A distributed fiber optic acoustic sensing apparatus and demodulation method

Abstract

The application provides a distributed optical fiber acoustic wave sensing device and a demodulation method, which comprises the following steps: a pulsed laser is shot into a to-be-detected optical fiber; a back Rayleigh scattering light signal is generated in the to-be-detected optical fiber, and the signal is divided into two parts; one part of the Rayleigh scattering light signal is coupled with the other part of the Rayleigh scattering light signal after frequency shift and time delay to form a Rayleigh scattering interference signal; the obtained Rayleigh scattering interference signal is converted into a digital signal; the discrete form of the obtained digital signal is obtained, and the phase change information at each point is calculated. The application can avoid the influence of low-frequency interference of a collection device, break through the limitation of the length of the optical fiber on the carrier frequency, and improve the bandwidth of the demodulated acoustic wave signal.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optical technology, and particularly relates to a distributed optical fiber acoustic wave sensing device and a demodulation method. BACKGROUND

[0002] There are still many deficiencies in the traditional distributed acoustic sensing (DAS) scheme based on the matched interferometer. Specifically, for the method based on 3x3 coupler, it needs to use 3 detectors at the receiving end, and at the same time, at least 3 channels of data acquisition card are needed, so the cost and complexity of the device are increased; at the same time, the interferometer signal returned through the 3x3 coupler does not carry the carrier, so the noise level of the demodulation result is not ideal in the low frequency band; in addition, the signal demodulation of the 3x3 coupler method depends on the 2 / 3pi phase difference between the 3 interference signals obtained, which is unstable in actual application and fluctuates with factors such as temperature, resulting in distortion of the demodulation result and deterioration of the performance of the device. For the PGC method, the most important limitation is the bandwidth of the recovered signal and the sensing distance. In its demodulation method, the interference signal scattered back by each point needs to be recovered on the slow time axis first, and then the interference signal is demodulated. The sampling rate of the interference signal recovered by each point is the pulse repetition frequency of the DAS, which is limited by the length of the optical fiber (the longer the distance, the lower the pulse repetition frequency), and the carrier frequency of the PGC is generally 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, so the bandwidth of the signal that can be recovered by the method is severely limited, and considering the quality of the recovered signal, the sensing distance is generally not long. For example, the DASA based on PGC given in document

[13] has a carrier frequency of only 1 kHz under the conditions of a sensing distance of 10 km and a pulse repetition frequency of 10 kHz, and the frequency of the demodulated signal is 120 Hz. SUMMARY

[0003] The present application aims to overcome the deficiencies in the prior art, and provides a distributed optical fiber acoustic wave sensing device and a demodulation method. The present application scheme can solve the problems existing in the prior art.

[0004] The technical solution of the present application is as follows:

[0005] According to the first aspect, a distributed optical fiber acoustic wave sensing device is provided, comprising a pulsed light source generating unit, an optical fiber, a circulator, a matched interferometer, an optical frequency shift unit, a signal processing unit and a data solving unit. The pulsed light source generates pulsed light, which is transmitted into the optical fiber. Backward Rayleigh scattering light signals are generated at different positions in the optical fiber. The signals are reversely transmitted to the circulator, and then exit from another port of the circulator and enter the matched interferometer. The matched interferometer has two arms. The Rayleigh scattering light signals enter the two arms respectively. The optical frequency shift unit is connected to one arm of the matched interferometer. A delay optical fiber is inserted into the same arm of the matched interferometer. After the Rayleigh scattering light signals pass through the optical frequency shift unit and the delay optical fiber, the frequency generated by the optical frequency shift unit is superimposed on the Rayleigh scattering light signals. The delay of the delay optical fiber delays the Rayleigh scattering light signals emitted from the arm as a whole. The processed Rayleigh scattering light signals and the Rayleigh scattering light signals emitted from the other arm are coupled and output to the signal processing unit. The signal processing unit converts the obtained Rayleigh scattering interference signals into digital signals and transmits them to the data solving unit. The data solving unit calculates the phase change information at several positions of the optical fiber according to a preset calculation formula.

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

[0007] Further, the distributed optical fiber acoustic wave sensing device further comprises a time synchronization unit, which realizes the time synchronization between the signal generator and the signal processing unit.

[0008] Further, the matched interferometer comprises 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 spatial resolution of the optical fiber is ΔL / 2. When the matched interferometer is a Michelson interferometer, the length of the delay optical fiber is ΔL, and the corresponding spatial resolution of the optical fiber is ΔL.

[0010] Further, the other arm of the matched interferometer is also connected to an optical frequency shift unit.

[0011] Furthermore, the signal processing unit includes a photodetector and a data converter. The photodetector receives Rayleigh scattering interference signals and converts them into electrical signals, which are then transmitted to the data converter. The data converter converts the obtained electrical signals into digital signals and transmits them to the data processing unit.

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

[0013]

[0014] Where m represents the number of sampling points of the Rayleigh scattering interference signal along the length of the optical fiber, and its corresponding sampling rate F s The original analog-to-digital conversion sampling rate of the acquisition device is denoted as F, with a value range of [1, M]. The direction of its increase is called the "fast time axis," and M is the number of discretized points along the entire fiber length. n is the group number of the received Rayleigh scattering signal. The device can only send the next group of pulse signals after it has completely received the signals returned from all points on the fiber. Therefore, the repetition frequency of the pulse signal is F. s / M, this frequency is defined as the pulse repetition frequency, and the value of n ranges from [1, +∞), with the direction of its increase being called the "slow time axis"; A m and B m This represents the DC and AC amplitudes of the interference signal formed at various points on the optical fiber. This represents the phase change information at the i-th point along the fiber distance direction, which is a waveform that varies with respect to n, i.e., along the slow time axis. First, the interference signal is demodulated using I / Q modulation along the "fast time axis". For example, by demodulating the k-th Rayleigh scattering interference signal, the sum of the phase information at each point at that slow time axis moment can be recovered. The phase information at position m can be obtained through differential operations, i.e. Finally, by sequentially solving for the phase signals at the same positions on the optical fiber from each group of Rayleigh scattering interference signals, the phase change information at the m-th point can be obtained.

[0015] According to the second aspect, the aforementioned distributed fiber optic acoustic wave sensing demodulation method is provided, comprising the following steps:

[0016] A pulsed laser is directed into the optical fiber under test;

[0017] Backscattered Rayleigh light signal is generated in the optical fiber under test, and the signal is divided into two parts;

[0018] One Rayleigh scattering signal is frequency-shifted and delayed, and then coupled with another Rayleigh scattering signal to form a Rayleigh scattering interference signal;

[0019] The obtained Rayleigh scattering interference signal is converted into a digital signal;

[0020] The discrete form of the obtained digital signal is used to calculate the phase change information at each point.

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

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

[0023] Formula for determining the discrete form of Rayleigh scattering interference signal:

[0024]

[0025] Where m represents the number of sampling points of the Rayleigh scattering interference signal along the length of the optical fiber, and its corresponding sampling rate F s The original analog-to-digital conversion sampling rate of the acquisition device is denoted as F, with a value range of [1, M]. The direction of its increase is called the "fast time axis," and M is the number of discretized points along the entire fiber length. n is the group number of the received Rayleigh scattering signal. The device can only send the next group of pulse signals after it has completely received the signals returned from all points on the fiber. Therefore, the repetition frequency of the pulse signal is F. s / M, this frequency is defined as the pulse repetition frequency, and the value of n ranges from [1, +∞), with the direction of its increase being called the "slow time axis"; A m and B m This represents the DC and AC amplitudes of the interference signal formed at various points on the optical fiber. This represents the phase change information at the i-th point in the fiber distance direction, which is a waveform that varies with n, i.e., along the slow time axis.

[0026] Along the "fast time axis," the Rayleigh scattering interferometer signal is demodulated using I / Q methods to recover the sum of phase information at each point at that slow time axis moment.

[0027] The phase information at position m can be obtained through differential operations, i.e.

[0028] By sequentially solving for the phase signals at the same positions on the optical fiber from each group of Rayleigh scattering interference signals, the phase change information at the m-th point can be obtained.

[0029] The beneficial effects of this invention compared to the prior art are as follows:

[0030] 1) Improve the noise performance of the demodulation results. By introducing a heterodyne frequency, the Rayleigh scattering interference signal is shifted to a higher frequency band, which can avoid the influence of low-frequency interference from the acquisition device.

[0031] 2) Overcoming the limitation of fiber length on carrier frequency and increasing the bandwidth of demodulated acoustic signals. By introducing a high-frequency heterodyne carrier into the Rayleigh scattering interference signal and combining it with a fast time-axis-first demodulation algorithm, a high sampling rate interference signal can be obtained, avoiding the limitation of pulse repetition frequency on the carrier frequency of the interference signal;

[0032] 3) In this invention, only two couplers are needed when using a Mach-Zehnder type interferometer, and only one coupler is needed when using a Michelson type interferometer. This reduces the number of couplers used and the number of corresponding detectors and data acquisition cards, thereby reducing the cost and complexity of the device. Attached Figure Description

[0033] The accompanying drawings, which form part of this specification, are provided to further illustrate embodiments of the invention and, together with the textual description, explain the principles of the invention. It is obvious that the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0034] Figure 1 A schematic diagram of a distributed fiber optic acoustic wave sensing device according to an embodiment of the present invention is shown.

[0035] Figure 2 The following are schematic diagrams of two unbalanced matched interferometer structures provided according to embodiments of the present invention;

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

[0037] Figure 4 A phase demodulation flowchart according to an embodiment of the present invention is shown;

[0038] Figure 5 A schematic diagram of the steps of a distributed optical fiber acoustic wave sensing demodulation method according to an embodiment of the present invention is shown. Detailed Implementation

[0039] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, 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 arrangement, numerical expressions, and values ​​of the components and steps set forth in these embodiments do not limit the scope of the invention. It should also be understood that, for ease of description, the dimensions of the various parts shown in the drawings are not drawn to actual scale. Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of the specification. In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters in the following figures denote similar items; therefore, once an item is defined in one figure, it need not be further discussed in subsequent figures.

[0042] like Figure 1As shown, according to a first aspect of the present invention, a distributed fiber optic acoustic wave sensing device is provided, comprising a pulse light source generating unit, an optical fiber, a circulator, a matched interferometer, an optical frequency shifting unit, a signal processing unit, and a data processing unit. The pulse light source generates pulsed light, which is transmitted into the optical fiber. Backscattered Rayleigh light signals are generated at different positions in the optical fiber. These signals are transmitted in the reverse direction to the circulator position, exit through the other port of the circulator, and enter the matched interferometer. The matched interferometer has two arms, and the Rayleigh scattered light signals are respectively injected into the two arms. The optical frequency shifting unit is connected to one arm of the matched interferometer. A delay fiber is also inserted into the same arm of the matching interferometer. After the Rayleigh scattering light signal passes through the optical frequency shifter and the delay fiber, the frequency signal generated by the optical frequency shifter unit is superimposed on the Rayleigh scattering light signal. The delay of the delay fiber delays the entire Rayleigh scattering light signal emitted from this arm. The processed Rayleigh scattering light signal is coupled with the Rayleigh scattering light signal emitted from the other arm 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 points on the fiber according to the preset calculation formula.

[0043] By applying this setup, an optical frequency shifting unit and a delay fiber are introduced into one arm of the matched interferometer. By introducing a heterodyne frequency, the Rayleigh scattering interference signal is shifted to a higher frequency band, thus avoiding the influence of low-frequency interference from the acquisition system.

[0044] In a further embodiment, the optical frequency shifting unit includes a signal generator, a driver, and an optical frequency shifter. The signal generator generates a desired frequency signal and sends it to the driver. The driver drives the optical frequency shifter to move, thereby obtaining an optical signal of the corresponding frequency. Preferably, in one embodiment, the frequency of the acquired optical signal is greater than 10MHz and is controlled by the signal generator. In specific engineering applications, this frequency can be specified by the user.

[0045] In a further embodiment, a distributed fiber optic acoustic wave sensing device also includes a time synchronization unit, which synchronizes the signal generator and the signal processing unit. This configuration ensures that the acquired data are on the same time axis, guaranteeing the quality of the demodulation results.

[0046] In a further embodiment, the matching interferometer includes a Mach-Zehnder type matching interferometer and a Michelson type matching interferometer.

[0047] In a further embodiment, when the matching interferometer is a Mach-Zehnder type interferometer, the length of the delay 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 fiber is ΔL, and the corresponding fiber spatial resolution is ΔL.

[0048] In a further embodiment, another optical frequency shifting unit is also connected to the other arm of the matched interferometer. By connecting an optical frequency shifting unit to another wall, the range of optical shifting frequencies can be increased, and the desired range of optical shifting frequencies can be obtained by combining two optical frequency shifting units, thereby increasing the flexibility of optical frequency shifting unit selection.

[0049] In a further embodiment, the signal processing unit includes a photodetector and a data converter. The photodetector receives the Rayleigh scattering interference signal and converts it into an electrical signal, which is then transmitted to the data converter. The data converter converts the obtained electrical signal into a digital signal and transmits it to the data processing unit.

[0050] In another embodiment, the preset calculation formula is:

[0051]

[0052] Where m represents the number of sampling points of the Rayleigh scattering interference signal along the length of the optical fiber, and its corresponding sampling rate F s The original analog-to-digital converter sampling rate of the acquisition device is [1, M]. The direction of increase is called the "fast time axis," where M is the number of discretized points along the entire fiber length; n is the group number of the received Rayleigh scattering signal. The device can only send the next pulse signal after it has completely received the signals returned from all points on the fiber. Therefore, the repetition frequency of the pulse signal is F. s / M, this frequency is defined as the pulse repetition frequency, and the value of n ranges from [1, +∞), with the direction of its increase being called the "slow time axis"; A m and B m This represents the DC and AC amplitudes of the interference signal formed at various points on the optical fiber. This represents the phase change information at the i-th point along the fiber distance direction, which is a waveform that varies with respect to n, i.e., along the slow time axis. First, the interference signal is demodulated using I / Q modulation along the "fast time axis". For example, by demodulating the k-th Rayleigh scattering interference signal, the sum of the phase information at each point at that slow time axis moment can be recovered. The phase information at position m can be obtained through differential operations, i.e. Finally, by sequentially solving for the phase signals at the same positions on the optical fiber from each group of Rayleigh scattering interference signals, the phase change information at the m-th point can be obtained.

[0053] This setting prioritizes demodulating phase information on the fast time axis and then recovers the waveforms at each point via the slow time axis, thereby avoiding the limitation of fiber length on signal demodulation bandwidth and enabling distributed sensing and demodulation of long-distance, wide-band acoustic signals.

[0054] According to the second aspect of the embodiment, such as Figure 5 As shown, the above-mentioned distributed fiber optic acoustic wave sensing demodulation method includes the following steps:

[0055] A pulsed laser is directed into the optical fiber under test;

[0056] Backscattered Rayleigh light signal is generated in the optical fiber under test, and the signal is divided into two parts;

[0057] One Rayleigh scattering signal is frequency-shifted and delayed, and then coupled with another Rayleigh scattering signal to form a Rayleigh scattering interference signal;

[0058] The obtained Rayleigh scattering interference signal is converted into a digital signal;

[0059] The discrete form of the obtained digital signal is used to calculate the phase change information at each point.

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

[0061] In a further embodiment, the method for calculating the phase change information at each point is as follows:

[0062] Formula for determining the discrete form of Rayleigh scattering interference signal:

[0063]

[0064] Where m represents the number of sampling points of the Rayleigh scattering interference signal along the length of the optical fiber, and its corresponding sampling rate F s The original analog-to-digital conversion sampling rate of the acquisition device is denoted as F, with a value range of [1, M]. The direction of its increase is called the "fast time axis," and M is the number of discretized points along the entire fiber length. n is the group number of the received Rayleigh scattering signal. The device can only send the next group of pulse signals after it has completely received the signals returned from all points on the fiber. Therefore, the repetition frequency of the pulse signal is F. s / M, this frequency is defined as the pulse repetition frequency, and the value of n ranges from [1, +∞), with the direction of its increase being called the "slow time axis"; A m and B m This represents the DC and AC amplitudes of the interference signal formed at various points on the optical fiber. This represents the phase change information at the i-th point in the fiber distance direction, which is a waveform that varies with n, i.e., along the slow time axis.

[0065] Along the "fast time axis," the Rayleigh scattering interferometer signal is demodulated using I / Q methods to recover the sum of phase information at each point at that slow time axis moment.

[0066] The phase information at position m can be obtained through differential operations, i.e.

[0067] By sequentially solving for the phase signals at the same positions on the optical fiber from each group of Rayleigh scattering interference signals, the phase change information at the m-th point can be obtained.

[0068] To gain a better understanding of the distributed optical fiber acoustic wave sensing device and demodulation method provided by the present invention, a detailed description is provided below with reference to specific examples and accompanying drawings.

[0069] A distributed acoustic wave sensing device and demodulation method based on heterodyne demodulation using an unbalanced matched interferometer, the device structure of which is as follows: Figure 1 As shown in the diagram. The laser is required to be a narrow-linewidth light source, providing a continuous light wave with low phase noise for the entire device; its optical frequency is denoted as f0. The emitted continuous light wave is modulated into pulsed light by a chopper, amplified to a suitable power by an erbium-doped fiber amplifier, and then filtered out by an optical bandpass filter to remove most of the ASE noise before being injected into the fiber under test through a circulator. The chopper can be a semiconductor optical amplifier or an acousto-optic modulator. To prevent the light wave from back propagating into the laser and causing damage, an isolator can be added between the chopper and the erbium-doped fiber amplifier.

[0070] When a pulsed light is injected into the fiber under test, backscattered Rayleigh light signals will be generated at different locations within the fiber. These signals are transmitted in the reverse direction to the circulator, exit through port 3 of the circulator, and enter the unbalanced matched interferometer. The specific structure of this unbalanced matched interferometer is as follows: Figure 2 As shown, where Figure 2(a) illustrates the structure of a Mach-Zehnder interferometer. The backscattered Rayleigh signal first passes through a 50:50 coupler 1 into the upper and lower arms of the Mach-Zehnder interferometer. In the upper arm, an optical frequency shifter 1 is inserted, causing a frequency shift of f1 in the light wave, meaning the frequency becomes f0+f1 after passing through the optical frequency shifter. Similarly, an optical frequency shifter 2 in the lower arm shifts the frequency of the passing light signal to f0+f2. Simultaneously, a delay fiber with a length of ΔL is inserted into the upper arm. This delay fiber matches the spatial resolution of the DAS, which 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 are no requirements on the specific form of the frequency shifter; (2) The frequency shift values ​​generated by the frequency shifter, namely f1 and f2, are controlled by any signal generator and driver in the device, and can be adjusted according to actual needs. The arbitrary signal generator and driver used here are all 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 have to be added. When there is optical frequency shifter 2, f1 will be introduced into the lower arm. c =|f1-f2| heterodyne carrier frequency, when optical frequency shifter 2 is absent, carrier frequency f c =f1. Finally, the optical signals from the upper and lower arms will be coupled together through 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 a matching interferometer structure built using the Michelson structure, the difference being: (1) Figure 2 (b) uses only one coupler (coupler 1), and is composed of Figure 2 (a) The 1×2 form becomes the 2×2 form; (2) Figure 2 (b) The Michelson interferometer shown uses two Faraday rotators (Faraday rotator 1 and Faraday rotator 2) to return the optical signal. Note that because the optical signal in the upper and lower arms passes through the optical frequency shifter twice, its optical frequency value is doubled. That is, when optical frequency shifter 2 is present in the lower arm, the obtained heterodyne carrier frequency is f. c = 2|f1-f2|; when the lower arm does not have optical frequency shifter 2, the heterodyne carrier frequency is f c =2f1; (3) At this time, there is still a delay fiber with a length of ΔL in the upper arm, but since the optical signal passes through the fiber twice, the corresponding spatial resolution is ΔL.

[0072] The Rayleigh scattering interference signal output from the matched 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 represents the number of sampling points of the Rayleigh scattering interference signal along the length of the optical fiber, and the corresponding sampling rate F s The original analog-to-digital conversion sampling rate of the acquisition device is denoted as F, with a value range of [1, M]. The direction of its increase is called the "fast time axis," and M is the number of discretized points along the entire fiber length. n is the group number of the received Rayleigh scattering signal. The device can only send the next pulse signal after it has completely received the signals returned from all points on the fiber. Therefore, the repetition frequency of the pulse signal is F. s / M, this frequency is defined as the pulse repetition frequency, and the value of n ranges from [1, +∞), and its increasing direction is called the "slow time axis". In formula (1), A m and B m This represents the DC and AC amplitudes of the interference signal formed at various points on the optical fiber. This represents the phase change information at the i-th point along the fiber distance direction, which is a waveform that varies with n, i.e., along the slow time axis.

[0075] As can be seen from formula (1), the received Rayleigh scattering interference signal can be regarded as a two-dimensional matrix, such as... Figure 3 As shown in the diagram. Here, m represents different positions on the optical fiber, with its increasing direction corresponding to the "fast time axis"; n represents the Rayleigh scattering interference signal received after emitting different subpulses, with its increasing direction corresponding to the "slow time axis". In this scheme, the signal demodulation steps are as follows: Figure 4 As shown. Unlike traditional signal demodulation methods, this scheme first requires I / Q demodulation of the interference signal along the "fast time axis". For example, demodulating the k-th Rayleigh scattering interference signal allows recovery of the sum of phase information at each point on the slow time axis. The phase information at position m can be obtained through differential operations, i.e. Finally, by sequentially solving for the phase signals at the same positions on the optical fiber from each group of Rayleigh scattering interference signals, the phase change information at the m-th point can be obtained. Phase change information at each point obtained by this algorithm The data can be uploaded to a computer for further analysis, and the computer can also control the data acquisition device. In this device, to ensure the quality of the demodulation results, clock synchronization needs to be achieved between the data acquisition device and any signal generator.

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

[0077] 1) Improve the noise performance of the demodulation results. By introducing a heterodyne frequency, the Rayleigh scattering interference signal is shifted to a higher frequency band, which can avoid the influence of low-frequency interference from the acquisition system.

[0078] 2) Overcoming the limitation of fiber length on carrier frequency and increasing the bandwidth of demodulated acoustic signals. By introducing a high-frequency heterodyne carrier into the Rayleigh scattering interference signal and combining it with a fast time axis-first demodulation algorithm, a high sampling rate interference signal can be obtained, avoiding the limitation of pulse repetition frequency on the carrier frequency of the interference signal, and realizing long-distance, wideband distributed sensing and demodulation of acoustic signals;

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

[0080] 4) Flexible and diverse implementation methods, easy to promote and apply. There are many different choices for the unbalanced interferometer type, frequency shifting device, and the position of the frequency shifting device used in the backend, to meet the application requirements of different occasions.

[0081] For ease of description, spatial relative terms such as "above," "on top of," "on the upper surface of," "above," etc., are used herein to describe the spatial positional relationship of a device or feature as shown in the figures to other devices or features. It should be understood that spatial relative terms are intended to encompass different orientations in use or operation beyond the orientation of the device as described in the figures. For example, if the device in the figures were inverted, a device described as "above" or "on top of" other devices or structures would subsequently be positioned as "below" or "under" other devices or structures. Thus, the exemplary term "above" can include both "above" and "below." The device may also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatial relative descriptions used herein will be interpreted accordingly.

[0082] Furthermore, it should be noted that the use of terms such as "first" and "second" to define components is merely for the purpose of distinguishing the corresponding components. Unless otherwise stated, the above terms have no special meaning and therefore should not be construed as limiting the scope of protection of this invention.

[0083] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. 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 distributed fiber optic acoustic sensing apparatus, comprising: The device comprises a pulse light source generating unit, an optical fiber, a circulator, a matching interferometer, an optical frequency shift unit, a signal processing unit and a data solving unit. The pulse light source generates pulse light, which is transmitted into the optical fiber. Backward Rayleigh scattering light signals are generated at different positions in the optical fiber. The signals are reversely transmitted to the position of the circulator, and then are emitted from the other port of the circulator and enter the matching interferometer. The matching interferometer has two arms. The Rayleigh scattering light signals enter the two arms respectively. The optical frequency shift unit is connected with one arm of the matching interferometer. A delay optical fiber is inserted into the same arm of the matching interferometer. The Rayleigh scattering light signals pass through the optical frequency shift unit and the delay optical fiber. The frequency generated by the optical frequency shift unit is superimposed on the Rayleigh scattering light signals. The delay of the delay optical fiber delays the Rayleigh scattering light signals emitted from the arm. The processed Rayleigh scattering light signals and the Rayleigh scattering light signals emitted from the other arm are coupled and output to the signal processing unit. The signal processing unit converts the obtained Rayleigh scattering interference signals into digital signals, which are transmitted to the data solving unit. The data solving unit calculates the phase change information of the positions of the optical fiber according to a preset calculation formula. The preset calculation formula is as follows: where m is the sampling point of Rayleigh scattering interference signal along the fiber length direction, which corresponds to the sampling rate F s is the original analog-to-digital conversion sampling rate of the used acquisition device, the value range is [1, M], the increasing direction is called "fast time axis", M is the number of points after discretization along the fiber length direction; n is the group number of received Rayleigh scattering signals, when the device completely receives the signals returned by each point on the fiber, the next group of pulse signals can be sent, therefore the repetition frequency of pulse signals is F s / M, this frequency is defined as the pulse repetition frequency, the value range of n is [1, +∞), the increasing direction is called "slow time axis"; A m and B m represent the direct current amplitude and alternating current amplitude of the interference signal formed by 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 about n, i.e. changes along the slow time axis; first, along the "fast time axis" direction, the interference signal is I / Q demodulated, and the phase information of each point position at this slow time axis moment can be recovered by demodulating the k-th Rayleigh scattering interference signal The phase information at the m position at this moment can be obtained by difference operation, that is Finally, the phase signals of the same position on the fiber solved by sequentially solving each group of Rayleigh scattering interference signals are solved, and the phase change information at the m-th point position is obtained f c is the heterodyne carrier frequency.

2. A distributed optical fibre acoustic sensor apparatus as claimed in claim 1, wherein, The optical frequency shift unit comprises a signal generator, a driver and an optical frequency shift device. The signal generator generates a required frequency signal and transmits the signal to the driver. The driver drives the optical frequency shift device to obtain optical signals with corresponding frequency shift.

3. A distributed optical fibre acoustic sensor apparatus as claimed in claim 2, wherein, The distributed optical fiber acoustic wave sensing device further comprises a time synchronization unit. The time synchronization unit realizes the time synchronization between the signal generator and the signal processing unit.

4. The distributed fiber optic acoustic sensor device of claim 1, wherein, The matching interferometer comprises a Mach-Zehnder type interferometer and a Michelson type interferometer.

5. A distributed optical fibre acoustic sensor apparatus as claimed in claim 4, wherein, When the interferometer is a Mach-Zehnder type interferometer, the length of the delay optical fiber is ΔL, and the corresponding spatial resolution of the optical fiber is ΔL / 2. When the matching interferometer is a Michelson type interferometer, the length of the delay optical fiber is ΔL, and the corresponding spatial resolution of the optical fiber is ΔL.

6. A distributed optical fibre acoustic sensor apparatus as claimed in claim 5, wherein, The other arm of the matching interferometer is also connected with an optical frequency shift unit.

7. The distributed fiber optic acoustic sensor device of claim 1, wherein, The signal processing unit comprises a photodetector and a data converter. The photodetector receives the Rayleigh scattering interference signals and converts the signals into electric signals, which are transmitted to the data converter. The data converter converts the obtained electric signals into digital signals, which are transmitted to the data solving unit.

8. A distributed optical fiber acoustic sensing demodulation method using the distributed optical fiber acoustic sensing apparatus according to any one of claims 1 to 7, characterized in that, The method comprises the following steps: Pulse laser is emitted into the optical fiber to be measured. Backward Rayleigh scattering light signals are generated in the optical fiber to be measured. The signals are divided into two parts. One part of the Rayleigh scattering light signals is coupled with the other part of the Rayleigh scattering light signals after frequency shift and delay to form Rayleigh scattering interference signals. The obtained Rayleigh scattering interference signals are converted into digital signals. The discrete form of the obtained digital signals is used to calculate the phase change information of each point.

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