CN105115939A - Tapered multimode interference-based high-sensitivity optical fiber methane sensing device - Google Patents

Abstract

A high-sensitivity optical fiber methane sensing device based on conical multimode interference, including a broadband light source, a conical multimode optical fiber sensor coated with a porous sensitive film of STMS structure, a test gas chamber, a switch valve, a mass flow controller, and a spectrometer analyzer and computer. Among them, the STMS structure optical fiber sensor is made of single-mode fiber-multi-mode fiber-single-mode fiber fusion and tapered multi-mode fiber. The outer surface of the tapered multi-mode fiber is pretreated with silane coupling agent; the sensitive film is a cage-shaped The α-hydrogen-ω-hydroxyl-polydimethylsiloxane porous methane sensitive film of molecule A; when the methane gas to be measured interacts with the porous methane sensitive film, the refractive index of the sensitive film changes rapidly, making the characteristic wavelength of the sensor interference spectrum λ _c moves, analyzing the movement amount Δλ _c of the characteristic wavelength of the interference spectrum before and after the sensor contacts with the methane gas, the concentration of the methane gas to be measured can be obtained. The invention has the characteristics of high sensitivity, fast response speed, good selectivity and stability, and the like.

Description

A high-sensitivity optical fiber methane sensing device based on cone-shaped multimode interference

technical field

The invention belongs to the technical field of optical fiber sensing, and in particular relates to a high-sensitivity optical fiber methane sensing method and device for obtaining methane concentration through conical multi-mode interference.

Background technique

With the rapid development of optical fiber sensing technology, the research of optical fiber gas sensor has received widespread attention. It is suitable for inflammable and explosive environments in harsh environments because of its good electrical insulation, anti-strong electromagnetic interference, explosion-proof, and long-distance long-term online measurement. Gas monitoring.

Single-mode fiber (SMF)-tapered multimode fiber (TMMF)-single-mode fiber (SMF) structure (STMS) is a fiber structure based on the mode interference mechanism of tapered multimode fiber, which is embedded by a section of multimode fiber It is made by tapering multimode fiber after reaching a section of standard single-mode fiber. This optical fiber sensor based on tapered optical fiber multimode interference has the advantages of simple structure, low cost, and high sensitivity. It can overcome the influence of light wave intensity fluctuations through wavelength modulation, and can be used for information detection such as humidity, hydrogen, ammonia, and oxygen. . At present, researchers have successively developed fiber optic sensors with different STMS structures. Wang Xueping et al [Master's degree thesis of China Jiji Institute, 2014.3.1] proposed a tapered optical fiber humidity sensor coated with polyvinyl alcohol (PVA) film, the humidity sensitive film is composed of multimode optical fiber tapered area in 5% PVA aqueous solution Coated, the refractive index of the humidity sensitive film will change with the humidity of the external environment, and the humidity information can be obtained according to the change of the optical power. The maximum sensitivity of the sensor is 0.251dB/%RH in the range of humidity 35-90%RH. Joel Villatoro et al. (Sensors and Actuators B, 2005, 110: 23-27; IEEE Sensors Journal, 2003, 3(4): 533-537) coated palladium on the surface of a tapered multimode fiber to form a highly sensitive hydrogen sensor with a detection limit of 0.3% and a response time of 90 seconds. David Monzón Hernández et al. (Sensors and Actuators B, 2010, 151: 219-222) tapered the optical fiber to 5-10 μm and plated palladium and gold films at 0.1 Å/s to form an optical fiber sensor that can quickly detect hydrogen, with a response time of 2 seconds. FuxingGu et al. (Advanced Optical Materials, 2014, 2:189-196) also coated single crystal palladium nanowires on the active area of the tapered optical fiber sensor, and the detection limit of palladium nanowires for hydrogen was as low as 0.2%. D.RitheshRaj et al. (OpticsCommunications, 2015, 340:86～92) coated nano-silver particles, polyvinylpyrrolidone and polyvinyl acetate on the surface of a tapered plastic optical fiber, and evaluated different nano-silver mass fractions (1.6, 3.3 and 6.6%) ) system is selective to ammonia. S.A.Ibrahim et al. (OpticsExpress, 2015, 23(3): 2837-2845) coated polyaniline nanofibers to the tapered multimode fiber sensing area to form an ammonia sensor, and the response and recovery times were 2.27 and 9.73 minutes, respectively. Good reproducibility and reversibility. RenataJarzebinska et al. (Analytical Letters, 2012, 45(10):1297～1309) proposed electrostatic self-assembly of alternating layers of tetra-(4-sulfophenyl)porphyrin (TSPP) and polyallylamine hydrochloride (PAH) into cones. The ammonia gas sensor is formed on the surface of the optical fiber with a beam waist diameter of 10 μm, a response time of 100 seconds, and a recovery time of 240 seconds. C. Pulido et al. (Sensors and Actuators B, 2013, 184:64-69) proposed to embed fluorophores into a tapered polymer fiber to form a fiber optic oxygen sensor, and use the fluorescence quenching signal caused by oxygen to measure the oxygen content, with a response time of 28 seconds. Rongsheng Chen et al. (PhysiologicalMeasurement, 2013, 34:N71～N81) also established an optical fiber sensing system that can detect human breathing. The principle is to coat the surface of the tapered optical fiber with a fluorophore mixed with a polymer, and oxygen has fluorescence quenching. Features, the sensor response time is 150 seconds, and the respiratory rate can be monitored up to 60 times per minute. Chen Daru et al. (ZL201110311888.7) disclosed a Bragg grating hydraulic sensing method based on a tapered fiber. The sensing system includes a broadband light source, an optical coupler, a spectrum analyzer and a Bragg grating of a tapered fiber. Placed in the hydraulic environment to be tested, the hydraulic pressure applied to the Bragg grating is obtained by measuring the shift of the center wavelength of the tapered fiber Bragg grating.

Methane is the main component of mine gas (accounting for about 83-89%), and it is the focus of mine safety monitoring. Cage molecular compounds (Cryptophanes) are the only new functional molecules discovered so far that have a direct photosensitive response to methane, and can be combined with optical fiber sensing technology to form a highly selective optical fiber methane sensitive thin film sensor, such as evanescent wave mode, fluorescence transmission, etc. Type, mode filter type, long period fiber grating type, photonic crystal fiber type and other methane sensors (Sensors and Actuators B, 2005, 107 (1): 32 ~ 39; Analytica Chimica Acta, 2009, 633 (2): 238 ~ 243; Chinese Optics Letters, 2010 , 8(5):482～484; OpticsExpress, 2011, 19(15):14696～14706). The analysis shows that the current research work is mainly to incorporate the cage molecule A or E into polysiloxane or styrene-acrylonitrile materials, and to coat them on plastic-clad silica cores, core-mismatched optical fibers or A fiber optic methane sensor formed on a long period fiber grating. However, the sensitive films of these methods are derived from the coating of cage molecules mixed with polysiloxane and styrene-acrylonitrile resins. Most of the cage molecules are wrapped in the film materials, and methane molecules are prone to appear in these films. The problems of high migration resistance and difficulty in contact with cage molecules in the material make the response speed of the sensor slow (2 to 5 minutes) and low sensitivity.

It can be seen that the above technologies all have a common problem, that is, the cage molecules are not directly exposed to methane gas, which affects the ability of the cage molecules to respond to methane molecules, making the sensor sensitivity low, and it is necessary to change the structure of the sensitive film.

Invention content:

In order to solve the above existing problems in the prior art, the present invention proposes a multi-mode interference optical fiber methane sensing method and device with high sensitivity and fast response speed. -ω-Hydroxy-polydimethylsiloxane porous film is used as a methane-sensitive material, and it is coated on the tapered multimode optical fiber sensing area of the SMF-TMMF-SMF structure, which can achieve high response speed and high sensitivity to methane gas , Highly selective detection.

For realizing above-mentioned purpose of the invention, the technical scheme that the present invention takes is as follows:

A high-sensitivity optical fiber methane sensing device based on conical multimode interference, including a broadband light source, a single-mode optical fiber, an optical fiber methane sensor with a TMMF of SMF-TMMF-SMF structure coated with a porous sensitive film, a test gas chamber, a switch valve, It consists of mass flow controller, spectrum analyzer and computer. The two ends of the multimode optical fiber MMF are respectively fused with the single-mode optical fiber SMF, and the multimode optical fiber is further tapered to form an SMF-TMMF-SMF structure, and the two ends of the single-mode optical fiber SMF connected with the TMMF are connected to the broadband light source, spectrum The analyzer and computer; the optical fiber sensor is located in the test gas chamber, the test gas chamber has an inlet and an outlet for the methane gas to be measured, and the gas inlet is connected to a mass flow controller for controlling the methane gas to be measured through a switch valve. The outer surface of the tapered multimode optical fiber of the SMF-TMMF-SMF structure optical fiber sensor is pretreated with a silane coupling agent aqueous solution and then coated with a methane sensitive film.

The methane-sensitive film is an alpha-hydrogen-omega-hydroxyl-polydimethylsiloxane porous methane-sensitive film containing a cage molecule A, which is the first optical grade alpha-hydrogen-omega-hydroxyl-polydimethylsiloxane The oxane, the cage molecule A, and the porogen ammonium bicarbonate were placed in a mixed solvent of dichloromethane and methanol, stirred at a speed of 2000 rpm for 0.5 hours, then further ultrasonically mixed, and then automatically pulled and coated on the conical multi- In the mode optical fiber sensing area, under the conditions of temperature 90°C and vacuum degree 0.08MPa, the mixed solvent of dichloromethane and methanol in the film and the thermal decomposition porogen ammonium bicarbonate are released to make the formed methane-sensitive film porous, with a porosity of 12 ~15%; the amount of reagents is 600 μmol of cage molecule A, 1 g of optical grade α-hydrogen-ω-hydroxy-polydimethylsiloxane, 0.1 g of porogen ammonium bicarbonate, and dichloromethane with a volume ratio of 5:1 12mL mixed solvent with methanol.

In the present invention, the sensitive film formed by mixing cage molecules and optical resins is made porous, and the transformation from a dense film to a porous film is realized by thermally decomposing the porogen in the sensitive film. The sensitive film has a relatively high porosity, and the exposed cages in the film The number of molecules is significantly increased; at the same time, the sensitive film is coated on the surface of the tapered multimode fiber and made porous, and the rapid change of the refractive index after the diffusion of methane molecules into the porous sensitive film causes the characteristic wavelength shift of the sensor transmission interference spectrum. When the methane to be measured When the gas interacts with the porous methane-sensitive film on the outer surface of the tapered multimode optical fiber, the refractive index of the sensitive film changes rapidly, which moves the characteristic wavelength of the sensor's interference spectrum, and can achieve high sensitivity, high response speed, and high selectivity for methane gas detection, etc. Purpose.

The diameter of the multimode optical fiber before tapering is 125 μm, and the beam waist diameter D after tapering is 25˜35 μm.

The length of the tapered multimode optical fiber (sensing region) is L, wherein the tapered length _LT is 0.55-0.85mm; the methane-sensitive film is coated on the surface of the tapered optical fiber and made porous, with a porosity of 12-15%. The change of the refractive index of the sensitive film can significantly affect the shift of the characteristic wavelength of the transmission interference spectrum and improve the sensitivity of the sensor.

The outer surface of the tapered multimode optical fiber is pretreated with an aqueous solution of KH-550 silane coupling agent (γ-aminopropyltriethoxysilane), the volume concentration is 0.1%, and the treatment time is 10 seconds to form a silane coupling agent. The nano-coating layer of the joint agent has a thickness of only nanometers, and is used to improve the adhesion between the methane-sensitive film and the outer surface of the optical fiber.

The porous polymer sensitive film is an α-hydrogen-omega-hydroxyl-polydimethylsiloxane sensitive film containing a cage molecule A, and the sensitive film only responds to methane gas, and responds to oxygen, nitrogen, carbon dioxide, There is almost no response to non-methane gases such as carbon monoxide and hydrogen sulfide, and the selectivity is good.

The α-hydrogen-ω-hydroxyl-polydimethylsiloxane-methane sensitive film containing the clathrate molecule A is a low-refractive-index sensitive material with a refractive index of about 1.41 and good toughness.

The broadband light source adopts DL-CS5014A super-radiation broadband light source SLD with a center wavelength of 1550nm and a bandwidth of 40nm.

The spectrum analyzer is an Agilent 86140B spectrum analyzer with a wavelength range of 600-1700nm.

The shift amount Δλ _c of the characteristic wavelength of the transmission interference spectrum of the sensor is the difference between the characteristic wavelength of the m-th order interference peak after the sensor is exposed to methane gas and before the exposure.

The working principle of the sensing device is as follows:

When the incident light of the fundamental mode LP ₀₁ enters the TMMF with a length of L through the SMF/tapered multimode fiber (TMMF) fusion interface from the single-mode fiber (SMF), multiple higher-order modes LP _0m will be excited in the TMMF , these modes will propagate along the TMMF to the beam waist with different propagation constants, and further reach the fusion interface between the TMMF and the output SMF, interfere with the fundamental mode and re-enter the SMF. For the excited LP _0m higher order mode, the electric field component is expressed as:

$\mathrm{<span\; class="notranslate">\; E.</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; i\&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

where E _m (r) and β _m are the electric field distribution and the propagation constant of the longitudinal m-order mode, and c _m is the excitation efficiency of the LP ₀₁ mode of the SMF to the LP _0m mode of the TMMF, which can be expressed as:

${\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; =</span>\frac{{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; \&infin;</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; r</span>}\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; r</span>}}{\sqrt{{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; \&infin;</span>}}{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; r</span>}\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; r</span>}{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; \&infin;</span>}}{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; r</span>}\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; r</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Among them, E _s (r) is the LP ₀₁ mode field distribution of SMF.

The power of the LP _0m high-order mode in TMMF is closely related to the coupling coefficient η _m , and the expression of η _m is Larger _ηm indicates higher power of LP _0m mode. Under the weak derivative approximation, the difference in longitudinal propagation constants of higher-order modes in TMMF can be expressed as:

${\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; ka</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

Among them: μ _m = π(m-1/4) and μ _n = π(n-1/4) are the roots of the zero-order Bessel function; n _T is the TMMF refractive index, and a is the waist radius of the multimode fiber bundle , k=2π/λ is the wavenumber of the incident light, and the subscripts m and n correspond to different modes of light, m-order mode and n-order mode. The phase difference between the two modes of light propagating to z is Δφ _mn =(β _m −β _n )z. When the length of the sensing area is L, the phase difference when the light propagates to the TMMF/SMF interface at the exit end is:

${\mathrm{<span\; class="notranslate">\; \&Delta;\&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 4</span>}{\mathrm{<span\; class="notranslate">\; \&pi;a</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; N</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

It can be known from formula (4) that the light propagation distance reaching the SMF is L, and the light of these different modes interferes due to the different phases, and the interference enhancement and interference weakening appear. There are characteristic peaks (peaks, troughs) in the interference spectrum, and the corresponding characteristic The wavelength satisfies

${\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 8</span>}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; o</span>}}{\mathrm{<span\; class="notranslate">\; a</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; N</span>}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&lsqb;</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; \&rsqb;</span>\mathrm{<span\; class="notranslate">\; L</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; ></span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

In the formula: L is the length of TMMF; N is a positive integer.

When the porous sensitive film coated on the outer surface of TMMF is in contact with methane gas, its refractive index will change rapidly, which will change the propagation constant and mode field distribution of light in TMMF, resulting in the shift of the characteristic wavelength λ _c of the interference spectrum. According to the magnitude of the characteristic wavelength shift Δλ _c , the concentration of the methane gas to be measured can be calculated.

The present invention has the characteristics of high sensitivity, fast response speed, good selectivity and stability, and the specific advantages are as follows:

1. The present invention directly fuses multimode optical fiber and single-mode optical fiber SMF-28, and then tapers the multimode optical fiber to form a tapered optical fiber, which can make the light transmitted in the multimode optical fiber core enter the tapered shape more The evanescent wave is formed at the interface of the optical fiber/sensitive film, which greatly improves the sensitivity and detection limit of the sensor.

2. The present invention proposes to realize the purpose of exposing clathrate molecules by making the methane-sensitive film porous, and the purpose of improving sensor sensitivity and response speed can be better solved by controlling the porosity of the sensitive film to 12-15%. It selects raw materials and adopts the most suitable process conditions. The methane-sensitive membrane solution used is composed of cage molecule A, optical grade α-hydrogen-ω-hydroxyl-polydimethylsiloxane, porogen ammonium bicarbonate, It is produced by high-speed stirring and ultrasonic treatment of a mixed solvent of dichloromethane and methanol. The solution is uniform and the viscosity is easy to adjust. The mixed solvent of dichloromethane and methanol in the film and the thermal decomposition of carbonic acid, a porogen, are removed from the film under the conditions of a temperature of 90 ° C and a vacuum of 0.08 MPa. ammonium hydrogen, thereby making the methane-sensitive film porous and improving the response speed of the sensor.

3. The methane-sensitive membrane adopted in the present invention is composed of cage molecule A and optical grade α-hydrogen-ω-hydroxyl-polydimethylsiloxane, fully utilizing α-hydrogen-ω-hydroxyl-polydimethylsiloxane The hydroxyl group (-OH) in the alkane molecular structure can coordinate with the oxygen atom in the cage molecule A structure, which is more sensitive to methane than the traditional cage molecule A formed with polysiloxane, styrene-acrylonitrile resin, etc. (Sensors and Actuators B, 2005, 107 (1): 32-39; Analytica Chimica Acta, 2009, 633 (2): 238-243; ZL200710093035.4; ZL201010593704.6) More uniform, stable, and higher quality film.

4. In order to enhance the binding force between the methane-sensitive film and the surface of the tapered multimode fiber, it is proposed to pretreat the surface of the tapered fiber with γ-aminopropyltriethoxysilane coupling agent, through the hydroxyl (- OH) and the silicon-oxygen bond (Si-O-) of the silane coupling agent, between the silicon-oxygen bond (Si-O-) and the hydroxyl group (-OH) of α-hydrogen-ω-hydroxyl-polydimethylsiloxane Interact with each other to form a nano-scale coating layer to achieve the purpose of enhancing the adhesion of the methane-sensitive film on the surface of the tapered optical fiber.

Description of drawings

Fig. 1 is a schematic diagram of the structure of the optical fiber methane sensing device based on the tapered multimode interference of the present invention.

Fig. 2 is a schematic structural diagram of the transmissive optical fiber methane sensor based on conical multimode interference in Fig. 1 .

Detailed ways

Below in conjunction with accompanying drawing 1 and Fig. 2, the present invention is described in further detail:

Referring to Figure 1, the structure of the optical fiber methane sensing device based on tapered multimode interference is to combine the tapered optical fiber methane sensor 2 of the α-hydrogen-ω-hydroxyl-polydimethylsiloxane sensitive film containing the cage molecule A Both ends are connected together with the single-mode optical fiber SMF-28, and the sensor 2 is assembled into the sensor device test air chamber 3 and sealed. The super-radiation broadband light source SLD1 is connected to the input end of the sensor 2, and the output end of the sensor 2 is connected to the Agilent86140B spectrum analyzer 4, and the spectrum analyzer 4 is connected to the computer 6 through the GPIB interface connection line 5. The test gas chamber 3 has an air inlet 7 and an air outlet 8 through which the methane gas 11 to be tested is connected.

Referring to FIG. 2 , the tapered optical fiber methane sensor 2 includes a SMF-28 single-mode optical fiber 21 , a tapered multimode optical fiber 22 , and a methane sensitive film 23 . To make the sensor, the two ends of the multimode fiber need to be fused with the single-mode fiber, and the multimode fiber is drawn into a tapered structure by using a taper machine. The length of the tapered multimode fiber (sensing area) is L, and the length of the taper is L _T is 0.55-0.85mm, and the beam waist diameter D is 25-35μm. The method of coating the surface of the tapered multimode optical fiber containing the α-hydrogen-ω-hydroxyl-polydimethylsiloxane sensitive film containing the cage molecule A and ammonium bicarbonate is the automatic pulling method; At 90°C and a vacuum of 0.08MPa, the mixed solvent of dichloromethane and methanol and the thermal decomposition of ammonium bicarbonate, a porogen, are removed to make the methane-sensitive film porous, with a porosity of 12-15%. The multimode fiber is Thorlabs AFS105/125Y gradient index MMF (core diameter 105 μm, cladding diameter 125 μm), length 42 mm; single mode fiber is Corning SMF-28 fiber, the core diameter is about 9 μm, and the cladding diameter is 125 μm.

The preparation process of optical fiber methane sensor based on tapered multimode interference includes welding to form SMF-MMF-SMF structure, tapering to form SMF-TMMF-SMF structure, TMMF tapered area coated with methane sensitive film and methane sensitive film porous:

(1) Take two sections of single-mode optical fiber SMF-28 and one section of multi-mode optical fiber MMF (both cladding diameters are 125 μm), use optical fiber strippers to remove the coating layers of these two types of optical fibers, and clean them with absolute ethanol Cut with an optical fiber cleaver to form a flat end face, and then use the automatic mode fusion of an optical fiber fusion splicer to cut the MMF to a length of 42mm to form a SMF-MMF-SMF structure.

(2) Place the optical fiber fused to form the SMF-MMF-SMF structure on a fusion tapered machine, the power of the CO ₂ laser is 30W at 10.6 μm, and the heating area is located in the middle of the MMF through a three-dimensional adjustment frame. Adjust the power of the _CO2 laser to 15W, and control the diameter of the _CO2 laser beam to 150 μm through a zinc selenide (ZnSe) cylindrical lens. The MMF heating area is melted and stretched by a constant tension of 3 grams of weight on the SMF connected to the MMF to form a tapered SMF-TMMF-SMF tapered optical fiber with a length _LT of 0.55-0.85 mm and a beam waist diameter D of 25-35 μm.

(3) Immerse the tapered area of the SMF-TMMF-SMF structured tapered optical fiber in an aqueous solution with a mass ratio of 0.1% sodium dodecylbenzenesulfonate and 5% sodium hydroxide at a temperature of 60°C for 30 minutes and wash with distilled water ; Immerse the degreased and degreased conical area in a volume ratio of 20% hydrofluoric acid aqueous solution for 5 minutes at room temperature, take it out and wash it thoroughly with distilled water; immerse it in an aqueous solution of 0.1% volume ratio KH-550 silane coupling agent for 10 seconds, A nanoscale silane coupling agent coating layer can be formed on the outer surface of the optical fiber, which can significantly improve the adhesion between the methane-sensitive film and the surface of the optical fiber.

(4) Prepare the coating liquid sensitive to methane, take 600 μmol of cage molecule A, 1 g of optical grade α-hydrogen-ω-hydroxyl-polydimethylsiloxane (molecular weight 4200, density 0.98g/mL, refractive index 1.41), Ammonium bicarbonate porogen 0.1g, mixed solvent of dichloromethane and methanol (volume ratio 5:1) 12mL, stirred with a high-speed stirrer for 30 minutes, stirring speed 2000 rpm, after further ultrasonic mixing, a uniform transparent solution was formed .

(5) Place the SMF-TMMF-SMF structure tapered optical fiber coated with silane coupling agent on the automatic pulling machine, TMMF is immersed in the coating solution of sensitive methane, under automatic mode (standstill time 3 minutes, lift Pulling speed 10-30 cm/hour) using an automatic pulling machine to form a methane-sensitive film with a uniform thickness of 0.2-0.5 μm, and finally release the dichloride in the film under the conditions of temperature 90 ° C, vacuum degree 0.08 MPa, and time 1 hour. The mixed solvent of methane and methanol and the pyrolysis porogen ammonium bicarbonate make the methane-sensitive film porous, with a porosity of 12-15%.

A fiber-optic methane sensor with a structure of SMF-TMMF-SMF coated with a methane-sensitive film was assembled in the test chamber. When the methane gas to be measured enters through the mass flow controller and the air inlet of the test chamber and interacts with the porous methane sensitive film on the surface of TMMF, the refractive index of the sensitive film changes rapidly; and the change of the refractive index of the sensitive film will directly change the sensor’s transmission. The characteristic wavelength λ _c of the interference spectrum. Use a spectrum analyzer to detect the movement of the characteristic wavelength of the interference spectrum, and analyze the movement of the characteristic wavelength of the interference spectrum Δλ _c before and after the sensor is in contact with the methane gas to obtain the concentration of the methane gas to be measured.

Example 1: A section of multimode fiber MMF and two sections of single-mode fiber SMF-28 are fused to form a SMF-MMF-SMF structure, the length of the multimode fiber is 42 mm, the tapered area of the multimode fiber after tapering is 0.68 mm, and the beam waist diameter is 30 μm ; After the surface of the tapered area is pre-coated with a silane coupling agent nano-covering layer, the surface of the tapered optical fiber is coated with α-hydrogen-ω-hydroxyl-polydimethylsiloxane containing cage molecule A with a thickness of 350nm Methane-sensitive membranes are porous. Taking methane standard gas with a concentration of 0-3.5% (v/v) as the object, it interacts with the sensitive film on the surface of the optical fiber respectively. The characteristic wavelength of the transmission interference spectrum moves to the short wavelength direction with the increase of the methane gas concentration, and the transmission interference spectrum There is a linear correlation between the trough characteristic wavelength shift Δλc near 1550nm and the methane concentration _c , and the linear regression equation is:

Δλ _c =kc+b

In the formula, c is the concentration of methane gas to be measured, Δλ _c is the movement of the characteristic wavelength of the trough of the sensor transmission interference spectrum, k is the slope, and b is the intercept.

In the experiment, methane standard gases with known concentrations of 0, 0.1, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5% (v/v) were used for detection respectively. For a certain methane concentration, according to the change of the characteristic wavelength of the trough of the transmission interference spectrum, the wavelength shift Δλ _c before and after the introduction of methane gas can be obtained _. 0.40, 0.80, 1.20, 1.60, 1.90, 2.40, 2.92, 3.30nm, the linear regression equation is: Δλ _c =0.8825c+0.2307, the correlation coefficient R ² =0.9903, that is, k and b in the linear regression equation are 0.8825, 0.2307.

When the methane gas to be measured is in contact with the sensor, the trough characteristic wavelength shift of the transmission interference spectrum Δλ _c is 1.82nm, the concentration of the methane gas to be measured can be calculated as c=1.80%, the response time is 41 seconds, and the recovery time is 43 seconds.

Embodiment 2: The experiment adopts the tapered optical fiber methane sensor with tapered region 0.55mm, beam waist diameter 25μm, sensitive film thickness 380nm, and with known concentration of 0, 0.1, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5% (v/v) methane standard gas is used for detection, and the characteristic wavelength of the trough of the corresponding transmission interference spectrum moves to the short wavelength direction with the increase of the methane gas concentration, and the movement amount of the characteristic wavelength of the trough of the interference spectrum Δλ _c is 0, 0.42, 0.82 respectively _. ^_

When the methane gas to be measured is in contact with the sensor, the trough characteristic wavelength shift of the transmission interference spectrum Δλ _c is 1.54nm, the concentration of the methane gas to be measured can be calculated as c=1.42%, the response time is 36 seconds, and the desorption time is 38 seconds.

Embodiment 3: adopt the tapered optical fiber methane sensor of tapered area 0.76mm, beam waist diameter 32 μ m, sensitive film thickness 330nm, and with known concentration 0,0.1,0.5,1.0,1.5,2.0,2.5,3.0,3.5 % (v/v) methane standard gas for detection, the corresponding transmission interference spectrum trough characteristic wavelength moves to the short wavelength direction with the increase of methane gas concentration, and the interferometric spectrum trough characteristic wavelength shift Δλ _c is 0, 0.42, 0.88, 1.24, 1.72, 2.16, 2.32, 3.04, 3.46nm, the linear regression equation is: Δλ _c =0.9115c+0.2654, the correlation coefficient R ² =0.9846, that is, k and b in the linear regression equation are 0.9115 and 0.2654 respectively.

When the methane gas to be measured is in contact with the sensor, the trough characteristic wavelength shift of the transmission interference spectrum Δλ _c is 3.18nm, the concentration c of the methane gas to be measured can be calculated to be 3.20%, the response time is 39 seconds, and the desorption time is 43 seconds.

Embodiment 4: adopt the tapered optical fiber methane sensor of tapered region 0.85mm, beam waist diameter 35 μm, sensitive film thickness 430nm, and with known concentration 0,0.1,0.5,1.0,1.5,2.0,2.5,3.0,3.5 % (v/v) methane standard gas for detection, the corresponding transmission interference spectrum trough characteristic wavelength moves to the short wavelength direction with the increase of methane gas concentration, and the interferometric spectrum trough characteristic wavelength shift Δλ _c is 0, 0.48, 0.88, 1.26, 1.82, 2.28, 2.32, 3.26, 3.58nm, the linear regression equation is: Δλ _c =0.9496c+0.2767, the correlation coefficient R ² =0.9783, that is, k and b in the linear regression equation are 0.9496 and 0.2767 respectively.

When the methane gas to be measured is in contact with the sensor, the trough characteristic wavelength shift of the transmission interference spectrum Δλ _c is 0.96nm, the concentration of the methane gas to be measured can be calculated as c=0.72%, the response time is 32 seconds, and the desorption time is 35 seconds.

Claims (4)

1., based on the high sensitivity optical fiber methane sensing device that tapered multimode is interfered, comprise wideband light source, methane optical fiber sensor, test air chamber, controlled valve, mass flow controller, spectroanalysis instrument and computing machine; Described wideband light source connecting fiber methane transducer, the interference signal of methane optical fiber sensor connects spectroanalysis instrument and computing machine through single-mode fiber; Described methane optical fiber sensor is positioned at test air chamber, and test air chamber has the air intake opening and gas outlet that pass into methane gas to be measured, and air intake opening is by the mass flow controller of controlled valve connection control methane gas to be measured;

It is characterized in that: described methane optical fiber sensor draws cone to form to multimode optical fiber by after multimode optical fiber two ends welding single-mode fiber further;

Described multimode optical fiber draws cone length L _tbe 0.55 ~ 0.85mm, beam waist diameter D is 25 ~ 35 μm; Tapered multimode fiber outside surface adopts the pre-service of KH-550 silane coupling agent aqueous solution to form silane coupling agent nanometer coat, KH-550 silane coupling agent aqueous liquid volume concentrations is 0.1%, 10 seconds processing times, coat thickness is nanoscale, apply porous methane sensitive thin film again, improve methane sensitive membrane and tapered multimode fiber outside surface adhesion; Described porous methane sensitive thin film is the alpha-hydro-omega-hydroxy-poly dimethyl siloxane porous methane sensitive thin film containing cage molecule A, its thickness 200 ~ 300nm; Described porous methane sensitive thin film first optical grade alpha-hydro-omega-hydroxy-poly dimethyl siloxane, cage molecule A, pore-foaming agent ammonium bicarbonate is placed in methylene chloride and methanol mixed solvent, ultrasonicly further after stirring 0.5 hour with 2000 revs/min of rotating speeds to mix, then lift is coated on tapered multimode fiber sensitive zones automatically, methylene chloride and methanol mixed solvent and thermal decomposition pore-foaming agent ammonium bicarbonate in film is deviate under temperature 90 DEG C and vacuum tightness 0.08MPa condition, make the methane sensitive thin film porous of formation, porosity is 12 ~ 15%; Reagent dosage is respectively cage molecule A600 μm ol, optical grade alpha-hydro-omega-hydroxy-poly dimethyl siloxane 1g, the methylene chloride of pore-foaming agent ammonium bicarbonate 0.1g, volume ratio 5:1 and methanol mixed solvent 12mL;

Described wideband light source adopts the superradiance wideband light source SLD of centre wavelength 1550nm, bandwidth 40nm.

2. high sensitivity optical fiber methane sensing device of interfering based on tapered multimode according to claim 1, it is characterized in that: the described alpha-hydro-omega-hydroxy-poly dimethyl siloxane methane sensitive thin film containing cage molecule A is low-refraction porous sensitive material, refractive index about 1.41, porosity 12 ~ 15%.

3. according to claim 1 and 2 based on tapered multimode interfere high sensitivity optical fiber methane sensing device, it is characterized in that: described multimode optical fiber two ends all with single-mode fiber SMF-28 welding.

4. high sensitivity optical fiber methane sensing device of interfering based on tapered multimode according to claim 1 and 2, is characterized in that: described spectroanalysis instrument is the spectroanalysis instrument of 600 ~ 1700nm wavelength coverage.