With the rapid development of augmented reality technology, people can clearly see the superposition of virtual and real world images at the same time. The diffractive optical waveguide has obvious advantages over the geometric optical waveguide because of its high diffraction efficiency, light weight and difficulty in generating ghost images. Therefore, the use of holographic gratings as coupling elements for planar waveguides has been widely used in head-mounted display systems. Because high efficiency grating is required as coupling element in coupling, we use slanted trapezoidal surface relief grating as coupling element of the planar waveguide. In this paper, a slanted trapezoidal surface relief grating has been designed as a high-efficiency coupling element with a grating period of 520 nm and a material of BaK3 glass with a refractive index of 1.54. By optimizing the structure of the slanted grating, the relationship between the groove parameters and the diffraction efficiency of the slanted rectangular grating and the slanted trapezoidal grating is analyzed in detail. The results show that under the normal incidence of light at 630 nm, the groove depth is 450 nm, and the slant angle θ1 is 40°, when the -1 order diffraction efficiency of the TE polarized light is higher than 80%, the ranges of the slant angle θ2 and of the maximum diffraction efficiency value would be obtained. This can greatly improve the coupling efficiency of the holographic planar waveguide.
Nowadays, the monitoring of CO2 concentration has gradually become the focus of scientists all over the world. In order to study the effects of CO2 on climate change and global ecosystems, hyperspectral and high spatial resolution CO2 detectors are necessary. Holographic grating is the core element of that CO2 detectors, and the immersion holographic grating can greatly improve the resolution of grating and reduce the volume of spectrometer. Therefore, it is necessary to take research on immersion grating. In this paper, we have designed and optimized a quartz immersion grating used in the CO2 detectors. We have designed and optimized the parameters of the grating, such as the width ratio and groove depth, according to the requirements of the spectrometer used and the actual fabrication errors, we designed and optimized rectangular and trapezoidal groove with different bottom angles to obtain high efficiency and low polarization-dependent immersion gratings. In the 2.04-2.08 μm band, with rectangular groove, the groove depth of the quartz immersion grating is 950 nm, the duty cycle is 0.7, the -1 order diffraction efficiency is over 82%, and the degree of polarization is below 12%. When the groove is trapezoidal and the bottom angle is 85 degrees, the -1 order diffraction efficiency is over 79% and the degree of polarization is below 10% at duty cycle of 0.63 and groove depth of 1050 nm. Then under the trapezoidal groove with a bottom angle of 80 degrees, when the duty cycle is 0.62 and the groove depth is 1100 nm, the -1 order diffraction efficiency is over 81% and the degree of polarization is less than 9%. Finally, we will fabricate a sample of immersion grating with a period of 1117 nm on a quartz substrate by holographic ion beam etching in late 2019.
The depolarization grating is needed particularly for use in imaging spectrometers used in sensing the atmosphere weak CO2 spectral band (1595nm - 1625 nm) at spectral resolution in the order of 0.1 nm whilst ensuring a high efficiency for both TE and TM polarizations. The diffraction characteristics of the depolarization grating have been investigated by using rigorous coupled wave theory. The simulation results show: the groove depth and duty cycle of the depolarization grating must be controlled within the range of 730nm-780nm and 0.3-0.37 respectively, in order to guarantee the 1 order diffraction efficiency is over 70% for both TE and TM polarizations at the atmosphere weak CO2 spectral band. The depolarization grating with the period of 869nm in a fused-silica substrate of 120 mm × 97mm will be fabricated by holographic lithography - ion beam etching in late 2018.
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