Research On Optical Waveguide Gas Sensing Technique Based On Infrared Absorption Spectroscopy

Compared with other detection methods,gas sensor based on infrared absorption spectroscopy has the advantages of high sensitivity,good selectivity and in-situ measurement.Deep space gas detection technology has important applications in planetary atmosphere detection and manned space micro-environment detection,so there is an urgent need to develop miniature,lightweight and integrated gas sensors.The discrete optical gas sensor has the advantages of large volume and weight,high power consumption,easy to be affected by vibration,and can not be used.The on-chip integrated optical waveguide gas sensor has the advantages of small size,light weight and low power consumption,so it is the optimal scheme for deep space gas detection.Due to the small size of the chip and weak gas absorption,the sensitivity of the on-chip sensor is low.In order to improve the sensitivity of on-chip gas sensor,in this thesis,new on-chip gas detection methods such as light field control based on chalcogenide glass suspended or slot waveguide,on-chip wavelength modulation based on coherent detection and localized surface plasmon resonance effect based on metal island film are proposed.The mid-infrared optical waveguide gas sensing systems are established,and the performance of optical waveguide gas sensors are analyzed by simulation and experiment.The feasibility of the above methods are proved.The main contents of this thesis are as follows:Firstly,the optical waveguide gas sensing technique based on direct absorption spectroscopy(DAS)is studied,and the optical field distribution is controlled by the cross-section structure of the waveguide to improve the sensitivity.(1)Because the nonsuspended vertical slot waveguide has small external confinement factor and the slot has large depth-to-width ratio,which has poor sensing performance and is difficult to fabricate.In order to solve the above problems,a suspended vertical slot waveguide gas sensor is proposed,and the structure of the waveguide is optimized.Using the ability of suspended / slot structure to control optical fields,the external confinement factor is85.77%.The depth-to-width ratio of slot is 0.5.The sensing performance is analyzed theoretically.When the optimal waveguide length is 1.45 cm,the limit of detection of CH4 is 1.70 ppm(parts per million).The effects of environmental and fabrication errors are also analyzed.(2)The chalcogenide suspended rib waveguide gas sensor is fabricated by two-step lift-off method,and the step of dry etching is omitted.The midinfrared gas sensing system is established,and the CH4 sensing experiment is carried out.The limit of detection of the suspended waveguide sensor is 30.3 ppm when the averaging time is 43.4 s,and the external confinement factor is 112%.Secondly,in order to reduce the noise of the sensor system and improve the performance of the sensor,the optical waveguide gas sensing technique based on wavelength modulation spectroscopy(WMS)is studied.The noise is suppressed by a coherent detection method to improve the sensitivity.The simulation model of second harmonic signal based on WMS is established.In order to reduce the light distributed in the lower cladding layer,improve the external confinement factor,and make the sensor operate at a long-wave mid-infrared band,a rectangular waveguide with Mg F2 as the lower cladding layer and chalcogenide glass as the core layer is fabricated by the lift-off method.(1)Using the absorption characteristic of CO2 at 4319 nm,the midinfrared CO2 sensing system is established,and the CO2 sensing experiment is carried out by using WMS technique.The limit of detection is 0.3%.Compared with the sensing system based on DAS,the sensing performance increases 8 times.(2)Using the absorption characteristic of CH4 at 3291 nm,the mid-infrared CH4 sensing system is established,and the CH4 sensing experiment based on WMS is carried out.The influence of waveguide parameters on the second harmonic signal is simulated and verified.When the averaging time is 0.2 s,the limit of detection of 2 cm-long waveguide is 1727.1 ppm,and the limit of detection of 1 cm-long waveguide is 6784.2ppm.Thirdly,in order to increase the optical path of the sensing waveguide and improve the performance of the sensor,the optical waveguide gas sensing technique based on microcavity enhanced absorption spectroscopy(MCEAS)is studied.The sensitivity is improved by microcavity resonance effect.In order to reduce the difficulty of calculating the effective optical path of microcavity waveguide,an equivalent model of microcavity to straight waveguide is proposed.In order to improve the external confinement factor and reduce the coupling difficulty of channel waveguide and microcavity waveguide,a chalcogenide suspended horizontal slot waveguide microcavity gas sensor is proposed.The external confinement factor is increased to44.63% by suspended horizontal slot waveguide structure.The performance of chalcogenide suspended horizontal slot waveguide microcavity gas sensor is analyzed theoretically.When the physical length of the resonant cavity is 710 μm and the transmission loss of the waveguide is set to 3 d B/cm,the limit of detection of CH4 is3.87 ppm.The effect of the loss caused by gas concentration on the effective optical path is studied.The effective optical path and quality factor decrease with the increase of gas concentration.When the gas concentration is less than 1000 ppm,the effective optical path is larger than 2.4 cm,and the optical path is increased by 30 times.Finally,in order to enhance the absorption of the waveguide,the optical waveguide gas sensing technique based on surface-enhanced infrared absorption(SEIRA)spectroscopy is studied.The sensitivity is improved by the localized surface plasmon resonance effect of the metal island film.Rectangular waveguides with chalcogenide glass core layer and Si O2 lower cladding layer were fabricated by lift-off method.Then4 kinds of silver island films with different thicknesses were fabricated on the surface of optical waveguides by oblique angle deposition method to introduce SEIRA effect.Using waveguides with different thicknesses of silver island films,a mid-infrared gas sensing system based on DAS was established,and CH4 sensing experiments were carried out at 3291 nm.The effect of silver island film thickness on the absorbance enhancement factor is analyzed,and the silver island film thickness affects the electric field enhancement effect and waveguide loss.When the thickness of the silver island film is 1.8 nm,the performance of the waveguide sensor is the best,and the absorbance enhancement factor is larger than 2.3.At the averaging time of 0.2 s,the limit of detection of CH4 is 4.11%.In addition,the sensor is used to detect the content of CH4 in shale gas at different fracturing temperatures.The experimental results show that the sensor has a good application prospect.The innovations of this thesis are as follows:(1)For the application of gas sensing,in order to solve the problem of small external confinement factor of chalcogenide non-suspended waveguide,suspended vertical slot waveguide structure and suspended rib waveguide structure are proposed.The external confinement factors of the two structures are 85.77% and 112%respectively,which are 10 times and 14 times higher than those of traditional chalcogenide rectangular waveguides.(2)In order to solve the problem that the optical waveguide gas sensor based on direct absorption spectroscopy is easily affected by system drift and 1/f noise,the wavelength modulation spectroscopy technique is applied to the on-chip gas sensor.The noise is suppressed by a coherent detection method.Compared with the direct absorption spectroscopy,this method increases the sensing performance by 8 times.(3)In order to reduce the plane size of the sensor,a racetrack resonator sensor is proposed,which uses the resonance effect of the detection beam in the micro-ring to enhance the absorption of gas molecules.This structure increases the effective optical path by 30 times.(4)Through the fabrication of silver island films on the surface of chalcogenide glass waveguides,the interaction between gas molecules and infrared light is enhanced by localized surface plasmon resonance effect.The effect of silver island film thickness on the absorbance enhancement factor is analyzed.The absorbance enhancement factor reaches the maximum(> 2.3)when the silver island film thickness was 1.8 nm.