Design,Fabrication And Application Of Linearly Variable Optical Filter

In recent years,with the rapid development of science and technology,portable spectrometers of small size and light weight have attracted more and more attention.In spectrometer,spectroscopical element such as prism and diffraction grating plays an important role to obtain quasi-monochromatic light from a wide band light source.To select a particular wavelength,mechanical displacement units are always needed to drive the prism or grating.The spectral accuracy of the spectrometer will deteriorate because of vibration and abrasion of the mechanical displacement parts.In addition,due to the large size and weight of the components,miniaturization and portability of spectrometer are difficult.The above difficulties can be overcome by using the filter technology,which uses the linear variable optical filter（LVOF）instead of the traditional optical splitter.On the one hand,the filter technology can greatly reduce the spectral resolution dependence of the device on the numerical aperture of the optical system.On the other hand,single-chip microspectrometers can be realized by preparing filters directly on photodetectors using technologies compatible with integrated circuits.Based on the above research background,the design,preparation and application of LVOF were deeply investigated in this paper.The methods to improve the performance of LVOF were particularly studied.Firstly,the materials used to prepare LVOF and the method of measuring the optical constants of the materials are introduced in detail.Based on the measured optical constants,both the all-dielectric type and induced-transmission LVOF were designed and the effects of structure parameters on their performance were studied.For the all-dielectric type LVOF,the influences of layer numbers,and cavity numbers were studied.For the induced transmission LVOF,a wide band LVOF covering 400-1000 nm with linearly varied thickness of Ag layer was designed.The LVOF sample was prepared by magnetron sputtering and a home-made wedge-shaped film preparation sample stage.The film system was（98 nm Si O₂ 61 nm Ti O₂）³ x Si O₂（61 nm Ti O₂98 nm Si O₂）³.The transmission spectrum was measured by a spectrophotometer in combining with a linear gradient filter testing auxiliary device designed and built by ourselves.The effects of annealing temperature and duration on transmission spectrum of the samples were studied.The working spectral range of the prepared sample is 510-700 nm,and the peak value of transmission peak at different positions of the sample shows a good linear relationship with the position of the sample.After one hour of vacuum annealing at 300°C,the transmittance of the sample is significantly improved,which is better than 60%,indicating that annealing can significantly improve the performance of the linear gradient filter.The fabricated LVOF was used as a spectroscopical element in a micro spectrometer system.The LVOF and linear CCD sensor array are integrated together and a light detection module is designed.After calibration,the emission spectra of a fluorescent glass were measured.The results can promote the development of LVOF-based micro-spectrometer and lay a research foundation for its prototype.