Design And Fabrication Of Narrow-band Filter For CO₂ Gas Detection

Narrow-band filter refers to relative bandwidth of filter?the ratio of half band-width to central wavelength?is less than 0.05,which play a role of selecting the spectrum of a particular pass-band.The specific band of light is passed,and the rest of the band is cut off.At present,it is mainly used in precision optical systems such as gas detectors and medical treatment.In recent years,the more serious greenhouse effect and the gradually raised mean temperature of the global surface made by greenhouse gases further increase the difficulty of the climate change predictions.So need to accurately grasp the CO₂ concentration and spatial distribution in the atmosphere.At the same time,metallurgy,household electrical appliances,agriculture,health services,environmental protection and other aspects all need the quantitative detection and control of CO₂,and in many cases the application environment is very poor.Therefore,the development of well-rectangularity,high transmittance of the narrow-band,super wide rejection band,and well optical properties of narrow-band filter for CO₂ gas detection and good stability,good selectivity,high sensitivity,miniaturization of portable CO₂ gas sensor has a great practical value.In narrow band-pass filter's design,some design parameters are usually adjusted and optimized to obtain better narrow-band filter,but some of the optimized designs are different from the results of preparation.The Macleod Essential software was used to simulate the relationship between the sensitivity and optical properties of narrow band-pass filter and high or low refractive index interval layer,number of reflectors,interference order and number of cavities.These effects were revealed and the law of influence has been obtained.Taking into account carbon dioxide?CO₂?characteristic absorption at 4.26 ?m,the single channel narrow-band filter for CO₂ gas detection can be designed and fabricated.To make use of the relative sensitivity analysis and design principle of Fabry-Perot filter can make pass band have great steepness,high transmittance,rectangularity and low relative sensitivity.Meanwhile,to use superimposed high-reflection film systems with different center wavelength can broaden the rejection band.The 4.26 ?m narrow band-pass filter with super wide rejection band was prepared by electron beam and resistance heating physical vaporous deposition?PVD?techniques using single crystal Si as substrate and Ge and SiO as coating materials.The narrow-band filter of only 62 layers is provided with average transmittance of over 80% at 4240-4280 nm,half band-width of 100±2 nm,average transmittance of the rejection zone of less than 0.05% at 1500-4180 nm and 4350-8000 nm and depth of rejection of less than 5×10-4.The characteristic absorption peak of CO₂ is not only at 4.26 um,but also has a strong absorption peak at 2.78 ?m and 14.95 ?m.So,the double-channel narrow-band filter of only 55 layers was designed with average transmittance of the rejection zone of 0.22%,0.11% and 0.02% at 2000-2700 nm,3850-4100 nm and 4400-8200,average transmittance of 99.13% at 2750-2800 nm,half band-width of 74±2 nm,and average transmittance of 99.60% at 4200-4300 nm,half band-width of 148±2 nm.At the same time,the error of the two sets of film system in design and preparation process was been analyzed,which verified the feasibility of the double-channel narrow-band filter for CO₂ gas detection in the preparation process.