Time-frequency division multiplexing Brillouin time domain reflectometer based on pulse sequence coding

Abstract

The invention belongs to the field of optical fiber sensing, relates to a distributed optical fiber sensing technology, and particularly provides a time-frequency division multiplexing Brillouin time domain reflectometer based on pulse sequence coding, which is used for improving the system performance of the Brillouin time domain reflectometer. According to the method, the optical pulse string with the time domain separated from the frequency domain is generated through the frequency shift ring, then pulse time domain coding is carried out on the optical pulses of different channels, on the premise that the number of the channels is increased, increase of the optical peak power is avoided, meanwhile, heterodyne detection of single intrinsic light has feasibility due to the reasonable frequency interval, and the detection accuracy is improved. The intrinsic optical power is maximized, so that the heterodyne detection signal-to-noise ratio is maximized; moreover, the time domain pulse codes between different channels are not mutually aliasing, so that different groups of codes can be almost simultaneously input into the optical fiber to be measured and can be decoded simultaneously, and the limitation of long measurement time of the traditional codes is broken through. And finally, the signal-to-noise ratio, the detection speed, the detection precision and other system performances of the system are effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing and relates to distributed fiber optic sensing technology. Specifically, it provides a time-frequency division multiplexing Brillouin time-domain reflectometer based on pulse sequence coding. Background Technology

[0002] Distributed fiber optic sensing technology benefits from the ease with which light in optical fibers can be modulated by the external field under test and the low transmission loss of optical fibers. By using optical fibers as both sensing units and information transmission media, the field at any point along the fiber under test can be measured. Among them, the Brillouin time domain reflectometer utilizes the spontaneous Brillouin echo generated by the probe light pulse in the fiber under test. By mapping the speed of the echo to time, different positions of the fiber under test are mapped. By analyzing the linear relationship between the Brillouin frequency shift and temperature, the stress or temperature at different positions of the fiber under test can be analyzed. Such systems have profound application prospects in fields such as industrial monitoring and health status analysis of large infrastructure.

[0003] Generally, a Brillouin time-domain analyzer modulates a continuous-wave laser into a single-frequency single-pulse, which is then driven into the fiber under test via a fiber circulator. The pulse generates spontaneous Brillouin scattering back light in the fiber. By shifting the laser source to a position near the Brillouin frequency shift and performing heterodyne detection with the beat frequency of the back light, the Brillouin frequency shift of the back light is determined, and the stress or temperature value along the fiber is calculated. To reduce the influence of noise, a conventional Brillouin time-domain reflectometer performs multiple averaging operations over time.

[0004] However, the spontaneous Burley-Y signal in optical fiber is weak, and the large-scale averaging required to improve the signal-to-noise ratio (SNR) increases the difficulty and time of data processing. To address this issue, the power of the optical pulse needs to be increased as much as possible without causing excessive interference nonlinear effects. Traditional wavelength division multiplexing (WDM) technology improves the system SNR by adding channels, thereby increasing the sensing distance. However, the simultaneous injection of multiple frequency signals leads to an increase in peak power, which may cause excessive Kerr effect. Furthermore, due to the excessive dispersion of frequency components in WDM technology, considering the bandwidth limitations of photodetectors, different intrinsic lights are required for coherent detection. Since photodetectors generally experience power saturation, the intrinsic light power is limited, making it impossible to maximize the SNR of heterodyne detection. These problems limit the performance improvement of traditional WDM technology. In addition, traditional time-division multiplexing techniques (such as pulse coding) have been widely used in BOTDR systems as another effective means of improving signal-to-noise ratio. Conventional coding schemes (such as complementary Golay coding, simplex coding, etc.) significantly improve the signal-to-noise ratio through multi-pulse superposition and decoding algorithms. Their coding gain is determined by the length of the sequence. However, in order to avoid information aliasing between different groups of codes, only one group of codes can be input for a single measurement, which greatly prolongs the measurement time and shortens the system response speed. Summary of the Invention

[0005] The purpose of this invention is to provide a time-frequency division multiplexing Brillouin time-domain reflectometer based on pulse sequence coding, which improves the system performance of a Brillouin time-domain analyzer. In this invention, optical pulse trains with separate time and frequency domains are generated by a frequency-shifting loop. Then, pulse time-domain coding is performed on the optical pulses from different channels. This avoids increasing the peak optical power while increasing the number of channels. Simultaneously, the reasonable frequency spacing makes heterodyne detection of individual intrinsic light feasible, maximizing the intrinsic optical power. Furthermore, the time-domain pulse codes between different channels do not overlap, allowing different sets of codes to be injected into the fiber under test almost simultaneously and decoded simultaneously, overcoming the limitation of long measurement times in traditional coding methods. Ultimately, this effectively improves the system's signal-to-noise ratio, detection speed, and detection accuracy.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A time-frequency division multiplexing Brillouin time-domain reflectometer based on pulse sequence coding, characterized in that it comprises: a narrow-linewidth laser 1, a first coupler 2, a frequency-shifting loop 3, a first intensity modulator 4, a first amplifier 5, a first filter 6, an optical fiber circulator 7, an optical fiber under test 8, a second amplifier 9, a second filter 10, a first polarization controller 11, an electro-optic modulator 12, a 2×2 coupler 13, a photodetector 14, and a signal processing unit; wherein:

[0008] The narrow linewidth laser 1 emits continuous laser light, which is divided into probe light and intrinsic light by the first coupler 2 and enters the probe path and intrinsic path respectively.

[0009] The detection route consists of a frequency shifting loop 3, a first intensity modulator 4, a first amplifier 5, and a first filter 6 connected in sequence. The frequency shifting loop 3 generates an optical pulse train composed of multiple frequency components.

[0010] The intrinsic route is formed by connecting the first polarization controller 11 and the electro-optic modulator 12;

[0011] The output of the detection path is output from one port of the fiber optic circulator 7 to the second port of the fiber optic circulator 7, and enters the fiber under test 8. The Brillouin return light of the fiber under test 8 is transmitted from the second port of the fiber optic circulator 7 to the third port of the fiber optic circulator 7 and then emitted to the return path.

[0012] The return light route is formed by connecting the second amplifier 9 and the second filter 10;

[0013] The outputs of the intrinsic path and the return path are both connected to the 2×2 coupler 13 and detected by the photodetector 14.

[0014] Furthermore, the frequency shifting loop 3 includes: a second intensity modulator 3-1, a second optical coupler 3-2, a second polarization controller 3-3, a polarization beam splitter 3-4, a first acousto-optic modulator 3-5, a third amplifier 3-6, a third filter 3-7, a time-delay fiber 3-8, and an optical isolator 3-9; wherein:

[0015] The probe light is modulated by the second acousto-optic modulator 3-1 to form a probe pulse with a preset pulse width, and then enters the 3dB coupler. One output of the 3dB coupler serves as the output of the frequency shift loop, and the other output enters the frequency shift loop. In the frequency shift loop, the probe pulse passes sequentially through the second polarization controller 3-3, the polarization beam splitter 3-4, the first acousto-optic modulator 3-5, the third amplifier 3-6, the third filter 3-7, the delay fiber 3-8, and the optical isolator 3-9 before passing through the 3dB coupler again. This process is repeated to modulate a single probe pulse into an optical pulse train composed of multiple frequency components.

[0016] Furthermore, the second intensity modulator 3-1 and the first acousto-optic modulator 3-5 are controlled by an arbitrary waveform generator, the pulse width of the second intensity modulator 3-1 corresponds to the sensing spatial resolution, and the modulation frequency of the first acousto-optic modulator 3-5 corresponds to the frequency interval of each frequency component in the optical pulse train.

[0017] Furthermore, the electro-optic modulator 12 is a carrier-suppressed single-sideband modulator.

[0018] Furthermore, the first intensity modulator 4 is controlled by an arbitrary waveform generator to modulate the output optical pulse train of the frequency shift loop 3 into a continuous and sequentially arranged pulse sequence encoding.

[0019] Furthermore, the signal processing unit stores the electrical signal of the photodetector and performs time-frequency analysis, including: Fast Fourier Transform, channel delay compensation, decoding, and Lorentz fitting; wherein:

[0020] The Fast Fourier Transform includes: performing Fast Fourier Transform sequentially over the entire cycle with the pulse width as the window to obtain the Brillouin gain spectrum of each frequency component at each moment of the entire cycle.

[0021] The channel delay compensation includes: compensating for the time delay of each frequency component based on the frequency shift ring length;

[0022] The decoding includes: using the corresponding decoding method for different encoding methods;

[0023] The Lorentz fitting includes: fitting the decoded result to a Lorentz curve, reconstructing the Brillouin gain spectrum, and calculating the temperature value along the fiber under test through the linear relationship between the Brillouin offset and temperature, thereby realizing distributed sensing of temperature or strain along the fiber under test.

[0024] Based on the above technical solution, the beneficial effects of the present invention are as follows:

[0025] This invention presents a novel frequency division multiplexing (FDM) technology based on a frequency shifting loop. This technology can expand the channel and reduce the number of time averaging operations, while simultaneously avoiding the increase in peak optical power caused by the superposition of multiple frequency components through delay lines, thereby reducing the noise impact of nonlinear effects. Due to the small spacing between the frequency components, compared to traditional multi-wavelength schemes, this system can not only optimize the signal-to-noise ratio (SNR) of the light source at the transmitter using a single amplifier and filter, but also maximize the intrinsic optical power at the receiver using a single intrinsic light for heterodyne detection, thus improving the system's heterodyne detection SNR and overcoming the theoretical limitation that traditional FDM cannot achieve the required SNR. The limitation of doubling the improvement.

[0026] This invention novelly combines frequency division multiplexing (FDM) with time division multiplexing (TDM), employing a multi-channel parallel transmission pulse sequence coding structure. The coding gain provided by this scheme enhances the system's signal-to-noise ratio. Furthermore, compared to traditional TDM techniques, which suffer from the limitation of long measurement times due to transmitting only one sequence code per measurement, this scheme significantly shortens the measurement time by using multiple channels to transmit multiple sequence codes in parallel, achieving high-precision and rapid measurement while maintaining spatial resolution.

[0027] This invention proposes a time-frequency division multiplexing (TFD) Brillouin time-domain reflectometer (BOTDR) based on pulse sequence coding. This system combines frequency division multiplexing with a coding scheme. By replacing traditional single pulses with coded sequences, the system significantly improves performance without sacrificing sensing accuracy or detection duration, enabling distributed sensing of temperature or strain over long distances with high precision. Simultaneously, the multi-frequency light source generated by the frequency-shifting loop avoids the problem of maximizing coherent detection gain inherent in traditional TFD methods. This technology not only effectively overcomes the limitations of traditional BOTDR systems but also provides a new approach to applying TFD in long-distance, high-precision distributed fiber optic sensing, possessing significant application value in fields such as structural health monitoring and environmental exploration. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the structure of the pulse sequence coding-based time-frequency multiplexing Brillouin time-domain reflectometer (BOTDR) in this invention.

[0029] Figure 2 This is a schematic diagram of the frequency shifting ring in this invention.

[0030] Figure 3 This is a schematic diagram of the multi-frequency sequence encoding generation in this invention. Detailed Implementation

[0031] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0032] This embodiment provides a time-frequency division multiplexing Brillouin time-domain reflectometer (BOTDR) based on pulse sequence coding, the structure of which is as follows: Figure 1 As shown, the system specifically includes: a narrow-linewidth laser 1, a first coupler 2, a frequency-shifting loop 3, a first intensity modulator 4, a first amplifier 5, a first filter 6, a fiber optic circulator 7, an optical fiber under test 8, a second amplifier 9, a second filter 10, a first polarization controller 11, an electro-optic modulator 12, a photodetector 14, and a signal processing unit; wherein, the narrow-linewidth laser 1 emits continuous laser light, which is divided into probe light and intrinsic light by the first coupler 2, and enters the probe path and intrinsic path respectively; the frequency-shifting loop 3, the first intensity modulator 4, the first amplifier 5, the first filter 6, the first filter 7, the fiber optic circulator 8, the fiber under test 9, the second amplifier 9, the second filter 10, the first polarization controller 11, the electro-optic modulator 12, the photodetector 14, and the signal processing unit; wherein, the narrow-linewidth laser 1 emits continuous laser light, which is divided into probe light and intrinsic light by the first coupler 2, and enters the probe path and intrinsic path respectively; the frequency-shifting loop 3, the first intensity modulator 4, the first amplifier 5, the first filter 6, the first filter 7, the fiber optic circulator 8, the fiber under test 9, the second amplifier 9, the second filter 10, the first polarization controller 11, the first polarization controller 12, the first polarization controller 13, the first polarization controller 14, the first polarization controller 15, the first polarization controller 16, the first polarization controller 17, the first polarization controller 18, the first polarization controller 19, the first polarization controller 10, the first polarization controller 11, the first polarization controller 12, the first polarization controller 13, the first polarization controller 14, the first polarization controller 12, the first polarization controller 13 The detector 5 and the first filter 6 are connected in sequence to form a detection path; the fiber optic circulator 7 has three ports, one port is connected to the end of the first fiber Bragg grating 6, the second port is connected to the fiber under test 8, the Brillouin return light of the fiber under test 8 is emitted from the third port, and the third port is connected to the return light path; the return light path is formed by the second amplifier 9 and the second filter 10 connected in sequence; the intrinsic path is formed by the first polarization controller 11 and the electro-optic modulator 12, and the output of the intrinsic path and the Brillouin return light are both connected to the 2×2 coupler 13 and detected by the photodetector 14.

[0033] Furthermore, the electro-optic modulator 12 is a carrier-suppressed single-sideband modulator, and the generated intrinsic light single-sideband frequency shift is near the Brillouin frequency shift; the first filter 6 can increase the extinction ratio of the pulse train and reduce the threshold of modulation instability; the second filter 10 is used to reduce Rayleigh scattering in the return light.

[0034] In this embodiment, the center wavelength of the narrow linewidth single-frequency laser 1 is 1550nm and the power is 20dBm. The coupling device 2 performs intensity distribution on the optical power output by the continuous laser according to the splitting ratio parameter. The splitting ratio is 50:50, and the generated continuous light is divided into two paths: the upper branch is the probe light and the lower branch is the intrinsic light.

[0035] Furthermore, the frequency shifting loop 3 is used to modulate the continuous probe light into pulses of different frequencies to generate multiple transmission channels. It is a key structure for the frequency division multiplexing deployment of this invention, and its specific structure is as follows: Figure 2As shown, it specifically consists of a second intensity modulator 3-1, a second coupler 3-2, a second polarization controller 3-3, a polarization beam splitter 3-4, a first acousto-optic modulator 3-5, a third amplifier 3-6, a third filter 3-7, a delay fiber 3-8, and an optical isolator 3-9. The continuous probe light is modulated by the second intensity modulator 3-1 into a probe pulse of a preset pulse duration, which enters the second coupler. Part of the pulse is output as a frequency shift loop, and the other part is retained in the frequency shift loop. After passing through the second polarization controller 3-3 and the polarization beam splitter 3-4, the light is then... The frequency of the probe pulse is modulated by the first acousto-optic modulator 3-5, then amplified by the third amplifier 3-6 and filtered by the third filter 3-7 to remove spontaneous emission noise. After passing through the delay fiber 3-8 and the optical isolator 3-9, it passes through the second coupler again. Part of it is output, and the other part continues to enter the frequency shifting loop for further frequency shifting. This process is repeated to modulate a single probe pulse into a pulse train composed of different frequency components. Subsequently, the frequency-division multiplexed pulse train is modulated into a pulse sequence encoding by the first intensity modulator 4 and then time-division multiplexed.

[0036] Furthermore, the polarization controller 3-3 and the polarization beam splitter 3-4 are used to control the polarization within the loop to stabilize the polarization-related loss within the loop; the third amplifier 3-6 is used to balance the loss within the loop; the third filter 3-7 is used to filter out the spontaneous emission noise caused by the amplification within the loop; the delay fiber 3-8 is used to avoid excessive Kerr effect caused by pulse overlap in the pulse train; and the optical isolator 3-9 is used to prevent backlight from damaging the devices within the loop.

[0037] Unlike traditional frequency division multiplexing systems, the frequency components in this embodiment are spaced 120MHz apart. Therefore, the same first amplifier 5 and first filter 6 can be used for amplification and filtering of the frequency channels, ensuring a high extinction ratio entering the fiber under test. This greatly reduces the impact of modulation instability on the system and improves the system signal-to-noise ratio at low cost. The spontaneous Brillouin return light generated by the fiber under test 8 is emitted from the three ports of the circulator 7. The scattered light is amplified by the second amplifier 9, and the Rayleigh scattering component is filtered out by the second filter 10. In the intrinsic light path, the intrinsic light is frequency-shifted by carrier-suppressed single-sideband modulation through the electro-optic modulator 12. The frequency shift is 10.8GHz, which is close to the frequency shift at room temperature. The frequency-shifted intrinsic light and the return light are coupled by a 2×2 coupler 13 and detected by a balanced photodetector 14. The detected electrical signal is further processed by time-frequency analysis.

[0038] Furthermore, in order to freely control the number of pulses in the pulse train while reducing the influence of noise within the loop, the second intensity modulator 3-1 and the first acousto-optic modulator 3-5 in the frequency shift loop 3 are controlled by an arbitrary waveform generator, and the control timing is as follows: Figure 3As shown; specifically, the pulse width and period modulated by the second acousto-optic modulator 3-1 determine the sensing spatial resolution and sensing distance; the modulation time of the first acousto-optic modulator 3-5 in the loop is slightly greater than the pulse width to ensure that the entire pulse can be completely modulated and frequency shifted, while its timing period corresponds to the entire loop length, thus ensuring that the first acousto-optic modulator 3-5 is only in working state to achieve pulse frequency shifting when the optical pulse passes through the first acousto-optic modulator 3-5 in the loop. When the first acousto-optic modulator is in the off state, the pulse energy in the loop will be quickly exhausted due to its large insertion loss, thus realizing the control of the number of pulses by controlling the timing of the first acousto-optic modulator.

[0039] In this embodiment, the modulation frequency of the first acousto-optic modulator 3-5 within the frequency shift loop 3 is 120MHz, meaning there are 16 channels (N=16) with a frequency interval of 120MHz, resulting in a total bandwidth of 1.92G. The encoding sequence uses 63-bit simplex encoding, with each symbol lasting 40ns (corresponding to a 4m spatial resolution). Therefore, the preset pulse duration of the probe pulse output by the second intensity modulator 3-1 is 40×64ns. A single measurement simultaneously inputs 16 sets of codes, and the entire measurement is repeated 4 times to ensure that all sets of codes are measured. Compared to traditional encoding schemes, this invention reduces the total measurement time by 16 times, maximizes the intrinsic power of coherent detection, and improves the signal-to-noise ratio due to the encoding gain. , where L is the encoding length.

[0040] The time-frequency analysis and signal acquisition processing section used in this embodiment mainly includes a balanced photodetector, a high-speed digital oscilloscope, and a computer. The balanced photodetector has a bandwidth of 2 GHz, which is greater than the total bandwidth required by the system. The photodetector converts the beat frequency signals of the spontaneously scattered light from the Leybow and the reference local light into electrical signals and sends them to the high-speed oscilloscope. The high-speed oscilloscope has a sampling rate of 5 GS / s@8-bit resolution, and then the computer completes the demodulation. The specific process is as follows:

[0041] Step 1: For the electrical signal acquired by the oscilloscope, slide the window with the symbol duration (40ns) as the step size, and perform fast Fourier transform on the extracted signal in sequence to obtain the Brillouin gain spectrum at each position along the optical fiber. The spatial interval between adjacent positions is 4m (equal to the spatial resolution) to prevent information loss.

[0042] Step 2: After the FFT processing of different time segments in Step 1, since there is a time delay between different frequency components in the pulse train, and the delay is determined by the length of the frequency shift loop, it is necessary to perform delay compensation for each frequency component before accumulating and averaging all frequency channels to align the different frequency components. The channel delay compensation process includes: taking the length of the frequency shift loop as the reference, denoted as t0, and then performing (i-1)×t0 delay compensation for the i-th frequency channel to compensate for the different time delays of each frequency component.

[0043] Step 3: After compensating for the delay, taking simplex encoding as an example, the signals of the 16 channels throughout the entire measurement time are divided according to the coding sequence period. For the data matrix formed, the intensity data of each time segment is linearly decoupled by applying the simplex inverse matrix. Finally, the 64 decoded single-pulse responses are time-aligned so that they correspond to the same position on the optical fiber. The aligned responses are accumulated and averaged to obtain the enhanced single-pulse time domain response.

[0044] Step 4: Decoding. Taking simplex encoding as an example, the signal within the entire measurement time T of N frequency channels is divided into M segments according to the encoding sequence period Ts, forming an L×Ts data matrix, where L=N×M. The intensity data of each time segment is linearly decoupled by applying the simplex inverse matrix.

[0045] Step 5: Time-domain alignment and cumulative averaging. The L decoded single-pulse responses are time-aligned so that they correspond to the same position on the optical fiber. The aligned responses are then cumulatively averaged to obtain the enhanced single-pulse time-domain response.

[0046] Step 6: Fit the enhanced response of each spatial point to a Lorentz curve, reconstruct the Brillouin gain spectrum, obtain the Brillouin offset of each spatial point of the fiber under test using the pulse time-of-flight method, and calculate the temperature value along the fiber under test by using the linear relationship between the Brillouin offset and temperature, thereby realizing distributed temperature sensing along the fiber under test.

[0047] In summary, this invention proposes a frequency division multiplexing (FDM) Brillouin time-domain reflectometer based on pulse sequence coding. By introducing a delay circuit, it effectively avoids the increase in peak optical power caused by the superposition of multiple frequency components, thereby suppressing the noise impact of nonlinear effects. Through the design of a frequency shift loop, the frequency spacing of the frequency channels is made small, allowing multiple frequency channels to achieve signal-to-noise ratio (SNR) optimization using a single amplifier and fiber Bragg grating. Simultaneously, at the receiving end, heterodyne detection is performed using a single intrinsic light, maximizing the intrinsic light power and improving the detection SNR. This overcomes the limitations of traditional FDM techniques in achieving theoretically high SNR. This solution overcomes the limitations of traditional frequency division multiplexing (FDM) and time division multiplexing (TDM) BOTDR systems by integrating a multi-channel parallel transmission pulse sequence coding structure. While maintaining spatial resolution and coding gain, it significantly shortens measurement time and improves the signal-to-noise ratio, achieving high-precision, fast, distributed sensing. This invention effectively overcomes the limitations of traditional FDM and TDM BOTDR systems and organically combines the two, providing an innovative approach for long-distance, high-precision temperature and strain monitoring. It has significant application value in fields such as structural health monitoring and environmental exploration.

[0048] The above description is merely a specific embodiment of the present invention. Any feature disclosed in this specification may be replaced by other equivalent or similar features unless otherwise specified. All disclosed features, or steps in all methods or processes, may be combined in any way except for mutually exclusive features and / or steps.

Claims

1. A time-frequency division multiplexed Brillouin time domain reflectometer based on pulse sequence coding, characterized in that, include: The system comprises a narrow-linewidth laser (1), a first coupler (2), a frequency-shifting loop (3), a first intensity modulator (4), a first amplifier (5), a first filter (6), an optical fiber circulator (7), an optical fiber under test (8), a second amplifier (9), a second filter (10), a first polarization controller (11), an electro-optic modulator (12), a 2×2 coupler (13), a photodetector (14), and a signal processing unit; wherein: The narrow linewidth laser (1) emits continuous laser light, which is divided into probe light and intrinsic light by the first coupler (2) and enters the probe path and intrinsic path respectively. The detection route frequency shift ring (3), the first intensity modulator (4), the first amplifier (5) and the first filter (6) are connected in sequence to form a light pulse train composed of multiple frequency components; The intrinsic route is formed by connecting the first polarization controller (11) and the electro-optic modulator (12); The output of the detection path is output from one port of the fiber optic circulator (7) to the second port and enters the fiber under test (8). The Brillouin return light of the fiber under test is transmitted from the second port of the fiber optic circulator (7) to the third port and emitted to the return path. The return path is formed by connecting the second amplifier (9) and the second filter (10); The outputs of the intrinsic path and the return path are both connected to a 2×2 coupler (13) and detected by a photodetector (14).

2. The time and frequency division multiplexed Brillouin time domain reflectometer based on pulse sequence coding according to claim 1, characterized in that, The frequency shifting loop (3) includes: a second intensity modulator (3-1), a second coupler (3-2), a second polarization controller (3-3), a polarization beam splitter (3-4), a first acousto-optic modulator (3-5), a third amplifier (3-6), a third filter (3-7), a time-delay fiber (3-8), and an optical isolator (3-9); wherein: The probe light is modulated by the second acousto-optic modulator to form a probe pulse with a preset pulse width, and then enters the 3dB coupler. One output of the 3dB coupler serves as the output of the frequency shift loop, and the other output enters the frequency shift loop. In the frequency shift loop, the probe pulse passes sequentially through the second polarization controller, the polarization beam splitter, the first acousto-optic modulator, the third amplifier, the third filter, the delay fiber and the optical isolator, and then passes through the second coupler again. This process is repeated to modulate a single probe pulse into an optical pulse train composed of multiple frequency components.

3. The time and frequency division multiplexed Brillouin time domain reflectometer based on pulse sequence coding according to claim 2, characterized in that, The second intensity modulator (3-1) and the first acousto-optic modulator (3-5) are controlled by an arbitrary waveform generator. The pulse width of the second intensity modulator corresponds to the sensing spatial resolution. The modulation frequency of the first acousto-optic modulator corresponds to the frequency interval of each frequency component in the optical pulse train.

4. The time and frequency division multiplexed Brillouin time domain reflectometer based on pulse sequence coding according to claim 1, characterized in that, The electro-optic modulator is a carrier-suppressed single-sideband modulator.

5. The time and frequency division multiplexed Brillouin time domain reflectometer based on pulse sequence coding according to claim 1, characterized in that, The first intensity modulator is controlled by an arbitrary waveform generator to modulate the output optical pulse train of the frequency shift loop into a continuous and sequentially arranged pulse sequence encoding.

6. The time and frequency division multiplexed Brillouin time domain reflectometer based on pulse sequence coding according to claim 1, characterized in that, The signal processing unit stores the electrical signal of the photodetector and performs time-frequency analysis, including: Fast Fourier Transform, channel delay compensation, decoding, and Lorentz fitting; wherein: The Fast Fourier Transform includes: performing Fast Fourier Transform sequentially over the entire cycle with the pulse width as the window to obtain the Brillouin gain spectrum of each frequency component at each moment of the entire cycle. The channel delay compensation includes: compensating for the time delay of each frequency component based on the frequency shift ring length; The decoding includes: using the corresponding decoding method for different encoding methods; The Lorentz fitting includes: fitting the decoded result to a Lorentz curve, reconstructing the Brillouin gain spectrum, and calculating the temperature value along the fiber under test through the linear relationship between the Brillouin offset and temperature, thereby realizing distributed sensing of temperature or strain along the fiber under test.

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