A method and device for detecting acoustic wave-induced micro-phase shift of optical fiber based on wide-spectrum light interference

Abstract

The application discloses a kind of acoustic wave causes optical fiber micro-phase shift detection method and device based on wide spectrum light interference, comprising: transmitting part, wide spectrum light source, circulator, Y waveguide, optical fiber delay line, sensing arm and reference arm, sensing arm and reference arm are connected in parallel in the output end of Y waveguide, the end of reference arm away from Y waveguide is connected with optical fiber delay line, the input end of Y waveguide is connected transmitting part by circulator, realize the function of acoustic wave signal detection caused optical fiber micro-phase shift, the application proposes a kind of high sensitivity, large dynamic range acoustic wave causes optical fiber micro-phase shift detection method and device based on low coherence light interference, provide a good solution for the development of high sensitivity optical fiber hydrophone.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing and detection technology, and more specifically, to a method and apparatus for detecting acoustic-induced micro-phase shifts in fiber optics based on broadband optical interference. Background Technology

[0002] When optical fibers are subjected to external physical information, changes occur in the optical path length and polarization of the light waves propagating within the fiber core. Fiber optic interferometers utilize this characteristic of optical fibers to sense these changes and demodulate the desired information by measuring the alteration in the light wave information. This sensing method offers high precision and stability, and has therefore been extensively researched and applied in the sensor field. Fiber optic hydrophones are an important application of fiber optic interferometers. In interferometric fiber optic hydrophones, the optical fiber on the sensing probe serves as the sensing unit, primarily responsible for modulating the received underwater acoustic signal onto the changes in the transmitted light wave. The refractive index and length of the optical fiber in the sensing unit change when subjected to the acoustic field in the external environment, causing a corresponding change in the phase of the light wave propagating in the fiber. Demodulating this phase change yields the desired acoustic information. Interferometric fiber optic hydrophone technology is currently a relatively mature technology. Compared to piezoelectric hydrophones, this type of fiber optic hydrophone offers a 2-3 order of magnitude improvement in sound pressure sensitivity and is easily multiplexed. However, most interferometric fiber optic hydrophones currently use narrowband lasers, which have a large coherence length. Although they can meet the interference conditions well and generate interference signals, the defects in the components and fiber drawing of the hydrophone will generate reflection points, thus causing parasitic secondary waves. As a result, there are large errors and noise in the output optical signal. Summary of the Invention

[0003] The present invention aims to provide a method and apparatus for detecting acoustic-induced microphase shift in optical fibers based on broadband optical interference, so as to solve or improve at least one of the above-mentioned technical problems.

[0004] In view of this, a first aspect of the present invention is to provide an acoustic wave-induced fiber micro-phase shift detection device based on broadband optical interference.

[0005] A second aspect of the present invention is to provide a method for detecting acoustic-induced microphase shifts in optical fibers based on broadband optical interference.

[0006] A first aspect of the present invention provides an acoustic wave-induced fiber micro-phase shift detection device based on broadband optical interferometry, comprising: a transmitter, a broadband light source, a circulator, a Y-waveguide, an optical fiber delay line, a sensing arm, and a reference arm. The sensing arm and the reference arm are connected in parallel to the output end of the Y-waveguide. The end of the reference arm away from the Y-waveguide is connected to the optical fiber delay line. The input end of the Y-waveguide is connected to the transmitter through the circulator. The measurement unit comprises: a photodetector whose receiving end is connected to the third port of the circulator; the transmitting end of the photodetector is electrically connected to a demodulation circuit module; the demodulation circuit module is electrically connected to the Y-waveguide, the optical fiber delay line, and a data acquisition computer, respectively. When acoustic wave detection is performed, the transmitter is a broadband light source, and the third port of the circulator is connected to the measurement unit. The measurement unit is configured to be responsible for detecting the acoustic waves sensitive to the sensing arm and controlling the opening and closing of the optical fiber delay line.

[0007] This invention provides an acoustic wave-induced fiber micro-phase shift detection device based on broadband optical interference. It achieves the detection of acoustic wave information by demodulating the acoustic wave signal sensitive by the sensing arm. This invention further improves the Michelson optical path structure by introducing a broadband light source and adding a Y waveguide to the device. It also uses a stepped wave phase modulation method to demodulate the acoustic wave signal.

[0008] Because a broadband light source is used in the optical path, its coherence length is relatively short, and the optical path difference between the sensing arm and the reference arm varies greatly under the influence of sound waves and the external environment, a fiber optic delay line with a large adjustment range is introduced into the optical path structure to ensure that the optical path difference between the two arms does not exceed the coherence length range of the light source. The fiber optic extension line has a large adjustment range, but to ensure that positive and negative compensation for the optical path difference can be achieved in the optical path, the driving section of the fiber optic delay line must first be adjusted to the center of its dynamic range.

[0009] The measuring unit is connected to the fiber optic delay line via a serial port, and then sends an adjustment command. The driving unit of the fiber optic delay line stops at the center of its dynamic range, thus completing the initial position adjustment of the fiber optic delay line.

[0010] During the initial arm length adjustment, the OTDR is connected to the circulator to measure the length of the two arms. The longer arm is then trimmed to ensure that the lengths of the two arms are similar. During the later acoustic wave detection, the OTDR is removed, and then the broadband light source is connected to the circulator.

[0011] In addition, the technical solutions provided by embodiments of the present invention may also have the following additional technical features:

[0012] In any of the above technical solutions, the demodulation circuit module processes the electrical signal converted by the photodetector and controls the Y-waveguide and the optical fiber delay line to detect the sensitive acoustic wave of the sensing arm.

[0013] In this technical solution, the measurement unit is equipped with a photodetector, a demodulation circuit module and a data acquisition computer. The demodulation circuit module changes the phase of the transmitted optical signal by applying voltage to control the change of the refractive index of the LiNbO crystal in the Y waveguide, thereby realizing the detection of the micro-phase of the optical fiber caused by the acoustic signal in the device.

[0014] The light waves output from the broadband light source pass through a circulator and then through a Y-waveguide into the sensing arm and reference arm, respectively. After receiving the acoustic signal, the light waves are reflected back into the sensing arm by its end mirror. The reference arm is insensitive to acoustic signals, and the light waves are directly reflected back to the reference arm by a mirror inside the fiber delay line. The light waves meet and interfere in the Y-waveguide, at which point the interference signal carries the acoustic information. The interfering light returns to the circulator and is transmitted to the photodetector. The received optical signal is finally converted into a easily processed electrical signal and output to the demodulation circuit module. The demodulation circuit module processes the electrical signal converted by the detector and controls the Y-waveguide and fiber delay line to adjust the optical path change, ultimately realizing the function of detecting the micro-phase shift in the fiber optic cable caused by the acoustic signal.

[0015] In any of the above technical solutions, a reflector is provided at the end of the sensing arm away from the Y waveguide.

[0016] In this technical solution, the reflector allows the signal emitted from one end of the sensing arm to return to the sensing arm and be transmitted back. By fixing it to the end of the sensing arm away from the Y-waveguide, the internal signal transmission settings of the sensing arm can be facilitated and signal loss can be reduced.

[0017] In any of the above technical solutions, the nominal value of the center wavelength of the broadband light source is 1530mm±5nm.

[0018] In this technical solution, the nominal center wavelength of the individual light source in the specification settings of each component of the device helps to ensure that the internal specification parameters of the broadband light source are not arbitrarily set, thus ensuring the processing effect of each stage. In addition, limiting the nominal center wavelength of the broadband light source also helps to ensure that the detection results in each stage and step meet the expectations, avoiding the situation where the nominal center wavelength of the light source in a single stage or step fails to meet the requirements, leading to unsatisfactory intervals and transmission in other stages, thus affecting the detection and transmission effects of other steps.

[0019] Broadband light sources have a shorter coherence length. Using broadband light sources in interferometric devices can suppress various types of noise, effectively reduce the influence of optical path noise such as backscattering and Kerr effect, and improve detection accuracy.

[0020] In any of the above technical solutions, the connection between the transmitter, the broadband light source, the circulator, the Y-waveguide, the fiber delay line, the sensing arm, and the reference arm is achieved by using polarization-maintaining fiber with a cladding diameter of 80 μm.

[0021] In this technical solution, the use of polarization-maintaining fiber can ensure that the linear polarization direction remains unchanged, improve the coherent signal-to-noise ratio, so as to achieve high-precision measurement of physical quantities, and limit the cladding diameter, which is beneficial to high acoustic sensitivity.

[0022] In any of the above technical solutions, both the sensing arm and the reference arm are wound with Φ80 / 135μm polarization-maintaining optical fiber.

[0023] In this technical solution, polarization-maintaining optical fiber with limited specifications is used for winding, which can further ensure the proximity of the sensing arm and the reference arm and ensure that the optical path difference between the two arms meets the interference requirements.

[0024] In any of the above technical solutions, the Y-waveguide is a lithium niobate phase modulator that integrates a coupler, a polarizer, and a phase modulator.

[0025] In this technical solution, the lithium niobate phase modulator is fabricated using the electro-optic effect of lithium niobate crystal (LiNbO) combined with optoelectronic integration technology, which has high response speed and low insertion loss.

[0026] In any of the above technical solutions, after the arm length measurement, the length difference between the sensing arm and the reference arm is no greater than 1 cm.

[0027] In this technical solution, after measuring and comparing the sensing arm and the reference arm, the maximum length difference is set to 1cm during the cutting process. This ensures that the optical path difference between the two arms meets the interference requirements and proves that the optical path can work in real time within the coherence length range of the light source in subsequent operations.

[0028] In any of the above technical solutions, when measuring the arm length, the transmitting unit is an OTDR, and the optical fiber delay line is provided with a driving unit for adjusting the position of its end; the driving unit is a motor.

[0029] In this technical solution, the entire motor is encapsulated inside the optical fiber delay line.

[0030] The second aspect of the present invention provides a method for detecting fiber micro-phase shift induced by acoustic waves based on broadband optical interference, comprising the following steps: S1, connecting the broadband light source (9) and the measuring part to the circulator (2) respectively, and the demodulation circuit module (10) outputting a stepped-wave phase modulation signal to the Y-waveguide (4), and changing the phase of the transmitted optical signal by applying a voltage to the Y-waveguide (4) to control the change in the refractive index of the LiNbO3 crystal inside; S2, when the optical path difference between the sensing arm (5) and the reference arm (6) in the optical path caused by acoustic waves or external environment exceeds the coherence length of the broadband light source, the demodulation circuit module (10) outputs a signal to control the phase of the transmitted optical signal. The optical fiber delay line (8) is dynamically compensated accordingly; S3, after the sensing arm (5) receives the acoustic signal, the light wave is returned to the optical path of the sensing arm (5) by the reflector (8) at the end; S4, the interference light returns to the circulator (2) and is transmitted to the photodetector (3). The received optical signal is converted into an easily processed electrical signal and output to the demodulation circuit module (10), and finally realizes the function of detecting the micro-phase shift of the optical fiber caused by the acoustic signal; wherein, the method for detecting the micro-phase shift of the optical fiber caused by the acoustic wave based on broadband optical interference is implemented by a device for detecting the micro-phase shift of the optical fiber based on broadband optical interference as described in any of the technical solutions in the first aspect.

[0031] A second aspect of the present invention provides a method for detecting acoustic-induced micro-phase shifts in optical fibers based on broadband optical interferometry. This method is implemented using one of the aforementioned broadband optical interferometry-based acoustic-induced micro-phase shift detection devices. Therefore, the method for detecting acoustic-induced micro-phase shifts in optical fibers proposed in this invention possesses all the beneficial effects of one of the aforementioned broadband optical interferometry-based acoustic-induced micro-phase shift detection devices, which will not be elaborated further here.

[0032] The beneficial effects of this invention are as follows:

[0033] (1) The detection technology of the present invention realizes the detection of weak sound wave signals and has high sensitivity. Theoretically, it can detect weak sound wave signals corresponding to optical path differences of less than 1 pm.

[0034] (2) This invention utilizes a fiber delay line with a large dynamic range to achieve dynamic compensation for the optical path difference of the optical path, which overcomes the problem of a small measurement dynamic range caused by the short coherence length of the broadband light source, ensures that the optical path works stably in the interference state, and achieves a detection range of not less than ±5mm for changes in optical path difference caused by acoustic waves.

[0035] Additional aspects and advantages of embodiments of the invention will become apparent in the following description or may be learned by practice of embodiments of the invention. Attached Figure Description

[0036] The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of the invention.

[0037] Figure 1 This is a schematic diagram illustrating the initial equal-length measurement principle of the sensing arm and reference arm of the present invention.

[0038] Figure 2 This is a schematic diagram of the present invention;

[0039] Figure 3 This is a flowchart of the fiber optic extension line compensation process of the present invention;

[0040] Figure 4 This is a flowchart illustrating the implementation of the present invention.

[0041] in, Figure 1-4 The correspondence between the reference numerals and component names in the attached drawings is as follows:

[0042] 1 OTDR, 2 Circulator, 3 Photodetector, 4 Y-waveguide, 5 Sensor arm, 6 Reference arm, 7 Mirror, 8 Fiber optic delay line, 9 Broadband light source, 10 Demodulation circuit module, 11 Acquisition computer. Detailed Implementation

[0043] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be combined with each other.

[0044] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and therefore the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0045] Please see Figure 1-4 The first aspect of the present invention provides an acoustic wave-induced fiber micro-phase shift detection device based on broadband optical interferometry, comprising: a transmitter, a broadband light source 9, a circulator 2, a Y-waveguide 4, an optical fiber delay line 8, a sensing arm 5, and a reference arm 6. The sensing arm 5 and the reference arm 6 are connected in parallel to the output end of the Y-waveguide 4. The end of the reference arm 6 away from the Y-waveguide 4 is connected to the optical fiber delay line 8. The input end of the Y-waveguide 4 is connected to the transmitter through the circulator 2. The measurement unit comprises: a photodetector 3 whose receiving end is connected to the third port of the circulator 2. The transmitting end of the photodetector 3 is electrically connected to a demodulation circuit module 10. The demodulation circuit module 10 is electrically connected to the Y-waveguide 4, the optical fiber delay line 8, and a data acquisition computer 11, respectively. When acoustic wave detection is performed, the transmitter is the broadband light source 9, and the third port of the circulator 2 is connected to the measurement unit. The measurement unit is configured to be responsible for detecting the acoustic waves sensitive to the sensing arm 5 and controlling the opening and closing of the optical fiber delay line 8.

[0046] This invention provides an acoustic wave-induced fiber micro-phase shift detection device based on broadband optical interference. It achieves the detection of acoustic wave information by demodulating the acoustic wave signal sensitive by the sensing arm 5. This invention further improves the Michelson optical path structure by introducing a broadband light source 9 and adding a Y waveguide 4 to the device. It uses a stepped wave phase modulation method to demodulate the acoustic wave signal.

[0047] Because a broadband light source 9 is used in the optical path, its coherence length is relatively short, and the optical path difference between the sensing arm 5 and the reference arm 6 varies greatly under the influence of sound waves and the external environment, in order to ensure that the optical path difference between the two arms does not exceed the coherence length range of the light source, a fiber delay line 8 with a large adjustment range is introduced into the optical path structure. The fiber extension line has a large adjustment range, but in order to ensure that positive and negative compensation for the optical path difference can be achieved in the optical path, the driving part of the fiber delay line 8 must first be adjusted to the center position of its dynamic range.

[0048] The measuring unit is connected to the fiber delay line 8 via a serial port, and then sends an adjustment command to drive the driving unit of the fiber delay line 8 to stop at the center of its dynamic range, thus completing the initial position adjustment of the fiber delay line 8.

[0049] During the initial arm length adjustment, the OTDR1 is connected to the circulator 2 to measure the length of the two arms. The longer arm is then trimmed to ensure that the lengths of the two arms are similar. During the later acoustic wave detection, the OTDR1 is removed, and the broadband light source 9 is connected to the circulator 2.

[0050] Specifically, the optical time-domain reflectometer (OTDR) 1 output port is connected to the input port of Y-waveguide 4. The output pigtail ports of Y-waveguide 4 are fused to the ports of reference arm 5 and sensing arm 6, respectively. The ports of sensing arm 5 and reference arm 6 are then fused together with the pigtails of reflector 7 and fiber delay line 8 using a fusion splicer. After alignment, the lengths of the two arms are measured using OTDR1. The two measurement peaks accurately determine the length difference between sensing arm 5 and reference arm 6. The longer arm is then cut off, ensuring the length difference between the two arms is less than 1 cm. Finally, the ports of sensing arm 5 and reference arm 6 are fused together with the pigtails of reflector 7 and fiber delay line 8 using a fusion splicer. This completes the initial equal-length adjustment of the optical path.

[0051] The application of broadband light sources is based on white-light interferometry. Demodulation is achieved through a Y-waveguide and demodulation circuit module. White-light interferometry, as a technique capable of accurately measuring phase shifts, is fundamentally based on partially coherent light theory. It analyzes and measures changes in interference fringes to obtain the corresponding phase shift. Utilizing the low coherence length of broadband light sources, white-light interferometry enables absolute measurement of physical quantities, a capability unmatched by traditional laser coherent measurement techniques. Furthermore, the low-coherence light source employed has strong anti-interference capabilities; the device resolution is independent of factors such as the stability of the light source wavelength, fluctuations in light source power, and fiber disturbances. Therefore, white-light interferometry offers advantages such as low detection background noise and high sensitivity.

[0052] Specifically, the fiber delay line 8 is a single-fiber reflective type. The fiber delay line 8 consists of input and output fiber collimators and an internal reflector. The fiber collimator projects light onto the reflector. The distance the reflector moves is controlled by a motor, which changes the optical path in the optical path structure, thereby achieving precise control of the distance the light travels in free space.

[0053] In any of the above embodiments, the demodulation circuit module 10 processes the electrical signal converted by the photodetector 3 and controls the Y-waveguide 4 and the fiber delay line 8 to detect the sensitive acoustic waves of the sensing arm 5.

[0054] In this embodiment, the measuring unit is equipped with a photodetector 3, a demodulation circuit module 10, and a data acquisition computer 11. The demodulation circuit module 10 changes the phase of the transmitted optical signal by applying a voltage to control the change of the refractive index of the LiNbO crystal in the Y waveguide 4, thereby realizing the detection of the micro-phase of the optical fiber caused by the acoustic signal in the device.

[0055] The light waves output from the broadband light source 9 pass through the circulator 2 and then through the Y-waveguide 4, entering the sensing arm 5 and the reference arm 6 respectively. After receiving the acoustic signal, the light waves are reflected back into the sensing arm 5 by its end mirror 7. The reference arm 6 is insensitive to acoustic signals, and the light waves are directly reflected back to the reference arm 6 by the mirror inside the fiber delay line 8. They meet and interfere in the Y-waveguide 4, at which point the interference signal carries the acoustic information. The interfering light returns to the circulator 2 and is transmitted to the photodetector 3. The finally received optical signal is converted into a easily processed electrical signal and output to the demodulation circuit module 10. The demodulation circuit module 10 processes the electrical signal converted by the detector and controls the Y-waveguide 4 and the fiber delay line 8 to adjust the optical path change, ultimately realizing the function of detecting the micro-phase shift in the fiber optic cable caused by the acoustic signal.

[0056] In any of the above embodiments, a reflector 7 is provided at the end of the sensing arm 5 away from the Y waveguide 4.

[0057] In this embodiment, the reflector 7 is configured so that the signal emitted from one end of the sensing arm 5 can be returned to the sensing arm 5 and transmitted back. It is fixedly installed at the end of the sensing arm 5 away from the Y waveguide 4, which facilitates the internal signal transmission settings of the sensing arm 5 and reduces signal loss.

[0058] In any of the above embodiments, the nominal wavelength of the center of the broadband light source 9 is 1530mm ± 5nm.

[0059] In this embodiment, the nominal value of the center wavelength of the individual light source in the specification settings of each component of the device helps to ensure that the internal specification parameters of the broadband light source 9 are not arbitrarily set, thus ensuring the processing effect of each stage. In addition, limiting the nominal value of the center wavelength of the broadband light source 9 also helps to ensure that the detection results in each stage and step meet the expectations, avoiding the situation where the nominal value of the center wavelength of the light source in a single stage or step fails to meet the requirements, resulting in unsatisfactory intervals and transmission in other stages, thus affecting the detection and transmission effects of other steps;

[0060] The broadband light source 9 has a short coherence length. Using the broadband light source 9 in the interferometer can suppress various noises, effectively reduce the influence of optical path noise such as backscattering and Kerr effect, and improve detection accuracy.

[0061] In any of the above embodiments, the connection between the transmitter, broadband light source 9, circulator 2, Y-waveguide 4, fiber delay line 8, sensing arm 5 and reference arm 6 is made by using polarization-maintaining fiber with a cladding diameter of 80 μm.

[0062] In this embodiment, the use of polarization-maintaining fiber can ensure that the linear polarization direction remains unchanged, improve the coherent signal-to-noise ratio, and enable high-precision measurement of physical quantities. Furthermore, limiting the cladding diameter is beneficial for high acoustic sensitivity.

[0063] In any of the above embodiments, both the sensing arm 5 and the reference arm 6 are wound with Φ80 / 135μm polarization-maintaining optical fiber.

[0064] In this embodiment, polarization-maintaining optical fiber with defined specifications is used for winding, which can further ensure the proximity of sensing arm 5 and reference arm 6, and ensure that the optical path difference between the two arms meets the interference requirements.

[0065] In any of the above embodiments, the Y-waveguide 4 is a lithium niobate phase modulator that integrates a coupler, a polarizer, and a phase modulator.

[0066] In this embodiment, the lithium niobate phase modulator is fabricated using the electro-optic effect of lithium niobate crystal (LiNbO) combined with optoelectronic integration technology, which results in high response speed and low insertion loss.

[0067] In any of the above embodiments, after the arm length measurement, the length difference between the sensing arm 5 and the reference arm 6 is no greater than 1 cm.

[0068] In this embodiment, after measuring and comparing the sensing arm 5 and the reference arm 6, the maximum length difference is set to 1cm during the cutting process. This ensures that the optical path difference between the two arms meets the interference requirements and proves that the optical path can work in real time within the coherence length range of the light source in subsequent operations.

[0069] In any of the above embodiments, when measuring the arm length, the transmitting part is OTDR1, and the fiber delay line 8 is provided with a drive part for adjusting the position of its end; the drive part is a motor.

[0070] In this embodiment, the motor is entirely encapsulated inside the optical fiber delay line 8.

[0071] The second aspect of the present invention provides a method for detecting fiber optic micro-phase shift induced by acoustic waves based on broadband optical interference, comprising the following steps: S1, connecting the broadband light source 9 and the measuring section to the circulator 2 respectively; the demodulation circuit module 10 outputs a stepped-wave phase modulation signal to the Y-waveguide 4; and changing the phase of the transmitted optical signal by applying a voltage to the Y-waveguide 4 to control the change in the refractive index of the LiNbO3 crystal in the Y-waveguide 4; S2, when the optical path difference between the sensing arm 5 and the reference arm 6 in the optical path caused by acoustic waves or external environment exceeds the coherence length of the broadband light source 9, the demodulation circuit module 10 outputs a signal to control... The fiber delay line 8 performs corresponding dynamic compensation; S3, after the sensing arm 5 receives the acoustic signal, the light wave is returned to the optical path of the sensing arm 5 by the reflector 7 at the end; S4, the interference light returns to the circulator 2 and is transmitted to the photodetector 3. The received optical signal is converted into an easily processed electrical signal and output to the demodulation circuit module 10, finally realizing the function of detecting the micro-phase shift of the optical fiber caused by the acoustic signal; wherein, an acoustic wave-induced micro-phase shift detection method based on broadband optical interference is implemented by an acoustic wave-induced micro-phase shift detection device based on broadband optical interference as described in any embodiment of the first aspect.

[0072] A second aspect of the present invention provides a method for detecting acoustic-induced micro-phase shifts in optical fibers based on broadband optical interferometry. The method is implemented using one of the acoustic-induced micro-phase shift detection devices based on broadband optical interferometry described in any of the above embodiments. Therefore, the method for detecting acoustic-induced micro-phase shifts in optical fibers based on broadband optical interferometry proposed in this invention possesses all the beneficial effects of one of the acoustic-induced micro-phase shift detection devices based on broadband optical interferometry described in any of the above embodiments, and will not be elaborated further here.

[0073] Furthermore, to ensure measurement accuracy, OTDR1 is connected to circulator 2 between tests. The lengths of sensing arm 5 and reference arm 6 are measured through OTDR1, and the longer arm is cut off to ensure that the length difference between the two arms is less than 1 cm.

[0074] Example 1

[0075] like Figure 3 and Figure 4 As shown, the specific detection steps of this device are as follows:

[0076] The light source input and the broadband light source output are connected to a circulator. The circulator is connected to a Y-waveguide and a photodetector. The output of the Y-waveguide is connected to the optical path after the initial equal length adjustment. At this point, the optical path for detecting fiber micro-phase shift based on acoustic wave signals with low coherence optical interference is completed and the light source is introduced through the broadband light source.

[0077] In the interference operation, the demodulation circuit module outputs the stepped-wave phase modulation signal to the Y-waveguide. By applying a voltage to the Y-waveguide, the refractive index of the LiNbO3 crystal is controlled to change the phase of the transmitted optical signal, thus ensuring that the system as a whole is in the interference operation state. At the same time, when the optical path difference between the two arms in the optical path exceeds the coherence length of the broadband light source due to sound waves or external environment, the demodulation circuit module outputs a signal to control the fiber delay line to perform corresponding dynamic compensation, so as to ensure that the optical path can work in real time within the coherence length range of the light source.

[0078] Delay line compensation: During the operation of the optical path system, the optical path difference between the sensing arm 5 and the reference arm 6 may exceed the coherence length range of the broadband light source 9 due to sound waves or external environmental factors. In order to ensure that the optical path system can work in the interference state for a long time, this invention introduces a large range of fiber delay lines 8 for dynamic compensation.

[0079] The demodulation output is calculated by the demodulation circuit module 10 for each Δt interval. The absolute value of the difference between the average value and the initial value is then checked to see if it exceeds the set coherence threshold L1. If the absolute value does not exceed the coherence threshold, the fiber optic delay line 8 does not start. If the signs are the same, the demodulation circuit module 10 issues a start command, and the fiber optic delay line 8 starts. During the motor's movement, the average value within Δt is checked again. When the absolute value of the difference between the average value and the initial value is less than the stop threshold L2, the demodulation circuit module 10 issues a stop command, and the fiber optic delay line 8 motor stops moving until the next start command is received.

[0080] In the description of this invention, it should be understood that the terms "longitudinal", "lateral", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0081] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made to the embodiments of the present invention by those skilled in the art without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for detecting acoustic-induced fiber optic micro-phase shift using an acoustic-induced fiber optic micro-phase shift detection device based on broadband optical interference, wherein the acoustic-induced fiber optic micro-phase shift detection device comprises: The transmitter, circulator (2), Y-waveguide (4), fiber delay line (8), sensing arm (5) and reference arm (6) are connected in parallel to the output end of the Y-waveguide (4). The end of the reference arm (6) away from the Y-waveguide (4) is connected to the fiber delay line (8). The input end of the Y-waveguide (4) is connected to the transmitter through the circulator (2). When performing acoustic wave detection, the emitting part is a broadband light source (9), and the third port of the circulator (2) is connected to the measuring part; The measuring unit includes: a photodetector (3) whose receiving end is connected to the third port of the circulator (2); the transmitting end of the photodetector (3) is electrically connected to the demodulation circuit module (10); and the demodulation circuit module (10) is electrically connected to the Y waveguide (4), the optical fiber delay line (8), and the acquisition computer (11). The measuring unit is configured to detect the sensitive acoustic waves of the sensing arm (5) and control the opening and closing of the optical fiber delay line (8). The demodulation circuit module (10) processes the electrical signal converted by the photodetector (3) and controls the Y waveguide (4) and the fiber delay line (8) to detect the sensitive acoustic waves of the sensing arm (5). A reflector is provided at the end of the sensing arm (5) away from the Y waveguide (4); Its features include the following steps: S1, connect the broadband light source (9) and the measurement part to the circulator (2) respectively, and the demodulation circuit module (10) outputs the stepped wave phase modulation signal to the Y waveguide (4). By applying voltage to the Y waveguide (4), the refractive index of the internal LiNbO3 crystal is controlled to change the phase of the transmitted optical signal. S2, when the optical path difference between the sensing arm (5) and the reference arm (6) in the optical path caused by the sound wave or the external environment exceeds the coherence length of the broadband light source, the demodulation circuit module (10) outputs a signal to control the fiber delay line (8) to perform corresponding dynamic compensation. The demodulation circuit module calculates the average value every Δt and determines whether the absolute value of the difference between the average value and the initial value exceeds the set coherence threshold L1. If the absolute value does not exceed the coherence threshold, the fiber delay line does not start; if the signs are the same, the demodulation circuit module issues a start command, and the fiber delay line starts. During the motor movement, it continues to determine the average value within Δt. When the absolute value of the difference between the average value and the initial value is less than the stop threshold L2, the demodulation circuit module issues a stop command, and the fiber delay line motor stops moving until the next start command is received. S3, after the sensing arm (5) receives the acoustic signal, the light wave is returned to the optical path of the sensing arm (5) by the reflector at the end; S4, the interference light returns to the circulator (2) and is transmitted to the photodetector (3). The received optical signal is converted into an easily processed electrical signal and output to the demodulation circuit module (10), thus realizing the function of detecting the micro-phase shift of the optical fiber caused by the acoustic signal.

2. The method for detecting acoustic-induced fiber optic micro-phase shift according to claim 1, characterized in that, The nominal center wavelength of the broadband light source (9) is 1530 mm ± 5 nm; and / or The connection between the transmitter, the broadband light source (9), the circulator (2), the Y-waveguide (4), the fiber delay line (8), the sensing arm (5), and the reference arm (6) is all made using polarization-maintaining fiber with a cladding diameter of 80µm; and / or Both the sensing arm (5) and the reference arm (6) are wound with polarization-maintaining optical fiber of Φ80 / 135μm.

3. The method for detecting acoustic-induced fiber optic micro-phase shift according to claim 1, characterized in that, The Y-waveguide (4) is a lithium niobate phase modulator that integrates a coupler, a polarizer, and a phase modulator.

4. The method for detecting acoustic-induced fiber optic micro-phase shift according to claim 1, characterized in that, After the arm length measurement, the length difference between the sensing arm (5) and the reference arm (6) is no greater than 1 cm.

5. The method for detecting acoustic-induced fiber optic micro-phase shift according to claim 1, characterized in that, When measuring arm length, the transmitting part is an OTDR (1), and the fiber delay line (8) is provided with a drive part for adjusting the position of its end. The drive unit is a motor.

Images