Design of Mid-infrared Negative Filter Against CO2 Radiation (Invited)

With the rapid development of infrared detection technology, infrared detectors have an extremely important role in the fields of aerospace,meteorological remote sensing and infrared guidance. However, during high speed flight,the front-end lens is disturbed by the CO2 gas radiation energy in the surge layer, and the temperature of the optical window of the infrared detector rises significantly. The infrared radiation generated during the transmission process is directly converged on the detector by the optical elements, resulting in a strong bright background on the infrared image, which significantly reduces the signal-to-noise ratio and causes violent jitter on the screen,seriously affecting the recognition accuracy of the target. There are currently more domestic studies for the 3.7 similar to 4.8 mu m band, but there are few reports on negative filters for blocking the interference of CO2 gas radiation energy. It is of great research significance that the analysis of the substrate material determines the use of aspheric single crystal silicon as the substrate,and the negative filter film system on the front surface is found through a calculation that Ge, ZnS as the coating material as a high and low refractive index material produces less absorption and can obtain higher transmittance. Ge, ZnS and YbF3 were chosen as the coating materials for the rear surface, and their ratio mismatch parameters were calculated to obtain the tooling values of the deposited materials. The anode voltage for the substrate cleaning was 170 V,the ion beam current was 3 mA, the reaction gas was argon,and the filling volume was 25 sccm. After plating,the spectral curves were tested and found that the negative filter film had too many secondary peaks in the passband,and the transmittance was not as designed. After further inversion analysis,it was found that the excessive error mainly caused the decrease in transmittance of the measured spectral curve in monitoring the thickness of layers 17 and 21. A random error of 1% to 3% was introduced to the most sensitive layers 17 and 21 of the negative filter film system to observe the effect of the error on their spectral curves. After the introduction of the error,there was almost no effect on the 4.15 similar to 4.4 mu m and 5.15 similar to 5.5 mu m cut-off regions,but the effect on the transmittance of their passband was stronger. From the experimental analysis,it was concluded that the best results were obtained when the crystal switch was set at the Ge film layer,i.e. 1= crystal control probe for layers 1 similar to 8, 2= crystal control probe for layers 9 similar to 12, 3= crystal control probe for layers 13 similar to 16, 4= crystal control probe for layers 17 similar to 20, 5= crystal control probe for layers 21 similar to 28, and 6= probe for layers 29 similar to 34,where the spectral characteristics best matched the design Spectra. This reduces the effect of sensitive layer thickness errors on the spectral profile. Solves the problem of excessive secondary peaks in the passband of negative filter films and reduced transmittance. The average transmittance is 65.05% at 3.65 similar to 3.95 mu m, 64.66% at 4.50 similar to 4.85 mu m, 2.09% at 4.15 similar to 4.4 mu m and 0.23% at 5.15 similar to 5.5 mu m. The average transmittance of the infrared antireflection coating on the back surface was 98% at the band of 3.65 similar to 4.85 mu m. On the basis of designing and optimizing the front and back surface experiments,a complete negative filter deposition experiment was carried out. The average transmittance of the spectral curve after double-sided plating in the high transmission target band was 0.87% and 0.23%,and the average transmittance in the high cut-off target band was 87.04% and 89.79%. The film has also passed the adhesion test and damp heat test. The experiments have proved that the film has good environmental adaptability and can work in complex working environments,improving the accuracy of target detection when the carrier is moving at high speed, and providing important technical guidance for the development of mid-infrared detection technology.