A method and device for optical fiber distributed detection of trace multi-component gas

Abstract

The present invention provides a method and device for distributed detection of trace multi-component gas in optical fiber. The detection method includes: the gas to be measured freely diffuses into the core of a hollow-core optical fiber with holes drilled on the side; the pump light and the seed light are coupled into the optical fiber from opposite directions; the gas to be measured is excited by the pump light and the seed light to generate a second Raman gain signal, the pump light is sequentially excited at the interface fused silica at both ends of the hollow-core optical fiber to generate the first and third Raman gain signals, and the first, second and third stimulated Raman gain signals are detected; the time when the second Raman gain signal reaches the photodetector is measured to determine the position of the gas to be measured; the intensity of the second Raman gain signal is measured to obtain the distributed concentration of the gas to be measured along the length of the hollow-core optical fiber. The device includes: multiple seed lasers, an optical switch, a polarization controller, an isolator, a hollow-core antiresonant optical fiber with holes drilled on the side, a circulator, a pump light filter, a photodetector, a pump laser, and fused silica at the optical fiber interface.

Description

Technical Field

[0001] The present invention belongs to the technical field of characteristic gas detection in electric power equipment, and in particular relates to an optical fiber distributed detection method and device for detecting multi-component characteristic gases by utilizing the stimulated Raman effect of gases. Background Art

[0002] Gaseous substances always accompany human production and life processes, but they are usually difficult to detect. Equipment operation, pipeline laying, and metabolism will release various gases into the environment. Real-time monitoring and precise positioning of trace gas substances are prerequisites for judging equipment failures, preventing pipeline leakage, and protecting the natural environment. In the power industry, various gases that can reflect the nature of equipment failures and insulation aging performance will be decomposed inside equipment such as power transformers and GIS used for power and nuclear energy; accurate detection of fault and aging characteristic gases is the key to realizing the diagnosis of the operating status of power transmission and transformation equipment; for example, GIL (gas insulation transmission line) is laid and applied on a large scale, and the safe operation and fault location of several kilometers of GIL rely on high-precision distributed detection of SF6, O2 and decomposed fault characteristic gases (H2S, SO2, SO2F2, CF4, etc.). Therefore, the establishment of a gas sensor network to realize the all-round, long-distance, real-time, multi-component, and high-sensitivity distributed detection of gas in power transmission and gas pipelines, large equipment, and confined spaces is of vital importance to the fields of energy and electricity.

[0003] In recent years, the mainstream detection methods for gas detection include: gas chromatography, nanomaterial gas sensing, fiber Bragg grating (FBG), Raman spectroscopy, laser absorption spectroscopy, photoacoustic spectroscopy, etc. Among them, gas chromatography has the disadvantages of complex structure, carrier gas and regular maintenance; although the nanomaterial gas sensing method has the advantages of simple structure, fast response rate and low power consumption, it has serious cross-interference; fiber Bragg grating (FBG) is only sensitive to H2 but cannot detect hydrocarbons; laser absorption spectroscopy is designed based on the characteristics that gas can directly absorb laser, but it requires a large gas absorption cell and is easily affected by light scattering and refraction. However, compared with spontaneous Raman scattering, stimulated Raman scattering has a more obvious threshold, better directionality (directionality), better monochromaticity and higher intensity of scattered light.

[0004] Therefore, the present invention adopts stimulated Raman detection technology as a gas detection method. At the same time, in order to reduce the stimulated Raman scattering threshold, the laser band is set in the visible light band with stronger Raman gain. In addition, the visible light band also has a high peak power laser, combined with the high laser damage threshold of the anti-resonant fiber, a higher detection sensitivity can be achieved.

[0005] Prior art document 1 (CN112683876A) discloses a method and system for optical thermal stimulated Raman spectroscopy detection of trace gases in optical fibers. The gas to be tested is filled into the hollow-core optical fiber by drilling holes on the side; the pump light and the first detection light are modulated and enter the hollow-core optical fiber through a coupling lens; the wavelength of the pump light is adjusted so that the energy difference between the laser wavelengths of the pump light and the first detection light is equal to the transition energy difference of the gas to be tested; the first detection light is used as a stimulated Raman trigger laser, and together with the periodically modulated pump light, it excites the gas to produce a stimulated Raman process, which affects the phase change of the second detection light; the second detection light forms a multi-beam interference light in the FP cavity, and the interference light phase signal is demodulated by a demodulator to obtain the gas concentration. The shortcoming of the prior art document 1 is that it only realizes the detection of single-point gas concentration; it uses the phase change of the detection light to reflect the gas concentration, but vibration, temperature, strain, etc. will cause the phase change of the light, resulting in large detection errors. The improvement of the present invention over the prior art is that it uses pulsed pump laser and seed light to excite the gas to generate stimulated Raman gain signals, and detects the time and intensity of the Raman gain signal reaching the photodetector to realize the detection of distributed concentration along the length of the hollow-core optical fiber. In addition, the stimulated Raman gain signal is basically not affected by vibration, temperature, strain, etc., and has high detection accuracy. Summary of the invention

[0006] The purpose of the present invention is to establish a multi-component gas stimulated Raman distributed sensing network to achieve high-precision distributed sensing of multi-component gases. The present invention provides a method and device for optical fiber distributed detection of trace multi-component gases.

[0007] In the first aspect, the present invention discloses a method for optical fiber distributed detection of trace multi-component gas, characterized in that the method is used for detecting the position and concentration of trace multi-component gas, and comprises the following steps:

[0008] Step 1, the gas to be tested freely diffuses into the core of the hollow-core optical fiber with holes punched on the side;

[0009] Step 2, coupling the pump light and seed light of the set frequency into the punched hollow core optical fiber from opposite directions;

[0010] Step 3, the gas to be measured is excited by the pump light and the seed light of the set frequency to generate the second Raman gain signal, the pump light is excited in turn at the interface fused silica at both ends of the hollow core optical fiber to generate the first Raman gain signal and the third Raman gain signal, and the first, second and third stimulated Raman gain signals are detected;

[0011] Step 4, measuring the time when the first and third Raman gain signals arrive at the photodetector to calibrate the position of the hollow core optical fiber;

[0012] Step 5, measuring the time when the second Raman gain signal reaches the photoelectric detector to determine the position of the gas to be measured;

[0013] Step 6, measuring the intensity of the second Raman gain signal to obtain the concentration of the gas to be measured; the obtained concentration of the gas to be measured is a distributed concentration along the length of the hollow-core optical fiber.

[0014] Preferably, in step 2, the pump laser is a pulsed light with a wavelength not greater than 532 nm; the seed light is a continuous light, the difference between its frequency and the frequency of the pump light is equal to the vibration or rotation energy level of the gas molecules to be measured;

[0015] The number of seed lights is equal to the type of gas to be detected.

[0016] Preferably, in step 4, the time when the first Raman gain signal reaches the photodetector for the first time is defined as time 0.

[0017] Preferably, by sequentially measuring the time t and the light speed c of the second Raman gain signal of the corresponding gas reaching the photodetector, the position D=t*c / 2 of each gas is obtained, thereby achieving distributed measurement of the gas to be measured.

[0018] Preferably, in step 5, an optical switch is used to quickly switch the seed laser to sequentially achieve a distributed concentration of each single gas hollow core fiber length;

[0019] Among them, the switching speed of the optical switch is less than 0.5ms.

[0020] Preferably, in step 5, the spatial resolution of the gas distributed concentration is determined by the hole density on the side of the hollow core optical fiber and the pulse width of the pump laser;

[0021] The detection distance of the gas distributed concentration is determined by the pulse frequency of the pump laser.

[0022] In a second aspect, the present invention further discloses a trace multi-component gas optical fiber distributed detection device, the device comprising:

[0023] A plurality of seed lasers (1-6), an optical switch (7), a polarization controller (8), an optical isolator (9), a hollow core anti-resonant optical fiber with a side hole drilled therein (10), a circulator (11), a pump optical filter (14), a photodetector (12), a pump laser (13), and fused silica (15) at the optical fiber interface;

[0024] The seed laser is efficiently coupled into the fiber core from the front end of the punched hollow core anti-resonant fiber via an optical switch, a fiber polarization controller, an optical isolator and a solid core quartz fiber;

[0025] The pump laser is efficiently coupled into the fiber core from the rear end of the punched hollow core anti-resonance fiber via a circulator and a solid core quartz fiber.

[0026] Preferably, the gas or interface fused silica stimulated Stokes Raman gain signal reaches the photodetector via the fiber circulator, the pump light filter and the solid core silica fiber.

[0027] Preferably, the fiber polarization controller is used to change the polarization state of the seed and pump lasers to increase the intensity of stimulated Raman gain;

[0028] The pump light filter is used for the residual pump light signal, so that the photodetector detects the reverse Stokes Raman gain signal.

[0029] Preferably, the interface fused silica is formed by melting a solid core silica fiber and a punched hollow core anti-resonant fiber during butt coupling.

[0030] 1. The beneficial effect of the present invention is that the position of the gas to be measured can be accurately located and the concentration of the gas to be measured can be identified;

[0031] 2. Rapid measurement of distributed concentration of multi-component gases can be achieved by rapidly switching the seed light;

[0032] 3. Filter the residual pump light signal so that the photodetector measures the reverse Stokes Raman gain signal as much as possible. By directly demodulating the seed light power difference, the error generation path is reduced, which is fast, convenient and highly accurate.

[0033] 4. Hollow-core antiresonant fiber has the characteristics of low transmission loss, wide transmission passband and high laser damage threshold. Hollow-core antiresonant fiber is used to adjust the polarization state of pump laser and seed laser to improve the Raman gain intensity;

[0034] 5. Setting the laser band to the visible light band with stronger Raman gain can achieve higher detection sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Some specific embodiments of the present invention will be described in detail below in an exemplary and non-restrictive manner with reference to the accompanying drawings. The same reference numerals in the accompanying drawings indicate the same or similar components or parts. It should be understood by those skilled in the art that these drawings are not necessarily drawn to scale.

[0036] Figure 1 It is a schematic diagram of the optical fiber distributed detection method for trace multi-component gases.

[0037] Figure 2 It is a schematic diagram of components of a trace multi-component gas optical fiber distributed detection device;

[0038] 1-6-seed laser, 7-optical switch, 8-polarization controller, 9-optical isolator, 10-hollow core antiresonant optical fiber with side holes drilled, 11-circulator, 12-photodetector, 13-pump laser, 14-pump light filter, 15-fused silica at the optical fiber interface.

[0039] Figure 3 This is a schematic diagram of the analysis of gas optical fiber distributed sensing signals based on backward stimulated Raman. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0041] like Figure 1 As shown, the optical fiber distributed detection method for trace multi-component gas according to the present invention comprises the following steps:

[0042] Step 1: The gas to be tested diffuses freely into the core of a long-distance side-punched hollow-core optical fiber.

[0043] That is, the gas to be measured is filled into the core of a long-distance hollow-core optical fiber with holes punched on the side.

[0044] Step 2, making the pump light and seed light of the set frequency efficiently coupled into the hollow core optical fiber from opposite directions;

[0045] The seed light of the set frequency is efficiently coupled into the fiber core from the front end of the punched hollow-core anti-resonant fiber through the optical switch, fiber polarization controller, optical isolator and solid-core quartz fiber. Among them, the pump light is a pulse light with a wavelength not greater than 532nm; the seed light is a continuous light, and the difference between its frequency and the frequency of the pump light is equal to the vibration or rotation energy level (Raman frequency shift number) of the gas molecules to be measured; the number of seed lights is equal to the type of gas to be measured.

[0046] Step 3, the gas to be measured is jointly excited by the set frequency pump light and seed light to produce stimulated Raman scattering effect, that is, when the difference between the pump laser output frequency and the seed laser frequency is just equal to the gas molecule vibration or rotation energy level (Raman frequency shift number), the gas will produce stimulated Raman effect and generate a second Raman gain signal; the pump light is sequentially stimulated at the interface fused silica at both ends of the hollow-core optical fiber to generate a first Raman gain signal and a third Raman gain signal, and the first, second, and third Raman gain signals are detected.

[0047] Step 4, measuring the time when the first and third Raman gain signals arrive at the photodetector to calibrate the position of the hollow core optical fiber;

[0048] Step 5, measuring the time when the second Raman gain signal reaches the photoelectric detector to determine the position of the gas to be measured;

[0049] like Figure 3 As shown in the figure, the group index of the fundamental mode in the hollow core fiber is ~1, so the pump light and seed light propagate at approximately the speed of vacuum light in the hollow core fiber. When the pump light enters the hollow core fiber from the single-mode quartz pigtail, it will first experience the backward stimulated Raman gain (first Raman gain signal) of the fused silica at the fiber interface, and the time when this gain reaches the photodetector is the starting time (t=0). After the pump light enters the fiber, it encounters the gas molecules to be measured at position D at time t (t=D / c). The backward Stokes Raman gain (second Raman gain signal) generated by the gas molecules to be measured at D is also captured by the photodetector after a distance of D. The time at this time is recorded as t1 (t1=2D / c). After the pump light encounters the gas to be measured in the hollow core fiber, it continues to move forward and finally reaches the output solid core quartz pigtail to generate the backward stimulated Raman gain (third Raman gain signal) of the fused silica at the other end. The time when this gain reaches the photodetector is recorded as the cut-off time t3 (t3=2L / c, where L is the length of the hollow core fiber). The distance D is obtained by measuring the time t1, D=t1*c / 2, thereby realizing the hollow-core optical fiber distributed measurement of the gas to be measured.

[0050] Step 6, measuring the intensity of the second Raman gain signal to obtain the concentration of the gas to be measured; the obtained concentration of the gas to be measured is a distributed concentration along the length of the hollow-core optical fiber.

[0051] Because the stimulated Raman effect will amplify the Stokes seed light, the amplification of the seed light is measured, and then the signal amplification value equation of the seed light is used.

[0052] ΔI S =∝g S I S I P ∝I S I P ΔN / Γ

[0053] Calculate the difference in number density of the gas molecules to be measured.

[0054] Among them, I S , I P is the input seed light and pump light power, the Raman line width is in Γ, ΔN is the gas molecule number density difference between the number of particles at different vibrational energy levels, g sThe Raman gain factor is related to polarization, because the amount of seed light amplification is proportional to the concentration of the gas to be measured, and then the concentration of the gas to be measured is obtained. When the seed light corresponding to the Stokes shift number of each gas exists at the same time, the stimulated Raman of each gas will compete. Therefore, the present invention uses an optical switch to quickly switch the seed light to avoid competition. The optical switch is used to quickly switch the seed laser to realize the distributed concentration of each single gas hollow core fiber length in turn. Among them, the switching speed of the optical switch is less than 0.5ms.

[0055] The spatial resolution of the gas distributed concentration is determined by the hole density on the side of the hollow core optical fiber and the pulse width of the pump laser; the detection distance of the gas distributed concentration is determined by the pulse frequency of the pump laser.

[0056] like Figure 2 As shown, the components of the trace multi-component gas hollow-core fiber distributed detection device of the present invention are shown, and the gas to be tested is filled into the core of the long-distance hollow-core fiber 10 with holes punched on the side. The seed light of the set frequency is efficiently coupled into the core from the front end of the punched hollow-core anti-resonant fiber through the optical switch 7, the polarization controller 8, the optical isolator 9 and the solid-core quartz fiber. At the same time, the pump light of the set frequency is efficiently coupled into the core from the rear end of the punched hollow-core anti-resonant fiber through the circulator 11 and the solid-core quartz fiber.

[0057] The fiber polarization controller 8 is used to change the polarization state of the seed and pump lasers to increase the intensity of stimulated Raman gain. The pump light filter 13 is used to filter the residual pump light signal so that the photodetector detects only reverse Stokes Raman gain signals.

[0058] The interface fused silica 15 is formed by melting the solid core silica fiber and the punched hollow core anti-resonant fiber 10 when they are butt-coupled.

[0059] The above description is only a specific implementation of the present invention and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement and improvement made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for optical fiber distributed detection of trace multi-component gases, characterized in that: The method is used to detect the position and concentration of trace multi-component gas, and includes the following steps: Step 1, the gas to be tested freely diffuses into the core of the hollow-core optical fiber with holes punched on the side; Step 2, coupling the pump light and seed light of the set frequency into the perforated hollow core optical fiber from opposite directions; the pump light is pulsed light; the seed light is continuous light, and the difference between its frequency and the frequency of the pump light is equal to the vibration or rotation energy level of the gas molecules to be measured; the number of the seed lights is equal to the type of gas to be measured; Step 3, the gas to be measured is excited by the pump light and the seed light of the set frequency to generate the second Raman gain signal, the pump light is excited in turn at the interface fused silica at both ends of the hollow core optical fiber to generate the first Raman gain signal and the third Raman gain signal, and the first, second and third stimulated Raman gain signals are detected; Step 4, measuring the time when the first and third Raman gain signals arrive at the photodetector to calibrate the position of the hollow core optical fiber; Step 5, measuring the time when the second Raman gain signal reaches the photodetector to determine the position of the gas to be measured; the spatial resolution of the gas distributed concentration is determined by the hole density on the side of the hollow core optical fiber and the pulse width of the pump laser; the detection distance of the gas distributed concentration is determined by the pulse frequency of the pump laser; Step 6, measuring the intensity of the second Raman gain signal to obtain the concentration of the gas to be measured; the concentration of the gas to be measured is a distributed concentration along the length of the hollow-core optical fiber.

2. The optical fiber distributed detection method for trace multi-component gas according to claim 1, characterized in that: In step 4, the time when the first Raman gain signal reaches the photodetector for the first time is defined as time 0.

3. The optical fiber distributed detection method for trace multi-component gas according to claim 2, characterized in that: By sequentially measuring the time t and light speed c of the second Raman gain signal of the corresponding gas reaching the photodetector, the position D=t*c / 2 of each gas is obtained, thereby realizing distributed measurement of the gas to be measured.

4. The optical fiber distributed detection method for trace multi-component gas according to claim 1, characterized in that: In step 5, an optical switch is used to quickly switch the seed laser to sequentially achieve distributed concentrations of each single gas hollow core fiber length; Among them, the switching speed of the optical switch is less than 0.5ms.

5. A trace multi-component gas optical fiber distributed detection device, the detection device adopts the trace multi-component gas optical fiber distributed detection method according to any one of claims 1 to 4, characterized in that: The device includes: A plurality of seed lasers (1-6), an optical switch (7), a polarization controller (8), an optical isolator (9), a hollow-core antiresonant optical fiber with a side hole drilled therein (10), a circulator (11), a pump light filter (14), a photodetector (12), a pump laser (13), and fused silica (15) at the optical fiber interface; The seed laser is efficiently coupled into the fiber core from the front end of the punched hollow core anti-resonant fiber via an optical switch, a fiber polarization controller, an optical isolator and a solid core quartz fiber; The pump laser is efficiently coupled into the fiber core from the rear end of the punched hollow core anti-resonance fiber via a circulator and a solid core quartz fiber.

6. The optical fiber distributed detection device for trace multi-component gas according to claim 5, characterized in that: The stimulated Stokes Raman gain signal of the gas or interface fused silica reaches the photodetector through the fiber circulator, pump light filter and solid core silica fiber.

7. The optical fiber distributed detection device for trace multi-component gas according to claim 5, characterized in that: The fiber polarization controller is used to change the polarization state of the seed and pump lasers to increase the intensity of stimulated Raman gain; The pump light filter is used for the residual pump light signal, so that the photodetector detects the reverse Stokes Raman gain signal.

8. The optical fiber distributed detection device for trace multi-component gas according to claim 5, characterized in that: The interface fused silica is formed by melting the solid core silica fiber and the punched hollow core anti-resonant fiber during butt coupling.

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