Algorithms for finely adjusting etch depths to improve the diffraction efficiency uniformity of large-aperture BSG

Abstract

Beam sampling gratings (BSGs) employed in high-power laser systems usually have large aperture so that the adequate uniformity of diffraction efficiency is difficult to obtain. We proposed a deterministic method using controllable non-uniform etch to improve the efficiency uniformity of large-aperture BSGs. During the ion beam etching (IBE) process, etch depths are finely adjusted by the dynamic leaf. The motion trajectory of the dynamic leaf is calculated using the fine adjustment algorithm. Simulations are conducted on the basis of a typical example. The simulation predictions show that the cumulative error is 0.067 nm and about 99.1% of depth differences are in the range of the required etch depth tolerance, which suggests that the diffraction efficiency uniformity of BSG is expected to be effectively improved and thus can meet the requirement of a RMS of 5%. As a cost-effective solution, it also has a broad prospect in many optical fabrication fields, especially for the fabrication of large optics.

Variable-spot ion beam figuring

“…In our previous work [18], we first found that there exists a correspondence relationship between the trapezoidal velocity profile of the driving motor and the S-shaped etching time profile along the scan path. The shape of an etching time profile can be adjusted by tuning control parameters of the corresponding trapezoidal velocity profile, which is the key point of S-curve transformation.…”

Fine-tuning the etch depth profile via dynamic shielding of ion beam

“…The shielding time of a point herein refers to the total elapsed time the point is shielded by the dynamic leaf. For clarity, the term, dwell time used in the previous paper [4], is renamed as shielding time hereafter. To calculate the shielding time, we should map the shielding rate profiles onto the spatial The Monte Carlo simulation [7,8], as is well known, is a general approach to analyze ion scattering from the leaf edge and its effect on etch depths.…”

“…As a state-of-art technique, IBE plays a crucial role in the ultra-precision fabrication of large optics such as X-ray mirrors [1], vacuum ultraviolet gratings [2], and beam sampling gratings (BSGs) [3,4]. Collimators or shutters are usually used to adjust the etch rate profile [1,5] in order to meet the desired Email addresses: wulx@mail.ustc.edu.cn (Lixiang Wu), blueleaf@ustc.edu.cn (Keqiang Qiu), sjfu@ustc.edu.cn (Shaojun Fu) Preprint submitted to arXiv.org September 11, 2018 arXiv:1512.09237v1 [physics.ins-det] 31 Dec 2015 etch depth map.…”

“…Hence, in our previous work [4,6], we proposed a new solution for fabricating the large-aperture BSG of high diffraction uniformity as well as algorithms for finely tuning the spatial distribution of etch depths.…”

“…In the fabrication of BSGs, we also encounter a similar scenario and need to adjust the spatial distribution of etch depths. But in contrast to the surfacing of X-ray mirrors, the adjustment of etch depths in the fabrication of BSGs is, to some extend, more difficult and requires more flexibility [3,4,6]. On one hand, the etch depths are almost randomly distributed.…”