Ultra-Precision Cutting and Characterization of Reflective Convex Spherical Blazed Grating Elements

Abstract

In this work, based on the diffraction principle of reflective blazed grating, the structure size of the convex spherical blazed grating unit is determined, the machining accuracy of the convex spherical blazed grating is formulated, the effects of tool nose radius and Poisson burr on the diffraction efficiency of the convex spherical blazed grating are analyzed, and the performances of cutting convex gratings with microcrystalline aluminum RSA6061 and RSA6061+ chemically plated NiP for two workpiece materials are compared. A convex spherical blazed grating with a radius of curvature R = 41.104 mm, substrate diameter 14 mm, grating density 53.97 line/mm, and blaze angle of roughly 3.8° is turned by a four-axis ultra-precision machining system by adjustment of the cutting tool, workpiece material, and cutting parameters, as well as modification of the layouts of the blazed grating on the convex sphere. The results of the testing of convex spherical blazed grating elements in both layouts show that the size error of the grating period is close for both layouts, the size error of grating height is smaller in the equal-along-arc layout, the blaze angle error in the equal-along-projection layout is only 0.74%, and the average roughness of the blazed surface is less than 5 nm to meet the processing quality requirements of the reflective convex spherical blazed grating. The greater the blaze angle accuracy of the blazed grating, the higher its diffraction efficiency, so the grating element with an equal-along-projection layout has a higher diffraction efficiency than the grating element with an equal-along-arc layout. RSA6061+ chemically plated NiP material is superior to RSA6061 material in Poisson burr height and blazed surface roughness, which is more suitable for Offner-type imaging spectrometers in the spectral range 0.95–2.5 μm (SWIR).

Miniaturized two-channel broadband spectrometer based on variable-spacing concave blazed gratings

“…The grating was used in the Offner configuration covering a spectral range from 500 to 1100 nm. In another work, Li et al 36 diamond tooled a convex spherical blazed grating with 54 lines∕mm grating density. According to the reported simulations, this grating can provide an efficiency higher than 50% in the spectral range from 1650 to 2500 nm.…”

“…The grating was used in the Offner configuration covering a spectral range from 500 to 1100 nm. In another work, Li et al 36 . diamond tooled a convex spherical blazed grating with 54 lines/mm grating density.…”

Spatially resolved scattering metrology to quantify losses induced by contamination and defects

“…The grooves are cut in Electroless Nickel plated (NiP) Aluminum alloy workpieces. NiP is recognized as the material providing best grooving results due to its higher hardness, limiting risks of Poisson burrs formation on the edge of the grooves caused by plastic flow of the material during the shaping process [6][10] [11].…”

Progress in the Preparation and Characterization of Convex Blazed Gratings for Hyper-Spectral Imaging Spectrometer: A Review

“…In a comparison of freeform gratings machined on RSA 6061 and RSA 443 substrates chemically coated with NiP surfaces with an average roughness of 2.5 nm RMS per blazed surface, Bourgenot C et al [ 68 ] discussed new opportunities for freeform gratings machined with diamond and identified NiP as the substrate material with the best roughness and profile quality. Li H et al [ 69 ] used a four-axis ultra-precision machining system for turning a convex spherical blazed grating with a curvature radius of R = 41.104 mm, a substrate diameter of 14 mm, a grating density of 53.97 lines/mm, and a blaze angle of about 3.8°. The blazed surface roughness is better than Ra2 nm.…”