CN115326756A - Micro-nano optical waveguide photothermal spectroscopy gas detection method and detection system - Google Patents

Abstract

The present application is applicable to the technical field of gas measurement, and provides a micro-nano optical waveguide photothermal spectroscopy gas detection method and detection system. The detection method provided by this application includes: inputting pump laser and probe laser into a micro-nano optical waveguide, the evanescent wave generated by the pump laser and the substance to be tested in the gas medium have photothermal effect and accompanying heat conduction, changing the gas medium and Refractive index of the micro-nano optical waveguide; detect the fundamental mode and the first high-order mode of the micro-nano optical waveguide generated by laser excitation, detect the phase difference between the fundamental mode and the first high-order mode after transmission in the micro-nano optical waveguide, and obtain the concentration of the substance to be tested . The present application can obtain a large proportion of evanescent field without harsh micro-nano optical waveguide preparation process conditions, and has high photothermal efficiency, small size, low cost and fast response speed.

Description

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

technical field

The application belongs to the technical field of gas measurement, and in particular relates to a micro-nano optical waveguide photothermal spectrum gas detection method and a detection system.

Background technique

Laser absorption spectroscopy is a highly sensitive and selective gas analysis method, which is widely used in the fields of environmental monitoring, energy and power, national defense and aerospace.

Based on various effects associated with light absorption, different methods of derivation of laser absorption spectra have been developed. Laser photothermal spectroscopy uses the photothermal effect and the thermo-optic modulation characteristics it brings to indirectly measure the gas concentration. It has the advantage of no background noise and effectively improves the sensitivity and accuracy of gas detection. In particular, the microstructured hollow-core optical fiber is used as a carrier, which greatly increases and improves the distance of light-matter interaction and the light energy density, greatly improves the heat generation efficiency of the photothermal effect, and achieves excellent gas detection performance.

However, the microstructured hollow core fiber needs to be prepared by a fiber drawing tower under precise temperature and pressure control conditions through a special structural design. In gas detection, on the one hand, microstructured hollow-core fibers need to be precisely aligned and connected reliably with standard single-mode fibers to form a gas detection system with an all-fiber structure. On the other hand, the gas fills the microstructured hollow-core fiber through free diffusion, which cannot meet the needs of real-time detection. Fabrication of microchannels can accelerate gas exchange, but sacrifices the mechanical strength of microstructured hollow-core fibers and introduces losses. In the above discussion, the high manufacturing cost and difficulty of the microstructured hollow core fiber, as well as the slow response speed limit its large-scale application.

Contents of the invention

The embodiment of the present application provides a micro-nano optical waveguide photothermal spectroscopy gas detection method and detection system, which can solve the problems of high manufacturing cost, difficulty and slow response speed of sensors in the prior art.

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

Input the pump laser and probe laser into the micro-nano optical waveguide; wherein, part of the energy of the pump laser and the probe laser will be transmitted in the gas medium outside the micro-nano optical waveguide in the form of evanescent waves, so The evanescent wave generated by the pump laser has a photothermal effect with the substance to be measured in the gas medium and then heats the gas medium, and the gas medium heats the micro-nano optical waveguide through heat conduction, changing the gas medium and the The refractive index of the micro-nano optical waveguide, the detection laser excitation generates the fundamental mode and the first high-order mode of the micro-nano optical waveguide;

Detecting the phase difference generated after the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide, and converting the phase difference to the concentration of the substance to be measured to obtain the substance to be measured in the gas medium true concentration.

Optionally, the central wavelength of the pumping laser is aligned 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.

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

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.

Optionally, the relationship between the phase difference generated after the fundamental mode and the first higher-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 (I)

Among them, δφ is the phase difference between the fundamental mode and the first high-order mode after transmission in the micro-nano optical waveguide; M is the photothermal coefficient, for a fixed-sized micro-nano optical waveguide and a fixed pump laser incident mode, the photothermal coefficient is fixed value; α ₀ is the absorption coefficient of the substance to be measured, and 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 substance to be measured concentration.

Among them, the phase difference generated after the fundamental mode and the first high-order mode are transmitted 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 probe laser emitted from the output end of the micro-nano optical waveguide , which can be converted to the concentration of the substance to be measured.

Compared with the prior art, the beneficial effect is: through the embodiment of the present application, since the gas medium containing the substance to be measured 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 light, it not only changes the refractive index of the gas medium, but also changes the refractive index of the micro-nano optical waveguide, and the thermo-optic coefficient of the micro-nano optical waveguide material (for example, silicon dioxide is about 10 ^-5 K ^-1 magnitude and above) is much larger than the thermo-optic coefficient of gas (for example, about -10 ^-6 K ^-1 at room temperature in gas medium), so under the same conditions, higher light than microstructured hollow core fiber can be obtained Thermal efficiency. At the same time, the gas medium usually has a lower thermal conductivity, which makes the covered micro-nano optical waveguide produce heat accumulation effect, further improves the photothermal efficiency, and improves the detection sensitivity of the gas concentration sensor. The micro-nano optical waveguide used supports high-order mode transmission, and the high-order mode has a larger evanescent field ratio. Therefore, while obtaining strong light-matter interaction, it avoids harsh process conditions and reduces manufacturing difficulty and cost. The micro-nano optical waveguide can be prepared by fused tapered standard single-mode optical fiber, or it can be mass-produced by a preparation process compatible with MEMS, which further reduces the difficulty and cost of production.

In the second aspect, the embodiment of the present application provides a micro-nano optical waveguide photothermal spectroscopy gas detection system, including a pump laser assembly for generating pump laser light, a detection laser assembly for generating detection laser light, and a beam combination Wavelength division multiplexer for pump laser and probe laser, micro-nano optical waveguide, gas collection chamber for collecting the substance to be measured and placing micro-nano optical waveguide, optical filter for filtering pump laser light, and analysis components ;

Among them, 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, and the input end and 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. connected, 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, part of the energy of the pump laser and the probe laser will be transmitted in the gas medium outside the micro-nano optical waveguide in the form of evanescent waves, and the pump laser The generated evanescent wave has a photothermal effect with the substance to be measured in the gas medium and heats the gas medium, and the gas medium heats the micro-nano optical waveguide through heat conduction, changing the gas medium and the micro-nano optical waveguide. The refractive index of the waveguide, the fundamental mode and the first high-order mode of the micro-nano optical waveguide generated by the excitation of the probe laser;

The analysis component is used to detect the phase difference generated after the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide, and obtain the phase difference according to the relationship between the phase difference and the concentration of the substance to be measured. The true concentration of the substance to be measured in the gaseous medium.

Optionally, the pump laser component 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 assembly includes an optical coupler, a first photodetector and a second photodetector, a lock-in amplifier, and an analysis terminal;

Wherein, 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 pumping light source are respectively connected with 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 component;

Wherein, the input end of the second laser driver is the input end of the detection laser component, the input end and the output end of the detection light source are respectively connected with 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 Probe the output of the laser assembly;

The input end of the optical coupler is the input end of the analysis component, the output end of the optical coupler is connected with the input end of the first photodetector and the second photodetector respectively, the output end of the first photodetector is connected with the lock-in amplifier The input end is connected, the output end of the second photodetector is connected with the input end of the analysis terminal, the output end of the lock-in amplifier is connected with the input end of the analysis terminal and the pump laser assembly respectively, and the output end of the analysis terminal is connected with the detection laser assembly. connected to the input.

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

Optionally, the cross-sectional size of the micro-nano optical waveguide is not greater than 10 times of the maximum light wavelength of both the pump laser and the probe laser.

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;

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 is excited to generate 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 simultaneously in the micro-nano optical waveguide in transmission.

Optionally, 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. Among them, a long-period grating is written at the input end of the micro-nano optical waveguide, and the long-period grating allows part of the energy of the fundamental mode to be coupled to the first high-order mode, so that the fundamental mode and the first high-order mode are simultaneously transmitted in the micro-nano optical waveguide.

Based on the above gas detection method, the detection system of the present invention has simple structure, convenient operation, high detection sensitivity, large dynamic range and fast signal response. It is tested by experiments that 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.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present application, the accompanying drawings that need to be used in the descriptions of the embodiments or the prior art will be briefly introduced below. Obviously, the accompanying drawings in the following description are only for the present application For some embodiments, those of ordinary skill in the art can also obtain other drawings based on these drawings without paying creative efforts.

Figure 1 is a schematic diagram of a photothermal effect generated on a micro-nano optical waveguide provided by the present application;

Fig. 2 is a cross-sectional view of a micro-nano optical waveguide provided by the present application;

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

Fig. 4 is a diagram of the automatic stabilization process of a micro-nano optical waveguide interferometer provided by the present application;

Fig. 5 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;

Fig. 6 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 and the pump power;

Fig. 7 is an Allan variance curve diagram of methane gas detection obtained based on 2-hour noise data measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application;

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

Fig. 9 is a curve diagram of the normalized photothermal signal measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method as a function of inflation time;

Fig. 10 is a graph of the measurement results of the dynamic range of the methane gas sensor at normal temperature and pressure measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application.

Detailed ways

In the following description, specific details such as specific system structures and technologies are presented for the purpose of illustration rather than limitation, so as to thoroughly understand the embodiments of the present application. It will be apparent, however, to one skilled in the art that the present application may be practiced in other embodiments without these specific details. In other instances, detailed descriptions of well-known systems, devices, circuits, and methods are omitted so as not to obscure the description of the present application with unnecessary detail.

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

It should also be understood that the term "and/or" used in the description of the present application and the appended claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes these combinations.

As used in this specification and the appended claims, the term "if" may be construed, depending on the context, as "when" or "once" or "in response to determining" or "in response to detecting ". Similarly, the phrase "if determined" or "if [the described condition or event] is detected" may be construed, depending on the context, to mean "once determined" or "in response to the determination" or "once detected [the described condition or event] ]” or “in response to detection of [described condition or event]”.

In addition, in the description of the specification and appended claims of the present application, the terms "first", "second", "third" and so on are only used to distinguish descriptions, and should not be understood as indicating or implying relative importance.

Reference to "one embodiment" or "some embodiments" or the like in the specification of the present application means that a particular feature, structure, or characteristic described in connection with the embodiment is included in one or more embodiments of the present application. Thus, appearances of the phrases "in one embodiment," "in some embodiments," "in other embodiments," "in other embodiments," etc. in various places in this specification are not necessarily All refer to the same embodiment, but mean "one or more but not all embodiments" unless specifically stated otherwise. The terms "including", "comprising", "having" and variations thereof mean "including but not limited to", unless specifically stated otherwise.

A micro-nano optical waveguide photothermal spectroscopy gas detection method provided in the present application will be exemplarily described below in conjunction with specific embodiments.

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

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.

Please refer to FIG. 1 . FIG. 1 is a schematic diagram of photothermal effect and heat conduction on a micro-nano fiber provided by the present application.

The gas concentration detection method in this embodiment is executed by a gas concentration sensor.

S101. Input pump laser light and probe laser light into the micro-nano optical waveguide; wherein, part of the energy of the pump laser light and the probe laser light will be transmitted in the gas medium outside the micro-nano optical waveguide in the form of evanescent waves , the evanescent wave generated by the pump laser has a photothermal effect with the substance to be measured in the gas medium and then heats the gas medium, and the gas medium heats the micro-nano optical waveguide through heat conduction, changing the gas medium and the refractive index of the micro-nano optical waveguide, the probe laser excitation generates the fundamental mode and the first high-order mode of the micro-nano optical waveguide.

S102. Detect the phase difference generated after the fundamental mode and the first higher-order mode are transmitted in the micro-nano optical waveguide, and convert the phase difference to the concentration of the substance to be measured to obtain the substance to be measured in the gas medium true concentration.

Specifically, in step S101, the pumping laser and the probing laser are input into the micro-nano optical fiber. For this step, reference may be made to the conventional method of injecting the laser into the optical fiber in the art, which is not specifically limited in this application. The purpose of this step is to make the pump laser and the probe laser respectively generate evanescent waves in the micro-nano fiber, and the generated evanescent waves are transmitted along the micro-nano fiber.

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

In the embodiment of the present application, the transmission direction of the pump laser light and the detection laser light in the micro-nano fiber is chosen to be the same direction.

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, for methane gas with a volume concentration of 1ppm, the substance to be measured is methane gas with a volume ratio of 1×10 ^-6 , and the gas medium with a volume ratio of (0.999999=1-1×10 ^-6 ) is nitrogen .

Optionally, the central wavelength of the pumping laser is aligned 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 away from the absorption line of the substance to be measured, so that the substance to be measured cannot absorb the detection laser.

Further, in the embodiment of the present application, the center wavelength of the pump laser is near the methane R3 absorption peak (ie, 1653.7 nanometers), and the center wavelength of the probe laser is near 1550 nanometers.

Optionally, the micro-nano fiber supports multi-mode transmission.

For example, please refer to FIG. 2 , which is a cross-sectional view of a micro-nano fiber provided by the present application. The diameter of the micro-nano fiber in Figure 2 is about 2.1 microns. The micro-nano fiber is prepared from a single-mode quartz fiber by two-step stretching methods of arc discharge and hydrogen-oxygen flame heating, and supports multiple transmission modes including fundamental mode HE ₁₁ and high-order mode HE ₁₂ .

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

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

Please refer to FIG. 3 . FIG. 3 is a schematic diagram of a micro-nano optical waveguide photothermal spectroscopy gas detection system provided by the present application. Micro-nano optical waveguide photothermal spectroscopy gas detection system includes:

A pump laser assembly for generating pump laser light, a probe laser assembly for generating probe laser light, a wavelength division multiplexer for combining the pump laser light and the probe laser light, and a micro-nano optical waveguide for A gas collection chamber for collecting the substance to be measured and placing the micro-nano optical waveguide, an optical filter for filtering out the pump laser light, and an analysis component;

Wherein, the output ends of the pump laser component and the detection laser component are respectively connected to the input ends of the wavelength division multiplexer, and the input ends and output ends of the micro-nano optical waveguide are respectively connected to the wavelength division multiplexer The output end of the user is connected to 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, part of the energy of the pump laser and the probe laser will be transmitted in the gas medium outside the micro-nano optical waveguide in the form of evanescent waves, and the pump laser The generated evanescent wave has a photothermal effect with the substance to be measured in the gas medium and heats the gas medium, and the gas medium heats the micro-nano optical waveguide through heat conduction, changing the gas medium and the micro-nano optical waveguide. The refractive index of the waveguide, the fundamental mode and the first high-order mode of the micro-nano optical waveguide generated by the excitation of the probe laser;

The analysis component is used to detect the phase difference generated after the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide, and obtain the phase difference according to the relationship between the phase difference and the concentration of the substance to be measured. The true concentration of the substance to be measured in the gaseous medium.

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 photodetector and a second photodetector, a lock-in amplifier, and an analysis terminal;

Wherein, the input end of the first laser driver is the input end of the pump laser assembly, and the input end and output end of the pump light source are respectively connected to the output end of the first laser driver and the laser amplifier. The input ends are connected, and the output end of the laser amplifier is the output end of the pump laser assembly;

Wherein, the input end of the second laser driver is the input end of the detection laser assembly, and the input end and output end of the detection light source are respectively connected with the output end of the second laser driver and the input of the polarization controller The terminals are connected, and the output terminal of the polarization controller is the output terminal of the detection laser component;

Wherein, the input end of the optical coupler is the input end of the analysis component, and the output end of the optical coupler is connected with the input end of the first photodetector and the input end of the second photodetector respectively. connected, the output of the first photodetector is connected to the input of the lock-in amplifier, the output of the second photodetector is connected to the input of the analysis terminal, and the output of the lock-in amplifier terminals are respectively connected to the input end of the analysis terminal and the input end of the pump laser assembly, and the output end of the analysis terminal is connected to the input end of the detection laser assembly.

In Figure 3, the pump laser and probe laser can enter from the input end of the micro-nano fiber. In Figure 3, there are first laser driver, pump light source, laser amplifier, wavelength division multiplexer, micro-nano fiber, gas collection chamber, detection light source, second laser driver, polarization controller, optical filter, and first light detector detector, second photodetector, lock-in amplifier and analysis terminal.

In a possible implementation, when the pump laser light enters the micro-nano fiber, the substance to be measured will absorb the pump laser light, thereby generating a photothermal effect. First, the gas medium outside the micro-nano fiber is heated to change the refractive index of the gas medium. Then the micro-nano fiber is heated by heat conduction, so that the refractive index of the micro-nano fiber is changed.

Specifically, firstly, methane gas with a volume concentration of 1% is filled in the gas collection chamber. The pump light source can be a distributed feedback laser of 1654 nm. The lock-in amplifier can input a 6 kHz sinusoidal modulation signal to the first laser driver, and at the same time, the first laser driver jointly drives the pump light source with a sawtooth wave scanning frequency of 0.01 Hz, outputs the modulated pump laser, and the modulated pump The pump laser can enter the micro-nano fiber through the laser amplifier and the wavelength division multiplexer, so that the interaction between the pump laser and the methane gas produces the photothermal effect and the accompanying heat conduction, causing the refractive index of the gas medium and the micro-nano fiber to be changed periodically. Among them, the laser amplifier is used to increase the energy of the pump laser.

In a possible implementation manner, the cross-sectional size of the micro-nano optical waveguide is not greater than 10 times of the maximum light wavelength of both the pump laser light and the probe laser light.

In a possible implementation, when the probe 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.

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 fiber.

When a non-adiabatic transition is used between the output end of the wavelength division multiplexer and the input end of the micro-nano optical fiber, when the probe laser is input from the output end of the wavelength division multiplexer to the input end of the micro-nano optical waveguide , so that the detection laser is excited to generate 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 simultaneously transmitted in the micro-nano optical waveguide.

When an adiabatic transition is used between the output end of the wavelength division multiplexer and the input end of the micro-nano optical fiber, when the probe laser is input from the output end of the wavelength division multiplexer to the input end of the micro-nano optical waveguide, such that The probe laser light 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 makes part of the energy of the fundamental mode coupled to The first high-order mode causes the fundamental mode and the first high-order mode to be simultaneously transmitted in the micro-nano optical waveguide.

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 fiber is a non-adiabatic transition, please refer to FIG. 3 . When the probe laser is input from the output end of the wavelength division multiplexer to the input end of the micro-nano fiber, the probe laser simultaneously excites the fundamental mode and the first high-order mode of the micro-nano fiber.

Specifically, the first higher-order mode is a higher-order transmission mode different from the fundamental mode. For example, the fundamental mode is the HE ₁₁ mode, and the first higher-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) anyone.

As an example and not a limitation, in the embodiment of the present application, HE ₁₂ is selected as the first higher-order mode.

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 fiber through a polarization controller and a wavelength division multiplexer. When the probe laser enters the micro-nano fiber, it excites the HE ₁₁ and HE ₁₂ modes of the micro-nano fiber.

Since the refractive index of the gas medium and the micro-nano fiber has changed. When the probe laser is transmitted along the micro-nano fiber, the phases of the HE ₁₁ and HE ₁₂ modes excited by the probe laser will be changed, and the phase difference between the HE ₁₁ and HE ₁₂ modes after transmission in the micro-nano fiber will also change accordingly.

Optionally, the relationship between the phase difference generated after the HE ₁₁ and HE ₁₂ modes are transmitted in the micro-nano fiber and the concentration of the substance to be measured can be expressed as:

δφ＝(M·α ₀ ·L·P)·C (2)

Among them, δφ is the phase difference generated after the HE ₁₁ and HE ₁₂ modes are transmitted in the micro-nano fiber; M is the photothermal coefficient, which is a constant value for a fixed-sized micro-nano fiber and a fixed pump laser incident mode; α ₀ is the absorption coefficient of methane. For a fixed absorption peak, its value 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.

For example, please refer to Figure 3. After receiving part of the detection laser light output by the micro-nano optical fiber, the second photodetector transmits the converted electrical signal to the analysis terminal, and the analysis terminal processes the feedback adjustment signal after receiving the electrical signal. , the feedback adjustment signal is transmitted to the second laser driver, and 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 fiber interferometer. Adjusting the polarization controller keeps the contrast of the interference fringes generated by the micro-nano fiber interferometer at the highest level, in order to keep the phase detection sensitivity optimal. Please refer to Figure 4. Figure 4 is a diagram of the automatic stabilization process of the micro-nano fiber interferometer. The micro-nano fiber interferometer gradually deviates from the operating point when it is running freely. After adding feedback adjustment, the micro-nano fiber interferometer quickly enters a stable state, that is, locking At the orthogonal operating point. The maximum phase detection sensitivity can be obtained by adjusting the micro-nano fiber interferometer to the orthogonal working point.

For example, please refer to FIG. 3 , after receiving part of the detection laser light output by the micro-nano fiber, the first photodetector transmits the converted electrical signal to a lock-in amplifier. After the lock-in amplifier receives the electrical signal, it can demodulate the harmonic signal from the electrical signal. The harmonic signal is an intermediate signal carrying the phase difference information generated after the HE ₁₁ and HE ₁₂ modes are 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, that is, the photothermal signal, which 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.

The measurement principle of the present application is exemplarily described below.

The propagation characteristics of optical modes in micro/nano fibers can be described by eigenequations. For the HE _lm mode, the eigenequation can be written as

d为微纳光纤的直径，β表示HE_lm模式在微纳光纤中的传播常数，(·)'表示求导，U、W、V为无量纲的模式参数。Among them, k ₀ =2π/λ, λ is the light wavelength in vacuum, n _silica and n _gas are the refractive index of quartz and gas medium respectively, J _l is the first kind of Bessel function of order l, K _l is the order l Modified Bessel functions of the second kind,

d is the diameter of the micro-nano fiber, β represents the propagation constant of the HE _lm mode in the micro-nano fiber, ( )' represents the derivative, and U, W, V are dimensionless mode parameters.

The effective index n _effi of mode _i can be expressed in the form of the propagation constant βi:

Among them, n represents the refractive index of micro-nano fiber or gas medium.

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

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

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

T(r, θ, z, t)∝α ₀ CP (6)

Then the effective refractive index modulation of the probe light HE _1m mode caused by the photothermal effect is

Δn _1m (z) = n _{eff, 1m} [n _max (r, θ, z), λ _s ]-n _{eff, 1m} [n _min (r, θ, z), λ _s ] (7)

In the formula, λ _s is the wavelength of the probe laser, n _max (r, θ, z) and n _min (r, θ, z) are the refractive index n of the gas medium at the longitudinal position z after the system enters the "steady state" ( r, θ, z, t) obtain the maximum and minimum lateral distribution.

After the probe light is transmitted through the micro-nano fiber with length L, the phase modulation accumulated in the HE _1m mode can be expressed as

For a micro-nano fiber with a fixed diameter d and a fixed HE _1m mode power ratio, it can also be expressed as a general form

Δφ _1m ＝α ₀ CLP·M _1m (f) (9)

In the formula, M _1m is the phase modulation coefficient of the HE _1m mode of the probe light, and f is the frequency of the pump laser.

The phase difference generated after the probe light HE ₁₁ and HE ₁₂ modes are transmitted in the micro-nano fiber can be expressed as

δφ＝|Δφ ₁₁ -Δφ ₁₂ |＝a ₀ CLP·M(f) (10)

In the formula, the photothermal coefficient M (=|M ₁₁ −M ₁₂ |) is the difference between the phase modulation coefficients of the HE ₁₁ and HE ₁₂ modes. 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 thermo-optic coefficient dn/dT of the fiber material can effectively enhance the mode phase difference modulation, so that the photothermal coefficient M in the micro-nano fiber is in The low-frequency band is larger than that of the microstructured hollow-core fiber, which means that the micro-nano fiber is more sensitive to the concentration C of the substance to be measured.

The beneficial effect of a micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application is further proved through some experimental data below.

Please refer to FIG. 5 . FIG. 5 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. In Fig. 5, the power of the pump laser corresponding to each curve is different. When the pump laser is modulated by wavelength, the obtained second harmonic signal shows that when the power of the pump laser is larger, the peak value of the second harmonic signal is The larger the peak value, the larger the photothermal signal.

Please refer to FIG. 6. FIG. 6 is a graph showing the relationship between the photothermal signal, system noise and pump power 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. When the wavelength of the pump laser is tuned to 1654.00 nm (that is, away from the methane gas absorption line), the mean square error of the obtained harmonic signal is the system noise. Experiments have proved that when the power of the pump laser is about 210 milliwatts, the power of the probe laser is 57 microwatts, and the integration time of the lock-in amplifier is 1 second, by calculating the signal-to-noise ratio, that is, the signal-to-noise ratio, it can be When the signal-to-noise ratio is 1, the corresponding minimum detectable sensitivity is about 1.6ppm.

Please refer to FIG. 7 . FIG. 7 is an Allan variance curve of methane gas detection based on 2-hour noise data measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application. It can be seen from Figure 7 that the detectable concentration limit keeps decreasing as the integration time increases. When the integration time is increased to 240 seconds, the minimum detectable sensitivity of methane gas of the present application can be increased to 0.44ppm. It embodies the effect of high-sensitivity detection of the detection system.

Please refer to FIG. 8 . FIG. 8 is a curve diagram of the second harmonic signal change 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. In the first 0.15 hours of amplification processing, the second harmonic signal generated by the methane gas absorption pump laser can be clearly observed, and the calculated fluctuation range of the photothermal signal in the 4 hours is about 1.6%. It reflects the effect of the stability of the detection system.

Please refer to FIG. 9 . FIG. 9 is a curve diagram of the normalized photothermal signal measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application as a function of gas filling time. In the figure, the curve starts to rise from 22 seconds until the gas coverage reaches 90% of the entire micro-nano fiber at about 29 seconds, then the curve area gradually stabilizes, and begins to decline at about 65 seconds until about 72 seconds The gas coating amount reaches 10% of the whole micro-nano fiber. The test process is as follows: 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 at a rate of 500 cubic centimeters per minute at 65 seconds Filled with pure nitrogen, the calculated response time of methane gas measurement is about 7 seconds. It embodies the effect of short response time of the detection system.

Please refer to FIG. 10 . FIG. 10 is a graph of the measurement results of the dynamic range of the methane gas sensor at normal temperature and pressure measured by a micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application. It can be seen from Figure 10 that during the change process of the volume concentration of methane gas from 4ppm to 1%, the photothermal signal is proportional to the concentration of methane gas, and 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 440 ppb, and the dynamic range of the system is as high as nearly 6 orders of magnitude (about 9.1×10 ⁵ ). It embodies the effect of ultra-high dynamic range.

It should be understood that the sequence numbers of the steps in the above embodiments do not mean the order of execution, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation to the implementation process of the embodiment of the present application.

Those skilled in the art can clearly understand that for the convenience and brevity of description, only the division of the above-mentioned functional units and modules is used for illustration. In practical applications, the above-mentioned functions can be assigned to different functional units, Completion of modules means that the internal structure of the device is divided into different functional units or modules to complete all or part of the functions described above. Each functional unit and module in the embodiment can be integrated into one processing unit, or each unit can exist separately physically, or two or more units can be integrated into one unit, and the above-mentioned integrated units can either adopt hardware It can also be implemented 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 protection scope of the present application. For the specific working process of the units and modules in the above system, reference may be made to the corresponding process in the foregoing method embodiments, and details will not be repeated here.

In the above-mentioned embodiments, the descriptions of each embodiment have their own emphases, and for parts that are not detailed or recorded in a certain embodiment, refer to the relevant descriptions of other embodiments.

Those skilled in the art can appreciate that the units and algorithm steps of the examples described in conjunction with the embodiments disclosed herein can be implemented by electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are executed by hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art may use different methods to implement the described functions for each specific application, but such implementation should not be regarded as exceeding the scope of the present application.

The above-described embodiments 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 foregoing embodiments, those of ordinary skill in the art should understand that: it can still implement the foregoing embodiments Modifications to the technical solutions described in the examples, or equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the various embodiments of the application, and should be included in the Within the protection scope of this application.

Claims (10)

1. A micro-nano optical waveguide photothermal spectroscopy gas detection method, characterized in that it comprises: Input the pump laser and probe laser into the micro-nano optical waveguide; wherein, part of the energy of the pump laser and the probe laser will be transmitted in the gas medium outside the micro-nano optical waveguide in the form of evanescent waves, so The evanescent wave generated by the pump laser has a photothermal effect with the substance to be measured in the gas medium and then heats the gas medium, and the gas medium heats the micro-nano optical waveguide through heat conduction, changing the gas medium and the The refractive index of the micro-nano optical waveguide, the detection laser excitation generates the fundamental mode and the first high-order mode of the micro-nano optical waveguide; Detecting the phase difference generated after the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide, and converting the phase difference to the concentration of the substance to be measured to obtain the substance to be measured in the gas medium true concentration. 2. The micro-nano optical waveguide photothermal spectroscopy gas detection method according to claim 1, wherein the central wavelength of the pump laser is aligned or scanned through the absorption peak of the substance to be measured, and the detection laser The central wavelength is far away from any absorption peak of the substance to be tested. 3. The micro-nano optical waveguide photothermal spectroscopy gas detection method according to claim 1, wherein the micro-nano optical waveguide supports multi-mode transmission. 4. The micro-nano optical waveguide photothermal spectroscopy gas detection method according to claim 1, wherein when the probe laser light enters the micro-nano optical waveguide, only the base of the micro-nano optical waveguide is excited. mode and the first higher-order mode. 5. The micro-nano optical waveguide photothermal spectroscopy gas detection method according to any one of claims 1-4, wherein the fundamental mode and the first high-order mode are generated after transmission in the micro-nano optical waveguide The relationship between the phase difference of and the concentration of the substance to be measured can be expressed as: 6φ＝(M·α ₀ ·L·P)·C Among them, δφ is the phase difference between the fundamental mode and the first high-order mode after transmission in the micro-nano optical waveguide; M is the photothermal coefficient, for a fixed-sized micro-nano optical waveguide and a fixed pump laser incident mode, the photothermal coefficient is fixed value; α ₀ is the absorption coefficient of the substance to be measured, and 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 substance to be measured concentration. 6. A micro-nano optical waveguide photothermal spectroscopy gas detection system, characterized in that it comprises: A pump laser assembly for generating pump laser light, a probe laser assembly for generating probe laser light, a wavelength division multiplexer for combining the pump laser light and the probe laser light, and a micro-nano optical waveguide for A gas collection chamber for collecting the substance to be measured and placing the micro-nano optical waveguide, an optical filter for filtering out the pump laser light, and an analysis component; Wherein, the output ends of the pump laser component and the detection laser component are respectively connected to the input ends of the wavelength division multiplexer, and the input ends and output ends of the micro-nano optical waveguide are respectively connected to the wavelength division multiplexer The output end of the user is connected to 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, part of the energy of the pump laser and the probe laser will be transmitted in the gas medium outside the micro-nano optical waveguide in the form of evanescent waves, and the pump laser The generated evanescent wave has a photothermal effect with the substance to be measured in the gas medium and heats the gas medium, and the gas medium heats the micro-nano optical waveguide through heat conduction, changing the gas medium and the micro-nano optical waveguide. The refractive index of the waveguide, the fundamental mode and the first high-order mode of the micro-nano optical waveguide generated by the excitation of the probe laser; The analysis component is used to detect the phase difference generated after the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide, and obtain the phase difference according to the relationship between the phase difference and the concentration of the substance to be measured. The true concentration of the substance to be measured in the gaseous medium. 7. The micro-nano optical waveguide photothermal spectroscopy gas detection system as claimed in claim 6, wherein the pumping laser assembly comprises a first laser driver, a pumping 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 photodetector and a second photodetector, a lock-in amplifier, and an analysis terminal; Wherein, the input end of the first laser driver is the input end of the pump laser assembly, and the input end and output end of the pump light source are respectively connected to the output end of the first laser driver and the laser amplifier. The input ends are connected, and the output end of the laser amplifier is the output end of the pump laser assembly; Wherein, the input end of the second laser driver is the input end of the detection laser assembly, and the input end and output end of the detection light source are respectively connected with the output end of the second laser driver and the input of the polarization controller The terminals are connected, and the output terminal of the polarization controller is the output terminal of the detection laser component; Wherein, the input end of the optical coupler is the input end of the analysis component, and the output end of the optical coupler is connected with the input end of the first photodetector and the input end of the second photodetector respectively. connected, the output of the first photodetector is connected to the input of the lock-in amplifier, the output of the second photodetector is connected to the input of the analysis terminal, and the output of the lock-in amplifier terminals are respectively connected to the input end of the analysis terminal and the input end of the pump laser assembly, and the output end of the analysis terminal is connected to the input end of the detection laser assembly. 8. The micro-nano optical waveguide photothermal spectroscopy gas detection system according to claim 6, wherein the micro-nano optical waveguide comprises any one of an integrated optical waveguide and a fiber optic waveguide. 9. The micro-nano optical waveguide photothermal spectroscopy gas detection system as claimed in claim 6, wherein the cross-sectional size of the micro-nano optical waveguide is not greater than the maximum light in both the pump laser and the detection laser. 10 times the wavelength. 10. The micro-nano optical waveguide photothermal spectroscopy gas detection system according to any one of claims 6-9, wherein the output end of the wavelength division multiplexer and the input end of the micro-nano optical waveguide There is a non-adiabatic transition between them; 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 generation of the micro-nano optical waveguide. The fundamental mode and the first higher-order mode, the fundamental mode and the first higher-order mode are simultaneously transmitted in the micro-nano optical waveguide; 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; the probe laser is input from the output end of the wavelength division multiplexer to the input of the micro-nano optical waveguide At the end, the probe laser light 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 makes the fundamental mode of the fundamental mode Part of the energy is coupled to the first high-order mode, so that the fundamental mode and the first high-order mode are simultaneously transmitted in the micro-nano optical waveguide.

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