Photo-thermal gas sensor based on single-hole double-core-shift optical fiber

Abstract

The invention discloses a photo-thermal gas sensor and process based on a single-hole double-core-shift optical fiber, and relates to a photo-thermal gas sensor in the technical field of photo-thermal spectrum and trace gas detection. The sensor comprises a single-hole double-core-shift optical fiber (1), a single-mode optical fiber (2), pump laser (3), probe light (4) and to-be-detected gas molecules (5), a to-be-detected gas molecule (5) is arranged between the single-hole double-eccentric-core optical fiber (1) and the single-mode optical fiber (2); the second detection light (4-2) and the third detection light (4-3) are finally input into a photoelectric detector (6), optical signals are converted into electric signals, then the electric signals are input into a lock-in amplifier (7) to detect second harmonic signals, and the concentration of the gas to be detected is calculated through inversion. The photo-thermal sensor has the advantages of high utilization rate, high sensitivity, all-fiber structure, good micro-packaging property, electromagnetic interference resistance, quick response and durability in a severe environment.

Description

Technical Field

[0001] The invention relates to a photothermal gas sensor in the technical field of photothermal spectroscopy and trace gas detection, and in particular to a photothermal gas sensor based on a single-hole double-eccentric core optical fiber. Background Art

[0002] Photothermal spectroscopy has attracted widespread attention due to its advantages such as high sensitivity, all-fiber structure, non-invasive detection, anti-electromagnetic interference, fast response and durability in harsh environments. This technology has been applied to many scenarios: aerospace, gas breathing detection, harmful gas leak detection, industrial emission gas detection and atmospheric environment monitoring. This technology can design the absorption spectrum characteristics of the target gas, identify the molecular fingerprint and its intensity, and then invert the type and concentration of the monitored gas. This technology involves a variety of physical processes: first, the wavelength of the pump laser must be modulated. When the gas to be measured absorbs the modulated laser energy, it will transition from a low energy level to a high energy level. Then it will return to a low energy level from an unstable high energy level. This process is also called non-radiative thermal relaxation. In the process of non-radiative thermal relaxation, a periodic heat source will be generated, causing the temperature, density and pressure in the gas chamber to change; thus, the effective refractive index of the gas will change, and a thermal signal, that is, a photothermal signal, will be generated. This means that the photothermal signal carries the internal information of the detected gas, and the gas concentration and type information can be inverted by detecting the gas effective refractive index change signal. It is worth noting that the original photothermal signal generated by this method is extremely weak, about 10 -5 to 10 -4 Even if a high-power pump laser is used to excite the gas to obtain a photothermal signal, the detection limit is not high.

[0003] Therefore, it is necessary to introduce hollow-core optical fiber to make the light and gas contact almost 100%, so that the gas can be fully excited and the detection light can quickly and comprehensively detect the photothermal signal. The principle of the pump-probe photothermal spectroscopy detection method is very simple. First, the two laser beams of pump laser and detection light are incident on the coupler and coupled together and incident on the gas chamber at the same time. Due to the different wavelengths of the pump laser and the detection light, the gas to be measured absorbs the pump laser but not the detection light. Since the gas absorbs the pump laser, the effective refractive index of the gas chamber will change. The detection light is not absorbed. The change in the phase of the detection light can be detected to invert the change in the effective refractive index of the gas. The phase change signal is transmitted to the phase-locked amplifier for signal processing to invert the concentration information of the gas to be measured. In the past few decades, the development of photothermal spectroscopy trace gas detection technology has not been rapid. Back in 1999, the detection light used a He-Ne laser with a power of about 1mw, and the pump laser used a carbon dioxide laser with a power of about 7W, allowing the light and gas to interact in a gas chamber with a length of 38cm. Since the length of the gas chamber is limited and the interaction between light and gas is not sufficient, its detection sensitivity is very limited. Later in 2018, people introduced multiple reflection technology or Fabry-Perot resonant cavity to design the gas chamber, the purpose of which is to increase the distance of light and gas interaction, thereby improving the detection sensitivity. However, traditional light chambers face the problems of low signal-to-noise ratio and insufficient sensitivity; later, due to the addition of hollow-core optical fiber, photothermal spectroscopy trace gas detection technology has developed rapidly. Hollow-core optical fiber allows light and gas to contact almost 100%, so that the gas is fully excited and the detection light can quickly and comprehensively detect the photothermal signal, thereby greatly improving the gas detection limit. However, using only hollow-core optical fiber as a gas chamber will make the gas chamber too long and difficult to integrate and package, which has certain defects. Therefore, making a photothermal gas chamber with high sensitivity and small size is of high value to photothermal spectroscopy technology.

[0004] Based on this document, Ni Wenjun, Yang Chunyong et al. designed a single-hole double-eccentric core fiber sensor based on anti-resonance effect combined with an inline Mach-Zehnder interferometer [Optics Express, 2017(11).DOI:10.1364 / oe.25.012372.] based on a single-hole double-eccentric core fiber and eccentric core fusion-excited Mach-Zehnder interferometer photothermal sensor. The sensor comprises a single-hole double-eccentric core fiber air inner core (1), a single-hole double-eccentric core fiber air hole (2), a single-hole double-eccentric core fiber cladding inner core (3), and a single-mode fiber core (4). The pump laser excites gas in the air hole (2) of the single-hole double-eccentric core optical fiber, and the detection light is transmitted in the air inner core (1) of the single-hole double-eccentric core optical fiber and the cladding inner core (3) of the single-hole double-eccentric core optical fiber through the alignment process of the single-mode optical fiber. The single-hole double-eccentric core optical fiber excites Mach-Zehnder interference through the eccentric core fusion to improve the measurement of the photothermal signal; the photothermal signal is collected by introducing the idea of ​​Mach-Zehnder interference, which will perform secondary amplification on the generated photothermal signal. Compared with the ordinary hollow-core optical fiber air chamber without the introduction of Mach-Zehnder interference, the sensor has higher sensitivity and faster response speed.

[0005] In summary, it is of great application value to design a photothermal spectroscopy trace gas sensor with ultrahigh sensitivity, all-fiber structure, non-invasive detection, anti-electromagnetic interference, fast response and resistance to harsh environments, low detection limit and good system performance. Summary of the invention

[0006] The purpose of the present invention is to overcome the weak sensitivity of photothermal signals in the gas chamber of the prior art and to provide a photothermal gas sensor based on a single-hole double-eccentric core optical fiber.

[0007] The present invention utilizes the geometric advantage of the air core in the middle of the hollow-core optical fiber, so that the excitation light and the gas can interact with each other almost 100%; the asymmetric distribution of the air core and the cladding core in the single-hole double-eccentric core optical fiber is introduced, which provides a perfect transmission medium for the beam transmission of the detection light and the excitation Mach-Zehnder interference, and performs secondary amplification of the photothermal signal, thereby improving the detection sensitivity and detection limit. It provides a new idea for the design and production of hollow-core optical fiber sensors, and has great application prospects in the field of photothermal spectrum trace gas detection.

[0008] To achieve the above object, the technical solution of the present invention is:

[0009] The core parts that affect the sensitivity of the photothermal sensor mainly include the excitation light source, the gas chamber and the detection light source. The present invention uses a single-hole double-eccentric core optical fiber and eccentric core fusion-excited Mach-Zehnder interference photothermal sensor as the source of the photothermal signal. Whether its design is reasonable directly affects the detection performance of the entire sensor.

[0010] 1. Photothermal gas sensor based on single-hole double-eccentric core fiber

[0011] The sensor includes a single-hole double-deflected core optical fiber, a single-mode optical fiber, a pump laser, a detection light and gas molecules to be detected;

[0012] The single-hole double-deflected core optical fiber includes an air inner core, an air hole and a cladding inner core;

[0013] Single-mode optical fiber includes a single-mode core;

[0014] The detection light includes a first detection light, a second detection light and a third detection light;

[0015] Its location and connection relationship are:

[0016] The central axes of the single-hole double-eccentric core optical fiber and the single-mode optical fiber are in the same vertical plane, and the air inner fiber and the cladding inner fiber core of the single-hole double-eccentric core optical fiber are in the same vertical plane as the single-mode fiber core; the single-mode optical fiber is translated 10μm in the vertical upward direction, and the horizontal interval between the single-hole double-eccentric core optical fiber and the single-mode optical fiber is 150μm;

[0017] Gas molecules to be measured are arranged between the single-hole double-eccentric core optical fiber and the single-mode optical fiber;

[0018] The first detection light comes from the detection structure and is input into the single-mode optical fiber;

[0019] The second detection light is input into the core inside the cladding;

[0020] The third detection light is input into the fiber core in the air;

[0021] The pump laser 3 is input from the detection mechanism to the air hole;

[0022] The second detection light and the third detection light are finally input into the photodetector to convert the optical signal into an electrical signal, which is then input into the phase-locked amplifier to detect the second harmonic signal and invert and calculate the concentration of the gas to be measured.

[0023] 2. Alignment process of photothermal gas sensor based on single-hole double-eccentric core fiber

[0024] ①. The diameter of the air core and the cladding core is 9.1μm, the diameter of the air hole is 40μm, and the diameter of the single-mode optical fiber core is 10μm;

[0025] ② The symmetry axis of the single-hole double-eccentric core optical fiber and the single-mode optical fiber are in the same vertical plane. At the same time, the core in the air and the core in the cladding are in the same vertical plane as the core of the single-mode optical fiber. The single-mode optical fiber is translated 10μm in the vertical upward direction.

[0026] The present invention has the following advantages and positive effects:

[0027] ① Based on the characteristics of the air hole in the middle of the single-hole double-eccentric core optical fiber and the double core, this photothermal sensor abandons the shortcomings of the traditional large air chamber and uneven contact between light and gas, enhances the photothermal signal, and solves the problems of low laser utilization and low detection performance of the existing air chamber sensor;

[0028] ② Through the alignment process of single-mode optical fiber, the detection light is transmitted almost evenly in the dual cores, and the Mach-Zehnder interference is stimulated by the partial core fusion, which greatly enhances the photothermal signal generated in the gas chamber and solves the problem of low sensor performance and detection limit;

[0029] ③ Compared with the existing hollow-core fiber photothermal sensors, this photothermal sensor has the advantages of high utilization rate, high sensitivity, low noise level, good system stability and fast response speed;

[0030] ④ The present invention provides a new idea for a novel high-sensitivity photothermal sensor and has great application prospects in the field of photothermal spectroscopy gas detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram of the structure of the photothermal sensor, in which:

[0032] 1—single-hole double-deflected core optical fiber,

[0033] 1-1—fiber core in air, 1-2—air hole, 1-3—fiber core in cladding;

[0034] 2—single-mode fiber,

[0035] 2-1—single-mode fiber core;

[0036] 3—Pump laser;

[0037] 4—Detection light,

[0038] 4-1—the first detection light, 4-2—the second detection light, 4-3—the third detection light;

[0039] 5—Gas molecules to be measured;

[0040] 6—Photodetector;

[0041] 7—Locked-in amplifier.

[0042] Figure 2 is a schematic cross-sectional view of a single-hole double-eccentric core optical fiber 1;

[0043] Figure 3 It is a schematic diagram of the fusion splicing of a single-mode optical fiber 2 and a single-hole double-eccentric core optical fiber 1, in which:

[0044] A—the first welding point;

[0045] B—The second welding point. DETAILED DESCRIPTION

[0046] The following is a detailed description with reference to the accompanying drawings and embodiments.

[0047] 1. Photothermal gas sensor based on single-hole double-eccentric core fiber

[0048] 1. Overall

[0049] like Figure 1 , the sensor comprises a single-hole double-deflected core optical fiber 1, a single-mode optical fiber 2, a pump laser 3, a detection light 4 and a gas molecule to be detected 5;

[0050] The single-hole double-deflected core optical fiber 1 comprises an air inner core 1-1, an air hole 1-2 and a cladding inner core 1-3;

[0051] The single-mode optical fiber 2 includes a single-mode core 2-1;

[0052] The detection light 4 includes a first detection light 4-1, a second detection light 4-2 and a third detection light 4-3;

[0053] Its location and connection relationship are:

[0054] The central axes of the single-hole double-eccentric core optical fiber 1 and the single-mode optical fiber 2 are in the same vertical plane, and the air inner fiber 1-1 and the cladding inner fiber core 1-3 of the single-hole double-eccentric core optical fiber 1 are in the same vertical plane as the single-mode fiber core 2-1; the single-mode optical fiber 2 is translated 10 μm in the vertical upward direction, and the horizontal interval between the single-hole double-eccentric core optical fiber 1 and the single-mode optical fiber 2 is 150 μm;

[0055] A gas molecule 5 to be measured is arranged between the single-hole double-eccentric core optical fiber 1 and the single-mode optical fiber 2;

[0056] The first detection light 4-1 comes from the detection structure and is input into the single-mode optical fiber 2;

[0057] The second detection light 4-2 is input into the core 1-3 in the cladding;

[0058] The third detection light 4-3 is input into the fiber core 1-1 in the air;

[0059] The pump laser 3 comes from the detection mechanism and is input into the air hole 1-2;

[0060] The second detection light 4-2 and the third detection light 4-3 are finally input to the photodetector 6, which converts the optical signal into an electrical signal, and then inputs it into the phase-locked amplifier 7 to detect the second harmonic signal and invert and calculate the concentration of the gas to be measured.

[0061] The working process is:

[0062] The gas molecules 5 to be measured diffuse into the sensor, and the pump laser 3 is incident into the air hole 1-2 to excite the gas 5 to be measured to generate a photothermal signal; the first detection light 4-1 is emitted from the single-mode optical fiber 2 and diverges into the second detection light 4-2 and the third detection light 4-3, the second detection light 4-2 is used as the reference detection light, and the third detection light 4-3 is used as the sensing detection light to detect the photothermal signal. After the sensing detection light detects the photothermal signal, it forms a Mach-Zehnder interference amplification with the reference detection light to obtain the photothermal signal; then the photodetector 6 is used to convert the optical signal into an electrical signal, which is input into the phase-locked amplifier 7 to detect the second harmonic signal and invert and calculate the concentration of the gas to be measured.

[0063] 2. Functional components

[0064] 1) Single hole double offset core optical fiber 1

[0065] The single-hole double-deflected core optical fiber 1 comprises an air inner core 1-1, an air hole 1-2 and a cladding inner core 1-3;

[0066] (1) Air core 1-1

[0067] The air core 1-1 is in the air core of the single-hole double-eccentric core optical fiber 1, and its refractive index is higher than that of the cladding and the air hole. It is used to transmit the third detection light 4-3 and receive the photothermal signal generated in the air hole 1-2, as well as the thermal deformation of the material to demodulate the third detection light 4-3 transmitted in the air core 1-1.

[0068] (2) Air holes 1-2

[0069] The air hole 1-2 is in the middle of the single-hole double-eccentric core optical fiber 1, and is used as an air chamber for the interaction between light and gas, so that the light and gas can interact with each other almost 100%, generate photothermal signals, and transmit the photothermal signals to the air core 1-1.

[0070] (3) Fiber core 1-3 in cladding

[0071] The fiber core 1-3 in the cladding is in the cladding of the single-hole double-eccentric core optical fiber 1, and its refractive index is higher than that of the cladding, which is equal to the fiber core 1-1 in the air, and is used to transmit the second detection light 4-2. Since the fiber core 1-3 in the cladding is far away from the air hole 1-2, the photothermal signal of the air hole 1-2 has little effect on it. The transmission of the second detection light 4-2 in the fiber core 1-3 in the cladding is in sharp contrast to the transmission of the third detection light 4-3 in the fiber core 1-1 in the air. Through the eccentric core welding, Mach-Zehnder interference is excited at the welding point, thereby increasing the sensitivity of detecting the photothermal signal.

[0072] 2) Single-mode fiber 2

[0073] The single-mode optical fiber 2 includes a single-mode core 2-1, which is used to emit detection light. After the alignment process of the single-mode optical fiber 2, the detection light is transmitted in the air core 1-1 and the cladding core 1-3 in almost equal amounts.

[0074] 3) Pump laser 3

[0075] Used to excite the gas to be tested and generate photothermal signals.

[0076] 4) First detection light 4-1

[0077] The first detection light 4-1 is emitted from the single-mode optical fiber 2 and incident into the single-hole double-eccentric core optical fiber 1 after an alignment process, and is used to detect changes in the phase of the light field.

[0078] 5) Second detection light 4-2

[0079] The first detection light 4-1 is incident into the single-hole double-eccentric core optical fiber 1 and is transmitted in the core 1-3 inside the cladding; it acts as a reference detection light and forms Mach-Zehnder interference with the third detection light 4-3.

[0080] 6) Third detection light 4-3

[0081] The first detection light 4-1 is incident into the single-hole double-eccentric core optical fiber 1, and is transmitted in the air core 1-1; it acts as a sensing detection light and forms Mach-Zehnder interference with the second detection light 4-2.

[0082] 7) Gas molecules to be measured

[0083] The gas to be tested in the experiment is excited by the pump laser 3 to generate a photothermal signal.

[0084] 3. Usage

[0085] ①Sensor preparation

[0086] Make sure that all components of the sensor are intact and that the single-hole double-eccentric core optical fiber 1 and the single-mode optical fiber 2 are aligned correctly.

[0087] ②Sensor calibration

[0088] Before use, the sensor needs to be calibrated to ensure its accuracy; for example, the sensor is calibrated by exposing it to a gas of known concentration until it can accurately detect the concentration of the gas.

[0089] ③Sensor deployment

[0090] Place the sensor in the specific environment where the gas needs to be detected.

[0091] ④Maintenance and inspection

[0092] Perform regular maintenance and inspections on the sensor to ensure it remains performing at its best; this may include cleaning the sensor, replacing damaged parts, and performing calibration checks.

[0093] It can be seen that the above-mentioned components each play an important role in the photothermal spectroscopy system, working together to achieve high-sensitivity, high-resolution and fast-response photothermal measurement and analysis.

[0094] Specifically:

[0095] In the photoacoustic and photothermal system, the modulated pump laser 3 is incident through the fusion point end of the single-hole double-eccentric core optical fiber 1, and the detection light 4 is incident from the single-mode optical fiber 2 in the opposite direction of the pump laser 3 after an alignment process. The pump laser 3 and the detection light 4 form a counter-reflection, and the incident pump laser 3 interacts with the gas to be measured 5 to generate a photothermal signal through the photothermal effect; the generated photothermal signal is transmitted to the fiber core 1 in the air, and the detection light 4 is transmitted and modulated in the fiber core 1 in the air and the fiber core 3 in the cladding, and Mach-Zehnder interference is excited at the fusion point to measure the photothermal signal.

[0096] In this system, the performance of the transmission detection light of the fiber core 1-1 in the air and the fiber core 1-3 in the cladding determines the system's comprehensive indicators such as the minimum detection limit, sensitivity and signal-to-noise ratio; the present invention stimulates Mach-Zehnder interference at the fusion point through biased core fusion, thereby increasing the sensitivity of detecting photothermal signals and making the system's performance better.

[0097] 2. Alignment process of photothermal gas sensor based on single-hole double-eccentric core fiber

[0098] ①, the diameter of the air core 1-1 and the cladding core 1-3 is 9.1μm, the diameter of the air hole 1-2 is 40μm, and the diameter of the single-mode fiber core 2-1 is 10μm (such as Figure 2 );

[0099] ②, the symmetry axes of the single-hole double-eccentric core optical fiber 1 and the single-mode optical fiber 2 are in the same vertical plane, and the air core 1-1 and the cladding core 1-3 are in the same vertical plane as the single-mode optical fiber core 2-1; the single-mode optical fiber 2 is translated 10μm in the vertical upward direction (such as Figure 3 ).

[0100] Specifically:

[0101] The diameters of the air inner core 1-1 and the air hole 1-2 of the double-eccentric core optical fiber 1 are 9.1 μm and 40 μm respectively, the air hole 1-2 is located at the center of the single-hole double-eccentric core optical fiber 1, the air inner core 1-1 is located at the inner edge of the air hole 1-2 and close to the air hole 1-2, and the cladding inner core 1-3 is close to the cladding center; the pump laser 3 can excite the gas in the air hole 1-2, and the detection light 4 is transmitted almost equally in the air inner core 1-1 of the single-hole double-eccentric core optical fiber 1 and the cladding inner core 1-3 of the single-hole double-eccentric core optical fiber 1 for better detection of photothermal signals;

[0102] Through the alignment process of the single-hole double-eccentric core optical fiber, the detection light 4 is transmitted in the air inner core 1-1 of the single-hole double-eccentric core optical fiber 1 and the cladding inner core 1-3 of the single-hole double-eccentric core optical fiber 1 respectively, and the single-hole double-eccentric core optical fiber 1 is fused with the eccentric core to excite Mach-Zehnder interference to improve the measurement of the photothermal signal;

[0103] The two combinations form a highly efficient photothermal sensor with the advantages of high utilization rate, high sensitivity, all-fiber structure, good miniaturization and encapsulation, resistance to electromagnetic interference, rapid response and durability in harsh environments.

[0104] 3. Working Mechanism

[0105] First, the rear end of the single-hole double-eccentric core fiber 1 and the front end of the single-mode fiber 2 are eccentrically fused, and the modulated pump laser 3 is incident through the fusion point of the single-hole double-eccentric core fiber 1. Due to the anti-resonance effect of the single-hole double-eccentric core fiber 1, the pump laser 3 is well locked in the air hole 1-2 to transmit and excite the gas 5 to be tested; when the gas 5 to be tested absorbs the modulated laser energy, it will transition from a low energy level to a high energy level; then it will return to a low energy level from an unstable high energy level. This process is also called non-radiative thermal relaxation. During the non-radiative thermal relaxation process, a periodic heat source will be generated; the generated heat source will be in the air holes 1-2 Heat transfer, heat conduction and heat radiation are carried out in the single-hole double-eccentric core optical fiber 1; since the air hole 1-2 is in the single-hole double-eccentric core optical fiber 1, and the heat transfer, heat conduction and heat radiation are divided into transverse and longitudinal directions, when the air core 1-1 is diffused transversely, the air core 1-1 will cause the temperature, density and pressure in the core to change due to the photothermal effect, thereby changing the effective refractive index of the optical fiber air core 1-1; at the same time, the air core 1-1 will produce thermal deformation due to the heating, and the thermo-optical coefficient of the optical fiber material will change; after the alignment process, the second detection light 4-2 emitted from the single-mode optical fiber core 2-1 , incident on the front end of the single-hole double-eccentric core optical fiber 1, so that the energy transmitted by the third detection light 4-3 and the second detection light 4-2 in the air core 1-1 and the cladding core 1-3 is almost equal, and at the same time, they are opposite to the pump laser 3; because the air core 1-1 is affected by both the photothermal signal and the change of its own material deformation thermo-optic coefficient, but the cladding core 1-3 is relatively far away from the air hole 1-2, the photothermal signal generated by the pump laser 3 to excite the gas is small and has little effect on it; when a beam of the third detection light 4-3 is transmitted in the air core 1-1, it is affected by the photothermal signal and the change of the thermo-optic coefficient of the material deformation. The signal modulation caused by the deformation of its own material, the other beam of the first detection light 4-1 is almost not modulated by the photothermal signal and the signal modulation caused by the deformation of its own material when it is transmitted in the fiber core 1-3 in the cladding. Due to the off-core fusion, the two detection lights meet at the fusion point to stimulate Mach-Zehnder interference; this makes the photothermal signal, the change of the thermo-optical coefficient of the material deformation itself and the Mach-Zehnder interference triple superimposed in this sensor to make the photothermal signal reach the extreme; the obtained photothermal signal is transmitted to the photodetector through the optical circulator, and then input into the phase-locked amplifier to detect the second harmonic signal to invert and obtain the concentration information of the gas to be measured.

[0106] This sensor uses a single-hole double-eccentric core optical fiber 1, abandoning the traditional anti-resonant hollow-core optical fiber and photonic crystal bandgap hollow-core optical fiber; it can better stimulate Mach-Zehnder interference while its own material deformation thermo-optical coefficient changes and stimulates the gas to be measured 5, enhancing the detection of photothermal signals. This feature can further improve the detection limit of the photothermal spectroscopy system in trace gas detection.

[0107] The diameter of the air core 1 and the cladding core 3 is 9.1 μm, the diameter of the air hole 2 is 40 μm, and the diameter of the single-mode fiber core 4 is 10 μm;

[0108] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A photothermal gas sensor based on a single-hole double-eccentric core optical fiber, characterized in that: The sensor comprises a single-hole double-deflected core optical fiber (1), a single-mode optical fiber (2), a pump laser (3), a detection light (4) and gas molecules to be detected (5); The single-hole double-eccentric core optical fiber (1) comprises an air inner core (1-1), an air hole (1-2) and a cladding inner core (1-3); The single-mode optical fiber (2) comprises a single-mode fiber core (2-1); The detection light (4) includes a first detection light (4-1), a second detection light (4-2) and a third detection light (4-3); Its location and connection relationship are: The central axes of the single-hole double-eccentric core optical fiber (1) and the single-mode optical fiber (2) are on the same vertical plane, and the air inner fiber (1-1) and the cladding inner fiber core (1-3) of the single-hole double-eccentric core optical fiber (1) are on the same vertical plane as the single-mode fiber core (2-1); the single-mode optical fiber (2) is translated by 10 μm in the vertical upward direction, and the horizontal interval between the single-hole double-eccentric core optical fiber (1) and the single-mode optical fiber (2) is 150 μm; Gas molecules (5) to be measured are arranged between the single-hole double-eccentric core optical fiber (1) and the single-mode optical fiber (2); The first detection light (4-1) comes from the detection structure and is input into the single-mode optical fiber (2); The second detection light (4-2) is input into the fiber core (1-3) in the cladding; The third detection light (4-3) is input into the air core (1-1); The pump laser (3) is input from the detection mechanism into the air hole (1-2); The second detection light (4-2) and the third detection light (4-3) are finally input into the photodetector (6), the optical signal is converted into an electrical signal, and then input into the phase-locked amplifier (7) to detect the second harmonic signal and invert and calculate the concentration of the gas to be measured.

2. The alignment process of the photothermal gas sensor based on the single-hole double-eccentric core optical fiber according to claim 1 is characterized in that: ①, the diameter of the air inner core (1-1) and the cladding inner core (1-3) is 9.1 μm, the diameter of the air hole (1-2) is 40 μm, and the diameter of the single-mode optical fiber core (2-1) is 10 μm; ②. The symmetry axes of the single-hole double-eccentric core optical fiber (1) and the single-mode optical fiber (2) are on the same vertical plane, and the air core (1-1) and the cladding core (1-3) are on the same vertical plane as the single-mode optical fiber core (2-1); the single-mode optical fiber (2) is translated by 10 μm in the vertical upward direction.

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