Fabrication of high-efficiency and low-stray-light grating by inductively coupled plasma(ICP) etching-polishing method

Fabrication of a large computer-generated hologram with high diffraction efficiency and high accuracy by scanning homogenization etching

The diffraction efficiency, defined as the ratio of diffracted power to incident power, is one of the key working indicators for a computer-generated hologram (CGH). The CGH with high diffraction efficiency could suppress stray light and eliminate ghost images, thus improving interferometric performance in aspherical testing of low-reflectivity or large off-axis distance surfaces. However, the high-efficiency CGH is hard to precisely fabricate by traditional reactive ion etching and focusing ion beam, because it requires high etching depth with a high uniformity and sub-nanometric roughness in the glass, especially in the fabrication of a large CGH with an aperture of up to 300 mm. In this study, fabrication of the above-mentioned CGH was demonstrated via what we believe to be a new method called scanning homogenization etching (SHE), in which the ion source with a Gaussian energy distribution accurately scans the glass surface to realize homogenization etching. Different from controlling dwell time at each etching point, this paper proposes to control the scanning rate to achieve not only uniform but also quantitative depth removal in a single scan. Moreover, the depth errors in deep etching across the whole glass surface can be remarkably reduced due to homogenization effects introduced by multiple scanning etching. Finally, the target etching depth of 692.3 nm with an etching uniformity of 2.2% in the etching of a 300 mm CGH was achieved. The roughness of the etched and unetched area both have Ra values of 0.3 nm. The diffraction efficiency of working order is 39.998%, achieving 98.6% of the theoretical diffraction efficiency. In addition, the SHE is not limited by the aperture of the ion source, so it can achieve even larger diffractive optical elements with high diffraction efficiency and high accuracy.

Low-stray-light gratings fabricated with scanning exposure method based on Lloyd's mirror for a high-contrast near-eye display in augmented reality

The stray light of gratings lowers the image contrast of augmented reality display devices based on lightguide gratings. We propose to reduce the stray light of gratings fabricated with the scanning exposure method in a Lloyd's mirror interferometer setup. The photoresist-coated substrate is moved in the longitudinal direction parallel to the exposure interference fringes during exposure to average out the laser speckle-induced noise. A phase locking module is designed to compensate for the unwanted lateral displacement caused by the straightness error and vibration of the translation stage. The stability and accuracy of phase locking in the Lloyd's mirror interferometer are analyzed with control system theory. Low-stray-light lightguide gratings were fabricated successfully. The stray light level was decreased by more than 50%, and the imaging contrast was increased from 65% to 85%.

Design of a High-Efficiency Multilayer Dielectric Diffraction Grating with Enhanced Laser Damage Threshold

Diffraction gratings are becoming increasingly widespread in optical applications, notably in lasers. This study presents the work on the characterization and evaluation of Multilayer Dielectric Diffraction Gratings (MDG) based on the finite element method using Comsol MultiPhysics software. The optimal multilayer dielectric diffraction grating structure using a rectangular three-layer structure consisting of an aluminum oxide Al₂O₃ layer sandwiched between two silicon dioxide SiO₂ layers on a multilayer dielectric mirror is simulated. Results show that this MDG for non-polarized lasers at 1064 nm with a significantly enhanced −1st diffraction efficiency of 97.4%, reaching 98.3% for transverse-electric (TE) polarization and 96.3% for transverse-magnetic (TM) polarization. This design is also preferable in terms of the laser damage threshold (LDT) because most of the maximum electric field is spread across the high LDT material SiO₂ for TE polarization and scattered outside the grating for TM polarization. This function allows the system to perform better and be more stable than normal diffraction grating under a high-intensity laser.