Distributed optical fiber sound wave sensing device and demodulation method

Abstract

The invention provides a distributed optical fiber sound wave sensing device and a demodulation method. The demodulation method comprises the following steps: injecting pulse laser into an optical fiber to be measured; a backward Rayleigh scattering light signal is generated in the optical fiber to be measured and is divided into two parts; one Rayleigh scattering light signal is coupled with the other Rayleigh scattering light signal after frequency shift and time delay to form a Rayleigh scattering interference signal; converting the obtained Rayleigh scattering interference signal into a digital signal; and calculating the phase change information of each point location according to the discrete form of the obtained digital signal. According to the invention, the influence of low-frequency interference of the acquisition device can be avoided, the limitation of the optical fiber length on the carrier frequency is broken through, and the bandwidth of demodulated acoustic signals is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of optics, and particularly relates to a distributed fiber optic acoustic wave sensing device and a demodulation method. Background Art

[0002] In traditional distributed fiber optic acoustic wave sensor (DAS) schemes based on a matched interferometer, there are still many deficiencies. Specifically, for the method based on a 3×3 coupler, three detectors are required at the receiving end, and at least a three-channel data acquisition card needs to be used simultaneously, thus increasing the cost and complexity of the device. At the same time, the interferometer signal returned by the 3×3 coupler does not carry a carrier, so the noise level of the demodulation result is not ideal in the low-frequency band. Moreover, the signal demodulation of the 3×3 coupler method depends on the 2 / 3π phase difference between the three interferometer signals obtained. In practical applications, this phase difference is unstable and will fluctuate with factors such as temperature, resulting in distortion of the demodulation result and deterioration of the device performance. For the PGC method, its main limitation lies in the bandwidth and sensing distance of the recovered signal. In its demodulation method, first, the interferometer signals scattered back at each point need to be recovered on the slow time axis, and then the interferometer signals are demodulated. The sampling rate of the interferometer signals recovered at each point is the pulse repetition frequency of the DAS, which is limited by the fiber length (the longer the distance, the lower the pulse repetition frequency). Generally, the carrier frequency of the PGC is lower than 1 / 8 of the pulse repetition frequency, and the bandwidth of the acoustic signal that can be demodulated is at least lower than 1 / 2 of the carrier frequency. Therefore, the bandwidth of the signal that can be recovered by this method is severely limited. Considering the quality of the recovered signal, its sensing distance is generally not very long. For example, for the DASA based on PGC given in reference

[13] , under the conditions of a sensing distance of 10 km and a pulse repetition frequency of 10 kHz, the carrier frequency can only be set at 1 kHz, and the frequency of the demodulated signal is 120 Hz. Summary of the Invention

[0003] The purpose of the present invention is to overcome the deficiencies in the prior art and provide a distributed fiber optic acoustic wave sensing device and a demodulation method. The solution of the present invention can solve the problems existing in the above prior art.

[0004] Technical solution of the present invention:

[0005] According to a first aspect, a distributed fiber optic acoustic sensing device is provided, including a pulsed light source generating unit, an optical fiber, a circulator, a matching interferometer, an optical frequency shifting unit, a signal processing unit, and a data calculation unit. The pulsed light source generates pulsed light, which is transmitted into the optical fiber. Backward Rayleigh scattering optical signals will be generated at different positions in the optical fiber. After the signal is transmitted backward to the circulator position, it exits through another port of the circulator and enters the matching interferometer. There are two arms in the matching interferometer, and the Rayleigh scattering optical signals are respectively incident into the two arms. The optical frequency shifting unit is connected to one arm of the matching interferometer, and a section of delay optical fiber is also inserted in the same arm of the matching interferometer. After the Rayleigh scattering optical signal passes through the optical frequency shifter and the delay optical fiber, the frequency generated by the optical frequency shifting unit is superimposed on the Rayleigh scattering optical signal. The delay of the delay optical fiber delays the overall Rayleigh scattering optical signal emitted from this arm. The processed Rayleigh scattering optical signal and the Rayleigh scattering optical signal emitted from the other arm are coupled and output to the signal processing unit. The signal processing unit converts the obtained Rayleigh scattering interference signal into a digital signal and transmits it to the data calculation unit; the data calculation unit calculates the phase change information at several positions of the optical fiber according to a preset calculation formula.

[0006] Further, the optical frequency shifting unit includes a signal generator, a driver, and an optical frequency shifter. The signal generator generates a required frequency signal and sends it to the driver, and the driver drives the optical frequency shifter to obtain an optical signal with a corresponding frequency shift.

[0007] Further, the distributed fiber optic acoustic sensing device further includes a time synchronization unit, and the time synchronization unit realizes the time synchronization between the signal generator and the signal processing unit.

[0008] Further, the matching interferometer includes a Mach-Zehnder interferometer and a Michelson interferometer.

[0009] Further, when the interferometer is a Mach-Zehnder interferometer, the length of the delay optical fiber is ΔL, and the corresponding fiber spatial resolution is ΔL / 2; when the matching interferometer is a Michelson interferometer, the length of the delay optical fiber is ΔL, and the corresponding fiber spatial resolution is ΔL.

[0010] Further, another arm of the matching interferometer is also connected with an optical frequency shifting unit.

[0011] Further, the signal processing unit includes a photodetector and a data converter. The photodetector receives the Rayleigh scattering interference signal, converts it into an electrical signal, and transmits it to the data converter. The data converter converts the obtained electrical signal into a digital signal and transmits it to the data calculation unit.

[0012] Further, the preset calculation formula is as follows:

[0013]

[0014] Where m is the sampling point of the Rayleigh scattering interference signal along the fiber length direction, and its corresponding sampling rate F s is the original analog-to-digital conversion sampling rate of the used acquisition device, with a value range of [1, M]. The increasing direction is called the "fast time axis", and M is the number of points after discretization in the entire fiber length direction; n is the group number of the received Rayleigh scattering signal. After the device completely receives the signals returned from each point on the fiber, it can send the next group of pulse signals. Therefore, the pulse repetition frequency is F s / M, and this frequency is defined as the pulse repetition frequency. The value range of n is [1, +∞), and its increasing direction becomes the "slow time axis"; A m and B m represent the DC amplitude and AC amplitude of the interference signal formed at each point on the fiber, represents the phase change information at the position of the i-th point in the fiber distance direction. It is a waveform that varies with n, that is, along the slow time axis; First, along the "fast time axis" direction, I / Q demodulation is performed on the interference signal. For example, when demodulating the k-th Rayleigh scattering interference signal, the sum of the phase information of each point position at this slow time axis moment can be restored The phase information at the m position at this time can be obtained through differential operation, that is, Finally, the phase signals at the same position on the fiber obtained by solving each group of Rayleigh scattering interference signals are solved in sequence, and the phase change information at the m-th point position can be obtained

[0015] According to the second aspect, there is provided the above-mentioned distributed fiber optic acoustic wave sensing demodulation method, including the following steps:

[0016] Inject pulsed laser light into the fiber to be measured;

[0017] Backward Rayleigh scattering optical signals are generated in the fiber to be measured, and the signals are divided into two parts;

[0018] One of the Rayleigh scattering optical signals is frequency-shifted and delayed and then coupled with the other Rayleigh scattering optical signal to form a Rayleigh scattering interference signal;

[0019] Convert the obtained Rayleigh scattering interference signal into a digital signal;

[0020] For the discrete form of the obtained digital signal, calculate the phase change information at each point position.

[0021] Furthermore, the frequency of the frequency shift is greater than or equal to 10 MHz.

[0022] Furthermore, the method for calculating the phase change information at each point position is as follows:

[0023] Determination formula for the discrete form of the Rayleigh scattering interference signal:

[0024]

[0025] where m is the sampling point of the Rayleigh scattering interference signal along the fiber length direction, and its corresponding sampling rate F s is the original analog-to-digital conversion sampling rate of the used acquisition device, with a value range of [1, M]. The increasing direction is called the "fast time axis", and M is the number of points after discretization in the entire fiber length direction; n is the group number of the received Rayleigh scattering signal. After the device completely receives the signals returned from each point on the fiber, it can send the next group of pulse signals. Therefore, the repetition frequency of the pulse signal is F s / M, and this frequency is defined as the pulse repetition frequency. The value range of n is [1, +∞), and its increasing direction becomes the "slow time axis"; A m and B m represent the DC amplitude and AC amplitude of the interference signal formed at each point on the fiber, represents the phase change information at the i-th point position in the fiber distance direction, which is a waveform varying with n, that is, along the slow time axis;

[0026] Along the "fast time axis" direction, perform I / Q demodulation on the Rayleigh scattering interference signal to recover the sum of the phase information at each point position at this slow time axis moment

[0027] The phase information at the m position at this time can be obtained through differential operation, that is,

[0028] Successively solve the phase signals at the same position on the fiber obtained from each group of Rayleigh scattering interference signals, and the phase change information at the m-th point position can be obtained

[0029] Advantages of the present invention compared with the prior art:

[0030] 1) Improve the noise performance of the demodulation result. By introducing the heterodyne frequency, the Rayleigh scattering interference signal is shifted to the high-frequency band, which can avoid the influence of low-frequency interference of the acquisition device on it;

[0031] 2) Break through the limitation of the fiber optic length on the carrier frequency and increase the bandwidth of the demodulable acoustic wave signal. By introducing a high-frequency heterodyne carrier into the Rayleigh scattering interference signal and combining with the demodulation algorithm that prioritizes the fast time axis, an interference signal with a high sampling rate can be obtained, avoiding the limitation of the pulse repetition frequency on the carrier frequency of the interference signal;

[0032] 3) In the present invention, only two couplers are required when using a Mach-Zehnder interferometer, and only one coupler is required when using a Michelson interferometer, reducing the number of couplers used and the corresponding number of detectors and data acquisition cards, and lowering the cost and complexity of the device. Description of the Drawings

[0033] The included drawings are used to provide a further understanding of the embodiments of the present invention, which form a part of the specification, are used to illustrate the embodiments of the present invention, and together with the written description, explain the principles of the present invention. Obviously, the drawings in the following description are only some embodiments of the present invention, and for those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0034] Figure 1 Shows a schematic structural diagram of a distributed fiber optic acoustic wave sensing device provided according to an embodiment of the present invention;

[0035] Figure 2 Shows schematic structural diagrams of two non-balanced matching interferometers provided according to an embodiment of the present invention;

[0036] Figure 3 Shows a schematic diagram of the received Rayleigh scattering interference signal provided according to an embodiment of the present invention;

[0037] Figure 4 Shows a phase demodulation flow chart provided according to an embodiment of the present invention;

[0038] Figure 5 Shows a schematic diagram of the steps of a distributed fiber optic acoustic wave sensing demodulation method provided according to an embodiment of the present invention. Detailed Embodiments

[0039] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments may be combined with each other. The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and in no way limits the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0040] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0041] Unless otherwise specifically stated, the relative arrangements, numerical expressions, and numerical values of the components and steps set forth in these embodiments do not limit the scope of the present invention. At the same time, it should be understood that, for the sake of convenience of description, the dimensions of the various parts shown in the drawings are not drawn in actual proportional relationships. Technologies, methods, and devices known to those of ordinary skill in the relevant art may not be discussed in detail, but where appropriate, the technologies, methods, and devices should be regarded as part of the authorized specification. In all the examples shown and discussed herein, any specific value should be construed as merely exemplary and not as a limitation. Therefore, other examples of the exemplary embodiments may have different values. It should be noted that like reference numerals and letters denote like items in the following drawings, and thus, once an item is defined in one drawing, it does not need to be further discussed in subsequent drawings.

[0042] Such as Figure 1As shown, according to an embodiment of the first aspect of the present invention, a distributed fiber optic acoustic wave sensing device is provided, which includes a pulsed light source generating unit, an optical fiber, a circulator, a matching interferometer, an optical frequency shifting unit, a signal processing unit, and a data calculation unit. The pulsed light source generates pulsed light, which is transmitted into the optical fiber. Backscattered Rayleigh light signals will be generated at different positions in the optical fiber. After the signal is transmitted backward to the circulator position, it is emitted from another port of the circulator and enters the matching interferometer. There are two arms in the matching interferometer, and the Rayleigh scattered light signals are respectively incident into the two arms. The optical frequency shifting unit is connected to one arm of the matching interferometer, and a section of delay optical fiber is also inserted in the same arm of the matching interferometer. After the Rayleigh scattered light signal passes through the optical frequency shifter and the delay optical fiber, the frequency signal generated by the optical frequency shifting unit is superimposed on the Rayleigh scattered light signal. The delay of the delay optical fiber delays the entire Rayleigh scattered light signal emitted from this arm. The processed Rayleigh scattered light signal and the Rayleigh scattered light signal emitted from the other arm are coupled and then output to the signal processing unit. The signal processing unit converts the obtained Rayleigh scattered interference signal into a digital signal and transmits it to the data calculation unit; the data calculation unit calculates the phase change information at several positions of the optical fiber according to a preset calculation formula.

[0043] With this setting, an optical frequency shifting unit and a delay optical fiber are introduced on one arm of the matching interferometer. By introducing the heterodyne frequency, the Rayleigh scattered interference signal is shifted to the high-frequency band, which can avoid the influence of low-frequency interference of the acquisition system on it.

[0044] Further, in an embodiment, the optical frequency shifting unit includes a signal generator, a driver, and an optical frequency shifter. The signal generator generates the required frequency signal and sends it to the driver, and the driver drives the optical frequency shifter to move to obtain an optical signal corresponding to the frequency. Preferably, in an embodiment, the frequency of the obtained optical signal is greater than 10 MHz, which is controlled by the signal generator and can be specified by the user in specific engineering applications.

[0045] Further, in an embodiment, a distributed fiber optic acoustic wave sensing device further includes a time synchronization unit, which realizes the time synchronization between the signal generator and the signal processing unit. With this setting, it is ensured that the obtained data is on the same time axis, ensuring the quality of the demodulation result.

[0046] Further, in an embodiment, the matching interferometer includes a Mach-Zehnder type matching interferometer and a Michelson type matching interferometer.

[0047] Further, in an embodiment, when the matching interferometer is a Mach-Zehnder type interferometer, the length of the delay optical fiber is ΔL, and the corresponding fiber spatial resolution is ΔL / 2; when the matching interferometer is a Michelson type interferometer, the length of the delay optical fiber is ΔL, and the corresponding fiber spatial resolution is ΔL.

[0048] Further, in one embodiment, another optical frequency shift unit is also connected to the other arm of the matching interferometer. By connecting the optical frequency shift unit to the other arm, the range of the optical moving frequency can be increased, and the required range of the optical moving frequency can be obtained through the combination of the two optical frequency shift units, thereby improving the flexibility of the selection of the optical frequency shift unit.

[0049] Further, in one embodiment, the signal processing unit includes a photodetector and a data converter. The photodetector receives the Rayleigh scattering interference signal, converts it into an electrical signal, and transmits it to the data converter. The data converter converts the obtained electrical signal into a digital signal and transmits it to the data calculation unit.

[0050] Further, in one embodiment, the preset calculation formula is:

[0051]

[0052] where m is the sampling point of the Rayleigh scattering interference signal along the fiber length direction, and its corresponding sampling rate F s is the original analog-to-digital conversion sampling rate of the used acquisition device. Here, the acquisition device refers to an analog-to-digital converter, and its value range is [1, M]. The increasing direction is called the "fast time axis", and M is the number of points after discretization in the entire fiber length direction; n is the group number of the received Rayleigh scattering signal. After the device completely receives the signals returned from each point on the fiber, it can send the next group of pulse signals. Therefore, the repetition frequency of the pulse signal is F s / M, and this frequency is defined as the pulse repetition frequency. The value range of n is [1, +∞), and its increasing direction becomes the "slow time axis"; A m and B m represent the DC amplitude and AC amplitude of the interference signal formed at each point on the fiber. represents the phase change information at the i-th point position in the fiber distance direction, which is a waveform that varies with n, that is, along the slow time axis; First, along the "fast time axis" direction, I / Q demodulation is performed on the interference signal. For example, when demodulating the k-th Rayleigh scattering interference signal, the sum of the phase information of each point position at this slow time axis moment can be restored The phase information at the m position at this time can be obtained through differential operation, that is Finally, the phase signals at the same position on the fiber obtained by solving each group of Rayleigh scattering interference signals are solved in turn, and the phase change information at the m-th point position can be obtained

[0053] With this setting, the phase information is preferentially demodulated on the fast time axis, and then the waveforms at each point are restored through the slow time axis, thus avoiding the limitation of the fiber length on the signal demodulation bandwidth and realizing the distributed sensing and demodulation of long-distance and wide-band acoustic wave signals.

[0054] According to the embodiments of the second aspect, as Figure 5 shown, the above-mentioned distributed fiber optic acoustic wave sensing and demodulation method is provided, including the following steps:

[0055] Inject pulsed laser into the fiber to be measured;

[0056] A backward Rayleigh scattering optical signal is generated in the fiber to be measured, and this signal is divided into two parts;

[0057] Shift the frequency and delay one of the Rayleigh scattering optical signals and then couple it with the other Rayleigh scattering optical signal to form a Rayleigh scattering interference signal;

[0058] Convert the obtained Rayleigh scattering interference signal into a digital signal;

[0059] Obtain the discrete form of the digital signal and calculate the phase change information at each point.

[0060] Furthermore, in one embodiment, the frequency of frequency shift is greater than 10 MHz, which is controlled by a signal generator and can be specified by the user in specific engineering applications.

[0061] Furthermore, in one embodiment, the method for calculating the phase change information at each point is as follows:

[0062] Determination formula for the discrete form of the Rayleigh scattering interference signal:

[0063]

[0064] where m is the sampling point of the Rayleigh scattering interference signal along the fiber length direction, and its corresponding sampling rate F s is the original analog-to-digital conversion sampling rate of the used acquisition device, with a value range of [1, M], and the increasing direction is called the "fast time axis", and M is the number of points after discretization in the entire fiber length direction; n is the group number of the received Rayleigh scattering signal. After the device completely receives the signals returned from each point on the fiber, it can send the next group of pulsed signals. Therefore, the repetition frequency of the pulsed signal is F s / M, and this frequency is defined as the pulse repetition frequency. The value range of n is [1, +∞), and the increasing direction becomes the "slow time axis"; A m and B m represent the DC amplitude and AC amplitude of the interference signal formed at each point on the fiber, Indicates the phase change information at the i-th point position in the fiber optic distance direction, which is a waveform varying with respect to n, i.e., along the slow time axis;

[0065] Along the "fast time axis" direction, perform I / Q demodulation on the Rayleigh scattering interference signal to recover the sum of the phase information of each point position at this slow time axis moment

[0066] The phase information at the m position at this time can be obtained through differential operation, that is,

[0067] Successively solve the phase signals at the same position on the optical fiber obtained from each group of Rayleigh scattering interference signals, and the phase change information at the m-th point position can be obtained

[0068] In order to have a further understanding of a distributed fiber optic acoustic wave sensing device and demodulation method provided by the present invention, the following will be described in detail with specific examples and accompanying drawings.

[0069] A distributed acoustic wave sensing device and demodulation method based on heterodyne demodulation and an unbalanced matching interferometer. The device structure is as Figure 1 shown. The laser is required to be a narrow linewidth light source, providing a continuous light wave with low phase noise for the entire device, and its optical frequency is denoted as f 0 . The continuous light wave emitted by it is modulated into pulsed light by a chopper, amplified to an appropriate power by an erbium-doped fiber amplifier, and after filtering out most of the ASE noise using an optical bandpass filter, it is injected into the fiber under test through a circulator. Here, the chopper can use a semiconductor optical amplifier or an acousto-optic modulator. In order to prevent the light wave from being transmitted back to the laser and causing damage to the laser, an isolator can be added between the chopper and the erbium-doped fiber amplifier.

[0070] When the pulsed light is injected into the fiber under test, backward Rayleigh scattering light signals will be generated at different positions in the fiber. After the signal is transmitted backward to the circulator position, it exits through the 3-port of the circulator and enters the unbalanced matching interferometer. The specific structure of this unbalanced matching interferometer is as Figure 2 shown, where Figure 2 (a) shows the structure of a Mach-Zehnder type interferometer. The backward Rayleigh scattering signal first enters the upper arm and the lower arm of the Mach-Zehnder type interferometer through a 50:50 coupler 1. In the upper arm, an optical frequency shifter 1 is inserted, which will cause the optical wave frequency to shift by a frequency of f 1 , that is, its frequency becomes f 0 +f 1 ; Similarly, the optical frequency shifter 2 in the lower arm will shift the frequency of the passing optical signal to f 0 +f2 Meanwhile, in the upper arm, an optical fiber with a delay length of ΔL is also inserted. This delay optical fiber will match the spatial resolution of the DAS, and its spatial resolution is ΔL / 2. It should be noted that: (1) The optical frequency shifter here can be an acousto-optic modulator or an electro-optic modulator, and there is no requirement for the specific form of the frequency shifter; (2) The frequency shift value generated by the frequency shifter, namely f 1 and f 2 , is controlled by any signal generator and driver in the device and can be adjusted according to actual needs. Any signal generator and driver used here are dual-channel to meet the driving requirements when there are optical frequency shifters in both the upper and lower arms; (3) The optical frequency shifter in the lower arm is optional, that is, it does not necessarily need to be added. When there is an optical frequency shifter 2, an external heterodyne carrier frequency of f c = |f 1 - f 2 | will be introduced. When there is no optical frequency shifter 2, the carrier frequency f c = f 1 . Finally, the optical signals in the upper arm and the lower arm will be coupled together by coupler 2 to form a Rayleigh scattering interference signal, which carries the phase information of each point along the optical fiber.

[0071] And Figure 2 (a) Similar, Figure 2 (b) shows the structure of a matching interferometer built using a Michelson structure. The differences are: (1) Figure 2 (b) only uses one coupler (coupler 1), and its form changes from 1×2 in Figure 2 (a) to 2×2; (2) Figure 2 The back end of the Michelson interferometer shown in Figure 2 (b) uses two Faraday rotators (Faraday rotator 1, Faraday rotator 2) to return the optical signal. Note that here, since the optical signals in the upper arm and the lower arm pass through the optical frequency shifter twice, the optical frequency value will be doubled. That is, when there is an optical frequency shifter 2 in the lower arm, the obtained external heterodyne carrier frequency is f c = 2|f 1 - f 2 |; when there is no optical frequency shifter 2 in the lower arm, the external heterodyne carrier frequency is f c = 2f 1 ; (3) At this time, there is still a section of optical fiber with a length of ΔL in the upper arm, but since the optical signal passes through this optical fiber twice, the corresponding spatial resolution is ΔL.

[0072] The Rayleigh scattering interference signal output from the matching interferometer is converted into an electrical signal by a photodetector and then into a digital signal by a data acquisition device. Considering the discrete form of the Rayleigh scattering interference signal, the following expression can be obtained:

[0073]

[0074] where m is the sampling point of the Rayleigh scattering interference signal along the fiber length direction, and the corresponding sampling rate F s is the original analog-to-digital conversion sampling rate of the used acquisition device, with a value range of [1, M]. The increasing direction is called the "fast time axis", and M is the number of points after discretization in the entire fiber length direction. n is the group number of the received Rayleigh scattering signal. After the device completely receives the signals returned from each point on the fiber, it can send the next group of pulse signals. Therefore, the repetition frequency of the pulse signal is F s / M. This frequency is defined as the pulse repetition frequency, and the value range of n is [1, +∞). The increasing direction becomes the "slow time axis". In formula (1), A m and B m represent the DC amplitude and AC amplitude of the interference signal formed at each point on the fiber, represents the phase change information at the position of the i-th point in the fiber distance direction, which is a waveform varying with n, that is, along the slow time axis.

[0075] It can be seen from formula (1) that the received Rayleigh scattering interference signal can be regarded as a two-dimensional matrix, as Figure 3 shown. Where m represents different positions on the fiber, and its increasing direction is the "fast time axis"; n represents the Rayleigh scattering interference signals received after emitting different pulses of light, and its increasing direction is the "slow time axis". In this solution, the signal demodulation steps are as Figure 4 shown. Different from the signal demodulation method in the traditional solution, in this solution, it is necessary to first perform I / Q demodulation on the interference signal along the "fast time axis" direction. For example, when demodulating the k-th Rayleigh scattering interference signal, the sum of the phase information of each point position at this slow time axis moment can be restored The phase information at the m position at this time can be obtained through differential operation, that is, Finally, the phase signals of the same position on the fiber obtained by solving each group of Rayleigh scattering interference signals are solved in turn, and the phase change information at the m-th point position can be obtained The phase change information at each point position obtained through this algorithm can be uploaded to the computer for further analysis, and at the same time, the computer can also complete the control of the data acquisition device. In this device, in order to ensure the quality of the demodulation result, it is necessary to achieve clock synchronization between the data acquisition device and any signal generator.

[0076] In summary, a distributed fiber optic acoustic wave sensing device and demodulation method provided by the present invention have at least the following advantages compared with the prior art:

[0077] 1) Improve the noise performance of the demodulation result. By introducing the heterodyne frequency, the Rayleigh scattering interference signal is shifted to the high-frequency band, which can avoid the influence of the low-frequency interference of the acquisition system on it;

[0078] 2) Break through the limitation of the fiber length on the carrier frequency and improve the bandwidth of the demodulable acoustic signal. By introducing a high-frequency heterodyne carrier into the Rayleigh scattering interference signal and combining the demodulation algorithm with the fast time axis priority, an interference signal with a high sampling rate can be obtained, avoiding the limitation of the pulse repetition frequency on the carrier frequency of the interference signal, and realizing the distributed sensing and demodulation of long-distance and wide-band acoustic signals;

[0079] 3) In the present invention, only two couplers are needed when using a Mach-Zehnder interferometer, and only one coupler is needed when using a Michelson interferometer, reducing the number of couplers used and the number of corresponding detectors and data acquisition cards, and reducing the cost and complexity of the device;

[0080] 4) The implementation form is flexible and diverse, and it is easy to promote and apply. For the form of the unbalanced interferometer used at the back end, the frequency shift device, the position where the frequency shift device is added, etc., there are a variety of different choices to meet the application requirements of different occasions.

[0081] For the sake of convenience in description, spatial relative terms such as "above...", "over...", "on the upper surface of...", "above" and the like can be used here to describe the spatial position relationship between a device or feature shown in the figure and other devices or features. It should be understood that the spatial relative terms are intended to include different orientations in use or operation in addition to the orientation described in the figure of the device. For example, if the device in the drawing is inverted, the device described as "above other devices or structures" or "over other devices or structures" will then be positioned as "below other devices or structures" or "under other devices or structures". Thus, the exemplary term "above..." can include both the orientations of "above..." and "below...". The device can also be positioned in other different ways (rotated 90 degrees or in other orientations), and the corresponding explanations are made for the spatial relative descriptions used here.

[0082] In addition, it should be noted that the use of words such as "first" and "second" to limit the components is only for the convenience of distinguishing the corresponding components. Without otherwise stating, the above words have no special meanings, so they cannot be understood as limiting the protection scope of the present invention.

[0083] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A distributed fiber optic acoustic wave sensing device, characterized in that, it includes a pulsed light source generating unit, an optical fiber, a circulator, a matching interferometer, an optical frequency shifting unit, a signal processing unit and a data calculation unit. The pulsed light source generates pulsed light and transmits it into the optical fiber. Backscattered Rayleigh optical signals will be generated at different positions in the optical fiber. After the signal is transmitted backward to the circulator position, it exits through another port of the circulator and enters the matching interferometer. There are two arms in the matching interferometer, and the Rayleigh scattered optical signals are respectively incident into the two arms. The optical frequency shifting unit is connected to one arm of the matching interferometer, and a section of delay optical fiber is also inserted in the same arm of the matching interferometer. After the Rayleigh scattered optical signal passes through the optical frequency shifter and the delay optical fiber, the frequency generated by the optical frequency shifting unit is superimposed on the Rayleigh scattered optical signal. The delay of the delay optical fiber delays the overall Rayleigh scattered optical signal emitted from this arm. The processed Rayleigh scattered optical signal and the Rayleigh scattered optical signal emitted from the other arm are coupled and output to the signal processing unit. The signal processing unit converts the obtained Rayleigh scattered interference signal into a digital signal and transmits it to the data calculation unit; the data calculation unit calculates the phase change information at several points on the optical fiber according to a preset calculation formula.

2. The distributed fiber optic acoustic wave sensing device according to claim 1, characterized in that, the optical frequency shifting unit includes a signal generator, a driver and an optical frequency shifter. The signal generator generates a required frequency signal and sends it to the driver, and the driver drives the optical frequency shifter to obtain an optical signal with a corresponding frequency shift.

3. The distributed fiber optic acoustic wave sensing device according to claim 2, characterized in that, the distributed fiber optic acoustic wave sensing device further includes a time synchronization unit, and the time synchronization unit realizes the time synchronization between the signal generator and the signal processing unit.

4. The distributed fiber optic acoustic wave sensing device according to claim 1, characterized in that, the matching interferometer includes a Mach-Zehnder type interferometer and a Michelson type interferometer.

5. The distributed fiber optic acoustic wave sensing device according to claim 4, characterized in that, when the interferometer is a Mach-Zehnder type interferometer, the length of the delay optical fiber is ΔL, and the corresponding optical fiber spatial resolution is ΔL / 2; when the matching interferometer is a Michelson type interferometer, the length of the delay optical fiber is ΔL, and the corresponding optical fiber spatial resolution is ΔL.

6. The distributed fiber optic acoustic wave sensing device according to claim 5, characterized in that, another arm of the matching interferometer is also connected with an optical frequency shifting unit.

7. The distributed fiber optic acoustic wave sensing device according to claim 1, characterized in that, The signal processing unit includes a photodetector and a data converter. The photodetector receives the Rayleigh scattering interference signal, converts it into an electrical signal, and transmits it to the data converter. The data converter converts the obtained electrical signal into a digital signal and transmits it to the data calculation unit.

8. A distributed fiber optic acoustic sensing device according to claim 7, wherein, the preset calculation formula is: Among them, m is the sampling point of the Rayleigh scattering interference signal along the fiber length direction, and the corresponding sampling rate F s is the original analog-to-digital conversion sampling rate of the used acquisition device, with a value range of [1, M]. The increasing direction is called the "fast time axis", and M is the number of points after discretization in the entire fiber length direction; n is the group number of the received Rayleigh scattering signal. After the device completely receives the signals returned from each point on the fiber, it can send the next group of pulse signals. Therefore, the repetition frequency of the pulse signal is F s / M. This frequency is defined as the pulse repetition frequency, and the value range of n is [1, +∞). The increasing direction becomes the "slow time axis"; A m and B m represent the DC amplitude and AC amplitude of the interference signal formed at each point on the fiber, represents the phase change information at the position of the i-th point in the fiber distance direction. It is a waveform that changes with respect to n, that is, along the slow time axis; First, along the "fast time axis" direction, I / Q demodulation is performed on the interference signal. For example, when demodulating the k-th Rayleigh scattering interference signal, the sum of the phase information of each point position at this slow time axis moment can be restored The phase information at the m position at this time can be obtained through differential operation, that is Finally, the phase signals at the same position on the fiber obtained by solving each group of Rayleigh scattering interference signals are solved in turn, and the phase change information at the m-th point position can be obtained 9. A distributed fiber optic acoustic sensing demodulation method using the distributed fiber optic acoustic sensing device according to any one of claims 1-8, wherein, the method includes the following steps: Inject pulsed laser light into the fiber to be measured; Backward Rayleigh scattering optical signals are generated in the fiber to be measured, and the signals are divided into two parts; One of the Rayleigh scattering optical signals is frequency-shifted and delayed and then coupled with the other Rayleigh scattering optical signal to form a Rayleigh scattering interference signal; Convert the obtained Rayleigh scattering interference signal into a digital signal; Obtain the discrete form of the digital signal and calculate the phase change information at each point.

10. A distributed fiber optic acoustic sensing device according to claim 9, wherein, the method for calculating the phase change information at each point is: Determination formula for the discrete form of the Rayleigh scattering interference signal: Among them, m is the sampling point of the Rayleigh scattering interference signal along the fiber length direction, and its corresponding sampling rate F s is the original analog-to-digital conversion sampling rate of the used acquisition device, with a value range of [1, M]. The increasing direction is called the "fast time axis", and M is the number of points after discretization in the entire fiber length direction; n is the group number of the received Rayleigh scattering signal. After the device completely receives the signals returned from each point on the fiber, it can send the next group of pulse signals. Therefore, the repetition frequency of the pulse signal is F s / M, and this frequency is defined as the pulse repetition frequency. The value range of n is [1, +∞), and its increasing direction becomes the "slow time axis"; A m and B m represent the DC amplitude and AC amplitude of the interference signals formed at each point on the fiber, represents the phase change information at the position of the i-th point in the fiber distance direction, which is a waveform that varies with n, that is, along the slow time axis; Along the "fast time axis" direction, perform I / Q demodulation on the Rayleigh scattering interference signal to recover the sum of the phase information at each point position at this slow time axis moment. The phase information at the m position at this time can be obtained through differential operation, that is By successively solving the phase signals at the same positions on the optical fibers for each group of Rayleigh scattering interference signals, the phase change information at the position of the m-th point can be obtained.

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