A distributed optical fiber acoustic wave sensing system based on double acousto-optic difference frequency modulation

Abstract

The present application relates to the field of optical time domain reflectometry, in particular to a distributed optical fiber acoustic wave sensing system based on double acousto-optic difference frequency modulation. The present application introduces a frequency shift device in the local reference light of the system, reduces the signal detection bandwidth: in the signal acquisition part, the difference frequency of two signal frequencies is used to complete the phase modulation of the probe light signal, which reduces the cost of the acquisition equipment required for signal restoration and reduces the burden of the data storage equipment; in the realization of frequency conversion, the frequency modulation is carried out on the optical signal 1 and 2, and the optical signal 2 is combined with the optical signal 1 to realize the frequency conversion together, and the frequency shift driving control of the optical signal 2 only needs to provide a driving signal alone, without considering the problem of whether the trigger is synchronous, in the case of ensuring the system restoration ability and spatial resolution unchanged, the signal acquisition pressure is greatly relieved, the hardware requirement is reduced, the system real-time performance is improved, and the distributed optical fiber sensing system with high signal-to-noise ratio is realized.

Description

Technical Field

[0001] This invention relates to the field of optical time-domain reflectometry, specifically to a distributed fiber optic acoustic wave sensing system based on dual acoustic-optical difference frequency modulation. Background Technology

[0002] Distributed sensing is one of the greatest advantages of fiber optic sensors because it can monitor changes in physical quantities along the sensing fiber in a distributed manner and accurately pinpoint the location of these changes. Distributed fiber optic sensing systems based on coherent optical time domain reflectometers (COTDRs) are particularly sensitive to the phase information of vibration signals. They can obtain external acoustic vibration signals through phase demodulation, offering advantages such as high sensitivity, resistance to electromagnetic interference, and long-distance continuous detection. This makes such systems a current research hotspot in distributed fiber optic sensing systems, and they are widely used in perimeter security, seismic wave detection, and marine monitoring.

[0003] The working principle of a distributed fiber optic sensing system based on a phase-sensitive optical time-domain reflectometer (OTDR) is based on detecting the phase change caused by coherent Rayleigh scattering at two points on the sensing fiber. This phase change is directly related to the magnitude of the strain between the two points. Any external interference (such as acoustic vibration) will alter the phase of the backscattered light; this phase change can be detected by monitoring the backscattered signal after the interaction. Currently, distributed fiber optic sensing systems based on phase-sensitive OTDs are mainly divided into two types: homodyne direct detection systems and heterodyne coherent detection systems. Heterodyne detection systems utilize beat frequency signals and do not require strict control of the frequency shifts of the two signals to be equal, thus offering greater versatility.

[0004] In heterodyne detection schemes, acousto-optic modulators with frequency shifts of several hundred megahertz are typically required. According to the Nyquist sampling theorem, the sampling rate must be greater than or equal to twice the highest frequency of the signal to completely reconstruct the original signal. This means the sampling rate of the acquisition card needs to be at least twice the frequency shift of the acousto-optic modulator. This places high demands on hardware circuit design, system data acquisition, and signal post-processing, is detrimental to noise suppression, and reduces the system's real-time performance.

[0005] Patent CN113970368A discloses a distributed optical fiber vibration acoustic wave sensing system based on down-conversion using dual acousto-optic modulators in series. The system achieves down-conversion distributed optical fiber vibration acoustic wave sensing through a series combination of acousto-optic modulators in the sensing optical path. Its fiber optic beam splitter divides the signal light into two beams: optical signal 1 (sensing light) enters the acousto-optic modulator combination for optical modulation, and optical signal 2 (local reference light) enters the fiber optic coupler as local light. Multiple frequency modulations of the single optical signal 1 require strict control of the trigger delay signal of the series frequency shifter, and multiple frequency modulations lead to severe laser intensity loss, necessitating higher gain to meet the requirements. Higher gain, in turn, implies higher gain noise. Summary of the Invention

[0006] To address the aforementioned problems and shortcomings, and to overcome the deficiencies caused by multiple frequency modulations in existing technologies, this invention provides a distributed fiber optic acoustic wave sensing system based on dual acoustic-optical difference frequency modulation. By adding a frequency shifting device to the local oscillator optical path signal and beating the sensing optical path signal, low-frequency modulation of the sensing optical signal is achieved, reducing the frequency modulation from hundreds of megahertz in traditional schemes to tens of megahertz. This reduces the system's requirements for data acquisition bandwidth and signal processing, decreases the system's data processing volume, and simultaneously increases the real-time performance of system phase demodulation.

[0007] A distributed fiber optic acoustic wave sensing system based on dual acoustic-optical difference frequency modulation includes a difference frequency modulation sensing system optical path module (1) and a signal demodulation module (2).

[0008] The optical path module (1) of the difference frequency modulation sensing system includes: a narrow linewidth laser, a coupler, an amplifier, a probe light pulse modulation module, a frequency shift modulation module, a circulator, and a signal receiving module.

[0009] The narrow linewidth laser serves as a light source to generate signal light. The signal light is split into two paths via an optical fiber coupler. One path (partial) of the signal light is output to the probe light pulse modulation module, and the other path (another part) of the signal light is output to the frequency shift modulation module.

[0010] The probe light pulse modulation module modulates the received signal light by frequency shift carrier modulation and outputs it to the amplifier to amplify the power of the signal light to meet the detection requirements. Then it is connected to one port of the circulator. The second port of the circulator receives the backscattered light from the fiber under test and outputs it to the signal receiving module through the third port.

[0011] The frequency shift modulation module modulates the received signal light using a frequency shift carrier and outputs it as reference light to the optical signal receiving module. The frequency shifts of the probe light pulse modulation module and the frequency shift modulation module are w1 and w2, respectively, with w1 ≠ w2.

[0012] The signal receiving module will beat the received backscattered light with the reference light and output it to the AD module.

[0013] The signal demodulation module (2) includes: a main control unit, a drive control module, an AD module, and a data transmission module.

[0014] The main control unit controls the drive control module to output a drive signal, thereby controlling the frequency shift of the probe light pulse modulation module and the frequency shift modulation module to be w1 and w2 respectively; and the AD module completes the analog-to-digital conversion of the signal, and transmits the converted signal to the main control unit for signal demodulation algorithm processing or uploads it to the host computer for processing via the data transmission module, so as to obtain the vibration information event of the fiber optic perimeter.

[0015] Furthermore, the coupler is a fiber optic coupler, the circulator is a fiber optic circulator, and the amplifier is a fiber optic amplifier, in order to facilitate the miniaturization of the entire system architecture.

[0016] Furthermore, the main control unit, drive control module, and data transmission module are implemented using FPGA chips to facilitate the subsequent loading of software and hardware and make integration easier.

[0017] Furthermore, the specific workflow of the distributed fiber optic acoustic wave sensing system based on dual acousto-optic difference frequency modulation is as follows:

[0018] 1. The narrow linewidth laser light source in the optical path module (1) of the difference frequency modulation sensing system generates signal light, which is then input into the coupler.

[0019] 2. The coupler splits the signal light into two paths and outputs them to the probe light pulse modulation module and the frequency shift modulation module, respectively. The probe light pulse modulation module modulates the signal light by frequency shift carrier w1 and outputs it to the amplifier for amplification. Then it is connected to one port of the circulator. The backscattered light of the fiber under test received by the second port of the circulator is output from its third port and interferes with the reference light after frequency shift carrier modulation w2 by the frequency shift modulation module in the optical signal receiving module.

[0020] 3. The main control unit controls the output drive signal of the drive module: controls the frequency shift of the probe light pulse modulation module and the frequency shift modulation module to w1 and w2 respectively; controls the AD module to be responsible for analog-to-digital conversion and transmits the converted signal to the main control unit.

[0021] 4. The main control unit receives the signal transmitted by the AD module, performs signal demodulation algorithm processing, or uploads it to the host computer for processing via the data transmission module, thereby obtaining vibration information events of the fiber optic perimeter.

[0022] This invention employs frequency shift modulation of the reference optical signal, and modulates and outputs the sensing optical signal through dual acousto-optic difference frequency modulation. In the signal acquisition section, the difference frequency of the two signal frequencies is used to complete the phase modulation of the probe optical signal, reducing the cost of the acquisition equipment required for signal reconstruction, alleviating the burden on the data storage equipment, and overcoming the control problem of trigger synchronization of the series acousto-optic modulator. In terms of the down-conversion implementation method, this invention performs frequency modulation on both optical signal 1 and optical signal 2, and combines optical signal 2 with optical signal 1 to jointly achieve down-conversion. Moreover, this invention only needs to provide a separate drive signal for the frequency shift drive controller (8) of optical signal 2, without considering the issue of trigger synchronization.

[0023] In summary, this invention reduces the signal detection bandwidth by introducing a frequency shifting device into the local reference light of the system. The signal light is divided into optical signal 1 and optical signal 2, which are output to the probe light pulse modulation module and the frequency shifting modulation module, respectively. By performing frequency modulation on both optical signal 1 and optical signal 2 to achieve down-conversion, the signal acquisition pressure is greatly alleviated, the hardware requirements are reduced, and the real-time performance of the system is improved while ensuring that the system's restoration capability and spatial resolution remain unchanged. This results in a high signal-to-noise ratio distributed fiber optic sensing system. Attached Figure Description

[0024] Figure 1 This is a diagram showing the overall system structure of the present invention;

[0025] Figure 2 This is a system framework diagram of an embodiment;

[0026] Figure 3 This is a schematic diagram of difference frequency modulation;

[0027] Figure 4 It is the spectrum of the interference traces before and after dual acoustic-optical difference frequency modulation;

[0028] Figure 5 The time-domain response and (b) PSD of a traditional COTDR system and a dual acousto-optic difference frequency modulation COTDR system are shown.

[0029] Figure 6 The (a) time domain and (b) PSD are obtained by driving PZT with sinusoidal signals of different frequencies.

[0030] Figure reference numerals: 1-Differential frequency modulation sensing system optical path module, 2-Signal demodulation module, 3-Narrow linewidth laser, 4-Coupled, 5-Detector light pulse modulation module, 6-Amplifier, 7-Circulator, 8-Frequency shift modulation module, 9-Signal receiving module, 10-AD module, 11-Drive control module, 12-Data transmission module, 13-Main control unit. Detailed Implementation

[0031] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0032] This embodiment is based on a distributed fiber optic acoustic wave sensing system with dual acousto-optic difference frequency modulation, including a difference frequency modulation sensing system optical path module (1) and a signal demodulation module (2). The difference frequency modulation sensing system optical path module consists of a narrow linewidth laser source (3), an optical fiber coupling device (4), a probe light pulse modulation module - acousto-optic modulator 1 (5), an optical fiber amplifier (6), an optical fiber circulator (7), a frequency shift modulation module - acousto-optic modulator 2 (8), and a signal receiving module - polarization diversity receiving module (9); the signal demodulation module (2) consists of an AD module (10), a drive control module (11), a data transmission module (12), and a main control unit (13).

[0033] The output of the narrow-linewidth laser source corresponds to the input of the 1*2 fiber coupler. The coupler inputs 99% of the input signal to the acousto-optic modulator 1 via output port one, and the remaining 1% to the acousto-optic modulator 2 via output port two. The output of the acousto-optic modulator 1 is connected to the input of the fiber amplifier, the output of the fiber amplifier is connected to port 1 of the fiber circulator, port 2 of the fiber circulator is connected to the fiber under test, and the reflected light is output from port 3 of the circulator to the polarization diversity receiver module (9). The output of the polarization diversity receiver is connected to the AD module, and the AD module outputs to the main control unit, which controls the drive control module and the data transmission module. The drive control module outputs control of the acousto-optic modulators 1 and 2.

[0034] In this embodiment, the narrow-linewidth light source is a signal light source with a wavelength of 1550.12nm, used to generate continuous light. The acousto-optic modulation module consists of two acousto-optic modulators 1 and 2, each with a different modulation frequency, which modulate the signal light and the local reference light, respectively. The main control unit is responsible for controlling the AD module, the drive control module, and the data transmission module. It also demodulates the signals acquired by the AD module or uploads them to the host computer for demodulation via the data transmission module.

[0035] A narrow-linewidth laser outputs continuous light with an optical frequency of ω. Due to the frequency noise generated by the laser, the frequency of the signal light input to the 1*2 coupler is ω+Δω.

[0036] The 1*2 coupler splits the signal light into two paths. One path is modulated by an 80MHz frequency-shifted acousto-optic modulator 1, resulting in a signal light frequency of ω+Δω+ω. c , here ω c =80MHz, and then input to the fiber amplifier for amplification to enhance the input optical power; another signal is modulated by a 100MHz frequency-shifted acousto-optic modulator 2, and the signal light frequency is ω+Δω+f1, where f1=100MHz. This signal is used as a local reference light input to one end of the polarization diversity receiver module.

[0037] The amplified signal light is input to the input end of the circulator, and output through port 2 to the fiber under test. The Rayleigh scattering light in the fiber under test returns to the circulator and is output through port 3 to the other end of the polarization diversity receiver module.

[0038] The returned backscattered Rayleigh light beats with the local reference light modulated by acousto-optic modulator 2 in the polarization diversity receiver module. The frequency of the difference frequency signal after beating is ω. c ′=|ω c -f1|, such as Figure 3 As shown, the signal is captured by a balanced photodetector in the polarization diversity receiver module, converting the optical signal into an electrical signal which is then input to the AD module for acquisition to obtain the spectrum of the beat frequency signal. The spectrum of the beat frequency signal before and after difference frequency modulation is as follows: Figure 4 As shown, the signal spectrum of a traditional COTDR before modulation is concentrated at 80MHz, while the signal spectrum of a COTDR using difference frequency modulation is concentrated at 20MHz. The data acquired by the AD module is output to an FPGA chip or a host computer for signal post-processing and demodulation, thereby obtaining vibration information events at the fiber optic perimeter.

[0039] In this embodiment, a laser source is modulated with a pulse signal with a repetition frequency of 10 kHz. Raw data from a conventional COTDR and a difference-frequency modulated COTDR are recorded for 1 second. The sampling frequencies of the AD module are 250 MSa / s and 62.5 MSa / s, respectively. Data acquisition and demodulation are performed over a fiber optic cable with a length of 2.25 km. The resulting signal data size and demodulation time are shown in Table 1.

[0040] Table 1 Comparison of data acquisition and demodulation times for different systems

[0041]

[0042] As can be seen, compared with the traditional COTDR scheme, the amount of data in the raw signal obtained by the difference frequency modulation COTDR is reduced by about 75%, while the demodulation time is reduced by about 77%. This was achieved using an Intel Core i9-12900k@3.2GHz CPU and MATLAB R2021b. However, their signal-to-noise ratios are almost identical. Figure 5 As shown, where Figure 5 (a) Time domain acquisition of 10Hz PZT signals by a conventional COTDR with 80MHz frequency shift and a difference frequency modulation COTDR with 20MHz frequency shift. Figure 5 (b) is its corresponding PSD. Experimental results show that the differential frequency modulation (COTDR) can reproduce phase change information very well, which means that the differential frequency modulation (COTDR) system can greatly reduce the demand for data storage devices while ensuring signal reproduction and increasing the real-time performance of signal demodulation.

[0043] In this implementation example, the time domain and PSD of the 10Hz, 50Hz, 100Hz, and 1kHz PZT signals are obtained through data acquisition and demodulation processing using the aforementioned dual-acoustic-optical difference-frequency modulation distributed fiber optic acoustic system, as shown below. Figure 6 As shown in (a) and (b), the distributed fiber optic acoustic wave sensing system based on dual acousto-optic difference frequency modulation of this invention uses dual acousto-optic modulators. One modulated beam is used as the probe light for subsequent vibration detection, while the other modulated beam is used as a local reference light to enter a 2*2 coupler for beat frequency. This reduces the frequency of the beat frequency signal. By controlling the frequency f1-f2 of the difference frequency signal according to actual needs, the signal sampling frequency required to satisfy the Nyquist sampling theorem can be greatly reduced. This means a significant reduction in data processing volume, while also reducing the hardware requirements for the sampling rate of the high-speed acquisition card, improving the real-time performance of the system signal processing, and achieving good phase signal recovery.

Claims

1. A distributed fiber optic acoustic wave sensing system based on dual acousto-optic difference frequency modulation, characterized in that: It includes a difference frequency modulation sensing system optical path module (1) and a signal demodulation module (2); The optical path module (1) of the difference frequency modulation sensing system includes: a narrow linewidth laser, a coupler, an amplifier, a probe light pulse modulation module, a frequency shift modulation module, a circulator, and a signal receiving module; The narrow linewidth laser serves as a light source to generate signal light. The signal light is split into two paths by a coupler. One path is output to the probe light pulse modulation module, and the other path is output to the frequency shift modulation module. The probe light pulse modulation module modulates the received signal light by frequency shift carrier modulation and outputs it to the amplifier to amplify the power of the signal light to meet the detection requirements. Then it is connected to one port of the circulator. The second port of the circulator receives the backscattered light from the fiber under test and outputs it to the signal receiving module through the third port. The frequency shift modulation module modulates the received signal light by a frequency shift carrier and outputs it as reference light to the optical signal receiving module; wherein the frequency shifts of the probe light pulse modulation module and the frequency shift modulation module are w1 and w2, respectively, and w1≠w2; The signal receiving module will beat the received backscattered light with the reference light and output it to the AD module; The signal demodulation module (2) includes: a main control unit, a drive control module, an AD module, and a data transmission module; The main control unit controls the drive control module to output a drive signal, thereby controlling the frequency shift of the probe light pulse modulation module and the frequency shift modulation module to be w1 and w2 respectively; and the AD module completes the analog-to-digital conversion of the signal, and transmits the converted signal to the main control unit for signal demodulation algorithm processing or uploads it to the host computer for processing via the data transmission module.

2. The distributed fiber optic acoustic wave sensing system based on dual acousto-optic difference frequency modulation as described in claim 1, characterized in that: The coupler is an optical fiber coupler, the circulator is an optical fiber circulator, and the amplifier is an optical fiber amplifier.

3. The distributed fiber optic acoustic wave sensing system based on dual acousto-optic difference frequency modulation as described in claim 1, characterized in that: The main control unit, drive control module, and data transmission module are implemented using FPGA chips.

4. The distributed fiber optic acoustic wave sensing system based on dual acousto-optic difference frequency modulation as described in claim 1, characterized in that, The specific workflow is as follows: 1) A narrow-linewidth laser source generates signal light, which is then input into a coupler; 2) The coupler splits the signal light into two paths and outputs them to the probe light pulse modulation module and the frequency shift modulation module, respectively. The probe light pulse modulation module modulates the signal light by frequency shift carrier w1 and outputs it to the amplifier for amplification. Then it is connected to one port of the circulator. The backscattered light of the fiber under test received by the second port of the circulator is output from its third port and interferes with the reference light after frequency shift carrier modulation w2 by the frequency shift modulation module in the optical signal receiving module. 3) The main control unit controls the drive control module to output drive signals: control the frequency shift of the probe light pulse modulation module and the frequency shift modulation module to w1 and w2 respectively; control the AD module to be responsible for analog-to-digital conversion and transmit the converted signal to the main control unit; 4) The main control unit receives the signal transmitted by the AD module, performs signal demodulation algorithm processing, or uploads it to the host computer for processing via the data transmission module.

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