CN115326756A - Micro-nano optical waveguide photo-thermal spectrum gas detection method and detection system - Google Patents
Micro-nano optical waveguide photo-thermal spectrum gas detection method and detection system Download PDFInfo
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Abstract
The application is suitable for the technical field of gas measurement, and provides a micro-nano optical waveguide photo-thermal spectrum gas detection method and a micro-nano optical waveguide photo-thermal spectrum gas detection system. The detection method provided by the application comprises the following steps: pumping laser and detection laser are input into the micro-nano optical waveguide, evanescent waves generated by the pumping laser and substances to be detected in the gas medium generate photothermal effect and accompanying heat conduction, and the refractive indexes of the gas medium and the micro-nano optical waveguide are changed; and exciting the detection laser to generate a basic mode and a first high-order mode of the micro-nano optical waveguide, and detecting the phase difference generated after the basic mode and the first high-order mode are transmitted in the micro-nano optical waveguide to obtain the concentration of the substance to be detected. The large-scale evanescent field can be obtained without harsh micro-nano optical waveguide preparation process conditions, and the preparation method has the advantages of high photo-thermal efficiency, small size, low cost and high response speed.
Description
Technical Field
The application belongs to the technical field of gas measurement, and particularly relates to a micro-nano optical waveguide photo-thermal spectrum gas detection method and system.
Background
The laser absorption spectrometry is a gas analysis method with high sensitivity and high selectivity, and has wide application in the fields of environmental monitoring, energy and power, national defense, aerospace and the like.
Based on various effects associated with light absorption, different laser absorption spectrum derivation methods have been developed. The laser photothermal spectroscopy utilizes the photothermal effect and the thermo-optic modulation characteristics brought by the photothermal effect to indirectly measure the gas concentration, has the advantage of no background noise, and effectively improves the sensitivity and accuracy of gas detection. Particularly, the micro-structure hollow optical fiber is used as a carrier, so that the interaction distance between light and a substance and the light energy density are greatly increased and improved, the heat production efficiency of a photothermal effect is greatly improved, and the excellent gas detection performance is realized.
However, microstructured hollow core optical fibers need to be prepared by a fiber draw tower under precise temperature and pressure control through special structural design. In gas detection, on one hand, the micro-structured hollow-core fiber needs to be precisely aligned and reliably connected with a standard single-mode fiber to form a gas detection system with a full-fiber structure. On the other hand, the gas fills the hollow optical fiber of the microstructure in a free diffusion mode, and the requirement of real-time detection cannot be met. Fabrication of microchannels can accelerate gas exchange, but can sacrifice mechanical strength and introduce losses in the microstructured hollow core fiber. In the above discussion, the cost and difficulty of fabricating microstructured hollow-core optical fibers, and the slow response speed, limit their large-scale application.
Disclosure of Invention
The embodiment of the application provides a micro-nano optical waveguide photo-thermal spectrum gas detection method and a detection system, and can solve the problems of high manufacturing cost and difficulty and low response speed of a sensor in the prior art.
In a first aspect, an embodiment of the present application provides a method for detecting a gas by a photothermal spectrum of a micro-nano optical waveguide, where the method includes:
inputting pumping laser and detection laser into the micro-nano optical waveguide; the method comprises the following steps that part of energy of pump laser and detection laser is transmitted in a gas medium outside a micro-nano optical waveguide in the form of evanescent waves, the evanescent waves generated by the pump laser and substances to be detected in the gas medium generate a photothermal effect and then heat the gas medium, the gas medium heats the micro-nano optical waveguide through heat conduction, the refractive indexes of the gas medium and the micro-nano optical waveguide are changed, and the detection laser is excited to generate a basic mode and a first high-order mode of the micro-nano optical waveguide;
and detecting the phase difference generated after the base mode and the first high-order mode are transmitted in the micro-nano optical waveguide, and converting according to the relation between the phase difference and the concentration of the substance to be detected to obtain the real concentration of the substance to be detected in the gas medium.
Optionally, the central wavelength of the pump laser is aligned with or scanned across the absorption peak of the substance to be detected, and the central wavelength of the probe laser is far away from any absorption peak of the substance to be detected.
Optionally, the micro-nano optical waveguide supports multimode 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, a relationship between a phase difference generated after the fundamental mode and the first high-order mode are transmitted in the micro-nano optical waveguide and a concentration of a substance to be measured may be represented as:
δφ＝(M·α 0 ·L·P)·C (I)
wherein, delta phi is a phase difference generated after the base mode and the first high-order mode are transmitted in the micro-nano optical waveguide; m is a photo-thermal coefficient, and the photo-thermal coefficient is a constant value for a micro-nano optical waveguide with a fixed size and a fixed pump laser incidence mode; alpha (alpha) ("alpha") 0 The absorption coefficient of the substance to be detected is a fixed value for the fixed absorption peak; 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 detected.
The phase difference generated after the base mode and the first high-order mode are transmitted in the micro-nano optical waveguide is in direct proportion to the concentration of the substance to be detected, and therefore the concentration of the substance to be detected can be obtained through conversion by detecting phase difference information carried in detection laser emitted from the output end of the micro-nano optical waveguide.
Compared with the prior art, the beneficial effects are that: by the embodiments of the present applicationThe micro-nano optical waveguide is covered by a gas medium containing a substance to be detected, so that the photo-thermal signal response is instantaneous, and the response speed is obviously improved. After the substance to be detected absorbs the pump laser, the refractive index of a gas medium is changed, the refractive index of the micro-nano optical waveguide is also changed, and the thermo-optic coefficient of the micro-nano optical waveguide material (for example, the silicon dioxide is about 10) -5 K -1 On the order of and above) much greater than the thermo-optic coefficient of a gas (e.g., a gaseous medium having a temperature of about-10 at room temperature) -6 K -1 ) Therefore, higher optical thermal efficiency can be obtained under the same conditions than that of the micro-structured hollow-core optical fiber. Meanwhile, the gas medium generally has a lower heat conduction coefficient, so that the coated micro-nano optical waveguide generates a heat accumulation effect, the photo-thermal efficiency is further improved, and the detection sensitivity of the gas concentration sensor is improved. The micro-nano optical waveguide supports high-order mode transmission, and the high-order mode has a larger evanescent field ratio, so that harsh process conditions are avoided while strong interaction between light and substances is obtained, and the manufacturing difficulty and cost are reduced. The micro-nano optical waveguide can be prepared by melting tapered standard single-mode optical fibers, and can also be produced in batches by a preparation process compatible with MEMS, so that the manufacturing difficulty and cost are further reduced.
In a second aspect, an embodiment of the application provides a micro-nano optical waveguide photothermal spectroscopy gas detection system, which includes a pumping laser component for generating pumping laser, a detection laser component for generating detection laser, a wavelength division multiplexer for combining the pumping laser and the detection laser, a micro-nano optical waveguide, a gas collection chamber for collecting substances to be detected and placing the micro-nano optical waveguide, an optical filter for filtering the pumping laser, and an analysis component;
the output ends of the pumping laser assembly and the detection laser assembly are respectively connected with the input end of the wavelength division multiplexer, the input end and the output end of the micro-nano optical waveguide are respectively connected with the output end of the wavelength division multiplexer and the input end of an optical filter, and the output end of the optical filter is connected with the input end of the analysis assembly;
a gas medium exists in the gas collection chamber, partial energy of the pump laser and the detection laser is 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 substances to be detected in the gas medium generate a photothermal effect and then heat the gas medium, the gas medium heats the micro-nano optical waveguide through heat conduction, the refractive indexes of the gas medium and the micro-nano optical waveguide are changed, and the detection laser excites and generates a base mode and a first high-order mode of the micro-nano optical waveguide;
the analysis component is used for detecting a phase difference generated after the base mode and the first high-order mode are transmitted in the micro-nano optical waveguide, and the real concentration of the substance to be detected in the gas medium is obtained through conversion according to the relation between the phase difference and the concentration of the substance to be detected.
Optionally, the pump laser assembly includes a first laser driver, a pump light source, and a laser amplifier;
the detection laser assembly comprises a detection light source, a second laser driver and a polarization controller;
the analysis component comprises an optical coupler, a first optical detector, 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 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 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 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 the output end of the detection 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 respectively connected with the input ends of the first optical detector and the second optical detector, the output end of the first optical detector is connected with the input end of the phase-locked amplifier, the output end of the second optical detector is connected with the input end of the analysis terminal, the output end of the phase-locked amplifier is respectively connected with the input ends of the analysis terminal and the pump laser component, and the output end of the analysis terminal is connected with the input end of the detection laser component.
Optionally, the micro-nano optical waveguide includes any one of an integrated optical waveguide and an optical fiber waveguide.
Optionally, the cross-sectional dimension of the micro-nano optical waveguide is not greater than 10 times of the maximum optical wavelength of the pump laser and the probe laser.
Optionally, non-adiabatic transition is formed 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 to the input end of the micro-nano optical waveguide from the output end of the wavelength division multiplexer, the detection laser is excited to generate a basic mode and a first high-order mode of the micro-nano optical waveguide, and the basic mode and the first high-order mode are transmitted in the micro-nano optical waveguide at the same time.
Optionally, the output end of the wavelength division multiplexer and the input end of the micro-nano optical waveguide are in adiabatic transition;
when the detection laser is input to the input end of the micro-nano optical waveguide from the output end of the wavelength division multiplexer, the detection laser only excites the fundamental mode of the micro-nano optical waveguide. The long-period grating is written in the input end of the micro-nano optical waveguide, and partial energy of the basic mode is coupled to the first high-order mode through the long-period grating, so that the basic mode and the first high-order mode are transmitted in the micro-nano optical waveguide at the same time.
The detection system is based on the gas detection method, and has the advantages of simple structure, convenient operation, high detection sensitivity, large dynamic range and quick signal response. Through experimental detection, the minimum detectable gas (methane) concentration of the detection system is as low as 440ppb, the dynamic range is close to 6 orders of magnitude, and the response time is only 7s.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments or the prior art descriptions will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present application, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without inventive exercise.
FIG. 1 is a schematic diagram of photothermal effect on 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;
FIG. 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 an 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 the micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application;
fig. 6 is a graph showing a relationship between a photothermal signal and system noise of methane gas with a volume concentration of 1% and pumping power measured by the micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application;
FIG. 7 is an Allan variance graph 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 application;
fig. 8 is a graph of a second harmonic signal variation 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;
FIG. 9 is a graph of normalized photo-thermal signals measured by the micro-nano optical waveguide photo-thermal spectrum gas detection method according to the present application as a function of inflation time;
fig. 10 is a graph of a measurement result of a dynamic range of a methane gas sensor at normal temperature and normal pressure measured by the micro-nano optical waveguide photo-thermal spectrum gas detection method provided by the application.
Detailed Description
In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular system structures, techniques, etc. in order to provide a thorough understanding of 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 that depart from 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 will be understood that the terms "comprises" and/or "comprising," when used in this specification and the appended claims, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
It should also be understood that the term "and/or" as used in this specification and the appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.
As used in this specification and the appended claims, the term "if" may be interpreted contextually as "when", "upon" or "in response to a determination" or "in response to a detection". Similarly, the phrase "if it is determined" or "if a [ described condition or event ] is detected" may be interpreted contextually to mean "upon determining" or "in response to determining" or "upon detecting [ described condition or event ]" or "in response to detecting [ described condition or event ]".
Furthermore, in the description of the present application and the appended claims, the terms "first," "second," "third," and the like are used for distinguishing between descriptions and not necessarily for describing or implying relative importance.
Reference throughout this specification to "one embodiment" or "some embodiments," or the like, 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," or the like, in various places throughout this specification are not necessarily all referring to the same embodiment, but rather "one or more but not all embodiments" unless specifically stated otherwise. The terms "comprising," "including," "having," and variations thereof mean "including, but not limited to," unless otherwise specifically stated.
The method for detecting gas by photothermal spectroscopy of a micro-nano optical waveguide provided by the present application is exemplarily described below with reference to specific embodiments.
In a possible implementation manner, the micro-nano optical waveguide comprises any one of an integrated optical waveguide and an optical fiber waveguide.
In the embodiment of the application, the micro-nano optical waveguide is a micro-nano optical fiber, namely, an optical fiber waveguide is adopted.
Referring to fig. 1, fig. 1 is a schematic diagram illustrating the photothermal effect and heat conduction generated on a micro-nano optical fiber according to the present application.
The main execution body of the gas concentration detection method in the present embodiment is a gas concentration sensor.
S101, inputting pumping laser and detection laser into a micro-nano optical waveguide; the method comprises the steps that part of energy of pump laser and detection laser is transmitted in a gas medium outside the micro-nano optical waveguide in the form of evanescent waves, the evanescent waves generated by the pump laser and substances to be detected in the gas medium heat the gas medium after the photothermal effect is generated, the gas medium heats the micro-nano optical waveguide through heat conduction, the refractive indexes of the gas medium and the micro-nano optical waveguide are changed, and the detection laser excites and generates a base mode and a first high-order mode of the micro-nano optical waveguide.
S102, detecting a phase difference generated after the base mode and the first high-order mode are transmitted in the micro-nano optical waveguide, and converting according to a relation between the phase difference and the concentration of the substance to be detected to obtain the real concentration of the substance to be detected in the gas medium.
Specifically, in step S101, the pump laser and the probe laser are input into the micro-nano fiber, and this step may refer to a conventional method for laser to enter the fiber in the art, which is not specifically limited in this application. The purpose of the step is to enable the pump laser and the detection laser to generate evanescent waves in the micro-nano optical fiber respectively, and the generated evanescent waves are transmitted along the micro-nano optical fiber.
It should be noted that the pumping laser and the detection laser may enter from the same side of the micro-nano fiber, or may enter from different sides of the micro-nano fiber. That is, the transmission directions of the pump laser and the detection laser in the micro-nano optical fiber are not limited, and the pump laser and the detection laser can be transmitted in the same direction or in the opposite direction.
In the embodiment of the application, the transmission directions of the pump laser and the detection laser in the micro-nano optical fiber are selected to be the same transmission direction.
By way of example and not limitation, in the present embodiment, the gas medium is nitrogen gas, and the substance to be measured is methane gas. For example, methane gas with a volume concentration of 1ppm, wherein the substance to be measured has a volume ratio of 1 × 10 -6 The residual volume ratio of (0.999999 =1-1 × 10) is -6 ) The gaseous medium of (a) is nitrogen.
Optionally, the central wavelength of the pump laser is aligned with or scanned across the absorption peak of the substance to be detected, and the central wavelength of the probe laser is far away from any absorption peak of the substance to be detected. The central wavelength of the pump laser can be tuned to the absorption peak of the substance to be detected, so that the substance to be detected can fully absorb the pump laser. The center wavelength of the probing laser can be tuned to an absorption line away from the substance to be tested, so that the substance to be tested cannot absorb the probing laser.
Further, in the present embodiment, the center wavelength of the pump laser is near the absorption peak of methane R3 (i.e., 1653.7 nm), and the center wavelength of the probe laser is near 1550 nm.
Optionally, the micro-nano optical fiber supports multimode transmission.
For example, referring to fig. 2, fig. 2 is a cross-sectional view of a micro-nano optical fiber provided in the present application. The diameter of the micro-nano optical fiber in fig. 2 is about 2.1 micrometers. The micro-nano optical fiber is prepared from a single-mode quartz optical fiber by an arc discharge and oxyhydrogen flame heating two-step drawing method, and the support comprises a basic mode HE 11 High order mode HE 12 Including multiple transmission modes.
In a possible implementation mode, the micro-nano optical waveguide is placed in the gas collection chamber, and a certain distance exists between the micro-nano optical waveguide and the inner wall of the gas collection chamber.
Further, the gas collecting chamber in the embodiment of the present invention is a plastic gas chamber of 12cm × 1.4cm × 1.5cm, and the gas pressure is always maintained at one atmospheric pressure during the measurement.
Referring to fig. 3, fig. 3 is a schematic diagram of a micro-nano optical waveguide photothermal spectroscopy gas detection system provided in the present application. The micro-nano optical waveguide photo-thermal spectrum gas detection system comprises:
the device comprises a pumping laser component for generating pumping laser, a detection laser component for generating detection laser, a wavelength division multiplexer for combining the pumping laser and the detection laser, a micro-nano optical waveguide, a gas collection chamber for collecting substances to be detected and placing the micro-nano optical waveguide, an optical filter for filtering the pumping laser and an analysis component;
the output ends of the pumping laser assembly and the detection laser assembly are respectively connected with the input end of the wavelength division multiplexer, the input end and the output end of the micro-nano optical waveguide are respectively connected with 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 with the input end of the analysis assembly;
a gas medium exists in the gas collection chamber, partial energy of the pump laser and the detection laser is 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 substances to be detected in the gas medium generate a photothermal effect and then heat the gas medium, the gas medium heats the micro-nano optical waveguide through heat conduction, the refractive indexes of the gas medium and the micro-nano optical waveguide are changed, and the detection laser excites and generates a base mode and a first high-order mode of the micro-nano optical waveguide;
the analysis component is used for detecting a phase difference generated after the base mode and the first high-order mode are transmitted in the micro-nano optical waveguide, and the real concentration of the substance to be detected in the gas medium is obtained through conversion according to the relation between the phase difference and the concentration of the substance to be detected.
The pump laser component comprises a first laser driver, a pump light source and a laser amplifier;
the detection laser assembly comprises a detection light source, a second laser driver and a polarization controller;
the analysis assembly comprises an optical coupler, a first optical detector, 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 component, the input end and the output end of the pump 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;
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 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 the output end of the detection 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 respectively connected with the input end of the first optical detector and the input end of the second optical detector, the output end of the first optical detector is connected with the input end of the lock-in amplifier, the output end of the second optical detector is connected with the input end of the analysis terminal, the output end of the lock-in amplifier is respectively connected with 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 with the input end of the detection laser component.
In fig. 3, the pump laser and the probe laser can enter from the input end of the micro-nano fiber. In fig. 3, there are a first laser driver, a pumping light source, a laser amplifier, a wavelength division multiplexer, a micro-nano fiber, a gas collection chamber, a detection light source, a second laser driver, a polarization controller, an optical filter, a first optical detector, a second optical detector, a lock-in amplifier and an analysis terminal.
In a possible implementation mode, after the pumping laser enters the micro-nano optical fiber, the substance to be detected absorbs the pumping laser and further generates a photo-thermal effect, a gas medium outside the micro-nano optical fiber is heated firstly, the refractive index of the gas medium is changed, and then the micro-nano optical fiber is heated through heat conduction, so that the refractive index of the micro-nano optical fiber is changed.
Specifically, first, a methane gas having a volume concentration of 1% was charged into the gas collection chamber. The 1654 nm distributed feedback laser can be selected as the pumping light source. The phase-locked amplifier can input a sinusoidal modulation signal of 6 kilohertz to the first laser driver, the first laser driver drives the pump light source together with a sawtooth wave scanning frequency of 0.01 hertz at the same time, modulated pump laser is output, and the modulated pump laser can enter the micro-nano optical fiber through the laser amplifier and the wavelength division multiplexer, so that the pump laser interacts with methane gas to generate a photothermal effect and accompanying heat conduction, and the refractive indexes of a gas medium and the micro-nano optical fiber are changed periodically. The laser amplifier is used for increasing 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 optical wavelength of the pump laser and the detection laser.
In a possible implementation mode, when the detection laser enters the micro-nano optical waveguide, only a basic mode and a first high-order mode of the micro-nano optical waveguide are excited.
Optionally, the output end of the wavelength division multiplexer and the input end of the micro-nano optical fiber may be in non-adiabatic transition, or in adiabatic transition.
When 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 to the input end of the micro-nano optical waveguide from the output end of the wavelength division multiplexer, the detection laser is excited to generate the base mode and the first high-order mode of the micro-nano optical waveguide, and the base mode and the first high-order mode are transmitted in the micro-nano optical waveguide at the same time.
When the output end of the wavelength division multiplexer and the input end of the micro-nano optical fiber are in adiabatic transition, and the detection laser is input to the input end of the micro-nano optical waveguide from the output end of the wavelength division multiplexer, the detection laser only excites the fundamental mode of the micro-nano optical waveguide; and writing a long-period grating into the input end of the micro-nano optical waveguide, wherein the long-period grating enables partial energy of the basic mode to be coupled to the first high-order mode, so that the basic mode and the first high-order mode are transmitted in the micro-nano optical waveguide at the same time.
In the embodiment of the present application, the non-adiabatic transition is selected as the transition between the output end of the wavelength division multiplexer and the input end of the micro-nano optical fiber, please refer to fig. 3. When the detection laser is input to the input end of the micro-nano optical fiber from the output end of the wavelength division multiplexer, the detection laser simultaneously excites the fundamental mode and the first high-order mode of the micro-nano optical fiber.
Specifically, the first high-order mode is a high-order transmission mode different from the fundamental mode. For example, the fundamental mode is HE 11 Mode, the first higher order mode may be HE lm (l, m is a natural number, wherein l ≠ 1,m ≠ 1), EH pq 、TE 0q Or TM 0q (p, q are natural numbers).
By way of example and not limitation, in the embodiments of the present application, the first higher-order mode is selected to be HE 12 。
In the embodiment of the application, the second laser driver drives the detection light source to output detection laser with the central wavelength of 1550 nanometers, and the detection laser enters the micro-nano optical fiber through the polarization controller and the wavelength division multiplexer. When the detection laser enters the micro-nano optical fiber, the detection laser excites the HE generating the micro-nano optical fiber 11 And HE 12 Mode(s).
The refractive indexes of the gas medium and the micro-nano optical fiber are changed. When the detection laser is transmitted along the micro-nano optical fiber, the HE excited by the detection laser is changed 11 And HE 12 Phase of mode, HE 11 And HE 12 The phase difference generated after the mode is transmitted in the micro-nano optical fiber is changed.
Alternatively, HE 11 And HE 12 The relation between the phase difference generated after the mode is transmitted in the micro-nano optical fiber and the concentration of the substance to be detected can be expressed as follows:
δφ＝(M·α 0 ·L·P)·C (2)
wherein δ φ is HE 11 And HE 12 The mode generates phase difference after being transmitted in the micro-nano optical fiber; m is a photo-thermal coefficient, and the value of the coefficient is a fixed value for a micro-nano optical fiber with a fixed size and a fixed pump laser incidence mode; alpha is alpha 0 The absorption coefficient of methane is a fixed value for a fixed absorption peak; l is the length of the micro-nano optical waveguide, P is the power of the pumping laser, and C is the concentration of the substance to be detected.
For example, referring to fig. 3, after receiving a part of the detection laser output by the micro-nano fiber, the second optical detector transmits the converted electrical signal to the analysis terminal, and after receiving the electrical signal, the analysis terminal processes the electrical signal to obtain a feedback adjustment signal, and transmits the feedback adjustment signal 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. And adjusting the polarization controller to keep the contrast of interference fringes generated by the micro-nano fiber interferometer to be the highest, so that the phase detection sensitivity is kept to be optimal. Referring to fig. 4, fig. 4 is a diagram of an automatic stabilization process of the micro-nano fiber interferometer, the micro-nano fiber interferometer gradually shifts a working point when freely running, and the micro-nano fiber interferometer rapidly enters a stable state after feedback adjustment is added, that is, the micro-nano fiber interferometer is locked at an orthogonal working point. The micro-nano fiber interferometer is adjusted to an orthogonal working point to obtain the maximum phase detection sensitivity.
For example, referring to fig. 3, after receiving a part of detection laser output by the micro-nano fiber, the first optical detector transmits a converted electrical signal to the lock-in amplifier. After receiving the electric signal, the lock-in amplifier can demodulate a harmonic signal from the electric signal, wherein the harmonic signal carries HE 11 And HE 12 And (4) generating a phase difference information intermediate signal after the mode is transmitted in the micro-nano optical fiber. After receiving the harmonic signal, the analysis terminal processes the harmonic signal to obtain a peak-to-peak value of the harmonic signal, namely a photo-thermal signal, wherein the photo-thermal signal is in direct proportion to the concentration of the substance to be detected. And finally, obtaining the real concentration of the substance to be detected according to the concentration relation between the photo-thermal signal and the substance to be detected.
The following is an exemplary description of the measurement principle of the present application.
The propagation characteristics of the optical mode in the micro-nano optical fiber can be described by an eigen equation. For HE lm The mode, eigen equation can be written
Wherein k is 0 =2 π/λ, λ being the wavelength of light in vacuum, n silica And n gas Refractive indices of quartz and gas medium, respectively, J l Is a Bessel function of order l, class I, K l For the order l modified bessel function of the second kind,
d is the diameter of the micro-nano optical fiber, and beta represents HE lm The propagation constant of the mode in the micro-nano fiber, (. Cndot.)' represents derivation, and U, W, V is a dimensionless mode parameter.
Effective refractive index n of mode i effi Can be expressed as a propagation constant beta i In the form of:
wherein n represents the refractive index of the micro-nano optical fiber or the gas medium.
Considering the photothermal effect generated by the absorption of the pump light by the substance to be measured and the influence thereof, 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)
in the formula, dn/dT and n 0 The thermo-optic coefficient and the initial refractive index of a gas medium (r is more than d/2) or quartz (r is less than or equal to d/2), wherein (r, theta and z) are cylindrical coordinates, and t is time. Amplitude of temperature change T caused by photothermal effect, pump light intensity P and absorption coefficient alpha of substance to be detected 0 And its concentration C is proportional, which can be expressed as
T(r，θ，z，t)∝α 0 CP (6)
Then detection light HE caused by photo-thermal effect 1m The effective refractive index of the mode is modulated as
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 For detecting the wavelength of the laser light, n max (r, theta, z) and n min (r, theta, z) are the transverse distributions of the refractive index n (r, theta, z, t) of the gaseous medium at the longitudinal position z after the system has entered "steady state" when it reaches a maximum and a minimum, respectively.
HE after the probe light is transmitted through the micro-nano optical fiber with the length of L 1m The phase modulation of the mode accumulation can be expressed as
For micro-nano optical fiber with fixed diameter d and fixed HE 1m The mode power ratio can also be expressed in a general form
Δφ 1m ＝α 0 CLP·M 1m (f) (9)
In the formula, M 1m For detecting light HE 1m The phase modulation factor of the mode, f, is the frequency of the pump laser.
Probe 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
δφ＝|Δφ 11 -Δφ 12 |＝a 0 CLP·M(f) (10)
In the formula, the photothermal coefficient M (= | M) 11 -M 12 I) is HE 11 And HE 12 Difference of mode phase modulation coefficients. The phase difference delta phi is in direct proportion to C, and 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 optical fiber material can effectively enhance mode phase difference modulation, so that the photo-optic coefficient M in the micro-nano optical fiber is larger than that in the micro-structure hollow optical fiber in a low frequency band, namely the micro-nano optical fiber is more sensitive to the concentration C of a substance to be detected.
The beneficial effects of the micro-nano optical waveguide photo-thermal spectrum gas detection method provided by the application are further proved by some experimental data.
Referring to fig. 5, fig. 5 is a second harmonic spectrum of methane gas with a volume concentration of 1% measured by the micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application. In fig. 5, the pump laser powers corresponding to each curve are different, and after the pump laser is subjected to wavelength modulation, the obtained second harmonic signal shows that, when the power of the pump laser is higher, the peak-to-peak value of the second harmonic signal is higher, that is, the photo-thermal signal is higher.
Referring to fig. 6, fig. 6 is a graph showing a relationship between a photothermal signal and system noise of methane gas with a volume concentration of 1% and pumping power measured by the 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 (i.e., away from the methane gas absorption line), the mean square error of the acquired harmonic signal is the system noise. Experiments prove 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 lock-in amplifier is 1 second, the corresponding minimum detectable sensitivity of about 1.6ppm when the signal-to-noise ratio is 1 can be obtained by calculating the signal-to-noise ratio, namely the signal-to-noise ratio.
Referring to fig. 7, fig. 7 is an allen variance graph of methane gas detection obtained based on 2-hour noise data measured by the micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application. As can be seen from fig. 7, the detectable concentration limit is constantly decreasing with increasing integration time. The minimum detectable sensitivity of methane gas in the present application can be increased to 0.44ppm as the integration time is increased to 240 seconds. The high-sensitivity detection effect of the detection system is reflected.
Referring to fig. 8, fig. 8 is a graph illustrating a second harmonic signal variation of methane gas with a volume concentration of 1% measured in 4 hours according to the micro-nano optical waveguide photothermal spectroscopy gas detection method provided by the present application. Amplifying the signal in the first 0.15 hours, clearly observing the second harmonic signal generated by the methane gas absorbing the pump laser, and calculating the fluctuation range of the photo-thermal signal in 4 hours to be about 1.6%. The effect of stable detection system is reflected.
Referring to fig. 9, fig. 9 is a graph showing a variation curve of a normalized photothermal signal with inflation time measured by the 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 coating amount reaches 90% of the whole micro-nano optical fiber at about 29 seconds, then the curve area gradually becomes stable and starts to fall at about 65 seconds until the gas coating amount reaches 10% of the whole micro-nano optical fiber at about 72 seconds. The test process is as follows: the gas collection chamber was first charged with pure nitrogen, 1% by volume methane gas was charged at a rate of 500 cubic centimeters per minute for 22 seconds, and 500 cubic centimeters per minute for 65 seconds, and the measured response time of methane gas was calculated to be about 7 seconds. The effect of short response time of the detection system is reflected.
Referring to fig. 10, fig. 10 is a graph of a measurement result of a methane gas sensor at normal temperature and normal pressure measured by the micro-nano optical waveguide photothermal spectroscopy gas detection method provided in the present application. In fig. 10, it can be seen that the photothermal signal is in direct proportion to the concentration of methane gas in the process of changing the volume concentration of methane gas from 4ppm to 1%, and a nonlinear relationship occurs when the concentration is greater than 1%. When the integration time is 240 seconds, the minimum detectable sensitivity of methane gas is about 440ppb, and the dynamic range of the system is up to nearly 6 orders of magnitude (about 9.1X 10) 5 ). The effect of ultrahigh dynamic range is embodied.
It should be understood that, the sequence numbers of the steps in the foregoing embodiments do not imply an execution sequence, and the execution sequence of each process should be determined by its function and inherent logic, and should not constitute any limitation to the implementation process of the embodiments of the present application.
It will be apparent to those skilled in the art that, for convenience and brevity of description, only the above-mentioned division of the functional units and modules is illustrated, and in practical applications, the above-mentioned function distribution may be performed by different functional units and modules according to needs, that is, the internal structure of the apparatus is divided into different functional units or modules to perform all or part of the above-mentioned functions. Each functional unit and module in the embodiments may be integrated in one processing unit, or each unit may exist alone physically, or two or more units are integrated in one unit, and the integrated unit may be implemented in a form of hardware, or in a form of software functional unit. In addition, specific names of the functional units and modules are only for convenience of distinguishing from each other, and are not used for limiting the protection scope of the present application. For the specific working processes of the units and modules in the system, reference may be made to the corresponding processes in the foregoing method embodiments, which are not described herein again.
In the above embodiments, the descriptions of the respective embodiments have respective emphasis, and reference may be made to the related descriptions of other embodiments for parts that are not described or illustrated in a certain embodiment.
Those of ordinary skill in the art will appreciate that the various illustrative elements and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware or combinations of computer software and electronic hardware. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the technical solution. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present application.
The above-mentioned embodiments are only used for illustrating the technical solutions of the present application, and not for limiting the same; although the present application has been described in detail with reference to the foregoing embodiments, it should be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; such modifications and substitutions do not substantially depart from the spirit and scope of the embodiments of the present application and are intended to be included within the scope of the present application.
Claims (10)
1. A micro-nano optical waveguide photo-thermal spectrum gas detection method is characterized by comprising the following steps:
inputting pumping laser and detection laser into the micro-nano optical waveguide; the method comprises the following steps that part of energy of pump laser and detection laser is transmitted in a gas medium outside a micro-nano optical waveguide in the form of evanescent waves, the evanescent waves generated by the pump laser and substances to be detected in the gas medium generate a photothermal effect and then heat the gas medium, the gas medium heats the micro-nano optical waveguide through heat conduction, the refractive indexes of the gas medium and the micro-nano optical waveguide are changed, and the detection laser is excited to generate a basic mode and a first high-order mode of the micro-nano optical waveguide;
and detecting the phase difference generated after the base mode and the first high-order mode are transmitted in the micro-nano optical waveguide, and converting according to the relation between the phase difference and the concentration of the substance to be detected to obtain the real concentration of the substance to be detected in the gas medium.
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 with or scanned through the absorption peak of the substance to be detected, and the central wavelength of the probe 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, 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 only the fundamental mode and the first high-order mode of the micro-nano optical waveguide are excited when the detection laser enters the micro-nano optical waveguide.
5. The method for detecting the photothermal spectrum gas of the micro-nano optical waveguide according to any one of claims 1 to 4, wherein the relationship between the phase difference generated after the transmission of the fundamental mode and the first high-order mode in the micro-nano optical waveguide and the concentration of a substance to be detected can be represented as follows:
6φ＝(M·α 0 ·L·P)·C
wherein, delta phi is a phase difference generated after the base mode and the first high-order mode are transmitted in the micro-nano optical waveguide; m is a photo-thermal coefficient, and the photo-thermal coefficient is a constant value for a micro-nano optical waveguide with a fixed size and a fixed pump laser incidence mode; alpha is alpha 0 The absorption coefficient of the substance to be detected is a fixed value for the fixed absorption peak; l is the length of the micro-nano optical waveguide, P is the power of the pumping laser, and C is the concentration of the substance to be detected.
6. A micro-nano optical waveguide photo-thermal spectrum gas detection system is characterized by comprising:
the device comprises a pumping laser component for generating pumping laser, a detection laser component for generating detection laser, a wavelength division multiplexer for combining the pumping laser and the detection laser, a micro-nano optical waveguide, a gas collection chamber for collecting substances to be detected and placing the micro-nano optical waveguide, an optical filter for filtering the pumping laser and an analysis component;
the output ends of the pumping laser assembly and the detection laser assembly are respectively connected with the input end of the wavelength division multiplexer, the input end and the output end of the micro-nano optical waveguide are respectively connected with 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 with the input end of the analysis assembly;
a gas medium exists in the gas collection chamber, partial energy of the pump laser and the detection laser is 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 substances to be detected in the gas medium generate a photothermal effect and then heat the gas medium, the gas medium heats the micro-nano optical waveguide through heat conduction, the refractive indexes of the gas medium and the micro-nano optical waveguide are changed, and the detection laser excites and generates a base mode and a first high-order mode of the micro-nano optical waveguide;
the analysis component is used for detecting a phase difference generated after the base mode and the first high-order mode are transmitted in the micro-nano optical waveguide, and the real concentration of the substance to be detected in the gas medium is obtained through conversion according to the relation between the phase difference and the concentration of the substance to be detected.
7. The micro-nano optical waveguide photothermal spectroscopy gas detection system according to claim 6, wherein the pump laser assembly comprises a first laser driver, a pump light source, a laser amplifier;
the detection laser component comprises a detection light source, a second laser driver and a polarization controller;
the analysis component comprises an optical coupler, a first optical detector, 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 component, the input end and the output end of the pump 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;
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 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 the output end of the detection 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 respectively connected with the input end of the first optical detector and the input end of the second optical detector, the output end of the first optical detector is connected with the input end of the phase-locked amplifier, the output end of the second optical detector is connected with the input end of the analysis terminal, the output end of the phase-locked amplifier is respectively connected with 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 with the input end of the detection laser component.
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 an optical fiber waveguide.
9. The micro-nano optical waveguide photothermal spectroscopy gas detection system according to claim 6, wherein the cross-sectional dimension of the micro-nano optical waveguide is not more than 10 times the maximum wavelength of the pump laser and the probe laser.
10. The micro-nano optical waveguide photothermal spectroscopy gas detection system according to any one of claims 6 to 9, wherein a non-adiabatic transition is provided 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 to the input end of the micro-nano optical waveguide from the output end of the wavelength division multiplexer, the detection laser is excited to generate the basic mode and the first high-order mode of the micro-nano optical waveguide, and the basic mode and the first high-order mode are transmitted in the micro-nano optical waveguide at the same time; or
The output end of the wavelength division multiplexer and the input end of the micro-nano optical waveguide are in adiabatic transition; when the detection laser is input to the input end of the micro-nano optical waveguide from the output end of the wavelength division multiplexer, the detection laser only excites the fundamental mode of the micro-nano optical waveguide; and writing a long-period grating into the input end of the micro-nano optical waveguide, wherein the long-period grating enables partial energy of the basic mode to be coupled to the first high-order mode, so that the basic mode and the first high-order mode are transmitted in the micro-nano optical waveguide at the same time.
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| CN117074366B (en) * | 2023-10-12 | 2023-12-22 | 朗思传感科技(深圳)有限公司 | Gas sensing device based on micro-nano optical fiber and concentration detection method |
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