Research On On-chip Gas Sensing Function Module And Signal Processing Method

Whether in industry,agriculture,medicine,or environmental science,etc.,all need high precision and high sensitivity gas sensors.The optical sensors based on infrared absorption have the advantages of high accuracy,good selectivity,fast response,and long service life.However,the commonly used optical gas sensors based on discrete devices are bulky,while the on-chip gas sensors have the advantages of compact structure and light weight,which have a good application prospect.In order to achieve the integration of on-chip gas sensors and improve the performance of on-chip gas sensing,optical sensing modules,electromechanical modules,and signal processing algorithms was researched:In order to calculate the sensing performance of the on-chip gas sensor,the effective index method（EIM）was used to calculate the effective refractive index of each mode of the waveguide.The differential method was used to calculate the transmission loss of different modes of the dielectric absorption waveguide.The transmission loss of the Silicon-On-Insulator（SOI）ridge waveguide was calculated for different concentrations of methane（CH₄）as the waveguide cladding.The waveguide length was optimized,and the waveguide sensing performance was analyzed.The limit of detection（Lo D）of CH₄ is 9115.15 parts per million（ppm）,when the waveguide length is 4.35 cm,and the SOI ridge waveguide was prepared for on-chip gas sensing.In order to improve the optical length of on-chip gas sensors,a SOI micro-ring resonator（MRR）CH₄ gas sensor working in the near infrared band was proposed.The performance of the MRR gas sensor was theoretically calculated by the transfer matrix method.When the loss of the waveguide is 1 d B/cm,the effective optical path is increased by 22.81 times compared with the straight waveguide with the same length.The Lo D of the sensor is 818.21 ppm.At the same time,verified that the series MRR can improve the performance of the sensor.Finally,a multi-gas sensing MRR was proposed,which can simultaneously monitor CH₄ and CO,and Lo D are 4315 ppm and1706 ppm,respectively.In order to realize the miniaturization and instrumentation of the on-chip gas sensor,mechanical and electrical modules for on-chip gas sensing were designed and developed,including:（1）Power source circuit for on-chip gas sensor was developed,which is powered by lithium battery and can output 5 V～12 V through modifying resistance value;（2）Temperature control module for on-chip gas sensor was developed and the corresponding mechanical structure was fabricated.The temperature stability of the sensing chip can reach±0.02°C in an open environment;（3）Mechanical gas cell for on-chip gas sensing was designed.The gas cell integrates temperature and pressure control functions.In order to reduce the noise and improve the performance of the on-chip gas sensor,wavelet denoising（WD）algorithm and Savitzky-Golay（SG）filtering algorithm were combined to process the noisy signal.With the fabricated SOI ridge waveguide,a waveguide CH₄ sensing system was developed,and the absorbance signal was processed by denoising algorithms.Experimental results show that compared with the traditional WD algorithm,the WD-SG algorithm has obvious advantages in suppressing low-frequency random noise and interference noise.Using WD-SG algorithm,the signal-to-noise ratio（SNR）of the absorbance signal is increased by 4.94times.According to the Allan deviation analysis,the 1σLo D of the sensor is reduced from 2929 ppm to 397 ppm when the WD-SG algorithm is adopted.At the same time,the sensor shows a fast response time of less than 2.5 s.The innovative points are as follows:（1）A MRR waveguide for on-chip gas sensing was designed and optimized,which increases the absorption optical path by 22.81 times and improves the on-chip gas detection performance.（2）WD-SG algorithm was applied on on-chip gas sensor signal processing,which filters the sensor signal noise.The SNR of the absorbance signal is increased by 4.94times,and the LOD of the sensor is reduced to 1/7.38 of the original value.