Illumination non-uniformity caused by the multiple total internal reflection has become an important factor restricting the development of grating- based AR devices. Most of the current solutions can effectively modulate the distribution of diffraction efficiency (DE) of different diffraction orders by adjusting the groove parameters of the diffraction grating, so as to optimize the exit pupil uniformity of the diffraction grating waveguide. In this paper, an out-coupling grating with variable duty cycle is fabricated by a new way to improve the performance of diffraction grating waveguides. The ashing process is chosen to modulate the duty cycle, which can chemically react with the photoresist without damaging the grating substrate. Based on the relationship between the ashing time and the shrinking rate of the duty cycle, the grating coupler can be fabricated to meet the optimized results. The diffraction efficiency of the gratings is tested. The results show that the method we proposed is reliable and effective to produce continuously variable duty cycle gratings. The diffraction efficiency of the grating is gradually improved along the direction of exit pupil expansion, which is beneficial to the display device to achieve more uniform output brightness.
We report on high-efficiency visible and near-infrared transmission gratings in fused silica generated by holographic recording and reactive ion beam etching technology. At a wavelength of 740 nm, near 100% diffraction efficiency is achieved under Littrow conditions. The design is based on the phenomenon of the high aspect ratio gratings by using the rigorous coupled wave analysis. A binary grating with the optimum grating period of 740 nm and groove depth of 1.55 had been fabricated in the paper. The grating wavelength bandwidth and angular bandwidth are extremely enhanced compared with conventional volume phase holographic gratings, making these gratings the key elements in high-resolution astronomical ground-based telescope spectrographs.
In this article, we designed and fabricated a high linear density concave variable line space holographic grating for the Lyman spectrometer, with level 3 working and the center line density is 3300line/mm. The focus curve is a circle with the grating vertex O as the center and a radius of 900 mm. We analyze the influence of the exposure error on the line density distribution and reduce the harm of errors through the method of multi-error compensation. To improve the diffraction efficiency, we use Finite Element Method software to get the best grating groove parameters. Finally, we initially prepared a holographic grating with a symmetrical arch groove with a groove depth of 175nm and a bottom duty cycle of 0.3.
As an essential component of the slitless spectrometer, the UV transmission blazed grating has the capability of high dispersion and high resolution. In this paper, a method for fabricating UV transmission blazed gratings by holography-ion beam etching is proposed. Holographic interference lithography is used to generate photoresist grating masks. The ion beam vertical etching transfers the photoresist mask pattern to the substrate to form a SiO2 grating mask. When the ion beam incident direction is at a certain angle to the normal direction of the substrate, the SiO2 mask is used to block the inclined ion beam, so that different parts of the mask bottom are bombarded by the ion beam with different fluxes, forming a blazing facet. When the mask is etched completely, the blazed grating is formed. Based on the idea of the line motion algorithm, the article establishes the geometric model of blazed grating etching, which provides the parameter guidance for precise control of the groove structure. Combined with the theoretical model, a UV transmission blazed grating with a line density of 333 lines/mm and a blazing angle of 13.2° is successfully fabricated.
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