Phase noise compensation device and method for OFDR (Optical Frequency Domain Reflectometer) system of optical fiber loop auxiliary interferometer

Abstract

The invention belongs to the technical field of distributed optical fiber sensing, and particularly relates to an OFDR system phase noise compensation device and method based on an or equivalent optical fiber loop auxiliary interferometer. The optical fiber 2 * 2 coupler and the delay optical fiber form a loop and serve as a delay optical path of the auxiliary interferometer, the optical delay can be doubled every time laser passes through the loop, and therefore a plurality of beat frequency signals with different optical delays are obtained. And according to the to-be-measured optical fibers at different distances on the main interferometer, the beat frequency signal of the auxiliary interferometer with the closest optical delay is selected for compensation. And splicing the compensated reflected signals at different distances to obtain a distance domain reflected signal of the whole sensing optical fiber, thereby realizing phase noise compensation in the full range of the OFDR system. The OFDR full-range high-precision measurement is realized, and the OFDR full-range high-precision measurement device has the characteristics of simple structure, stable performance, low manufacturing cost and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of distributed optical fiber sensing, and more specifically, relates to an OFDR system phase noise compensation device and method based on an optical fiber loop-assisted interferometer. Background Art

[0002] Optical frequency domain reflectometry (OFDR) is a fiber-optic distributed positioning technology that uses the principle of beat frequency interferometry to locate the reflected signal at different positions of the optical fiber to be tested. Optical frequency domain reflectometry has the advantages of simple structure, high spatial resolution, high signal-to-noise ratio, and anti-electromagnetic interference. It is widely used in structural health monitoring, composite material defect detection, three-dimensional shape sensing, oil and gas pipeline detection and other fields. It is an important branch of the field of distributed optical fiber sensing.

[0003] However, the phase noise of the tunable laser source will seriously affect the spatial resolution of the optical frequency domain reflectometer. Generally, an external auxiliary interferometer is used to measure and compensate for the phase noise. The traditional auxiliary interferometer uses an unbalanced Mach-Zehnder interferometer with a single optical delay. This method has a good compensation effect when the length of the optical fiber to be measured on the main interferometer is close to the length of the delay optical fiber of the auxiliary interferometer. However, when the length of the optical fiber to be measured differs too much from the length of the delay optical fiber, the compensation effect of this method will deteriorate, resulting in a deterioration in the spatial resolution of long-distance measurements. Summary of the invention

[0004] In view of the defects of the prior art, the purpose of the present invention is to provide an OFDR system phase noise compensation device and method based on a fiber loop auxiliary interferometer. The fiber 2x2 coupler and the delay fiber of the present invention form a loop and serve as the delay optical path of the auxiliary interferometer. Each time the laser undergoes a loop optical delay, it will double, thereby obtaining multiple beat frequency signals with different optical delays, which solves the problem that the existing auxiliary interferometer only has a good compensation effect on the phase noise within a certain measurement range of the optical fiber to be measured. The present invention also has a good compensation effect on the phase noise of long-distance optical fibers.

[0005] The technical solution of the present invention is: a phase noise compensation device for an OFDR system based on an optical fiber loop auxiliary interferometer, comprising: a tunable laser source, used to output a frequency-linearly changing swept laser and a sampling trigger signal; a first optical fiber 1x2 coupler, used to divide the swept laser into two paths, which enter the optical fiber loop auxiliary interferometer and the main interferometer respectively; characterized in that the optical fiber loop auxiliary interferometer is used to make the swept laser entering the auxiliary interferometer generate beat frequency interference with different optical delays; the main interferometer is used to make the swept laser entering the main interferometer generate beat frequency interference after being reflected by the optical fiber to be measured; a balanced photodetector is used to convert the optical beat frequency signal of the optical fiber loop auxiliary interferometer into an electrical signal, and at the same time remove the DC amount in the beat frequency signal; the balanced photodetector is used to convert the optical beat frequency signal output by the main interferometer into an electrical signal, and at the same time remove the DC amount in the beat frequency signal; an oscilloscope is used to balance Sampling of electrical signals output by a photodetector and a balanced photodetector; a computer for processing data collected by an oscilloscope; the computer includes filtering of beat frequency signals under different optical delays of an auxiliary interferometer; performing noise compensation on signals at different distances of the optical fiber by a resampling algorithm; splicing compensated signals in different regions to obtain a complete reflection signal in the distance domain of the optical fiber to be measured; an optical fiber loop auxiliary interferometer includes an auxiliary optical fiber 1x2 coupler, an auxiliary optical fiber 2x2 coupler, a delay optical fiber, and an auxiliary optical fiber 2x2 coupler; the delay optical fiber is connected to two ports on different sides of the auxiliary optical fiber 2x2 coupler, and the other two ports of the auxiliary optical fiber 2x2 coupler are respectively connected to one of the branch ends of the auxiliary optical fiber 1x2 coupler and the auxiliary optical fiber 2x2 coupler to form a delay loop; the other branch end of the auxiliary optical fiber 1x2 coupler is connected to the auxiliary optical fiber 2x2 coupler to form an auxiliary reference optical path.

[0006] An OFDR system phase noise compensation device of an equivalent fiber loop auxiliary interferometer comprises: a tunable laser source, used for outputting a frequency-sweeping laser with linear frequency variation and a sampling trigger signal; a first fiber 1x2 coupler, used for dividing the frequency-sweeping laser into two paths, respectively entering the fiber loop auxiliary interferometer and the main interferometer; characterized in that the fiber loop auxiliary interferometer is used for causing the frequency-sweeping laser entering the auxiliary interferometer to generate beat frequency interference with different optical delays; the main interferometer is used for causing the frequency-sweeping laser entering the main interferometer to generate beat frequency interference after being reflected by the optical fiber to be measured; a balanced photodetector is used for converting the optical beat frequency signal of the fiber loop auxiliary interferometer into an electrical signal, and removing the DC amount in the beat frequency signal; the balanced photodetector is used for converting the optical beat frequency signal output by the main interferometer into an electrical signal, and removing the DC amount in the beat frequency signal; an oscilloscope is used for balancing The invention discloses a method for sampling the output electrical signals of the weighing photodetector and the balanced photodetector; a computer is used for processing the data collected by the oscilloscope; the computer includes filtering the beat frequency signals of the auxiliary interferometer under different optical delays; performing a resampling algorithm to compensate for the noise of the signals at different distances of the optical fiber; splicing the compensated signals in different areas to obtain the complete reflection signal in the distance domain of the optical fiber to be measured; the delay optical path of the optical fiber loop auxiliary interferometer includes an auxiliary optical fiber 1x2 coupler, an auxiliary optical fiber circulator, an array optical fiber grating and an auxiliary optical fiber 2x2 coupler; the laser enters the array optical fiber grating through the auxiliary optical fiber circulator, and the gratings at different positions in the array optical fiber grating will respectively reflect a part of the power of the light, and the reflected signal enters the auxiliary optical fiber 2x2 coupler through the auxiliary optical fiber circulator; the other path in the branch end of the auxiliary optical fiber 1x2 coupler is connected to the auxiliary optical fiber 2x2 coupler to form an auxiliary reference optical path.

[0007] According to the above-mentioned OFDR system phase noise compensation device based on or equivalent to the fiber loop auxiliary interferometer, the main interferometer comprises a main fiber 1x2 coupler, a main fiber circulator, and the fiber to be tested is connected to the main fiber 2x2 coupler; one of the branch ends of the main fiber 1x2 coupler is connected to the main 2x2 coupler 34 to form a main reference optical path; the other branch end of the main fiber 1x2 coupler is connected to the a port of the main fiber circulator, and the b port and the c port of the main fiber circulator are respectively connected to the fiber to be tested and the main fiber 2x2 coupler to form a measurement optical path; the reflected signals at different distances on the fiber to be tested in the main interferometer have different optical delays compared with the main reference optical path, and the main optical delay τ F satisfy:

[0008]

[0009] Among them, n eff is the effective refractive index of the optical fiber core to be tested; c is the speed of light in vacuum; l F are different distances on the optical fiber to be tested.

[0010] According to the above-mentioned OFDR system phase noise compensation device based on fiber loop auxiliary interferometer, it is characterized in that the optical delay generated by the Nth time passing through the delay loop is called the Nth order auxiliary optical delay, and the auxiliary optical delay τ N satisfy:

[0011]

[0012] Among them, n eff is the effective refractive index of the delay fiber core; l is the length of the delay fiber; c is the speed of light in vacuum; N is a positive integer.

[0013] According to the above-mentioned OFDR system phase noise compensation device of an equivalent fiber loop auxiliary interferometer, the reflection signal of the Nth grating has an auxiliary light delay τ N The calculation formula is:

[0014]

[0015] Among them, n eff is the effective refractive index of the fiber grating array core, c is the speed of light in vacuum; l FBG is the spacing length between adjacent gratings on the fiber grating array.

[0016] According to the above-mentioned OFDR system phase noise compensation device based on or equivalent to the fiber loop assisted interferometer, it is characterized in that the starting trigger end of the tunable laser source is connected to the sampling trigger end of the oscilloscope to form a trigger circuit.

[0017] The present invention also discloses a method for compensating phase noise of an OFDR system based on an optical fiber loop-assisted interferometer, which is characterized by comprising the following steps:

[0018] Step 1. Calculate the maximum main light delay τ of the fiber under test Fmax ,

[0019]

[0020] Among them, n eff is the effective refractive index of the optical fiber core to be tested, c is the speed of light in vacuum, l Fmax is the length of the optical fiber to be tested;

[0021] Step 2. Calculate the first-order auxiliary light delay τ in the auxiliary interferometer 1 ,

[0022]

[0023] Among them, n effis the effective refractive index of the delay fiber core, c is the speed of light in vacuum; l is the length of the delay fiber;

[0024] Step 3. Calculate the upper limit N of the order N max ,

[0025] N max =round[τ Fmax / τ 1 ]

[0026] Among them, N max The minimum value is 1; round[] means rounding to the nearest integer;

[0027] Step 4. Use filtering to reduce the order to no more than N max The beat frequency signals corresponding to each order of auxiliary light delay are filtered out;

[0028] Step 5. Use the beat frequency signals corresponding to different auxiliary light delays to respectively measure the beat frequency signal S collected by the main interferometer. main (t) to make compensation;

[0029] Step 6. For the distance domain reflection signals of the main interferometer after compensation for different auxiliary light delays, only the signals within a suitable distance range are selected as effective compensation reflection signals;

[0030] Step 7. Connect the effective compensation reflection signals within different distance ranges end to end from small to large in the distance domain to form a whole distance-reflection intensity curve.

[0031] According to the above-mentioned OFDR system phase noise compensation method based on optical fiber loop auxiliary interferometer, the specific process of step 4 is as follows: the beat frequency signals corresponding to different auxiliary light delays have different frequencies; the original beat frequency signal of the auxiliary interferometer collected by the oscilloscope is Fourier transformed by a computer to obtain different frequency components in the beat frequency signal; the frequency components corresponding to different auxiliary light delays are filtered respectively by a rectangular window function, and the filtered frequency signals are inverse Fourier transformed to separate and obtain the beat frequency signals corresponding to different auxiliary light delays.

[0032] According to the above-mentioned OFDR system phase noise compensation method based on fiber loop auxiliary interferometer, the feature is that step 5 specifically comprises: performing Hilbert transform on the beat frequency signal corresponding to different auxiliary light delays, converting the real cosine signal into a complex exponential signal; performing arc tangent transform on the complex exponential signal to obtain the instantaneous phase of the beat frequency signal; differentiating the phase to obtain the instantaneous output frequency of the tunable laser source under different auxiliary light delays; using a cubic spline interpolation function to interpolate the beat frequency signal S of the main interferometer. main(t) performing equal frequency resampling; performing Fourier transform on the resampled beat frequency signal to obtain a reflection signal in the distance domain after the beat frequency signal corresponding to different auxiliary light delays of the optical fiber to be tested is compensated.

[0033] According to the above-mentioned OFDR system phase noise compensation method based on fiber loop auxiliary interferometer, the auxiliary fiber 2x2 coupler and the delay fiber are replaced by auxiliary fiber circulator and array fiber grating, and the length l of the delay fiber in the formula is replaced by 2l FBG Substitute into the calculation.

[0034] Compared with the prior art, the above technical solutions conceived by the present invention can achieve the following

[0035] Beneficial effects:

[0036] 1. Compared with other auxiliary interferometers, the fiber loop auxiliary interferometer of the present invention can obtain beat frequency signals with different optical delays with only one delay optical fiber. Because other auxiliary interferometers simply connect the delay optical fiber in series on one of the paths, only the beat frequency signal with a single optical delay can be obtained, so that the phase noise compensation effect is only good near a single position of the optical fiber to be tested, while the phase noise compensation effect for other farther positions is poor. If you want to have a better compensation effect on the optical fiber to be tested at a long distance, you can only replace the delay optical fiber with a longer and matching length and measure again, which increases the measurement time and material cost, which is not conducive to real-time, high spatial resolution, and large-scale distributed optical fiber sensing.

[0037] 2. The fiber loop assisted interferometer of the present invention connects a single delayed fiber and a fiber 2x2 coupler into a loop, and only a single measurement is required to obtain the beat frequency interference signal of different optical delays, thereby achieving a good noise compensation effect at different positions of the measured fiber. The present invention realizes full-range and high-precision OFDR measurement, and has the characteristics of simple structure, stable performance and low production cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a structural schematic diagram of a device according to an embodiment of the present invention;

[0039] Figure 2 It is a structural schematic diagram of a device of a second embodiment of the present invention;

[0040] Figure 3 Schematic diagram of the optical fiber length to be measured and the auxiliary light delay matching;

[0041] Description of reference numerals: Figure 1In it, there are a tunable laser source 1, a first optical fiber 1x2 coupler 11, a trigger circuit 12, an auxiliary interferometer 2, a main interferometer 3, an oscilloscope 4, and a computer 5; in the auxiliary interferometer, there are an auxiliary optical fiber 1x2 coupler 21, an auxiliary optical fiber 2x2 coupler 22, a delay optical fiber 23, an auxiliary optical fiber 2x2 coupler 24, and a balanced photodetector 25; in the main interferometer, there are a main optical fiber 1x2 coupler 31, a main optical fiber circulator 32, an optical fiber to be tested 33, a main optical fiber 2x2 coupler 34, and a balanced photodetector 35.

[0042] Figure 2 In the figure, there are auxiliary interferometer 2, auxiliary fiber circulator 26, and array fiber grating 27. DETAILED DESCRIPTION

[0043] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0044] The embodiment of the present invention provides an OFDR system phase noise compensation device based on a fiber loop auxiliary interferometer, such as Figure 1 As shown, it includes: a tunable laser source 1, an auxiliary interferometer 2, a main interferometer 3, an oscilloscope 4, and a computer 5.

[0045] The start trigger terminal of the tunable laser source 1 of the present invention is connected to the sampling trigger terminal of the oscilloscope 4 to form a trigger circuit 12.

[0046] The tunable laser source 1 outputs linearly swept continuous light, which enters the auxiliary interferometer 2 and the main interferometer 3 respectively through the first optical fiber 1x2 coupler 11; in the main interferometer 3, the laser is divided into two paths through the main optical fiber 1x2 coupler 31, one path enters the main optical fiber circulator 32, and the other path enters the main reference optical path. After the laser enters the a port of the main optical fiber circulator 32, it is output from the b port and enters the optical fiber to be tested 33. The reflected light in the optical fiber to be tested 33 returns to the b port and is output from the c port. The output light here is the signal light. The main reference light and the signal light undergo beat frequency interference in the main optical fiber 2x2 coupler 34 and are received by the balanced photodetector 35. The reflected signals at different distances on the optical fiber to be tested 33 in the main interferometer have different optical delays compared to the main reference optical path. The main light delay τ F satisfy:

[0047]

[0048] Among them, n eff is the effective refractive index of the optical fiber core to be tested; c is the speed of light in vacuum; l F is the different distances on the optical fiber to be tested, and the value range is 0≤l F ≤lFmax ; l Fmax is the total length of the optical fiber to be tested.

[0049] The laser in the auxiliary interferometer 2 is divided into two paths through the auxiliary fiber 1x2 coupler 21. One path enters the auxiliary fiber 2x2 coupler 22 and the delay fiber 23 to form a delay loop, and the other path enters the auxiliary reference optical path. Each time the swept laser passes through the delay loop, the optical delay compared to the auxiliary reference optical path will double. At the same time, half of the optical power of the laser in the delay loop will be output to the auxiliary fiber 2x2 coupler 24 through the auxiliary fiber 2x2 coupler 22, and the other half of the optical power of the laser will continue to be transmitted in the loop. In this way, lasers with different optical delays can be generated and output in a reciprocating cycle. The optical delay generated by the Nth time through the delay loop is called the Nth-order auxiliary optical delay, and the auxiliary optical delay τ N satisfy:

[0050]

[0051] Among them, n eff is the effective refractive index of the core of the delay optical fiber 23; l is the length of the delay optical fiber 23; c is the speed of light in vacuum; N is a positive integer, ranging from 1≤N≤N max , represents the number of times the light passes through the delay loop; N max is the upper limit of the order N, defined as:

[0052] N max =round[τ Fmax / τ 1 ]

[0053] Among them, N max The minimum value is 1; round[] means rounding to the nearest integer; τ Fmax Represents the maximum optical delay of the optical fiber under test; τ 1 It is the first-order auxiliary light delay.

[0054] N generated by the delay loop max The lasers with different auxiliary light delays and the laser of the auxiliary reference light path generate beat frequency interference in the auxiliary optical fiber 2x2 coupler 24 , and the interference signal is received by the balanced photodetector 25 .

[0055] As a second specific implementation method, Figure 2As shown, the delay optical path of the auxiliary interferometer 2 includes an auxiliary fiber 1x2 coupler 21, an auxiliary fiber circulator 26, an array fiber grating 27 and an auxiliary fiber 2x2 coupler 24. The array fiber grating 27 is a section of optical fiber engraved with a grating array. The length of the array fiber grating 27 should be greater than or equal to the length of the optical fiber to be measured in the main interferometer. Fiber grating is a passive reflective optical device, and a number of weak reflectivity fiber gratings are evenly distributed on the axial distance of the optical fiber to form a grating array. The laser enters the array fiber grating 27 through the circulator 26, and the array fiber grating 27 reflects a part of the power of the light, and the reflected signal enters the fiber coupler 24 through the circulator 26; the other path in the branch end of the auxiliary fiber 1x2 coupler 21 is connected to the auxiliary 2x2 coupler 24 to form an auxiliary reference optical path. Signals reflected at different positions will have different optical delays when they arrive at the balanced photodetector. The reflection signal of the Nth grating has an auxiliary optical delay τ N The calculation formula is:

[0056]

[0057] Among them, n eff is the effective refractive index of the core of the array fiber grating 27, c is the speed of light in vacuum; l FBG is the spacing length between adjacent gratings on the array fiber grating 27.

[0058] The difference between the two specific implementations is that the first specific implementation uses the laser to pass through the loop multiple times, so that the laser has different optical delays when it reaches the balanced photodetector. The second specific implementation uses a fiber grating array with a reflective effect, and the laser reflected at different distances will have different optical delays when it reaches the balanced photodetector.

[0059] In both embodiments, all optical fiber components are connected together through optical fiber jumpers and flanges. The working bandwidth of all optical fiber couplers should be sufficient to cover the frequency scanning range of the tunable laser source.

[0060] Based on the device of the above embodiment, the present invention also provides a method for phase noise compensation of an OFDR system based on a fiber loop assisted interferometer, comprising the following steps:

[0061] Step 1. Measure the length of the optical fiber to be tested l Fmax , calculate the maximum main light delay τ of the optical fiber to be tested Fmax ,

[0062]

[0063] Among them, n eff is the effective refractive index of the optical fiber core to be tested, and c is the speed of light in vacuum.

[0064] Step 2. Measure the length l of the delay fiber in the auxiliary interferometer and calculate the first-order auxiliary light delay τ 1 ,

[0065]

[0066] Among them, n eff is the effective refractive index of the delay fiber core, and c is the speed of light in vacuum.

[0067] Step 3. Calculate the upper limit N of the order N max ,

[0068] N max =round[τ Fmax / τ 1 ]

[0069] Among them, N max The minimum value is 1; round[] means rounding to the nearest integer;

[0070] Step 4. Use filtering to reduce the order to no more than N max The beat frequency signals corresponding to each order of auxiliary light delay are filtered out. The specific method is: the beat frequency signals corresponding to different auxiliary light delays have different frequencies. For example, the beat frequency signal S corresponding to the Nth order of auxiliary light delay N The frequency of (t) is:

[0071] f N =τ N *γ

[0072] Wherein, γ is the sweeping speed of the linear swept laser output by the tunable laser source 1 .

[0073] The signal S received by the photodetector in the auxiliary interferometer ref (t) = ∑S N (t) Perform Fourier transform fft to obtain the frequency information ∑F(f N ):

[0074]

[0075] The frequency components corresponding to different auxiliary light delays are filtered separately through the window function:

[0076]

[0077] Performing inverse Fourier transform ifft on the filtered frequency components can separate and obtain the beat frequency signals corresponding to different auxiliary light delays. The beat frequency signal S corresponding to the Nth order auxiliary light delay is N (t) as an example:

[0078]

[0079] Step 5. Use the beat frequency signals corresponding to different auxiliary light delays to respectively measure the beat frequency signal S collected by the main interferometer. main The specific method is: perform Hilbert transform on the beat frequency signals corresponding to different auxiliary light delays, convert the real cosine signal into a complex exponential signal, and use the beat frequency signal S corresponding to the Nth order auxiliary light delay as the N (t) as an example:

[0080]

[0081] Perform an inverse tangent transform on the complex exponential signal to obtain the instantaneous phase of the beat frequency signal

[0082]

[0083] Among them, imag() means taking the imaginary part, and real() means taking the real part.

[0084] Differentiate the phase to obtain the instantaneous output frequency f of the tunable laser source 1 under the Nth order auxiliary light delay N (t):

[0085]

[0086] According to the instantaneous output frequency f of the tunable laser source 1 N (t), using the cubic spline interpolation function spline to calculate the beat frequency signal S of the main interferometer main (t) Perform equal frequency resampling:

[0087]

[0088] The resampled beat frequency signal S main,N (t) Perform Fourier transform to obtain the beat frequency signal S corresponding to the Nth-order auxiliary light delay of the optical fiber under test N (t) Reflected signal F in the distance domain after compensation main,N (l F ):

[0089]

[0090] Among them, l F are different distances on the optical fiber to be tested.

[0091] Step 6. For the distance domain reflection signal of the main interferometer after different auxiliary light delay compensation, only the signal within the appropriate distance range is selected as the effective compensation reflection signal. main,N (lF ) as an example, only take

[0092]

[0093] The signal within the distance range is used as the effective compensation reflection signal. Where l is the length of the delay fiber; especially note that when N = 1, F main,1 (l F ) Only take

[0094]

[0095] The signal within the distance range is used as the effective compensation reflection signal. In order to more clearly explain the relationship between different positions of the optical fiber to be tested and different auxiliary light delays, the following is provided: Figure 3 Schematic diagram of the optical fiber length to be measured and the auxiliary light delay matching.

[0096] Step 7. Connect the effective compensation reflection signals within different distance ranges end to end from small to large in the distance domain to form a whole distance-reflection intensity curve.

[0097] It should be noted that the above steps 1-7 are the method of OFDR system phase noise compensation based on fiber loop assisted interferometer, which is applicable to the first specific implementation. For the second specific implementation, just replace l in the above steps with 2l FBG Just substitute it into the calculation.

[0098] In summary, the present invention discloses a phase noise compensation device and method for an OFDR system based on an optical fiber loop auxiliary interferometer. An optical fiber 2x2 coupler and a delay optical fiber form a loop and serve as a delay optical path of an auxiliary interferometer. Each time the light passes through the loop, a fixed optical delay will be added, and the coupler will output a portion of the power of the light. According to the optical fibers to be tested at different distances on the main interferometer, the auxiliary interferometer beat frequency signal with the closest optical delay order is selected for compensation. The reflection signal of the distance domain of the entire optical fiber can be obtained by splicing the reflection signals of the optical fibers to be tested at different distances after compensation.

[0099] It will be easily understood by those skilled in the art that the embodiments described herein are only used to illustrate the technology of the present invention and are not used 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 protection scope of the present invention.

Claims

1. A phase noise compensation device for an OFDR system based on a fiber loop assisted interferometer, comprising: A tunable laser source (1) is used to output a frequency-sweeping laser with linear frequency variation and a sampling trigger signal; The first optical fiber 1x2 coupler (11) is used to divide the frequency sweeping laser into two paths, which enter the auxiliary interferometer (2) and the main interferometer (3) respectively; the auxiliary interferometer (2) is used to make the frequency sweeping laser entering the auxiliary interferometer generate beat frequency interference with different optical delays; the main interferometer (3) is used to make the frequency sweeping laser entering the main interferometer generate beat frequency interference after being reflected by the optical fiber to be measured (33); the balanced photodetector (25) is used to convert the optical beat frequency signal of the optical fiber loop auxiliary interferometer into an electrical signal, and remove the DC amount in the beat frequency signal; the balanced photodetector (35) is used to convert the optical beat frequency signal output by the main interferometer into an electrical signal, and remove the DC amount in the beat frequency signal; the oscilloscope (4) is used to sample the electrical signals output by the balanced photodetector (25) and the balanced photodetector (35); the computer (5) is used to process the data collected by the oscilloscope; The computer (5) includes filtering the beat frequency signal of the auxiliary interferometer under different optical delays; performing a resampling algorithm to perform noise compensation on the signals at different distances of the optical fiber; and splicing the compensated signals in different regions to obtain the reflection signal in the complete distance domain of the optical fiber to be measured; the auxiliary interferometer (2) includes an auxiliary optical fiber 1x2 coupler (21), an auxiliary optical fiber 2x2 coupler (22), a delay optical fiber (23), and an auxiliary optical fiber 2x2 coupler (24); the delay optical fiber (23) is connected to two ports on different sides of the auxiliary optical fiber 2x2 coupler (22), and the other two ports of the auxiliary optical fiber 2x2 coupler (22) are respectively connected to one of the branch ends of the auxiliary optical fiber 1x2 coupler (21) and the auxiliary optical fiber 2x2 coupler (24) to form a delay loop; the other branch end of the auxiliary optical fiber 1x2 coupler (21) is connected to the auxiliary optical fiber 2x2 coupler (24) to form an auxiliary reference optical path.

2. An OFDR system phase noise compensation device of an equivalent fiber loop assisted interferometer, comprising: A tunable laser source (1) is used to output a frequency-sweeping laser with linear frequency variation and a sampling trigger signal; The first optical fiber 1x2 coupler (11) is used to divide the frequency sweeping laser into two paths, which enter the auxiliary interferometer (2) and the main interferometer (3) respectively; the auxiliary interferometer (2) is used to make the frequency sweeping laser entering the auxiliary interferometer generate beat frequency interference with different optical delays; the main interferometer (3) is used to make the frequency sweeping laser entering the main interferometer generate beat frequency interference after being reflected by the optical fiber to be measured (33); the balanced photodetector (25) is used to convert the optical beat frequency signal of the optical fiber loop auxiliary interferometer into an electrical signal, and remove the DC amount in the beat frequency signal; the balanced photodetector (35) is used to convert the optical beat frequency signal output by the main interferometer into an electrical signal, and remove the DC amount in the beat frequency signal; the oscilloscope (4) is used to sample the electrical signals output by the balanced photodetector (25) and the balanced photodetector (35); the computer (5) is used to collect data from the oscilloscope The computer (5) includes filtering the beat frequency signals of the auxiliary interferometer under different optical delays; performing a resampling algorithm to perform noise compensation on the signals at different distances of the optical fiber; splicing the compensated signals in different regions to obtain the complete reflection signal in the distance domain of the optical fiber to be measured; the delay optical path of the auxiliary interferometer (2) includes an auxiliary optical fiber 1x2 coupler (21), an auxiliary optical fiber circulator (26), an array optical fiber grating (27) and an auxiliary optical fiber 2x2 coupler (24); the laser enters the array optical fiber grating (27) through the auxiliary optical fiber circulator (26), and the gratings at different positions in the array optical fiber grating (27) will respectively reflect a part of the power of the light, and the reflected signal enters the auxiliary optical fiber 2x2 coupler (24) through the auxiliary optical fiber circulator (26); the other path in the branch end of the auxiliary optical fiber 1x2 coupler (21) is connected to the auxiliary optical fiber 2x2 coupler (24) to form an auxiliary reference optical path.

3. The OFDR system phase noise compensation device based on or equivalent to a fiber loop assisted interferometer according to claim 1 or 2, characterized in that: The main interferometer comprises a main optical fiber 1x2 coupler (31), a main optical fiber circulator (32), an optical fiber to be tested (33) and a main optical fiber 2x2 coupler (34); one of the branch ends of the main optical fiber 1x2 coupler (31) is connected to the main 2x2 coupler 34 to form a main reference optical path; the other branch end of the main optical fiber 1x2 coupler (31) is connected to the a port of the main optical fiber circulator (32), and the b port and the c port of the main optical fiber circulator (32) are respectively connected to the optical fiber to be tested (33) and the main optical fiber 2x2 coupler (34) to form a measurement optical path; in the main interferometer, reflected signals at different distances on the optical fiber to be tested (33) have different optical delays compared with the main reference optical path, and the main optical delay τ F satisfy: Among them, n eff is the effective refractive index of the optical fiber core to be tested; c is the speed of light in vacuum; l F are different distances on the optical fiber to be tested.

4. The OFDR system phase noise compensation device based on fiber loop assisted interferometer according to claim 1, characterized in that: The optical delay generated by the Nth time passing through the delay loop is called the Nth-order auxiliary optical delay. N satisfy: Among them, n eff is the effective refractive index of the core of the delay optical fiber (23); l is the length of the delay optical fiber (23); c is the speed of light in vacuum; and N is a positive integer.

5. The OFDR system phase noise compensation device of an equivalent fiber loop assisted interferometer according to claim 2, characterized in that: The reflected signal of the Nth grating has an auxiliary light delay τ N The calculation formula is: Among them, n eff is the effective refractive index of the core of the array fiber grating (27), c is the speed of light in vacuum; l FBG is the spacing length between adjacent gratings on the array fiber grating (27).

6. The OFDR system phase noise compensation device based on or equivalent to a fiber loop assisted interferometer according to claim 1 or 2, characterized in that: The start trigger terminal of the tunable laser source is connected to the sampling trigger terminal of the oscilloscope to form a trigger circuit (12).

7. A method for compensating phase noise of an OFDR system based on a fiber loop assisted interferometer, characterized in that: The following steps are involved: Step 1. Calculate the maximum main light delay τ of the fiber under test Fmax , Among them, n eff is the effective refractive index of the optical fiber core to be tested, c is the speed of light in vacuum, l Fmax is the length of the optical fiber to be tested; Step 2. Calculate the first-order auxiliary light delay τ1 in the auxiliary interferometer, Among them, n eff is the effective refractive index of the delay fiber core, c is the speed of light in vacuum; l is the length of the delay fiber; Step 3. Calculate the upper limit N of the order N max , N max =round[τ Fmax / t1] Among them, N max The minimum value is 1; round[] means rounding to the nearest integer; Step 4. Use filtering to reduce the order to no more than N max The beat frequency signals corresponding to each order of auxiliary light delay are filtered out; Step 5. Use the beat frequency signals corresponding to different auxiliary light delays to respectively measure the beat frequency signal S collected by the main interferometer. main (t) to make compensation; Step 6. For the distance domain reflection signals of the main interferometer after compensation for different auxiliary light delays, only the signals within a suitable distance range are selected as effective compensation reflection signals; Step 7. Connect the effective compensation reflection signals within different distance ranges end to end from small to large in the distance domain to form a whole distance-reflection intensity curve.

8. The OFDR system phase noise compensation method based on fiber loop assisted interferometer according to claim 7, characterized in that: The specific process of step 4 is as follows: the beat frequency signals corresponding to different auxiliary light delays have different frequencies; the original beat frequency signal of the auxiliary interferometer collected by the oscilloscope (4) is Fourier transformed by the computer (5) to obtain different frequency components in the beat frequency signal; the frequency components corresponding to different auxiliary light delays are filtered respectively by a rectangular window function, and the filtered frequency signals are inversely Fourier transformed to separate and obtain the beat frequency signals corresponding to different auxiliary light delays.

9. The OFDR system phase noise compensation method based on fiber loop assisted interferometer according to claim 7, characterized in that: Step 5 is specifically as follows: perform Hilbert transform on the beat frequency signal corresponding to different auxiliary light delays to convert the real cosine signal into a complex exponential signal; perform inverse tangent transform on the complex exponential signal to obtain the instantaneous phase of the beat frequency signal; differentiate the phase to obtain the instantaneous output frequency of the tunable laser source (1) under different auxiliary light delays; use a cubic spline interpolation function to calculate the beat frequency signal S of the main interferometer. main (t) performing equal frequency resampling; performing Fourier transform on the resampled beat frequency signal to obtain a reflection signal in the distance domain after the beat frequency signal corresponding to different auxiliary light delays of the optical fiber to be tested is compensated.

10. The OFDR system phase noise compensation method based on fiber loop assisted interferometer according to claim 7, characterized in that: The auxiliary fiber 2x2 coupler (22) and the delay fiber (23) are replaced by an auxiliary fiber circulator (26) and an array fiber grating (27), and the length l of the delay fiber in the formula is replaced by 2l FBG Substitute into the calculation.

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