Research On Optical Waveguide Carbon Dioxide Sensor Based On Refractive Index Sensing

With the high-speed development of economy and science,people’s requirements for daily living standards is improving,and people pay more and more attention to the problems of environment,health and safety.The country has put forward two major strategic decisions to strive to achieve carbon peak by 2030 and achieve carbon neutrality by 2060,so it is urgent to reduce carbon emissions.In atmospheric gas,CO₂is not only one of the main components,but also one of the main causes of the greenhouse effect.Due to the massive use of fossil fuels such as coal and oil and the destruction of the forest environment,the proportion of CO₂ in the atmosphere increases year by year,resulting in global warming and extreme meteorological events,and destroying the ecology of the sea and land.In addition,high CO₂ content also has an impact on people’s health,too high concentration of CO₂ will make people uncomfortable and even lead to death.In the field of agricultural production,controlling the concentration of CO₂ in agricultural greenhouses can control the growth of crops and improve crop yields.Therefore,the research of sensors that can monitor environmental CO₂ content has important application value.Commonly used gas detection technologies such as Tunable Diode Laser Absorption Spectroscopy（TDLAS）and Non-Distancepersive Infrared（NDIR）absorption spectroscopy generally use a large gas chamber,while optical waveguide sensor has the advantages of small volume,easy to manufacture,and can be integrated with lasers and detectors on the chip,which has a high research value.In this paper,two optical waveguide refractive index sensors were designed,and one of them was fabricated and tested.The specific work is as follows:First of all,the optical biosensor was described briefly,and the classification of optical waveguide sensor was described in detail,and the research progress of optical waveguide refractive index sensor was summarized.The mode of waveguide was theoretically analyzed,and the sensing principle of MZI and subwavelength grating refractive index sensor was described in detail.The Mode solution software of Lumerical platform was used to simulate the single Mode condition of SU-8 rectangular waveguide and SOI slot waveguide,which lays a foundation for the subsequent design.Secondly,a double slot subwavelength grating structure was proposed to combine with the PHMB for CO₂ sensing.The characteristic size of subwavelength grating was optimized by FDTD software,the influence of characteristic size on the output spectrum of subwavelength grating was analyzed,and the referable fabrication process was given.Besides,the performance of the sensor was analyzed,the sensitivity and detection limit of the sensor were calculated,and the effect of fabrication error on the sensitivity of the sensor was also analyzed.Then,the fabrication process of SU-8 optical waveguide was described in detail,and the problems and solutions of each step of the process were shown in our experiment.Finally,the process parameters of SU-8 optical waveguide with good shape were obtained.The optical waveguide testing system was introduced.The performance of DFB laser was tested by using power meter and Fourier infrared spectrometer.In addition,the mode field of conical lens fiber was tested by using near-field beam quality analyzer.In the end,an asymmetric MZI optical waveguide refractive index sensor was designed.The sensor structure was simulated by using Rsoft Beam PROP.The length difference between the two arms of asymmetric MZI was optimized by using matlab software,and the transmission loss of cosine structure was simulated.The mask of the sensor was designed by using L-Edit software,and the sensor with good morphology was made by using the optimized SU-8 preparation process.The various components of the sensor were tested by infrared camera.The interference effect of the sensor was verified by using deionized water and anhydrous ethanol.A refractive index sensor for CO₂ detection is designed by combining asymmetric MZI and PHMB,and the sensitivity of the sensor is verified theoretically.The main innovations of the paper are as follows:two optical waveguide CO₂refractive index sensors were designed to meet the needs of environmental CO₂monitoring,and the performance of the sensors was verified theoretically.The fabrication process of SU-8 optical waveguide was optimized,and the SU-8asymmetric MZI refractive index sensor was fabricated.