Optical fiber temperature sensing system based on photon counting optical time domain reflectometry

Abstract

The present invention provides an optical fiber temperature sensing system based on a photon counting optical time domain reflectometer, which is characterized in that it consists of a pulse laser, a three-port optical fiber circulator, an optical fiber under test (FUT), an optical fiber sensor, a single photon detector, a signal readout circuit, a time correlated single photon counter (TCSPC), and a system control and data processing module; it can be used for real-time monitoring of the external environment temperature and can be widely used in the fields of metallurgy, construction, power systems, petrochemicals, etc.

Description

(I) Technical field

[0001] The present invention relates to an optical fiber temperature sensing system based on a photon counting optical time domain reflectometer, which can be used for real-time monitoring of external temperature and can be widely used in fields such as metallurgy, construction, power systems and petrochemicals, belonging to the field of optical fiber sensing technology. (II) Background technology

[0002] Fiber optic sensing technology is a new technology developed in the 1970s. With the large-scale practical application of optical fiber, fiber optic sensors have been widely used due to their small size, light weight, low transmission loss, no electromagnetic interference, and the ability to work in harsh environments such as flammable and explosive. During the transmission of light, optical fiber is easily affected by the external environment (temperature, strain, deformation), which causes changes in optical parameters such as light intensity, phase, wavelength, frequency and polarization state. Fiber optic sensing technology is developed by monitoring the changes in these optical parameters to obtain the corresponding information to be measured.

[0003] A distributed fiber optic sensing system is a sensing system that uses the optical fiber under test on a fiber optic link as a sensing medium. It is capable of remote and real-time monitoring of objects within the range of the fiber optic link. It integrates sensing and optical signal transmission to achieve long-distance measurement and real-time monitoring. A one-dimensional distribution map of the entire optical fiber can be obtained through only one measurement. Only one set of system is needed to obtain a large amount of data information from a fiber optic link of tens or even hundreds of kilometers.

[0004] Optical Time Domain Reflectometer (OTDR) is a measuring instrument based on the backscattering theory and used to measure the transmission characteristics of the optical fiber under test. Its basic principle is that the pulsed light signal emitted by the pulsed laser generates Rayleigh backscattering signal and Fresnel reflection during the transmission along the optical fiber under test, and the information of the echo signal is received by the photoelectric detector, which is then processed to obtain the characteristic information of the optical fiber under test.

[0005] Traditional optical time domain reflectometers use analog detection technology, which is limited by the bandwidth and response time of analog detection technology. The most prominent contradiction between large dynamic range and high spatial resolution has greatly restricted the application scenarios of traditional optical time domain reflectometers.

[0006] The optical time domain reflectometer based on photon counting solves the contradiction between dynamic range and spatial resolution very well. Compared with the traditional optical time domain reflectometer based on linear photoelectric detection, the optical time domain reflectometer based on photon counting benefits from the extremely high detection sensitivity of the single-photon detector (the internal gain of the detector is 3-4 orders of magnitude higher than that of the traditional photoelectric detector), which greatly improves the weak light detection capability, thereby improving the detection speed of the system, reducing the detection time of the system, and effectively reducing or eliminating the test blind area of ​​the system, thereby obtaining high spatial resolution and large dynamic range at the same time.

[0007] The current temperature sensing system mainly uses the temperature characteristics of different types of scattered light in optical fibers to achieve the purpose of temperature sensing. It can be divided into three categories: distributed temperature sensors based on Rayleigh backscattering, distributed temperature sensors based on Raman scattering, and distributed temperature sensors based on Brillouin scattering.

[0008] Distributed temperature sensors based on Raman scattering have been widely used in the energy industry. For example, in the literature, “High-resolution single-mode fiber-optic distributed Raman sensor for absolute temperature measurement using superconducting nanowire single-photondetectors”, the system can obtain high-precision, high-spatial-resolution temperature measurements without the need for a separate temperature calibration standard. However, this type of temperature sensor requires complex frequency shift processing, has high manufacturing costs, and has relatively complex system results.

[0009] Distributed temperature sensors based on Brillouin scattering are a new type of temperature sensing technology. For example, the literature "A Distributed Brillouin Temperature Sensor sing a Single-PhotonDetector" proposes using the Rayleigh / Anti-Stokes ratio to measure temperature information. This type of temperature sensor has the characteristics of a large dynamic range and high sensitivity, but it also involves the frequency shift of backscattered light, requiring complex frequency shift processing. The system is complex and the manufacturing cost is high, making it difficult to achieve large-scale commercial applications.

[0010] Distributed temperature sensors based on Rayleigh scattering optical time-domain reflectometry technology have also been widely studied in recent years. For example, the document "A low-cost new distributed temperature sensor based on temperature-sensitive optical fiber and OTDR technology" proposes relevant design and verification; although no complex frequency shift processing is required and the system structure is relatively simple, the Rayleigh scattering in the optical fiber is less sensitive to temperature, so only special optical fibers with temperature-sensitizing capabilities can be used; or the optical fiber under test is irradiated and annealed at high temperature, making the optical fiber under test sensitive to temperature. This type of optical fiber has high manufacturing costs and is difficult to use on a large scale.

[0011] In order to solve the above problems, the present invention discloses an optical fiber temperature sensing system based on a photon counting optical time domain reflectometer, which can be used for real-time monitoring of the external environment temperature and can be widely used in the fields of metallurgy, smelting, construction, power system and petrochemical industry. The invention is based on a single photon counting optical time domain reflectometer. Benefiting from the excellent performance of the single photon detector, the system can achieve low-noise operation with low dark count rate and low after-pulse rate, improve the signal-to-noise ratio of the single photon counting optical time domain reflectometer system, greatly reduce the test time required for the system, and improve the measurement speed. At the same time, the present invention proposes to add an optical fiber sensor to the optical fiber under test, use the relationship between the number of photons and temperature to characterize the external temperature change, and monitor the external temperature change in real time while obtaining the characteristic information of the optical fiber under test. Compared with the distributed temperature sensor based on Rayleigh backscattering, the distributed temperature sensor based on Raman scattering and the distributed temperature sensor based on Brillouin scattering, neither complex frequency shift processing nor special optical fiber is required. The system has the characteristics of simple design, low price, stable system and strong practicality. (III) Summary of the invention

[0012] The purpose of the present invention is to provide a fiber optic temperature sensing system based on a photon counting optical time domain reflectometer which can be used to monitor the external temperature in real time, has low cost, simple design and strong stability; and can be widely used in the fields of metallurgy, construction, power systems and petrochemical industry.

[0013] The object of the present invention is achieved in that:

[0014] The optical fiber temperature sensing system based on photon counting optical time domain reflectometer is composed of a pulse laser (1), a three-port optical fiber circulator (2), an optical fiber under test (FUT) (3), an optical fiber sensor (4), a single photon detector (5), a signal readout circuit (6), a time-correlated single photon counter (TCSPC) (7) and a system control and data processing module (8); the pulse laser (1) outputs a synchronous electrical signal (SYNC) while outputting a pulse optical signal to trigger the time-correlated single photon counting system (TCSPC) (7) to start counting, the pulse optical signal enters through the a port of the three-port optical fiber circulator (2), and is output to the optical fiber under test (3) through the b port, n optical fiber sensors (4) are connected in the optical fiber under test, and the backscattered optical signal generated in the optical fiber under test (FUT) (3) and the optical fiber sensor (4) is coupled again and enters the three-port optical fiber circulator (2) The b port is output to the single photon detector (5) through the c port. After receiving each pulse light signal, the single photon detector (5) reads out and outputs a corresponding electrical signal together with the signal readout circuit (6) and transmits it to the time-correlated single photon counter (TCSPC) (7) to stop counting. The time-correlated single photon counter (TCSPC) (7) records the time difference between the synchronous electrical signal and the electrical signal output by the single photon detector on the one hand; on the other hand, it counts the number of detected photons and transmits the counted photon count-time histogram data to the system control and data processing module (8) through USB for processing. By demodulating the change in the number of photons at the optical fiber sensor, the purpose of real-time monitoring of the external temperature change is achieved. At the same time, the system control and data processing module 7 can change the maximum detection length and detection accuracy of the system by modulating the pulse repetition frequency and pulse width of the pulse laser.

[0015] When the pulse laser outputs a pulse light signal, it also outputs a synchronous electrical signal to trigger the time-correlated single photon counting system to start counting.

[0016] The pulse optical signal enters through the a port of the three-port optical fiber circulator and is output to the optical fiber under test through the b port.

[0017] There are n optical fiber sensors connected to the optical fiber under test. The optical fiber sensor is a cascade structure consisting of single-mode fiber (SMF)-hollow-core fiber (COF)-single-mode fiber (SMF). Since the core of the hollow-core fiber is injected with colloidal substances, which have a different refractive index from the core of the single-mode fiber, two reflection interfaces are formed at the two melting points of the single-mode fiber and the hollow-core fiber, which are denoted as M1 and M2 respectively. When the pulse light signal is incident on the reflection interface, the reflectance R1 and R2 and the transmittance T1 and T2 of M1 and M2 are obtained by equations (1) and (2) respectively:

[0018]

[0019]

[0020] Where n1 refers to the core refractive index of the single-mode optical fiber, and n2 refers to the refractive index of the colloid substance in the hollow-core optical fiber.

[0021] The reflected power at the reflection interface of the optical fiber sensor has two main values. The first value is the reflected light power when the optical signal passes through the first reflection interface. The second value is the reflected light power that passes through the first reflection interface, is reflected back by two reflection surfaces, and then is transmitted back again. Assuming that the power of the pulse light signal to the first reflection interface is P, the reflected power P1 here can be obtained by formula (3):

[0022]

[0023] Then the equivalent reflectivity R of the optical fiber sensor can be obtained from formula (4):

[0024]

[0025] When the external temperature changes, the refractive index of the colloidal substance in the hollow-core optical fiber of the optical fiber sensor will change with the temperature, resulting in a change in the amount of backscattered light signals returned at the fusion point (reflection interface).

[0026] The backscattered light signal is input again through the b port of the three-port fiber circulator and output to the single-photon detector through the c port. Every time the single-photon detector receives an optical signal, it reads out and outputs an electrical signal corresponding to the signal readout circuit and transmits it to the time-correlated single-photon counter (TCSPC) to stop counting. During the signal detection process, the detection noise depends on the dark count rate of the single-photon detector, the photon counting noise equivalent power (minimum detection power) NEP pc From formula (5), we can get:

[0027]

[0028] Where h is Planck's constant, v is the photon frequency, η is the detection efficiency of the single-photon detector, and p dc is the dark count rate of the single-photon detector.

[0029] Every time a time-correlated single-photon counter (TCSPC) detects an electrical signal, it measures the arrival time of the photons in that cycle (the time difference between the synchronous electrical signal and the electrical signal output by the single-photon detector) and adds 1 to the corresponding time channel. Each time channel is a storage unit, and the channel width determines the minimum timing accuracy of the system. After recording a large number of single-photon events over many cycles, the total count in each storage unit corresponds to the signal intensity in the time channel, from which a photon number-time distribution histogram can be established to obtain the original signal waveform.

[0030] The original signal waveform is processed by the system control and data processing module to obtain the relationship between temperature and photon number. Then, by demodulating the change in photon number at the optical fiber sensor, the purpose of real-time monitoring of external temperature changes is achieved.

[0031] Depending on the length of the optical fiber being measured, the system control and data processing module can control the pulse repetition frequency and pulse width of the pulse laser, thereby changing the maximum measurable range and positioning accuracy of the system; at the same time, the pulse repetition frequency of the pulse laser must ensure that only one light pulse propagates in the optical fiber being measured at the same time, and at the same time send out a synchronous electrical signal to trigger the time-correlated single-photon counter to ensure the clock synchronization of the two.

[0032] Compared with the prior art, the present invention has the following beneficial inventive features.

[0033] The present invention discloses an optical fiber temperature sensing system based on a photon counting optical time domain reflectometer. The single photon detector in the system can realize low-noise operation with low dark count rate and low after-pulse rate, thereby improving the signal-to-noise ratio of the single photon counting optical time domain reflectometer, improving the measurement speed, greatly reducing the measurement time required for the system to obtain data, and having a larger dynamic range and higher spatial resolution. At the same time, the present invention proposes adding an optical fiber sensor to the optical fiber under test, using the corresponding relationship between the number of photons and temperature to characterize the external temperature change, and achieving the purpose of real-time monitoring of the external temperature change by demodulating the change in the number of photons at the optical fiber sensor. The system has the characteristics of simple design, low price, stable system, strong practicality, etc. (IV) Description of the drawings

[0034] Figure 1 The invention relates to an optical fiber temperature sensing system based on a photon counting optical time domain reflectometer; the system comprises a pulse laser (1), a three-port optical fiber circulator (2), an optical fiber under test (FUT) (3), an optical fiber sensor (4), a single photon detector (5), a signal readout circuit (6), a time correlated single photon counter (TCSPC) (7), and a system control and data processing module (8).

[0035] Figure 2It is a structural schematic diagram of a distributed optical fiber temperature sensor in the optical fiber under test; the optical fiber sensor is formed by cascading single-mode optical fiber (SMF) 4-1-hollow core optical fiber (COF) 4-4-single-mode optical fiber (SMF) 4-1; in the production of the optical fiber temperature sensor, a hollow core optical fiber injected with a colloidal substance is melted into two single-mode optical fibers through an optical fiber fusion splicer, thereby achieving efficient coupling between the single-mode optical fiber and the hollow core optical fiber; the diameter of the cladding 4-2 of the single-mode optical fiber 4-1 is 125μm, and the diameter of the core 4-3 is 9μm; the diameter of the cladding 4-5 of the hollow core optical fiber 4-4 is 125μm, and the core diameter of the core 4-6 is 9μm.

[0036] Figure 3 The present invention is a schematic diagram of an embodiment of a fiber optic temperature sensing system based on photon counting; the system comprises a pulsed laser (1), a three-port fiber optic circulator (2), a fiber under test (FUT) (3), a fiber optic sensor (4), an avalanche photodiode detector (5), an active quenching and reset integrated circuit (AQR-IC) (6), a time-correlated single photon counter (TCSPC) (7), and a system control and data processing module (8).

[0037] Figure 4 This is the temperature-photon number correspondence diagram obtained by the optical fiber sensor composed of single-mode fiber (SMF)-hollow core fiber (COF)-single-mode fiber (SMF) with a hollow core fiber length of 50μm under measurement time of (a) T=2s; (b) T=5s; (c) T=10s; (d) T=20s.

[0038] Figure 5 This is the temperature-photon number correspondence diagram obtained by the optical fiber sensor composed of single-mode fiber (SMF)-hollow core fiber (COF)-single-mode fiber (SMF) with a hollow core fiber length of 500μm under measurement time of (a) T=2s; (b) T=5s; (c) T=10s; (d) T=20s. (V) Specific implementation methods

[0039] In order to make the purpose, technical solution and advantages of the embodiment of the present invention more clear, the counting scheme in the embodiment of the present invention will be clearly described below in conjunction with the drawings in the embodiment of the present invention.

[0040] Figure 3An embodiment of a distributed optical fiber temperature system based on photon counting is provided. The system is composed of a pulse laser (1), a three-port optical fiber circulator (2), an optical fiber under test (FUT) (3), an optical fiber sensor (4), an avalanche photodiode detector (5), an active quenching and reset integrated circuit (AQR-IC) (6), a time-correlated single photon counter (TCSPC) (7), and a system control and data processing module (8); the pulse laser (1) outputs a synchronous electrical signal (SYNC) while outputting a pulse optical signal to trigger the time-correlated single photon counting system (TCSPC) (7) to start counting, the pulse optical signal enters through the a port of the three-port optical fiber circulator (2), and is output to the optical fiber under test (3) through the b port, n optical fiber sensors (4) are connected to the optical fiber under test, and the backscattered optical signal generated in the optical fiber under test (3) and the optical fiber sensor (4) is coupled again to enter the b port of the three-port optical fiber circulator (2), and is output through the c port. To the avalanche photodiode detector (5), after receiving each pulse light signal, the avalanche photodiode detector (5) reads out and outputs a corresponding electrical signal together with the active quenching and reset integrated circuit (AQR-IC) (6) and transmits it to the time-correlated single photon counter (TCSPC) (7) to stop counting. The time-correlated single photon counter (TCSPC) (7) records the time difference between the synchronous electrical signal and the electrical signal output by the single photon detector; on the other hand, it counts the number of detected photons and transmits the photon count-time histogram data to the system control and data processing module (8) through USB for processing. By demodulating the change in the number of photons at the optical fiber sensor, the purpose of real-time monitoring of the external temperature change is achieved; at the same time, the system control and data processing module 7 can change the maximum detection length and detection accuracy of the system by modulating the pulse repetition frequency and pulse width of the pulse laser.

[0041] When the pulse laser outputs a pulse light signal, it also outputs a synchronous electrical signal to trigger the time-correlated single-photon technology system to start counting; the pulse repetition frequency of the pulse laser is modulated by the system control and data processing module, so that only one light pulse propagates in the optical fiber under test at the same time, and at the same time, a synchronous electrical signal is sent out to trigger the time-correlated single-photon counter to ensure that the clocks of the two are synchronized.

[0042] The pulse optical signal enters through the a port of the three-port optical fiber circulator and is output to the optical fiber under test through the b port.

[0043] Two optical fiber sensors are connected to the optical fiber under test. The sensor is a cascade structure consisting of single-mode fiber (SMF)-hollow-core fiber (COF)-single-mode fiber (SMF). Since colloidal substances are injected into the core of the hollow-core fiber, which has a different refractive index from the core of the single-mode fiber, two reflection interfaces are formed at the two melting points of the single-mode fiber and the hollow-core fiber. When a pulsed light signal is incident on the reflection interface, its refractive index changes, resulting in a change in the number of photons.

[0044] When the external temperature changes, the refractive index of the colloidal substance in the hollow-core optical fibers of the two optical fiber sensors changes with the change of the external temperature, thereby causing the amount of backscattered light signals returned at the reflection interface to change accordingly.

[0045] The backscattered light signal is input again through the b port of the three-port fiber circulator, and then output to the avalanche photodiode detector through the c port. The avalanche photodiode detector together with the active quenching and reset integrated circuit (AQR-IC) reads it out and outputs an electrical signal to the time-correlated single photon counting system (TCSPC) to stop counting. Every time the time-correlated single photon counting system detects an electrical signal, it measures the arrival time of the photons in that period and adds 1 to the corresponding time channel.

[0046] Each time channel is a storage unit. After many cycles of recording a large number of single-photon events, the total count in each storage unit corresponds to the signal intensity in the time channel. From this, a photon number-time distribution histogram can be established to obtain the original signal waveform.

[0047] The original signal waveform is processed by the system control and data processing module to obtain the relationship between temperature and photon number. Then, by demodulating the change in photon number at the optical fiber sensor, the purpose of real-time monitoring of external temperature changes is achieved.

Claims

1. An optical fiber temperature sensing system based on photon counting optical time domain reflectometry; characterized by: It consists of a pulse laser (1), a three-port optical fiber circulator (2), an optical fiber to be tested (3), an optical fiber sensor (4), a single-photon detector (5), a signal readout circuit (6), a time-correlated single-photon counter (7), and a system control and data processing module (8); in the system, the pulse laser (1) outputs a synchronous electrical signal while outputting a pulse optical signal to trigger a time-correlated single-photon counting system (7) to start counting; the pulse optical signal enters through the a port of the three-port optical fiber circulator (2), and is output to the optical fiber to be tested (3) through the b port; n optical fiber sensors (4) are connected to the optical fiber to be tested; the backscattered optical signal generated in the optical fiber to be tested (3) and the optical fiber sensor (4) is coupled again to enter the b port of the three-port optical fiber circulator (2), and is output to the single-photon counter through the c port; In the detector (5), after receiving each pulse light signal, the single photon detector (5) reads out and outputs a corresponding electrical signal together with the signal readout circuit (6) and transmits it to the time-correlated single photon counter (7) to stop counting. The time-correlated single photon counter (7) records the time difference between the synchronous electrical signal and the electrical signal output by the single photon detector on the one hand; on the other hand, it counts the number of detected photons and transmits the counted photon count-time histogram data to the system control and data processing module (8) via USB for processing. By demodulating the change in the number of photons at the optical fiber sensor, the purpose of real-time monitoring of the external temperature change is achieved; at the same time, the system control and data processing module 7 can respectively change the maximum detection length and detection accuracy of the system by modulating the pulse repetition frequency and pulse width of the pulse laser; The optical fiber sensor (4) is a cascade structure consisting of a single-mode optical fiber-hollow-core optical fiber-single-mode optical fiber. In the optical fiber sensor (4), a colloidal substance is injected into the core of the hollow-core optical fiber, and its refractive index is different from that of the core of the single-mode optical fiber. After being cured by ultraviolet light, it is fused and connected between two sections of single-mode optical fiber by an optical fiber fusion splicer to form two reflection interfaces. When a pulse light signal is incident on the reflection interface, its refractive index changes, resulting in a change in the number of backscattered photons.

2. The optical fiber temperature sensing system based on photon counting optical time domain reflectometry according to claim 1 is characterized in that: The pulse laser (1) outputs a synchronous electrical signal while outputting a pulse light signal to trigger a time-correlated single-photon counting system to start counting. The pulse width and pulse repetition frequency of the pulse laser are modulated by a system control and data processing module. The pulse laser can be a yttrium aluminum garnet laser, a ruby ​​laser or an excimer laser.

3. The optical fiber temperature sensing system based on photon counting optical time domain reflectometry according to claim 1, characterized in that: The single photon detector (5) converts the optical signal into an electrical signal, and may be any one of a photomultiplier tube, a single photon avalanche photodiode or a superconducting nanowire single photon detector.

4. The optical fiber temperature sensing system based on photon counting optical time domain reflectometry according to claim 1 is characterized in that: The signal readout circuit (6) reads out the backscattered light signal received by the single-photon detector and outputs an electrical signal correspondingly for transmission to a time-correlated single-photon counter.

5. The optical fiber temperature sensing system based on photon counting optical time domain reflectometry according to claim 1, characterized in that: The time-correlated single-photon counter (7) can be used to record the time difference between the synchronous electrical signal and the electrical signal output by the single-photon detector; it can be used to count the number of detected photons, thereby characterizing the radiation flux of light; at the same time, the time-correlated single-photon counter (7) will transmit the counted photon count-time histogram data to the system control and data processing module via USB for processing, and the system resolution accuracy and detection time of the time-correlated single-photon counter (7) are controlled by the system control and data processing module.

6. The optical fiber temperature sensing system based on photon counting optical time domain reflectometry according to claim 1, characterized in that: The system control and data processing module (8) can be used to modulate the pulse width and pulse repetition frequency of the pulse light signal emitted by the pulse laser, thereby achieving the purpose of changing the positioning accuracy and maximum detection length of the system; can be used to control the system resolution accuracy and detection time of the time-correlated single photon counter; can be used to process the photon count-time histogram data counted by the time-correlated single photon counter, and achieve the purpose of real-time monitoring of external temperature changes by demodulating the photon number changes at the optical fiber sensor.

Images