into the reference arm 32 as a reference beam, and the other beam is injected into the delay arm 33 as a test beam. The reference arm 32 and the delay arm 33 are... The beam output from the delayed arm 33 is combined in the second coupler 35 and then split into two beams, which are then differentially detected by the first balanced detector 36. The beam in the first coupler 61 is injected into the auxiliary interferometer module 4 after passing through the second output pigtail 61c of the first coupler. The polarization beam splitting and frequency shifting interferometer signal 37 output from the polarization beam splitting and frequency shifting interferometer module 3 and the auxiliary interferometer signal 46 output from the auxiliary interferometer module 4 are jointly injected into the acquisition unit 50 in the signal acquisition and analysis module 5 for acquisition. The obtained signal then enters the signal processing unit 51 for data analysis and processing.

[0014] When light is injected into the device under test module 2, it passes through the polarizer input pigtail 20a, polarizer 20, polarizer output pigtail 20b, first connection point 21, device under test input pigtail 23a, second connection point 22, device under test 23, third connection point 24, device under test output pigtail 23b and fourth connection point 25 in sequence.

[0015] When the measured parameter of the device under test 23 includes a high extinction ratio, the fusion angle of the first connection point 21 is θ1 = 45°. When the measured parameter of the device under test 23 is polarization crosstalk at a general coupling point, the fusion angle of the first connection point 21 is θ1 = 0° ± 2° or θ1 = 90° ± 2°. The fusion angle of the fourth connection point 25 is θ2 = 0° ± 2° or θ2 = 90° ± 0°. The optical path difference of the device under test module 2 is X1, and the frequency shift is f1.

[0016] After light is injected into the device under test (DUT), the coupling mode and transmission mode are orthogonal at the input pigtail 30 of the polarization beamsplitter. The polarization beamsplitter 31 completely separates them and injects them into the reference arm 32 and the delay arm 33 of the interferometer, respectively. Due to the presence of the frequency shifter 34 in the delay arm, a certain time delay is generated in the delay arm as light is injected. The polarization beamsplitter 31 and the second coupler 35 are connected to the fifth connection point 32a and the sixth connection point 33a via a flange. The polarization is adjusted by the polarization controller 33b. The polarization state of the beam splitter interferometer is used to obtain a signal with the largest possible amplitude; the optical path difference of the polarization beam splitter interferometer module 3 is X2. After the light passes through the frequency shifter 34, the frequency shift amount f2 is generated. From the optical path difference and time delay correspondence τ2=X2 / c and the time delay amount τ2=1 / f2, we can get f2=c / X2; similarly, we can get the frequency shift amount of the device under test module (2) as f1=c / X1. Then, from X2>2X1, we can know that the relationship between the frequency shift amount introduced by the frequency shifter and the frequency shift amount introduced by the device under test is 2f2<f1;

[0017] The beam injected into the third coupler 40 is split into two beams. One beam is injected into the reference arm 41 as a reference beam, and the other beam is injected into the delay arm 42 as a test beam. The beams are then passed through the test fiber 43. The beams output from the reference arm 41 and the delay arm 42 are combined in the fourth coupler 44 and then split into two beams. The beams are then differentially detected by the second balanced detector 45. The optical path difference of the auxiliary interferometer module 4 is X3, where X3 > 2X2.

[0018] The signal acquisition unit 50 converts the received analog signal auxiliary interferometer signal 46 and polarization beam-splitting interferometer signal 37 into digital signals, and both are input to the signal processing unit 51. The parameter settings of the acquisition software need to be consistent with the hardware parameter settings to achieve complete data acquisition. The data processing unit 51 processes the data by performing nonlinear correction of the light source on the polarization beam-splitting interferometer signal 37 based on the acquired auxiliary interferometer signal 46, filtering out most of the intensity noise inside the frequency sweep light source, extracting the effective information from the corrected signal, and then using Fourier transform to demodulate the position and intensity information of polarization crosstalk in the frequency domain signal, finally obtaining a signal that meets the test requirements.

[0019] The distributed polarization crosstalk measurement process of the polarization beamsplitter interferometer module with polarization beamsplitter 31 is as follows: After light passes through the device under test (DUT) 23, the DUT pigtail 23b carries the polarization crosstalk information inside the DUT. Most of the energy is transmitted in the transmission axis (fast axis) of the device, and a small portion of the energy jumps from the transmission mode to the coupling mode due to the presence of internal defects or coupling points. As the light propagates in the DUT, the waveforms 23c in the two modes will be offset in position, i.e., delay is generated. The output pigtail 23b of the DUT and the input pigtail 30 of the polarization beamsplitter are fused to the fourth connection point 25 by a polarization-maintaining welding machine. In the polarization-maintaining welding machine, the axis angle 30° of the two pigtails is 0°-0°. At this time, the light of the transmission mode and the coupling mode introduced by the DUT can be injected by the polarization beamsplitter 31 into the reference arm 32 and the delay arm 33 of the Mach-Zehnder interferometer, respectively. Let the light transmitted in the reference arm 32 be 31b, and the light transmitted in the delay arm be 31c. After the light transmitted in the delay arm passes through the frequency shifter 34, the optical path difference between it and the reference arm increases, thus significantly increasing the beat frequency effect when transitioning from the distance domain to the wavelength domain, and also significantly increasing the length requirements of the device under test. The two beams of light are finally combined and interfere at the second coupler 35. After differential detection by the first balanced detector 36, the digital signal is acquired by the acquisition card and converted into an analog signal. After processing, the polarization crosstalk coupling peak inside the device under test can be obtained.

[0020] The polarization coupling measurement method calibrated using polarization beamsplitter 31 is used to obtain the relative dynamic range. Traditional polarization crosstalk measurement devices calibrate the coupling peak by normalizing the main peak to obtain dynamic range and sensitivity. Since the main interference peak is suppressed after using polarization beamsplitter 31, we perform two different alignments between the pigtail of the device under test 23 and the pigtail of polarization beamsplitter 31 for calibration. When the alignment angle is 0°-45°, polarization beamsplitter 31 acts as a 50:50 coupler. The information from the device under test is uniformly injected into the fast and slow axes of the fiber after passing through polarization beamsplitter 31. The fast and slow axes in reference arm 32 and delay arm 33 interfere with each other, forming the desired main interference peak. A significant coupling peak appears symmetrically to the left and right of the main interference peak; we select this peak as the calibration peak. Alignment is performed again, using a welding machine to achieve an alignment angle of 0°-0°. From the actual welding length, we infer that the coupling peak next to the original main interference peak still exists, but its amplitude changes. By comparing the data obtained from the pigtail of the device under test and the pigtail of the polarization beam splitter at two different welding angles, the coupling peak obtained at the axial angle of 0°-45° is used to calibrate the coupling peak at the axial angle of 0°-0°. Since the original main interference peak is suppressed after using the polarization beam splitter 31, the phase noise introduced by the main interference peak is also suppressed, thereby greatly improving the sensitivity.

[0021] Compared with the prior art, the advantages of the present invention are as follows:

[0022] 1. This 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 sweep frequency, effectively improving the system signal-to-noise ratio.

[0023] 2. By using the frequency shifter in the polarization beam splitter interferometer module to shift the signal to a higher frequency, the influence of 1 / f flicker noise at low frequencies and the influence of DC component introduced by the acquisition signal system are eliminated. While avoiding the noise aliasing problem from low frequencies, it meets the requirements for testing long-distance devices. The frequency shifter is equivalent to increasing the length of the delay fiber, which can effectively improve the sensing performance of the system.

[0024] 3. This 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 delay 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 coupled modes exists. By using the coupling peak for calibration, the sensitivity of the device can be improved to -130dB, providing more options for testing devices with high extinction ratios.

[0025] 4. This invention utilizes a device for rapid measurement of distributed polarization crosstalk with a polarization beam splitter, overcoming the limitations caused by phase noise. Under the premise of sufficiently high light intensity, the sensitivity can be continuously improved as the light intensity detected by the detector increases. Attached image description:

[0026] Figure 1 This is a schematic diagram of an optical frequency domain polarimeter with an ultra-high sensitivity polarization beam splitter structure for frequency shift detection.

[0027] Figure 2 This is a schematic diagram illustrating the working principle of the polarization beam-splitting interferometer in an optical frequency domain polarimeter with ultra-high sensitivity polarization beam-splitting structure frequency shift detection.

[0028] Figure 3 This is a schematic diagram of an optical frequency domain polarimeter for frequency shift detection using an ultra-sensitive polarization beam splitter structure for Y-waveguides.

[0029] To more clearly illustrate the optical frequency domain polarimeter and method for ultra-high sensitivity polarization beam splitting structure frequency shift detection proposed in this invention, the following description, in conjunction with embodiments and accompanying drawings, further illustrates the invention, but should not be construed as limiting the scope of protection of this invention.

[0030] In a specific embodiment, an optical frequency domain polarimeter provides ultra-high sensitivity frequency shift detection of a polarization beam splitter structure in a Y-waveguide in the optical frequency domain, as shown in the attached figure. Figure 3 As shown:

[0031] It consists of four parts: a tunable laser source module 1, a device under test module 2, a polarization beam splitter frequency shifter interferometer module 3, an auxiliary interferometer module 4, and a signal acquisition and analysis module 5.

[0032] 1) The tunable laser source module 1 outputs linear frequency modulated continuous light 60, which is injected into the first coupler 61 from the input end 61a and split into two beams;

[0033] 2) The beam output from the first output pigtail 61b of the first coupler is injected into the device under test module 2;

[0034] 3) The beam output from the device under test module 2 is injected into the polarization beam splitter frequency shifter module 3. The beam is injected into the polarization beam splitter 31 through the polarization beam splitter input terminal 30 and split into two beams of orthogonal light. One beam is injected into the reference arm 32 as a reference beam, and the other beam is injected into the delay arm 33 as a test beam. The beams output from the reference arm 32 and the delay arm 33 are combined in the second coupler 35 and then split into two beams. Then, differential detection is performed by the first balanced detector 36.

[0035] The beam output from the second output pigtail 61c of the first coupler is injected into the auxiliary interferometer module 4;

[0036] The polarization beam-splitting interferometer signal 37 output from polarization beam-splitting interferometer module 3 and the auxiliary interferometer signal 46 output from 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 consists of acquisition unit 50 and signal processing unit 51. Acquisition unit 50, programmed and controlled by LabVIEW software, synchronously acquires and stores the analog voltage signal obtained from the photoelectric conversion of the detector, converting it into a digital signal. Signal processing unit 51 controls signal generation and reception, as well as processes and demodulates the digital signal.

[0037] The main optoelectronic devices selected and their parameters for this scheme are shown below:

[0038] 1) The tunable laser source module 1 is a narrow linewidth tunable laser capable of continuous wavelength scanning. The wavelength tuning range is 1480-1640nm. When the coherent mode is enabled, the wavelength scanning rate is 40nm / s and the wavelength scanning time is 4s.

[0039] 2) The photosensitive material of the first balanced detector 36 and the second balanced detector 45 is InGaAs, the common-mode rejection ratio is 25dB, the optical detection range is 900~1700nm, the maximum detection bandwidth is 80MHz, the saturated differential detection power is 55uW, the transimpedance gain is 50000V / A, the minimum noise equivalent power is 1A / W, and the peak responsivity is 1A / W. For example, the 1817 balanced detector from NewFocus can be used.

[0040] 3) The 16-bit sampling rate of the acquisition unit 50 is 45MHz, and the sampling time is about 4s. Combined with external triggering by the laser, the sampling time is longer than the wavelength scanning time, and the parameter design is reasonable.

[0041] 4) The splitting ratio of the first coupler 61 is 5:95, and the splitting ratios of the second coupler 35, the third coupler 40, and the fourth coupler 44 are 50:50. The extinction ratios are all greater than 20dB, the insertion losses are all less than 0.5dB, and the operating wavelength covers the 1550nm band. All couplers are single-mode couplers. The output pigtail of the polarization beam splitter is a polarization-maintaining fiber, which is connected to the single-mode input pigtail of the second coupler 35 via a flange at the fifth connection point 32a and the sixth connection point 33a. The three-ring polarization controller 33b is used to control the polarization state of the polarization beam-shifting interferometer to make the signal as stable as possible to obtain a higher dynamic range.

[0042] 5) The polarizer 20 operates in the 1550nm band, with a polarization angle of 0°, insertion loss less than 1dB, and extinction ratio greater than 30dB. The input pigtail 20a is a single-mode fiber, and the output pigtail 20b is a 125µm diameter panda-type polarization-maintaining fiber with a length of l1 = 15m. The corresponding optical path length S1 is calculated to be approximately 7.5 × 10⁻⁶ m. 3 um, where the linear birefringence Δn of the polarization-maintaining fiber is taken as 5×10 -4 ;

[0043] 6) The polarization beam splitter 31 operates in the 1550nm band. The input pigtail 30 is a 125µm diameter panda-shaped polarization-maintaining fiber with a length of l2 = 10m. The corresponding optical path length S2 is calculated to be approximately l2 × Δn ≈ 5 × 10. 3 um, where the linear birefringence Δn of the polarization-maintaining fiber is taken as 5×10 -4 ;

[0044] 7) The welding angle of the first connection point 21 is θ1 = 45°, and the welding angle of the fourth connection point 25 is θ2 = 0°;

[0045] 8) The Y-waveguide 23Y of the device under test uses a lithium niobate crystal as the chip substrate, with an extinction ratio of 90 dB and operates on the fast axis. The geometric length of the Y-waveguide input pigtail 23a is l3 = 1 m, and the diameter is 125 μm. Calculate the corresponding optical path length S3 = l3 × Δn = 5 × 10 2 The geometric length of the Y-waveguide output pigtail 23b is l4 = 2m, and its diameter is 125um. Calculate the corresponding optical path S4 = l4 × Δn = 1 × 10⁻⁶. 3 Given a Y-waveguide chip with length l5 = 0.04 m, calculate the corresponding optical path length S = S3 + S4 + S5 = l3 × Δn + l4 × Δn + l5 × Δn Y ≈5236um, where the linear birefringence Δn of the polarization-maintaining fiber is taken as 5×10. -4 The linear birefringence Δn of the Y-waveguide chip Y Take 9.34 × 10 -2 ;

[0046] 9) The polarization beam splitter interferometer module 3 is a Mach-Zehnder fiber interferometer structure with a frequency shifter in one arm. The optical path difference X2 corresponding to the polarization beam splitter interferometer module 3 in this embodiment is calculated to be 11648000um; the optical path difference X1 corresponding to the device under test module 2 in this embodiment is calculated to be S1+S2+S=17736um, which satisfies X2>2X1, and the parameter design is reasonable.

[0047] 10) The auxiliary interferometer module 4 uses an extended optical fiber 43 as l refGiven a Mach-Zehnder fiber optic interferometer structure with a length of 20m, calculate the optical path difference X3 corresponding to the auxiliary interferometer module 4 in this embodiment. ref ×Δn=29.12×10 6 um, where the refractive index n of the single-mode fiber is 1.456; therefore, X3 > 2X2 is satisfied, and the parameter design is reasonable;

[0048] 11) The position corresponding to the characteristic peak of the extinction ratio interference is |X2+S1+S2+S3+S4+S5|=11665736um;

[0049] 12) After the beam is coupled once through the first connection point 21, the second connection point 22, the third connection point 24, and the fourth connection point 25, the positions of the four first-order polarization crosstalk interference characteristic peaks are |X2+S1|=11655500um, |X2+S1+S3|=11656000um, |X2+S2+S4|=11654000um, and |X2+S2|=11653000um, respectively. Since l1>l2, the position distribution of the interference peaks is |X2+S2|<|X2+S2+S4|<|X2+S1|<|X2+S1+S3|.

[0050] 13) After the beam is coupled twice through the first connection point 21 and the third connection point 24, the first connection point 21 and the fourth connection point 25, the second connection point 22 and the third connection point 24, and the second connection point 22 and the fourth connection point 25, the positions of the four second-order polarization crosstalk interference characteristic peaks are |X2+S1+S2+S4|=11661500um, |X2+S1+S2|=11660500um, |X2+S1+S2+S3+S4|=11662000um, and |X2+S1+S2+S3|=11661000um.

[0051] Excluding the time spent changing the axis angle of the solder joint between the output pigtail of the device under test and the input pigtail of the polarization beam splitter, the duration of one test is approximately 3.95 seconds.

[0052] Those skilled in the art will readily understand 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 within the scope of protection of the present invention.

Claims

1. A high-sensitivity optical frequency domain polarimeter for frequency shift detection using a polarization beam splitter structure, comprising a tunable laser source module (1), a device under test module (2), a polarization beam splitter 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) sends linearly frequency-modulated continuous light (60) into the first coupler (61) from the input end (61a) and splits it into two beams; the beam in the first coupler is injected into the device under test module (2) after passing through the first output end (61b); the beam output from the device under test module (2) is injected into the polarization beam splitter interferometer module (3), and then injected into the polarization beam splitter (31) through the polarization beam splitter input pigtail (30) and split into two beams of orthogonal light. One of the beams is injected into the first reference arm (32) of the polarization beam splitter interferometer module (3) as a reference light, and the other beam is injected into the first delay of the polarization beam splitter interferometer module (3) as a test light. The beams output from the first reference arm (32) and the first delay arm (33) are combined in the second coupler (35) and then split into two beams, which are then differentially detected by the first balanced detector (36). The beam in the first coupler is injected into the auxiliary interferometer module (4) after passing through the second output pigtail (61c) of the first coupler. The polarization beam splitting frequency shifting interferometer signal (37) output from the polarization beam splitting frequency shifting interferometer module (3) and the auxiliary interferometer signal (46) output from the auxiliary interferometer module (4) are jointly injected into the acquisition unit (50) in the signal acquisition and analysis module (5) for acquisition. The obtained signal is then entered into the signal processing unit (51) for data analysis and processing. After the light is injected into the device under test, the coupling mode and the transmission mode are orthogonal at the input pigtail (30) of the polarization beam splitter. The polarization beam splitter (31) completely separates them and injects them into the first reference arm (32) and the first delay arm (33) of the interferometer, respectively. Due to the presence of the frequency shifter (34), the signal can generate a certain time delay, which allows us to obtain a larger beat frequency interference signal without using a delay fiber. At this time, since there are no other modes interfering in a single arm, only the first reference arm (32) and the first delay arm (33) exist. Interference occurs when the two modes reach the first balanced detector (36) after passing through the second coupler (35); the optical path difference of the polarization beam splitter interferometer module (3) is X2, and after passing through the frequency shifter (34), a frequency shift amount f2 is generated. From the optical path difference and time delay correspondence τ2=X2 / c and the time delay amount τ2=1 / f2, we can get f2=c / X2. We can get the frequency shift amount of the device under test module (2) as f1=c / X1. Then, from X2>2X1, we can know that the frequency shift amount introduced by the frequency shifter and the frequency shift amount introduced by the device under test are related as 2f2. <f1。 2. The optical frequency domain polarimeter with ultra-high sensitivity polarization beam splitter structure frequency shift detection as described in claim 1, characterized in that: When light is injected into the device under test module (2), it passes through the device under test module (2) in sequence via the polarizer input pigtail (20a), polarizer (20), polarizer output pigtail (20b), first connection point (21), device under test input pigtail (23a), second connection point (22), device under test (23), third connection point (24), device under test output pigtail (23b) and fourth connection point (25).

3. The optical frequency domain polarimeter with ultra-high sensitivity polarization beam splitter structure frequency shift detection as described in claim 2, characterized in that: When the measured parameter of the device under test (23) includes a high extinction ratio, the welding angle of the first connection point (21) of the device under test module (2) is θ1 = 45° ± 2°. When the measured parameter of the device under test (23) is polarization crosstalk of a general coupling point, the welding angle of the first connection point (21) is θ1 = 0° ± 2° or θ1 = 90° ± 2°. The welding angle of the fourth connection point (25) is θ2 = 0° ± 2° or θ2 = 90° ± 2°.

4. The optical frequency domain polarimeter with ultra-high sensitivity polarization beam splitter structure frequency shift detection as described in claim 1, characterized in that: The beam injected into the third coupler (40) is split into two beams. One beam is injected as a reference beam into the second reference arm (41) of the auxiliary interferometer module (4), and the other beam is injected as a test beam into the second delay arm (42) of the auxiliary interferometer module (4). The beams are then passed through an auxiliary extension fiber (43). The beams output from the second reference arm (41) and the second delay arm (42) are combined in the fourth coupler (44) and then split into two beams. The beams are then differentially detected by the second balanced detector (45). The optical path difference of the auxiliary interferometer module (4) is X3, where X3 > 2X2.

5. The optical frequency domain polarimeter with ultra-high sensitivity polarization beam splitter structure frequency shift detection according to claim 1, characterized in that: The signal acquisition and analysis module (5) includes an acquisition unit (50) and a signal processing unit (51). The acquisition unit (50) converts the received analog signal-assisted interferometer signal (46) and polarization beam splitter frequency shifter signal (37) into digital signals, which are then input into the signal processing unit (51). The signal processing unit (51) processes the data by performing nonlinear phase correction of the light source on the polarization beam-splitting interferometer signal (37) based on the acquired auxiliary interferometer signal (46), performing nonlinear correction on the signal obtained from the long-distance test using a deskewing filtering algorithm, and then using Fourier transform to demodulate the position and intensity information of the polarization crosstalk in the frequency domain signal, finally obtaining a signal that meets the test requirements.

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