CN101419161A - Gas detecting method and gas sensor based on plane annular micro-cavity - Google Patents

Abstract

The invention relates to a gas detection method and a gas sensor based on a planar annular microcavity. The main feature is that a coupler composed of two planar annular microcavities and a biconical optical fiber is used, one is a gas detection coupler, and the other is a coupler in a vacuum environment. ; The gas detection method is based on the principle that the absorption of different gases by the evanescent wave changes the peak value of the transmission spectrum. The transmission spectrum of the detection gas coupler of the gas to be detected is compared with the transmission spectrum of the coupler in a vacuum environment. The spectral changes of the two are used to detect the type and concentration of the gas to be measured. The gas sensor designed by this method has the advantages of short response time, relatively simple structure, and high sensitivity to highly toxic and harmful gases with extremely low concentrations.

Description

Gas detection method and gas sensor based on planar annular microcavity

1. Technical field

The invention belongs to a method sensor for detecting gas with an optical signal, in particular to a gas detection method and a gas sensor based on a planar annular microcavity.

2. Background technology

With the development of society and the progress of industry, especially the development of petroleum, coal, automobile industry and chemical industry, the types and quantities of gases emitted in life and industry are increasing day by day. Therefore, the detection of gas concentration and species is of great significance in production and life. Such as the detection of gas in coal mines and mines; the detection of harmful chemical gases in chemical workshops; the detection of harmful gases in interior decoration, etc. Especially harmful and highly toxic gases, a small amount in the air will cause great harm to the human body and even threaten life. For example: Arsine is a highly toxic gas, and when its concentration reaches 0.5 mg per cubic meter of air, it will cause acute poisoning. The Guizhou channel of Xinhuanet reported an incident of hydrogen arsenide poisoning on July 7: In a small electrolytic zinc factory with an annual output of only a few thousand tons of zinc, more than 20 workers were collectively poisoned in a week, and 3 of them died due to ineffective rescue. . Therefore, it is of great significance to manufacture high-sensitivity gas sensors, especially for harmful gases.

Gas detection methods and sensors are divided into optical detection, semiconductor detection, solid electrolytic detection, electrochemical detection and so on. Compared with several other types, optical detection has the advantages of high sensitivity, fast response speed, and high anti-interference performance. The most important type of optical gas sensor is the absorption spectrum type. This type of sensor is based on the famous "Beer-Lambert" theorem, and the gas concentration can be obtained from the change of light input and output. The sensing medium mostly chooses ordinary optical fiber, special optical fiber and so on. For example: Chinese Patent Application No. 200410037099 discloses a gas concentration sensor using a nanoscale microporous optical fiber. Optical fiber, the other ends of the two optical fibers are respectively connected to the detector." "The optical fiber with nano-scale micro-hole structure is a kind of micro-hole with nano-scale connection or part connection inside, which can transmit light"; Chinese patent application number Patent No. 200510012344.5 discloses a fiber optic gas sensor, which is characterized by "the optical fiber in the gas absorption cell and the contrast fiber are microstructured hollow rays"; Chinese patent application No. 200610012988.9 discloses a hollow photonic crystal fiber gas sensor The sensor is characterized in that "the light-guiding gas chamber adopts a hollow photonic crystal fiber with a micron-sized air-permeable microhole on it".

The gas detection principles of the above-mentioned sensors are all based on the detection principle of direct absorption, and have the advantages of simple structure and easy application. The difference is that different absorption media are used. It is not easy to control the absorption optical path in the process of deriving the gas concentration by the "Beer-Lambert" theorem. When the concentration of the gas to be measured is very low, this detection method cannot quickly and accurately measure the gas concentration. Therefore, its detection is restricted by the gas concentration to be measured. However, extremely low concentrations of harmful and highly toxic gases require low concentrations, high sensitivity, and rapid response. Therefore, how to detect extremely low-concentration gases through effective means has become the key to detection.

3. Contents of the invention

The object of the present invention is to design and provide a sensor with high sensitivity, short response time and relatively simple structure on the basis of overcoming the above-mentioned gas sensor deficiency. There are also gas detection methods and gas sensors based on planar annular microcavities with extremely high sensitivity.

The present invention is realized by adopting the following technical solutions:

The gas detection method based on the planar annular microcavity is characterized in that: a coupler composed of two planar annular microcavities and a biconical fiber is used, one is a gas detection coupler, and the other is a coupler in a vacuum environment; the gas detection method It is based on the principle that the absorption of different gases by the evanescent wave changes the peak value of the transmission spectrum. The transmission spectrum of the detection gas coupler of the gas to be detected is compared with the transmission spectrum of the coupler in a vacuum environment, and the spectral changes of the two are used. To realize the detection of the type and concentration of the gas to be measured.

A gas detection sensor based on a planar annular microcavity, comprising an optical fiber, a laser transmitter, an optical filter, a beam splitter, an absorption gas chamber, a vacuum chamber, a photodetector and a signal processor; it is characterized in that:

The optical fiber is a double-tapered optical fiber; each of the absorption chamber and the vacuum chamber has a coupler composed of a planar annular microcavity and a biconical optical fiber, and the planar annular microcavity of the coupler is located in the described The tapered region of the double-tapered optical fiber; the coupler in the absorption gas chamber is a detection gas coupler, and the coupler in the vacuum chamber is a coupler in a vacuum environment;

●The absorption gas chamber is composed of a detection gas coupler and a dust-proof breathable cover;

●The vacuum chamber is composed of a coupler and a vacuum cover in a vacuum environment;

The laser transmitter is connected to the optical filter, and then connected to the input end of the beam splitter. The output end of the beam splitter is connected to the input end of the double-tapered optical fiber of the gas absorption chamber through a single-mode optical fiber, and the other is connected to the comparison signal The double-tapered optical fiber input end of the vacuum chamber used; the double-tapered optical fiber output end of the absorption air chamber and the vacuum chamber are respectively connected to two photodetectors through a single-mode optical fiber, and the optical signal is converted into an electrical signal by the two detectors. signal and connected to the signal processing unit.

Compared with the prior art, the present invention has the following advantages:

1. Using the structure of planar annular microcavity and double-tapered optical fiber coupler as the core technology of detection, it can realize high-sensitivity detection of gas.

2. The coupling process of the coupler is completed through the evanescent waves on the two surfaces, so as to realize real-time detection, save the gas diffusion time of the previous fiber optic gas sensor, and improve the response speed.

3. Strong scalability, replace the planar annular microcavity (mainly the diameter parameter) and the light source according to the test gas environment. This sensor network can be set up under special circumstances, so as to grasp the gas concentration distribution state in detail.

4. Description of drawings

Fig. 1 and Fig. 2 are the structural representations of the present invention of two forms;

Fig. 3 is a system device diagram of the present invention.

In the figure: planar annular microcavity 1; double-tapered optical fiber 2; dust-proof and breathable cover 3; laser emitter 4; optical filter 5; beam splitter 6; absorption gas chamber 7; vacuum chamber 8; photoelectric detector 9 ; Signal processing circuit 10 ; Single-mode optical fiber 11 .

5. Specific implementation

Embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings.

The present invention is based on the gas sensor of planar annular microcavity and tapered optical fiber structure, comprises optical fiber, laser emitter 4, optical filter 5, beam splitter 6, absorption gas chamber 7, vacuum chamber 8, photodetector 9 and signal Processor 10;

The optical fiber is a double-tapered optical fiber; each of the absorption chamber and the vacuum chamber has a coupler composed of a planar annular microcavity and a biconical optical fiber, and the planar annular microcavity of the coupler is located in the described The tapered region of the bitapered fiber is shown in Figure 1 and Figure 2. The coupler in the absorption gas chamber is a detection gas coupler, and the coupler in the vacuum chamber (8) is a coupler in a vacuum environment.

●The absorption gas chamber is composed of a detection gas coupler and a dust-proof breathable cover.

●The vacuum chamber is composed of a coupler and a vacuum cover in a vacuum environment.

The laser transmitter is connected to the optical filter, and then connected to the input end of the beam splitter. The output end of the beam splitter is connected to the double-tapered optical fiber input end of the gas absorption chamber through the single-mode optical fiber 11, and the other is connected to the comparison The double-tapered optical fiber input end of the vacuum chamber for signals; the double-tapered optical fiber output end of the absorption chamber and the vacuum chamber are respectively connected to two photodetectors through single-mode optical fibers, and the two detectors convert the optical signal into The electrical signal is connected to the signal processing unit, as shown in Figure 3.

The core technology of the present invention is a coupler composed of a planar annular microcavity 1 and a biconical optical fiber 2. The planar annular microcavity is located in the tapered region of the biconical optical fiber, and the two ends of the biconical optical fiber are respectively connected to the common gas absorption chamber. single-mode fiber. The laser transmitter and the optical fiber are connected through an optical fiber adapter (flange). The bitapered optical fiber can be a nanofiber, a photonic crystal optical fiber, or a coupling tapered optical fiber, etc., and the planar annular microcavity can be designed as a disc microcavity, as shown in FIG. 1 . Or design as a racetrack microcavity, as shown in Figure 2.

The planar annular microcavity 1 is made by modern MEMS processing technology, the material is SiO ₂ , the diameter is 60-200 μm, and the shape is planar annular. The material of the double-tapered optical fiber 2 is SiO ₂ , which is made by melting and drawing by using a hydrogen-oxygen flame machine. The characteristic of the optical fiber taper made by the fusion drawing method is that the diameters of the cladding and the core of the fiber gradually become thinner along the fiber axis. Generally, it can be considered that the ratio of the diameter of the cladding to the core remains constant throughout the tapered region. The tapered region length of the bi-tapered fiber is 200 μm, the tip core radius of the tapered fiber used is 1 μm, the cladding radius is 2.5 μm, and the coupling efficiency is about 86% when the taper angle is 11 degrees. Both the absorption chamber 7 and the vacuum chamber 8 are equipped with a coupler with the same size planar annular microcavity 1 and double-tapered optical fiber 2 structure. The difference between the two is that the absorption gas chamber 7 is covered with a dust-proof and breathable cover 3, so that the measured ambient gas and the coupler can fully contact, while the vacuum chamber 8 is a closed vacuum environment, and the vacuum chamber 8 is for the gas absorption pool. The coupler detection in provides a comparison signal.

How the invention works

The planar annular microcavity has a super high quality factor (10 ⁸ ) due to its unique structure and surface finish. When light is transmitted in a tapered fiber, due to the existence of evanescent waves, and the small distance (0μm-2μm) between the planar ring microcavity and the tapered fiber, there will be light coupling when the transmission is close to the planar ring microcavity It enters the planar ring microcavity and continues to propagate, and forms resonance in the planar ring microcavity, and then transmits out through the evanescent wave coupling into the tapered optical fiber again. Since the planar annular microcavity is extremely sensitive to light, a small change in the incident light will also cause a significant change in the resonance peak of the transmission spectrum. Therefore, the coupling structure is placed in the gas chamber (or gas absorption cell). When the incident light passes through the tapered optical fiber, it is coupled into the planar annular microcavity in the form of evanescent waves, and passes through the tiny gap between the tapered optical fiber and the planar annular microcavity. Since the gas in the measured environment is different from the vacuum, the type and concentration of the gas will absorb the evanescent wave differently, which will affect the incident light coupled into the planar annular microcavity, and finally affect the resonance peak of the transmission spectrum. By comparing with the resonance peak of the transmission spectrum of the vacuum chamber, the type and concentration of the gas to be measured in the measured environment can be obtained.

When a beam of monochromatic light with an intensity of I ₀ and a wavelength of λ is incident on the gas cell, if the sample in the gas cell has an absorption line or band at λ, the light will be attenuated when passing through the gas cell, according to Bill-Lang According to Burger's law, the intensity of light exiting the gas cell is:

I(λ)=I ₀ (λ)exp(-αCl)

In the formula: I ₀ (λ) is the light intensity of monochromatic light with a wavelength of λ before entering the gas chamber; I(λ) is the light intensity of the light emitted from the gas chamber; C is the concentration of the gas to be measured; α is the concentration of the gas Absorption coefficient; l is the length of the gas chamber (action length of light and gas).

At the same time, the tapered optical fiber has a good coupling effect on light due to its special structure.

Common processing methods for tapered optical fibers include chemical etching, grinding and melting. The tapered fiber geometry and its transmission characteristics have the following relationship:

$\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; arctan</span>}\frac{\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; b</span>}}{\mathrm{<span\; class="notranslate">\; l</span>}}$

Where: l is the length of the light cone, α is the taper of the light cone, a is the radius of the butt end of the fiber cone, and b is the radius of the tip. It can be seen that when other parameters have been set, the larger the tip diameter, the larger α; the shorter the tapered transition zone of the optical fiber, that is, the smaller the l value, the larger the relative cone angle α, and the sharper the taper change.

After placing the sensor in a certain environment, since the outer shell of the gas absorption chamber 7 is a dustproof breathable cover 3, the ambient gas can quickly fill the entire gas chamber evenly (the gas concentration in the gas chamber is the same as the ambient gas concentration). At this time, the laser transmitter 4 is turned on, and the light source is adjusted so that the light source emits laser light to meet the resonance conditions of the gas absorption chamber and the planar annular microcavity in the vacuum chamber. Since the coupler is composed of a planar annular microcavity 1 and a tapered optical fiber 2, when the light wave in the tapered optical fiber 2 satisfies the resonance condition of the planar annular microcavity 1, a large number of light waves will propagate to the planar annular microcavity in the form of evanescent waves. microcavity1. Among them, the evanescent wave is a wave that propagates in the near field. Therefore, only when the distance between the planar annular microcavity fiber 1 and the light cone 2 is within a few microns, the coupling can occur. Moreover, the coupled transmission spectrum is easily affected. When there is a gas with an absorption peak for the coupling wavelength in the environment, the transmission spectra of the coupling structure in the gas absorption chamber 7 and the coupling structure 4 in the vacuum chamber 8 will change significantly. Different concentrations of gases have different effects on the coupling structure. The photodetector 9 receives different detection signals and sends them to the signal processing unit 10 to complete gas detection.

Claims (5)

1, based on the gas detection method of plane annular micro-cavity, it is characterized in that: the coupling mechanism that adopts two plane annular micro-cavities (1) and biconical fiber (2) to constitute, one is the detected gas coupling mechanism, another is a coupling mechanism in the vacuum environment; This gas detection method is based on evanescent wave has changed the transmitted spectrum peak value to the absorption of gas with various principle, utilize that coupling mechanism institute transmitted spectrum compares in detected gas coupling mechanism institute's transmitted spectrum of gas to be detected and the vacuum environment, and realize detecting the kind and the concentration of gas to be measured by the two spectrum change.

2,, comprise optical fiber (2), generating laser (4), optical filter (5), beam splitter (6), absorb air chamber (7), vacuum room (8), photodetector (9) and signal processor (10) based on the gas detection sensor of plane annular micro-cavity; It is characterized in that:

● described optical fiber is double-tapered fiber; A coupling mechanism that is made of plane annular micro-cavity and double-tapered fiber is respectively arranged in described absorption air chamber and the vacuum room, and the plane annular micro-cavity of coupling mechanism is positioned at the awl district of described double-tapered fiber; Coupling mechanism in the described absorption air chamber is the detected gas coupling mechanism, and the coupling mechanism in the vacuum room is a coupling mechanism in the vacuum environment;

● described absorption air chamber is made up of detected gas coupling mechanism and dustproof ventilating cover (3);

● described vacuum room is made up of coupling mechanism in the vacuum environment and vacuum (-tight) housing;

● described generating laser connects optical filter, connect the input end of beam splitter then, the output terminal of beam splitter is by single-mode fiber (11), and one the tunnel connects the double-tapered fiber input end of gas absorption chamber, the double-tapered fiber input end of the vacuum room that other one tunnel connection contrast signal is used; Absorb air chamber and be connected two photodetectors respectively by single-mode fiber, light signal is converted to electric signal and receives unit, signal place by two detectors with the double-tapered fiber output terminal of vacuum room.

3, the gas detection sensor based on plane annular micro-cavity according to claim 2 is characterized in that: described double-tapered fiber is a nano optical fibers, perhaps is photonic crystal fiber.

4, the gas detection sensor based on plane annular micro-cavity according to claim 2 is characterized in that: described plane annular micro-cavity is the collar plate shape microcavity, perhaps is the racetrack microcavity.

5, the gas detection sensor based on plane annular micro-cavity according to claim 2 is characterized in that: the spacing of described coupling mechanism midplane annular micro-cavity and double-tapered fiber is 0～2um.

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