High-precision demodulation system of long-distance optical fiber sensor

Abstract

The invention provides a high-precision demodulation system of a long-distance optical fiber sensor, which relates to the technical field of distributed optical fiber sensing and comprises a sweep frequency laser, a power divider, a semiconductor optical amplifier, a circulator, a grating array, a photoelectric detector, a data acquisition card and a computer, the system is characterized in that the frequency sweep laser is used for scanning wavelength in a stepping mode, and one trigger pulse is output to the power divider when one wavelength value is stepped; the power divider is used for outputting the trigger pulse in two paths, and one path is transmitted to the electric input end of the semiconductor optical amplifier; according to the invention, by optimizing a system architecture, namely a synchronous triggering and demodulation algorithm of a sweep frequency laser, a semiconductor optical amplifier and a data acquisition card, namely matrix reconstruction and local highest peak searching, wavelength and position information of a weak grating array in a long-distance optical fiber can be accurately extracted at the same time, and the limitation that only single-point demodulation can be realized or synchronous positioning cannot be realized in the prior art is broken through; and the distributed multi-point monitoring requirement is met.

Description

Technical Field

[0001] This invention relates to the field of distributed optical fiber sensing technology, and in particular to a high-precision demodulation system for long-distance optical fiber sensors. Background Technology

[0002] In recent years, distributed fiber optic sensing systems have developed rapidly due to their advantage of long-distance real-time detection of environmental variables. Based on optical time domain reflectometry (OTDR), they monitor environmental variables along the entire fiber by detecting backscattered light at various points in the fiber. They have been widely used in long-distance, large-scale monitoring scenarios such as oil and gas pipelines, tunnels, and bridges. To improve measurement accuracy and flexibly adjust monitoring points, existing technologies propose etching grating arrays in optical fibers. Among them, identical weak grating arrays have become the core component of long-distance distributed fiber optic sensing technology due to their low reflection loss, weak signal crosstalk, and high multiplexing capacity. However, their demodulation process is easily affected by grating reflection spectrum distortion caused by device imperfections, making it difficult to achieve accurate demodulation of wavelength and position simultaneously.

[0003] Existing demodulation technologies have significant limitations: the fiber optic grating wavelength demodulation method based on DFB lasers disclosed in patent 201610296552.0, although requiring no complex equipment and having low requirements for reflected light power, is only applicable to demodulation of a single point or a limited number of gratings with different center wavelengths, and cannot meet the needs of long-distance distributed multi-point synchronous monitoring; the light intensity step frequency modulation demodulation device and method disclosed in patent 202411202656.1 achieves the testing of identical weak grating arrays through spectral stitching, but it does not effectively solve the impact of system noise on demodulation accuracy, and it is difficult to ensure the synchronous and accurate identification of the position and wavelength of weak gratings in long-distance scenarios, which cannot meet the high requirements of data accuracy and stability of long-distance fiber optic sensing systems. Therefore, this invention proposes a high-precision demodulation system for long-distance fiber optic sensors to solve the problems existing in the prior art. Summary of the Invention

[0004] To address the aforementioned issues, this invention proposes a high-precision demodulation system for long-distance fiber optic sensors. This system optimizes the system architecture: a swept-frequency laser, a semiconductor optical amplifier, and a data acquisition card, and uses a synchronous triggering and demodulation algorithm: matrix reconstruction + local peak finding. This allows for the simultaneous and accurate extraction of wavelength and position information of weak grating arrays in long-distance optical fibers, overcoming the limitations of existing technologies that can only perform single-point demodulation or cannot provide synchronous positioning, thus meeting the needs of distributed multi-point monitoring.

[0005] To achieve the objectives of this invention, the following technical solution is provided: A high-precision demodulation system for a long-distance fiber optic sensor, comprising a swept-frequency laser, a power divider, a semiconductor optical amplifier, a circulator, a grating array, a photodetector, a data acquisition card, and a computer. The swept-frequency laser is used to scan wavelengths in a step-by-step manner, and outputs a trigger pulse to the power divider for each step of a wavelength value. The power divider is used to split the trigger pulse into two outputs: one output is transmitted to the electrical input terminal of the semiconductor optical amplifier, and the other output is transmitted to the trigger input terminal of the data acquisition card. The optical input terminal of the semiconductor optical amplifier is connected to the optical output terminal of the swept-frequency laser, and is used to generate pulsed light according to the received trigger pulse and input it to port I of the circulator.

[0006] The circulator's port 2 is connected to the grating array, used to transmit pulsed light to the grating array and receive the sensing light signal reflected by the grating array, then outputting it from port 3; the circulator's port 3 is connected to the optical input port of the photodetector, which converts the sensing light signal into an electrical signal and transmits it to the analog input terminal of the data acquisition card; the data acquisition card's communication output terminal is connected to the communication input terminal of a computer, which processes the electrical signal to demodulate the wavelength and position of each weak grating in the grating array.

[0007] A further improvement is that the sweep frequency parameters of the sweep laser include the starting wavelength. Termination wavelength, wavelength step length With step time interval The starting wavelength The step time interval is set according to the Bragg wavelength of the grating array, and the termination wavelength is determined by the step wavelength. satisfy ,in Let L be the refractive index of the fiber, L be the total length of the fiber containing the grating array, and c be the speed of light. .

[0008] A further improvement is that the semiconductor optical amplifier is in external modulation mode, and the pulse width output by the semiconductor optical amplifier is... satisfy ,in The distance between two adjacent weak gratings in the grating array is denoted as .

[0009] A further improvement is that the data acquisition card has two acquisition modes: single-trigger acquisition and continuous-trigger acquisition. Single-trigger acquisition involves setting the number of sampling points based on the time of one sweep of the frequency-sweeping laser. The data acquisition card begins acquisition after receiving the first trigger pulse and ends acquisition after the frequency-sweeping laser completes its sweep. Continuous-trigger acquisition involves setting the number of sampling points based on the time of one step of the frequency-sweeping laser. The data acquisition card begins acquisition after receiving each trigger pulse and waits for the next trigger pulse after acquisition is complete.

[0010] A further improvement is that the sampling rate of the data acquisition card is not less than twice the highest frequency of the sensing optical signal, and the total number of sampling points triggered by the data acquisition card in one acquisition is [not specified]. ,in Where is the sampling rate, T is the frequency sweep period, m is the number of times the frequency sweep laser is triggered, and n is the number of sampling points in a single step time.

[0011] A further improvement lies in the following: the computer's data processing of electrical signals includes a preprocessing step: reconstructing the one-dimensional electrical signal array transmitted by the data acquisition card into an m×n matrix, where m is the number of triggers of the frequency-sweeping laser, and n is the number of sampling points within a single step time; the horizontal axis of the matrix is... Convert to wavelength, ordinate by Converted to distance, t is the time from when the swept-frequency laser emits a pulse of light to when it receives the reflected signal.

[0012] A further improvement is that the computer's data processing of the electrical signal also includes a smoothing filtering step: performing smoothing filtering on each row of the m×n matrix to eliminate system noise.

[0013] A further improvement is that the computer's data processing of electrical signals also includes a peak-finding and identification step: using a matrix window that matches the size of the grating's reflection spectrum to traverse the m×n matrix, and when the center point of the matrix window is a local peak of the m×n matrix, the coordinates of that center point are recorded. ,coordinate These represent the wavelength and position of the weak grating, respectively.

[0014] Further improvements include: an optical attenuator is provided between the 3-port of the circulator and the optical input port of the photodetector; the optical connection between the sweep laser, the semiconductor optical amplifier, and the circulator is achieved through fiber optic patch cords and flanges; and the electrical connection between the power divider, the semiconductor optical amplifier, and the data acquisition card is achieved through radio frequency cables.

[0015] A further improvement is that the computer is also used to calibrate the temperature of the weak grating: the weak grating is placed in a water bath constant temperature chamber, the temperature of the water bath constant temperature chamber is set from 35℃ to 85℃, the wavelength of the weak grating is recorded every 10℃, the correspondence between temperature and wavelength is linearly fitted, and the fitting result is written into the temperature demodulation algorithm to calculate the temperature of its location through the wavelength of the weak grating.

[0016] The beneficial effects of this invention are as follows:

[0017] 1. This invention optimizes the system architecture: a frequency-sweeping laser-semiconductor optical amplifier-data acquisition card synchronous triggering and demodulation algorithm: matrix reconstruction + local peak finding, which can simultaneously and accurately extract the wavelength and position information of weak grating arrays in long-distance optical fibers. This breaks through the limitations of existing technologies that can only demodulate at a single point or cannot be synchronously located, and meets the needs of distributed multi-point monitoring.

[0018] 2. This invention features an optical attenuator to protect the photodetector, eliminates system noise through a smoothing filtering algorithm, and precisely designs the SOA pulse width and sweep frequency step time according to fiber parameters to effectively avoid signal crosstalk and reflection spectrum distortion, ensuring demodulation accuracy. Data acquisition supports two modes, which can be flexibly adapted according to fiber length and grating density, enhancing system applicability.

[0019] 3. This invention calibrates the temperature-wavelength linear relationship of the weak grating using a water bath constant temperature chamber and incorporates the calibration results into the demodulation algorithm, enabling the system to simultaneously monitor multiple parameters such as temperature and strain; the maximum detection length of the system is determined by the frequency sweep step time. It can be adapted to different long-distance monitoring scenarios and is highly practical. Attached Figure Description

[0020] Figure 1 This is the front view of the present invention;

[0021] Figure 2 This is a schematic diagram of a data acquisition method according to the present invention;

[0022] Figure 3 This is a schematic diagram of the second data acquisition method of the present invention;

[0023] Figure 4 This is a schematic diagram of an m×n matrix according to the present invention;

[0024] Figure 5 This is a schematic diagram of the smoothing filtering process of the present invention;

[0025] Figure 6 This is a schematic diagram showing the position and wavelength of the weak grating in Embodiment 1 of the present invention;

[0026] Figure 7 This is a schematic diagram showing the position and wavelength of the weak grating in Embodiment 2 of the present invention;

[0027] Figure 8 This is a schematic diagram showing the correspondence between temperature and wavelength in this invention. Detailed Implementation

[0028] To enhance understanding of the present invention, the present invention will be further described in detail below with reference to embodiments. These embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0029] Example 1

[0030] according to Figure 1 , 2 As shown in Figures 3, 4, 5, 6, 7, and 8, this embodiment proposes a high-precision demodulation system for long-distance fiber optic sensors:

[0031] A frequency-sweeping laser scans wavelengths in steps, outputting a trigger pulse to a power divider for each wavelength increment. The power divider sends one trigger pulse to the electrical input of a semiconductor optical amplifier (SOA) and another to a data acquisition card. The optical input of the SOA is connected to the frequency-sweeping laser, and the SOA generates pulsed light based on the received trigger pulses, which enters port 1 of a circulator. Port 2 of the circulator outputs to a grating array, generating a sensing light signal that is reflected back to the circulator and enters the optical input port of a photodetector from port 3. The output of the photodetector is connected to the input of the data acquisition card, and the output of the data acquisition card is connected to a computer. Finally, the sensing signal data is processed on the computer. See the schematic diagram for a detailed scheme. Figure 1 .

[0032] The present invention provides two data acquisition methods: one is to set the number of sampling points of the data acquisition card according to the time it takes for the frequency sweep laser to complete one frequency sweep, and the data acquisition card starts one acquisition after receiving the first pulse emitted by the frequency sweep laser, and the acquisition ends after the frequency sweep laser completes the frequency sweep; the other is to set the number of sampling points of the data acquisition card according to the time it takes for the laser to complete one step, and the data acquisition card starts acquisition every time it receives a trigger pulse, and waits for the next trigger after the acquisition is completed.

[0033] A frequency-sweeping laser sends a trigger pulse for each wavelength increment. Assuming the laser steps i times, it amplifies its output light and generates a single optical pulse that enters the grating array. Since the frequency-sweeping laser operates at fixed time intervals... The sweeping laser's starting wavelength is denoted as [wavelength value]. The frequency-sweeping laser increases the wavelength by one step. When the collection time is At this time, the wavelength is .

[0034] This invention proposes a high-precision demodulation algorithm for long-distance fiber optic sensors:

[0035] First, the sensor optical signal acquired by the data acquisition card is preprocessed. When the data acquisition card uses the first acquisition method, after the frequency sweep laser completes one frequency sweep, the acquired data uploaded by the data acquisition card to the computer is a one-dimensional array. This array is then timed according to the wavelength. Divide into equal parts, with m trigger counts. If n sampling points are collected within a given time period, the array is reconstructed into an m×n matrix for the next step of demodulation, such as... Figure 4 As shown. When the data acquisition card adopts the first acquisition method, the acquired data uploaded by the data acquisition card to the computer is a one-dimensional array of m segments. The array is concatenated and reconstructed into an m×n matrix for the next step of demodulation.

[0036] According to the formula, the horizontal axis of an m×n matrix can be converted into wavelength, and the vertical axis is determined by the formula... Where n1 is the refractive index of the optical fiber, D represents the test distance, c is the speed of light (3×10⁸ m / s), and t represents the time taken from the emission of the light pulse to the receipt of the reflected light signal, which can be converted into distance. Therefore, each local peak represents a weak grating.

[0037] Smoothing filtering is performed on each row of the m×n matrix to eliminate the influence of system noise, which can effectively improve demodulation accuracy.

[0038] Using a small window whose size matches the grating's reflection spectrum, the entire matrix is ​​traversed. When the center of the matrix satisfies the characteristics of a grating reflection peak, the position of that point is recorded. By finding the coordinates of all local peaks, the positions and wavelengths of all weak gratings can be demodulated. Figure 6 As shown.

[0039] Specifically, it includes the following steps:

[0040] The start and end wavelength settings of the swept laser must encompass all Bragg wavelengths of the entire weak grating array. The time interval for wavelength stepping by the swept laser depends on the total length of the fiber etched with the weak grating array. Each wavelength step generates a trigger signal, and the step time between two trigger signals is... To ensure that the reflected signal from the previous trigger signal has returned to the photodetector after passing through the end of the optical fiber, the interval time is calculated as: optical fiber length × 2 refractive index / speed of light.

[0041] The semiconductor optical amplifier (SOA) operates in external modulation mode, meaning it outputs a light pulse of a specified width upon receiving a trigger signal. The pulse width depends on the distance between two adjacent gratings in the grating array; a single pulse width can only contain one grating. Specifically, the pulse width = grating spacing × refractive index / speed of light.

[0042] Reference Figure 1 The various instruments are connected to build a device for wavelength and position measurement of weak fiber optic grating arrays, as proposed in this invention, and the acquired sensor signals are processed by a computer.

[0043] Configure the parameters of the sweep laser: start wavelength, end wavelength, step period (time for one wavelength step), step length (wavelength for one wavelength step), and number of sweeps; configure the data acquisition card acquisition mode: single-trigger acquisition, continuous-trigger acquisition, and number of sampling points.

[0044] The frequency sweep laser begins a frequency sweep task from the starting wavelength to the ending wavelength. When the acquisition card triggers acquisition once, with a sampling rate of S0, the sampling time of one frequency sweep cycle is T, the total number of sampling points S is S0×T, the number of times the frequency sweep laser is triggered is m, and the number of points acquired in one step cycle is... The total number of sampling points S is equal to m×n.

[0045] The data acquisition card uploads a one-dimensional array of length S to the computer. The computer reconstructs the array into an m×n matrix, according to the formula... and In this matrix, the number of columns m represents the wavelength, and the number of rows n represents the position.

[0046] Each row of the m×n matrix is ​​smoothed and filtered to eliminate system noise.

[0047] Based on the size of the area occupied by a weak raster in a two-dimensional matrix, a matrix window of the corresponding size is set. This window will traverse the entire matrix. If the center point of the small window is a local peak of the two-dimensional matrix, the x and y coordinates of that point are recorded. The horizontal axis represents wavelength, and the vertical axis represents position. The coordinates of each weak grating are recorded one by one to complete the wavelength and position demodulation of the weak grating array.

[0048] Example 2

[0049] according to Figure 1 , 2As shown in Figures 3, 4, 5, 6, 7, and 8, this embodiment proposes a high-precision demodulation system for long-distance fiber optic sensors, involving the construction and application of a fiber optic sensing and measurement system. This system uses a tunable laser as a trigger source to simultaneously drive a semiconductor optical amplifier (SOA) and trigger a data acquisition card. The data acquisition card can acquire the sensing optical signal in two ways and finally upload it to a computer for data processing.

[0050] The maximum detection fiber length of the system is determined by the sweeping speed of the frequency-sweeping laser. The specific formula for ensuring that the optical pulse signal of that wavelength completes one round trip in the fiber optic link is as follows: Where c is the speed of light, generally considered to be 3 × 10⁸ m / s; n1 is the refractive index of the fiber; and Δt is the time taken for the swept laser to advance one wavelength per step.

[0051] The specific implementation steps are as follows:

[0052] according to Figure 1 The device's electrical and optical links are connected as shown; the device parameters are set; the acquired optical signals are preprocessed; and the wavelength and position of the weak grating array are extracted using the proposed demodulation algorithm.

[0053] To synchronize the start of data acquisition by the data acquisition card with the start of laser frequency sweep, the trigger output port of the tunable laser is connected to the electrical input of the semiconductor optical amplifier (SOA) and the trigger input of the data acquisition card via a power divider.

[0054] To generate an optical pulse signal for input into the long-distance fiber optic link under test, the semiconductor optical amplifier (SOA) is set to external trigger mode. The trigger signal from the sweep laser triggers the SOA to output an optical pulse signal. The optical output of the SOA is reflected back to the circulator through a weak grating and output from port 3 of the circulator to a photodetector. The photodetector converts the optical signal into an electrical signal, which is then input to the analog input of the data acquisition card.

[0055] All electrical connections are made via radio frequency cables.

[0056] The optical link connection is as follows: the optical output of the swept laser is connected to the optical input of the semiconductor optical amplifier (SOA); the optical output of the SOA is further connected to port 1 of the circulator; port 2 of the circulator is connected to the grating array under test; and port 3 of the circulator is connected to the optical input of the photodetector.

[0057] The optical connection is connected via fiber optic patch cords and flanges. An optical attenuator needs to be connected between the circulator's three ports and the photodetector to avoid affecting the photodetector's performance.

[0058] The parameters set for a frequency-sweeping laser include the start wavelength, end wavelength, step period (time for one wavelength step), step size (wavelength for one wavelength step), and number of sweeps. The start and end wavelengths are set based on the center wavelength of the weak grating array contained in the sensing fiber. The sweep speed can be set relatively slowly, but the range of sweep wavelengths set cannot exceed the sweep period time obtained by the sweep speed alone, which is less than the time that the data acquisition card can acquire data in one pass.

[0059] Based on the turn-on voltage required by the semiconductor optical amplifier (SOA), the amplitude of the trigger pulse generated by the sweep laser should be greater than the trigger voltage amplitude of 3.3V.

[0060] The pulse width t2 of the semiconductor optical amplifier (SOA) is determined by the distance L1 between gratings on the fiber optic link of the long-distance fiber optic sensor, as shown in the formula: .

[0061] According to the Nyquist sampling theorem, the data acquisition card samples the sensing optical signal at a sampling rate of no less than twice the highest frequency of the signal. The resulting discrete sampled values ​​can accurately determine the original signal. Taking the optical pulse width as an example, if the optical pulse width is 10ns, the sampling rate needs to be no less than 200MS / s in order to accurately restore the signal.

[0062] The weak grating array to be tested contains a set of weak gratings with different or the same central Bragg wavelengths, and the corresponding reflection peak waveforms can be detected by the method of the present invention.

[0063] The sensing fiber contains 100 weak gratings, all with the same central Bragg wavelength. A laser initiates a frequency sweep to trigger a data acquisition card to acquire the sensing optical signal. The signal undergoes matrix reconstruction preprocessing and noise removal. Using a local maximum peak finding algorithm, the positions and wavelengths of the 100 weak gratings are extracted. The results are as follows: Figure 7 As shown.

[0064] It also includes temperature testing methods and demodulation algorithms: a weak grating is placed in a water bath constant temperature chamber, the chamber temperature is set to continuously increase from 35℃ to 85℃, the wavelength demodulated by the weak grating is recorded every 10℃, and the correspondence between temperature and wavelength is plotted. Figure 8 Linear fitting was performed, achieving a linearity of 0.9995. By calibrating the temperature response of the weak gratings, the calibration results were written into the temperature demodulation algorithm, allowing the temperature at each grating location to be calculated using wavelength.

[0065] This high-precision demodulation system for long-distance fiber optic sensors optimizes the system architecture: a swept-frequency laser, a semiconductor optical amplifier, and a data acquisition card, with synchronous triggering and demodulation algorithms including matrix reconstruction and local peak finding. This allows for the simultaneous and accurate extraction of wavelength and position information from weak grating arrays in long-distance optical fibers, overcoming the limitations of existing technologies that only offer single-point demodulation or cannot provide synchronous positioning, thus meeting the needs of distributed multi-point monitoring. Furthermore, this invention incorporates an optical attenuator to protect the photodetector, eliminates system noise through a smoothing filtering algorithm, and precisely designs the SOA pulse width and swept-frequency step time according to fiber parameters, effectively avoiding signal crosstalk and reflection spectrum distortion, ensuring demodulation accuracy. Data acquisition supports two modes, which can be flexibly adapted according to fiber length and grating density, enhancing system applicability. Simultaneously, this invention calibrates the temperature-wavelength linear relationship of the weak grating using a water bath constant temperature chamber and integrates the calibration results into the demodulation algorithm, enabling the system to simultaneously monitor multiple parameters such as temperature and strain. The maximum detection length of the system is determined by the swept-frequency step time. It can be adapted to different long-distance monitoring scenarios and is highly practical.

[0066] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A high-precision demodulation system for a long-distance fiber optic sensor, comprising a swept-frequency laser, a power divider, a semiconductor optical amplifier, a circulator, a grating array, a photodetector, a data acquisition card, and a computer, characterized in that: The frequency-sweeping laser is used to scan wavelengths in a step-by-step manner, and outputs a trigger pulse to the power divider for each wavelength step. The power divider is used to split the trigger pulse into two outputs: one output is transmitted to the electrical input terminal of the semiconductor optical amplifier, and the other output is transmitted to the trigger input terminal of the data acquisition card. The optical input terminal of the semiconductor optical amplifier is connected to the optical output terminal of the frequency-sweeping laser, and is used to generate pulsed light according to the received trigger pulse and input it to port 1 of the circulator. The circulator's port 2 is connected to the grating array, used to transmit pulsed light to the grating array and receive the sensing light signal reflected by the grating array, then outputting it from port 3; the circulator's port 3 is connected to the optical input port of the photodetector, which converts the sensing light signal into an electrical signal and transmits it to the analog input terminal of the data acquisition card; the data acquisition card's communication output terminal is connected to the communication input terminal of a computer, which processes the electrical signal to demodulate the wavelength and position of each weak grating in the grating array.

2. The high-precision demodulation system for a long-distance fiber optic sensor according to claim 1, characterized in that: The frequency sweeping parameters of the frequency sweeping laser include the starting wavelength. Termination wavelength, wavelength step length With step time interval The starting wavelength The step time interval is set according to the Bragg wavelength of the grating array and the termination wavelength is determined by the step time interval. satisfy ,in Let L be the refractive index of the fiber, L be the total length of the fiber containing the grating array, and c be the speed of light. .

3. The high-precision demodulation system for a long-distance fiber optic sensor according to claim 1, characterized in that: The semiconductor optical amplifier is in external modulation mode, and the pulse width output by the semiconductor optical amplifier is... satisfy ,in The distance between two adjacent weak gratings in the grating array is denoted as .

4. The high-precision demodulation system for a long-distance fiber optic sensor according to claim 1, characterized in that: The data acquisition card has two acquisition modes: single-trigger acquisition and continuous-trigger acquisition. The single-trigger acquisition is as follows: the number of sampling points is set according to the time of one frequency sweep of the frequency sweeping laser; the data acquisition card starts acquisition after receiving the first trigger pulse; and the acquisition ends after the frequency sweeping laser completes the frequency sweep. The continuous trigger acquisition is as follows: the number of sampling points is set according to the step time of one cycle of the frequency sweep laser, and the data acquisition card starts acquisition every time it receives a trigger pulse, and waits for the next trigger pulse after the acquisition is completed.

5. The high-precision demodulation system for a long-distance fiber optic sensor according to claim 4, characterized in that: The sampling rate of the data acquisition card is not less than twice the highest frequency of the sensing optical signal, and the total number of sampling points triggered by the data acquisition card in one operation is [not specified]. ,in Where is the sampling rate, T is the frequency sweep period, m is the number of times the frequency sweep laser is triggered, and n is the number of sampling points in a single step time.

6. The high-precision demodulation system for a long-distance fiber optic sensor according to claim 1, characterized in that: The computer's data processing of electrical signals includes a preprocessing step: reconstructing the one-dimensional electrical signal array transmitted by the data acquisition card into an m×n matrix, where m is the number of triggers of the frequency-sweeping laser and n is the number of sampling points within a single step time; the horizontal axis of the matrix is... Convert to wavelength, ordinate by Converted to distance, t is the time from when the swept-frequency laser emits a pulse to when it receives the reflected signal.

7. The high-precision demodulation system for a long-distance fiber optic sensor according to claim 6, characterized in that: The computer's data processing of the electrical signal also includes a smoothing filtering step: performing smoothing filtering on each row of the m×n matrix to eliminate system noise.

8. The high-precision demodulation system for a long-distance fiber optic sensor according to claim 7, characterized in that: The computer's data processing of electrical signals also includes a peak-finding identification step: traversing the m×n matrix using a matrix window matching the size of the grating's reflection spectrum, and recording the coordinates of the center point when the center point of the matrix window is a local peak of the m×n matrix. ,coordinate These represent the wavelength and position of the weak grating, respectively.

9. A high-precision demodulation system for a long-distance fiber optic sensor according to claim 1, characterized in that: An optical attenuator is provided between the 3-port of the circulator and the optical input port of the photodetector; the optical connection between the sweep laser, the semiconductor optical amplifier, and the circulator is achieved through fiber optic patch cords and flanges, and the electrical connection between the power divider, the semiconductor optical amplifier, and the data acquisition card is achieved through radio frequency cables.

10. A high-precision demodulation system for a long-distance fiber optic sensor according to claim 1, characterized in that: The computer is also used to calibrate the temperature of the weak grating: the weak grating is placed in a water bath constant temperature chamber, the temperature of the water bath constant temperature chamber is set from 35℃ to 85℃, the wavelength of the weak grating is recorded every 10℃, the correspondence between temperature and wavelength is linearly fitted, and the fitting result is written into the temperature demodulation algorithm to calculate the temperature of its location through the wavelength of the weak grating.

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