Design And Fabrication Of Volume Phase Holographic Grating Used In Near Infrared Wavebands

In recent years, with the intensification of greenhouse effect, more and more countries start to use the infrared absorption method to monitor the concentration of greenhouse gases, for its wide measuring range, high sensitivity, high accuracy, short response time,good selectivity and strong anti-interference ability. In order to measure the concentrations of different greenhouse gases, multiple dielectric multilayer film filters are used to filter the absorption spectra bands of different greenhouse gases, traditionally. To achieve high efficiency, it requires the filter to be coated with dozens or even hundreds of film layers.However, the more film layers are coated, the more complicated the manufacture process are. Also the defective rate and price will increase. Instead, using multiple volume phase holographic grating(VPHG) recorded in dichromated gelatin(DCG) can significantly decrease the complexity of manufacture process. Only tens of microns thick layer are needed to achieve ideal diffraction efficiency. Several VPHGs can be used as a filter for beam splitter which has many advantages, such as high optical density, narrow bandwidth,steep band edge and high optical through output, compared with multiple dielectric multilayer film filters. So it is important to design and fabricate high efficiency VPHG that can be used in near-infrared wavebands.In this paper, the background of VPHG is introduced. The theory of VPHG is stated briefly, including Kogelnik’ coupled wave theory and rigorous coupled wave analysis(RCWA). A novel kind of beam splitter which is consisted of two reflection VPHGs as core components and used in near-infrared waveband is proposed. The design idea of the device is described in detail. As an example, two wavebands of methane and carbon dioxide(1.574μm-1.617μm and 1.636μm-1.682μm, respectively) that need to be separated in nearinfrared spectrum region are taken into account. The parameters of two reflection VPHGs are determined and optimized using Kogelnik’ coupled wave theory and rigorous coupled wave analysis. DCG is used as holographic recording media to fabricate reflection VPHGs.Measuring setup is built up to test the diffraction performance of two reflection VPHGs,and the experiment results are compared with theoretical calculations. The experiment results verify the feasibility of this kind of beam splitter, which is prospected to be used in spectrometers for greenhouse gases monitoring.