A noise-suppressed frequency-division multiplexed optical frequency domain reflectometer with multiple reference arms

Abstract

The present invention provides a noise-suppressed, multi-reference arm frequency-division multiplexing 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 multi-reference arm frequency-division multiplexing module. The device is characterized in that reference arms of varying lengths are constructed in the main interferometer test module, and light passing through the multiple reference arms generates reference lights of varying time delays, which interfere with the test light to obtain multiple groups of beat signals, each with independent frequencies and no crosstalk. These signals are synthesized into the same acquisition signal, achieving frequency-division multiplexing of multiple beat signals in a single scan. After frequency sweep nonlinearity correction, the acquisition signal is transformed into the frequency domain, intensity-corrected, and frequency-equally spaced to obtain multiple beat signals. These signals are then integrated into a single signal through average calculation, effectively suppressing the influence of external noise and improving the sensitivity and accuracy of short-distance optical fiber device measurements. Furthermore, the device has a compact structure and is easy to implement.

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 multi-reference arm structure and frequency division multiplexing. Background Art

[0002] Optical reflectometry is a fiber optic system testing technology that utilizes the principles of fiber backscattering, such as Rayleigh scattering, Brillouin scattering, and Raman scattering. Currently, Rayleigh scattering is primarily used for fiber optic system testing and sensing. 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 primary limitations of OTDR technology, resulting in a spatial resolution of approximately meters.

[0003] Optical frequency domain reflectometry (OFDR) technology has been proposed for this purpose. This technology, which spans the fields of fiber-optic sensing, testing, and communications, is used to test a variety of high-performance devices. It 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, phase, and polarization 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, changes in polarization state, and external environmental noise can cause differences in each set of measurement data, making it difficult to accurately splice multiple sets of data to improve 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. The measurement results are susceptible to residual sidebands from the electro-optic modulator, thereby reducing the sensitivity of OFDR testing. Furthermore, filtering out high-order sidebands increases optical power loss, and amplification using an optical amplifier introduces additional phase noise. Furthermore, if the optical comb modulation frequency exceeds the operating wavelength range of the coupler, performance degradation can occur. If a master interferometer with switchable optical path is used for measurement, the polarization state and sweep rate of the laser source fluctuate with each frequency sweep, making it susceptible to environmental influences on the test and failing to improve the sensitivity of optical frequency domain reflectometry.

[0008] The present invention is based on improvements in the prior art and discloses an optical frequency domain reflectometer with a noise-suppressed multi-reference arm structure and frequency division multiplexing. The optical source wavelength is scanned in the optical frequency domain to implement testing of the optical fiber information to be tested. By setting N main interferometer reference arms of different lengths, N reference lights with different time delays are obtained to interfere with the test light respectively, and N beat signals at different positions in the frequency domain can be obtained. Each set of information carries complete information about the device to be tested. The basic idea of the frequency division multiplexing used is that the beat signal generated by the optical frequency sweep has a limited bandwidth, and the total bandwidth obtained by the acquisition is much larger than the beat signal bandwidth. The main interferometer uses N reference arms, which can modulate the beat signal to different frequency domain positions of the acquisition signal without overlapping each other, thereby realizing frequency division multiplexing of each sub-frequency signal in the acquisition signal and achieving the purpose of simultaneously transmitting multiple beat signals in the acquisition channel.

[0009] The signal is then collected using an acquisition card and subjected to nonlinear correction. The corrected signal undergoes a Fourier transform to the frequency domain, where it is segmented and normalized according to signal frequency, corrected for peak intensity, and processed using an average value filtering algorithm. This achieves frequency division multiplexing of the beat frequency signal in a single scan, expanding the utilization of the channel bandwidth and achieving higher optical frequency domain reflectometry sensitivity. Furthermore, the present invention utilizes a fiber coupler, resulting in low signal loss in the optical fiber under test and immunity to modulator electrical noise. This method is compact and easy to implement. Summary of the Invention

[0010] The purpose of the present invention is to provide a noise-suppressed multi-reference arm frequency division multiplexing optical frequency domain reflectometer that can obtain multiple beat frequency signals in a single scan for multiplexing based on the principle of optical frequency domain reflectometry, simplify the optical path structure and eliminate the influence of additional electrical noise, suppress external noise after signal acquisition and analysis, improve test sensitivity, and enhance detection accuracy.

[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 multi-reference arm 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 multi-reference arm frequency division multiplexing module 5, wherein:

[0013] 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 split into two beams of light for output;

[0014] 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 device under test 203. 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 The test light 203b enters the multi-reference arm frequency division multiplexing module 5 along the first input end 501a of the third coupler, passes through the third coupler 501 and is split into N beams of light, which are respectively injected into the first delay fiber 502, the second delay fiber 503, ..., and the Nth delay fiber 504. The test light 203b is output through the fourth coupler 505 to obtain the first reference light 506a, the second reference light 506b, ..., and the Nth reference light 506c, which are then injected into one arm of the fifth coupler 204. The test light 203b and the multiple reference lights are combined and split by the fifth coupler 204. The optical signal is output and detected by the second balanced photodetector 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 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 balanced photodetector 304 to convert 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 converts the signal into a frequency domain signal 405a, and then performs intensity correction 406 to compensate for the frequency intensity of each beat signal in the acquisition signal. Algorithm processing: select the circulator reflection peak intensity of the first beat signal 407a as the standard calibration, normalize the peak intensity of the circulator reflection peaks of the remaining beat signals according to the calibration point, and compensate each beat signal reflection peak to the same intensity before segmentation processing. Then, the segmentation interception unit 407 segments the collected signal according to a fixed frequency domain interval to obtain the first beat signal 407a, the second beat signal 407b, ..., and the Nth beat signal 407c. These are then input into the averaging operation unit 408 for averaging filtering processing. After the optical frequency domain reflectometry measurement is completed, the process ends 409.

[0017] The length of the delay fiber 302 in the auxiliary interferometer module 3 is l0, and the corresponding optical path is L0. The optical path difference between the two arms of the auxiliary interferometer is ΔL0=nl0, where n is the refractive index of the single-mode fiber.

[0018] The length of the device under test 203 in the main interferometer test module 2 is l3, and the corresponding round-trip optical path is L3=2nl3, where n is the refractive index of the single-mode fiber. The length of the first delay fiber 502 is l 01 And the optical path is L 01 , the length of the second delay fiber 503 is l 02 And the optical path is L 02 , ..., the length l of the Nth delayed optical fiber 504 0N And the optical path is L 0N The optical path difference between the first reference light 506a and the test light 203b is ΔL1=L 01 -L3 corresponds to a beat frequency of f b1 =2γΔL1 / c, the optical path difference between the second reference light 506b and the test light 203b is ΔL2=L 02 -L3 corresponds to a beat frequency of f b2 =2γΔL2 / c, ..., the optical path difference between the Nth reference light 506c and the test light 203b is ΔL N =L 0N -L3 corresponds to a beat frequency of f bN =2γΔL N / c, required to satisfy ΔL x-1 +L3<ΔL x And f b(x-1) <f bx , where x = 2, 3, ..., N.

[0019] The device under test 203 is a short-distance optical fiber device, and its length l3 is ≤ 10 m.

[0020] The length l0 of the delay fiber 302 of the auxiliary interferometer module 3 corresponds to the optical path L0, and the optical path difference between the two arms of the interferometer is ΔL0, which is required to satisfy 5ΔL0≥ΔL N .

[0021] 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 b x).

[0022] 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.

[0023] The average calculation unit 408 performs an average value algorithm on the frequency domain signal 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, and the obtained N groups of beat frequency data are filtered by average value. The positions of each point of each group of frequency signals are read and the arithmetic average is performed to obtain the intensity of each point of the synthetic signal. z represents the horizontal coordinate position of the data after each segmentation, and finally the output signal results are plotted and analyzed;

[0024] The principle and process of performing optical frequency domain reflectometry on the device under test 203 are as follows:

[0025] The relationship between time delay and optical path difference is Δτ=2nΔL / c. The scanning test process of the present invention is shown in the attached figure. Figure 2 As shown in the figure, each straight line has the same slope γ, which represents the laser wavelength scanning rate, τ represents the delay of the test light 203b, τ1 represents the delay of the first reference light 506a, τ2 represents the delay of the second reference light 506b, ..., τ N represents the delay of the Nth reference light 506c. Interference beat signals are generated between different reference lights and the test light 203b, which is expressed as f b1 =γ(τ1-τ), f b2 =γ(τ2-τ),……,f bN =γ(τ N -τ), which is converted into frequency domain spectrum through Fourier transform;

[0026] The data processing process is as follows Figure 3 As shown in the figure, a fixed-width frequency domain spacing is selected, the beat frequency signal is segmented and intercepted into N groups of data, and the intercepted data is averaged using an average filtering algorithm to suppress noise, thereby improving the sensitivity of optical frequency domain reflectometry. The information carried by the optical fiber of the device under test is accurately obtained, and the sensitivity of optical frequency domain reflectometry is improved.

[0027] Compared with the prior art, the advantages of the technology of the present invention are:

[0028] (1) The present invention is a noise-suppressed, multi-reference arm structure frequency-division multiplexed optical frequency domain reflectometer. Based on the principle of optical frequency domain reflectometer, a tunable laser outputs linear frequency-sweep light. A main interferometer multi-reference arm structure is adopted. Based on the different time delays of the reference arms, multiple beat frequency signals are obtained in a single scan and frequency-division multiplexed in the bandwidth of the same acquisition signal. Each beat frequency signal carries complete information about the device to be tested. The frequency-division multiplexed acquisition signal is subjected to equidistant normalization correction and segmented interception, and is processed by an average value filtering algorithm. This 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, stable performance, and an easy-to-implement structure.

[0029] (2) The present invention builds multiple reference arms of different lengths in the main interferometer to achieve multiple representations of the device under test information in the collected signal under a single scan, overcoming the influence of electrical noise and large optical loss introduced by the use of electrical modulators and acousto-optic modulators, working within the effective wavelength range of the coupler and optical fiber device, reducing the increase in optical path insertion loss caused by the wavelength offset of the light source, and effectively achieving an improvement in measurement sensitivity;

[0030] (3) The present invention adopts a main interferometer multi-reference arm structure to obtain multiple interference beat frequency signals, corrects them through the signals output by the auxiliary interferometer, and then performs intensity correction, segmented truncation and average value filtering on the data, which can effectively suppress the influence of frequency sweep nonlinearity and external environmental noise, and can fuse multiple groups of optical fiber data to be tested, thereby realizing the suppression of optical frequency domain reflection measurement noise and the optimization of optical frequency domain reflection measurement signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a noise-suppressed multi-reference arm structure frequency division multiplexing optical frequency domain reflectometer summary diagram;

[0032] Figure 2 This is a schematic diagram of the principle of an optical frequency domain reflectometer with frequency division multiplexing using a multi-reference arm structure for noise suppression;

[0033] Figure 3 It is a schematic diagram of the data processing process of the collected signal;

[0034] Figure 4It is a schematic diagram of a device for performing optical frequency domain reflection measurement on an optical fiber to be tested;

[0035] Figure 5 It is a schematic diagram of the specific structure of the optical fiber to be tested. DETAILED DESCRIPTION

[0036] For a clearer explanation, the present invention proposes a noise-suppressed multi-reference arm 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.

[0037] Specific implementation method: A noise-suppressed multi-reference arm structure frequency division multiplexing optical frequency domain reflectometer performs optical frequency domain reflection measurement on the optical fiber 208A to be tested, as shown in the attached figure. Figure 4 shown.

[0038] Its structure consists of 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 multi-reference arm frequency division multiplexing module 5, wherein:

[0039] 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 split into two beams of light for output;

[0040] In the multi-reference arm frequency division multiplexing module 5, the number of reference arms N is set to 3, which consists of a third coupler 501, a first delay fiber 502, a second delay fiber 503, a third delay fiber 504a and a fourth coupler 505;

[0041] 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 device under test 203. 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, and the other arm of the second coupler 201 output is injected into the third arm of the third coupler 204. A beam of light is input into the multi-reference arm frequency division multiplexing module 5, enters the third coupler 501 along the first input end 501a of the third coupler and is split into two beams of light, which are respectively injected into the first delay fiber 502, the second delay fiber 503, and the third delay fiber 504a. The beams are output through the fourth coupler 505 to obtain the first reference light 506a, the second reference light 506b, and the third reference light 506d. The beams are then injected into one arm of the fifth coupler 204. The test light 203b and the three reference lights are combined and split by the fifth coupler 204. The optical signals are output and detected by the second balanced photodetector 205, realizing the conversion of the optical signals into electrical signals.

[0042] 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 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 balanced photodetector 304 to convert the optical signals into electrical signals.

[0043] 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 a first acquisition signal 403a. The signal of 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 nonlinearity 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 a frequency domain signal 405a and performs intensity correction 406. The signal is then input into the segmentation interception unit 407 for segmentation interception according to the signal frequency to obtain a first beat frequency signal 407a, a second beat frequency signal 407b, and a third beat frequency signal 407d. The signal is then input into the mean operation unit 408 for algorithm average filtering processing. After the optical frequency domain reflectance measurement is completed, the process ends 409.

[0044] The intensity correction unit 406 performs a compensation algorithm on the frequency intensity of each beat signal in the collected signal, normalizes the peak intensity of the circulator reflection peaks of the remaining beat signals according to the calibration points, and then inputs them into the segmentation unit 407, which segments them according to fixed frequency domain intervals to obtain three groups of beat signals;

[0045] 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, performs arithmetic averaging, and synthesizes 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.

[0046] The selection and parameters of the main optoelectronic devices in this solution are as follows:

[0047] 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 1540nm-1550nm, the wavelength scanning rate is 10nm / s, and the wavelength scanning time is 1s;

[0048] 2) The maximum detection bandwidth of the photodetector is 200MHz, the saturated differential detection power is 55uW, and the common mode rejection ratio is 25dB;

[0049] 3) The acquisition card has a 16-bit sampling bit, a sampling rate of 100 MHz, 100M sampling points, a sampling time of 1 s, and a triggering method using Labview software triggering.

[0050] 4) The first coupler 501 and the third coupler 505 are 1×2 couplers with a splitting ratio of 50:50. The extinction ratio of the couplers used is greater than 20 dB, the insertion loss is less than 0.5 dB, and the operating wavelength covers the 1550 nm band;

[0051] 5) The operating wavelength of the three-port circulator 202 is 1550 nm, the insertion loss is 0.8 dB, and the isolation is greater than 50 dB;

[0052] 6) As attached Figure 5 As 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.

[0053] 7) A reference arm consisting of three delay fibers is provided in the reference arm of the main interferometer. The length of the first delay fiber 502 is l 01 =1m corresponds to an optical path length of L 01 =1.456m; the length of the second delay fiber 503 is l 02 =10m corresponds to the optical path L 02 =14.56m, the length of the third delay fiber 504a is l 03 =30m corresponding to the optical path L 03 =43.68m, the optical path difference between the first reference light 506a and the test light 203b is ΔL1=L 01 -L3, the optical path difference between the second reference light 506b and the test light 203b is ΔL2=L 02 -L3, the optical path difference between the third reference light 506d and the test light 203b is ΔL3 = L 03 -L3;

[0054] 8) The auxiliary interferometer module 3 adopts a fiber interferometer structure with a delay fiber 302 length of l0=10m and an optical path of L0=14.56m. The calculated optical path difference between the two arms of the auxiliary interferometer module 3 is ΔL0=14.56m;

[0055] 9) Main interferometer test module 2, wherein the swept frequency optical signal enters the optical fiber under test 203A after passing through the second port 202b of the three-port circulator, and the optical path difference of each delayed fiber is set to satisfy ΔL1≤5ΔL0 and ΔL2≤5ΔL0, and the parameter design is reasonable;

[0056] This multi-reference arm structure frequency division multiplexing optical frequency domain reflectometer can achieve the effect of frequency division multiplexing in the collected signal. Each beat frequency signal is not aliased and carries complete information of the optical fiber 203A to be tested.

[0057] 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. Effectively improve measurement sensitivity and suppress test noise.

[0058] 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 multi-reference arm 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 multi-reference arm frequency division multiplexing module (5), characterized in that: A tunable laser source module (1) outputs linearly modulated light (101), which is divided into two beams of light through a second coupler (201), one beam of light being input into a device under test (203) and a three-port circulator (202) and returning as test light (203b), and the other beam of light entering a third coupler (501) and being divided into N beams of light. The first reference light (506a), the second reference light (506b), ..., and the Nth reference light (506c) are obtained through a first delay optical fiber (502), a second delay optical fiber (503), ..., and an Nth delay optical fiber (504), and the first reference light (506a), the second reference light (506b), ..., and the Nth reference light (506c) are respectively interfered with the test light (203b) to obtain multiple groups of beat frequency signals, each beat frequency signal being a plurality of beat frequency signals. The signals all carry complete information of the device under test (203), and their frequency positions do not overlap. They are multiplexed in the same acquisition signal under a single light source sweep. Then the signal acquisition and analysis module (4) receives the signal and performs time-frequency conversion. Then the segmented interception unit (407) intercepts the frequency signal at equal intervals and separates it into multiple beat frequency signals. Based on the circulator reflection peak of the first beat frequency signal, the peak intensity of each beat frequency signal is corrected. Then, the average value operation unit (408) performs average value filtering algorithm processing on the multiple groups of beat frequency signals and merges them into a group of averaged signals. The length of the first delay optical fiber (502) in the multi-reference arm frequency division multiplexing module (5) is l 01 The corresponding optical path is L 01 , the length of the second delay optical fiber (503) is l 02 The corresponding optical path is L 02 , ..., the length l of the Nth delayed optical fiber (504) 0N The corresponding optical path is L 0N The optical path difference between the first reference light (506a) and the test light (203b) is ΔL1=L 01 -L3, whose beat frequency is f b1 =2γΔL1 / c, the optical path difference between the second reference light (506b) and the test light (203b) is ΔL2=L 02 -L3, whose beat frequency is f b2 =2γΔL2 / c, ..., the optical path difference between the Nth reference light (506c) and the test light (203b) is ΔL N =L 0N -L3, whose beat frequency is f bN =2γΔL N / c, the delay fiber length of each reference arm of the multi-reference arm frequency division multiplexing module (5) is required to meet ΔL x-1 +L3<ΔL x And, where x=2,3,......,N.

2. The noise-suppressed multi-reference arm 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), and the other beam of light output from the second coupler (201) enters the device under test (203). A multi-reference arm frequency division multiplexing module (5) is input into the third coupler (501) along the first input end (501a) of the third coupler and divided into N beams of light, which are respectively injected into the first delay optical fiber (502), the second delay optical fiber (503), ..., and the Nth delay optical fiber (504). The first reference light (506a), the second reference light (506b), ..., and the Nth reference light (506c) are output through the fourth coupler (505), and then injected into one arm of the fifth coupler (204). The test light (203b) and the multiple reference lights are combined and split through the fifth coupler (204). The optical signal is output and detected by the second balanced photodetector (205), thereby realizing conversion of the optical signal into an electrical signal. Another beam of light is output to the auxiliary interferometer module (3) through the first coupler (601) and the first output end (601b) of the first coupler, and is divided 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 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 balanced photodetector (304), thereby realizing the conversion of 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 multi-reference arm frequency division multiplexing optical frequency domain reflectometer according to claim 2, characterized in that: In the main interferometer test module (2), the second port (202b) of the three-port circulator is connected to the device under test (203), light is scattered by the device under test (203), and test light (203b) is output from the third port (202c) of the three-port circulator, and the length of the device under test satisfies l3≤10m.

4. The noise-suppressed multi-reference arm frequency division multiplexing optical frequency domain reflectometer according to claim 2, characterized in that: A signal acquisition and analysis module (4) is provided, wherein 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 a first acquisition signal (403a); the signal of the auxiliary interferometer module (3) is input into the second acquisition unit (402) to obtain a second acquisition signal (402a); the first acquisition signal (403a) is converted into a frequency domain signal (405a) through the light source nonlinear correction unit (404) and the spectrum analysis unit (405); then the first acquisition signal (403a) is subjected to intensity correction (406) and input into the segmented interception unit (407) to obtain a first beat frequency signal (407a), a second beat frequency signal (407b), ..., an Nth beat frequency signal (407c), respectively; then the signals are input into the mean value operation unit (408) to be processed into a group of signals; and the optical frequency domain reflection measurement is completed and the method ends (409).

5. The noise-suppressed multi-reference arm frequency division multiplexing optical frequency domain reflectometer according to claim 4, characterized in that: The light source nonlinear correction unit (404) of the signal acquisition and analysis module (4) adopts a resampling algorithm and modulates the first acquisition signal (403a) to an equal frequency sweep interval by means of calibration of the second acquisition signal (402a), thereby eliminating the influence of the light source nonlinearity.

6. The noise-suppressed multi-reference arm frequency division multiplexing optical frequency domain reflectometer according to claim 4, 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 calibration, and calibrates the peak intensity of the circulator reflection peaks of the remaining beat signals according to the calibration point, and compensates each beat signal reflection peak to the same intensity.

7. The noise-suppressed multi-reference arm frequency division multiplexing optical frequency domain reflectometer according to claim 4, characterized in that: The segmentation interception unit (407) of the signal acquisition and analysis module (4) locates the position of each beat signal, takes the reflection peak of the circulator of each beat signal as the starting point of the segmentation, and then performs segmentation interception of the frequency domain length according to a fixed frequency domain interval to obtain N groups of beat signals.

8. The noise-suppressed multi-reference arm frequency division multiplexing optical frequency domain reflectometer according to claim 4, 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), ..., the Nth beat frequency signal (407c), reads the position of each point of each group of frequency signals, performs arithmetic averaging, and synthesizes the strength of each point of the signal. z represents the horizontal coordinate position of the data after each segmentation. The synthesized signal can be expressed as the average of multiple beat frequency signals. Finally, the signal results are graphed and analyzed.

Images