In augmented reality near-to-eye display devices based on grating waveguides, the uniformity of grating duty cycles has a great influence on the uniformity of virtual images. In holographic exposure, the non-uniform intensity distribution of the Gaussian beam results in a non-uniform exposure dose distribution and a non-uniform duty cycle distribution. In Lloyd's mirror interferometer which is widely used in grating waveguide fabrication, the non-uniformity problem is even worse due to the setup that the center of the beam is aligned with the edge of the substrate. The main approach to this problem is to expand the beam diameter, i.e., to use only the center part of the Gaussian beam which is more uniform, but this will reduce the overall intensity and increase the exposure time. Recently, the Gaussian beam in some studies is converted to a flat-top beam by a beam shaper to increase the duty cycle uniformity. However, the flat-top beam cannot be spatially filtered, and the high-frequency noise will cause the stray light of gratings and lower the image contrast of the grating waveguide. In this study, we propose an exposure system based on Lloyd's mirror interferometer to improve the duty cycle uniformity of holographic grating masks. The system utilizes the methods of moving baffles and changing exposure intensity to compensate for the non-uniform duty cycle distribution caused by the non-uniform intensity distribution of the Gaussian beam during exposure.

The exposure dose equals the integration of exposure intensity over time. Therefore, the modulation of both the exposure intensity and the exposure time can change the exposure dose distribution. The exposure intensity is adjusted by a rotating half-wave plate and a polarization beam splitter. The rotation angle of the half-wave plate is decided by the error between the pre-calculated target intensity and the real-time intensity detected by the photoelectric detector. The exposure time of each point on the substrate is modulated by a pair of moving baffles. The two baffles are placed and moved symmetrically to ensure that their shadows overlap completely, so as to keep a high exposure contrast. After the intensity distribution of the exposure beam is measured, numerical optimization is executed to obtain uniform exposure dose distribution. The objective function is the maximum exposure dose reduction percentage within the exposure area. The optimization variables are the space-varying shape of the baffles, the time-varying velocity of the baffles, and the time-varying exposure intensity. In Lloyd's mirror interferometer, a simplification is performed, and rectangular baffles and constant velocity of the baffles are used.

The non-uniformity of the exposure dose is reduced from 11.7% to 3.07% after optimization (Fig. 2). Two-dimensional grating waveguides with grating periods of 456 nm and 329 nm are successfully fabricated for red and blue color displays, respectively. At the microscopic level, an atomic force microscope is used to measure the duty cycle (Fig. 5). From position 1 to position 5 (farther from the center of the Gaussian beam in order), gratings without exposure dose modulation have a decreasing duty cycle distribution, which exceeds the target duty cycle range of 0.47-0.53. The range of duty cycle of gratings fabricated in this system is decreased by 66.7% and 78.3% compared with conventional Lloyd's mirror exposure (Table 1). At the macroscopic level, the fabricated grating-based waveguides are applied in augmented reality display tests. The imaging position non-uniformity is reduced by 38.3% and 31.7%, and the angular non-uniformity is reduced by 42.9% and 59.0% (Table 2). The solid color virtual images intuitively show the imaging uniformity improvement.

We have proposed a holographic exposure system with adjustable exposure dose distribution based on a Lloyd's mirror interferometer. The system moves baffles and rotates a half wave plate to modulate the exposure dose distribution across the substrate, so as to compensate for the non-uniform duty cycle distribution of the gratings. Grating waveguides are fabricated, and the imaging uniformity is tested to verify the system. The experimental results show that the duty cycle range is reduced by over 66% compared with conventional Lloyd's mirror exposure. The imaging uniformity is improved by over 30% when applying the fabricated grating-based waveguides to augmented reality display. Besides increasing the duty cycle uniformity, different shapes of baffles and different intensity modulation can result in different exposure dose distributions, which indicates the possibility of fabricating gratings with more complex duty cycle distributions. The method of moving baffles and rotating a half-wave plate to modulate the exposure dose distribution can also be adapted to a dual-beam interferometer.