Sweep-frequency-coding-free Brillouin optical time domain reflectometer based on coherent detection

Abstract

The invention relates to a frequency-sweeping-free coding Brillouin optical time domain reflectometer based on coherent detection, which is characterized in that an optical coupler divides laser generated by a continuous laser into a local reference light branch and a detection light branch, and a signal generation circuit module generates a radio frequency signal of a single side band coding sequence with frequency deviation and drives the detection light branch; the detection light branch generates single-side-band coding laser and inputs the single-side-band coding laser to the sensing optical fiber to be detected through the optical circulator; the spontaneous Brillouin scattering light generated by the sensing optical fiber to be measured is subjected to beat frequency superposition with the reference local light through the optical circulator; the signal acquisition circuit samples the beat frequency signal, obtains a Brillouin scattering spectrum through a short-time Fourier algorithm, and reconstructs a Brillouin gain spectrum based on a coding demodulation algorithm. Compared with the prior art, the method has the advantages that the signal-to-noise ratio of the beat frequency signal is increased, the problem of time consumption of traditional frequency sweeping operation is avoided, and the measurement speed and efficiency of distributed temperature / strain sensing are improved.

Description

Technical Field

[0001] The present invention relates to the field of optical measurement and sensing technology, and in particular to a sweep-free frequency-coded Brillouin optical time-domain reflectometer based on coherent detection. Background Art

[0002] Brillouin optical time-domain reflectometry (BOTDR), as an important branch of distributed fiber optic sensing technology, can achieve long-distance, high-precision temperature and strain sensing. However, the intensity of spontaneous Brillouin scattering is much lower than that of stimulated Brillouin scattering, resulting in a low signal-to-noise ratio (SNR). Especially in harsh environments or long-distance sensing scenarios, it is difficult to achieve effective parameter detection. For example, at a sensing distance of more than 20 kilometers, the inherent loss of the optical fiber (about 0.2dB / km) and environmental noise will significantly reduce the signal-to-noise ratio of the far-end signal, making the extraction of Brillouin frequency shift extremely difficult. In addition, traditional BOTDR systems usually use sweeping frequency operations to obtain Brillouin scattering spectra. This process is time-consuming and difficult to meet real-time requirements. In addition, the system complexity is high, which limits its application in engineering practice.

[0003] To improve the signal-to-noise ratio (SNR) of BOTDR systems, coherent detection technology has been introduced as an effective solution. Compared to traditional direct detection, coherent detection significantly improves the intensity and SNR of the detection signal by introducing local reference light to interfere with the signal light. Specifically, coherent detection technology uses high-power local reference light to beat the weak Brillouin scattering signal, converting the optical signal into an electrical signal for amplification and processing. Compared to single-pulse solutions, coherent detection technology can achieve a higher SNR at the same input optical power, providing stronger noise immunity for sensing over long distances and in complex environments.

[0004] At the same time, coding technology has been widely used in BOTDR systems as another effective means to improve the signal-to-noise ratio. Conventional coding schemes (such as complementary Golay coding, Simplex coding, etc.) significantly improve the SNR without changing the spatial resolution and detection time through multi-pulse superposition and decoding algorithms. Compared with single-pulse schemes, coding technology can provide higher coding gain (determined by the length of the sequence) under the same conditions, effectively improving the detection performance of the system. However, existing coding schemes still rely on traditional frequency sweeping operations, which are difficult to meet real-time requirements. For example, the frequency sweeping process requires gradually adjusting the local mixer reference frequency to obtain a complete Brillouin scattering spectrum, which is time-consuming and increases the complexity and cost of the system. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects of the above-mentioned prior art, such as low signal-to-noise ratio and long time consumption, and to provide a sweep-free coded Brillouin optical time domain reflectometer based on coherent detection.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A sweep-free coded Brillouin optical time-domain reflectometer based on coherent detection, comprising a continuous laser, an optical coupler, a signal generation circuit module, a detection optical branch, an optical circulator, a sensing optical fiber to be tested, and a signal acquisition circuit;

[0008] The optical coupler divides the laser light generated by the continuous laser into two branches: a local reference light and a detection light. The signal generation circuit module generates a radio frequency signal with a single-sideband coding sequence with a frequency offset and drives the detection light branch.

[0009] The detection optical branch generates a single-sideband coded laser and inputs it into the sensing optical fiber to be tested via an optical circulator;

[0010] The spontaneous Brillouin scattered light generated by the sensing optical fiber to be tested is subjected to beat frequency superposition with the reference local light through an optical circulator;

[0011] The signal acquisition circuit samples the beat frequency signal, obtains the Brillouin scattering spectrum through the short-time Fourier algorithm, and reconstructs the Brillouin gain spectrum based on the coding demodulation algorithm.

[0012] As a preferred technical solution, the detection light branch includes a light intensity modulation control unit and a polarization controller;

[0013] The light intensity modulation control unit generates a radio frequency signal with a single sideband coding sequence having a frequency offset based on the signal generation circuit module, generates a single sideband coded laser, and inputs the signal to the input end of the polarization controller;

[0014] The polarization controller performs polarization modulation on the single sideband coded laser to avoid polarization orthogonality between the single sideband coded laser and the reference local light. The polarization modulated single sideband coded laser is input to the sensing optical fiber to be measured via the optical circulator.

[0015] As a preferred technical solution, the light intensity modulation control unit includes: an IQ modulator, an optical filter and an acousto-optic modulator;

[0016] The IQ modulator performs down-conversion single-sideband modulation on the incident continuous detection light, and the frequency offset between the single-sideband modulated light and the incident light is determined by the frequency of the continuous radio frequency signal provided by the signal generating circuit module, and the frequency of the continuous radio frequency signal is the Brillouin shift of the sensing fiber to be measured;

[0017] The optical filter is used to suppress sidebands and noise;

[0018] The acousto-optic modulator modulates the intensity envelope of the single-sideband modulated light, and the signal generating circuit module provides a coded sequence radio frequency signal to determine whether the acousto-optic modulator outputs or not, thereby generating a coded pulse sequence light.

[0019] As a preferred technical solution, the signal generating circuit module is equipped with two independent output channels and one synchronous trigger channel;

[0020] One of the independent output channels generates a coded sequence radio frequency signal to drive an acousto-optic modulator; the other independent output channel outputs a continuous radio frequency signal with a frequency offset to drive an IQ modulator.

[0021] As a preferred technical solution, the signal generating circuit module includes an arbitrary waveform generator;

[0022] The arbitrary waveform generator outputs a coded waveform, which is used as a radio frequency signal to drive an acousto-optic modulator to generate coded laser light.

[0023] As a preferred technical solution, the signal generating circuit module includes an arbitrary waveform generator and an acousto-optic driver adapted to the acousto-optic modulator;

[0024] The arbitrary waveform generator provides a switching signal to be input into the acousto-optic driver, and the acousto-optic driver then generates a radio frequency signal to drive the acousto-optic modulator.

[0025] As a preferred technical solution, the signal generating circuit module includes a signal generator; the signal generator outputs a continuous radio frequency signal to drive the IQ modulator in the light intensity modulation control unit.

[0026] As a preferred technical solution, the signal acquisition circuit includes a photodetector, a high-speed digital acquisition card and a computer

[0027] The photoelectric detector converts the beat frequency signal of the spontaneous Brillouin scattered light and the reference local light into an electrical signal and transmits it to a high-speed digital acquisition card;

[0028] The computer obtains a Brillouin scattering spectrum based on a short-time Fourier algorithm for the collected beat frequency signal; extracts the time domain intensity distribution at each frequency from the Brillouin scattering spectrum, and reconstructs the Brillouin gain spectrum using a corresponding coding and demodulation algorithm.

[0029] As a preferred technical solution, the computer reconstructs the Brillouin gain spectrum as follows:

[0030] Slice the beat frequency signal, perform short-time Fourier transform on each slice signal to obtain the time-frequency scattering spectrum, and fill zeros at both ends;

[0031] Take the single-sided scattering spectrum that is consistent with the driving frequency of the IQ modulator, extract the time domain distribution waveform at the same frequency, and multiply it with the inverse matrix of the driving signal sequence of the acousto-optic modulator to obtain the fiber time domain distribution matrix;

[0032] Adjust the corresponding delay according to the sequence of the pulses into the optical fiber to be tested, and obtain the time domain response of the optical fiber single pulse at the current frequency through cumulative averaging.

[0033] The time domain response of the optical fiber single pulse at each frequency is reorganized according to the frequency dimension to obtain the Brillouin gain spectrum, and the Brillouin frequency shift of the sensing fiber to be measured is obtained by searching the peak value through Lorentz fitting.

[0034] As a preferred technical solution, when external temperature / strain acts on the sensing optical fiber to be measured, the computer obtains the change in Brillouin frequency shift and reversely determines the applied temperature / strain, thereby realizing distributed temperature / strain sensing.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] 1) The proposed coherent detection-based, sweep-free coded BOTDR system solution combines coherent detection technology with a coding scheme. The system replaces the traditional single pulse with a coding sequence, significantly improving the signal-to-noise ratio of the scattered signal without sacrificing sensing accuracy or detection time, thereby achieving high-precision monitoring of distributed temperature / strain. At the same time, the short-time Fourier transform algorithm is introduced to achieve direct spectrum measurement, avoiding the time-consuming problem of traditional sweep-frequency operation. This technology not only effectively overcomes the limitations of traditional BOTDR systems, but also provides a new solution for long-distance, high-precision distributed fiber optic sensing, with important application value in structural health monitoring, perimeter security and other fields.

[0037] 2) The proposed system optimizes the reference optical path design, achieving simultaneous Brillouin frequency shift compensation for pulse modulation. This shifting unit from the conventional local reference path to the detection path not only simplifies system hardware costs but also minimizes noise interference with the reference signal, improving the signal-to-noise ratio of the beat signal. Furthermore, the signal received by the detector is converted to spontaneous Brillouin scattered light, which is then superimposed on the reference local light directly output by the laser through an optical circulator. This ensures high-power reference light output with a high signal-to-noise ratio, significantly improving the quality of the beat signal and ensuring higher measurement accuracy.

[0038] 3) The proposed system incorporates a short-time Fourier transform (SFT) algorithm to enable direct spectrum measurement. This algorithm acquires the Brillouin scattering spectrum, and directly obtains the time-domain distribution of the beat frequency scattering spectrum through time-frequency analysis, significantly improving demodulation speed and detection accuracy. This avoids the time-consuming nature of traditional frequency sweeping operations, simplifies system design, and significantly improves measurement speed and efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Schematic diagram of the structure of the sweep-free coded Brillouin optical time domain reflectometer based on coherent detection of the present invention;

[0040] Figure 2 Schematic diagram of the structure of a sweep-free coded Brillouin optical time domain reflectometer based on coherent detection in one embodiment of the present invention;

[0041] Figure 3 This is a diagram showing the actual effect test results of an embodiment of the present invention;

[0042] The numbers in the figure are as follows: 1. Continuous laser, 2. Optical coupler, 3. Signal generation circuit module, 4. Light intensity modulation control unit, 5. Polarization controller, 6. Optical circulator, 7. Sensing fiber to be tested, 8. Signal acquisition circuit. DETAILED DESCRIPTION

[0043] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0044] Example 1

[0045] The present invention proposes a sweep-free coded Brillouin optical time domain reflectometer based on coherent detection, such as Figure 1 As shown, the reflectometer system includes: a continuous laser 1, an optical coupler 2, a signal generation circuit module 3, a light intensity modulation control unit 4, a polarization controller 5, an optical circulator 6, a sensing optical fiber to be measured 7, and a signal acquisition circuit 8. In the system, a continuous laser 1 first generates laser light; the laser light is divided into two branches, a reference local light and a detection light, after passing through an optical coupler 2; the detection light is converted into a single-sideband coded laser with a frequency offset after passing through an optical intensity modulation control unit 4 driven by a signal generation circuit module 3; the polarization state of the coded laser light is changed by a polarization controller 5; the polarization-modulated coded laser light passes through an optical circulator 6 and is incident on a sensing optical fiber to be measured 7; the spontaneous Brillouin scattered light generated in the sensing optical fiber to be measured 7 passes through the optical circulator 6 and is beat-frequency superimposed with the reference local light; the beat frequency signal is sampled and received by a signal acquisition circuit 8, and a Brillouin scattering spectrum is obtained based on a short-time Fourier algorithm; the time domain intensity distribution at each frequency is extracted from the Brillouin scattering spectrum, and the Brillouin gain spectrum is reconstructed using a corresponding coding and demodulation algorithm; the Brillouin frequency shift of the sensing optical fiber to be measured 7 is determined by the Brillouin gain spectrum, and effective temperature / strain sensing is achieved through the change in the Brillouin frequency shift.

[0046] The present invention replaces the traditional single pulse with a coded sequence, significantly improving the signal-to-noise ratio of the scattered signal without sacrificing sensing accuracy and detection time, thereby realizing high-precision monitoring of distributed temperature / strain; at the same time, coherent detection technology is used to optimize the reference optical path design to ensure high-power and high signal-to-noise ratio reference light output, further improving the beat frequency signal quality and measurement accuracy; in addition, by introducing the short-time Fourier transform algorithm to achieve direct spectrum measurement, the time-consuming problem of traditional sweeping operation is avoided, and the measurement speed and system efficiency are greatly improved; ultimately, this system can be used as a high-performance monitoring solution, playing an important role in structural health monitoring, perimeter security and other fields, and has outstanding engineering application value and practical significance.

[0047] As one of the specific implementation examples of the present invention, Figure 2 As shown, a continuous laser 11 generates laser light which is incident on the input end of an optical coupler 2; the optical coupler 2 divides the laser light into two branches, a local reference light and a detection light, which are incident on the optical input end of an optical intensity modulation control unit 4 and the optical input end of a signal acquisition circuit 8, respectively; a signal generation circuit module 3 generates a radio frequency signal with a single-sideband coding sequence having a frequency offset and inputs it into the optical intensity modulation control unit 4; the optical intensity modulation control unit 4 generates a single-sideband coded laser which is input into the input end of a polarization controller 5; the polarization controller 5 performs polarization modulation on the coded laser light and inputs it into a sensing fiber 7 to be tested via an optical circulator 6; the spontaneous Brillouin scattered light generated by the sensing fiber 7 to be tested is subjected to beat frequency superposition with the reference local light via the optical circulator 6; the signal acquisition circuit 8 samples the beat frequency signal, obtains the Brillouin scattering spectrum through a short-time Fourier transform algorithm, and reconstructs the Brillouin gain spectrum based on a coding demodulation algorithm, and finally realizes efficient sensing of temperature / strain through the change in Brillouin frequency shift.

[0048] The continuous laser 1 is a narrow-linewidth single-frequency laser with a nominal output frequency of 1550.12 nm, an output linewidth of less than 3 kHz, and an output power of 50 mW.

[0049] Optical coupler 2 distributes the intensity of the optical power output by the continuous laser according to the splitting ratio parameter. In this embodiment, a polarization-maintaining optical coupler 2 is selected, and its nominal splitting ratio is 20:80. The 20% end is incident on the signal acquisition circuit 8 as the reference local light, and the 80% end is incident on the light intensity modulation control unit 4 as the detection light.

[0050] The optical intensity modulation control unit 4 consists of an IQ modulator, an optical filter, and an acousto-optic modulator. The IQ modulator has a nominal modulation bandwidth of 8-12 GHz and an insertion loss of less than 6 dB. The optical filter has a nominal filtering range of 1530 nm to 1610 nm, a filtering bandwidth of 0.08 nm to 4 nm, and an insertion loss of less than 3 dB. The acousto-optic modulator uses a 1550 nm fiber acousto-optic modulator with a nominal acousto-optic crystal frequency shift of 200 MHz, an extinction ratio greater than 50 dB, and an insertion loss of less than 3 dB. The IQ modulator down-converts the incident continuous probe light to single-sideband modulation, achieving a carrier suppression ratio exceeding 25dB. The frequency offset between the SSB modulated light and the incident light is determined by the continuous drive signal frequency provided by the signal generation circuit module 3. The drive signal frequency is the Brillouin shift of the sensing fiber 7 to be measured, approximately 10.8GHz. An optical filter is used to further suppress sidebands and noise, improving the signal-to-noise ratio of the detection signal, with a filter bandwidth of 0.08nm. The acousto-optic modulator modulates the intensity envelope of the SSB continuous light to generate a coded pulse sequence. The signal generation circuit module 3 provides a high- and low-level sequence as the coded drive signal to determine whether the AOM outputs. When the drive signal is continuously high, the AOM generates an optical signal. When the drive signal is continuously low, the AOM remains silent and waits for no output, ultimately producing a "0, 1" intensity modulated code sequence.

[0051] Signal generation circuit module 3 is equipped with two independent output channels and a synchronous trigger channel, comprising an arbitrary waveform generator (AWG) and a signal generator. The AWG nominally has a sampling rate of 1GSa / s, an analog bandwidth of 120MHz, and a storage capacity of 64MSa. It generates a coded waveform through one of its independent output channels, which serves as the RF signal to drive the AOM to produce the coded laser. To ensure the proper operation of the AOM, the frequency of the sequence waveform output by the AWG must match the AOM's frequency shift characteristics. Furthermore, an AOD driver compatible with the AOM can be selected, nominally capable of outputting an RF signal with a stability better than 100ppm when receiving a digitally modulated signal input. Since the coded waveform used in this system is essentially a digital intensity modulated pulse sequence, two approaches are possible: The AWG directly generates the RF signal to drive the AOM; or the AWG provides a switching signal to the AOM, which then generates the RF signal to drive the AOM. The former offers a simpler hardware structure, while the latter offers greater ease of operation. Both approaches provide equivalent results. The coding sequence uses 63-bit simplex encoding, with each symbol duration of 40 ns (corresponding to a 4m spatial resolution) and a single sequence period of 40 ns*63 + 100 us = 102.52 us, ensuring the absence of optical aliasing. The signal generator has a nominal signal frequency range of 8-12 GHz, an output level upper limit of 16 dBm, and a frequency accuracy of 0.01 Hz. The signal generator outputs a continuous RF signal through another independent output channel to drive the IQ modulator in the optical intensity modulation control unit 4. The continuous RF signal frequency is approximately 10.8 GHz, which is close to the Brillouin frequency shift of the sensing fiber 7 to be measured.

[0052] Polarization controller 5 uses an optical polarization scrambler with a nominal modulation operating range exceeding 100nm@1550nm, an output polarization degree less than 5%, and an average polarization mode dispersion less than 0.05ps. The polarization scrambler modulates the polarization state of the input coded laser to ensure that it is non-orthogonal to the polarization state of the reference local light, otherwise a normal beat frequency signal cannot be obtained.

[0053] Optical circulator 6 uses a single-mode optical circulator with a nominal insertion loss of less than 1.0 dB, an isolation exceeding 60 dB, and a return loss exceeding 50 dB. Port 1 of optical circulator 6 is connected to polarization controller 5, which guides the coded laser light through port 2 and into the sensor fiber 7 to be tested.

[0054] The sensing fiber 7 to be tested uses G.652.D single-mode fiber, which has a nominal sensing distance of approximately 10.05 km and a transmission loss of less than 0.2 dB / km@1550 nm. The coded laser generates spontaneous Brillouin scattered light in the single-mode fiber, which is then superimposed with the reference local light through port 3 of the optical circulator 6.

[0055] The signal acquisition circuit 8 primarily comprises a photodetector, a high-speed digital acquisition card, and a computer. The photodetector is an InGaAs balanced photodetector with a nominal bandwidth of 30kHz-1.6GHz and a cross-group gain of 10kV / W at 1550nm. The photodetector converts the beat frequency signal of the spontaneous Brillouin scattered light and the reference local light into an electrical signal and transmits it to the high-speed digital acquisition card. The high-speed acquisition card is a programmable high-speed acquisition card with a nominal analog bandwidth of 400MHz and a sampling rate of 1GS / s at 16-bit resolution. The high-speed acquisition card has a trigger period of 102.52µs and a repeated accumulation average detection time of 50ms. The subsequent demodulation operation is as follows: 50ms of the beat frequency signal are sliced, using a 102.52µs sequence period as the time unit. A short-time Fourier transform (SFT) was performed on each slice signal to obtain a time-frequency scattering spectrum. The SFT window was 32 ns (first, the slice length must be smaller than the spatial resolution to prevent information leakage; second, integer powers of 2 facilitate computational speed). Zeros were padded on both ends to 1024 points (to improve frequency resolution), and the number of sliding points was 5. Based on the scattering spectrum (the SFT algorithm yields a symmetrical double-sided spectrum; only the single-sided spectrum corresponding to the IQ modulator drive frequency is required), the time-domain distribution waveforms at each frequency were extracted and multiplied with the inverse simplex matrix (which is the inverse matrix of the acousto-optic modulator drive signal sequence and strictly matches the drive sequence). After obtaining the fiber time-domain distribution matrix, the single-pulse time-domain response of the fiber at the current frequency was obtained by adjusting the corresponding delay according to the sequence injected into the photosensitive fiber to be tested. The cumulative average was then used to obtain the single-pulse time-domain response of the fiber at the current frequency. The single-pulse time-domain responses at each frequency were reorganized along the frequency dimension to obtain the Brillouin gain spectrum. The Brillouin frequency shift of the sensing fiber 7 to be tested was then determined by searching for peaks using a Lorentz fit. When external temperature / strain acts on the sensing optical fiber 7 to be measured, the Brillouin frequency shift changes linearly. The amount of temperature / strain applied can be inversely determined by the change in the Brillouin frequency shift, thereby achieving distributed temperature / strain sensing.

[0056] For the sweep-free coded Brillouin optical time domain reflectometer system solution based on coherent detection in the embodiment of the present invention, Figure 3 Firstly, the fiber time domain response / SNR comparison diagram of the traditional single pulse system and the proposed system at the Brillouin frequency shift is given ( Figure 3 (a)) to verify that this system solution provides a stable signal-to-noise ratio gain for the Brillouin optical time domain analyzer and can complete distributed temperature / strain sensing in a shorter time. In order to verify the sensing capability of this system solution, hot spot events are applied to the end of the sensing fiber 7 to be tested, which are 31.6℃, 39.6℃, 49.9℃, 62.5℃, and 76.8℃ respectively. At this time, the Brillouin frequency shift of the sensing fiber 7 to be tested is linearly offset, and the offset is linearly mapped to the temperature change, as shown in the following figure. Figure 3(b); at the same time, Figure 3 (b) The rising / falling edges of the heating / cooling zone (10%-90%) confirm that the spatial resolution of this system is approximately 4.2 m, which is close to the 40 ns duration of a single symbol in the simplex sequence. The horizontal axis of the image is the distance of the sensing fiber 7 to be measured. Figure 3 The vertical axis in (a) is the normalized Brillouin gain and signal-to-noise ratio, Figure 3 The vertical axis in (b) is the Brillouin frequency shift.

[0057] The above describes in detail the preferred embodiments of the present invention. It should be understood that those skilled in the art can make numerous modifications and variations based on the concepts of the present invention without inventive effort. Therefore, any technical solutions that can be derived by those skilled in the art through logical analysis, reasoning, or limited experimentation based on the concepts of the present invention and the prior art should be within the scope of protection defined by the claims.

Claims

1. A sweep-free coded Brillouin optical time-domain reflectometer based on coherent detection, characterized in that: include: Continuous laser (1), optical coupler (2), signal generation circuit module (3), detection optical branch, optical circulator (6), sensing optical fiber to be tested (7) and signal acquisition circuit (8); The optical coupler (2) divides the laser light generated by the continuous laser (1) into two branches, namely, a local reference light and a detection light. The signal generation circuit module (3) generates a radio frequency signal with a single-sideband coding sequence having a frequency offset and drives the detection light branch. The detection optical branch generates a single-sideband coded laser and inputs the laser into the sensing optical fiber (7) to be tested via an optical circulator (6); The spontaneous Brillouin scattered light generated by the sensing optical fiber (7) to be measured is passed through the optical circulator (6) and is subjected to beat frequency superposition with the reference local light; The signal acquisition circuit (8) samples the beat frequency signal, obtains the Brillouin scattering spectrum through a short-time Fourier algorithm, and reconstructs the Brillouin gain spectrum based on a coding demodulation algorithm.

2. The sweep-free coded Brillouin optical time domain reflectometer based on coherent detection according to claim 1, characterized in that: The detection light branch includes a light intensity modulation control unit (4) and a polarization controller (5); The light intensity modulation control unit (4) generates a radio frequency signal with a frequency-shifted single-sideband coding sequence based on the signal generation circuit module (3), generates a single-sideband coded laser, and inputs the signal to the input end of the polarization controller (5); The polarization controller (5) performs polarization modulation on the single sideband coded laser to prevent the single sideband coded laser from being orthogonal to the polarization of the reference local light. The polarization-modulated single sideband coded laser is input to the sensing optical fiber (7) to be measured via the optical circulator (6).

3. The sweep-free coded Brillouin optical time domain reflectometer based on coherent detection according to claim 2, characterized in that: The light intensity modulation control unit (4) comprises: an IQ modulator, an optical filter and an acousto-optic modulator; The IQ modulator performs down-conversion single-sideband modulation on the incident continuous detection light, and the frequency offset between the single-sideband modulated light and the incident light is determined by the frequency of the continuous radio frequency signal provided by the signal generating circuit module (3), and the frequency of the continuous radio frequency signal is the Brillouin offset of the sensing optical fiber (7) to be measured; The optical filter is used to suppress sidebands and noise; The acousto-optic modulator modulates the intensity envelope of the single-sideband modulated light, and the signal generating circuit module (3) provides a coded sequence radio frequency signal to determine whether the acousto-optic modulator outputs or not, thereby generating a coded pulse sequence light.

4. The sweep-free coded Brillouin optical time domain reflectometer based on coherent detection according to claim 3, characterized in that: The signal generating circuit module (3) is equipped with two independent output channels and one synchronous trigger channel; One of the independent output channels generates a coded sequence radio frequency signal to drive an acousto-optic modulator; the other independent output channel outputs a continuous radio frequency signal with a frequency offset to drive an IQ modulator.

5. The sweep-free coded Brillouin optical time domain reflectometer based on coherent detection according to claim 4, characterized in that: The signal generating circuit module (3) includes an arbitrary waveform generator; The arbitrary waveform generator outputs a coded waveform, which is used as a radio frequency signal to drive an acousto-optic modulator to generate coded laser light.

6. The sweep-free coded Brillouin optical time domain reflectometer based on coherent detection according to claim 4, characterized in that: The signal generating circuit module (3) includes an arbitrary waveform generator and an acousto-optic driver adapted to the acousto-optic modulator; The arbitrary waveform generator provides a switching signal to be input into the acousto-optic driver, and the acousto-optic driver then generates a radio frequency signal to drive the acousto-optic modulator.

7. The sweep-free coded Brillouin optical time domain reflectometer based on coherent detection according to claim 4, characterized in that: The signal generating circuit module (3) comprises a signal generator; the signal generator outputs a continuous radio frequency signal to drive the IQ modulator in the light intensity modulation control unit (4).

8. The sweep-free coded Brillouin optical time domain reflectometer based on coherent detection according to claim 1, characterized in that: The signal acquisition circuit (8) includes a photoelectric detector, a high-speed digital acquisition card and a computer The photoelectric detector converts the beat frequency signal of the spontaneous Brillouin scattered light and the reference local light into an electrical signal and transmits it to a high-speed digital acquisition card; The computer obtains a Brillouin scattering spectrum based on a short-time Fourier algorithm for the collected beat frequency signal; extracts the time domain intensity distribution at each frequency from the Brillouin scattering spectrum, and reconstructs the Brillouin gain spectrum using a corresponding coding and demodulation algorithm.

9. The sweep-free coded Brillouin optical time domain reflectometer based on coherent detection according to claim 8, characterized in that: The computer reconstructs the Brillouin gain spectrum as follows: Slice the beat frequency signal, perform short-time Fourier transform on each slice signal to obtain the time-frequency scattering spectrum, and fill zeros at both ends; Take the single-sided scattering spectrum that is consistent with the driving frequency of the IQ modulator, extract the time domain distribution waveform at the same frequency, and multiply it with the inverse matrix of the driving signal sequence of the acousto-optic modulator to obtain the fiber time domain distribution matrix; Adjust the corresponding delay according to the sequence of the pulses into the optical fiber to be tested, and obtain the time domain response of the optical fiber single pulse at the current frequency through cumulative averaging. The time domain response of the optical fiber single pulse at each frequency is reorganized according to the frequency dimension to obtain the Brillouin gain spectrum, and the Brillouin frequency shift of the sensing optical fiber (7) to be measured is obtained by searching the peak value through Lorentz fitting.

10. The sweep-free coded Brillouin optical time domain reflectometer based on coherent detection according to claim 9, characterized in that: When external temperature / strain acts on the sensing optical fiber (7) to be measured, the computer obtains the Brillouin frequency shift change and reversely determines the temperature / strain applied amount, thereby realizing distributed temperature / strain sensing.

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