A high-spatial-resolution OTDR optical fiber temperature measurement system

Abstract

The present invention provides an OTDR optical fiber temperature measurement system with high spatial resolution, which includes a host computer, a data transmission line, a pulsed light source, a wavelength division multiplexing device, a photoelectric detector, a high-speed acquisition card, and a sensing and temperature measurement optical fiber. It is characterized in that: the external clock input end of the high-speed acquisition card is connected to the output end of a radio frequency signal source, and the radio frequency signal source emits a periodic pulse signal that is synchronized with the trigger pulse of the pulsed light source set in the external trigger mode; the radio frequency signal source is programmable, and the host computer controls the frequency of the signal output by the radio frequency signal source. On the basis of the original hardware structure of the distributed temperature measurement system, the present invention adds a radio frequency signal source, obtains multiple sampling results by adjusting the sampling frequency of the acquisition card, shortens the interval between sampling points, reconstructs the temperature curve with high spatial resolution, and realizes the improvement of spatial resolution.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber signal detection, and particularly relates to an OTDR optical fiber temperature measurement system with high spatial resolution. Background Art

[0002] In the future development of the traditional thermal power industry, new-era big data processing technologies, network communication technologies, advanced detection technologies, etc. will play important roles. The combustion optimization technology based on advanced detection technology is the main method for boiler combustion optimization in thermal power plants and is also one of the important contents of the construction of smart power plants. The distributed monitoring system for the application scenario of high-temperature parallel pipelines in thermal power plants provides the possibility for full-coverage temperature monitoring of the main steam in boilers and plays an important guiding role in the healthy operation of superheaters and reheaters in thermal power plants. The superheater consists of a large number of parallel pipelines and functions to absorb the heat of flue gas in the boiler system to raise the temperature of the water vapor in the pipelines. Research shows that ash deposition will lead to an increase in the thermal resistance during the heat transfer process and hinder the flow of flue gas; the metal of the heating surface will corrode, making the pipe wall brittle. When the main steam temperature reaches above 650 °C, serious high-temperature ash deposition and corrosion will occur after a long time of operation. Maintaining the temperature of the water vapor at about 540 °C can significantly inhibit the external corrosion and high-temperature ash deposition of the superheater and reheater.

[0003] Currently, the existing high-temperature measurement methods mainly include thermocouple sensors, optical radiation method, sapphire optical fiber temperature measurement, etc. The optical radiation method can collect energies of different wavelengths and calculate the temperature by the colorimetric method. This temperature measurement method uses an infrared optical system, which has a relatively complex structure and requires a lot of time to build a reasonable calculation model; for a large number of parallel high-temperature pipelines, the installation workload of the optical system is extremely large, and it is difficult to transmit optical signals over long distances. The sapphire temperature measurement optical fiber uses a sapphire single crystal optical fiber as the sensing head and measures the temperature by collecting the radiation signal in the optical fiber. There are currently various development directions for the sapphire temperature measurement optical fiber technology, but its application scenarios are limited by disadvantages such as high cost, large sensor volume, and the measured value being the ambient average temperature.

[0004] The detection of steam pipelines in high-temperature heat exchangers and superheaters in thermal power plants usually adopts the single-point measurement technology of thermocouples. There are defects in the way of using thermocouples to measure the temperature of high-temperature pipelines. Since the manufacturing materials of thermocouple temperature measurement equipment are mostly precious metals such as platinum and rhodium, the cost of large-scale temperature monitoring is relatively high. In a complex environment, thermocouples do not have good corrosion resistance and electromagnetic interference resistance. For the application scenario of a large number of parallel high-temperature pipelines, there are problems of complex wiring and high cost of long-distance electrical signal transmission. Because the detection object is a large number of parallel pipelines with a narrow pipeline interval and a dense demand for temperature measurement points, the thermocouple temperature measurement, due to its high cost and complex wiring technology, results in a limited number of detection points, which cannot represent the temperature change of local pipelines and affects the detection effect.

[0005] The fiber optic temperature measurement system based on Raman scattering has the advantage of distributed temperature measurement, can achieve full-coverage temperature measurement for the steam pipeline to be measured, and plays an important guiding role in the healthy operation of thermal power plants. The system utilizes the Raman scattering generated along the line when pulsed light passes through multimode fiber to realize the on-line monitoring of the temperature field of the entire length of the fiber. Raman scattering is a phenomenon in which photons interact with molecules, causing new spectra to appear on both sides of the incident light frequency, which are respectively called Stokes light and anti-Stokes light. By using the principle of optical time domain reflectometry combined with the temperature-sensitive effect of Raman scattering, distributed fiber optic temperature measurement can be realized. Foreign research on distributed fiber optic temperature measurement systems started earlier. Currently, Sensonet in the UK is at the leading level of this technology. For example, the Halo-DTS distributed temperature measurement system developed by this company can achieve a spatial resolution of 1m and a temperature accuracy of 0.01°C at a temperature measurement distance of 60km. In addition, Fujikura in Japan, CESO in Germany, Agilent in the United States, etc. have also launched mature products. Domestic research began in the 1980s, and China Jiliang University, Chongqing University, etc. have successively started relevant scientific research work. In recent years, with the maturity of this technology, domestic manufacturers have started to produce related products. For example, the DTS system of Shanghai Senshore Technology can ensure a temperature measurement accuracy of ±1°C and a positioning accuracy of ±1m at a measurement distance of 4km. The DTS system of Huaguang Tianrui can reach a measurement of 30km, and the temperature measurement range is -40°C - 120°C.

[0006] However, for the existing optical fiber temperature measurement technologies, due to the overly wide spatial resolution in the technical specifications, that is, the length of the sensing optical fiber section required for measuring a single pipeline, they cannot adapt to special application scenarios such as temperature monitoring of a large number of high-temperature parallel pipelines. For the large number of high-temperature parallel pipelines in superheaters and heat exchangers in thermal power plants, due to the inherent installation structure and environmental limitations, the length of the part of the sensing optical fiber that can be installed on a single pipeline is often less than 1 m, and they are arranged compactly and densely according to regions. In order to achieve temperature detection accurate to each pipeline and ensure full coverage of the distributed temperature measurement system, it is necessary to further improve the spatial resolution of the distributed temperature measurement system to adapt to this special application scenario in thermal power plants. Currently, most of the methods for improving the spatial resolution focus on two aspects: improving the hardware indicators and using software algorithms to improve the signal-to-noise ratio; the hardware factors directly restricting the spatial resolution of the distributed temperature measurement system include the sampling frequency of the acquisition card, the pulse width of the pulse light source signal, and the bandwidth of the photoelectric conversion circuit. Improving these indicators often requires hardware upgrades, bringing huge costs; although there have been many studies on software algorithms, the final effect can only make the spatial resolution of the distributed temperature measurement system infinitely close to the theoretical value limited by the hardware, and there cannot be further breakthroughs. Summary of the Invention

[0007] In order to overcome the defect that the optical fiber spatial resolution is low during the detection of high-temperature parallel pipelines in the prior art, and the length of the sensing optical fiber section required for measuring a single pipeline cannot adapt to the installation environment, the purpose of the present invention is to provide an OTDR optical fiber temperature measurement system with high spatial resolution. On the basis of the original hardware structure of the distributed temperature measurement system, a radio frequency signal source is added. By adjusting the sampling frequency of the acquisition card, multiple sampling results are obtained, the interval between sampling points is shortened, and a temperature curve with high spatial resolution is reconstructed to achieve an improvement in spatial resolution.

[0008] To solve the above technical problems, the present invention is realized through the following technical solutions:

[0009] An OTDR optical fiber temperature measurement system with high spatial resolution, characterized in that it includes a host computer, a data transmission line, a pulse light source, a wavelength division multiplexing device, a photodetector, a high-speed acquisition card, and a sensing and temperature measurement optical fiber; it is characterized in that the external clock input end of the high-speed acquisition card is connected to the output end of the radio frequency signal source, and the radio frequency signal source emits a periodic pulse signal that is synchronized with the trigger pulse of the pulse light source set in the external trigger mode; the radio frequency signal source is programmable, and the host computer controls the frequency of the signal output by the radio frequency signal source; the high-speed acquisition card realizes variable-frequency sampling of the optical fiber Raman backscattered light signal through the radio frequency signal source providing a variable external clock signal, thereby obtaining the optical fiber Raman backscattered light signal at different sampling frequencies;

[0010] The host computer manages the timing coordination between the high-speed acquisition card and the radio frequency signal source. When the system is working: First, the host computer sends a set of control signals to control the frequency of the pulse signal output by the radio frequency signal source, and at this time, a frequency setting is completed. After the high-speed acquisition card completes data acquisition and uploads it to the host computer at the current frequency, the host computer saves the set of acquired data.

[0011] Then, the host computer sends the next set of control signals to control the radio frequency signal source to adjust the frequency output, thereby changing the sampling frequency of the high-speed acquisition card, and performing the next set of data acquisition and data saving. Through multiple frequency settings, the host computer completes the optical signal data acquired at multiple different sampling frequencies.

[0012] Using the optical time domain reflectometry principle, the time information of the acquired signal is converted into position information in space. At the same time, the sampling interval of the acquisition card is also converted from a fixed time interval to an arrangement of fixed intervals distributed in space based on different sampling frequencies. The change in the sampling interval at different sampling frequencies results in a dense distribution of sampling points. Using the reconstruction algorithm, the spatial resolution of the distributed temperature measurement system is designed according to an appropriate expected value, the sampling point closest to the ideal sampling point in terms of spatial distance is found, and the temperature value at this point is calculated.

[0013] Using the reconstruction algorithm to select the sampling points at appropriate positions at each sampling frequency to reconstruct the high-spatial-resolution signal; demodulating the temperature signal according to the Raman scattering temperature-sensitive principle.

[0014] The temperature measurement optical fiber is a multimode optical fiber with a metal coating layer, the core diameter is 62.5μm, and the cladding outer diameter is 125μm, and it can work in a high-temperature environment for a long time. The temperature measurement optical fiber is fixed on the high-temperature parallel pipeline through a stainless steel sleeve and fixed with high-temperature glue.

[0015] The optical time domain reflectometry principle is that the propagation speed of light in the optical fiber is the speed of light in vacuum divided by the effective refractive index of the optical fiber core, which is determined by the physical properties of the optical fiber. When the optical signal enters the optical fiber, according to the time difference between the time when the incident light is emitted and the time when the backward Raman scattering signal is received, the position relationship between the scattering point and the incident end of the optical fiber can be calculated. According to the time of the received scattered light, the time series of the sampling points is converted into an arrangement in terms of distance according to the formula, and further the measurement values along the entire length of the optical fiber are obtained. In the temperature measurement system, the pulsed light source emits pulsed light with a fixed pulse width at a certain frequency, and periodic backward spontaneous Raman scattering optical signals are generated in the multimode optical fiber. The position information of the sampling points can be obtained from the received time of the scattered light. Generally speaking, the narrower the pulse width of the pulsed light is reduced, the more accurate the measurement result will be.

[0016] The temperature-sensitive principle of Raman scattering is that when light propagates in an optical fiber, due to the inhomogeneity of the medium, a part of the incident light will interact with the molecular motion in the optical fiber material, changing its original trajectory. This phenomenon is called light scattering. Among them, the process that causes the frequency of the scattered light to increase or decrease is called inelastic scattering. When the incident light is scattered, since the vibration energy level of the corresponding medium molecules is transferred to the next stable energy level, a part of the emitted optical molecules generates Stokes light with a frequency smaller than that of the incident light, and a part of the absorbed optical molecules generates anti-Stokes light with a frequency larger than that of the incident light. The anti-Stokes light has a higher sensitivity to temperature than the Stokes light. Therefore, the temperature measurement system can measure the temperature field along the temperature measurement optical fiber by demodulating the temperature information in the anti-Stokes light.

[0017] The external clock input terminal of the high-speed acquisition card is connected to the signal output terminal of the radio frequency signal source, and the external clock is selected as the reference clock. In the parameter settings, the frequency division factor is set to 1. The upper limit of the sampling frequency of the high-speed acquisition card is 500 MHz. The radio frequency signal source is programmable and can emit a dynamic high-frequency signal as the external clock of the acquisition card according to the instructions of the upper computer. The signal frequency range is 350 - 450 MHz, and the minimum frequency change of the signal is 1 MHz. The upper computer realizes communication with the pulse light source, high-speed acquisition card, and radio frequency signal source through the serial port, stores the data uploaded by the acquisition card multiple times, and integrates, processes, and reconstructs the high-spatial-resolution signal curve.

[0018] The high-speed acquisition card has a built-in accumulation algorithm. Since the optical signal is doped with a large amount of white noise caused by the photodetector and amplifier circuit, and the optical intensity of the Raman signal is very weak, it is easily affected by noise during transmission and acquisition, resulting in a low signal-to-noise ratio. Theoretical research shows that the result of calculating the mean value of white noise is approximately zero, and the above-mentioned pulse light source emits pulsed optical signals periodically, making it easy to obtain periodic repetitive signals. Therefore, the accumulation algorithm is used to denoise the signal.

[0019] Furthermore: The pulse light source periodically emits pulsed light with a nanosecond-level pulse width, which is injected into the sensing and temperature measurement optical fiber through a wavelength division multiplexer. Fiber Raman backscattered light signals are generated along the sensing and temperature measurement optical fiber and return to the wavelength division multiplexer. The wavelength division multiplexer divides the fiber Raman backscattered light signals into two paths of Stokes light and anti-Stokes light. The two paths of scattered light signals are converted into electrical signals by a photoelectric converter and uploaded to the upper computer by the high-speed acquisition card.

[0020] The wavelength division multiplexer can distinguish optical signals of specific wavelengths and divide the composite backscattered Raman light signals in the multimode optical fiber into two paths of Stokes light and anti-Stokes light. The photoelectric converter utilizes the avalanche breakdown of the APD avalanche photodiode to amplify the weak optical signal into an electrical signal that can be read by the acquisition card.

[0021] Furthermore, for the temperature field data of the temperature measurement optical fiber obtained by the reconstruction algorithm, there is a phenomenon that the Raman signal decays as the fiber distance increases, and there are differences in the attenuation trends of the Stokes light and the anti-Stokes light. To ensure the accuracy of the demodulated temperature data, a compensation algorithm is used to eliminate temperature drift. Before actually installing the sensing optical fiber, the entire temperature measurement optical fiber is placed in a constant temperature environment, and the Stokes light and anti-Stokes light signals in the optical fiber at this temperature are measured. The polynomial obtained by fitting the ratio curve of the two using the least squares method is used as the compensation formula, which is preset in the program of the upper computer and compensated during the temperature demodulation process to eliminate the temperature drift caused by the attenuation of the scattered light signal.

[0022] Furthermore, the RF signal source communicates with the upper computer through the serial port, and the high-speed acquisition card communicates with the upper computer through USB3.0. During the frequency conversion process, the RF signal source adjusts the output frequency according to the control signal sent by the upper computer instruction. When the output frequency is stable, the RF signal source returns the data carrying the frequency information to the upper computer to ensure the accuracy of the output signal.

[0023] Furthermore, the frequency conversion sampling can obtain groups of optical fiber Raman backscattered light signal data at multiple different sampling frequencies. Based on the principle of optical time domain reflectometry, different sampling intervals are converted into different spatial intervals. Since the sensing temperature measurement optical fiber is fixed on the object to be measured, that is, the initial position of the sampling signal is fixed, the sampling point sequence can be rearranged according to the position relationship.

[0024] When the set sampling frequency changes little, the sampling point sequence shows a dense distribution phenomenon within a specific interval. According to the distribution of the sampling points, a set of sampling points at specific positions is selected to form an ideal sampling point set, which can avoid the limitation of the acquisition card sampling frequency on the spatial resolution.

[0025] There are various ways to select sampling points: (1) Select the actual positions of the sampling points at different frequencies to establish an ideal sampling point set; (2) Select an ideal sampling point set according to the expected spatial resolution at a fixed interval; (3) Select corresponding sampling points according to the actual measured positions of the temperature measurement optical fiber in the distributed temperature measurement system.

[0026] Furthermore, the reconstruction algorithm is used to determine the distribution of the sampling point set at a specific position according to the expected spatial resolution at a fixed interval and the distribution of the ideal sampling points at the expected spatial resolution. The method is as follows:

[0027] (1) Let the set of positions of the sampling points on the sensing temperature measurement optical fiber be {x i | i = 1, 2,..., k}, where x iStarting from 0, it is an arithmetic sequence with an interval of Δx, representing the distance between the ideal sampling points and the initial point, that is, the expected spatial resolution at this time is Δx; first, calculate the deviation between each ideal sampling point and the actual sampling point closest to it at a single sampling frequency. Let the set of actual sampling points at each frequency form the data set A:

[0028]

[0029] where a m，n is the position of the nth sampling point from the initial point at frequency m, and there is an index matrix of sampling points at each frequency:

[0030]

[0031] For any n ∈ [1, N], it satisfies where i ∈ [1, k], m ∈ {390, 391, …, 400}

[0032] (2) At different sampling frequencies, find the actual sampling points closest to each ideal sampling point to obtain the final index matrix:

[0033]

[0034] For any m ∈ {390, 391, 392, …, 400}, it satisfies

[0035] In the above index matrix F, the first row F 1，i represents the sampling frequency selected for the ith point of the reconstructed data curve, and the second row F 2，i represents the F 1，i th sampling point at frequency F 2，i , from which a set of sampling signals with high spatial resolution can be reconstructed.

[0036] The reconstruction algorithm first calculates the positions of the ideal sampling points at the expected spatial resolution; using the principle of optical time domain reflectometry, it converts the sampling intervals at different sampling frequencies into a spatial distribution relationship, determines the positions of each sampling point, selects the actual sampling point closest to the ideal sampling point, and imports its corresponding sampling frequency and serial number into the sampling point index array; according to the index array, the points in multiple groups of data are selected and reconstructed.

[0037] Further: when the frequency conversion range of the sampling frequency is between 390 MHz and 400 MHz, there is a temperature measurement blind area in the sensing temperature measurement optical fiber for the reconstruction algorithm. Taking 50 m as a cycle, in the interval with a length of 6 - 45 m, compared with the data collected at a single sampling frequency of 400 MHz, it can significantly improve the resolution. The actual temperature measurement points with a position deviation less than 0.01 m can be found in the 11 groups of collected data, and a spatial resolution of 0.1 m can be achieved.

[0038] Further: the interval between ideal sampling points at the expected spatial resolution is 0.1 m and they are evenly distributed; the upper limit of the sampling frequency of the high-speed acquisition card is 500 MHz, the pulse width of the pulse light source is 1 ns, and the pulse light wavelength is 1550 nm. This pulse-width-limited spatial resolution does not constitute a limiting condition for the optimization result of the reconstruction algorithm; the photoelectric converter uses an APD avalanche photodiode, and the bandwidth of the photoelectric converter does not constitute a limiting condition for the optimization result of the reconstruction algorithm; the host computer controls the radio frequency signal source, with a frequency conversion range of 350 - 450 MHz, a minimum frequency change of 1 MHz, and 11 groups of data are collected each time. A high-spatial-resolution signal is reconstructed according to the sampling point index matrix.

[0039] The pulse width of the pulse light source is a periodic pulse optical signal between 1 - 30 ns. The optical signal intensity, frequency, and pulse width can all be controlled by the host computer, and an external synchronization signal input port is provided.

[0040] Further: the sensing temperature measurement optical fiber is fixed on the high-temperature parallel pipeline of the thermal power plant by high-temperature glue or iron wire. The length of the temperature measurement optical fiber on each pipeline is not less than the positioning accuracy of the fiber optic temperature sensor; the output of the pulse light source is connected to the 1550 nm input end of the wavelength division multiplexer; the sensing temperature measurement optical fiber is connected to the COM end of the wavelength division multiplexer; the 1450 nm output end of the wavelength division multiplexer is connected to the 1450 nm photoelectric signal input end of the photoelectric converter, and the 1660 nm output end of the wavelength division multiplexer is connected to the 1660 nm photoelectric signal input end of the photoelectric converter; the 1450 nm output end of the photoelectric converter is connected to the CH1 analog signal input of the acquisition card, and the 1660 nm output end of the photoelectric converter is connected to the CH2 analog signal input of the acquisition card; all fiber optic signal paths use FC-type fiber optic connectors; the output port of the radio frequency signal source is connected to the external clock input of the acquisition card; the radio frequency signal source is connected to the external trigger signal input end of the pulse light source and the external trigger input end of the acquisition card for synchronization signals.

[0041] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0042] Different from the traditional method of improving the spatial resolution of a distributed temperature measurement system by improving hardware indicators, such as shortening the optical pulse width of a pulsed light source, increasing the upper limit of the sampling frequency of a high-speed acquisition card, or adding algorithms to improve the signal-to-noise ratio, the present invention retains the original hardware and adds a radio frequency signal source as the external clock input of the acquisition card to the system, which has no impact on the rest of the hardware structure of the system and has a low modification cost.

[0043] The high-speed acquisition card of the present invention adopts a variable external clock, thus having different sampling frequencies, changing the interval between sampling points, adjusting the spatial position distribution corresponding to the sampling points, and effectively improving the problem of too low spatial resolution caused by the sampling frequency limitation of the high-speed acquisition card. On the selected temperature measurement optical fiber section, the sampling frequency of the high-speed acquisition card can be flexibly adjusted, and the sampling point with the smallest distance from the set sampling point can be selected to improve the spatial resolution on the premise of ensuring measurement accuracy. In the temperature monitoring of high-temperature pipelines in thermal power plants, it can effectively solve the limitations caused by the complex installation environment and the too short installable length of a single pipeline. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 is a flow chart of the present invention;

[0045] Figure 2 is a schematic structural diagram of the present invention;

[0046] Figure 3 is a schematic diagram of the experimental device of the present invention;

[0047] Figure 4 is a schematic diagram of frequency modulation sampling of the present invention;

[0048] Figure 5 is a flow chart of the algorithm used in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0049] In order to enable those skilled in the art to better understand the technical solution of the present invention, the preferred implementation of the present invention will be described below in conjunction with specific embodiments. However, it should be understood that the drawings are only for illustrative purposes and cannot be construed as a limitation to the present invention; for better illustration of this embodiment, some components in the drawings will be omitted, enlarged or reduced, which does not represent the size of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted. The description of the positional relationship in the drawings is only for illustrative purposes and cannot be construed as a limitation to the present invention.

[0050] The present invention will be further described below in conjunction with the drawings and embodiments, but it shall not be used as a basis for limiting the present invention.

[0051] As Figures 1 to 5As shown in the figure, a high-spatial-resolution OTDR optical fiber temperature measurement system includes a pulsed light source, a wavelength division multiplexer, a photoelectric converter, a high-speed acquisition card, a radio frequency signal source, and a sensing and temperature-measuring optical fiber. The pulsed light source periodically emits pulsed light with a wavelength of 1550 nm and a pulse width in the nanosecond range. The pulsed light is injected into the sensing and temperature-measuring optical fiber through the wavelength division multiplexer. Raman backscattered light signals are generated along the optical fiber and return to the wavelength division multiplexer. The wavelength division multiplexer divides the Raman backscattered light signals into two paths: Stokes light and anti-Stokes light. The two scattered light signals are converted into electrical signals by the photoelectric converter and collected by the high-speed acquisition card and uploaded to the upper computer. The radio frequency signal source emits a PWM wave signal that is synchronized with the pulsed light source and the high-speed acquisition card in an external trigger mode. The output end of the radio frequency signal source is connected to the external clock input end of the high-speed acquisition card.

[0052] Among them, for the high-temperature aerobic environment where the superheater and heat exchanger are located in the thermal power plant, a special protection method is adopted to ensure the long-term stable operation of the optical fiber. The temperature-measuring optical fiber is nested inside a hollow stainless steel tube, and is bent into a suitable shape according to the specific layout interval and length of the high-temperature steam pipelines of the superheater and reheater in the thermal power plant, and is bonded to the pipeline surface using high-temperature glue. The length of the temperature-measuring optical fiber on each pipeline should not be less than the positioning accuracy of the optical fiber temperature sensor.

[0053] Among them, after the Stokes light and anti-Stokes light pass through the photoelectric converter, they are converted into electrical signals that can be read by the computer. To address the problem of a large amount of white noise in the weak Raman signal, the accumulation averaging method is used to denoise the signal. The high-speed acquisition card collects and uploads the accumulated data at the current frequency to the upper computer. After completing the measurement of all frequencies, the Stokes light and anti-Stokes light data with high spatial resolution are obtained according to the reconstruction algorithm.

[0054] To avoid light source fluctuations and the sensitivity of the optical fiber to bending and stress, the Stokes light is used as the reference channel and the anti-Stokes light is used as the main channel for temperature demodulation. This demodulation method only calculates the ratio of the intensities of the two lights, and since they are of the same nature of scattering, it can avoid the possibility of the stress difference during installation affecting the measurement accuracy. The demodulation formula is as follows:

[0055]

[0056] In the formula, T0 is the reference temperature, k is the Boltzmann constant, k = 1.3806505×10 -23 J / K, h is the Planck constant, h = 6.62606896×10 -34 J·s, Δv is the Raman frequency shift of the optical fiber molecules, which is 13.2 THz, P as is the intensity of the anti-Stokes light, and P s is the intensity of the Stokes light.

[0057] Due to the different attenuation trends of the Stokes light and the anti-Stokes light, the measured data needs to be compensated with reference to the attenuation curve under a constant temperature environment. The present invention uses the least squares method to fit the ratio of the two signals, eliminate the temperature drift caused by attenuation, and then demodulate the temperature information. Assuming that the given data is the data measured for the Stokes light and the anti-Stokes light of the Raman signal under a constant temperature environment, and their ratio is where d i is the measurement position, i = 1, 2, 3, …, m, a linear model of the compensation curve for polynomial fitting is established, and its mathematical expression is:

[0058] y i = a0 + a1d i + … + a k d i k

[0059] In the formula, a0, a1, …, a k are the fitting polynomial coefficients, and y i is the fitting result. The deviation of the approximate compensation curve at the point d i is According to the least squares principle, the fitting curve is selected according to the principle of the minimum sum of squared deviations to determine the coefficients.

[0060]

[0061] After the coefficients are determined, the compensation curve is written into the host computer to compensate each measurement result and eliminate the influence of the attenuation of the scattered light signal on the temperature measurement accuracy.

[0062] Among them, the frequency modulation acquisition is realized by the integrated control of the host computer. When the measurement part of the distributed temperature measurement system is working normally, the host computer issues an instruction through the serial port to control the frequency of the signal output by the radio frequency signal source. At this time, the external clock input of the high-speed acquisition card is the output of the radio frequency signal source. With the clock source determined, the sampling frequency is determined; the acquisition card accumulates and removes the white noise from the data at the current frequency and uploads it to the host computer to complete one acquisition at a single frequency; after receiving this data, the host computer sends an instruction to the radio frequency signal source to change the output of the radio frequency signal source, thereby changing the sampling frequency of the acquisition card. The acquisition card uploads the accumulated and denoised data at the updated sampling frequency; this process is repeated until the entire set of sampling is completed.

[0063] In each set of sampling processes, with 1 MHz as the change unit and within the range of 390 - 400 MHz, the host computer sequentially saves 11 times of data uploaded by the acquisition card;

[0064] The reconstruction algorithm calculates the sampling point index matrix offline according to the expected spatial resolution, and selects sampling points from 11 groups of data stored in the acquisition card to reconstruct a temperature curve with higher spatial resolution; the host computer stores the sampling data of different frequencies in a two-dimensional matrix DATA:

[0065]

[0066] The rows of this matrix represent the sampling frequencies, and the columns represent the sampling points sorted in time series, which are converted into an arrangement according to spatial positions through the optical time domain reflectometry principle. According to the index matrix, the ideal sampling point closest to each ideal sampling point is selected to form a high-resolution reconstructed signal. Through simulation calculations, the present invention can significantly improve the resolution in the interval of 6 - 45m on the measurement length with a period of 50m for the temperature measurement optical fiber, compared with the data collected at a single frequency of 400MHz.

[0067] Among them, the index matrix is obtained by calculating through the reconstruction algorithm, and it is necessary to first design the ideal sampling point set of the system composition signal curve;

[0068] The designed ideal point set is the sampling points at specific positions selected. Let the position set of sampling points on the optical fiber be {x i |i = 1, 2,..., k}, where x i is a preset specific value representing the distance between the ideal sampling point and the initial point;

[0069] The ideal sampling point set is arranged according to the expected spatial resolution. The present invention presets the expected spatial resolution to be 0.1m. Let the position set of sampling points on the optical fiber be {x i |i = 1, 2,..., k}, where x i is an arithmetic sequence starting from 0 with an interval of 0.1m, representing the distance between the ideal sampling point and the initial point.

[0070] Among them, for the reconstruction algorithm, first calculate the deviation between each ideal sampling point and the actual sampling point closest to it at a single sampling frequency. Let the actual sampling point sets at each frequency form the data set A:

[0071]

[0072] where a m，n is the position of the nth sampling point from the initial point at frequency m, and there are sampling point index matrices at each frequency:

[0073]

[0074] For any n ∈ [1, N], it satisfies where i ∈ [1, k], m ∈ {390, 391,..., 400}

[0075] At different sampling frequencies, find the actual sampling points closest to each ideal sampling point to obtain the final index matrix.

[0076]

[0077] For any m ∈ {390, 391, 392,..., 400}, it satisfies

[0078] In the above index matrix F, the first row F 1，i represents the sampling frequency selected for the i-th point of the reconstructed data curve, and the second row F 2，i represents the F 1，i -th sampling point at frequency F. Given the data sets DATA collected at different sampling frequencies, a set of high-spatial-resolution sampling signals can be reconstructed therefrom. 1，i {DATA(F

[0079] , F 1，0 , F 2，0 ), DATA(F 1，1 , F 2，1 ), …, DATA(F 1，i , F 2，i ), where i = 1, 2, …, k

[0080] The above is a row vector composed of a set of high-spatial-resolution sampling signals. The elements in the row vector are distributed in the spatial order of the set expected spatial resolution, and the element values are the sampling values at the spatial positions corresponding to their serial numbers.

[0081] According to the description and drawings of the present invention, those skilled in the art can easily manufacture or use a high-spatial-resolution OTDR optical fiber temperature measurement system of the present invention and can produce the positive effects recorded in the present invention.

[0082] The above are only the preferred embodiments of the present invention and do not impose any form of limitation on the present invention. Any simple modification or equivalent change made to the above embodiments based on the technical essence of the present invention falls within the protection scope of the present invention.

Claims

1. An OTDR optical fiber temperature measurement system with high spatial resolution, comprising a host computer, a data transmission line, a pulsed light source, a wavelength division multiplexing device, a photodetector, a high-speed acquisition card, and a sensing and temperature-measuring optical fiber; characterized in that: The external clock input terminal of the high-speed acquisition card is connected to the output terminal of the radio frequency signal source, and the radio frequency signal source emits a periodic pulse signal that is synchronized with the trigger pulse of the pulse light source set to the external trigger mode; The radio frequency signal source is programmable, and the host computer controls the frequency of the output signal of the radio frequency signal source; the high-speed acquisition card realizes frequency conversion sampling of the fiber Raman backscattering optical signal through the variable external clock signal provided by the radio frequency signal source, thereby obtaining the fiber Raman backscattering optical signal at different sampling frequencies; The host computer manages the timing coordination between the high-speed acquisition card and the radio frequency signal source. When the system works: First, the host computer sends a set of control signals to control the frequency of the pulse signal output by the radio frequency signal source, and at this time, a frequency setting is completed; at the current frequency, after the high-speed acquisition card completes data acquisition and uploads it to the host computer, the host computer saves the set of acquired data; Then, the host computer sends the next set of control signals to control the radio frequency signal source to adjust the frequency output, thereby changing the sampling frequency of the high-speed acquisition card, and performing the next set of data acquisition and data saving; through multiple frequency settings, the host computer completes the optical signal data acquired at multiple different sampling frequencies; Using the optical time domain reflectometry principle, the time information of the acquired signal is converted into spatial position information, and at the same time, the sampling interval of the acquisition card is also converted from a fixed time interval to an arrangement of fixed intervals distributed in space based on different sampling frequencies. The change in the sampling interval at different sampling frequencies results in a dense distribution of sampling points; using a reconstruction algorithm, the spatial resolution of the distributed temperature measurement system is designed according to an appropriate expected value, the sampling point closest to the ideal sampling point in terms of spatial distance is found, and the temperature value of this point is calculated; The frequency conversion sampling can obtain data sets of fiber Raman backscattering optical signals at multiple different sampling frequencies. Based on the optical time domain reflectometry principle, different sampling intervals are converted into different spatial intervals; since the sensing and temperature measurement optical fiber is fixed on the object to be temperature measured, that is, the initial position of the sampling signal is fixed, the sampling point sequence can be rearranged according to the position relationship; When the set sampling frequency changes little, the sampling point sequence shows a dense distribution phenomenon within a specific interval; According to the distribution of the sampling points, a set of sampling points at specific positions is selected to form an ideal sampling point set, which can avoid the limitation of the sampling frequency of the acquisition card on the spatial resolution; There are various ways to select sampling points: (1) Select the actual positions of the sampling points at different frequencies to establish an ideal sampling point set; (2) Select an ideal sampling point set according to the expected spatial resolution at a fixed interval; (3) Select corresponding sampling points according to the actual measured position of the temperature measurement optical fiber of the distributed temperature measurement system; The reconstruction algorithm is used to establish the distribution of the sampling point set at a specific position according to the expected spatial resolution at a fixed interval and the distribution of the ideal sampling points at the expected spatial resolution. The method is as follows: (1) Let the set of positions of the sampling points on the sensing and temperature - measuring optical fiber be {x i | i = 1, 2, …, k}, where x i is an arithmetic sequence starting from 0 with an interval of Δx, representing the distance between the ideal sampling point and the initial point, that is, the expected spatial resolution at this time is Δx; First, calculate the deviation between each ideal sampling point and the actual sampling point closest to it at a single sampling frequency. Let the set of actual sampling points at each frequency form a data set A: where a m，n is the position of the nth sampling point from the initial point at frequency m, and there is a sampling point index matrix at each frequency: For any n ∈ [1, N], it holds that where i ∈ [1, k] and m ∈ {390, 391, …, 400} (2) At different sampling frequencies, find the actual sampling points closest to each ideal sampling point to obtain the final index matrix; For any m ∈ {390, 391, 392, …, 400}, it satisfies In the above index matrix F, the first row F 1，i represents the sampling frequency selected for the i-th point of the reconstructed data curve, and the second row F 2，i represents the frequency F 1，i at the F 2，i -th sampling point. From this, a set of sampling signals with high spatial resolution can be reconstructed.

2. The OTDR optical fiber temperature measurement system with high spatial resolution according to claim 1, characterized in that: The pulsed light source periodically emits pulsed light with a nanosecond-level pulse width in wavelength, which is injected into the sensing and temperature-measuring optical fiber through a wavelength division multiplexer. The fiber Raman backscattered light signal is generated along the sensing and temperature-measuring optical fiber and returns to the wavelength division multiplexer. The wavelength division multiplexer divides the fiber Raman backscattered light signal into two paths of Stokes light and anti-Stokes light. The two paths of scattered light signals are converted into electrical signals through a photoelectric converter and uploaded to the host computer by a high-speed acquisition card.

3. The OTDR optical fiber temperature measurement system with high spatial resolution according to claim 2, characterized in that: There are different attenuations in the transmission of the Raman backscattered light signal and the anti-Stokes light signal in the optical fiber. To ensure the temperature measurement accuracy, an attenuation compensation algorithm is added. First, the temperature-measuring optical fiber is placed in a constant-temperature environment to collect the normal attenuation signal of Raman scattering, and the attenuation curve polynomial is fitted by the least square method. Then, during actual measurement, the temperature signal is compensated according to the fitted curve.

4. The OTDR optical fiber temperature measurement system with high spatial resolution according to claim 1, characterized in that: The radio frequency signal source realizes communication with the host computer through the serial port, and the high-speed acquisition card realizes communication with the host computer through USB3.

0. During the frequency conversion process, the radio frequency signal source adjusts the output frequency according to the control signal sent by the host computer instruction. When the output frequency is stable, the radio frequency signal source returns the data carrying frequency information to the host computer to ensure the accuracy of the output signal.

5. An OTDR optical fiber temperature measurement system with high spatial resolution according to claim 1, characterized in that: When the frequency conversion range of the sampling frequency is between 390 MHz and 400 MHz, there is a temperature measurement blind area of the reconstruction algorithm on the sensing and temperature-measuring optical fiber. Taking 50 m as a cycle, in the interval with a length of 6 - 45 m, compared with the data collected at a single sampling frequency of 400 MHz, it has an obvious effect of improving the resolution. The actual temperature measurement point with a position deviation less than 0.01 m can be found in the 11 groups of data collected, and a spatial resolution of 0.1 m can be achieved.

6. The high spatial resolution OTDR optical fiber temperature measurement system according to claim 2, characterized in that: The interval between ideal sampling points at the expected spatial resolution is 0.1 m and is evenly distributed. The upper limit of the sampling frequency of the high-speed acquisition card is 500 MHz. The pulse width of the pulsed light source is 1 ns, and the pulsed light wavelength is 1550 nm. The spatial resolution limited by this pulse width does not constitute a limiting condition for the optimization result of the reconstruction algorithm. The photoelectric converter uses an APD avalanche photodiode, and the bandwidth of the photoelectric converter does not constitute a limiting condition for the optimization result of the reconstruction algorithm. The host computer controls the radio frequency signal source, with a frequency conversion range of 350 - 450 MHz, a minimum frequency change of 1 MHz, and 11 groups of data are collected each time. A high-spatial-resolution signal is reconstructed according to the sampling point index matrix.

7. An OTDR optical fiber temperature measurement system with high spatial resolution according to claim 4, characterized in that: The temperature-sensing optical fiber is fixed on the high-temperature parallel pipeline of the thermal power plant by high-temperature glue or iron wire, and the length of the temperature-sensing optical fiber on each pipeline is not less than the positioning accuracy of the optical fiber temperature sensor; the output of the pulsed light source is connected to the 1550nm input end of the wavelength division multiplexer; the temperature-sensing optical fiber is connected to the COM end of the wavelength division multiplexer; the 1450nm output end of the wavelength division multiplexer is connected to the 1450nm optical and electrical signal input end of the optical and electrical converter, and the 1660nm output end of the wavelength division multiplexer is connected to the 1660nm optical and electrical signal input end of the optical and electrical converter; the 1450nm output end of the optical and electrical converter is connected to the CH1 analog signal input of the acquisition card, and the 1660nm output end of the optical and electrical converter is connected to the CH2 analog signal input of the acquisition card; all the optical fiber signal paths adopt FC-type optical fiber connectors; the output port of the radio frequency signal source is connected to the external clock input of the acquisition card; the radio frequency signal source is connected to the external trigger signal input end of the pulsed light source and the external trigger input end of the acquisition card for synchronizing signals.

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