CN115711633A - Phase noise accurate correction optical frequency domain reflectometer of loop structure reference interferometer - Google Patents
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Abstract
The invention discloses a phase noise accurate correction optical frequency domain reflectometer of a loop structure reference interferometer. The system comprises a tunable laser source, an auxiliary interferometer module, a loop structure module, a main interferometer module, a device module to be tested and a signal processing module, wherein the loop structure module is used for generating a plurality of signals with different time delays; the auxiliary interferometer module is used for generating a plurality of beat frequency signals with different frequencies by interfering with the signals in the loop structure module; the main interferometer is used for receiving beat frequency signals carrying information of the device module to be tested; and the signal processing module is used for selecting a proper auxiliary interferometer beat frequency signal for data processing and respectively and accurately correcting the nonlinearity of the light source frequency sweep and the phase noise. The invention can accurately correct the nonlinearity of the light source frequency sweep and the phase noise simultaneously by manufacturing the multi-time delay under the condition of measuring the long-distance device to be measured, and can improve the spatial resolution of long-distance test and sensing.
Description
Technical Field
The invention belongs to the field of distributed optical fiber sensing, and relates to an optical frequency domain reflectometer.
Background
Optical Frequency Domain reflectometry (ofdr) is an optical fiber sensing technology based on optical heterodyne detection and Frequency Domain analysis, which realizes accurate measurement of fusion joints, bends, breakpoints and the like along a link by detecting rayleigh scattering signals generated at different positions in an optical fiber.
In the OFDR technique, the scattering data at the near end is mainly affected by the nonlinear frequency sweep, while the scattering data at the far end is mainly limited by phase noise, and the single-delay auxiliary interferometer is difficult to consider the fiber scattering spectrum correction of different lengths.
In the OFDR technique, there is a phenomenon that a tunable light source generates a nonlinear tuning effect during linear tuning, which causes beat frequency interference signals in an interferometer to be not equal optical frequency intervals at the same time interval, so that directly performing fourier transform on signals collected in the interferometer generates errors.
According to the traditional method for eliminating the nonlinearity of the light source frequency sweep, an auxiliary interferometer is built, the length of a delay ring of the auxiliary interferometer is short, the auxiliary interferometer can detect the change of the light intensity of an interference signal, the change of the phase information of the light source along with the time can be obtained, and the nonlinearity of the light source frequency sweep can be eliminated through data processing.
Phase noise refers to the fact that the sinusoidal oscillation of light is unstable and random jump of phase occurs somewhere in time. The phase noise may cause the line width of the light source to be widened. The traditional OFDR distributed optical fiber sensing equipment has frequency offset due to problems of phase noise interference and the like, so that the positioning of a reflection point or a scattering point is not accurate enough.
In 2019, zhanbi et al of the core hua chuang (wuhan) opto-electronic technology limited proposed an OFDR detection method (an OFDR detection method, 201910862111.6), which can perform accurate phase noise compensation for specific abnormal frequency points on an optical fiber by setting a delay value of an adjustable delay interference arm on an adjustable delay auxiliary interferometer.
In 2019, zhanbi et al of wayor optical technology ltd, wayor, china, has proposed an OFDR detection device (an OFDR detection device, 201910862124.3), which adopts an adjustable delay-assisted interferometer, and can measure phase noise at different frequency points under different delays through multiple measurements, thereby compensating for phase noise at different abnormal frequency points.
In 2022, gazeh et al, university of electronic technology, proposed a method for reducing the nonlinear phase effect of OFDR light source (a system and method for reducing the nonlinear phase effect of OFDR light source, 202011249634.2), which uses a test fiber as a reference fiber of an auxiliary interferometer to enable long-distance and high spatial resolution signal detection in OFDR system.
In 2021, qinyu, jade and the like at Guangdong university of industry proposed a distributed bidirectional polarization measurement device based on a double-beat-frequency single-auxiliary interferometer (a distributed bidirectional polarization measurement device based on a double-beat-frequency single-auxiliary interferometer, 202110941018.1), which uses a half-reflecting half-transparent mirror and a total reflector through an auxiliary interferometer, and generates double-beat-frequency signals by using different optical path differences to respectively match a transmission module and a reflection module, thereby correcting the sweep frequency nonlinearity of a light source and rapidly obtaining the transflective information of an optical fiber device. Qin and Yuwen et al also propose a distributed bidirectional polarization measuring device based on matching correction optical frequency domain interference (a distributed bidirectional polarization measuring device based on matching correction optical frequency domain interference, 202110817517. X), the device utilizes a plurality of supplementary interferometers with different optical path differences to match transmission module and reflection module respectively to realize correcting the light source sweep frequency nonlinearity, promoted the spatial resolution of system, obtain the transflective information of optical fiber device fast. The two devices can only correct the non-linearity of the frequency sweep of the light source, but can not correct the phase noise when testing the long-distance optical fiber.
The OFDR detection device used at present uses a fixed delay auxiliary interferometer to compensate phase noise and light source frequency sweep nonlinearity when testing a long-distance device to be tested, and the auxiliary interferometer needs a longer delay optical fiber when measuring a long distance and has larger phase noise, so that the light source frequency sweep nonlinearity and the phase noise cannot be accurately corrected through a fixed delay value.
The invention discloses a phase noise accurate correction optical frequency domain reflectometer of a loop structure reference interferometer based on the prior art improvement, wherein when a long-distance optical fiber is used for testing and sensing, a beat frequency signal containing information of a device to be tested is generated in a main interferometer; meanwhile, two sections of beat frequency signals containing different phase noise information are obtained by arranging two scaled extension optical fibers in the auxiliary interferometer, one section of the extension optical fiber is shorter and used for correcting the sweep frequency nonlinearity of a light source, and the other section of the extension optical fiber is matched with the length of the optical fiber to be detected and used for correcting the phase noise. Therefore, the nonlinear frequency sweeping and phase noise of the light source can be accurately corrected in long-distance test and sensing simultaneously, and the reflection information of the long-distance device to be tested with higher precision can be obtained.
Disclosure of Invention
The invention aims to provide a phase noise accurate correction optical frequency domain reflectometer of a loop structure reference interferometer, which solves the problem that in the prior art, the frequency sweep nonlinearity and the phase noise of a light source are difficult to be accurately corrected at one time during long-distance test.
A phase noise precision-corrected optical frequency domain reflectometer for loop structure reference interferometers comprises five modules: tunable laser source 1, supplementary interferometer module 2, loop structure module 3, main interferometer module 4, device module 5, the signal processing module 6 that awaits measuring, characterized by:
a signal light is emitted from a tunable laser source 1, and is injected into an auxiliary interferometer module 2 and a main interferometer module 3 through a first coupler 101, after being injected into the auxiliary interferometer module 2, the optical signal is divided into two beams by a second coupler 201, one beam is output from a first output end 201a of the second coupler to a first input end 203a of a sixth coupler, the other beam is injected into a loop structure module 3 from a second output end 201b of the second coupler, the light is split in a fifth coupler 301, a part of the light is output from a second output end 301d of the fifth coupler, the other part of the light is output from a first output end 301c of the fifth coupler, the light is injected into an extension optical fiber 303 through a first welding point 302a, the light is output to a second input end 301b of the fifth coupler through a second welding point 302b, and finally enters a fifth coupler 301, after being split again, a part of the light is output through a second output end 301d of the fifth coupler again, the other part of the light is injected into an extension optical fiber 303 through a first output end 301c of the fifth coupler, the light enters a second input end 301b of the fifth coupler, the light enters the sixth coupler after being repeatedly coupled, the loop structure module 3, and the loop structure module 3 is output from the sixth coupler; after the optical signal is injected into the main interferometer module 4, the optical signal is injected into the device module to be tested 5 through the circulator 402, and the reflected signal light is injected back into the main interferometer module 4 through the circulator 402 again; the signals collected from the main interferometer module 4 and the auxiliary interferometer module 2 are transmitted to the signal processing module 6, and the accurate measurement value of the device to be measured is obtained after data processing.
The tunable laser source 1 is characterized in that: the linearly tunable continuous light emitted by the tunable laser source 1 is divided into two beams by the first coupler 101, and the two beams are output from the first output end 101a of the first coupler and the second output end 101b of the first coupler and enter the second coupler 201 and the third coupler 401, respectively.
The auxiliary interferometer module 2 is characterized in that: after being injected into the auxiliary interferometer module 2, the optical signal is divided into two beams by the second coupler 201, and one beam is output from the first output end 201a of the second coupler to the first input end 203a of the sixth coupler and is used as reference light; the other beam is injected into the loop structure module 3 from the second coupler second output end 201b, and is injected into the sixth coupler 203 from the sixth coupler second input end 203b after being output from the loop structure module 3; the signal detected by the first balanced detector 204 is then acquired by the first acquisition unit 601.
In the loop structure module 3, an optical signal is input from the first input end 301a of the fifth coupler, light is split in the fifth coupler 301, a part of the light is output from the second output end 301d of the fifth coupler, another part of the light is output from the first output end 301c of the fifth coupler, is injected into the extension optical fiber 303 through the first welding point 302a, is output to the second input end 301b of the fifth coupler through the second welding point 302b, and finally enters the fifth coupler 301, after the light is split again, a part of the light is output from the second output end 301d of the fifth coupler again, another part of the light is injected into the extension optical fiber 303 through the first output end 301c of the fifth coupler again, and then enters the fifth coupler 301 through the second input end 301b of the fifth coupler, the cycle is repeated in this way, the number of times of passing through the extension optical fiber 303 is m, the length of the extension optical fiber 303 is L, and L is less than 10m.
The fifth optical fiber coupler 301 is characterized in that: the splitting ratio of the fifth fiber coupler 301 is a: b, i.e. the ratio of the output optical power of the second output end 301d of the fifth fiber coupler to the output optical power of the first output end 301c of the fifth fiber coupler, and the output power P of the second output end 301d of the fifth fiber coupler out Satisfy the requirement of
The splitting ratio of the fifth fiber coupler 301 is selected to satisfy P out ≥P BPD ，P BPD The minimum detection power for the first balanced detector 204.
The loop structure module 3 preferably uses one coupler for light splitting, and can use a plurality of couplers for light splitting along with the change of the requirement.
The main interferometer module 4 is characterized in that: an optical signal is injected into the main interferometer module 4 through the second output end 101b of the first coupler 101, and is divided into two beams after entering the third coupler 401, one beam enters the first port 402a of the circulator, light is injected into the device module to be tested 5 through the second port of the circulator, and then the reflected light is injected into the main interferometer module 4 again through the third port 402c of the circulator and then injected into the seventh coupler 404; the other beam passes through a main interferometer delay fiber 403 and then is injected into a seventh coupler 404; interference occurs in the seventh coupler 404, and an interference signal thereof is detected by the second balanced detector 405 and then acquired by the second acquisition unit 602.
The signal processing module 6 is characterized in that: the signal detected by the first balanced detector 204 is acquired by the first acquisition unit 601, the auxiliary interferometer signal is filtered, and the low-pass filter 603 filters out a first beat signal 603a in the auxiliary interferometer signal, where m =1; the bandpass filter 604 filters out the mth beat signal 604a in the auxiliary interferometer signal; the signal detected by the second balanced detector 405 is collected by the second collecting unit 602, and the collected beat signal 602a of the main interferometer is obtained; a beat signal 602a of a main interferometer firstly enters a light source nonlinear correction unit 605, the light source sweep frequency nonlinearity is corrected by acquiring information in a first beat signal 603a, then the beat signal 605a of the main interferometer after the light source nonlinearity is corrected enters a phase noise correction unit 606 for correcting phase noise, and the phase noise is corrected by acquiring information in a second beat signal 604 a; after the data is corrected, the position and amplitude of the scattering are obtained, and finally the data enters the spectrum analysis unit 607 to be analyzed, so as to obtain the accurate reflection information of the device to be measured.
The light source nonlinearity correction unit 605 is characterized in that: the light source sweep nonlinearity is corrected by using a resampling method, the optical frequency information of the tunable light source is obtained by using the beat signal 511 output by the auxiliary interferometer 514, the equal optical frequency position can be obtained, the signal of the main interferometer 515 is resampled by using an interpolation method, and finally the main interferometer signal with the equal optical frequency interval is obtained.
The phase noise correction 606 is characterized by: the phase noise term carried by the mth beat frequency signal 604a obtained by the light in the loop structure auxiliary interferometer module 2 interfering with the light on the reference arm m times through the extension fiber 303 is phi m (t)-Φ m (t-τ m ) The phase noise term carried by the beat signal 602a of the main interferometer generated by the device under test module 4 and the reference arm 401b of the main interferometer is Φ 0 (t)-Φ 0 (t-τ z ) Let phi m (t)-Φ m (t-τ m ) And phi 0 (t)-Φ 0 (t-τ z ) And matching to eliminate the phase noise in the beat signal of the main interferometer.
A master interferometer beat signal 602aI generated by the reflected light in the device under test module 5 and the reference light in the master interferometer reference arm 401b 0 The specific formula of (t) is:
wherein E is 0 Is the amplitude of the beat light signal of the master interferometer; r (tau) z ) Is the reflection coefficient of the fiber; f. of 0 Is the initial frequency of the light source; γ is the tunable rate of the light source; tau is z Is the time delay of the test fiber and the reference arm of the main interferometer; wherein tau is z =2nz/c; where c is the propagation speed of light in vacuum; n is the effective refractive index of the fiber; z is the length of the test fiber; phi 0 (t)-Φ 0 (t-τ z ) Is the phase noise term induced at the main interferometer.
The first beat signal 603aI generated by the loop structure module 3 and the auxiliary interferometer reference arm 201a 1 The specific formula of (t) is:
I 1 (t)＝2E 1 2 cos(2πf 0 τ 1 +2πγτ 1 t+πγτ 1 2 +Φ 1 (t)-Φ 1 (t-τ 1 ))
wherein m =1,E 1 Is the amplitude of the beat light signal of the auxiliary interferometer; f. of 0 Is the initial frequency of the light source; tau. 1 The time delay between one cycle of the loop structure module and the optical fiber of the reference arm is obtained; wherein τ is 1 ＝nL 1 C; where c is the propagation speed of light in vacuum; n is the effective refractive index of the fiber; l is a radical of an alcohol 1 Is the length of the delay fiber; phi 1 (t)-Φ 1 (t-τ 1 ) Is the phase noise term caused by the first cycle of the loop structure module.
The m-th delay optical fiber 303 and a second beat frequency signal 604aI generated by the auxiliary interferometer reference arm 201a 2 The specific formula of (t) is:
I m (t)＝2E 1 2 cos(2πf 0 τ m +2πγτ m t+πγτ m 2 +Φ m (t)-Φ m (t-τ z ))
wherein E is 1 Is the amplitude of the beat light signal of the auxiliary interferometer; f. of 0 Is the initial frequency of the light source; tau. m The time delay between the loop structure circulation m times and the reference arm optical fiber; wherein τ is m ＝nmL 1 C; where c is the propagation speed of light in vacuum; n is the effective refractive index of the fiber; l is 1 Is the length of the delay fiber; phi (phi) of m (t)-Φ m (t-τ m ) Phase noise term caused by m cycles of the loop structure module.
The sweep frequency nonlinear correction unit 605 is characterized in that: the loop structure module circulates the primary light through the extended fiber only once, L 1 The optical fiber length of (2) is shorter, and the phase noise is small, so that the light source frequency sweep nonlinearity is corrected by adopting the loop structure module to circulate once and the beat frequency signal generated by the reference arm.
The phase noise correction 606 is characterized in that: the phase noise term caused by the self nonlinearity of the sweep frequency light source in the main interferometer is phi 0 (t)-Φ 0 (t-τ z ) Z is the maximum test distance of the device to be tested, so that the loop structure module is circulated for m times to delay time
Can pass through the phase noise term phi of the auxiliary interferometer m (t)-Φ m (t-τ m ) To eliminate the phase noise term phi of the main interferometer 0 (t)-Φ 0 (t-τ z )。
The principle of Optical Frequency Domain Reflection (OFDR) measurement is shown in fig. 3: the chirped continuous light emitted by the tunable laser source 1 is divided into two beams after passing through the coupler 210, one beam is injected into a first output end 211 of the coupler and is injected into a device to be measured 215 through a three-port circulator 213, and because Rayleigh scattering and Fresnel reflection exist in an optical fiber to be measured, a part of light is reflected back and enters the coupler 216 as the light to be measured through the three-port circulator 213; the other beam is injected into the second input end 212 of the coupler as reference light, an optical path difference is generated in the coupler 216 after passing through the delay fiber 214 and the light to be measured, after a time delay tau, a frequency difference is generated between the light to be measured and the reference light, namely a beat frequency signal, the beat frequency signal is received by the balance detector 217 for differential detection, the acquisition card 218 performs data synchronous acquisition and storage on the optical signal, after correcting sweep frequency nonlinearity, the FFT is performed to convert optical frequency domain information into a distance domain, and finally Rayleigh scattering peak amplitude and position information corresponding to a single defect point on the device to be measured are obtained.
Compared with the prior art, the invention has the advantages that:
(1) The invention relates to a light frequency domain reflectometer measuring device based on a loop structure reference interferometer, which is based on the light frequency domain interference principle, improves the testing rate and the measuring accuracy at one time through the quick linear frequency scanning of a tunable laser source, and quickly obtains the reflection information of a device;
(2) The invention utilizes the auxiliary interferometer with a loop structure to obtain a plurality of sections of beat frequency signals containing different phase noise information, and can accurately correct the phase noise and nonlinearity of a device to be tested under long-distance test, so that the frequency spectrum does not widen, the spatial resolution of the system is improved, and the measurement precision of the system is improved;
(3) The invention utilizes the auxiliary interferometer with a loop structure, can accurately correct the sweep frequency nonlinearity of the light source, realizes equal optical frequency interval sampling, and improves the system measurement precision.
Description of the drawings:
FIG. 1 is a simplified schematic diagram of a phase noise accurate optical frequency domain reflectometer for a loop structure reference interferometer;
FIG. 2 is an illustration of a swept frequency nonlinear correction resampling method;
FIG. 3 is a schematic diagram of an Optical Frequency Domain Reflectometry (OFDR) measurement technique for single defect point measurement;
FIG. 4 is an optical frequency domain reflectometer for a loop structure reference interferometer for fiber optic measurements.
The specific implementation mode is as follows:
in order to make the objects, technical solutions and advantages of the present application more clearly understood, the present application is further described in detail below with reference to the accompanying drawings and embodiments. The specific embodiments described herein are merely illustrative of the present application and are not intended to be limiting of the present application.
The implementation method comprises the following steps: the phase noise of the loop structure reference interferometer, which measures 5km of fiber, accurately corrects the optical frequency domain reflectometer.
The main photoelectric devices of the device are selected and the parameters are as follows:
the light source is a narrow-linewidth tunable laser source, the relative intensity noise RIN-145dB/Hz, the linewidth of the light source is 60kHz (coherent control switch), the frequency sweep range is 10nm, the wavelength tuning range is 1545 nm-1555 nm, the frequency sweep rate is 10nm/s, and the frequency sweep time is 1s;
the photosensitive materials of the first differential detector 206 and the second differential detector 304 are InGaAs, the light detection range is 900-1700 nm, the responsivity is larger than 1A/W, and the typical value of the common mode rejection ratio is 25dB. If a balanced optical fiber receiver of 1817-FC type of Newport company is adopted, the maximum detection bandwidth of the detector is 80MHz, the saturation differential detection power is 55uW, and the common mode rejection ratio is 25dB;
the maximum sampling rate of the acquisition card 501 is 400MHz/s, and the acquisition card can simultaneously acquire four channels and has 2 independent full-function ports, for example, an M4i.4471-x8 acquisition card of SPECTRUM company is adopted; the number of sampling points is 400M, the sampling time is 1s, and the triggering mode is triggered by the external part of a laser;
the splitting ratio of the first coupler 101 is 1: 99, the working center wavelength is 1550nm, the bandwidth is +/-40nm, the 99% end insertion loss is less than or equal to 0.2dB, and the 1% end insertion loss is less than or equal to 24.1dB;
the splitting ratio of the fifth optical fiber coupler 202 in the loop structure is selected to be 50: 50, the working center wavelength is 1550nm, and the bandwidth is +/-40 nm.
The light splitting ratio of the rest couplers is 50: 50, the working center wavelength is 1550nm, the bandwidth is +/-40 nm, and the insertion loss is less than or equal to 3.6dB;
the working wavelength of the circulator 402 is 1550nm, the insertion loss is 0.8dB, and the isolation is greater than 50dB;
the main interferometer module 4 adopts a Mach-Zehnder type interferometer with an optical path difference S FUT ＝L FUT n =7280m, refractive index n =1.456 of the single-mode fiber;
device to be tested module 5: the device to be tested is a single-mode optical fiber with the diameter of 250 mu m and the length of L FUT ＝5km。
At this time L FUT The time delay brought is:
n is the effective refractive index of the fiber, n =1.456; c is the propagation speed of light in vacuum.
The auxiliary interferometer module 2 is a Mach-Zehnder type interferometer, and is provided with a loop structure, wherein the length of the extension optical fiber 303 is L, and L is less than 10m.
The arm length difference of the loop structure circulating once is L, which is used for correcting the nonlinearity of the light source frequency sweep, in this case, L =5m.
Obtaining optical frequency information of a tunable light source from beat frequency signals output by the L and the auxiliary interferometer reference arm, obtaining equal optical frequency positions, resampling a signal of the main interferometer by using an interpolation method, and finally obtaining a main interferometer signal with equal optical frequency intervals.
The delay with the reference arm after the loop structure has cycled m times is: tau is m =2nmL/c, in this case, take
So take m =100.
The phase noise term caused by the nonlinearity of the swept-frequency light source in the main interferometer is
L FUT Maximum test distance for the device under test is L FUT =5km, length of passage after circulating loop structure m times is L m =500m, phase noise term Φ (t) - Φ (t- τ) by means of an auxiliary interferometer m ) To the phase noise term of the main interferometer
And carrying out accurate correction.
After being corrected by the light source nonlinearity correction unit 605 and the phase noise correction unit 606, the optical fiber enters the spectrum analysis unit 607 to be analyzed, and reflection information of the 5km long-distance optical fiber is obtained.
Although the present invention has been described with reference to a preferred embodiment, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (10)
1. A phase noise precision correction optical frequency domain reflectometer for loop structure reference interferometer comprises five modules: tunable laser source (1), supplementary interferometer module (2), loop structure module (3), main interferometer module (4), device module to be measured (5), signal processing module (6), characterized by:
the signal light is emitted from a tunable laser source (1), and is injected into an auxiliary interferometer module (2) and a main interferometer module (3) through a first coupler (101), after being injected into the auxiliary interferometer module (2), the optical signal is divided into two beams by a second coupler (201), one beam is output to a first input end (203 a) of a sixth coupler from a first output end (201 a) of the second coupler, the other beam is injected into a loop structure module (3) from a second output end (201 b) of the second coupler, the optical signal is split in a fifth coupler (301), one part of the optical signal is output through a second output end (301 d) of the fifth coupler, the other part of the optical signal is output through a first output end (301 c) of the fifth coupler, injecting the extension optical fiber (303) through the first welding point (302 a), outputting the extension optical fiber (303) to the second input end (301 b) of the fifth coupler through the second welding point (302 b), finally entering the fifth coupler (301), splitting the light again, outputting a part of light through the second output end (301 d) of the fifth coupler again, injecting the other part of light into the extension optical fiber (303) through the first output end (301 c) of the fifth coupler again, then entering the fifth coupler (301) through the second input end (301 b) of the fifth coupler, repeating the above steps, and injecting the light into the sixth coupler (203) from the second input end (203 b) of the sixth coupler after being output from the loop structure module (3); after an optical signal is injected into the main interferometer module (4), the optical signal is injected into the device module to be tested (5) through the circulator (402), and the reflected signal light is injected back into the main interferometer module (4) through the circulator (402); and signals collected from the main interferometer module (4) and the auxiliary interferometer module (2) are transmitted to the signal processing module (6), and an accurate measurement value of the device to be measured is obtained after data processing.
2. The tunable laser source (1) according to claim 1, characterized in that: linear tunable continuous light emitted by a tunable laser source (1) is divided into two beams by a first coupler (101), and the two beams are output from a first output end (101 a) of the first coupler and a second output end (101 b) of the first coupler and respectively enter a second coupler (201) and a third coupler (401).
3. The auxiliary interferometer module (2) of claim 1, wherein: after being injected into the auxiliary interferometer module (2), the optical signal is divided into two beams by the second coupler (201), and one beam is output from the first output end (201 a) of the second coupler to the first input end (203 a) of the sixth coupler to be used as reference light; the other beam is injected into the loop structure module (3) from the second coupler second output end (201 b), and is injected into the sixth coupler (203) from the sixth coupler second input end (203 b) after being output from the loop structure module (3); the signal detected by the first balanced detector (204) is then acquired by the first acquisition unit (601).
4. The loop structure module (3) according to claim 1, wherein the optical signal is input from a first input end (301 a) of the fifth coupler, split in the fifth coupler (301), and a portion of the optical signal is output through a second output end (301 d) of the fifth coupler, and another portion of the optical signal is output through the first output end (301 c) of the fifth coupler, and then injected into the extension optical fiber (303) through a first welding point (302 a), and then output to a second input end (301 b) of the fifth coupler through a second welding point (302 b), and finally enter the fifth coupler (301), and after the light splitting again, a portion of the optical signal is output through a second output end (301 d) of the fifth coupler, and another portion of the optical signal is injected into the extension optical fiber (303) through a first output end (301 c) of the fifth coupler, and then enters the fifth coupler (301) through a second input end (301 b) of the fifth coupler, and the cycle is repeated, wherein the number of the light passing through the extension optical fiber (303) is m, and the length of the extension optical fiber (303) is L, and L10 < m.
5. A fifth fiber optic coupler (301) as claimed in claim 4, wherein: the splitting ratio of the fifth optical fiber coupler (301) is a: b, namely the output optical power of the second output end (301 d) of the fifth optical fiber coupler is compared with the output optical power of the first output end (301 c) of the fifth optical fiber coupler, and the output power P of the second output end (301 d) of the fifth optical fiber coupler is compared with the output optical power of the first output end (301 c) of the fifth optical fiber coupler out Satisfy the requirement of
The splitting ratio of the fifth optical fiber coupler (301) is selected to satisfy P out ≥P BPD ，P BPD The minimum detection power for the first balanced detector (204).
6. The loop structure module (3) according to claim 1, wherein preferably one coupler is used for splitting, and a plurality of couplers are used for splitting as the requirement changes.
7. A main interferometer module (4) as claimed in claim 1, wherein: an optical signal is injected into a main interferometer module (4) through a second output end (101 b) of a first coupler (101), and is divided into two beams after entering a third coupler (401), one beam enters a first port (402 a) of a circulator, light is injected into a device module to be tested (5) through the second port of the circulator, and then reflected light is injected into the main interferometer module (4) again through a third port (402 c) of the circulator and then injected into a seventh coupler (404); the other beam passes through a delay optical fiber (403) of the main interferometer and then is injected into a seventh coupler (404); interference occurs in the seventh coupler (404), and an interference signal thereof is detected by the second balanced detector (405) and then acquired by the second acquisition unit (602).
8. The signal processing module (6) according to claim 1, characterized in that: the signal detected by the first balance detector (204) is acquired by a first acquisition unit (601), the auxiliary interferometer signal is subjected to filtering processing, and a low-pass filter (603) filters out a first beat frequency signal (603 a) in the auxiliary interferometer signal, wherein m =1; a band-pass filter (604) filters out an mth beat signal (604 a) in the auxiliary interferometer signal; the signal detected by the second balanced detector (405) is collected by a second collecting unit (602), and a collected beat frequency signal (602 a) of the main interferometer is obtained; the method comprises the following steps that a main interferometer beat frequency signal (602 a) enters a light source nonlinearity correction unit (605) firstly, the light source sweep frequency nonlinearity is corrected by acquiring information in a first beat frequency signal (603 a), then the main interferometer beat frequency signal (605 a) after the light source nonlinearity is corrected enters a phase noise correction unit (606) to be corrected for phase noise, and the phase noise is corrected by acquiring information in a second beat frequency signal (604 a); after the data is corrected, the position amplitude of scattering is obtained, and finally the data enters a spectrum analysis unit (607) for analysis to obtain accurate reflection information of the device to be measured.
9. The light source non-linearity correction unit (605) of claim 8, characterized by: the light source frequency sweep nonlinearity is corrected by using a resampling method, the optical frequency information of the tunable light source is obtained by using a frequency sweep signal (511) output by the auxiliary interferometer (514), the equal optical frequency position can be obtained, the signal of the main interferometer (515) is resampled by using an interpolation method, and finally the main interferometer signal reaching the equal optical frequency interval is obtained.
10. The phase noise correction (606) of claim 8, wherein: the phase noise term carried by the mth beat frequency signal (604 a) obtained by the light in the loop structure module (3) interfering with the light on the reference arm m times through the extension optical fiber (303) is phi m (t)-Φ m (t-τ m ) The phase noise term carried by a master interferometer beat frequency signal (602 a) generated by the device module (4) to be tested and the master interferometer reference arm (401 b) is phi 0 (t)-Φ 0 (t-τ z ) Let phi m (t)-Φ m (t-τ m ) And phi 0 (t)-Φ 0 (t-τ z ) And matching to eliminate the phase noise in the beat signal of the main interferometer.
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