A Noise-Suppressed Loop-Reference-Structure Frequency-Division-Multiplexed Optical Frequency Domain Reflectometer

Abstract

The present invention provides a noise-suppressed, loop-reference-structure frequency-division-multiplexed optical frequency domain reflectometer, belonging to the field of optical fiber testing. The device comprises a tunable laser source module, a main interferometer test module, an auxiliary interferometer module, a signal acquisition and analysis module, and a ring cavity frequency-division-multiplexing module. The device is characterized in that the ring cavity frequency-division-multiplexing module employs a ring cavity structure. After light is input into the ring cavity, a portion of the light is directly output, while the other portion circulates in the ring cavity before being output. Multiple cycles generate reference light with different time delays, which interfere with the test light. In a single laser scan, multiple groups of beat frequency signals are collected. After frequency sweep nonlinearity correction and frequency domain conversion, signal intensity correction and segmented interception are performed to achieve frequency division multiplexing of the optical signal in a single scan. After averaging processing, the signals are integrated into a group, effectively improving the sensitivity of optical frequency domain reflectometry measurements of short-distance optical fiber devices, suppressing noise influences, and maintaining a compact structure.

Description

Technical Field

[0001] The invention belongs to the technical field of distributed optical fiber measurement, and in particular relates to an optical frequency domain reflectometer with a noise-suppressed loop reference structure and frequency division multiplexing. Background Art

[0002] Optical reflectometry is a fiber optic detection technology that uses the principles of fiber backscattering, such as Rayleigh scattering, Brillouin scattering, and Raman scattering. Currently, Rayleigh scattering is primarily used for testing and sensing fiber optic systems. One type of optical reflectometry technology is optical time domain reflectometry (OTDR), which uses optical pulse detection. OTDR technology boasts long fiber detection distances, typically reaching tens of kilometers, and a simple system structure. Commercial products are already available. The spatial resolution of OTDR technology depends on the width of the laser pulse; the narrower the pulse, the higher the spatial resolution. Laser performance and the broadening effect of optical fiber transmission are the main limitations of OTDR technology, resulting in a spatial resolution of approximately meters.

[0003] To this end, Optical Frequency Domain Reflectometry (OFDR) technology was proposed. It spans the fields of fiber optic sensing, testing, and communications, and is used to test a variety of high-performance devices. OFDR uses swept-frequency light to scan fiber systems, measuring Rayleigh scattered light within the device under test or the fiber system. This technology obtains distributed amplitude and phase information about the device under test and extracts environmental variations at various locations in the fiber system, such as temperature and strain, enabling distributed testing and sensing.

[0004] In recent years, researchers have conducted extensive research on improving the structure and actual test performance of optical frequency domain reflectometer devices, focusing on parameters such as measurement sensitivity and signal-to-noise ratio. They have also adopted a variety of measurement devices and methods to improve the measurement sensitivity of OFDR devices.

[0005] Sensitivity can be derived from the intensity of the noise floor behind the signal, representing the ability of fiber optic detection technology to detect minimum signal intensity information. It is related to the noise of the collected signal. Suppressing signal noise can effectively improve the sensitivity of the test. In the optical frequency domain reflectometry test, high sensitivity represents the high-precision detection of Rayleigh scattered light by the fiber optic system, which can effectively obtain various phase frequency, amplitude frequency, and polarization characteristics inside the device under test. It can be effectively applied to high-sensitivity test systems such as strain testing and temperature testing. In 2008, the literature reported the use of a single-sideband modulator to modulate a narrow-linewidth laser to obtain a linear swept-frequency light source with a wide range, realizing high-sensitivity testing of optical frequency domain reflectometry [J. Lightwave Technol. 6, 3287-3294 (2008)]. This solution has now become the mainstream choice for externally modulated OFDR systems. In 2014, a literature report proposed the use of phase optical frequency domain reflectometry (Phase-OFDR) technology to achieve high spatial resolution strain positioning with a sensitivity of -40dB [Optics Express 25(22): 27913(2017)].

[0006] After searching existing technologies, researchers have proposed various optical frequency domain reflectometry devices to improve the accuracy and sensitivity of optical frequency domain reflectometry testing. In 2016, He Zuyuan et al. from Shanghai Jiao Tong University employed electro-optical modulators and acousto-optic modulators, using optical comb signals for frequency sweeping and performing frequency synthesis in the digital domain to increase the frequency sweep range (Optical Frequency Domain Reflectometry Method and System Based on Frequency Synthesis, 201610004146.2), thereby improving the spatial resolution of the system. In 2018, Shi Yi et al. from Shantou University connected an electro-optical modulator to the OFDR main interferometer. The laser performed multiple sweeps, shifting the beat frequency signal to different frequency positions and performing frequency domain segmented splicing (A Distributed Fiber Optic Sensing System and Sensing Method Based on Multi-Frequency OFDR, 201810554954.5), which can increase the length of the test fiber. In 2020, inventors Wang Huiwen and others used a master interferometer reference arm with switchable optical path lengths to perform multiple light source sweeps. Before each sweep, they switched the reference arm optical switch to match the length of each segment of the device under test and perform distance domain splicing (Apparatus and Method for Long-Distance Measurement Using OFDR Segmented Acquisition, 202010401588.7). This approach enables long-distance sensing and improved accuracy. In 2021, Peng Xin and others from Beijing University of Posts and Telecommunications established multiple master interferometers and processed the beat frequency signals in parallel (A Multi-Channel Fiber Optic Sensing System Based on OFDR Technology, 202120686441.7), enabling simultaneous measurement of changes in sensing quantities across multiple fiber segments. In 2021, Yu Zhangjun and others from Guangdong University of Technology used optical frequency domain polarization measurement technology to collect transmitted light by setting up multiple independent main interferometer modules to measure distributed polarization crosstalk (a distributed polarization crosstalk measurement device based on optimal frequency shift interference in the optical frequency domain 202110817547.0), achieving long-distance device testing while maintaining test sensitivity.

[0007] However, existing optical frequency domain reflectometry device solutions still face some issues that limit measurement sensitivity. For example, during each frequency sweep of a tunable laser in optical path testing, the nonlinearity of the light source sweep and external environmental noise can cause differences in each set of measurement data, making it difficult to accurately splice multiple sets of data to increase test length and sensitivity. For example, when using acousto-optic modulators and electro-optic modulators to generate optical comb signals for frequency shifting and synthesis, the external modulator introduces additional electrical noise and sidebands, making the measurement results susceptible to the residual sidebands of the electro-optic modulator. Furthermore, filtering out high-order sidebands increases optical power loss, reducing test sensitivity. For example, using an optical amplifier for amplification introduces additional phase noise, resulting in performance degradation. When using a main interferometer with switchable optical path for measurement, the polarization state and sweep speed of the laser source fluctuate during each frequency sweep, making them susceptible to external environmental noise and unable to improve the sensitivity of optical frequency domain reflectometry.

[0008] The present invention introduces a ring cavity structure into the reference arm of the main interferometer. Under a single scan of a tunable light source, the reference lights with different time delays generated after different cycles of the ring cavity respectively perform beat interference with the test light, thereby obtaining multiple discrete beat signals in the frequency domain, realizing frequency division multiplexing in the acquisition bandwidth, and compounding the beat signal frequency bandwidth into the acquisition bandwidth. The multiple beat signals exist at different positions in the frequency domain and do not overlap with each other, realizing frequency division multiplexing of each sub-frequency signal, and achieving the purpose of transmitting multiple signals simultaneously in one acquisition channel.

[0009] Based on improvements to existing technologies, the present invention discloses a noise-suppressed, loop-reference-structure frequency-division-multiplexed optical frequency domain reflectometer. A single light source wavelength scan is performed in the optical frequency domain to test the information of the optical fiber to be tested. Light circulates N-1 times through a ring cavity structure formed by connecting delayed optical fibers at both ends of a coupler, generating N reference lights with different time delays. The test light interferes with the N reference lights, generating N beat signals at different locations in the frequency domain. Each signal carries complete information about the device to be tested, and each beat signal is located at a different frequency domain location within the acquisition bandwidth, enabling the simultaneous transmission of multiple beat signals within a single acquisition channel. The signal is then Fourier transformed into the frequency domain, corrected for signal strength, segmented by frequency, and processed using an averaging algorithm. This achieves frequency division multiplexing of the optical signal in a single scan, expands the utilization of the channel bandwidth, suppresses noise in the optical frequency domain reflectometry system and external environmental noise, and achieves higher sensitivity in the optical frequency domain reflectometry. In addition, the present invention adopts an optical fiber coupler, the optical fiber signal loss to be measured in the optical path is low and is not affected by the noise of the electrical modulator, and has the characteristics of compact structure, stable system, easy implementation, etc. Summary of the Invention

[0010] The present invention aims to provide an optical frequency domain reflectometer with a noise-suppressed loop reference structure and frequency-division multiplexing, which can obtain multiple beat frequency signals from a single frequency sweep of a light source based on the principle of optical frequency domain reflectometry, perform frequency division multiplexing, simplify the optical path structure and eliminate the influence of additional electrical noise, suppress the test system noise and external environmental noise after signal acquisition and analysis, and improve the test sensitivity.

[0011] The purpose of the present invention is to be achieved through the following measures:

[0012] An optical frequency domain reflectometer with a noise-suppressed loop reference structure and frequency division multiplexing includes a tunable laser source module 1, a main interferometer test module 2, an auxiliary interferometer module 3, a signal acquisition and analysis module 4, and a loop reference structure module 5, and is characterized by:

[0013] The linearly modulated light 101 output by the tunable laser source module 1 is injected into the first coupler 601 from the first coupler input end 601a and is split into two light beams for output;

[0014] One of the light beams is output to the main interferometer test module 2 through the second output port 601c of the first coupler, and then split into two light beams through the second coupler 201. One of the light beams is injected into the three-port circulator 202 along the first output port 201b of the second coupler, output from the second port 202b of the three-port circulator and injected into the device under test 203. The generated Rayleigh scattered light propagates in the reverse direction, is re-input from the second port 202b of the three-port circulator, and is emitted from the third port 202c of the three-port circulator to obtain test light 203b. The other light beam output from the second coupler 201 enters the loop reference structure module 5, and the light beam is output from the third coupler. The first input end 501a of the combiner inputs the third coupler 501 and the first delay fiber 502. The light passes through the fourth coupler 503, with a portion of the light returning to the second input end 501b of the third coupler, and the other portion of the light output from the second output end 503c of the fourth coupler. The light circulates multiple times in the ring cavity to generate a first reference light 504a, a second reference light 504b, ..., and an Nth reference light 504c. The reference light and the test light 203b then pass through the fifth coupler 204 for beam combining and splitting. The output optical signal is detected by the second photoelectric balanced detector 205, realizing the conversion of the optical signal into an electrical signal.

[0015] The other light beam output by the first coupler 601 passes through the first output end 601b of the first coupler and is output to the auxiliary interferometer module 3. It is then split into two light beams by the sixth coupler 301. One of the light beams passes through the first output end 301b of the sixth coupler as intrinsic light, and the other light beam is emitted from the second output end 301c of the sixth coupler and passes through the second delay fiber 302 as delayed light. The two light beams are then combined and split by the seventh coupler 303. The output optical signals are detected by the first photoelectric balanced detector 304, thereby converting the optical signals into electrical signals.

[0016] The signal of the main interferometer test module 2 is input into the first acquisition unit 401 and passes through the bandpass filter 403 to obtain the first acquisition signal 403a. The signal of the auxiliary interferometer module 3 is input into the second acquisition unit 402 to obtain the second acquisition signal 402a. Then, in the light source nonlinear correction unit 404, the second acquisition signal 402a is used to correct the light source frequency sweep nonlinearity of the first acquisition signal 403a. Then, the spectrum analysis unit 405 performs time-frequency conversion to a frequency domain signal 405a, and then performs intensity correction 406. The frequency intensity of each beat signal in the acquisition signal is processed by a compensation algorithm, and the first beat signal 407a is selected. The circulator reflection peak intensity is used as a standard calibration point, and the circulator reflection peaks of the remaining beat signals are normalized according to the calibration point. Before segmentation, each beat signal reflection peak is compensated to the same intensity. Then, the segmentation interception unit 407 locates the position of each beat signal and uses the circulator reflection peak of each beat signal as the starting point of the segmentation. The collected signal is evenly spaced and frequency signals are intercepted, separating it into a first beat signal 407a, a second beat signal 407b, ..., and an Nth beat signal 407c. Then, the average calculation unit 408 performs an average filtering algorithm on the obtained N beat signals and integrates them into a set of averaged results.

[0017] The length of the second delay fiber 302 of the auxiliary interferometer module 3 is l0, corresponding to an optical path length L0. The optical path difference between the two arms of the interferometer is ΔL0=L0, which is required to satisfy 5ΔL0≥ΔL1.

[0018] The device under test 203 is a short-distance optical fiber device, and its length l2≤20m.

[0019] The first acquisition unit 401 and the second acquisition unit 402 of the signal acquisition and analysis module 4 have a sampling time of t and a sampling rate of f. s , the number of sampling points is M, according to the sampling theorem, it is required to satisfy f s ≥2max(f bx ).

[0020] The light source nonlinearity correction unit 404 of the signal acquisition and analysis module 4 uses the second acquisition signal 402a as a calibration to compensate for the light source nonlinear fluctuations in the first acquisition signal 403a. Through time domain stretching and contraction, the light source nonlinearity correction unit 404 achieves equal optical frequency interval acquisition of the main interferometer signal, eliminating the influence of the light source nonlinearity correction.

[0021] The average calculation unit 408 performs an average value algorithm on the frequency domain signal 405a obtained by the segmentation interception unit 407. The frequency domain signal has a total of f b1 、f b2 、……、f bN There are N groups of effective beat frequency signals, according to f b1The center peak position and amplitude of the beat frequency are normalized, and then the peak value of each group of segmented data is processed by the average value filter algorithm on the obtained N groups of beat frequency data. The position of each point of each group of frequency signals is read and the arithmetic average is performed to synthesize the intensity of each point of the signal. z represents the horizontal coordinate position of the data after each segmentation, and finally the output signal results are graphed and analyzed.

[0022] The principle and process of performing optical frequency domain reflectometry on the device under test 203 are as follows:

[0023] The length of the device under test 203 in the main interferometer test module 2 is l2, corresponding to a round-trip optical path of L2=2nl2. The loop reference structure module 5 is composed of a third coupler 501, a first delay fiber 502, and a fourth coupler 503. The light generates reference lights with different delays after multiple cycles. The optical path difference between the test light 203b and the reference lights with different delays is ΔL1=|xL1-L2|, where x represents the number of times the light circulates in the ring cavity structure, and x=0, 1, 2, 3..., N-1. The frequency of the generated beat signal is f b(x+1) =2γΔL1 / c=2γ|xL1-L2| / c, f x represents the beat signal frequency of the reference light and the test light 203b after x cycles, and the beat signal frequencies are f b1 、f b2 、……、f bN The optical path L1 of the first delay fiber 502 and the optical path L2 of the device under test 203 are required to satisfy |(x-1)L1-L2|+L2<|xL1-L2|;

[0024] The scanning test process of the present invention is shown in the attached Figure 2 As shown, the time delay caused by the first delay fiber 502 is τ, and the time delays of the first reference light 504a, the second reference light 504b, ..., and the Nth reference light 504c are τ1=0, τ2=1τ, ..., τ N =(N-1)τ, let x = 0, 1, ..., (N-1), the interference beat signal frequency can be expressed as f bx =2γΔL1 / c=γτ x , delay τ x =2ΔL1 / xc=f bx / γ, where n is the refractive index of the single-mode fiber, γ is the scanning speed, and interference beat signals are generated between different reference lights and test lights 203b. The frequencies of the beat signals can be expressed as f b1 =γτ, f b2 =γ(τ1-τ),......,f bN =γ(τ N-1 -τ);

[0025] The data processing process is as follows Figure 3 As shown, the collected signal is converted into a frequency domain spectrum through Fourier transform, which will generate beat frequency signals at N different positions. The bandpass filter 403 is used to filter out unnecessary signals, and the second collected signal 402a is used to perform light source nonlinear correction, and then intensity correction and frequency domain segmentation are performed, and then the average algorithm is processed to accurately obtain the information carried by the optical fiber of the device under test, and improve the sensitivity of optical frequency domain reflection and test accuracy.

[0026] Compared with the prior art, the advantages of the present invention are:

[0027] (1) The present invention is an optical frequency domain reflectometer with a noise-suppressed loop reference structure and frequency division multiplexing. Based on the principle of optical frequency domain reflection measurement, a tunable laser outputs linear frequency sweep light, and a ring cavity structure is adopted in the reference arm of the main interferometer. The light passes through the ring cavity for different times and is output in a cycle to obtain reference light with different time delays. Multiple reference lights interfere with the test light respectively, and multiple groups of beat frequency signals are obtained in a single scan, and are frequency-division multiplexed in the bandwidth of the same acquisition signal. Each group of beat frequency signals carries complete information of the device to be tested. The acquisition signal of the frequency division multiplexed is intercepted and normalized in equal distance segments, and is processed by an average value filtering algorithm. It can more accurately and effectively characterize the internal characteristics of the device, improve the sensitivity of optical frequency domain reflection measurement, and has lower insertion loss, compact structure, reliable test system and good stability.

[0028] (2) The present invention utilizes a coupler and an extended optical fiber to form a ring cavity structure in the main interferometer, thereby realizing multiple representations of the information of the device under test in the collected signal under a single scan, overcoming the influence of electrical noise and large optical loss introduced by the use of an electrical modulator and an acousto-optic modulator, operating within the effective wavelength range of the coupler and the optical fiber device, reducing the increase in optical path insertion loss caused by the wavelength offset of the light source, and effectively realizing the improvement of measurement sensitivity;

[0029] (3) The present invention adopts a circulator structure composed of a coupler and a delay fiber, and uses it as the reference arm of the main interferometer. After interfering with the test light of the main interferometer, multiple interference beat frequency signals are obtained. The intensity of the signal output by the auxiliary interferometer is measured, and then the data is intensity corrected, segmented and intercepted, and averaged filtered. This effectively suppresses the influence of frequency scanning nonlinearity, internal noise of the system, and external environmental noise. It can fuse multiple groups of optical fiber data to be tested, and realize the suppression of optical frequency domain reflection measurement noise and the optimization of optical frequency domain reflection measurement signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a summary diagram of the optical frequency domain reflectometer with frequency division multiplexing of the loop reference structure with noise suppression;

[0031] Figure 2 This is a schematic diagram of the principle of optical frequency domain reflectometry based on frequency division multiplexing of a loop reference structure;

[0032] Figure 3 It is a schematic diagram of the data processing process of the collected signal;

[0033] Figure 4 This is a schematic diagram of an optical frequency domain reflectometry device for an optical fiber to be tested;

[0034] Figure 5 It is a schematic diagram of the specific structure of the optical fiber to be tested. DETAILED DESCRIPTION

[0035] For a clearer explanation, the present invention proposes a noise-suppressed loop reference structure frequency division multiplexing optical frequency domain reflectometer, which is further described with reference to examples and drawings, but the scope of protection of the present invention should not be limited thereto.

[0036] Specific implementation method, a noise suppression loop reference structure frequency division multiplexing optical frequency domain reflectometer to measure the optical fiber 203A, as shown in the attached Figure 4 shown.

[0037] Its structure consists of a tunable laser source module 1, a main interferometer test module 2, an auxiliary interferometer module 3, a signal acquisition module 4 and a loop reference structure module 5, wherein:

[0038] The tunable laser source module 1 outputs linearly modulated light 101, which is injected into the first coupler 601 from the first coupler input end 601a and is divided into two light beams for output;

[0039] Assume that the number of effective beat frequency signals N=3, set the splitting ratio parameters of the third coupler 501 to a=50, b=50, and the parameters of the fourth coupler 503 to c=90, d=10;

[0040] One of the beams is output to the main interferometer test module 2 through the second output port 601c of the first coupler, and then split into two beams through the second coupler 201. One beam is injected into the three-port circulator 202 along the first output port 201b of the second coupler, output from the second port 202b of the three-port circulator and injected into the optical fiber 203A to be tested. The generated Rayleigh scattered light propagates in the reverse direction, re-enters the second port 202b of the three-port circulator, and is emitted from the third port 202c of the three-port circulator to obtain the test light 203b. The test light 203b is then injected into one arm of the fifth coupler 204. The other beam output from the second coupler 201 enters In loop reference structure module 5, a light beam is input from the first input end 501a of the third coupler into the third coupler 501 and the first delay fiber 502. The light passes through the fourth coupler 503, with a portion of the light returning to the second input end 501b of the third coupler, and the remaining portion of the light being output from the second output end 503c of the fourth coupler. The light circulates multiple times in the ring cavity to generate a first reference light 504a, a second reference light 504b, and a third reference light 504d. The reference light and the test light 203b are then combined and split by the fifth coupler 204. The output optical signal is detected by the second photoelectric balanced detector 205, converting the optical signal into an electrical signal.

[0041] The other light beam output by the first coupler 601 is output to the auxiliary interferometer module 3 through the first output terminal 601b of the first coupler. It is then split into two light beams by the sixth coupler 301. One light beam passes through the first output terminal 301b of the sixth coupler to obtain intrinsic light, and the other light beam is emitted from the second output terminal 301c of the sixth coupler and passes through the second delay fiber 302 to obtain delayed light. The two light beams are combined and split by the seventh coupler 303. The output optical signals are detected by the first photoelectric balanced detector 304 to convert the optical signals into electrical signals.

[0042] The signal acquisition and analysis module 4 includes a signal input from the main interferometer test module 2 into the first acquisition unit 401 and passes through a bandpass filter 403 to obtain a first acquisition signal 403a. The signal from the auxiliary interferometer module 3 is input into the second acquisition unit 402 to obtain a second acquisition signal 402a. Then, in the light source nonlinear correction unit 404, the second acquisition signal 402a is used to correct the light source frequency sweep nonlinearity of the first acquisition signal 403a. Then, the spectrum analysis unit 405 converts the signal into the frequency domain and performs intensity correction 406 to correct the beat frequency signal in the acquisition signal. The circulator reflection peak intensity of the first beat signal 407a is selected as the standard calibration, and the circulator reflection peaks of the remaining beat signals are normalized according to the calibration point. Before the segmented processing, the reflection peaks of each beat signal are compensated to the same intensity. Then, the segmented interception unit 407 intercepts the signals in segments according to the signal frequency to obtain the first beat signal 407a, the second beat signal 407b, and the third beat signal 407d. The signals are then input into the average calculation unit 408 for algorithmic averaging and filtering. After the optical frequency domain reflectometry is completed, the process ends 409.

[0043] The selection and parameters of the main optoelectronic devices in this solution are as follows:

[0044] 1) The tunable laser source module 1 is a narrow-linewidth tunable laser capable of continuous wavelength scanning. The wavelength tuning range is set to 1540-1550nm, the wavelength scanning rate is 10nm / s, and the wavelength scanning time is 1s;

[0045] 2) The maximum detection bandwidth of the photodetector is 80MHz, the saturated differential detection power is 55uW, and the common mode rejection ratio is 25dB;

[0046] 3) The acquisition card has a 16-bit sampling bit, a sampling rate of 200MHz, 200M sampling points, a sampling time of 1s, and a trigger mode of software triggering using Labview;

[0047] 4) The first coupler 601 has a splitting ratio of 1:99, wherein the first output end 601b of the first coupler outputs 1% of the light, and the second output end 601c of the first coupler outputs 99% of the light. The extinction ratio of the coupler is greater than 20dB, the insertion loss is less than 1dB, and the operating wavelength covers the 1550nm band;

[0048] 5) The operating wavelength of the three-port circulator 202 is 1550 nm, the insertion loss is 0.5 dB, and the isolation is greater than 60 dB;

[0049] 6) As attached Figure 5As shown, the optical fiber 203A to be tested is composed of a first single-mode optical fiber 71, a second single-mode optical fiber 72, a connection point 701, and an optical fiber end face 702. The total length of the optical fiber is 2 m, the coil radius is 6 cm, and the round-trip optical path of the optical fiber 203A to be tested is L3 = 5.824 m. The refractive index of the single-mode optical fiber is n = 1.456. Light enters from the input end 701a of the optical fiber to be tested, is scattered by the first connection point 701 of the optical fiber to be tested and the optical fiber end face 702, and forms a reflection peak.

[0050] 7) The length of the first delay fiber 502 of the reference arm of the main interferometer is l1 = 10 m, corresponding to an optical path L1 = 14.56 m;

[0051] 8) The auxiliary interferometer module 3 uses a second delay optical fiber 302 with a length of l0=20m, corresponding to an optical path of L0=29.12m. The optical path difference between the two arms of the auxiliary interferometer in this embodiment is calculated to be ΔL0=29.12m.

[0052] 8) Main interferometer test module 2, wherein the swept frequency optical signal enters the optical fiber to be tested 203A after passing through the second port 202b of the three-port circulator, and its scattered light returns along the original path and is emitted from the third port 202c of the three-port circulator. The beat frequency generated by the interference between the test light 203b and the first reference light 504a is f b1 =2γΔL1 / c, the beat frequency generated by the test light 203b and the second reference light 504b is f b2 =2γΔL2 / c, the test light 203b and the third reference light 504d generate a beat frequency of f b3 =2γΔL3 / c;

[0053] The optical frequency domain reflectometer with frequency division multiplexing of the loop reference structure can achieve the effect of frequency division multiplexing in the collected signal. The beat frequency signals are not aliased and all carry complete information of the optical fiber 203A to be tested.

[0054] The segmented interception unit 407 first performs a normalization algorithm on the frequency intensity of each beat signal. The calibration point selects the circulator reflection peak of the first beat signal 407a, and the circulator reflection peaks of the remaining beat signals are normalized according to the calibration point. Then, the segmented interception is performed according to a fixed frequency domain interval to obtain three groups of beat signals.

[0055] The average calculation unit 408 performs an average value algorithm on the first beat frequency signal 407a, the second beat frequency signal 407b, and the third beat frequency signal 407d obtained by the segmented interception unit 407, reads the position of each point of each group of frequency signals, and performs arithmetic averaging to synthesize the intensity of each point of the signal. z represents the horizontal coordinate position of the data after each segmentation, and finally the signal results are graphed and analyzed.

[0056] 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 noise-suppressed loop reference structure frequency division multiplexing optical frequency domain reflectometer, comprising a tunable laser source module (1), a main interferometer test module (2), an auxiliary interferometer module (3), a signal acquisition and analysis module (4) and a loop reference structure module (5), characterized in that: When the tunable laser source module (1) performs a single light source frequency sweep, the light passes through the second coupler (201) and is divided into two beams of light, one of which is input into the device under test (203) and the three-port circulator (202) and is scattered back as test light (203b), and the other beam of light enters the loop reference structure module (5), and the light beam is input into the third coupler (501) from the first input end (501a) of the third coupler and passes through the fourth coupler (503), wherein a portion of the light passes through the first delay fiber (50 2) returns to the second input end (501b) of the third coupler, and the other part of the light is output from the second output end (503c) of the fourth coupler. The light circulates multiple times in the ring cavity to generate the first reference light (504a), the second reference light (504b), ..., the Nth reference light (504c), which interfere with the test light (203b) respectively to obtain beat signals with non-overlapping frequency positions. Under a single light source frequency sweep, the beat signals are multiplexed in the same acquisition signal, and then the signal acquisition and analysis module (4) is connected The signal is received and time-frequency conversion is performed, intensity correction (406), and the frequency intensity of each beat signal in the collected signal is processed by a compensation algorithm. Then, the segmented interception unit (407) intercepts the frequency signal at equal intervals and separates it into a first beat signal (407a), a second beat signal (407b), ..., and an Nth beat signal (407c). Then, the average operation unit (408) performs an average value filtering algorithm on the obtained N beat signals and integrates them into a group of averaged results to improve the sensitivity and test accuracy of optical frequency domain reflection measurement. The loop reference structure module (5) is composed of a third coupler (501), a first delay optical fiber (502) and a fourth coupler (503). The light is circulated for multiple times to generate reference light with different time delays. The optical path difference between the test light (203b) and the reference light with different time delays is ΔL1=|xL1-L2|, where x represents the number of times the light circulates in the ring cavity structure, and x=0,1,2,3...,N-1. The frequency of the generated beat signal is f b(x+1) =2γΔL1 / c=2γ|xL1-L2| / c, f x represents the frequency of the beat signal generated by the reference light and the test light (203b) after x cycles, and the beat signal frequencies are f b1 、f b2 、……、f bN The optical path of the first delay fiber (502) of the loop reference structure module (5) is L1, and the optical path of the device under test (203) is L2, which is required to satisfy |(x-1)L1-L2|+L2<|xL1-L2|.

2. The noise-suppressed loop reference structure frequency division multiplexing optical frequency domain reflectometer according to claim 1, characterized in that: The linearly modulated light (101) output by the tunable laser source module (1) is injected into the first coupler (601) from the first coupler input end (601a) and is divided into two light beams for output; One of the beams of light is output to the main interferometer test module (2) through the second output end (601c) of the first coupler, and then is divided into two beams of light through the second coupler (201). One of the beams of light is injected into the three-port circulator (202) along the first output end (201b) of the second coupler, output from the second port (202b) of the three-port circulator and injected into the device under test (203). The generated Rayleigh scattered light propagates in the reverse direction, is re-input from the second port (202b) of the three-port circulator, and is emitted from the third port (202c) of the three-port circulator to obtain the test light (203b). The test light (203b) is then injected into one arm of the fifth coupler (204). The other beam of light output from the second coupler (201) enters the loop parameter. Considering the structural module (5), a light beam is inputted from the first input end (501a) of the third coupler into the third coupler (501) and the first delay optical fiber (502), and the light passes through the fourth coupler (503), wherein a portion of the light returns to the second input end (501b) of the third coupler, and the other portion of the light is outputted from the second output end (503c) of the fourth coupler. The light circulates multiple times in the ring cavity to generate a first reference light (504a), a second reference light (504b), ..., an Nth reference light (504c), and then the reference light and the test light (203b) are combined and split through the fifth coupler (204), and the output optical signal is detected by the second photoelectric balance detector (205), thereby realizing conversion of the optical signal into an electrical signal. Another beam of light output by the first coupler (601) is output to the auxiliary interferometer module (3) through the first output end (601b) of the first coupler, and is split into two beams of light through the sixth coupler (301), one beam of light passing through the first output end (301b) of the sixth coupler to obtain intrinsic light, and the other beam of light is emitted from the second output end (301c) of the sixth coupler and passes through the second delay optical fiber (302) to obtain delayed light. The two beams of light are combined and split through the seventh coupler (303), and the output optical signal is detected by the first photoelectric balance detector (304), thereby converting the optical signal into an electrical signal. The output signal of the main interferometer test module (2) enters the first acquisition unit (401) of the signal acquisition and analysis module (4), the signal of the auxiliary interferometer module (3) enters the second acquisition unit (402) and performs light source nonlinear correction on the main interferometer acquisition signal, the first acquisition signal (403a) passes through the bandpass filter (403), the light source nonlinear correction unit (404), the spectrum analysis unit (405), the intensity correction (406), the segmented interception unit (407) and the mean value operation unit (408) in sequence, and ends after the optical frequency domain reflection test is completed (409).

3. The noise-suppressed loop reference structure frequency division multiplexing optical frequency domain reflectometer according to claim 2, characterized in that: The device (203) to be tested of the main interferometer test module (2) is a short-distance optical fiber device with a length l2≤20m, and a corresponding round-trip optical distance L=2nl2, where n is the refractive index of the single-mode optical fiber.

4. The noise-suppressed loop reference structure frequency division multiplexing optical frequency domain reflectometer according to claim 2, characterized in that: The length of the second delay optical fiber (302) of the auxiliary interferometer module (3) is l0, and the corresponding optical path is L0. The optical path difference between the two arms of the interferometer is ΔL0=L0, which is required to meet 5ΔL0≥ΔL1.

5. The noise-suppressed loop reference structure frequency division multiplexing optical frequency domain reflectometer according to claim 2, characterized in that: A signal acquisition and analysis module (4) is provided, wherein a signal of a main interferometer test module (2) is input into a first acquisition unit (401) and passes through a bandpass filter (403) to obtain a first acquisition signal (403a); a signal of an auxiliary interferometer module (3) is input into a second acquisition unit (402) to obtain a second acquisition signal (402a); then, in a light source nonlinear correction unit (404), the second acquisition signal (402a) is used to correct the light source frequency sweep nonlinearity of the first acquisition signal (403a); then, a spectrum analysis unit (405) performs time-frequency conversion into a frequency domain signal (405a); a segmented interception unit (407) normalizes the acquisition signal according to the reflection peak intensity of the first beat frequency signal as a standard; then, a frequency signal is intercepted at equal intervals to separate the signal into a first beat frequency signal (407a), a second beat frequency signal (407b), ..., and an Nth beat frequency signal (407c); then, a mean value operation unit (408) performs mean value filtering algorithm processing on the obtained N beat frequency signals to integrate them into a group of averaged results.

6. The noise-suppressed loop reference structure frequency division multiplexing optical frequency domain reflectometer according to claim 5, characterized in that: The light source nonlinearity correction unit (404) of the signal acquisition and analysis module (4) uses the second acquisition signal (402a) as calibration to compensate for the light source nonlinear fluctuations of the first acquisition signal (403a), and realizes equal optical frequency interval acquisition of the main interferometer signal through time domain stretching and contraction, thereby correcting the light source nonlinearity.

7. The noise-suppressed loop reference structure frequency division multiplexing optical frequency domain reflectometer according to claim 5, characterized in that: The intensity correction (406) of the signal acquisition and analysis module (4) performs compensation algorithm processing on the frequency intensity of each beat signal in the acquired signal, selects the circulator reflection peak intensity of the first beat signal (407a) as the standard peak calibration, normalizes the circulator reflection peaks of the remaining beat signals according to the standard peak, and compensates each beat signal reflection peak to the same intensity before segmentation processing.

8. The noise-suppressed loop reference structure frequency division multiplexing optical frequency domain reflectometer according to claim 5, characterized in that: The mean value calculation unit (408) of the signal acquisition and analysis module (4) performs mean value algorithm processing on the first beat frequency signal (407a), the second beat frequency signal (407b), ..., and the Nth beat frequency signal (407c) obtained by the segmented interception unit (407), performs mean value processing on each position value of the signal, and the synthesized signal can be expressed as the average of multiple beat frequency signals. Finally, the signal result is image-drawn and analyzed.

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