Study On High Precision And Wide Spectrum Optical Thin Film Monitoring System

With the continuous development of science and technology,Optical thin films have received extensive attention due to the applications in diverse fields such as fiber optics communication,automatics driving,military industry and infrared thermometry.T Among the factors affecting the spectral performance is optical thickness,which determines the spectral quality of the films.Our aim is to develop a broad-spectrum optical film thickness monitoring system covering visible to mid-infrared band with control error less than 0.015%.Using such a monitoring system to control the optical thin film thickness,we can precisely determine the high-reflectance zone in narrow band filters where we have infrared band.The software and hardware structure of the system is studied in detail and in depth.The hardware of the system consists of a highly stable light source luminescence device,a collimated optical path transmission system combining optical waveguide and free-form surface,a combined silicon-based photodetector and indium-arsenic-tellurium photovoltaic detector,a narrow linewidth spectral analysis device,a transimpedance amplifier,a lock-in amplifier,and an industrial control computer.The software development is based on Lab VIEW language,using the phase-locked voltage-reflectivity coefficient introduction method for parameter fitting method for fitting parameter,the Savitzky-Golay fitting method combined with the least squares method for filtering,and the photoelectric polarization method and the fitting pre-critical stop method for determining the film thickness termination,as follows.A high stability light source lamination device combining visible light and mid-infrared light is developed independently according to the wavelength and response requirements of the monitoring system.Based on Planck’s law of radiation,a suitable blackbody radiation source is selected for the infrared light source,which makes the light source transmission spectrum close to the blackbody and avoids the error of light collection due to thermal radiation,and broadens the spectral monitoring range from visible light to mid-infrared band,i.e.,400～5000 nm.The fiber optical waveguide combined with the free-form lens set is used to realize the optical path transmission method that meets the requirements of the device through the evaluation function of light trajectory diagram and dispersion covariance displacement,and the collimated optical path system is designed by combining the optical system design software ZEMAX and mechanical design software Solid Works,and the construction of the optical system is completed to realize the collimated characteristics of the optical path system,so that the optical waveguide can transmit light in the full wavelength band.The optical path collimation of the optical waveguide in the full waveband（400～5000 nm）is as high as6.59×10^-4.By combining a silicon-based photodetector with an indium-arsenic-tellurium photovoltaic detector,the spectral width has been extended.For the photoelectric conversion stability of the photodetector,the temperature control method of NTC thermal sensor is used to reduce the photoelectric conversion error due to temperature field change and the light brightness value acquisition error due to temperature drift,respectively,and to improve the control accuracy of the optical film thickness monitoring system.Based on the Fresnel formula and the combined optical conductance of the dielectric film substrate,the reflectance formula applicable to the optical thickness monitoring system is derived using algorithms such as topological analysis and variable optimization.A phase-locked voltage-reflectivity coefficient introduction method is proposed to fit by varying the values of the constant parameters,and a Savitzky-Golay fitting method combined with the least-squares method is used to realize the fitting and filtering processing of the actual acquisition curve,which improves the data acquisition accuracy to more than 9.0×10^-7.The control accuracy of the developed optical thin film monitoring system was verified by the preparation of infrared narrow band filter film.The IR narrow-band filter film was prepared by combining the"e"electron gun and Hall ion source with the Opti Layers thin film design software and physical vapor deposition method.The sample was tested by Fourier spectroscopy,and the peak transmittance of the IR narrowband filter film was 92%at 3.2μm with a bandwidth of 45 nm,and the control error was less than 0.015%,which realized the accurate control of the thickness of the mid-infrared film.