Research On Short-wavelength Infrared Dual-band-pass Filter And Its Spectrum At Cryogenic Temperature

In the optical remote sensing field,the remote sensing instruments has a tendency of miniaturization and integration,dual-spectral detection is a powerful technology to adopt the trend.Dual-band-pass filters(DBPF)are key optical components in dual-spectral detection.The DBPF is transparent on two working wavebands through one aperture surface,and it intercepts all other spectral energy.The primary goal of this article is developing the DBPF that meets the requirements of the remote sensing,and its spectral characteristic at cryogenic temperature is also studied.For the development of any interference optical filter,its primary task is to design a proper multilayer film stack.There are several methods to design dual-band-pass optical filters,such as automatic optimization,design using buffer and combinatorial layers,Fabry-Perot(F-P)filter with thick spacer layers,repeating blocks of F-P filter with double peaks and F-P filter with a fractal structure,etc,but they are not suitable to develop the DBPFs that the center wavelengths are located at 2.95 u m,4.41 u m and the bandwidths are 0.09 times of the center wavelengths.So a method that based on combination of F-P films is presented.With this method,two passbands of DBPF can be designed separately,both width and location of the passbands can be adjusted conveniently,the problem of designing a wide band-width and large channel span DBPF is effectively solved.The designed film stack is more concise compared with other methods,and has a good technological practicability.The method and main technology of film deposition is introduced.The accruate monitoring of film deposition is the key of developing the DBPF successfully.The percent of optical extrema monitoring(POEM)strategy,which can be applied in direct optical monitoring with a single wavelength,is introduced in this thesis.The main reason to use this strategy is to make sure there is enough monitoring accuracy for the higher extinction coefficient material Ge at the wave region of near infrared.According to the monitoring strategy,the film stack is adjusted,using only 34 layers.The monitoring curves that meet the requirement of monitoring strategy are also designed.The film layers are deposited according to the monitoring curves,and the tested spectrum of the sample show that: The shorter pass-band's average transmittance is above 84%,the longer one is above 88%,and four of the pass-bands' edge steepness values are 1.4%,2.8%,1.9%,and 1.7%,respectively.The main reason that the tested spectra of the sample are inconsistent with the theoretical design spectra is the errors of layer thickness.The sources of errors are analyzed,as well as the principle of error generation when using transmittance monitoring.The influence of overall random errors(ORE)to different type of film stack's spectrum is analyzed,and the influence of specific film thickness error to the spectrum performance of film stack is also analyzed.The central wavelength of band-pass filter can be adjusted by the monitoring wavelength,and transmittance of the pass band is the key performance indicator,therefor the maximum transmittance difference(MTD)and the average transmittance difference(ATD)at the top of the pass band are presented to evaluate influence of specific film thickness error to the filter spectrum.The ORE analyses of single band-pass filters that deposited at the different side of substrate are made,and the MTD and ATD analyses at the top of pass band and transparent band are also made.The error analysis is made combined with the tested spectrum of the sample.The DBPF is used at cryogenic temperature,therefore,the cryogenic temperature properties need to be studied.The cryogenic spectrum test device are introduced,the change rule from room temperature to cryogenic temperature of Ge and SiO's optical constants are obtained from spectrum test at room temperature and cryogenic temperature(100 K),their temperature coefficient's dispersion curves of optical constants are calculated.With the designed film stack,the shift feature of the spectrum from room temperature to cryogenic temperature is obtained.There are some differences between theoretical simulation spectrum shift and the tested spectrum shift,the accuracy of optical constants at cryogenic temperature need to be improved.