A micro-nano optical waveguide photothermal spectroscopy gas detection method and detection system

Abstract

The present application is applicable to the field of gas measurement technology, and provides a micro-nano optical waveguide photothermal spectroscopy gas detection method and detection system. The detection method provided in the present application includes: inputting a pump laser and a detection laser into a micro-nano optical waveguide, the evanescent wave generated by the pump laser generates a photothermal effect and accompanying heat conduction with a substance to be measured in a gas medium, and changes the refractive index of the gas medium and the micro-nano optical waveguide; the detection laser excites the fundamental mode and the first high-order mode of the micro-nano optical waveguide, and the phase difference generated after the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide is detected to obtain the concentration of the substance to be measured. The present application can obtain a large proportion of evanescent field without harsh micro-nano optical waveguide preparation process conditions, with high photothermal efficiency, small size, low cost, and fast response speed.

Description

Technical Field

[0001] The present application belongs to the field of gas measurement technology, and in particular relates to a micro-nano optical waveguide photothermal spectroscopy gas detection method and detection system. Background Art

[0002] Laser absorption spectroscopy is a highly sensitive and selective gas analysis method with wide applications in environmental monitoring, energy and electricity, national defense, aerospace and other fields.

[0003] Based on the various effects associated with light absorption, people have developed different laser absorption spectroscopy derivative methods. Laser photothermal spectroscopy uses the photothermal effect and the thermo-optical modulation characteristics it brings to indirectly measure gas concentration. It has the advantage of no background noise and effectively improves the sensitivity and accuracy of gas detection. In particular, using microstructured hollow-core optical fibers as carriers greatly increases and improves the distance and light energy density of the interaction between light and matter, greatly improves the heat generation efficiency of the photothermal effect, and achieves excellent gas detection performance.

[0004] However, microstructured hollow-core optical fibers require special structural designs and are prepared by optical fiber drawing towers under precise temperature and pressure control conditions. In gas detection, on the one hand, microstructured hollow-core optical fibers need to be precisely aligned and reliably connected with standard single-mode optical fibers to form a gas detection system with an all-fiber structure. On the other hand, gas fills microstructured hollow-core optical fibers by free diffusion, which cannot meet the needs of real-time detection. Processing microchannels can speed up gas exchange, but it will sacrifice the mechanical strength of microstructured hollow-core optical fibers and introduce losses. In the above discussion, the high cost and difficulty of producing microstructured hollow-core optical fibers, as well as the slow response speed, limit their large-scale application. Summary of the invention

[0005] The embodiments of the present application provide a micro-nano optical waveguide photothermal spectroscopy gas detection method and detection system, which can solve the problems of high sensor manufacturing cost and difficulty and slow response speed in the prior art.

[0006] In a first aspect, the present application provides a micro-nano optical waveguide photothermal spectroscopy gas detection method, which includes:

[0007] Inputting a pump laser and a detection laser into the micro-nano optical waveguide; wherein part of the energy of the pump laser and the detection laser will be transmitted in the gas medium outside the micro-nano optical waveguide in the form of evanescent waves, the evanescent waves generated by the pump laser and the substance to be measured in the gas medium will heat the gas medium after undergoing a photothermal effect, the gas medium will heat the micro-nano optical waveguide through heat conduction, and the refractive index of the gas medium and the micro-nano optical waveguide will be changed, and the detection laser will excite and generate the fundamental mode and the first high-order mode of the micro-nano optical waveguide;

[0008] The phase difference between the fundamental mode and the first high-order mode after being transmitted in the micro-nano optical waveguide is detected, and the real concentration of the substance to be measured in the gas medium is obtained by conversion according to the relationship between the phase difference and the concentration of the substance to be measured.

[0009] Optionally, the central wavelength of the pump laser is aligned with or scanned through the absorption peak of the substance to be detected, and the central wavelength of the detection laser is far away from any absorption peak of the substance to be detected.

[0010] Optionally, the micro-nano optical waveguide supports multi-mode transmission.

[0011] Optionally, when the detection laser enters the micro-nano optical waveguide, only the fundamental mode and the first high-order mode of the micro-nano optical waveguide are excited.

[0012] Optionally, the relationship between the phase difference generated after the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide and the concentration of the substance to be measured can be expressed as:

[0013] δΦ=(M·α 0 ·L·P)·C (I)

[0014] Among them, δΦ is the phase difference between the fundamental mode and the first higher-order mode after transmission in the micro-nano optical waveguide; M is the photothermal coefficient. For a micro-nano optical waveguide of fixed size and a fixed pump laser incident mode, the photothermal coefficient is a constant; α 0 is the absorption coefficient of the substance to be measured. For a fixed absorption peak, the absorption coefficient of the substance to be measured is a constant value; L is the length of the micro-nano optical waveguide, P is the power of the pump laser, and C is the concentration of the substance to be measured.

[0015] Among them, the phase difference generated by the fundamental mode and the first high-order mode after transmission in the micro-nano optical waveguide is proportional to the concentration of the substance to be measured. Therefore, by detecting the phase difference information carried in the detection laser emitted from the output end of the micro-nano optical waveguide, the concentration of the substance to be measured can be converted.

[0016] Compared with the prior art, the beneficial effects are as follows: through the embodiment of the present application, since the gas medium containing the substance to be tested is wrapped in the micro-nano optical waveguide, the photothermal signal response is instantaneous, which significantly improves the response speed. After the substance to be tested absorbs the pump laser, it not only changes the refractive index of the gas medium, but also changes the refractive index of the micro-nano optical waveguide. The thermo-optical coefficient of the micro-nano optical waveguide material (for example, silicon dioxide is about 10 -5 K -1 The thermo-optic coefficient of the gas is much larger than that of the gas at room temperature (e.g., about -10 -6 K -1), so under the same conditions, a higher photothermal efficiency can be obtained than that of microstructured hollow-core optical fibers. At the same time, gas media usually have a lower thermal conductivity coefficient, which causes the wrapped micro-nano optical waveguide to produce a heat accumulation effect, further improving the photothermal efficiency and the detection sensitivity of the gas concentration sensor. The adopted micro-nano optical waveguide supports high-order mode transmission, and high-order modes have a larger evanescent field ratio. Therefore, while obtaining a strong interaction between light and matter, it avoids harsh process conditions and reduces the difficulty and cost of production. Micro-nano optical waveguides can be prepared by melt-tapering standard single-mode optical fibers, and can also be mass-produced by a preparation process compatible with MEMS, further reducing the difficulty and cost of production.

[0017] In a second aspect, an embodiment of the present application provides a micro-nano optical waveguide photothermal spectroscopy gas detection system, including a pump laser component for generating a pump laser, a detection laser component for generating a detection laser, a wavelength division multiplexer for combining the pump laser and the detection laser, a micro-nano optical waveguide, a gas collection chamber for collecting a substance to be detected and placing the micro-nano optical waveguide, an optical filter for filtering out the pump laser, and an analysis component;

[0018] The output ends of the pump laser component and the detection laser component are respectively connected to the input end of the wavelength division multiplexer, the input end and the output end of the micro-nano optical waveguide are respectively connected to the output end of the wavelength division multiplexer and the input end of the optical filter, and the output end of the optical filter is connected to the input end of the analysis component;

[0019] There is a gas medium in the gas collection chamber, and part of the energy of the pump laser and the detection laser will be transmitted in the gas medium outside the micro-nano optical waveguide in the form of evanescent waves. The evanescent waves generated by the pump laser heat the gas medium after photothermal effect with the substance to be measured in the gas medium. The gas medium heats the micro-nano optical waveguide through heat conduction, changing the refractive index of the gas medium and the micro-nano optical waveguide. The detection laser excites and generates the fundamental mode and the first high-order mode of the micro-nano optical waveguide.

[0020] The analysis component is used to detect the phase difference between the fundamental mode and the first high-order mode after transmission in the micro-nano optical waveguide, and convert the actual concentration of the substance to be measured in the gas medium according to the relationship between the phase difference and the concentration of the substance to be measured.

[0021] Optionally, the pump laser assembly includes a first laser driver, a pump light source, and a laser amplifier;

[0022] The detection laser assembly includes a detection light source, a second laser driver, and a polarization controller;

[0023] The analysis component includes an optical coupler, a first optical detector and a second optical detector, a lock-in amplifier, and an analysis terminal;

[0024] The input end of the first laser driver is the input end of the pump laser assembly, the input end and the output end of the pump light source are respectively connected to the output end of the first laser driver and the input end of the laser amplifier, and the output end of the laser amplifier is the output end of the pump laser assembly;

[0025] The input end of the second laser driver is the input end of the detection laser assembly, the input end and the output end of the detection light source are respectively connected to the output end of the second laser driver and the input end of the polarization controller, and the output end of the polarization controller is the output end of the detection laser assembly;

[0026] The input end of the optical coupler is the input end of the analysis component, the output end of the optical coupler is connected to the input ends of the first photodetector and the second photodetector respectively, the output end of the first photodetector is connected to the input end of the phase-locked amplifier, the output end of the second photodetector is connected to the input end of the analysis terminal, the output end of the phase-locked amplifier is connected to the analysis terminal and the input end of the pump laser component respectively, and the output end of the analysis terminal is connected to the input end of the detection laser component.

[0027] Optionally, the micro-nano optical waveguide includes any one of an integrated optical waveguide and an optical fiber waveguide.

[0028] Optionally, the cross-sectional size of the micro-nano optical waveguide is not greater than 10 times the maximum wavelength of the pump laser and the detection laser.

[0029] Optionally, there is a non-adiabatic transition between the output end of the wavelength division multiplexer and the input end of the micro-nano optical waveguide;

[0030] When the detection laser is input from the output end of the wavelength division multiplexer to the input end of the micro-nano optical waveguide, the detection laser excites the fundamental mode and the first high-order mode of the micro-nano optical waveguide, and the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide at the same time.

[0031] Optionally, an adiabatic transition is provided between the output end of the wavelength division multiplexer and the input end of the micro-nano optical waveguide;

[0032] When the detection laser is input from the output end of the wavelength division multiplexer to the input end of the micro-nano optical waveguide, the detection laser only excites the fundamental mode of the micro-nano optical waveguide. A long period grating is written at the input end of the micro-nano optical waveguide, and the long period grating couples part of the energy of the fundamental mode to the first high-order mode, so that the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide at the same time.

[0033] The detection system of the present invention is based on the above-mentioned gas detection method, has a simple structure, is easy to operate, and has high detection sensitivity, a large dynamic range, and a fast signal response. According to experimental tests, the minimum detectable gas (methane) concentration of the detection system of the present invention is as low as 440ppb, the dynamic range is close to 6 orders of magnitude, and the response time is only 7s. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0035] Figure 1 This is a schematic diagram of generating a photothermal effect on a micro-nano optical waveguide provided by the present application;

[0036] Figure 2 is a cross-sectional view of a micro-nano optical waveguide provided by the present application;

[0037] Figure 3 It is a schematic diagram of a micro-nano optical waveguide photothermal spectroscopy gas detection system provided by the present application;

[0038] Figure 4 This is a diagram of an automatic stabilization process of a micro-nano optical waveguide interferometer provided by the present application;

[0039] Figure 5 It is a second harmonic spectrum of methane gas with a volume concentration of 1% measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application;

[0040] Figure 6 This is a graph showing the relationship between the photothermal signal and system noise of methane gas with a volume concentration of 1% and the pump power measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided in the present application;

[0041] Figure 7 This is an Allan variance curve of methane gas detection obtained based on 2 hours of noise data measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided in this application;

[0042] Figure 8 It is a second harmonic signal change curve of methane gas with a volume concentration of 1% measured within 4 hours by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application;

[0043] Fig. 9 This is a curve diagram of the change of normalized photothermal signal with filling time measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided in the present application;

[0044] Fig.10 This is a curve chart of the dynamic range measurement results of a methane gas sensor at normal temperature and pressure measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided in the present application. DETAILED DESCRIPTION

[0045] In the following description, specific details such as specific system structures, technologies, etc. are provided for the purpose of illustration rather than limitation, so as to provide a thorough understanding of the embodiments of the present application. However, it should be clear to those skilled in the art that the present application may also be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to prevent unnecessary details from obstructing the description of the present application.

[0046] It should be understood that when used in the present specification and the appended claims, the term "comprising" indicates the presence of described features, wholes, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, components and / or combinations thereof.

[0047] It should also be understood that the term “and / or” used in the specification and appended claims refers to any and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0048] As used in the specification and appended claims of this application, the term "if" can be interpreted as "when" or "uponce" or "in response to determining" or "in response to detecting", depending on the context. Similarly, the phrase "if it is determined" or "if [described condition or event] is detected" can be interpreted as meaning "uponce it is determined" or "in response to determining" or "uponce [described condition or event] is detected" or "in response to detecting [described condition or event]", depending on the context.

[0049] In addition, in the description of the present application specification and the appended claims, the terms "first", "second", "third", etc. are only used to distinguish the descriptions and cannot be understood as indicating or implying relative importance.

[0050] References to "one embodiment" or "some embodiments" etc. described in the specification of this application mean that one or more embodiments of the present application include specific features, structures or characteristics described in conjunction with the embodiment. Therefore, the statements "in one embodiment", "in some embodiments", "in some other embodiments", "in some other embodiments", etc. that appear in different places in this specification do not necessarily refer to the same embodiment, but mean "one or more but not all embodiments", unless otherwise specifically emphasized in other ways. The terms "including", "comprising", "having" and their variations all mean "including but not limited to", unless otherwise specifically emphasized in other ways.

[0051] In the following, in conjunction with specific embodiments, a micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application is exemplarily described.

[0052] In a possible implementation, the micro-nano optical waveguide includes any one of an integrated optical waveguide and an optical fiber waveguide.

[0053] In the embodiment of the present application, the micro-nano optical waveguide is a micro-nano optical fiber, that is, an optical fiber waveguide is used.

[0054] See also Figure 1 , Figure 1 This is a schematic diagram of generating photothermal effect and heat conduction on micro-nano optical fiber provided in the present application.

[0055] The gas concentration detection method in this embodiment is performed by a gas concentration sensor.

[0056] S101, inputting a pump laser and a detection laser into the micro-nano optical waveguide; wherein part of the energy of the pump laser and the detection laser will be transmitted in the gas medium outside the micro-nano optical waveguide in the form of evanescent waves, the evanescent waves generated by the pump laser react with the substance to be measured in the gas medium to heat the gas medium after photothermal effect, the gas medium heats the micro-nano optical waveguide through heat conduction, and the refractive index of the gas medium and the micro-nano optical waveguide is changed, and the detection laser excites and generates the fundamental mode and the first high-order mode of the micro-nano optical waveguide.

[0057] S102, detecting a phase difference between the fundamental mode and the first high-order mode after transmission in the micro-nano optical waveguide, and converting the actual concentration of the substance to be measured in the gas medium according to the relationship between the phase difference and the concentration of the substance to be measured.

[0058] Specifically, in step S101, the pump laser and the probe laser are input into the micro-nano optical fiber. This step can refer to the conventional method of laser input into optical fiber in the art, and is not specifically limited in this application. The purpose of this step is to make the pump laser and the probe laser generate evanescent waves in the micro-nano optical fiber respectively, and the generated evanescent waves are transmitted along the micro-nano optical fiber.

[0059] It should be noted that the pump laser and the probe laser can enter from the same side of the micro-nano optical fiber, or from different sides of the micro-nano optical fiber. That is, the present application does not limit the transmission direction of the pump laser and the probe laser in the micro-nano optical fiber, and they can be transmitted in the same direction or in the opposite direction.

[0060] In the embodiment of the present application, the pump laser and the detection laser are transmitted in the same direction in the micro-nano optical fiber.

[0061] As an example but not a limitation, in the embodiment of the present application, the gas medium is nitrogen and the substance to be tested is methane gas. For example, the volume concentration of methane gas is 1 ppm, wherein the substance to be tested is 1×10 -6of methane gas, the remaining volume is (0.999999 = 1-1×10 -6 ) The gas medium is nitrogen.

[0062] Optionally, the central wavelength of the pump laser is aligned with or scanned through the absorption peak of the substance to be tested, and the central wavelength of the detection laser is far away from any absorption peak of the substance to be tested. The central wavelength of the pump laser can be tuned to the absorption peak of the substance to be tested, so that the substance to be tested can fully absorb the pump laser. The central wavelength of the detection laser can be tuned to an absorption line far away from the substance to be tested, so that the substance to be tested cannot absorb the detection laser.

[0063] Furthermore, in the embodiment of the present application, the central wavelength of the pump laser is near the methane R3 absorption peak (ie, 1653.7 nanometers), and the central wavelength of the detection laser is near 1550 nanometers.

[0064] Optionally, micro-nano optical fibers support multimode transmission.

[0065] For example, see Figure 2 , Figure 2 This is a cross-sectional view of a micro-nano optical fiber provided in this application. Figure 2 The diameter of the medium-micro-nano optical fiber is about 2.1 microns. The micro-nano optical fiber is made of single-mode quartz optical fiber through arc discharge and hydrogen-oxygen flame heating two-step stretching method, supporting including fundamental mode HE 11 , High Order Mode HE 12 Various transmission modes including.

[0066] In a possible implementation, the micro-nano optical waveguide is placed in the gas collecting chamber, and there is a certain distance between the micro-nano optical waveguide and the inner wall of the gas collecting chamber.

[0067] Furthermore, the gas collection chamber in the embodiment of the present invention is a plastic gas chamber of 12 cm×1.4 cm×1.5 cm, and the gas pressure is always maintained at one atmosphere during the measurement process.

[0068] See also Figure 3 , Figure 3 This is a schematic diagram of a micro-nano optical waveguide photothermal spectroscopy gas detection system provided by the present application. The micro-nano optical waveguide photothermal spectroscopy gas detection system includes:

[0069] A pump laser assembly for generating a pump laser, a detection laser assembly for generating a detection laser, a wavelength division multiplexer for combining the pump laser and the detection laser, a micro-nano optical waveguide, a gas collection chamber for collecting a substance to be tested and placing the micro-nano optical waveguide, an optical filter for filtering out the pump laser, and an analysis assembly;

[0070] The output ends of the pump laser component and the detection laser component are respectively connected to the input end of the wavelength division multiplexer, the input end and the output end of the micro-nano optical waveguide are respectively connected to the output end of the wavelength division multiplexer and the input end of the optical filter, and the output end of the optical filter is connected to the input end of the analysis component;

[0071] There is a gas medium in the gas collection chamber, and part of the energy of the pump laser and the detection laser will be transmitted in the gas medium outside the micro-nano optical waveguide in the form of evanescent waves. The evanescent waves generated by the pump laser heat the gas medium after photothermal effect with the substance to be measured in the gas medium. The gas medium heats the micro-nano optical waveguide through heat conduction, changing the refractive index of the gas medium and the micro-nano optical waveguide. The detection laser excites and generates the fundamental mode and the first high-order mode of the micro-nano optical waveguide.

[0072] The analysis component is used to detect the phase difference between the fundamental mode and the first high-order mode after transmission in the micro-nano optical waveguide, and convert the actual concentration of the substance to be measured in the gas medium according to the relationship between the phase difference and the concentration of the substance to be measured.

[0073] The pump laser assembly includes a first laser driver, a pump light source, and a laser amplifier;

[0074] The detection laser assembly includes a detection light source, a second laser driver, and a polarization controller;

[0075] The analysis component includes an optical coupler, a first optical detector and a second optical detector, a lock-in amplifier, and an analysis terminal;

[0076] The input end of the first laser driver is the input end of the pump laser assembly, the input end and the output end of the pump light source are respectively connected to the output end of the first laser driver and the input end of the laser amplifier, and the output end of the laser amplifier is the output end of the pump laser assembly;

[0077] The input end of the second laser driver is the input end of the detection laser assembly, the input end and the output end of the detection light source are respectively connected to the output end of the second laser driver and the input end of the polarization controller, and the output end of the polarization controller is the output end of the detection laser assembly;

[0078] Among them, the input end of the optical coupler is the input end of the analysis component, the output end of the optical coupler is respectively connected to the input end of the first light detector and the input end of the second light detector, the output end of the first light detector is connected to the input end of the phase-locked amplifier, the output end of the second light detector is connected to the input end of the analysis terminal, the output end of the phase-locked amplifier is respectively connected to the input end of the analysis terminal and the input end of the pump laser component, and the output end of the analysis terminal is connected to the input end of the detection laser component.

[0079] Figure 3 In the process, the pump laser and the detection laser can enter from the input end of the micro-nano optical fiber. Figure 3 The invention comprises a first laser driver, a pump light source, a laser amplifier, a wavelength division multiplexer, a micro-nano optical fiber, a gas collection chamber, a detection light source, a second laser driver, a polarization controller, an optical filter, a first light detector, a second light detector, a phase-locked amplifier and an analysis terminal.

[0080] In one possible implementation, when the pump laser enters the micro-nano optical fiber, the material to be tested will absorb the pump laser, thereby generating a photothermal effect, first heating the gas medium outside the micro-nano optical fiber, changing the refractive index of the gas medium, and then heating the micro-nano optical fiber through heat conduction, causing the refractive index of the micro-nano optical fiber to be changed.

[0081] Specifically, first, methane gas with a volume concentration of 1% is filled into the gas collection chamber. A 1654-nanometer distributed feedback laser can be selected as the pump light source. The phase-locked amplifier can input a 6-kHz sinusoidal modulation signal to the first laser driver, and the first laser driver drives the pump light source at a sawtooth scanning frequency of 0.01 Hz to output a modulated pump laser. The modulated pump laser can pass through the laser amplifier and the wavelength division multiplexer into the micro-nano optical fiber, so that the pump laser interacts with the methane gas to produce a photothermal effect and accompanying heat conduction, causing the refractive index of the gas medium and the micro-nano optical fiber to be periodically changed. Among them, the laser amplifier is used to increase the energy of the pump laser.

[0082] In a possible implementation, the cross-sectional size of the micro-nano optical waveguide is not greater than 10 times the maximum wavelength of the pump laser and the detection laser.

[0083] In a possible implementation, when the detection laser enters the micro-nano optical waveguide, only the fundamental mode and the first high-order mode of the micro-nano optical waveguide are excited.

[0084] Optionally, there may be a non-adiabatic transition or an adiabatic transition between the output end of the wavelength division multiplexer and the input end of the micro-nano optical fiber.

[0085] When a non-adiabatic transition is adopted between the output end of the wavelength division multiplexer and the input end of the micro-nano optical fiber, when the detection laser is input from the output end of the wavelength division multiplexer to the input end of the micro-nano optical waveguide, the detection laser excites the fundamental mode and the first high-order mode of the micro-nano optical waveguide, and the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide at the same time.

[0086] When an adiabatic transition is adopted between the output end of the wavelength division multiplexer and the input end of the micro-nano optical fiber, when the detection laser is input from the output end of the wavelength division multiplexer to the input end of the micro-nano optical waveguide, the detection laser only excites the fundamental mode of the micro-nano optical waveguide; wherein a long period grating is written at the input end of the micro-nano optical waveguide, and the long period grating couples part of the energy of the fundamental mode to the first high-order mode, so that the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide at the same time.

[0087] In the embodiment of the present application, the transition between the output end of the wavelength division multiplexer and the input end of the micro-nano optical fiber is a non-adiabatic transition, see Figure 3 When the detection laser is input from the output end of the wavelength division multiplexer to the input end of the micro-nano optical fiber, the detection laser simultaneously excites the fundamental mode and the first high-order mode of the micro-nano optical fiber.

[0088] Specifically, the first high-order mode is a high-order transmission mode different from the fundamental mode. 11 mode, the first high-order mode can be HE lm (l, m are natural numbers, where l≠1, m≠1), EH pq TE 0q or TM 0q (p, q are natural numbers).

[0089] As an example but not a limitation, in the embodiment of the present application, the first high-order mode is HE 12 .

[0090] In the embodiment of the present application, the second laser driver drives the detection light source to output a detection laser with a central wavelength of 1550 nanometers, and the detection laser enters the micro-nano optical fiber through a polarization controller and a wavelength division multiplexer. When the detection laser enters the micro-nano optical fiber, it excites the HE of the micro-nano optical fiber. 11 and HE 12 model.

[0091] Since the refractive index of the gas medium and the micro-nano optical fiber has changed, the HE excited by the detection laser will be changed when the detection laser is transmitted along the micro-nano optical fiber. 11 and HE 12 The phase of the mode, HE 11 and HE 12 The phase difference generated after the mode is transmitted in the micro-nano optical fiber also changes accordingly.

[0092] Optional, HE 11 and HE 12 The relationship between the phase difference generated after the mode is transmitted in the micro-nano optical fiber and the concentration of the substance to be measured can be expressed as:

[0093] δΦ=(M·α 0 ·L·P)·C (2)

[0094] Among them, δΦ is HE 11 and HE 12 The phase difference generated after the mode is transmitted in the micro-nano optical fiber; M is the photothermal coefficient, which is a constant for micro-nano optical fibers of fixed size and fixed pump laser incidence mode; α 0 is the absorption coefficient of methane, and for a fixed absorption peak, its value is a constant; L is the length of the micro-nano optical waveguide, P is the power of the pump laser, and C is the concentration of the substance to be measured.

[0095] For example, see Figure 3 After receiving part of the detection laser output by the micro-nano optical fiber, the second optical detector transmits the converted electrical signal to the analysis terminal. After receiving the electrical signal, the analysis terminal processes it to obtain a feedback adjustment signal, which is transmitted to the second laser driver. The second laser driver drives the detection light source to tune the central wavelength of the detection laser to the working wavelength corresponding to the 90° working point phase (orthogonal working point) of the micro-nano optical fiber interferometer. The polarization controller is adjusted to keep the contrast of the interference fringes generated by the micro-nano optical fiber interferometer at the highest level, in order to keep the phase detection sensitivity optimal. Please refer to Figure 4 , Figure 4 This is the automatic stabilization process diagram of the micro-nano fiber interferometer. The micro-nano fiber interferometer gradually deviates from the working point during free operation. After adding feedback adjustment, the micro-nano fiber interferometer quickly enters a stable state, that is, locked at the orthogonal working point. The micro-nano fiber interferometer can obtain the maximum phase detection sensitivity when adjusted to the orthogonal working point.

[0096] For example, see Figure 3 After receiving part of the detection laser output by the micro-nano optical fiber, the first optical detector transmits the converted electrical signal to the phase-locked amplifier. After receiving the electrical signal, the phase-locked amplifier can demodulate the harmonic signal from the electrical signal. The harmonic signal is the signal carrying the HE 11 and HE 12 The intermediate signal of the phase difference information generated after the mode is transmitted in the micro-nano optical fiber. After receiving the harmonic signal, the analysis terminal processes the peak-to-peak value of the harmonic signal, which is the photothermal signal. The photothermal signal is proportional to the concentration of the substance to be measured. Finally, the true concentration of the substance to be measured is obtained through the relationship between the photothermal signal and the concentration of the substance to be measured.

[0097] The measurement principle of the present application is described below by way of example.

[0098] The propagation characteristics of optical modes in micro-nano optical fibers can be described by eigenvalue equations. lm mode, the eigenvalue equation can be written as

[0099]

[0100] Among them, k 0 =2π / λ, λ is the wavelength of light in vacuum, n silica and n gas are the refractive indices of quartz and gas medium, J l is the first-kind Bessel function of order l, K l is the l-th order modified Bessel function of the second kind, d is the diameter of the micro-nano optical fiber, β represents HE lm The propagation constant of the mode in the micro-nano optical fiber, (·)' represents the derivative, and U, W, and V are dimensionless mode parameters.

[0101] The effective refractive index n of mode i effi It can be expressed as the propagation constant β i In the form of:

[0102]

[0103] Wherein, n represents the refractive index of the micro-nano optical fiber or gas medium.

[0104] Considering the photothermal effect and its impact caused by the absorption of pump light by the material to be tested, the instantaneous refractive index n of the material can be expressed as:

[0105] n(r,θ,z,t)=n 0 +dn / dT·T(r,θ,z,t) (5)

[0106] Where dn / dT and n 0 is the thermo-optic coefficient and initial refractive index of the gas medium (r>d / 2) or quartz (r≤d / 2), (r, θ, z) is the cylindrical coordinate, and t is the time. The amplitude of the temperature change T caused by the photothermal effect is related to the pump light intensity P and the absorption coefficient α of the substance to be measured. 0 is proportional to its concentration C and can be expressed as

[0107] T(r,θ,z,t)∝α 0 CP (6)

[0108] Then the detection light HE caused by the photothermal effect 1m The effective refractive index modulation of the mode is

[0109] Δn 1m (z) = neff，1m [n max (r, θ, z), λ s ]-n eff，1m [n min (r, θ, z), λ s ] (7)

[0110] In the formula, λ s is the wavelength of the detection laser, n max (r, θ, z) and n min (r, θ, z) are the lateral distributions of the refractive index n(r, θ, z, t) of the gas medium at the longitudinal position z when the system enters the "steady state" and reaches the maximum and minimum respectively.

[0111] After the detection light is transmitted through the micro-nano optical fiber with a length of L, HE 1m The phase modulation of the mode accumulation can be expressed as

[0112]

[0113] For a micro-nano optical fiber with a fixed diameter d and a fixed HE 1m Mode power ratio can also be expressed in the general form

[0114] Δφ 1m =α 0 CLP·M 1m (f) (9)

[0115] Where M 1m HE is the detection light 1m The phase modulation coefficient of the mode, f is the frequency of the pump laser.

[0116] Detection light HE 11 and HE 12 The phase difference generated after the mode is transmitted in the micro-nano optical fiber can be expressed as

[0117] δΦ=|ΔΦ 11 -ΔΦ 12 |=a 0 CLP·M(f) (10)

[0118] In the formula, the photothermal coefficient M(=|M 11 -M 12 |) is HE 11 and HE 12 The difference in the mode phase modulation coefficient. The phase difference δφ is proportional to C. The theoretical analysis results show that the heat accumulation effect of the micro-nano optical waveguide and the large thermal-optical coefficient dn / dT of the optical fiber material can effectively enhance the mode phase difference modulation, making the photothermal coefficient M in the micro-nano optical fiber larger than that in the microstructure hollow-core optical fiber in the low frequency band, that is, the micro-nano optical fiber is more sensitive to the concentration C of the substance to be measured.

[0119] The following experimental data further demonstrates the beneficial effects of the micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application.

[0120] See also Figure 5 , Figure 5 It is a second harmonic spectrum of methane gas with a volume concentration of 1% measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided in the present application. Figure 5 In the figure, each curve corresponds to a different pump laser power. After the pump laser is wavelength modulated, the obtained second harmonic signal is expressed as follows: when the pump laser power is greater, the peak-to-peak value of the second harmonic signal is greater, that is, the photothermal signal is greater.

[0121] See also Figure 6 , Figure 6 This is a graph showing the relationship between the photothermal signal and system noise of methane gas with a volume concentration of 1% measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application and the pump power. When the wavelength of the pump laser is tuned to 1654.00 nanometers (i.e., far away from the absorption line of methane gas), the mean square error of the acquired harmonic signal is the system noise. Experiments have shown that when the power of the pump laser is about 210 milliwatts, the power of the detection laser is 57 microwatts, and the integration time of the phase-locked amplifier is 1 second, by calculating the signal-to-noise ratio, that is, the signal-to-noise ratio, it can be obtained that the minimum detectable sensitivity corresponding to a signal-to-noise ratio of 1 is about 1.6ppm.

[0122] See also Figure 7 , Figure 7 This is an Allan variance curve of methane gas detection obtained based on 2 hours of noise data measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided by this application. Figure 7 It can be seen that as the integration time increases, the detectable concentration limit continues to decrease. When the integration time increases to 240 seconds, the minimum detectable sensitivity of methane gas in the present application can be increased to 0.44ppm. This reflects the high-sensitivity detection effect of the detection system.

[0123] See also Figure 8 , Figure 8 This is a graph of the change in the second harmonic signal of methane gas with a volume concentration of 1% measured within 4 hours by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application. The first 0.15 hours are amplified, and the second harmonic signal generated by the absorption of pump laser by methane gas can be clearly observed. The fluctuation amplitude of the photothermal signal in 4 hours is calculated to be about 1.6%. This reflects the stability of the detection system.

[0124] See also Fig. 9 , Fig. 9This is a graph of the change of the normalized photothermal signal with the filling time measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application. In the figure, the curve starts to rise from 22 seconds until the gas coverage reaches 90% of the entire micro-nano optical fiber at about 29 seconds, and then the curve area gradually stabilizes and begins to decline at about 65 seconds until the gas coverage reaches 10% of the entire micro-nano optical fiber at about 72 seconds. The test process is: first fill the gas collection chamber with pure nitrogen, then fill it with methane gas with a volume concentration of 1% at a rate of 500 cubic centimeters per minute at 22 seconds, and fill it with pure nitrogen at a rate of 500 cubic centimeters per minute at 65 seconds. The measurement response time of methane gas is calculated to be about 7 seconds. It reflects the effect of short response time of the detection system.

[0125] See also Fig.10 , Fig.10 This is a curve chart of the dynamic range measurement results of a methane gas sensor at normal temperature and pressure measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided in the present application. Fig.10 It can be seen that when the volume concentration of methane gas changes from 4ppm to 1%, the photothermal signal is proportional to the concentration of methane gas. When the concentration is greater than 1%, a nonlinear relationship appears. When the integration time is 240 seconds, the minimum detectable sensitivity of methane gas is about 440ppb. At this time, the dynamic range of the system is as high as nearly 6 orders of magnitude (about 9.1×10 5 ). This shows the effect of ultra-high dynamic range.

[0126] It should be understood that the size of the serial numbers of the steps in the above embodiments does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0127] The technicians in the relevant field can clearly understand that for the convenience and simplicity of description, only the division of the above-mentioned functional units and modules is used as an example for illustration. In practical applications, the above-mentioned function allocation can be completed by different functional units and modules as needed, that is, the internal structure of the device can be divided into different functional units or modules to complete all or part of the functions described above. The functional units and modules in the embodiment can be integrated in a processing unit, or each unit can exist physically separately, or two or more units can be integrated in one unit. The above-mentioned integrated unit can be implemented in the form of hardware or in the form of software functional units. In addition, the specific names of the functional units and modules are only for the convenience of distinguishing each other, and are not used to limit the scope of protection of this application. The specific working process of the units and modules in the above-mentioned system can refer to the corresponding process in the aforementioned method embodiment, which will not be repeated here.

[0128] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described or recorded in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0129] Those of ordinary skill in the art will appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0130] The embodiments described above are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, a person 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. Such 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 application, and should all be included in the protection scope of the present application.

Claims

1. A micro-nano optical waveguide photothermal spectroscopy gas detection method, It is characterized in that include: Inputting a pump laser and a detection laser into the micro-nano optical waveguide; wherein part of the energy of the pump laser and the detection laser will be transmitted in the gas medium outside the micro-nano optical waveguide in the form of evanescent waves, the evanescent waves generated by the pump laser and the substance to be measured in the gas medium will heat the gas medium after undergoing a photothermal effect, the gas medium will heat the micro-nano optical waveguide through heat conduction, and the refractive index of the gas medium and the micro-nano optical waveguide will be changed, and the detection laser will excite and generate the fundamental mode and the first high-order mode of the micro-nano optical waveguide; The phase difference between the fundamental mode and the first high-order mode after being transmitted in the micro-nano optical waveguide is detected, and the real concentration of the substance to be measured in the gas medium is obtained by conversion according to the relationship between the phase difference and the concentration of the substance to be measured.

2. The micro-nano optical waveguide photothermal spectroscopy gas detection method according to claim 1, It is characterized in that The central wavelength of the pump laser is aligned with or scanned through the absorption peak of the substance to be detected, and the central wavelength of the detection laser is far away from any absorption peak of the substance to be detected.

3. The micro-nano optical waveguide photothermal spectroscopy gas detection method according to claim 1, It is characterized in that The micro-nano optical waveguide supports multi-mode transmission.

4. The micro-nano optical waveguide photothermal spectroscopy gas detection method according to claim 1, It is characterized in that When the detection laser enters the micro-nano optical waveguide, only the fundamental mode and the first high-order mode of the micro-nano optical waveguide are excited.

5. The micro-nano optical waveguide photothermal spectroscopy gas detection method according to any one of claims 1 to 4, It is characterized in that The relationship between the phase difference generated after the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide and the concentration of the substance to be measured can be expressed as: δΦ=(M·α 0 ·L·P)·C Among them, δΦ is the phase difference between the fundamental mode and the first higher-order mode after transmission in the micro-nano optical waveguide; M is the photothermal coefficient. For a micro-nano optical waveguide of fixed size and a fixed pump laser incident mode, the photothermal coefficient is a constant; α 0 is the absorption coefficient of the substance to be measured. For a fixed absorption peak, the absorption coefficient of the substance to be measured is a constant value; L is the length of the micro-nano optical waveguide, P is the power of the pump laser, and C is the concentration of the substance to be measured.

6. A micro-nano optical waveguide photothermal spectroscopy gas detection system, It is characterized in that include: A pump laser assembly for generating a pump laser, a detection laser assembly for generating a detection laser, a wavelength division multiplexer for combining the pump laser and the detection laser, a micro-nano optical waveguide, a gas collection chamber for collecting a substance to be tested and placing the micro-nano optical waveguide, an optical filter for filtering out the pump laser, and an analysis assembly; The output ends of the pump laser component and the detection laser component are respectively connected to the input end of the wavelength division multiplexer, the input end and the output end of the micro-nano optical waveguide are respectively connected to the output end of the wavelength division multiplexer and the input end of the optical filter, and the output end of the optical filter is connected to the input end of the analysis component; There is a gas medium in the gas collection chamber, and part of the energy of the pump laser and the detection laser will be transmitted in the gas medium outside the micro-nano optical waveguide in the form of evanescent waves. The evanescent waves generated by the pump laser heat the gas medium after photothermal effect with the substance to be measured in the gas medium. The gas medium heats the micro-nano optical waveguide through heat conduction, changing the refractive index of the gas medium and the micro-nano optical waveguide. The detection laser excites and generates the fundamental mode and the first high-order mode of the micro-nano optical waveguide. The analysis component is used to detect the phase difference between the fundamental mode and the first high-order mode after transmission in the micro-nano optical waveguide, and convert the actual concentration of the substance to be measured in the gas medium according to the relationship between the phase difference and the concentration of the substance to be measured.

7. The micro-nano optical waveguide photothermal spectroscopy gas detection system as claimed in claim 6, It is characterized in that The pump laser assembly includes a first laser driver, a pump light source, and a laser amplifier; The detection laser assembly includes a detection light source, a second laser driver, and a polarization controller; The analysis component includes an optical coupler, a first optical detector and a second optical detector, a lock-in amplifier, and an analysis terminal; The input end of the first laser driver is the input end of the pump laser assembly, the input end and the output end of the pump light source are respectively connected to the output end of the first laser driver and the input end of the laser amplifier, and the output end of the laser amplifier is the output end of the pump laser assembly; The input end of the second laser driver is the input end of the detection laser assembly, the input end and the output end of the detection light source are respectively connected to the output end of the second laser driver and the input end of the polarization controller, and the output end of the polarization controller is the output end of the detection laser assembly; Among them, the input end of the optical coupler is the input end of the analysis component, the output end of the optical coupler is respectively connected to the input end of the first light detector and the input end of the second light detector, the output end of the first light detector is connected to the input end of the phase-locked amplifier, the output end of the second light detector is connected to the input end of the analysis terminal, the output end of the phase-locked amplifier is respectively connected to the input end of the analysis terminal and the input end of the pump laser component, and the output end of the analysis terminal is connected to the input end of the detection laser component.

8. The micro-nano optical waveguide photothermal spectroscopy gas detection system as claimed in claim 6, It is characterized in that The micro-nano optical waveguide includes any one of an integrated optical waveguide and an optical fiber waveguide.

9. The micro-nano optical waveguide photothermal spectroscopy gas detection system according to claim 6, It is characterized in that The cross-sectional size of the micro-nano optical waveguide is not greater than 10 times the maximum wavelength of the pump laser and the detection laser.

10. The micro-nano optical waveguide photothermal spectroscopy gas detection system according to any one of claims 6 to 9, It is characterized in that There is a non-adiabatic transition between the output end of the wavelength division multiplexer and the input end of the micro-nano optical waveguide; when the detection laser is input from the output end of the wavelength division multiplexer to the input end of the micro-nano optical waveguide, the detection laser excites the fundamental mode and the first high-order mode of the micro-nano optical waveguide, and the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide at the same time; or There is an adiabatic transition between the output end of the wavelength division multiplexer and the input end of the micro-nano optical waveguide; when the detection laser is input from the output end of the wavelength division multiplexer to the input end of the micro-nano optical waveguide, the detection laser only excites the fundamental mode of the micro-nano optical waveguide; wherein a long period grating is written at the input end of the micro-nano optical waveguide, and the long period grating couples part of the energy of the fundamental mode to the first high-order mode, so that the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide at the same time.

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