Distributed optical fiber sound wave sensing system and device based on frequency shift ring

Abstract

The invention provides a distributed optical fiber sound wave sensing system and device based on a frequency shift ring. The distributed optical fiber sound wave sensing system comprises a narrow linewidth laser source module, a sweep frequency optical pulse generation module, a radio frequency signal generation module, a circulator, a coherent receiving and photoelectric conversion module and a signal acquisition and processing module. The sweep frequency optical pulse generation module comprises an acousto-optic modulator, a frequency shift ring, an optical fiber coupler and an optical amplifier, the acousto-optic modulator generates narrow-bandwidth sweep frequency optical signals, the frequency shift ring is composed of an acousto-optic frequency shifter, a time delay optical fiber and the optical amplifier, the acousto-optic frequency shifter achieves frequency shift, the time delay optical fiber achieves time delay of the optical signals, and the optical amplifier compensates loss of the optical signals. And the spliced optical pulse enters a sensing optical fiber, returned Rayleigh backscattered light and local light are subjected to beat frequency, a matched filter generated by deconvolution is used for processing a beat product signal, and a vibration signal is demodulated. According to the invention, the frequency-shift ring is spliced to obtain a wide-bandwidth sweep-frequency optical signal, and high-spatial-resolution vibration detection is realized.

Description

Technical Field

[0001] The present invention relates to the field of optical sensing technology, and in particular to a distributed optical fiber acoustic wave sensing system and device based on a frequency shift ring. Background Art

[0002] Distributed Fiber-optic Acoustic Sensing (DAS) is widely used in industrial monitoring, seismic exploration and safety detection. The DAS system can continuously sense external physical changes such as vibration and pressure along the entire optical fiber, and has the advantages of not requiring a large number of sensors to be arranged, being inherently safe, and being resistant to environmental interference. There are two main technical implementation methods for existing DAS systems: Phase-sensitive Optical Time-domain Reflectometry (φ-OTDR) and Time-gated Digital Optical Frequency Domain Reflectometry (TGD-OFDR). In the φ-OTDR system, the sensing distance and spatial resolution are inversely related to the pulse width, respectively, so the sensing distance and spatial resolution are mutually restricted. TGD-OFDR decouples the relationship between spatial resolution and detection pulse width, thereby solving the contradictory relationship between spatial resolution and sensing length and signal-to-noise ratio in φ-OTDR, and can simultaneously obtain high spatial resolution, long sensing distance and high signal-to-noise ratio.

[0003] In DAS based on TGD-OFDR, the spatial resolution is determined by the frequency sweep range of the detection signal. Using an acousto-optic modulator (AOM) with excellent modulation performance cannot achieve a wide range of frequency sweeps. Using an electro-optic modulator (EOM) that can achieve a wide range of frequency sweeps will reduce the modulation performance of the signal, thereby reducing the signal-to-noise ratio and effective monitoring distance. In order to improve the performance of the system, a technology that can expand the frequency sweep range, improve the spatial resolution and signal-to-noise ratio is urgently needed. Summary of the invention

[0004] In view of the defects in the prior art, an object of the present invention is to provide a distributed optical fiber acoustic wave sensing system and device based on a frequency shift ring.

[0005] The distributed optical fiber acoustic wave sensing system based on the frequency shift ring provided by the present invention comprises: a narrow line width laser source module, a frequency sweep optical pulse generation module, a radio frequency signal generation module, a circulator, a coherent receiving and photoelectric conversion module and a signal acquisition and processing module;

[0006] Wherein: the narrow linewidth laser source module is respectively connected to the swept frequency optical pulse generation module and the coherent receiving and photoelectric conversion module to transmit the narrow linewidth laser source; the radio frequency signal generation module is respectively connected to the swept frequency optical pulse generation module and the signal acquisition and processing module to transmit the swept frequency radio frequency pulse signal and the frequency shifted radio frequency pulse signal to the swept frequency optical pulse generation module, and transmit the trigger signal and the clock synchronization signal to the signal acquisition and processing module; the swept frequency optical pulse module is connected to the sensing optical fiber through a circulator to output the swept frequency optical pulse train, the sensing optical fiber transmits the Rayleigh backscattered light through the circulator to the coherent receiving and photoelectric conversion module, the coherent receiving and photoelectric conversion module converts the beat frequency signal into an electrical signal, and is connected to the signal acquisition and processing module to transmit the electrical signal;

[0007] The frequency range of the swept pulse is expanded by a frequency shifting ring to achieve high spatial resolution vibration detection of the sensing optical fiber. The Rayleigh backscattered light returned from the sensing optical fiber beats with the reference light, and a high spatial resolution beat signal is obtained by using a matched filter generated by deconvolution. Vibration information on the optical fiber is then obtained through signal processing.

[0008] Preferably, the narrow linewidth laser source module comprises a connected narrow linewidth fiber laser and a first fiber coupler, wherein: the narrow linewidth fiber laser is connected to the swept frequency optical pulse generating module and the coherent receiving and photoelectric conversion module through the first fiber coupler, and transmits detection light and reference light to the swept frequency optical pulse generating module and the coherent receiving module respectively.

[0009] Preferably, the RF signal generating module comprises a connected RF signal generator and a RF signal amplifier, wherein: the RF signal generator is connected to the frequency sweeping optical pulse generating module via the RF signal amplifier, and inputs and amplifies the frequency sweeping RF pulse signal and the frequency shifting RF signal pulse respectively.

[0010] Preferably, the swept frequency optical pulse generating module comprises an acousto-optic modulator, a frequency shift ring and an erbium-doped fiber amplifier, wherein: the acousto-optic modulator is respectively connected to the narrow linewidth laser light source module and the frequency shift ring, the frequency shift ring is connected to the erbium-doped fiber amplifier, and the amplified signal is connected to the sensing fiber through a circulator; the frequency shift ring comprises a connected fiber coupler, a delay fiber, an optical amplifier and an acousto-optic frequency shifter, the delay fiber realizes the time delay of the optical signal, the optical amplifier compensates for the loss of the optical signal, and the acousto-optic frequency shifter realizes the frequency shift.

[0011] Preferably, the swept frequency optical pulse generating module generates a swept frequency signal with a bandwidth of B0 through an acousto-optic modulator. The swept frequency signal is frequency-shifted through a frequency shifting ring and time-shifted through a delay optical fiber, thereby splicing each signal segment to form a continuous large-range swept frequency signal.

[0012] Preferably, the optical amplifier is a semiconductor optical amplifier.

[0013] Preferably, after the small-range frequency-sweeping optical pulse passes through the second optical fiber coupler and enters the frequency-shifting loop, it first enters the first acousto-optic frequency shifter, which generates a single-frequency signal to achieve frequency shift of the frequency-sweeping optical signal. The frequency-shifted signal passes through the third optical fiber coupler, enters the sensing optical cable in one way, and enters the first time-delay optical fiber and the first optical amplifier in the other way. The signal after time-delay amplification passes through the second optical fiber coupler again and enters the first acousto-optic frequency shifter for frequency shifting, and the above process is repeated.

[0014] Preferably, the small-range frequency-sweeping optical pulse passes through the fourth optical fiber coupler, enters the sensing optical cable in one way and enters the frequency shifting loop in the other way, passes through the second delay optical fiber, the second acousto-optic frequency shifter, and the third optical amplifier in turn, and then enters the fourth optical fiber coupler, and the above process is repeated.

[0015] Preferably, the swept frequency signal generated by the acousto-optic modulator is frequency-shifted and spliced ​​by a frequency shift loop to obtain a continuous swept frequency signal with a large bandwidth, thereby realizing vibration detection with high spatial resolution and high signal-to-noise ratio, which specifically includes the following steps:

[0016] Step 1: The AOM modulates to generate a swept frequency signal: E1(t) = exp j[2π(f c +f0)t+πkt 2 ]g(t); where f c is the center frequency of the laser, f0 is the starting frequency of the sweep signal, k is the sweep rate of the AOM 3; j represents the sign of the imaginary part of the complex number, t represents time;

[0017] Step 2: After passing through the frequency shift loop, the signal circulates M times in the frequency shift loop, and the spliced ​​signal is: Where: the whole of the spliced ​​signal, or any of them, is regarded as a complete detection signal; where M represents the number of cycles; T represents the cycle period of the signal in the frequency shift loop; B0 represents the sweep bandwidth of the acousto-optic modulator;

[0018] Step 3: The beat frequency signal of the Rayleigh signal returned from the position where the round-trip time delay is τ on the sensing fiber and the local light is: The scattered light signal containing multiple cycles is compressed by the corresponding matched filter h(t) to obtain the distribution information of the Rayleigh scattering signal on the sensing optical fiber with high spatial resolution, and then the vibration information on the optical fiber is obtained through the intensity and phase information of the Rayleigh scattering signal.

[0019] The distributed optical fiber acoustic wave sensing device based on a frequency shift ring provided by the present invention comprises the distributed optical fiber acoustic wave sensing system based on a frequency shift ring.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) The use of an acousto-optic modulator has a higher extinction ratio and can achieve high signal-to-noise ratio acoustic wave detection;

[0022] (2) A frequency shifting ring is used to shift and splice multiple frequency-sweeping sub-pulses with different center frequencies to generate a large-range frequency-sweeping optical pulse, thus achieving high spatial resolution acoustic wave detection.

[0023] (3) Compared with the conventional DAS based on electro-optic modulator to realize wide-range frequency scanning, the present invention can obtain a higher signal-to-noise ratio while achieving high spatial resolution. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0025] Figure 1 It is a schematic diagram of the structure of a DAS system based on a frequency shift loop of the present invention;

[0026] Figure 2 (a) is a time domain diagram of the spliced ​​wide-range frequency-sweep optical signal sequence. There are 5 wide-range frequency-sweep optical pulses in the figure. Figure 2 (c) is the spectrum of the spliced ​​wide-range swept frequency optical pulse. Figure 2 (b) in FIG. 1 shows that each wide-range frequency-sweep optical pulse contains m=10 frequency-sweep sub-pulses. Figure 2 (d) in Figure 2 (b) shows the frequency information of the corresponding frequency sweep sub-pulse.

[0027] Wherein: the modulation pulse time width of the first acousto-optic modulator is t1, the frequency sweep bandwidth is B0, the propagation speed of light in the optical fiber is c, and the length of the delay optical fiber is L=ct1;

[0028] In the figure: a narrow linewidth laser light source module, comprising: a narrow linewidth laser 1, a first optical fiber coupler 2;

[0029] The frequency-sweeping optical pulse generation module comprises: an acousto-optic modulator 3, a first frequency-shifting ring 31, a second optical amplifier 9; a circulator 10; a coherent receiving and photoelectric conversion module 11; and a signal acquisition and processing module 12;

[0030] The first frequency shifting loop 31 comprises: a second optical fiber coupler 4, a first acousto-optic frequency shifter 5, a third optical fiber coupler 6, a first delay optical fiber 7, and a first optical amplifier 8;

[0031] The radio frequency signal generating module includes: a radio frequency signal generator 13, a first radio frequency signal amplifier 14, a second radio frequency signal amplifier 15; a sensing optical cable 20; and a second frequency shifting ring 32;

[0032] The second frequency shifting loop 32 includes: a second acousto-optic frequency shifter 16 , a third optical amplifier 17 , a fourth optical fiber coupler 18 , and a second delay optical fiber 19 . DETAILED DESCRIPTION

[0033] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0034] Example

[0035] like Figure 1 As shown, the present invention relates to a distributed fiber acoustic wave sensing system based on a frequency shift ring, wherein a narrow linewidth laser 1 generates continuous light, which passes through a first fiber coupler 2 and is used as a reference light and a detection light. The detection light passes through an acousto-optic modulator 3 to generate a small range of swept frequency light pulses, which enter the first frequency shift ring 31. The bandwidth of the swept frequency light pulse modulated by the acousto-optic modulator 3 is B0, and the pulse width is t1.

[0036] There are two ways to implement the frequency shift loop. In the first way, after a small-range frequency-sweeping optical pulse enters the first frequency-shifting loop 31 through the second fiber coupler 4, it first enters the first acousto-optic frequency shifter 5, which generates a single-frequency signal to realize the frequency shift of the frequency-sweeping optical signal. The frequency-shifting frequency is the same as the bandwidth of the frequency-sweeping signal. The frequency-shifted signal passes through the third fiber coupler 6, one way through the second optical amplifier 9 and the circulator 10, and then enters the first delay optical fiber 7. The length of the delay optical fiber is basically the same as the length of the optical pulse, ensuring that the spliced ​​signal does not overlap in the time domain. The first optical amplifier 8 compensates for the insertion loss of the acousto-optic modulator 3 and the transmission loss of the delay optical fiber, ensuring the power stability of the detection optical signal. The signal after delay amplification passes through the second fiber coupler 4 again and enters the first acousto-optic frequency shifter 5 for frequency shifting, and the above process is repeated.

[0037] In the second frequency shift ring structure, a small-range frequency sweeping optical pulse passes through the fourth optical fiber coupler 18, one way passes through the second optical amplifier 9 and the circulator 10 and then enters the sensing optical cable 20, and the other way enters the second frequency shift ring 32. After passing through the second delay optical fiber 19, the second acousto-optic frequency shifter 16 generates a frequency shift signal, and the third optical amplifier 17 compensates for the optical loss in the ring. In this method, the first frequency sweeping sub-pulse does not pass through the second acousto-optic frequency shifter 16.

[0038] The driving RF signals of the first acousto-optic frequency shifter 5 and the second acousto-optic frequency shifter 16 both contain a cutoff signal to control the number of cycles of the optical signal. The RF signals of all acousto-optic modulators 3 are generated by a RF signal generator 13, which is also connected to a signal acquisition and processing module 12 to transmit a trigger signal and a clock synchronization signal. The parameters are adjusted to ensure that the spliced ​​signal has continuity in both the time domain and the frequency domain. The signal generated by the RF signal generator 13 passes through a first RF signal amplifier 14 and a second RF signal amplifier 15 to drive the acousto-optic modulator 3 and the first acousto-optic frequency shifter 5, respectively.

[0039] The first acousto-optic frequency shifter 5 and the second acousto-optic frequency shifter 16 are identical acousto-optic modulators. The RF drive signal is a single-frequency signal with a signal frequency of B0, and the center frequency of each small-range sweep pulse is changed by B0. The number of cycles of the pulse signal in the frequency shift loop is m, and the working duration of the first acousto-optic frequency shifter 5 and the second acousto-optic frequency shifter 16 is t2=mt1, and the duration is greater than t1 when it is cut off. The sweep optical pulse is as follows Figure 2 .

[0040] The first delay optical fiber 7 and the second delay optical fiber 19 have the same length, which is L=ct1, where c is the propagation speed of light in the optical fiber.

[0041] The first optical amplifier 8 and the third optical amplifier 17 are preferably semiconductor optical amplifiers to avoid possible gaps in the splicing of sub-pulses, which may cause transient effects of erbium-doped fiber amplifiers.

[0042] The first optical fiber coupler 2 preferably has a splitting ratio of 95:5.

[0043] The second optical fiber coupler 4 , the third optical fiber coupler 6 , and the fourth optical fiber coupler 18 preferably have a splitting ratio of 50:50.

[0044] The Rayleigh backscattered signal and the local light beat frequency returned by the sensing optical cable 20 are received by the coherent receiving and photoelectric conversion module 11 , converted into electrical signals and processed by the signal acquisition and processing module 12 .

[0045] The vibration sensing method of the system comprises the following steps:

[0046] Step 1: The AOM modulates to generate a swept frequency signal: E1(t) = exp j[2π(f c +f0)t+πkt 2 ]g(t); where f c is the center frequency of the laser, f0 is the starting frequency of the sweep signal, k is the sweep rate of the AOM 3; j represents the sign of the imaginary part of the complex number, t represents time;

[0047] Step 2: After passing through the frequency shift loop, the signal circulates M times in the frequency shift loop, and the spliced ​​signal is: Where: the whole of the spliced ​​signal, or any of them, is regarded as a complete detection signal; where M represents the number of cycles; T represents the cycle period of the signal in the frequency shift loop; B0 represents the sweep bandwidth of the acousto-optic modulator;

[0048] Step 3: The beat frequency signal of the Rayleigh signal returned from the position where the round-trip time delay is τ on the sensing fiber and the local light is: The scattered light signal containing multiple cycles is compressed by the corresponding matched filter h(t) to obtain the distribution information of the Rayleigh scattering signal on the sensing optical fiber with high spatial resolution, and then the vibration information on the optical fiber is obtained through the intensity and phase information of the Rayleigh scattering signal.

[0049] Compared with the prior art, the present invention realizes high spatial resolution sound wave detection under the premise of using an acousto-optic modulator to ensure a high signal-to-noise ratio.

[0050] In the description of the present application, it should be understood that the terms "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0051] Those skilled in the art know that, in addition to implementing the system, device and its various modules provided by the present invention in a purely computer-readable program code, it is entirely possible to implement the same program in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers and embedded microcontrollers by logically programming the method steps. Therefore, the system, device and its various modules provided by the present invention can be considered as a hardware component, and the modules included therein for implementing various programs can also be considered as structures within the hardware component; the modules for implementing various functions can also be considered as both software programs for implementing the method and structures within the hardware component.

[0052] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A distributed optical fiber acoustic wave sensing system based on a frequency shift ring, characterized in that: include: Narrow linewidth laser source module, swept frequency optical pulse generation module, radio frequency signal generation module, circulator, coherent receiving and photoelectric conversion module and signal acquisition and processing module; Wherein: the narrow linewidth laser source module is respectively connected to the swept frequency optical pulse generation module and the coherent receiving and photoelectric conversion module to transmit the narrow linewidth laser source; the radio frequency signal generation module is respectively connected to the swept frequency optical pulse generation module and the signal acquisition and processing module to transmit the swept frequency radio frequency pulse signal and the frequency shifted radio frequency pulse signal to the swept frequency optical pulse generation module, and transmit the trigger signal and the clock synchronization signal to the signal acquisition and processing module; the swept frequency optical pulse module is connected to the sensing optical fiber through a circulator to output the swept frequency optical pulse train, the sensing optical fiber transmits the Rayleigh backscattered light through the circulator to the coherent receiving and photoelectric conversion module, the coherent receiving and photoelectric conversion module converts the beat frequency signal into an electrical signal, and is connected to the signal acquisition and processing module to transmit the electrical signal; The frequency range of the swept pulse is expanded by a frequency shifting ring to achieve high spatial resolution vibration detection of the sensing optical fiber. The Rayleigh backscattered light returned from the sensing optical fiber beats with the reference light, and a high spatial resolution beat signal is obtained by using a matched filter generated by deconvolution. Vibration information on the optical fiber is then obtained through signal processing.

2. The distributed optical fiber acoustic wave sensing system based on the frequency shift ring according to claim 1 is characterized in that: The narrow linewidth laser source module includes a connected narrow linewidth fiber laser and a first fiber coupler, wherein the narrow linewidth fiber laser is connected to a frequency sweeping optical pulse generating module and a coherent receiving and photoelectric conversion module through the first fiber coupler, and transmits detection light and reference light to the frequency sweeping optical pulse generating module and the coherent receiving module respectively.

3. The distributed optical fiber acoustic wave sensing system based on the frequency shift ring according to claim 2 is characterized in that: The RF signal generating module includes a connected RF signal generator and a RF signal amplifier, wherein the RF signal generator is connected to the frequency sweeping optical pulse generating module through the RF signal amplifier, and inputs and amplifies the frequency sweeping RF pulse signal and the frequency shifting RF signal pulse respectively.

4. The distributed optical fiber acoustic wave sensing system based on the frequency shift ring according to claim 3 is characterized in that: The swept frequency optical pulse generating module comprises an acousto-optic modulator, a frequency shifting ring and an erbium-doped fiber amplifier, wherein: the acousto-optic modulator is respectively connected to the narrow linewidth laser light source module and the frequency shifting ring, the frequency shifting ring is connected to the erbium-doped fiber amplifier, and the amplified signal is connected to the sensing fiber through a circulator; the frequency shifting ring comprises a connected fiber coupler, a delay fiber, an optical amplifier and an acousto-optic frequency shifter, the delay fiber realizes the time delay of the optical signal, the optical amplifier compensates for the loss of the optical signal, and the acousto-optic frequency shifter realizes the frequency shift.

5. The distributed optical fiber acoustic wave sensing system based on the frequency shift ring according to claim 4 is characterized in that: The frequency sweep optical pulse generating module generates a frequency sweep signal with a bandwidth of B0 through an acousto-optic modulator. The frequency sweep signal is frequency shifted by a frequency shifting ring and time shifted by a delay optical fiber, thereby splicing each signal segment to form a continuous large-range frequency sweep signal.

6. The distributed optical fiber acoustic wave sensing system based on a frequency shift ring according to claim 4, characterized in that: The optical amplifier is a semiconductor optical amplifier.

7. The distributed optical fiber acoustic wave sensing system based on a frequency shift ring according to claim 4, characterized in that: After the small-range frequency-sweeping optical pulse passes through the second optical fiber coupler and enters the frequency-shifting loop, it first enters the first acousto-optic frequency shifter. The first acousto-optic frequency shifter generates a single-frequency signal to realize the frequency shift of the frequency-sweeping optical signal. The frequency-shifted signal passes through the third optical fiber coupler, enters the sensing optical cable in one way, and enters the first delay optical fiber and the first optical amplifier in the other way. The signal after time-delay amplification passes through the second optical fiber coupler again and enters the first acousto-optic frequency shifter for frequency shifting, and the above process is repeated.

8. The distributed optical fiber acoustic wave sensing system based on a frequency shift ring according to claim 4, characterized in that: The small-range frequency-sweeping optical pulse passes through the fourth optical fiber coupler, one way enters the sensing optical cable, and the other way enters the frequency shifting loop, passes through the second delay optical fiber, the second acousto-optic frequency shifter, and the third optical amplifier, and then enters the fourth optical fiber coupler, repeating the above process.

9. The distributed optical fiber acoustic wave sensing system based on a frequency shift ring according to claim 4, characterized in that: The frequency-shifting loop is used to shift and splice the swept frequency signals generated by the acousto-optic modulator to obtain a continuous swept frequency signal with a large bandwidth, thereby realizing vibration detection with high spatial resolution and high signal-to-noise ratio. The specific steps include: Step 1: The AOM modulates to generate a swept frequency signal: E1(t) = exp j[2π(f c +f0)t+πkt 2 ]g(t); where f c is the center frequency of the laser, f0 is the starting frequency of the sweep signal, k is the sweep rate of the AOM 3; j represents the sign of the imaginary part of the complex number, t represents time; Step 2: After passing through the frequency shift loop, the signal circulates M times in the frequency shift loop, and the spliced ​​signal is: Where: the whole of the spliced ​​signal, or any of them, is regarded as a complete detection signal; where M represents the number of cycles; T represents the cycle period of the signal in the frequency shift loop; B0 represents the sweep bandwidth of the acousto-optic modulator; Step 3: The beat frequency signal of the Rayleigh signal returned from the position where the round-trip time delay is τ on the sensing fiber and the local light is: The scattered light signal containing multiple cycles is compressed by the corresponding matched filter h(t) to obtain the distribution information of the Rayleigh scattering signal on the sensing optical fiber with high spatial resolution, and then the vibration information on the optical fiber is obtained through the intensity and phase information of the Rayleigh scattering signal.

10. A distributed optical fiber acoustic wave sensing device based on a frequency shift ring, characterized in that: A distributed optical fiber acoustic wave sensing system based on a frequency shift ring comprising any one of claims 1 to 9.

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