Grating-based lightguide with variable duty cycle designed and fabricated for AR head-mounted displays

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.