Objective Diffractive optical elements are more and more widely used in infrared optical system, which requires higher processing quality of diffractive structure. High-precision diffractive microstructure surface can be machined directly by single point diamond turning. However, the position error and surface quality of diffraction structure have great influence on its optical properties. The diffraction efficiency of diffractive optical element is mainly affected by surface profile quality and surface roughness. The surface profile error and roughness error will produce shadow and scattering effect, which will reduce the diffraction efficiency of diffractive optical element. In order to improve the performance of diffractive optical components, there have been many researches on improving the turning quality of diffractive optical components, but the influence of the surface morphology of the diffraction plane has been ignored. In order to improve the performance of diffractive optical elements, it is necessary to control their turning errors accurately.
Methods The factors affecting the surface quality of the diffraction element in the process of SPDT processing were analyzed. On this basis, a mathematical model among the ring position error, diffraction surface shape and tool radius was established (Fig.1), which was used to simulate and calculate the relationship between the size of the machining residue area and the turning surface roughness and tool radius. The selection of machining parameters of diffraction optical element turning is guided. Combined with the simulation model and roughness influence parameters, the selection of turning tool radius is guided. It provides technical support for obtaining the high-precision surface topography of diffractive optical elements and is beneficial to improve the imaging quality of diffractive optical elements. Then, based on the simulation results, the machining capability of SPDT on the diffraction surface and the validity of the simulation model are verified by experiments, so as to provide technical support for the high-precision mass production of diffraction optical components.
Results and Discussions Finally, based on the simulation results, a semi arc tool with a radius of 0.02 mm is selected for machining. The shape error of diffraction element is 292 nm (Fig.6), the maximum position error of diffraction band is 55 nm, the maximum height error is 16 nm (Fig.7), and the roughness is 5.6 nm (Fig.9). Experimental results indicate that the prediction model can guide the acquisition of high precision surface topography of diffractive optical elements, which is beneficial to improve the imaging quality of the optical system. The research results provide technical support for the development of high-precision diffractive optical elements and have a wide range of engineering applications.
Conclusions Based on the error analysis of single point diamond turning processing diffraction structure, the diffraction element machined by semicircular arc tool has higher machining accuracy and position accuracy. Before the diffraction element is processed, the proper tool radius is selected by using the simulation model of the position error of the annular band of the diffraction structure with a semicircular tool combined with the influence factors of surface roughness. High-precision diffractive optical element can be obtained by controlling the shape of the base plane. According to the design of the diffraction structure plane shape and simulation model, the semi-circular diamond tool with the tool radius of 0.02 mm was selected for turning. The surface shape error of the diffraction element was 292 nm, the position error of the diffraction ring was less than 55 nm, the height error was less than 16 nm, and the roughness was 5.6 nm. The experimental results show that the prediction model can guide the acquisition of high-precision surface topography of diffractive optical elements and improve the imaging quality of diffractive optical elements. The results provide technical support for the development of high-precision diffractive optical elements and have a wide range of engineering applications.
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