Ultra-precision manufacturing plays a crucial role in supporting the successful development of many high-tech fields, affecting profoundly the process of human society. Blazed gratings with periodically inclined structures exhibit satisfactory characteristics concentrating most of diffracted light to a single nonzero order. In addition to the well-known high diffraction efficiency, blazed gratings can also greatly improve the sensitivity, resolution and measurement range of grating measurement system, and have become core functional components to improve the level of ultra-precision manufacturing. Global efforts from the scientific and industrial circles have been devoted to the manufacture of blazed gratings. This review describes the physical principles of blazed gratings, specific approaches and the achievements of typical processing methods, including mechanical ruling, holographic ion beam etching, electron beam lithography, wet etching, etc. The strength and weakness of these fabricating methods were addressed, and prospective roads were proposed in order to improve the manufacturing of blazed gratings.

https://iopscience.iop.org/article/10.1088/1361-6463/abfd6a/pdf

A Review on Fabrication of Blazed Gratings

The convex reflective diffraction grating is an essential optical component in Offner systems, which has been widely used in imaging spectrometers. We propose a new design and optimization method for the convex blazed grating in the Offner imaging spectrometer. The method integrates the macro- and microdesign of the optical system, and it can be used to design and optimize the convex blazed grating with high diffraction efficiency. Traditional geometric optics theory and image quality evaluation methods are used to design the macro-optical structure parameters of the Offner system. And then the incident ray information, such as the incident angle and the polarization states are calculated by using the three-dimensional polarization ray-tracing method. To improve the diffraction efficiency, we combine rigorous coupled wave analysis and a particle swarm optimization algorithm to optimize the microstructure parameters of the convex-blazed grating. Further, a convex-blazed grating in a mid-wave infrared Offner imaging spectrometer is designed as an example to illustrate our design method in detail. The design results indicate that the Offner imaging spectrometer has good imaging quality, and the average diffraction efficiency of the -1st diffraction order of the convex-blazed grating in the spectral coverage 3-5 µm is 82.24%. Compared to the traditional design method, the lowest spectral diffraction efficiency is improved from 59.88% to 69.24%, the highest spectral diffraction efficiency is improved from 90.45% to 91.84%, and the standard deviation is reduced from 7.82 to 6.62.

Design and optimization method of a convex blazed grating in the Offner imaging spectrometer.

The study of spectral signatures of various elements has been enabled with the ability to diffract light using optical gratings, coupled with the ability to control light using lenses and mirrors. As technological devices seek to become more compact while maintaining or increasing operational efficiency, integration of functions becomes necessary to reduce the overall component count. However, due to the lack of axial flexibility, integrated microstructures on complex forms have shown to be challenging to manufacture. Thus, the need for fabrication techniques to be able to produce such microstructures on non-planar surfaces arises. The Continuous Rotating Freeform Shaping (CRFS) algorithm proposed in this paper utilizes the axial flexibility and precision of Ultra-Precision Machining (UPM) to directly fabricate optical quality blazed gratings on complex profiles which can directly be used as mirrors or molds. The developed algorithm not only maintains the tool along the primary profile of the optical surface, but also the rake angle of the tool at every point along the surface for a constant cutting mechanism. This is coupled with a rotation of the tool to maintain the blazed angle across the surface, optimizing the surface for the efficiency of the grating for a specific wavelength. In this paper, the developed algorithm is presented with experimental verifications of the generation of blazed gratings on a high curvature spherical surface and a freeform tri-axial ellipsoidal surface, followed by the corresponding analysis.

Diamond shaping of blazed gratings on freeform surfaces

Convex blazed gratings, which can effectively broaden the spectral range and improve spectral resolution, have gradually evolved into a crucial optical component for lightweight and compact imaging spectroscopy instruments. Their design, processing, and testing involve multidisciplinary interdisciplinary scientific issues, and they continue to be a major area of research in imaging optics applications. This paper summarizes the effects of various grating groove shapes and structural parameters on the spectral range and diffraction efficiency of convex blazed gratings, after providing a brief introduction to the typical functions and applications of convex blazed gratings. Firstly, the latest progress in typical processing methods for convex blazed gratings is reviewed. It focuses on the current fabrication processes and reviews their capabilities in creating convex blazed gratings from three main types of technologies, namely ultra-precision machining, high-energy density beam processing, and chemically assisted fabrication processes. Secondly, the adaptability of the manufacturing process for convex blazed gratings on different scales is summarized, analyzing the adaptation of current procedures to various grating fabrication scales and their bottlenecks. Finally, the characterization methods and future feasible characterization methods for convex blazed gratings are reviewed. The development trend of efficient and precise preparation of convex blazed gratings is pointed out.

/pdf/progress-in-the-preparation-and-characterization-of-convex-3exc0ug3.pdf

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

Abstract We present an azimuthal-rotation-controlled dynamic nanoinscribing (ARC-DNI) process for continuous and scalable fabrication of asymmetric nanograting structures with tunable periods and shape profiles. A sliced edge of a nanograting mold, which typically has a rectangular grating profile, slides over a polymeric substrate to induce its burr-free plastic deformation into a linear nanopattern. During this continuous nanoinscribing process, the “azimuthal angle,” that is, the angle between the moving direction of the polymeric substrate and the mold’s grating line orientation, can be controlled to tailor the period, geometrical shape, and profile of the inscribed nanopatterns. By modulating the azimuthal angle, along with other important ARC-DNI parameters such as temperature, force, and inscribing speed, we demonstrate that the mold-opening profile and temperature- and time-dependent viscoelastic polymer reflow can be controlled to fabricate asymmetric, blazed, and slanted nanogratings that have diverse geometrical profiles such as trapezoidal, triangular, and parallelogrammatic. Finally, period- and profile-tunable ARC-DNI can be utilized for the practical fabrication of diverse optical devices, as is exemplified by asymmetric diffractive optical elements in this study. 

Azimuthal rotation-controlled nanoinscribing for continuous patterning of period- and shape-tunable asymmetric nanogratings

A swing ion-beam etching method to fabricate convex blazed gratings used in shortwave infrared hyperspectral imaging spectrometers is presented. This method solves the consistency problem of blaze angles by swing etching through the meridian direction of the gratings. The mathematical relationship of the curvature, aperture, and diffraction efficiency of convex gratings is studied to demonstrate the limitation of conventional translational lithography and the necessity of swing etching. A geometric model is built to analyze the influence of swinging speed and beam slit width on groove evolution. Convex gratings with a 45.5 gr/mm groove density, 67 mm aperture, 156.88 mm radius of curvature, and 2.2° blaze angle have been fabricated and measured where the peak and average diffraction efficiency in the shortwave infrared band reach 90% and 70%, respectively. Experimental results validate that high-efficiency convex gratings of small blaze angle and high groove consistency can be produced by swing etching, which satisfy the requirements for high spectral resolution and miniaturization of imaging spectrometers.

Convex blazed grating of high diffraction efficiency fabricated by swing ion-beam etching method.

The influence of the removal depth of a silicon modification layer on grating structures and mirrors is studied. The removal depth 6-14 μm is the optimization result for Si-modified reaction-sintered silicon carbide (RS-SiC) used as mirror substrates, but the removal depth 9-12 μm is the optimization result for Si-modified RS-SiC used as grating substrates. The diffraction efficiency and stray light of the gratings fabricated in the Si-modified RS-SiC substrates with removal depth 9-12 μm is 90.5%-94% and 5.30×10-7-5.45×10-7, respectively. Additionally, the number and scale of high-reflection points can be used as the basis for judging the removal depth of the Si-modified RS-SiC used as grating substrates. These results and the regularity have guiding significance for the application of Si-modified RS-SiC as a microstructural substrate.

Study of influence of the removal depth of the silicon modification layer on grating structures in reaction-sintered silicon carbide substrate and improvement method.

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Convex blazed grating of high diffraction efficiency fabricated by swing ion-beam etching method.

Study of influence of the removal depth of the silicon modification layer on grating structures in reaction-sintered silicon carbide substrate and improvement method.