Ultra-precision rotary scratching of concave blazed gratings

Abstract

Concave blazed gratings serve a fundamental role in miniaturized spectrometers, fulfilling the duties of both a plane grating and focusing mirror. However, common fabrication methods are often limited in terms of efficiency, precision or the range of machinable materials. To address the issue, the ultra-precision rotary scratching method for generating the complex structures is suggested. A continuous and smooth tool path is utilized to preclude acceleration movements across various machine axes during fabrication. A diamond tool with the blaze angle incorporated into the tool geometry is employed. To avoid tool interference when fabricating concave structures, the real-time position of points where the tool may interfere with the processed surface is calculated. The interference checks are integrated into the tool path generation process. The analysis of four potential error sources in path generation is conducted and corresponding error modeling is performed. The influence of errors on the form and position of the path is investigated and compensations are made accordingly. This method offers high flexibility, efficiency, and precision in machining concave gratings. Successful fabrication of concave blaze gratings with 1.8° blazed angle, 1/3 mm grating pitch and 49.375 mm radius of the base sphere is achieved, and the surface roughness (Sa) of top facets reaches 9 nm, accompanied by a blaze angle error of 0.004° and a period error of 0.1984 %.