A device and method for compensating laser phase noise for optical frequency domain reflectometry

Abstract

The application discloses a compensation device and method for laser phase noise of an optical frequency domain reflectometer, and relates to the technical field of distributed optical fiber sensing. The compensation device comprises a sweep laser source module, a main interferometer module, an auxiliary interferometer module and a signal processing module. The sweep laser source module is used for outputting a first sweep laser beam and a second sweep laser beam. The main interferometer module receives the first sweep laser beam and generates a first beat frequency signal related to the position of a scattering point of a sensing optical fiber. The auxiliary interferometer module receives the second sweep laser beam and generates a second beat frequency signal related to the position of a weak reflection point of an auxiliary optical fiber. The signal processing module collects the first beat frequency signal and the second beat frequency signal, extracts phase terms of the sensing optical fiber and the auxiliary optical fiber respectively, and realizes phase noise compensation. The compensation method realizes the function of the compensation device, that is, realizes phase noise compensation, analyzes the spectrum of compensation information again, and demodulates strain information along the sensing optical fiber. The application realizes uniform compensation of phase noise, and improves the system measurement distance, spatial resolution and strain accuracy.

Description

Technical Field

[0001] This invention belongs to the field of distributed optical fiber sensing technology, specifically relating to a laser phase noise compensation device and method for optical frequency domain reflectometers. Background Technology

[0002] Distributed fiber optic sensing and laser measurement technology uses optical fiber as the sensing and transmission medium. It utilizes physical mechanisms such as Rayleigh scattering, Brillouin scattering, and Raman scattering in optical fiber to analyze backscattered signals, thereby achieving high-resolution measurement of physical quantities such as temperature and strain along the optical fiber. It is widely used in fields such as energy and power system monitoring, large-scale structural health monitoring, and natural disaster early warning, and has broad development prospects and application value.

[0003] Optical Frequency Domain Reflectometry (OFDR) is widely used in the field of distributed fiber optic sensing. Due to its ability to achieve millimeter-level spatial resolution and με-level strain accuracy, OFDR has shown potential for extending to long-distance monitoring over kilometers and has attracted widespread attention in fields requiring precise distributed sensing. However, it is important to note that swept-frequency nonlinearity and laser intrinsic phase noise are key limiting factors that directly affect core performance indicators such as spatial resolution, strain measurement accuracy, and maximum effective sensing distance. Both swept-frequency nonlinearity and laser intrinsic phase noise exist as additional phase terms in linearly swept frequency-modulated continuous wave signals, impacting measurement performance. Therefore, swept-frequency nonlinearity and laser intrinsic phase noise are collectively referred to as phase noise.

[0004] Currently, the main approach to phase noise compensation in OFDR systems is to introduce an auxiliary interferometer and use a resampling algorithm to extract equiphase intervals from the auxiliary interferometer, then resample the phase of the main interferometer to achieve phase noise compensation. However, phase noise extraction is limited by the length of the auxiliary fiber: for longer auxiliary fibers, the compensation range is limited; shorter auxiliary fibers can extend the compensation range to some extent, but reduce the system's sensitivity to phase noise and affect compensation accuracy. This presents a length matching problem between the auxiliary and main interferometers: the portion of the sensing fiber that matches the length of the auxiliary fiber has a better compensation effect, while the greater the distance difference between the two, the worse the compensation effect; when the length difference is large, compensation is impossible, and the unevenness of the compensation effect leads to large fluctuations in the signal-to-noise ratio. Regarding the compensation length matching problem of the resampling algorithm, the phase generation method generates the phase through translation and accumulation, which alleviates the unevenness of compensation to some extent, but the compensation effect deteriorates as the generated phase length increases, still resulting in a limited compensation range. Furthermore, this method increases data processing time due to the need for phase generation via concatenation.

[0005] To address the aforementioned issues, the main current solution is to increase the number of auxiliary interferometers and use auxiliary optical fibers of different lengths to perform segmented compensation on the sensing optical fiber. However, some unresolved problems still exist:

[0006] 1. Due to the length matching problem between auxiliary optical fiber and compensation optical fiber, the more compensation optical fibers of different lengths there are and the finer the compensation segments are, the better the compensation effect will be. However, each length of auxiliary optical fiber needs to be configured with an independent interferometer arm. The number of auxiliary optical fibers is limited by the system complexity, and the number of auxiliary interferometers is difficult to increase significantly. Ultimately, the number of reference signals is limited, and it is impossible to achieve the effect of uniform compensation in long-distance sensing.

[0007] 2. When there are many auxiliary interferometers, mutual interference and aliasing will occur between auxiliary fiber signals of different lengths, making it difficult to correctly extract the phase noise information of the target length from the aliased signals. This is more obvious in long-distance sensing, making it difficult to achieve accurate compensation for the entire length of the fiber in long-distance sensing.

[0008] Therefore, it is urgent to develop a new phase noise compensation device and method to address the shortcomings and deficiencies of current technology. Summary of the Invention

[0009] To address the aforementioned problems, the main objective of this invention is to provide a laser phase noise compensation device and method for optical frequency domain reflectometers. This method solves the problems of limited compensation range and uneven compensation in traditional compensation methods, achieving uniform compensation of phase noise and improving key performance aspects of the system such as measurement distance, spatial resolution, and strain accuracy.

[0010] To achieve the above objectives, the present invention adopts the following technical solution.

[0011] As a first aspect of the present invention, a laser phase noise compensation device for an optical frequency domain reflectometer is proposed, comprising: a swept-frequency laser source module 1 for outputting a swept-frequency laser, and equipped with a first coupler 106 for splitting the swept-frequency laser into a first swept-frequency laser beam and a second swept-frequency laser beam; a main interferometer module 2, the input end of which is connected to the first coupler 106, receiving the first swept-frequency laser beam, splitting it into a probe beam and a first local oscillator beam, the probe beam being injected into a sensing fiber 203 to form a backscattered Rayleigh signal, interfering with the first local oscillator beam, and generating a first beat frequency signal related to the scattering point position of the sensing fiber 203; and an auxiliary interferometer module 2. The auxiliary interferometer module 3, with its input end connected to the first coupler 106, receives the second swept laser beam, splits it into a compensation beam and a second local oscillator beam, and the compensation beam is encoded and modulated into a modulated beam and injected into the auxiliary fiber 307 to form a weak back reflection signal, which interferes with the second local oscillator beam to generate a second beat frequency signal related to the position of the weak reflection point of the auxiliary fiber 307; the signal processing module 4, with its input end connected to the output ends of the main interferometer module 2 and the auxiliary interferometer module 3 respectively, collects the first beat frequency signal and the second beat frequency signal, extracts the phase terms of the sensing fiber 203 and the auxiliary fiber 307 respectively, and performs phase noise compensation.

[0012] In specific implementation, the auxiliary interferometer module of the present invention improves upon existing auxiliary interferometers by using a fiber Bragg grating (FBG) instead of the existing multi-interferometer arm structure, effectively increasing the number of reference signals used for compensation.

[0013] In specific implementation, the compensation light in the auxiliary interferometer module of the present invention is encoded and modulated to solve the problem that the existing technology cannot demodulate in the case of multiple points due to interference between different reference points, and can correctly extract the reference signals at different positions of the sensing fiber.

[0014] As a second aspect of the present invention, a method for compensating laser phase noise for an optical frequency domain reflectometer is also proposed. The method uses the aforementioned compensation device for laser phase noise compensation, and includes the following steps: a swept-frequency laser source module emits a swept-frequency laser and splits it into two beams; the first swept-frequency laser beam is injected into a main interferometer module, and the second swept-frequency laser beam is injected into an auxiliary interferometer module; the first swept-frequency laser beam is split into a probe beam and a first local oscillator beam within the main interferometer module, and after beat frequency interference, a first beat frequency signal related to the scattering point position of the sensing fiber is obtained; the second swept-frequency laser beam is split into a compensation beam and a second local oscillator beam within the auxiliary interferometer module, and the compensation beam is encoded and modulated to generate modulated light; the pseudo-random noise code used for encoding is stored locally; the modulated light and... After the second local vibrator undergoes beat frequency interferometry, a second beat frequency signal related to the weak reflection point position of the auxiliary fiber is obtained. The signal processing module acquires the first and second beat frequency signals respectively, mixes the second beat frequency signal with pseudo-random noise code to obtain the decoded signal, performs Hilbert transform on the first and decoded signals, and extracts the first phase term of the first beat frequency signal and the second phase term of the decoded signal respectively. The phase terms of the first and decoded signals are extracted again by resampling, which are the first and second phase terms respectively. The second phase term is resampled and compensated to the first phase term to perform phase noise compensation of the sensing fiber. Based on the compensation information, spectrum analysis is performed to demodulate the strain information along the sensing fiber.

[0015] The device and method described in this invention can achieve accurate compensation of sensing optical fibers over long distances, thereby improving sensing accuracy and sensing distance.

[0016] Beneficial effects

[0017] Based on the above technical solution, it can be seen that the laser phase noise compensation device and method of the present invention, compared with the prior art, has at least one of the following beneficial effects:

[0018] 1. This invention improves the auxiliary interferometer by using a fiber Bragg grating (FBG) instead of the existing multi-interferometer arm structure, effectively increasing the number of reference signals used for compensation;

[0019] 2. This invention introduces a coding modulation method to solve the problem in existing technologies where demodulation is impossible at multiple points due to interference between different reference points, and can correctly extract reference signals at different locations of the sensing fiber;

[0020] 3. Compared with the prior art, the present invention can accurately compensate for the sensing fiber over long distances, thereby improving performance indicators such as sensing accuracy and sensing distance. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the laser phase noise compensation device of the present invention;

[0023] Figure 2 This is a schematic diagram comparing the number of reference signals in the prior art with that of the present invention;

[0024] Figure 3 This is a schematic diagram illustrating the effects of existing technologies and the present invention on the extraction of reference signals that interfere with each other at different locations;

[0025] In the above figures, the meanings of the reference numerals are as follows:

[0026] 1. Frequency sweep laser source module, 101. First linear frequency sweep signal, 102. Tunable laser, 103. Single frequency laser, 104. Modulator, 105. Second linear frequency sweep signal, 106. First coupler.

[0027] 2. Main interferometer module, 201. Second coupler, 202. First circulator, 203. Sensing fiber, 204. Third coupler, 205. First balanced photodetector;

[0028] 3 Auxiliary interferometer module, 301 fourth coupler, 302 light intensity modulator, 303 bias point controller, 304 signal generator, 305 second circulator, 306 fifth coupler, 307 auxiliary optical fiber, 308 second balanced photodetector.

[0029] 4 Signal processing modules, 401 Second signal acquisition unit, 402 First signal acquisition unit, 403 Decoding unit, 404 First phase extraction unit, 405 Second phase extraction unit, 406 Phase noise compensation unit, 407 Spectrum analysis unit, 408 Strain demodulation unit. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0031] The terminology used in this invention is for the purpose of describing particular embodiments only and is not intended to limit the embodiments of the invention. The singular forms “a,” “the,” and “the” as used in the embodiments of the invention and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0032] Optical Frequency Domain Reflectometry (OFDR) uses a linearly modulated laser beam split into two paths. The first path serves as a probe beam injected into the fiber under test (BUT), while the second path acts as a local oscillator reference beam. The probe beam, after passing through the BUT, undergoes Rayleigh scattering and coherently mixes with the local oscillator reference beam. The beat frequency signal obtained from this coherent mixing is detected by a photodetector. The frequency of the beat frequency signal is proportional to the position of the scattering point in the BUT. By processing and analyzing the beat frequency signal, distributed measurements of parameters such as strain along the fiber can be achieved.

[0033] Spatial resolution reflects the minimum distance between two distinguishable targets on the fiber under test. The theoretical limit is determined by the frequency sweep range of the light source, while actual resolution is affected by phase noise, leading to a decrease in effective resolution. Measurement distance refers to the farthest distance at which the OFDR can detect a target, which is related to the coherence length of the laser and directly affected by phase noise. Strain accuracy refers to the minimum strain that the system can resolve. As the measurement distance approaches the coherence length, phase noise gradually increases, masking the Rayleigh scattering signal and further causing a sharp deterioration in phase demodulation accuracy. Since phase demodulation accuracy is crucial for calculating parameters such as strain, phase noise becomes one of the main bottlenecks restricting OFDR strain demodulation accuracy. In summary, phase noise directly affects many key technical indicators of OFDR, and compensating for phase noise is one of the key ways to improve OFDR performance.

[0034] Current phase noise compensation devices and methods have the following problems:

[0035] 1. Due to the limited number of auxiliary optical fibers, it is difficult to obtain phase information from multiple points. The long sensing optical fiber can only be compensated by a very limited number of reference signals, resulting in poor compensation effect. In addition, increasing the number of interferometer arms leads to a more complex structure of the auxiliary interferometer.

[0036] 2. When there are many auxiliary optical fibers, signals at different locations interfere with each other, making it difficult to correctly extract phase information from the superimposed signals, which limits the compensation range and compensation accuracy.

[0037] 3. Due to issues such as limited compensation range and uneven compensation effect, the key performance characteristics of OFDR, such as sensing distance and sensing accuracy, are limited.

[0038] This application proposes an all-fiber domain phase noise compensation scheme to achieve uniform compensation for phase noise, thereby improving the system's key performance characteristics such as measurement distance, spatial resolution, and strain accuracy.

[0039] As the first part of this invention, a laser phase noise compensation device for optical frequency domain reflectometers is proposed, such as... Figure 1 As shown, it includes: a swept-frequency laser source module 1, used to output a swept-frequency laser, and equipped with a first coupler 106 to split the swept-frequency laser into a first swept-frequency laser beam and a second swept-frequency laser beam; a main interferometer module 2, whose input end is connected to the first coupler 106, receiving the first swept-frequency laser beam, splitting it into a probe beam and a first local oscillator beam, injecting the probe beam into the sensing fiber 203 to form a backscattered Rayleigh signal, interfering with the first local oscillator beam, and generating a first beat frequency signal related to the scattering point position of the sensing fiber 203; and an auxiliary interferometer module 3, whose input end is connected to the first coupler 106. 06 connects to receive the second swept laser beam, splitting it into a compensation beam and a second local oscillator beam. The compensation beam is encoded and modulated into a modulated beam and injected into the auxiliary fiber 307 to form a weak backward reflection signal, which interferes with the second local oscillator beam to generate a second beat frequency signal related to the position of the weak reflection point of the auxiliary fiber 307. The signal processing module 4 has its input end connected to the output ends of the main interferometer module 2 and the auxiliary interferometer module 3, respectively. It collects the first beat frequency signal and the second beat frequency signal, extracts the phase terms of the sensing fiber 203 and the auxiliary fiber 307, and performs phase noise compensation.

[0040] The sensing fiber is a long-distance (several km to tens of km) fiber. The auxiliary fiber uses fiber Bragg gratings (FBGs) instead of the traditional multi-arm auxiliary interferometer structure. Each weak-reflection grating corresponds to an auxiliary interferometer. Digital encoding is used to extract weak-reflection aliasing signals. A single measurement can generate a large number of reference signals, ultimately achieving high-precision phase noise compensation across the entire sensing fiber distance. For example... Figure 2 As shown, Figure 2 In (a), the existing technology uses a four-arm auxiliary interferometer structure, which can only extract a limited number of reference signals. Figure 2 In (b), a fiber Bragg grating structure is used, which significantly increases the number of reference signals extracted. Specifically, existing technologies can only handle three or four compensation points. If noise compensation is performed on a 1km fiber, it would require compensation every few hundred meters. The spacing between compensation points is large, and the fiber within the spacing cannot be compensated, resulting in significant non-uniformity. However, the technical solution of this invention can achieve compensation every 10m, simplifying the equipment structure while improving the compensation effect, and is also suitable for phase noise compensation of long-distance fibers.

[0041] In a preferred embodiment, such as Figure 1As shown, the swept-frequency laser source module 1 has the following structures: First structure a: The swept-frequency laser source module 1 includes a first linear swept-frequency signal 101 and a tunable laser 102. The first linear swept-frequency signal 101 is used to internally modulate the tunable laser 102 to emit a linear swept-frequency laser. Second structure b: The swept-frequency laser source module 1 includes a single-frequency laser 103, a modulator 104, and a second linear swept-frequency signal 105. The second linear swept-frequency signal 105 is used to externally modulate the single-frequency laser 103 through the modulator 104 to emit a linear swept-frequency laser. The technical solution of this invention can compensate for the laser phase noise of the optical frequency domain reflectometer with the above two structures, and is not limited to the formation conditions of the linear swept-frequency laser.

[0042] In a preferred embodiment, the first coupler 106 is a fiber optic coupler with a splitting ratio of 50:50. It achieves optical signal distribution through the coupling effect between optical fibers, offering advantages such as uniform optical power distribution, bidirectional operation, low insertion loss, and high return loss. This ensures that the first and second swept-frequency laser beams are proportional beams with the same optical power, and also guarantees the quality and stability of the optical signal transmission.

[0043] In a preferred embodiment, such as Figure 1 As shown, the main interferometer module 2 includes: a second coupler 201 whose input end is connected to the output end of the first coupler 106; a first circulator 202 and a third coupler 204 whose first port is connected to the output end of the second coupler 201; a sensing fiber 203 connected to the second port of the first circulator 202; and a first balanced photodetector 205 whose input end is connected to the output end of the third coupler 204. The third port of the first circulator 202 is connected to the input end of the third coupler 204, and the output end of the first balanced photodetector 205 is connected to the signal processing module 4.

[0044] Specifically, the second coupler 201 receives the first swept laser beam and splits it into a probe beam and a first local oscillator beam; the third coupler 204 receives the local oscillator beam; the first circulator 202 receives and transmits the probe beam to the sensing fiber 203; the sensing fiber 203 generates a backscattered Rayleigh signal based on the probe beam; wherein, the backscattered Rayleigh signal enters the third coupler 204 through the first circulator 202 and interferes with the first local oscillator beam to generate a first beat frequency signal related to the scattering point position of the sensing fiber 203, which is received by the first balanced photodetector 205 and transmitted to the signal processing module 4. The third coupler 204 uses a fiber optic coupler with a splitting ratio of 50:50 to facilitate the first balanced photodetector 205 in receiving optical signals; the second coupler 201 uses a fiber optic coupler with a splitting ratio of 90:10, in which 90% of the light is the probe light and 10% is the first local oscillator light. Since the backscattered Rayleigh signal formed by the probe light is weak, it is necessary to increase the optical power of the probe light entering the sensing fiber 203, so fiber optic couplers with different splitting ratios are selected.

[0045] In a preferred embodiment, such as Figure 1 As shown, the auxiliary interferometer module 3 includes: a fourth coupler 301 whose input end is connected to the output end of the first coupler 106; a light intensity modulator 302 and a fifth coupler 306 whose input end is connected to the output end of the fourth coupler 301; a second circulator 305 whose first port is connected to the output end of the light intensity modulator 302; an auxiliary optical fiber 307 connected to the second port of the second circulator 305; and a second balanced photodetector 308 whose input end is connected to the output end of the fifth coupler 306. The other input ends of the light intensity modulator 302 are connected to the bias point controller 303 and the signal generator 304, respectively. The third port of the second circulator 305 is connected to the input end of the fifth coupler 306. The output end of the second balanced photodetector 308 is connected to the signal processing module 4.

[0046] Specifically, the fourth coupler 301 receives the second swept laser beam and splits it into a compensation beam and a second local oscillator beam; the fifth coupler 306 receives the second local oscillator beam; the light intensity modulator 302 encodes and modulates the compensation beam to generate modulated light, which is then injected into the auxiliary fiber 307 through the second circulator 305; the auxiliary fiber 307 generates a weak back reflection signal based on the modulated light; the weak back reflection signal enters the fifth coupler 306 through the third port of the second circulator 305 and interferes with the second local oscillator beam, generating a second beat frequency signal related to the weak reflection point position of the auxiliary fiber 307, which is received by the second balanced photodetector 308 and transmitted to the signal processing module 4. The fifth coupler 306 uses a fiber optic coupler with a splitting ratio of 50:50 to facilitate the second balanced photodetector 308 in receiving optical signals; the third coupler 204 uses a fiber optic coupler with a splitting ratio of 90:10, of which 90% of the light is compensation light and 10% is the second local oscillator light. Since the back reflection signal formed by the compensation light is relatively weak, it is necessary to increase the optical power of the compensation light entering the auxiliary fiber 307. Therefore, fiber optic couplers with different splitting ratios are selected.

[0047] In a preferred embodiment, the light intensity modulator 302 encodes and modulates the compensation light to generate modulated light, including: using a bias point controller 303 to control the bias point so that the light intensity modulator 302 operates at a zero operating point; and using a signal generator 304 to generate a pseudo-random noise code to encode the compensation light, forming the modulated light. The pseudo-random noise code (PRN code) is a binary sequence with characteristics similar to random noise. Due to its ideal autocorrelation properties, the PRN code can accurately extract the signal at the receiver through the peak value of the autocorrelation function. The bias point controller is an MZM (Mach-Zehnder Modulator) bias point controller. In conjunction with the bias point controller, the output voltage of the pseudo-random noise code is set to achieve the encoding and modulation of the compensation light. Taking an m-sequence as an example, the output voltage is set to... During compensated optical modulation, the phase of the optical field signal corresponding to the code element "0" in the m-sequence remains unchanged, while the phase of the optical field signal corresponding to the code element "1" in the m-sequence is changed by a phase shift of π = 180°. For subsequent decoding, the pseudo-random noise code is stored locally.

[0048] In a preferred embodiment, such as Figure 1As shown, the signal processing module 4 includes: a second signal acquisition unit 401 and a first signal acquisition unit 402, whose input terminals are respectively connected to the output terminals of the auxiliary interferometer module 3 and the main interferometer module 2, for acquiring the second beat frequency signal and the first beat frequency signal, respectively; a decoding unit 403, whose input terminal is connected to the output terminal of the second signal acquisition unit 401, for decoding the second beat frequency signal to obtain a decoded signal; a second phase extraction unit 405, whose input terminal is connected to the output terminal of the decoding unit 403, for performing a Hilbert transform on the decoded signal to extract the second phase term of the second beat frequency signal; and a first phase extraction unit 404, whose input terminal is connected to the output terminal of the first signal acquisition unit 402, for... A Hilbert transform is performed on the first beat frequency signal to extract the first phase term of the first beat frequency signal. A phase noise compensation unit 406, whose input is connected to the output of the second phase extraction unit 405 and the first phase extraction unit 404 respectively, is used to perform phase noise compensation on the first beat frequency signal using a resampling method based on the second phase term and the first phase term to obtain the compensated time-domain signal. A spectrum analysis unit 407, whose input is connected to the output of the phase noise compensation unit 406, is used to perform spectrum analysis based on the time-domain signal to obtain the frequency-domain signal. A strain demodulation unit 408, whose input is connected to the output of the spectrum analysis unit 407, is used to demodulate the strain information along the sensing fiber 203 based on the frequency-domain signal.

[0049] Specifically, when decoding the second beat frequency signal, the decoding unit 403 mixes the locally stored pseudo-random noise code (i.e., m-sequence) with the second beat frequency signal to obtain the decoded signal. Due to the autocorrelation characteristics of the pseudo-random noise code, the autocorrelation value is maximum at zero offset and close to zero at other offsets, enabling accurate decoding of the optical signal. Figure 3 The diagram shows signals interfering with each other at different locations. Figure 3 (a) illustrates that existing technologies cannot extract mutually interfering signals. Figure 3 Figure (b) demonstrates that the present invention can correctly extract the signal of each weak reflection point. Due to the autocorrelation characteristics of the m-sequence, τ=τ i The weak reflection signal at the specified location is extracted, while signals at other locations are suppressed due to the spreading effect; the degree of suppression depends on the autocorrelation coefficient of the m-sequence. Therefore, this invention uses coded modulation to correctly extract phase information from weak reflection signals interfering with each other at multiple locations, improving the compensation range and accuracy.

[0050] The second part of this invention proposes a method for compensating laser phase noise in an optical frequency domain reflectometer. The method uses the aforementioned compensation device to perform laser phase noise compensation, and includes the following steps:

[0051] S1: The frequency sweep laser source module emits a frequency sweep laser and splits it into two beams. The first frequency sweep laser beam is injected into the main interferometer module, and the second frequency sweep laser beam is injected into the auxiliary interferometer module.

[0052] S2: The main interferometer module is connected to the sensing fiber. After performing beat frequency interference on the first sweeping laser beam, the first beat frequency signal related to the position of the scattering point of the sensing fiber is obtained.

[0053] S3: The auxiliary interferometer module is connected to the auxiliary optical fiber. After splitting the second sweeping laser beam, one beam is encoded and modulated into a modulated light, and then beat-frequency interference is performed with the other beam to obtain the second beat-frequency signal related to the position of the weak reflection point of the auxiliary optical fiber.

[0054] S4: The signal processing module acquires the first beat frequency signal and the second beat frequency signal, decodes the second beat frequency signal and extracts its phase as the second phase term, extracts the phase of the first beat frequency signal as the first phase term, and performs phase noise compensation of the sensing fiber based on the second phase term and the first phase term.

[0055] In a preferred embodiment, the compensation method of the laser phase noise compensation device of the present invention is as follows:

[0056] The frequency-sweeping laser source module emits a frequency-sweeping laser and splits it into two beams. The first frequency-sweeping laser beam is injected into the main interferometer module, and the second frequency-sweeping laser beam is injected into the auxiliary interferometer module.

[0057] The first sweeping laser beam is split into a probe beam and a first local oscillator beam in the main interferometer module. After beat frequency interference, the first beat frequency signal related to the position of the scattering point of the sensing fiber is obtained.

[0058] The second sweeping laser beam is split into a compensation beam and a second local oscillator beam within the auxiliary interferometer module. The compensation beam is encoded and modulated to generate the modulated beam. The pseudo-random noise code used for encoding is stored locally.

[0059] After the modulated light and the second local oscillator light undergo beat frequency interference, a second beat frequency signal related to the position of the weak reflection point of the auxiliary optical fiber is obtained;

[0060] The signal processing module acquires the first beat frequency signal and the second beat frequency signal respectively, and mixes the second beat frequency signal with a pseudo-random noise code to obtain the decoded signal.

[0061] Perform Hilbert transform on the first beat frequency signal and the decoded signal to extract the phase of the first beat frequency signal and the decoded signal, namely the first phase term and the second phase term;

[0062] The extracted second phase term is resampled at equal phase intervals to compensate for the first phase term, thereby compensating for the phase noise of the sensing fiber. Based on the compensation information, spectral analysis is performed to demodulate the strain information along the sensing fiber.

[0063] In a preferred embodiment, the first beat frequency signal is the sum of all backscattered Rayleigh scattering signals from the scattering point m of the sensing fiber, where the time delay corresponding to the scattering point m is... The photocurrent at that point for:

[0064]

[0065] In the formula, the symbol This indicates that the left and right ends of the symbol are proportional; It is the amplitude of the optical field of the photoelectric signal after interference by the main interferometer module. It is the Rayleigh scattering coefficient at scattering point m in the sensing fiber. It is the starting frequency of the swept laser. It is the frequency tuning rate of the swept laser. It is the phase noise term at scattering point m in the sensing fiber. It is the laser phase noise at time t; yes The laser phase noise at time t; t is the time after the sweep laser is started.

[0066] In practice, the position of scattering point m and the time delay corresponding to scattering point m are as follows: and the Rayleigh scattering coefficient at scattering point m Correspondingly, once the location of the scattering point m is determined, its corresponding time delay... ; and Rayleigh scattering coefficient All have been confirmed.

[0067] In a preferred embodiment, the compensation light is encoded and modulated to generate modulated light. for:

[0068]

[0069] In the formula, It is the amplitude of the optical field of the optical signal modulated by the auxiliary interferometer. is a pseudo-random noise code, t is the time after the sweep laser is started, and j is the imaginary unit.

[0070] In a preferred embodiment, the second beat frequency signal is the sum of all backward weak reflection signals at the weak reflection point of the auxiliary optical fiber, wherein the sum of all backward weak reflection signals... for:

[0071]

[0072] In the formula, The time delay corresponds to the weak reflection point i. It is the starting frequency of the swept laser. It is the frequency tuning rate of the swept laser. It is the reflection coefficient corresponding to the weak reflection point i. This is the phase noise term corresponding to the weak reflection point i. It is the pseudo-random noise code corresponding to the weak reflection point i.

[0073] In a preferred embodiment, the phases of the first beat frequency signal and the decoded signal are extracted, and are respectively designated as the first phase term. Second phase term They are respectively:

[0074]

[0075]

[0076] in, , For constant terms;

[0077] The first phase term is processed using a resampling method, which is then used to compensate for the second phase term.

[0078] The second phase term after resampling Rewritten as:

[0079]

[0080] in The phase constant term at time zero.

[0081] The resampled second phase term Substitute the first phase term Rewritten as:

[0082]

[0083] in, For linear phase terms, This is the residual phase noise. This is a constant term. From this equation, we can deduce that when τ... m With τ i The compensation effect is best when the lengths of the auxiliary optical fiber and the sensing optical fiber are equal; while when τ m As the difference between the phase noise and τi increases, the compensation effect of phase noise gradually deteriorates. Existing technologies have a limited number of compensation points, which can only provide coarse compensation for a relatively large range. This invention, in conjunction with the coding modulation of an auxiliary interferometer, solves the above difficulties, enabling precise division of the compensation range and achieving high-precision compensation across the entire range of the sensing fiber.

[0084] The present invention will be further illustrated below through specific embodiments. It should be noted that the following embodiments are merely illustrative and not intended to limit the scope of the invention. All other embodiments obtained by those skilled in the art based on the embodiments shown below without inventive effort are within the scope of protection of the embodiments of the present invention.

[0085] Example 1

[0086] The internally modulated sweeping laser source module 1 generates a linear sweeping laser. The sweeping laser is split into two beams by the first coupler 106. One beam enters the main interferometer module 2 equipped with a sensing fiber 203 for sensing, and the other beam enters the auxiliary interferometer module 3 equipped with an FBG auxiliary fiber 307 as a reference signal for phase noise compensation of the sensing fiber 203.

[0087] The swept laser entering the main interferometer module 2 is split into two beams by the second coupler 201, entering the two arms of the main interferometer module 2: a probe beam and a first local oscillator beam. The probe beam enters through one port of the first circulator 202 and is injected into the sensing fiber 203 from the second port. The backscattered Rayleigh scattering from the sensing fiber 203 enters the main interferometer module 2 from the third port of the first circulator 202, and undergoes beat frequency interference with the first local oscillator beam at the third coupler 204, generating a first beat frequency signal related to the scattering point position of the sensing fiber 203. This signal is received by the first balanced photodetector 205, and the received photocurrent signal is the sum of all backscattered Rayleigh scattering signals.

[0088] The swept laser entering the auxiliary interferometer module 3 is split into two beams by the fourth coupler 301, which then enter the two arms of the auxiliary interferometer module 3: a compensation beam and a second local oscillator beam. The compensation beam is encoded and modulated by the intensity modulator 302. The bias point is controlled by the MZM bias controller 303, ensuring the intensity modulator 302 operates at a zero (null) operating point to guarantee modulation requirements. A pseudo-random noise code is generated by the signal generator 304 to modulate the compensation beam, resulting in a modulated light signal.

[0089] The modulated optical signal is injected into the auxiliary optical fiber 307 by the second circulator 305. The weak back reflection signal of the auxiliary optical fiber 307 enters the auxiliary interferometer module 3 through the second circulator 305 and interferes with the second local oscillator beam of the auxiliary interferometer module 3 at the fifth coupler 306, generating a second beat frequency signal related to the position of the weak reflection point of the auxiliary optical fiber 307. This signal is received by the second balanced photodetector 308, and the received photocurrent signal is the sum of all reflected signals. :

[0090]

[0091] in, The time delay corresponds to the weak reflection point i. It is the starting frequency of the swept laser. It is the frequency tuning rate of the swept laser. It is the reflection coefficient corresponding to the weak reflection point i. This is the phase noise term corresponding to the weak reflection point i. It is the pseudo-random noise code corresponding to the weak reflection point i, i.e., the m-sequence.

[0092] The first signal acquisition unit 402 and the second signal acquisition unit 401 acquire the first beat frequency signal and the second beat frequency signal, respectively. The second beat frequency signal is decoded by the decoding unit 403 using a local m-sequence. The signal is mixed with the second beat frequency signal of the auxiliary interferometer module 3 to obtain the decoded signal.

[0093] The second phase extraction unit 405 and the first phase extraction unit 404 respectively perform Hilbert transform on the decoded signal and the first beat frequency signal to extract the first phase term of the sensing fiber 203. and the second phase term of the decoded signal They are respectively:

[0094]

[0095]

[0096] in, , For constant terms;

[0097] After obtaining the aforementioned phase term, the phase noise compensation unit 406 performs phase noise compensation on the sensing fiber 203. Specifically, the second phase term of the decoded signal is resampled using a resampling method, and the resampled second phase term... Rewritten as:

[0098]

[0099] in The phase constant term at time zero.

[0100] The resampled second phase term Substitute the first phase term First phase term Rewritten as:

[0101]

[0102] in, For linear phase terms, This is the residual phase noise. This is a constant term.

[0103] From this formula, we can deduce that when τ m With τ i The compensation effect is best when the lengths of the auxiliary optical fiber and the sensing optical fiber are equal; while when τ m With τ i As the gap widens, the phase noise compensation effect gradually deteriorates. This demonstrates that the laser phase noise during the sensing fiber measurement process has been compensated.

[0104] After compensation by the phase noise compensation unit 406, a frequency domain signal is obtained. This frequency domain signal enters the spectrum analysis unit 407 for Fourier transform analysis, yielding another frequency domain signal. The frequency domain signal is then demodulated by the strain demodulation unit 408 to extract strain information along the sensing fiber. The strain information refers to the strain occurring at a specific location along the fiber, including the strain location and magnitude.

[0105] Compared with the prior art, the main technical advantages of this invention are:

[0106] 1. This invention improves the auxiliary interferometer by using FBG instead of the existing multi-interferometer arm structure, effectively increasing the number of reference signals used for compensation;

[0107] 2. This invention introduces coded modulation. Existing technologies cannot demodulate at multiple points due to interference between different reference points, while this invention can correctly extract reference signals from different locations.

[0108] 3. Thanks to the accurate compensation of the sensing fiber over long distances, the present invention improves performance indicators such as sensing accuracy and sensing distance compared with the prior art.

[0109] In the description of the embodiments of the present invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In the embodiments of the present invention, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in a suitable manner in any one or more embodiments or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in the embodiments of the present invention, as well as the features of the different embodiments or examples.

[0110] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of embodiments of the present invention, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0111] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., 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 laser phase noise compensation device for an optical frequency domain reflectometer, characterized in that, include: The frequency sweep laser source module (1) is used to output frequency sweep laser and is provided with a first coupler (106) to split the frequency sweep laser into a first frequency sweep laser and a second frequency sweep laser. The main interferometer module (2) is connected to the first coupler (106) at its input end. It receives the first swept laser beam, splits it into a probe beam and a first local oscillator beam, and injects the probe beam into the sensing fiber (203) to form a backscattered Rayleigh signal. The probe beam interferes with the first local oscillator beam to generate a first beat frequency signal related to the position of the scattering point of the sensing fiber (203). The auxiliary interferometer module (3) has its input end connected to the first coupler (106), receives the second sweeping laser beam, splits it into compensation light and second local oscillator light, and the compensation light is encoded and modulated into modulation light and injected into the auxiliary optical fiber (307) to form a weak back reflection signal, which interferes with the second local oscillator light to generate a second beat frequency signal related to the position of the weak reflection point of the auxiliary optical fiber (307). The signal processing module (4) has its input end connected to the output ends of the main interferometer module (2) and the auxiliary interferometer module (3) respectively. It collects the first beat frequency signal and the second beat frequency signal, extracts the phase terms of the sensing fiber (203) and the auxiliary fiber (307) respectively, and performs phase noise compensation. The auxiliary interferometer module (3) includes: a fourth coupler (301) whose input end is connected to the output end of the first coupler (106); a light intensity modulator (302) and a fifth coupler (306) whose input end is connected to the output end of the fourth coupler (301); a second circulator (305) whose first port is connected to the output end of the light intensity modulator (302); an auxiliary optical fiber (307) connected to the second port of the second circulator (305); and a second balanced photodetector (308) whose input end is connected to the output end of the fifth coupler (306). The other input ends of the light intensity modulator (302) are connected to a bias point controller (303) and a signal generator (304), respectively. The third port of the second circulator (305) is connected to the input end of the fifth coupler (306), and the output end of the second balanced photodetector (308) is connected to the signal processing module (4). The fourth coupler (301) receives the second swept laser beam and splits it into the compensation light and the second local oscillator light; the fifth coupler (306) receives the second local oscillator light; the light intensity modulator (302) encodes and modulates the compensation light to generate modulated light, and injects the modulated light into the auxiliary optical fiber (307) through the second circulator (305); the auxiliary optical fiber (307) generates a weak back reflection signal based on the modulated light; wherein, the weak back reflection signal enters the fifth coupler (306) through the third port of the second circulator (305) and interferes with the second local oscillator light to generate a second beat frequency signal related to the weak reflection point position of the auxiliary optical fiber (307), which is received by the second balanced photodetector (308) and transmitted to the signal processing module (4). The light intensity modulator (302) encodes and modulates the compensation light to generate modulated light, including: using a bias point controller (303) to control the bias point so that the light intensity modulator (302) operates at a zero operating point; and generating pseudo-random noise code through a signal generator (304) to encode the compensation light to form the modulated light.

2. The compensation device according to claim 1, characterized in that, The main interferometer module (2) includes: A second coupler (201) whose input end is connected to the output end of the first coupler (106), a first circulator (202) and a third coupler (204) whose first port is connected to the output end of the second coupler (201), the sensing optical fiber (203) connected to the second port of the first circulator (202), and a first balanced photodetector (205) whose input end is connected to the output end of the third coupler (204); wherein, The third port of the first circulator (202) is connected to the input of the third coupler (204). The output terminal of the first balanced photodetector (205) is connected to the signal processing module (4).

3. The compensation device according to claim 2, characterized in that, The second coupler (201) receives the first swept laser beam and splits the first swept laser beam into the probe beam and the first local oscillator beam; The third coupler (204) receives the local oscillator light; The first circulator (202) receives and transmits the probe light to the sensing fiber (203). The sensing fiber (203) generates a backscattered Rayleigh signal based on the probe light; wherein... The backscattered Rayleigh signal enters the third coupler (204) through the first circulator (202) and interferes with the first local oscillator beam, generating a first beat frequency signal related to the scattering point position of the sensing fiber (203), which is received by the first balanced photodetector (205) and transmitted to the signal processing module (4).

4. The compensation device according to claim 1, characterized in that, The signal processing module (4) includes: The second signal acquisition unit (401) and the first signal acquisition unit (402), whose input terminals are respectively connected to the output terminals of the auxiliary interferometer module (3) and the main interferometer module (2), are used to acquire the second beat frequency signal and the first beat frequency signal, respectively. The decoding unit (403), whose input terminal is connected to the output terminal of the second signal acquisition unit (401), is used to decode the second beat frequency signal to obtain a decoded signal. The second phase extraction unit (405), whose input terminal is connected to the output terminal of the decoding unit (403), is used to perform Hilbert transform on the decoded signal and extract the second phase term of the second beat frequency signal. The first phase extraction unit (404), whose input terminal is connected to the output terminal of the first signal acquisition unit (402), is used to perform Hilbert transform on the first beat frequency signal and extract the first phase term of the first beat frequency signal. A phase noise compensation unit (406), whose input terminals are respectively connected to the output terminals of the second phase extraction unit (405) and the first phase extraction unit (404), is used to perform phase noise compensation on the first beat frequency signal using a resampling method based on the second phase term and the first phase term, to obtain a compensated time-domain signal. The spectrum analysis unit (407), whose input terminal is connected to the output terminal of the phase noise compensation unit (406), is used to perform spectrum analysis based on the time-domain signal to obtain the frequency-domain signal. The strain demodulation unit (408), whose input end is connected to the output end of the spectrum analysis unit (407), is used to demodulate the strain information along the sensing fiber (203) according to the frequency domain signal.

5. A method for compensating laser phase noise in an optical frequency domain reflectometer, characterized in that, Laser phase noise compensation using the compensation device according to any one of claims 1-4 includes the following steps: The frequency-sweeping laser source module emits a frequency-sweeping laser and splits it into two beams. The first frequency-sweeping laser beam is injected into the main interferometer module, and the second frequency-sweeping laser beam is injected into the auxiliary interferometer module. The first sweeping laser beam is split into a probe beam and a first local oscillator beam in the main interferometer module. After beat frequency interference, the first beat frequency signal related to the position of the scattering point of the sensing fiber is obtained. The second sweeping laser beam is split into a compensation beam and a second local oscillator beam within the auxiliary interferometer module. The compensation beam is encoded and modulated to generate the modulated beam. The pseudo-random noise code used for encoding is stored locally. After the modulated light and the second local oscillator light undergo beat frequency interference, a second beat frequency signal related to the position of the weak reflection point of the auxiliary optical fiber is obtained; The signal processing module acquires the first beat frequency signal and the second beat frequency signal respectively, and mixes the second beat frequency signal with a pseudo-random noise code to obtain the decoded signal. Perform Hilbert transform on the first beat frequency signal and the decoded signal, and extract the phases of the first beat frequency signal and the decoded signal respectively, which are the first phase term and the second phase term, respectively; The second phase term is resampled and compensated to the first phase term to achieve phase noise compensation of the sensing fiber. Based on the compensation information, spectral analysis is performed to demodulate the strain information along the sensing fiber.

6. The compensation method according to claim 5, characterized in that, The first beat frequency signal is the sum of all backscattered Rayleigh scattering signals from the scattering point of the sensing fiber.

7. The compensation method according to claim 6, characterized in that, The second beat frequency signal is the sum of all backward weak reflection signals at the weak reflection point of the auxiliary optical fiber.

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