Calibration device and method based on Raman distributed optical fiber temperature measurement in low-temperature environment

Abstract

The invention provides a calibration device based on Raman distributed optical fiber temperature measurement in a low-temperature environment, and belongs to the field of temperature detection in the low-temperature environment, the calibration device comprises an isothermal block, the isothermal block is located in a vacuum chamber, a standard thermometer and an optical fiber are placed on the isothermal block, and the vacuum chamber is placed in a liquid helium temperature zone. Controlling the internal temperature of the vacuum chamber to slowly rise to the room temperature, acquiring the temperature of the standard thermometer and the Raman signal of the optical fiber, and fitting to obtain the corresponding relationship between the Raman light intensity and the temperature, so as to realize the calibration of the whole temperature zone from the liquid helium temperature zone to the room temperature. As the Raman scattering signal is only influenced by the temperature, the corresponding relation between the Raman light intensity and the temperature from the liquid helium temperature zone to the room temperature environment is accurately calibrated by fitting the standard temperature and the Raman signal, and the error generated by calculating the relation between the Raman signal and the temperature by adopting a formula method is eliminated. Therefore, the Raman temperature measurement system can be used for low-temperature or ultralow-temperature temperature measurement.

Description

Technical Field

[0001] The present invention relates to the technical field of temperature detection in a low-temperature environment, and in particular to a calibration device and method based on Raman distributed optical fiber temperature measurement in a low-temperature environment. Background Art

[0002] In recent years, with the construction of large scientific facilities in various places, refrigeration and cryogenic technology research has been widely used in various scientific fields and is an important support for cutting-edge basic theories. In order to ensure the reliable operation of low-temperature refrigeration and the reasonable distribution of low-temperature transmission, temperature detection in extremely low temperature environments is becoming increasingly critical.

[0003] At present, temperature measurement in the liquid helium temperature range (4K) and liquid nitrogen temperature range (77K) is mainly carried out by measuring the resistance of thermistors with four-wire system. The resistance of thermistors changes with temperature fluctuations at low temperatures hundreds of times that at room temperature, and the four-wire measurement method can eliminate the error caused by the wire resistance. Therefore, this method has very high measurement accuracy at low temperatures. Thermistors are also widely purchased and used as core components of cryogenic devices. However, this measurement method based on thermistors has very obvious shortcomings. Due to its very high price, it is impossible to measure the temperature distribution of large cryogenic devices on a large scale. At the same time, since the resistance wire will generate interference magnetic field when the four-wire system is used to measure the resistance, it cannot be used in occasions that are sensitive to magnetic fields.

[0004] In addition, temperature measurement technology based on infrared imaging has gradually been applied in low-temperature environments. Its principle is mainly based on the convective heat transfer effect between the metal surface and the air. This also limits the application of this technology in a vacuum environment. At the same time, its accuracy improvement methods and defect identification still need to be improved.

[0005] Fiber Bragg grating, as a new type of temperature sensor, is gradually being used in extremely low temperature environments. Compared with traditional temperature sensors, it has obvious advantages, such as small size, easy embedding in structures, and no interference from electromagnetic fields. However, the sensor is affected by stress and the variable components of stress need to be demodulated, so the calibration and demodulation of the system are relatively complicated.

[0006] The above temperature measurement methods all belong to discrete temperature measurement systems, that is, point-to-point temperature measurement methods. They can only measure the temperature changes of one point at a time and cannot measure the distribution of the temperature field in a timely manner. The Raman distributed temperature measurement system can simultaneously detect the temperature distribution of all points on the sensing optical fiber. However, the temperature accuracy and distance accuracy of the currently mature Raman temperature measurement system are very low, so it is rarely used in the low-temperature field.

[0007] For example, the Chinese invention patent application with publication number CN110987231A, "A distributed optical fiber rapid monitoring superconductor temperature system", includes a distributed single-mode optical fiber Raman temperature measurement device, a superconductor, a polyimide single-mode optical fiber, a liquid nitrogen tank, a function generator and an oscilloscope. The surface of the superconductor is grooved, the polyimide single-mode optical fiber is installed in the surface groove of the superconductor, the polyimide single-mode optical fiber is connected to the distributed single-mode optical fiber Raman temperature measurement device, the superconductor is placed in the liquid nitrogen tank, the function generator is connected to one end of the superconductor, and the oscilloscope is connected to the other end of the superconductor to realize the state monitoring of the superconductor in a liquid nitrogen environment. The temperature system should only be used for Used in the liquid nitrogen temperature zone (77k), demodulated by an oscilloscope, the demodulation method is to use the initial temperature T0 as a reference, and the temperature-Raman signal relationship is calculated by the formula method, but the reliability of the formula method is only valid from 0℃ to 100℃, but at low temperatures of -196℃ to 0℃, the temperature-Raman signal relationship does not satisfy its derived formula function at all. The demodulation temperature in the sub-zero temperature zone must be calibrated at multiple points, and the temperature measurement value obtained by only using the initial normal temperature as a reference is completely unreliable. In addition, the temperature system is demodulated by an oscilloscope because Raman noise interference is not eliminated, and its measurement accuracy is almost impossible to apply to low-temperature environments. Summary of the invention

[0008] The technical problem to be solved by the present invention is how to solve the problem that the Raman temperature measurement system cannot be applied in the low temperature field due to its low temperature measurement accuracy.

[0009] The present invention solves the above technical problems through the following technical solutions: a calibration device based on Raman distributed optical fiber temperature measurement in a low-temperature environment, the calibration device comprises an isothermal block, the isothermal block is located in a vacuum chamber, a standard thermometer and an optical fiber are placed on the isothermal block, the vacuum chamber is placed in a liquid helium temperature zone, the internal temperature of the vacuum chamber is controlled to slowly rise to room temperature, the temperature of the standard thermometer and the Raman signal of the optical fiber are collected and fitted to obtain the corresponding relationship between the Raman light intensity and the temperature, so as to realize full temperature zone calibration from the liquid helium temperature zone to the room temperature.

[0010] The present invention places a standard thermometer and an optical fiber on the same isothermal block, and the isothermal block is placed in a vacuum chamber. The internal temperature of the vacuum chamber is controlled to gradually and slowly rise from a liquid helium temperature zone to room temperature, and the temperature of the standard thermometer is collected as the real temperature for calibration, and the Raman scattering signal returned from the optical fiber is collected. Since the Raman scattering signal is only affected by the temperature, the corresponding relationship between the Raman light intensity and the temperature in the liquid helium temperature zone to the room temperature environment can be accurately calibrated by fitting the standard temperature with the Raman signal, thereby eliminating the error generated by the formula method used in the prior art to calculate the relationship between the Raman signal and the temperature. Since the present invention can realize the full temperature zone calibration from the liquid helium temperature zone to the room temperature, the Raman temperature measurement system can be used for low temperature or ultra-low temperature temperature measurement and the measurement accuracy can be guaranteed.

[0011] Preferably, the isothermal block has a hole chamber, a standard thermometer is placed in the hole chamber, and the optical fiber is wound around the outer wall of the isothermal block.

[0012] Preferably, the vacuum chamber comprises an outer thermal insulation barrier, an inner thermal insulation barrier, a heat sink, and a lead heat sink, wherein the lead heat sink is located on the heat sink and is used for wiring of a standard thermometer, the isothermal block is fixed at one end of the heat sink, the inner thermal insulation barrier wraps the isothermal block and the lead of the standard thermometer and its open end is fixed at the other end of the heat sink, and the outer thermal insulation barrier wraps the inner thermal insulation barrier and its open end is fixed at the other end of the heat sink.

[0013] Preferably, the calibration device further comprises a fiber optic vacuum flange and a liquid helium dewar, the vacuum chamber is placed in the liquid helium dewar, the fiber optic vacuum flange is fixed to the open end of the liquid helium dewar, and the optical fiber passes through the fiber optic vacuum flange to connect to the vacuum chamber.

[0014] Preferably, the calibration device also includes a temperature measuring host and an optical fiber temperature measuring system. The temperature measuring host is connected to a standard thermometer to monitor the temperature of the standard thermometer, and controls the temperature of the standard thermometer to slowly rise so as to control the internal temperature of the vacuum chamber to slowly rise to room temperature. The optical fiber temperature measuring system is connected to the optical fiber to collect the Raman signal of the optical fiber.

[0015] Preferably, the optical fiber temperature measurement system includes a pulse light source, a wavelength division multiplexer, a photoelectric detector, an acquisition card, and a host computer. The pulse light source is respectively connected to the wavelength division multiplexer and the acquisition card. The pulse light source automatically emits pulse light at a set frequency. The pulse light enters the optical fiber after being screened by the wavelength division multiplexer. The Raman scattered light of the optical fiber returns and enters the wavelength division multiplexer. The photoelectric detector is connected to the wavelength division multiplexer to detect the Raman scattered light and convert the light intensity analog signal into a level analog signal. The acquisition card is connected to the photoelectric detector, and the host computer is connected to the acquisition card. The acquisition card acquires the level analog signal at a frequency of not less than 200 MHz, and converts the level analog signal into a level digital signal after amplifying the level analog signal and inputting it into the host computer.

[0016] The present invention also provides a calibration method based on Raman distributed optical fiber temperature measurement in a low-temperature environment, the calibration method comprising: placing a standard thermometer and an optical fiber on an isothermal block respectively, placing the isothermal block in a vacuum chamber, placing the vacuum chamber in a liquid helium temperature zone and controlling the internal temperature of the vacuum chamber to slowly rise to room temperature, collecting the temperature of the standard thermometer and the Raman signal of the optical fiber and fitting to obtain the corresponding relationship between the Raman light intensity and the temperature, and realizing full temperature zone calibration from the liquid helium temperature zone to the room temperature.

[0017] Preferably, the process of placing the vacuum chamber in the liquid helium temperature zone and controlling the internal temperature of the vacuum chamber to rise slowly to room temperature includes: inserting the vacuum chamber into a liquid helium dewar, inputting liquid helium into the liquid helium dewar to reduce the internal temperature of the vacuum chamber to 8K, continuing to input cold helium into the vacuum chamber to reduce the internal temperature of the vacuum chamber to 4.3K, then extracting cold helium from the liquid helium dewar to reduce the temperature of the isothermal block in the vacuum chamber to 3.5K, operating the temperature measuring host to control the temperature of the standard thermometer to rise slowly to control the internal temperature of the vacuum chamber to rise slowly to room temperature, and collecting data for full temperature zone calibration from the liquid helium temperature zone to room temperature.

[0018] Preferably, the process of collecting the temperature of the standard thermometer and the Raman signal of the optical fiber and fitting to obtain the corresponding relationship between the Raman light intensity and the temperature includes:

[0019] Step 1: The temperature measuring host collects the temperature of the standard thermometer, and the optical fiber temperature measuring system collects the Raman signal of the optical fiber;

[0020] Step 2, performing partition denoising on the Raman signal of the optical fiber to obtain a denoised Raman signal;

[0021] Step 3: Divide the denoised Raman signal into a set number of temperature segments, and sequentially fit the denoised Raman signal and temperature of each temperature segment to obtain a fitting function, thereby obtaining a corresponding relationship between Raman light intensity and temperature.

[0022] Preferably, the partition denoising process in step 2 includes: setting the number of samples for each sampling to N, dividing the N sampling points into a Raman sampling interval, a truncation interval and a noise interval, for each sampling, the sampling points collected on the optical fiber are included in the Raman sampling interval, the sampling value of the noise interval is the noise signal collected this time, the signals in the noise interval are accumulated and averaged to obtain the average value of the noise interval, and the signal in the Raman sampling interval is subtracted from the average value of the noise interval to obtain the denoised Raman signal.

[0023] Preferably, the calibration method also includes: calculating the position of the Raman reflection point corresponding to a certain sampling point based on the time elapsed from each start of sampling to the sampling point, obtaining the corresponding distance of the sampling point on the optical fiber, obtaining the temperature and distance relationship of all sampling points on the optical fiber, and detecting the temperature of all sampling points on the optical fiber at the same time.

[0024] The advantages provided by the present invention are:

[0025] (1) The present invention places a standard thermometer and an optical fiber on the same isothermal block, and the isothermal block is placed in a vacuum chamber. By controlling the internal temperature of the vacuum chamber to gradually and slowly rise from the liquid helium temperature zone to room temperature, the temperature of the standard thermometer is collected as the actual temperature for calibration, and the Raman scattering signal returned from the optical fiber is collected. Since the Raman scattering signal is only affected by temperature, the corresponding relationship between the Raman light intensity and the temperature in the liquid helium temperature zone to the room temperature environment can be accurately calibrated by fitting the standard temperature with the Raman signal, thereby eliminating the error caused by the formula method used in the prior art to calculate the relationship between the Raman signal and the temperature. Since the present invention can realize the full temperature range calibration from the liquid helium temperature zone to the room temperature, the Raman temperature measurement system can be used for low temperature or ultra-low temperature temperature measurement and the measurement accuracy can be guaranteed.

[0026] (2) The present invention can accurately demodulate the correspondence between Raman signals and temperature. Placing the optical fiber in a large low-temperature device can realize distributed temperature measurement, without the need to install expensive temperature-sensitive resistors in the large low-temperature device, thereby solving the problem that temperature measurement equipment in the low-temperature field is expensive and difficult to detect temperature distribution.

[0027] (3) The present invention performs partition denoising on the Raman signal of the optical fiber, and divides N sampling points into a Raman sampling interval, a truncation interval and a noise interval. For each N sampling, the sampling point collected on the temperature measuring optical fiber is included in the Raman sampling interval, and the sampling value of the noise interval is the noise signal collected this time. The denoised Raman signal can be obtained by subtracting the average value of the noise interval from the Raman sampling interval signal, thereby eliminating the optical noise caused by ambient light and incident light, optimizing the calibration process and improving the denoising accuracy, further improving the temperature measurement accuracy, and no need to add an additional optical fiber dedicated to measuring the noise value. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A schematic diagram of a calibration device based on Raman distributed optical fiber temperature measurement in a low temperature environment provided by an embodiment of the present invention;

[0029] Figure 2 A top view of a calibration device based on Raman distributed optical fiber temperature measurement in a low temperature environment provided by an embodiment of the present invention;

[0030] Figure 3 A cross-sectional view of a calibration device based on Raman distributed optical fiber temperature measurement in a low temperature environment provided by an embodiment of the present invention;

[0031] Figure 4 A schematic diagram of an optical fiber temperature measurement system in a calibration device based on Raman distributed optical fiber temperature measurement in a low temperature environment provided by an embodiment of the present invention;

[0032] Figure 5 A sub-flow chart of a calibration method based on Raman distributed optical fiber temperature measurement in a low temperature environment provided by an embodiment of the present invention;

[0033] Figure 6 A sub-flow chart of a calibration method based on Raman distributed optical fiber temperature measurement in a low temperature environment provided by an embodiment of the present invention;

[0034] In the figure: 10 isothermal block, 11 hole chamber, 20 external thermal insulation barrier, 30 internal thermal insulation barrier, 40 heat sink, 50 lead heat sink. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the technical solution of the present invention is clearly and completely described below in combination with specific embodiments and with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] Temperature measurement in the liquid helium temperature zone (4K) and liquid nitrogen temperature zone (77K) is mainly carried out by measuring the resistance of thermistors in a four-wire system. Thermistors can only achieve point-to-point temperature measurement. If you want to measure the temperature distribution of large low-temperature devices, you need to install thermistors on a large scale. However, since thermistors are very expensive, the temperature measurement equipment will be very expensive, making it impossible to measure the temperature distribution of large low-temperature devices on a large scale. At the same time, the resistance wire of thermistors will generate interference magnetic fields when they are energized, making them unable to be used in places that are sensitive to magnetic fields. Based on the Raman distributed temperature measurement system, the temperature distribution of all points on the sensing optical fiber can be detected simultaneously. The principle of Raman temperature measurement is that light is not always transmitted forward in the optical fiber. During the transmission process, light running back will be generated, which is called backscattered light. The wavelength of these backscattered lights will deviate from the wavelength of the incident light. The backscattered light with a large deviation is called Raman scattered light. The characteristic of Raman scattered light is that it is only affected by temperature and is almost unaffected by stress. While emitting a pulse of light at the incident end of the optical fiber, the intensity of the returned Raman scattered light is monitored at the incident end. The location of the scattering point in the optical fiber can be inferred based on the time when the light intensity is detected, and the temperature of the point can be demodulated based on the intensity of the Raman light. Therefore, after emitting a beam of pulsed light, the temperature of each point on the optical fiber can be detected in theory.

[0037] However, the temperature demodulation commonly used in the existing Raman temperature measurement system is to directly demodulate after determining several reference temperatures by formula method. This method has very low reliability at low temperatures and is almost impossible to be applied to low temperature or ultra-low temperature environments. The present invention, through the designed Raman distributed optical fiber temperature measurement calibration device and method, calibrates the full temperature range from the low temperature liquid helium temperature range to the room temperature (4k to 300k), so that the Raman temperature measurement system can be applied to the temperature measurement in low temperature or ultra-low temperature environments. When the Raman temperature measurement system is used for temperature measurement in low temperature or ultra-low temperature environments, the Raman temperature measurement system is calibrated by the Raman distributed optical fiber temperature measurement calibration device and method of the present invention, and the corresponding relationship between the Raman signal and the temperature can be accurately demodulated, which continues the distributed measurement advantage of optical fiber, and there is no need to install expensive temperature-sensitive resistors in large low temperature devices, thereby solving the problem that the temperature measurement equipment in the low temperature field is very expensive and difficult to detect the temperature distribution. The technical solution of the present invention will be described in detail below:

[0038] Example 1

[0039] like Figure 1-3 As shown, this embodiment provides a calibration device based on Raman distributed optical fiber temperature measurement in a low temperature environment, including an isothermal block 10, which is located in a vacuum chamber and is used to maintain temperature stability. The isothermal block 10 has at least one hole chamber 11, and a standard thermometer is placed in the hole chamber 11. The outer wall of the isothermal block 10 is provided with a plurality of grooves, and the optical fiber is wound in the grooves of the outer wall of the isothermal block 10 and fixed by a low-temperature tape. The vacuum chamber is placed in a liquid helium temperature zone, and the vacuum chamber includes an outer insulation barrier 20, an inner insulation barrier 30, a heat sink 40, and a lead heat sink 50. The lead heat sink 50 is located on the heat sink 40 for wiring the standard thermometer and reducing the cold loss of the standard thermometer. The heat sink 40 is used to reduce the cold loss. The isothermal block 10 is fixed to one end of the heat sink 40, the inner insulation barrier 30 wraps the isothermal block 10 and the lead wire of the standard thermometer and its open end is fixed to the other end of the heat sink 40, the outer insulation barrier 20 wraps the inner insulation barrier 30 and its open end is fixed to the other end of the heat sink 40, the inner insulation barrier 30 is made of copper, which is used to ensure the sealing of the vacuum chamber and plays the role of wrapping the lead wire, and the outer insulation barrier 20 is made of stainless steel, which is used to ensure the sealing of the internal chamber. The internal temperature of the vacuum chamber is controlled to rise slowly to room temperature, the temperature of the standard thermometer and the Raman signal of the optical fiber are collected and fitted to obtain the corresponding relationship between Raman light intensity and temperature, so as to realize the full temperature range calibration from liquid helium temperature range to room temperature.

[0040] The present invention places a standard thermometer and an optical fiber on the same isothermal block, and the isothermal block is placed in a vacuum chamber. The internal temperature of the vacuum chamber is controlled to gradually and slowly rise from a liquid helium temperature zone to room temperature, and the temperature of the standard thermometer is collected as the real temperature for calibration, and the Raman scattering signal returned from the optical fiber is collected. Since the Raman scattering signal is only affected by the temperature, the corresponding relationship between the Raman light intensity and the temperature in the liquid helium temperature zone to the room temperature environment can be accurately calibrated by fitting the standard temperature with the Raman signal, thereby eliminating the error generated by the formula method used in the prior art to calculate the relationship between the Raman signal and the temperature. Since the present invention can realize the full temperature zone calibration from the liquid helium temperature zone to the room temperature, the Raman temperature measurement system can be used for low temperature or ultra-low temperature temperature measurement and the measurement accuracy can be guaranteed.

[0041] The calibration device based on Raman distributed optical fiber temperature measurement in a low temperature environment also includes an optical fiber vacuum flange, a liquid helium dewar, a temperature measurement host and an optical fiber temperature measurement system. The vacuum chamber is placed in the liquid helium dewar. The liquid helium dewar adopts a flat liquid helium dewar, which can be inserted into the vacuum chamber to provide a liquid helium cold source (4k) for the vacuum chamber. The vacuum chamber is connected to the open end of the liquid helium dewar through a slender cavity tube. The slender cavity tube is used for wiring of the standard thermometer and the optical fiber. The optical fiber vacuum flange is fixed at the open end of the liquid helium dewar, and the optical fiber passes through the optical fiber vacuum flange to access the vacuum chamber. An aviation plug is also welded at the open end of the liquid helium dewar to connect the inside and outside of the vacuum chamber. The temperature measurement host is connected to the standard thermometer to monitor the temperature of the standard thermometer, and the temperature of the standard thermometer is slowly raised to control the internal temperature of the vacuum chamber to slowly rise to room temperature. The optical fiber temperature measurement system is connected to the optical fiber to collect the Raman signal of the optical fiber.

[0042] The standard thermometer of the present invention adopts a temperature-sensitive resistor sensor, model cernox1050, and the temperature measurement accuracy of this model of standard thermometer in the liquid helium temperature range can reach ±5mK. The model of the temperature measurement host is Lakeshore336, which can not only monitor and save the temperature data of the standard thermometer in real time, but also control the temperature of the standard thermometer to slowly rise.

[0043] like Figure 4 As shown, the optical fiber temperature measurement system includes a pulse light source, a wavelength division multiplexer, a photoelectric detector, an acquisition card, and a host computer. The pulse light source is connected to the wavelength division multiplexer and the acquisition card respectively. The pulse light source automatically emits pulse light at a set frequency. The pulse light enters the optical fiber after being screened by the wavelength division multiplexer. The Raman scattered light of the optical fiber returns and enters the wavelength division multiplexer. The photoelectric detector is connected to the wavelength division multiplexer to detect the Raman scattered light and convert the light intensity analog signal into a level analog signal. The acquisition card is connected to the photoelectric detector, and the host computer is connected to the acquisition card. The acquisition card is used to amplify the level analog signal and convert it into a level digital signal before inputting it into the host computer.

[0044] The optical fiber of the present invention adopts 62.5 / 125nm multimode optical fiber, and the length of the optical fiber measured in the calibration section is 300m. The pulse frequency of the pulse light source is 10khz, and the wavelength is 1550nm. Since the time when the light intensity is detected represents the distance between the temperature measurement point and the incident end of the optical fiber, the pulse light source emits a light beam and the acquisition card starts to collect signals at the same time. A single-chip microcomputer can send a start signal to the acquisition card and the pulse light source at the same time, or the pulse light source can be adjusted to internal triggering, that is, automatically emit pulse light at a certain frequency, and then send a trigger signal to the acquisition card each time it emits light, thereby realizing synchronization between the acquisition card and the light source. The wavelength division multiplexer uses a CWDM coarse wavelength division multiplexer with Raman-specific sub-bands of 1550nm and 1310nm. The wavelength division multiplexer can filter and classify the incoming light according to the wavelength range. For example, the light source emits a 1550nm beam into the wavelength division multiplexer, and then outputs it from the 1550nm filter port to the measuring optical fiber. The Raman scattered light (1660nm) of the measuring optical fiber returns to the wavelength division multiplexer and is filtered and separated by the 1660nm filter port. The Raman light filtered by the wavelength division multiplexer is a light intensity analog signal, which is converted into a level analog signal through a photoelectric detector. The photoelectric detector uses an avalanche detector with a frequency of 10Ghz and an amplification factor of 10 times. Since the level analog signal is too weak to be captured by the computer, the level analog signal is amplified and converted into a digital signal through a digital acquisition card. The distance accuracy of Raman temperature measurement depends on the sampling frequency of the acquisition card. The sampling frequency of the acquisition card must be above 200MHz. The sampling frequency of the acquisition card is set to 500MHz in the present invention.

[0045] Example 2

[0046] This embodiment provides a calibration method based on Raman distributed optical fiber temperature measurement in a low-temperature environment, including: placing a standard thermometer and an optical fiber on an isothermal block respectively, placing the isothermal block in a vacuum chamber, placing the vacuum chamber in a liquid helium temperature zone and controlling the internal temperature of the vacuum chamber to slowly rise to room temperature, collecting the temperature of the standard thermometer and the Raman signal of the optical fiber and fitting to obtain the corresponding relationship between the Raman light intensity and the temperature, so as to realize full temperature range calibration from the liquid helium temperature zone to the room temperature.

[0047] The process of placing the vacuum chamber in the liquid helium temperature zone and controlling the internal temperature of the vacuum chamber to slowly rise to room temperature includes: inserting the vacuum chamber into the liquid helium dewar, inputting liquid helium into the liquid helium dewar to reduce the internal temperature of the vacuum chamber to 8K, continuously inputting cold helium into the vacuum chamber to reduce the internal temperature of the vacuum chamber to 4.3K, then extracting the cold helium in the liquid helium dewar to reduce the temperature of the isothermal block in the vacuum chamber to 3.5K, operating the temperature measuring host to control the temperature of the standard thermometer to slowly rise to control the internal temperature of the vacuum chamber to slowly rise to room temperature, and collecting data for full temperature zone calibration from the liquid helium temperature zone to room temperature.

[0048] like Figure 5 As shown, the process of collecting the temperature of the standard thermometer and the Raman signal of the optical fiber and fitting to obtain the corresponding relationship between the Raman light intensity and the temperature includes:

[0049] Step 1: The temperature measuring host collects the temperature of the standard thermometer, and the optical fiber temperature measurement system collects the Raman signal of the optical fiber; the Raman signal and standard temperature data obtained during the calibration process are saved and integrated in the form of an xlsx table.

[0050] Step 2, partition and denoise the Raman signal of the optical fiber to obtain a denoised Raman signal; set the number of samples for each time to N, and divide the N sampling points into a Raman sampling interval, a truncation interval and a noise interval. For each sampling, the sampling points collected on the optical fiber are included in the Raman sampling interval, and the sampling value of the noise interval is the noise signal collected this time. The signals in the noise interval are accumulated and averaged to obtain an average value of the noise interval, and the signal in the Raman sampling interval is subtracted from the average value of the noise interval to obtain a denoised Raman signal.

[0051] The Raman value measured during the calibration process will contain noise interference. These optical noises are not randomly distributed along both sides of the axis, and their noise average value changes with temperature. Usually, calibration requires an additional optical fiber dedicated to measuring the noise value, but this is not only inconvenient to operate, but can only eliminate ambient light noise, and cannot filter out the optical noise generated by the incident light. The present invention performs partition denoising on the Raman signal of the optical fiber. For example, for the calibration of a 300m optical fiber, the number of N samplings is set to 2000. For these 2000 sampling points, the first 1000 sampling points are called Raman sampling intervals, the middle 500 sampling points are called truncation intervals, and the last 500 sampling points are called noise intervals. For each N sampling, the sampling points collected on the temperature measuring optical fiber are included in the Raman sampling interval, and the sampling values ​​of the noise interval can be regarded as the noise signal collected this time. The denoised Raman signal can be obtained by subtracting the average value of the noise interval from the Raman sampling interval signal, thereby eliminating the optical noise caused by ambient light and incident light, optimizing the calibration process, improving the denoising accuracy, and further improving the temperature measurement accuracy.

[0052] Step 3, the denoised Raman signal is divided into a set number of temperature segments, and the denoised Raman signal and temperature of each temperature segment are fitted in turn to obtain a fitting function, and the corresponding relationship between the Raman light intensity and the temperature is obtained. For example, the denoised Raman signal is divided into 100 temperature segments, and each temperature segment is imported into the fitting software for fitting in turn. The fitting software can be Matlab or labview, or can be fitted by calling a function library in python. The present invention adopts Matlab program fitting, selects a 10th power function as the fitting function, and finally obtains a 10th power fitting function with 11 coefficients.

[0053] like Figure 6 As shown in the figure, n is the number of sampling times in each N sampling process, and the initial value of n is 0. When it is determined that there is a trigger signal, the value of n is increased by 1 and the nth sampling is started. The temperature corresponding to the Raman signal is obtained through step 3. In order to correspond all sampling points on the optical fiber to the temperature one by one, the position of the Raman reflection point corresponding to the sampling point is calculated according to the time from each sampling to a certain sampling point by the following formula:

[0054]

[0055] Among them, L is the distance corresponding to the nth sampling point, c is the speed of light, n * is the effective refractive index of the optical fiber, and f is the sampling frequency of the acquisition card.

[0056] The corresponding distance of the sampling point on the optical fiber is obtained, the temperature and distance relationship of all sampling points on the optical fiber is obtained, and the temperature of all sampling points on the optical fiber is detected at the same time.

[0057] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A calibration device based on Raman distributed optical fiber temperature measurement in a low temperature environment, characterized in that: The calibration device comprises an isothermal block, which is located in a vacuum chamber. A standard thermometer and an optical fiber are placed on the isothermal block. The vacuum chamber is placed in a liquid helium temperature zone. The internal temperature of the vacuum chamber is controlled to slowly rise to room temperature. The temperature of the standard thermometer and the Raman signal of the optical fiber are collected and fitted to obtain the corresponding relationship between the Raman light intensity and the temperature, thereby realizing full temperature zone calibration from the liquid helium temperature zone to the room temperature.

2. The calibration device based on Raman distributed optical fiber temperature measurement in a low temperature environment according to claim 1, characterized in that: The isothermal block is provided with a hole chamber, a standard thermometer is placed in the hole chamber, and an optical fiber is wound around the outer wall of the isothermal block.

3. The calibration device based on Raman distributed optical fiber temperature measurement in a low temperature environment according to claim 1, characterized in that: The vacuum chamber comprises an outer thermal insulation barrier, an inner thermal insulation barrier, a heat sink, and a lead heat sink, wherein the lead heat sink is located on the heat sink and is used for wiring of a standard thermometer, an isothermal block is fixed at one end of the heat sink, the inner thermal insulation barrier wraps the isothermal block and the lead of the standard thermometer and its open end is fixed at the other end of the heat sink, and the outer thermal insulation barrier wraps the inner thermal insulation barrier and its open end is fixed at the other end of the heat sink.

4. The calibration device based on Raman distributed optical fiber temperature measurement in a low temperature environment according to claim 1, characterized in that: The calibration device also includes an optical fiber vacuum flange and a liquid helium dewar. The vacuum chamber is placed in the liquid helium dewar. The optical fiber vacuum flange is fixed to the open end of the liquid helium dewar. The optical fiber passes through the optical fiber vacuum flange and is connected to the vacuum chamber.

5. The calibration device based on Raman distributed optical fiber temperature measurement in a low temperature environment according to claim 1, characterized in that: The calibration device also includes a temperature measuring host and an optical fiber temperature measuring system. The temperature measuring host is connected to a standard thermometer to monitor the temperature of the standard thermometer, and controls the temperature of the standard thermometer to slowly rise so as to control the internal temperature of the vacuum chamber to slowly rise to room temperature. The optical fiber temperature measuring system is connected to an optical fiber to collect the Raman signal of the optical fiber.

6. The calibration device based on Raman distributed optical fiber temperature measurement in a low temperature environment according to claim 5, characterized in that: The optical fiber temperature measurement system includes a pulse light source, a wavelength division multiplexer, a photoelectric detector, an acquisition card, and a host computer. The pulse light source is respectively connected to the wavelength division multiplexer and the acquisition card. The pulse light source automatically emits pulse light at a set frequency. The pulse light enters the optical fiber after being screened by the wavelength division multiplexer. The Raman scattered light of the optical fiber returns and enters the wavelength division multiplexer. The photoelectric detector is connected to the wavelength division multiplexer to detect the Raman scattered light and convert the light intensity analog signal into a level analog signal. The acquisition card is connected to the photoelectric detector, and the host computer is connected to the acquisition card. The acquisition card collects the level analog signal at a frequency of not less than 200 MHz, and converts the level analog signal into a level digital signal after amplifying the level analog signal and inputs it into the host computer.

7. A calibration method based on Raman distributed optical fiber temperature measurement in a low temperature environment, characterized in that: The calibration method comprises: placing a standard thermometer and an optical fiber on an isothermal block respectively, placing the isothermal block in a vacuum chamber, placing the vacuum chamber in a liquid helium temperature zone and controlling the internal temperature of the vacuum chamber to slowly rise to room temperature, collecting the temperature of the standard thermometer and the Raman signal of the optical fiber and fitting to obtain the corresponding relationship between the Raman light intensity and the temperature, and realizing full temperature zone calibration from the liquid helium temperature zone to the room temperature.

8. The calibration method based on Raman distributed optical fiber temperature measurement in a low temperature environment according to claim 7, characterized in that: The process of placing the vacuum chamber in the liquid helium temperature zone and controlling the internal temperature of the vacuum chamber to slowly rise to room temperature includes: inserting the vacuum chamber into a liquid helium dewar, inputting liquid helium into the liquid helium dewar to reduce the internal temperature of the vacuum chamber to 8K, continuously inputting cold helium into the vacuum chamber to reduce the internal temperature of the vacuum chamber to 4.3K, then extracting cold helium from the liquid helium dewar to reduce the temperature of the isothermal block in the vacuum chamber to 3.5K, operating a temperature measuring host to control the temperature of a standard thermometer to slowly rise so as to control the internal temperature of the vacuum chamber to slowly rise to room temperature, and collecting calibration data for the entire temperature zone from the liquid helium temperature zone to room temperature.

9. The calibration method based on Raman distributed optical fiber temperature measurement in a low temperature environment according to claim 7, characterized in that: The process of collecting the temperature of the standard thermometer and the Raman signal of the optical fiber and fitting to obtain the corresponding relationship between the Raman light intensity and the temperature includes: Step 1: The temperature measuring host collects the temperature of the standard thermometer, and the optical fiber temperature measuring system collects the Raman signal of the optical fiber; Step 2, performing partition denoising on the Raman signal of the optical fiber to obtain a denoised Raman signal; Step 3: Divide the denoised Raman signal into a set number of temperature segments, and sequentially fit the denoised Raman signal and temperature of each temperature segment to obtain a fitting function, thereby obtaining a corresponding relationship between Raman light intensity and temperature.

10. The calibration method based on Raman distributed optical fiber temperature measurement in a low temperature environment according to claim 9, characterized in that: The partition denoising process in step 2 includes: setting the number of samples for each time to N, dividing the N sampling points into a Raman sampling interval, a truncation interval and a noise interval, for each sampling, the sampling points collected on the optical fiber are included in the Raman sampling interval, the sampling value of the noise interval is the noise signal collected this time, accumulating the signals in the noise interval and averaging them to obtain an average value of the noise interval, and subtracting the average value of the noise interval from the signal in the Raman sampling interval to obtain a denoised Raman signal.

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