Phase noise compensation apparatus and method for ofdr system of fiber loop assisted interferometer

Abstract

The present application belongs to the field of distributed optical fiber sensing technology, and particularly relates to a kind of OFDR system phase noise compensation device and method based on or equivalent optical fiber loop auxiliary interferometer.The optical fiber 2x2 coupler and delay optical fiber of the present application constitute a loop and serve as the delay optical path of auxiliary interferometer, and the optical delay of laser will be doubled after each loop, thereby obtaining multiple beat signals with different optical delays.According to the different distances of the optical fiber to be measured on the main interferometer, the beat signal of the auxiliary interferometer with the closest optical delay is selected for compensation.The distance domain reflection signal of the whole sensing optical fiber can be obtained by splicing the compensated reflection signals at different distances, thereby realizing the phase noise compensation of the OFDR system in the whole range.The present application realizes the full-range and high-precision measurement of OFDR, and has the characteristics of simple structure, stable performance, low manufacturing cost and the like.

Description

Technical Field

[0001] This invention belongs to the field of distributed optical fiber sensing technology, and more specifically, relates to a phase noise compensation device and method for an OFDR system based on an optical fiber loop-assisted interferometer. Background Technology

[0002] Optical frequency domain reflectometry (OFDR) is a fiber optic distributed positioning technology that uses the beat frequency interference principle to locate the reflected signals at different positions on the fiber under test. OFDRs have advantages such as simple structure, high spatial resolution, high signal-to-noise ratio, and resistance to electromagnetic interference. They are widely used in structural health monitoring, composite material defect detection, three-dimensional shape sensing, and oil and gas pipeline inspection, and represent an important branch of distributed fiber optic sensing.

[0003] However, the phase noise of tunable laser sources can severely affect the spatial resolution of optical frequency domain reflectometers. A common method is to use an external auxiliary interferometer to measure and compensate for this phase noise. Traditional auxiliary interferometers use a single, unbalanced Mach-Zehnder interferometer with optical delay. This method provides good compensation when the length of the fiber under test in the main interferometer is close to the length of the delay fiber in the auxiliary interferometer. However, when the lengths of the fiber under test and the delay fiber differ significantly, the compensation effect deteriorates, leading to a decrease in spatial resolution over long distances. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a phase noise compensation device and method for an OFDR system based on a fiber optic loop-assisted interferometer. In this invention, a 2x2 fiber coupler and a delay fiber form a loop, which serves as the delay optical path for the auxiliary interferometer. The optical delay doubles each time the laser passes through the loop, thereby obtaining multiple beat frequency signals with different optical delays. This overcomes the limitation of existing auxiliary interferometers, which only provide good compensation for phase noise within a certain measurement range of the fiber under test. The present invention also provides good compensation for phase noise in long-distance fibers.

[0005] The technical solution of this invention is: a phase noise compensation device for an OFDR system based on a fiber optic loop-assisted interferometer, comprising: a tunable laser source for outputting a frequency-sweeping laser with linearly varying frequency and a sampling trigger signal; a first fiber optic 1x2 coupler for splitting the frequency-sweeping laser into two paths, which enter the fiber optic loop-assisted interferometer and the main interferometer, respectively; characterized in that the fiber optic loop-assisted interferometer is used to generate beat frequency interference with different optical delays in the frequency-sweeping laser entering the auxiliary interferometer; the main interferometer is used to generate beat frequency interference in the frequency-sweeping laser entering the main interferometer after reflection by the fiber under test; a balanced photodetector for converting the optical beat frequency signal of the fiber optic loop-assisted interferometer into an electrical signal, while removing the DC component from the beat frequency signal; a balanced photodetector for converting the optical beat frequency signal output by the main interferometer into an electrical signal, while removing the DC component from the beat frequency signal; and an oscilloscope for balancing... The system includes: sampling of the output electrical signals of photodetectors and balanced photodetectors; a computer for processing the data acquired by the oscilloscope; filtering of the beat frequency signals under different optical delays of the auxiliary interferometer; noise compensation of signals at different fiber distances using a resampling algorithm; and splicing the compensated signals from different regions to obtain the complete reflection signal over the distance domain of the fiber under test. The fiber loop auxiliary interferometer includes an auxiliary fiber 1x2 coupler, an auxiliary fiber 2x2 coupler, a delay fiber, and an auxiliary fiber 2x2 coupler. The delay fiber is connected to two ports on different sides of the auxiliary fiber 2x2 coupler, and the other two ports of the auxiliary fiber 2x2 coupler are respectively connected to one of the branch ends of the auxiliary fiber 1x2 coupler and the auxiliary fiber 2x2 coupler to form a delay loop. The other branch end of the auxiliary fiber 1x2 coupler is connected to the auxiliary fiber 2x2 coupler to form an auxiliary reference optical path.

[0006] A phase noise compensation device for an equivalent fiber optic loop-assisted interferometer (OFDR) system includes: a tunable laser source for outputting a linearly varying frequency sweep laser and a sampling trigger signal; a first fiber optic 1x2 coupler for splitting the sweep laser into two paths, which enter the fiber optic loop-assisted interferometer and the main interferometer, respectively; characterized in that the fiber optic loop-assisted interferometer generates beat frequency interference with different optical delays from the sweep laser entering the auxiliary interferometer; the main interferometer generates beat frequency interference from the sweep laser entering the main interferometer after reflection by the fiber under test; a balanced photodetector for converting the optical beat frequency signal of the fiber optic loop-assisted interferometer into an electrical signal while removing DC from the beat frequency signal; a balanced photodetector for converting the optical beat frequency signal output by the main interferometer into an electrical signal while removing DC from the beat frequency signal; and an oscilloscope for flattening the optical beat frequency signal. The system includes sampling of the output electrical signals of the balanced photodetector and the balanced photodetector; a computer for processing the data acquired 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 noise in signals at different distances in the optical fiber; splicing the compensated signals from different regions to obtain the complete reflection signal in the distance domain of the optical fiber under test; the delay optical path of the fiber loop auxiliary interferometer includes an auxiliary fiber 1x2 coupler, an auxiliary fiber circulator, an arrayed fiber grating, and an auxiliary fiber 2x2 coupler; the laser enters the arrayed fiber grating through the auxiliary fiber circulator, and the gratings at different positions in the arrayed fiber grating reflect a portion of the power of the light, and the reflected signal enters the auxiliary fiber 2x2 coupler through the auxiliary fiber circulator; the other path in the branch end of the auxiliary fiber 1x2 coupler is connected to the auxiliary fiber 2x2 coupler to form an auxiliary reference optical path.

[0007] According to the claims above, an OFDR system phase noise compensation device based on or equivalent to a fiber optic loop-assisted interferometer is characterized in that the main interferometer includes a main fiber 1x2 coupler, a main fiber circulator, and a fiber under test connected to a main fiber 2x2 coupler; one branch of the main fiber 1x2 coupler is connected to the main 2x2 coupler to form a main reference optical path; the other branch of the main fiber 1x2 coupler is connected to port a of the main fiber circulator, and ports b and c of the main fiber circulator are respectively connected to the fiber under test and the main fiber 2x2 coupler to form a measurement optical path; the reflected signals at different distances on the fiber under test in the main interferometer have different optical delays than the main reference optical path, and the main optical delay τ F satisfy:

[0008]

[0009] Where, n eff c is the effective refractive index of the fiber core under test; l is the speed of light in vacuum; F These represent different distances on the optical fiber under test.

[0010] According to the claim above, an OFDR system phase noise compensation device based on a fiber optic loop-assisted interferometer is characterized in that the optical delay generated by the delay loop for the Nth time is called the Nth-order auxiliary optical delay, and the auxiliary optical delay τ N satisfy:

[0011]

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

[0013] According to the claim above, an OFDR system phase noise compensation device for an equivalent fiber optic loop-assisted interferometer is characterized in that the reflected signal of the Nth grating has an auxiliary optical delay τ. N The calculation formula is:

[0014]

[0015] Where, n eff l is the effective refractive index of the fiber core of the arrayed fiber grating, and c is the speed of light in vacuum; FBG It is the spacing length between adjacent gratings on the arrayed fiber Bragg grating.

[0016] According to the claims, an OFDR system phase noise compensation device based on or equivalent to a fiber optic loop-assisted interferometer is characterized in that the start-up trigger terminal of the tunable laser source is connected to the sampling trigger terminal of the oscilloscope to form a trigger circuit.

[0017] This invention also discloses a phase noise compensation method for an OFDR system based on a fiber optic loop-assisted interferometer, characterized by comprising the following steps:

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

[0019]

[0020] Where, n eff ρ is the effective refractive index of the fiber core under test, c is the speed of light in vacuum, and l is the effective refractive index of the fiber core under test. Fmax The length of the optical fiber to be measured;

[0021] Step 2. Calculate the first-order auxiliary optical delay τ1 in the auxiliary interferometer.

[0022]

[0023] Where, n eff The effective refractive index of the delay fiber core is c, the speed of light in vacuum is l, and the length of the delay fiber is l.

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

[0025] N max =round[τ Fmax / τ1]

[0026] Where, N max The minimum value is 1; round[] represents rounding to the nearest integer.

[0027] Step 4. Filter the elements whose order is not greater than N. max The beat frequency signals corresponding to each order of auxiliary optical delay are filtered out;

[0028] Step 5. Use the beat frequency signals corresponding to different auxiliary optical delays to analyze the beat frequency signals S acquired by the main interferometer. main (t) is used for compensation;

[0029] Step 6. For the range domain reflection signals of the master interferometer after different auxiliary optical delay compensations, only the signals within a suitable range are selected as the effective compensation reflection signals;

[0030] Step 7. Connect the effective compensation reflection signals within different distance ranges from smallest to largest in the distance domain to form a complete distance-reflection intensity curve.

[0031] According to the above-described method for phase noise compensation of an OFDR system based on a fiber optic loop-assisted interferometer, the specific process of step 4 is as follows: the beat frequency signals corresponding to different auxiliary optical delays have different frequencies; the original beat frequency signals of the auxiliary interferometer acquired by the oscilloscope are subjected to Fourier transform by a computer to obtain different frequency components in the beat frequency signals; the frequency components corresponding to different auxiliary optical delays are filtered by a rectangular window function, and the beat frequency signals corresponding to different auxiliary optical delays are separated and obtained by performing an inverse Fourier transform on the filtered frequency signals.

[0032] According to the aforementioned method for phase noise compensation of an OFDR system based on a fiber optic loop-assisted interferometer, step 5 specifically comprises: performing a Hilbert transform on the beat frequency signals corresponding to different auxiliary optical delays to convert the real cosine signal into a complex exponential signal; performing an arctangent 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 optical delays; and using a cubic spline interpolation function to adjust the beat frequency signal S of the main interferometer. main (t) Perform equal-frequency resampling; perform Fourier transform on the resampled beat frequency signal to obtain the distance domain reflection signal after the beat frequency signal is compensated for by different auxiliary light delays of the optical fiber under test.

[0033] According to the claim above, a phase noise compensation method for an OFDR system based on a fiber optic loop-assisted interferometer is characterized in that the auxiliary fiber 2x2 coupler and the delay fiber are replaced by an auxiliary fiber circulator and an arrayed fiber grating, and the length l of the delay fiber in the formula is replaced by 2l. FBG Substitute the values ​​into the calculation.

[0034] Compared with the prior art, the above-described technical solutions conceived in this invention can achieve the following results.

[0035] Beneficial effects:

[0036] 1. Compared with other auxiliary interferometers, the fiber optic loop-assisted interferometer of this invention can obtain beat frequency signals with different optical delays using only a single delay fiber. Other auxiliary interferometers simply connect the delay fiber in series on one path, thus only obtaining the beat frequency signal of a single optical delay. This results in good phase noise compensation only near a single location on the fiber under test, while the compensation effect for phase noise at other, more distant locations is poor. To achieve good compensation for distant fiber under test, a longer and more matching delay fiber must be used, and the measurement must be repeated, increasing measurement time and material costs. This is not conducive to real-time, high spatial resolution, and large-scale distributed fiber optic sensing.

[0037] 2. The fiber optic loop-assisted interferometer of this invention connects a single delay fiber to a 2x2 fiber coupler to form a loop. It can obtain beat frequency interference signals with different optical delays in a single measurement, thereby achieving good noise compensation at different locations on the fiber under test. This invention achieves full-range, high-precision OFDR measurement and features simple structure, stable performance, and low manufacturing cost. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the structure of an embodiment of the device of the present invention;

[0039] Figure 2 This is a schematic diagram of the structure of the device according to the second embodiment of the present invention;

[0040] Figure 3 Schematic diagram of matching the length of the fiber under test with the delay of the auxiliary light;

[0041] Explanation of reference numerals in the attached figures: Figure 1In the instrument, there are: 1. Tunable laser source; 11. First fiber optic 1x2 coupler; 12. Trigger circuit; 2. Auxiliary interferometer; 3. Main interferometer; 4. Oscilloscope; 5. Computer; 6. Auxiliary interferometer: 21. Auxiliary fiber optic 1x2 coupler; 22. Auxiliary fiber optic 2x2 coupler; 23. Delay fiber; 24. Auxiliary fiber optic 2x2 coupler; 25. Balanced photodetector; 7. Main interferometer: 31. Main fiber optic 1x2 coupler; 32. Main fiber optic circulator; 33. Fiber under test; 34. Main fiber optic 2x2 coupler; 35. Balanced photodetector.

[0042] Figure 2 In the middle, there is an auxiliary interferometer 2, an auxiliary fiber optic circulator 26, and an array of fiber optic gratings 27. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0044] This invention provides a phase noise compensation device for an OFDR system based on a fiber optic loop-assisted 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-up 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] A tunable laser source 1 outputs linearly swept continuous light, which enters the auxiliary interferometer 2 and the main interferometer 3 respectively via a first fiber optic 1x2 coupler 11. In the main interferometer 3, the laser is split into two paths by the main fiber optic 1x2 coupler 31: one path enters the main fiber optic circulator 32, and the other path enters the main reference optical path. After entering port a of the main fiber optic circulator 32, the laser is output from port b and enters the fiber under test 33. The reflected light in the fiber under test 33 returns to port b and is output from port c; this output light is the signal light. The main reference light and the signal light undergo beat frequency interference in the main fiber optic 2x2 coupler 34 and are received by the balanced photodetector 35. The reflected signals at different distances on the fiber under test 33 in the main interferometer have different optical delays compared to the main reference optical path; the main optical delay τ... F satisfy:

[0047]

[0048] Where, n eff c is the effective refractive index of the fiber core under test; l is the speed of light in vacuum; F The values ​​represent different distances on the optical fiber under test, with a range of 0 ≤ l. F ≤lFmax ;l Fmax The total length of the optical fiber to be tested.

[0049] In auxiliary interferometer 2, the laser beam is split into two paths via auxiliary fiber 1x2 coupler 21. One path enters auxiliary fiber 2x2 coupler 22 and delay fiber 23 to form a delay loop, while 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 doubles. Simultaneously, half the optical power of the laser in the delay loop is output to auxiliary fiber 2x2 coupler 24 via auxiliary fiber 2x2 coupler 22, while the other half continues to propagate within the loop. This cycle repeats to generate and output laser beams with different optical delays. The optical delay generated by the Nth pass through the delay loop is called the Nth-order auxiliary optical delay, τ. N satisfy:

[0050]

[0051] Where, n eff is the effective refractive index of the core of delay fiber 23; l is the length of delay fiber 23; c is the speed of light in vacuum; N is a positive integer, with a value range of 1≤N≤N max N represents the number of times the light passes through the delay loop; max The order N is the upper limit of the possible values, defined as:

[0052] N max =round[τ Fmax / τ1]

[0053] Where, N max The minimum value is 1; round[] represents rounding to the nearest integer; τ Fmax τ represents the maximum optical delay of the fiber under test; τ1 is the first-order auxiliary optical delay.

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

[0055] As a second specific implementation method, such as 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 arrayed fiber grating 27, and an auxiliary fiber 2x2 coupler 24. The arrayed fiber grating 27 is a section of optical fiber etched with a grating array. The length of the arrayed fiber grating 27 should be greater than or equal to the length of the fiber under test in the main interferometer. A fiber grating is a passive reflective optical device; several weakly reflective fiber gratings are uniformly distributed along the axial distance of the fiber to form a grating array. The laser light enters the arrayed fiber grating 27 through the circulator 26, and the arrayed fiber grating 27 reflects a portion of the power of the light. 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 from different positions will have different optical delays when they reach the balanced photodetector. The reflected signal of the Nth grating has an auxiliary optical delay τ. N The calculation formula is:

[0056]

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

[0058] The difference between the two specific implementation methods lies in the following: the first implementation method utilizes the laser to pass through the loop multiple times, resulting in different optical delays when the laser reaches the balanced photodetector. The second implementation method utilizes a fiber optic grating array with reflective properties, so that the laser reflected at different distances will have different optical delays when it reaches the balanced photodetector.

[0059] In both implementations, all fiber optic devices are connected together via fiber optic patch cords and flanges. The operating bandwidth of all fiber optic couplers should be sufficient to encompass the frequency scanning range of the tunable laser source.

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

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

[0062]

[0063] Where, n eff denoted as ρ, where ρ is the effective refractive index of the fiber core under test, 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 optical delay τ1.

[0065]

[0066] Where, n eff ρ is the effective refractive index of the fiber core of the time-delay fiber, 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 / τ1]

[0069] Where, N max The minimum value is 1; round[] represents rounding to the nearest integer.

[0070] Step 4. Filter the elements whose order is not greater than N. max The beat frequency signals corresponding to each order of auxiliary optical delay are filtered out. Specifically, different auxiliary optical delays correspond to different beat frequency signals, for example, the beat frequency signal S corresponding to the Nth order auxiliary optical delay... N The frequency of (t) is:

[0071] f N =τ N *γ

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

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

[0074]

[0075] The frequency components corresponding to different auxiliary optical delays are filtered using a window function:

[0076]

[0077] By performing an inverse Fourier transform (IFFT) on the filtered frequency components, the beat frequency signals corresponding to different auxiliary optical delays can be separated and obtained. The beat frequency signal S corresponding to the Nth order auxiliary optical delay is used as an example. N For example, (t):

[0078]

[0079] Step 5. Use the beat frequency signals corresponding to different auxiliary optical delays to analyze the beat frequency signals S acquired by the main interferometer. main (t) is compensated. Specifically, the beat frequency signals corresponding to different auxiliary optical delays are subjected to Hilbert transform, converting the real cosine signals into complex exponential signals, and the beat frequency signal S corresponding to the Nth order auxiliary optical delay is used. N For example, (t):

[0080]

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

[0082]

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

[0084] Differentiating the phase yields the instantaneous output frequency f of the tunable laser source 1 under the Nth-order auxiliary optical delay. N (t):

[0085]

[0086] Based on the instantaneous output frequency f of the tunable laser source 1 N (t), using the cubic spline interpolation function spline to interpolate 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 a Fourier transform to obtain the beat frequency signal S corresponding to the Nth order auxiliary optical delay of the optical fiber under test. N (t) The reflected signal F in the range domain after compensation main,N (l F ):

[0089]

[0090] Among them, l F These represent different distances on the optical fiber under test.

[0091] Step 6. For the range-domain reflection signals of the master interferometer after different auxiliary optical delay compensations, only signals within a suitable range are selected as effective compensation reflection signals. The range-domain reflection signal F of the master interferometer after Nth-order auxiliary optical delay compensation is used as an example. main,N (l F For example, only take

[0092]

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

[0094]

[0095] Signals within the distance range serve as effective compensation reflection signals. To more clearly explain the relationship between different positions of the fiber under test and different auxiliary light delays, the following is provided: Figure 3 A schematic diagram showing the matching of the fiber length under test with the auxiliary light delay.

[0096] Step 7. Connect the effective compensation reflection signals within different distance ranges from smallest to largest in the distance domain to form a complete distance-reflection intensity curve.

[0097] It should be noted that steps 1-7 above are a method for phase noise compensation in an OFDR system based on a fiber optic loop-assisted interferometer, applicable to the first specific implementation. For the second specific implementation, simply replace 'l' with '2l' in the above steps. FBG Simply substitute the values ​​into the calculation.

[0098] In summary, this invention discloses a phase noise compensation device and method for an OFDR system based on a fiber optic loop-assisted interferometer. A 2x2 fiber coupler and a delay fiber form a loop, which serves as the delay optical path for the auxiliary interferometer. Each time the light passes through the loop, a fixed optical delay is added, and the coupler outputs a portion of the power. Based on the different distances of the fiber under test on the main interferometer, the beat frequency signal of the auxiliary interferometer with the closest optical delay order is selected for compensation. By splicing the compensated reflection signals from the fiber under test at different distances, the range-domain reflection signal of the entire fiber can be obtained.

[0099] It will be readily understood by those skilled in the art that the embodiments described herein are merely illustrative of the present invention and are not intended to limit the invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A phase noise compensation device for an OFDR system based on a fiber optic loop-assisted interferometer, comprising: A tunable laser source (1) is used to output a frequency-sweeping laser with linearly varying frequency and a sampling trigger signal; a first fiber optic 1x2 coupler (11) is used to split the frequency-sweeping laser into two paths, which enter the auxiliary interferometer (2) and the main interferometer (3) respectively; characterized in that the auxiliary interferometer (2) is used to generate beat frequency interference with different optical delays in the frequency-sweeping laser entering the auxiliary interferometer; the main interferometer (3) is used to generate beat frequency interference after the frequency-sweeping laser entering the main interferometer is reflected by the fiber under test (33); a balanced photodetector (25) is used to convert the optical beat frequency signal of the fiber optic loop auxiliary interferometer into an electrical signal, and at the same time remove the direct current in the beat frequency signal. The system includes: a balanced photodetector (35) for converting the optical beat frequency signal output by the main interferometer into an electrical signal, and removing DC from the beat frequency signal; an oscilloscope (4) for sampling the electrical signals output by the balanced photodetector (25) and the balanced photodetector (35); a computer (5) for processing the data acquired by the oscilloscope; the computer (5) includes filtering the beat frequency signal under different optical delays of the auxiliary interferometer; performing a resampling algorithm to compensate for noise in signals at different distances of the optical fiber; and splicing the compensated signals from different regions to obtain the complete reflection signal in the distance domain of the optical fiber under test; the auxiliary interferometer (2) includes an auxiliary optical... The main interferometer includes a 1x2 fiber coupler (21), an auxiliary 2x2 fiber coupler (22), a delay fiber (23), and an auxiliary 2x2 fiber coupler (24). The delay fiber (23) is connected to two ports on different sides of the auxiliary 2x2 fiber coupler (22). The other two ports of the auxiliary 2x2 fiber coupler (22) are connected to one of the branch ends of the auxiliary 1x2 fiber coupler (21) and the auxiliary 2x2 fiber coupler (24) to form a delay loop. The other branch end of the auxiliary 1x2 fiber coupler (21) is connected to the auxiliary 2x2 fiber coupler (24) to form an auxiliary reference optical path. The main interferometer includes a main 1x2 fiber coupler. (31) Main fiber circulator (32), fiber under test (33) and main fiber 2x2 coupler (34); one of the branch ends of the main fiber 1x2 coupler (31) is connected to the main 2x2 coupler 34 to form the main reference optical path; the other branch end of the main fiber 1x2 coupler (31) is connected to port a of the main fiber circulator (32), and ports b and c of the main fiber circulator (32) are connected to the fiber under test (33) and the main fiber 2x2 coupler (34) respectively to form the measurement optical path; the reflected signal at different distances on the fiber under test (33) in the main interferometer has a different optical delay than the main reference optical path, and the main optical delay is... satisfy: in, c is the effective refractive index of the fiber core under test; c is the speed of light in vacuum. These represent different distances on the optical fiber under test.

2. A phase noise compensation device for an OFDR system of an equivalent fiber optic loop-assisted interferometer, comprising: A tunable laser source (1) is used to output a frequency-sweeping laser with linearly varying frequency and a sampling trigger signal; a first fiber optic 1x2 coupler (11) is used to split the frequency-sweeping laser into two paths, which enter the auxiliary interferometer (2) and the main interferometer (3) respectively; characterized in that the auxiliary interferometer (2) is used to generate beat frequency interference with different optical delays in the frequency-sweeping laser entering the auxiliary interferometer; the main interferometer (3) is used to generate beat frequency interference after the frequency-sweeping laser entering the main interferometer is reflected by the fiber under test (33); a balanced photodetector (25) is used to convert the optical beat frequency signal of the fiber optic loop auxiliary interferometer into an electrical signal, and at the same time remove the beat frequency signal. The DC current in the main interferometer; a 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 current in the beat frequency signal; an oscilloscope (4) is used to sample the electrical signals output by the balanced photodetector (25) and the balanced photodetector (35); a computer (5) is used to process the data acquired by the oscilloscope; the computer (5) includes filtering the beat frequency signal under different optical delays of the auxiliary interferometer; performing a resampling algorithm to perform noise compensation on the signal at different distances of the optical fiber; splicing the compensated signals from different regions to obtain the complete reflection signal in the distance domain of the optical fiber under test; the auxiliary interferometer (2 The delay optical path of the main interferometer includes an auxiliary fiber 1x2 coupler (21), an auxiliary fiber circulator (26), an arrayed fiber grating (27), and an auxiliary fiber 2x2 coupler (24). The laser enters the arrayed fiber grating (27) through the auxiliary fiber circulator (26). The gratings at different positions in the arrayed fiber grating (27) will reflect a portion of the power of the light. The reflected signal enters the auxiliary fiber 2x2 coupler (24) through the auxiliary fiber circulator (26). The other path of the branch end of the auxiliary fiber 1x2 coupler (21) is connected to the auxiliary fiber 2x2 coupler (24) to form an auxiliary reference optical path. The main interferometer includes a main fiber 1x2 coupler (31). The main optical fiber circulator (32), the optical fiber under test (33) and the 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 the main reference optical path; the other branch end of the main optical fiber 1x2 coupler (31) is connected to port a of the main optical fiber circulator (32), and ports b and c of the main optical fiber circulator (32) are connected to the optical fiber under test (33) and the main optical fiber 2x2 coupler (34) respectively to form the measurement optical path; the reflected signal at different distances on the optical fiber under test (33) in the main interferometer has a different optical delay than the main reference optical path, and the main optical delay is... satisfy: in, c is the effective refractive index of the fiber core under test; c is the speed of light in vacuum. These represent different distances on the optical fiber under test.

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

4. The phase noise compensation device for an OFDR system of an equivalent fiber optic loop-assisted interferometer according to claim 2, characterized in that, The reflected signal of the Nth grating has an auxiliary optical delay. The calculation formula is: in, is the effective refractive index of the fiber core of the arrayed fiber grating (27), and c is the speed of light in vacuum. is the spacing length between adjacent gratings on the array fiber grating (27).

5. The phase noise compensation device for an OFDR system based on a fiber optic loop-assisted interferometer according to claim 1, characterized in that, The start-up trigger terminal of the tunable laser source is connected to the sampling trigger terminal of the oscilloscope to form a trigger circuit (12).

6. The phase noise compensation device for an OFDR system of an equivalent fiber optic loop-assisted interferometer according to claim 2, characterized in that, The start-up 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 phase noise compensation in an OFDR system based on a fiber optic loop-assisted interferometer, characterized in that, Includes the following steps: Step 1. Calculate the maximum main optical delay of the fiber under test. , in, ρ is the effective refractive index of the fiber core under test, and c is the speed of light in vacuum. To measure the length of the optical fiber; Step 2. Calculate the first-order auxiliary optical delay in the auxiliary interferometer. , in, Where c is the effective refractive index of the fiber core of the delay fiber, and c is the speed of light in vacuum. The length of the delay fiber; Step 3. Calculate the upper limit of the order N. , in, The minimum value is 1; round[] represents rounding to the nearest integer. Step 4. Filter the elements whose order is not greater than [a certain value]. The beat frequency signals corresponding to each order of auxiliary optical delay are filtered out; Step 5. Use the beat frequency signals corresponding to different auxiliary optical delays to analyze the beat frequency signals acquired by the main interferometer. Provide compensation; Step 6. For the range domain reflection signals of the master interferometer after different auxiliary optical delay compensations, only the signals within a suitable range are selected as the effective compensation reflection signals; Step 7. Connect the effective compensation reflection signals within different distance ranges from smallest to largest in the distance domain to form a complete distance-reflection intensity curve.

8. The phase noise compensation method for an OFDR system based on a fiber optic loop-assisted interferometer according to claim 7, characterized in that, Step 4 is as follows: the beat frequency signals corresponding to different auxiliary optical delays have different frequencies; the computer (5) performs a Fourier transform on the original beat frequency signal of the auxiliary interferometer acquired by the oscilloscope (4) to obtain different frequency components in the beat frequency signal; the frequency components corresponding to different auxiliary optical delays are filtered by a rectangular window function, and the beat frequency signals corresponding to different auxiliary optical delays can be separated and obtained by performing an inverse Fourier transform on the filtered frequency signals.

9. The phase noise compensation method for an OFDR system based on a fiber optic loop-assisted interferometer according to claim 7, characterized in that, Step 5 specifically involves: performing a Hilbert transform on the beat frequency signals corresponding to different auxiliary optical delays to convert the real cosine signals into complex exponential signals; performing an arctangent transform on the complex exponential signals to obtain the instantaneous phase of the beat frequency signals; differentiating the phase to obtain the instantaneous output frequency of the tunable laser source (1) under different auxiliary optical delays; and using a cubic spline interpolation function to evaluate the beat frequency signals of the main interferometer. Equal-frequency resampling is performed; Fourier transform is applied to the resampled beat frequency signal to obtain the distance domain reflection signal after beat frequency signal compensation corresponding to different auxiliary light delays of the optical fiber under test.

10. A phase noise compensation method for an OFDR system based on a fiber optic loop-assisted interferometer according to claim 7, characterized in that, The auxiliary fiber 2x2 coupler (22) and delay fiber (23) are replaced with an auxiliary fiber circulator (26) and an arrayed fiber grating (27), and the length of the delay fiber in the formula is... Replace with Substitute the values ​​into the calculation.

Images