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10.1 Introduction
Many infrared components are manufactured by diamond turning. For that reason some remarks will be included in this text. These remarks are intended as guidelines for the systems design phase. Understanding the process provides the designer with the means to investigate whether lens elements can be eliminated or system performance can be improved. At the same time, an awareness of the process limitation can save one from disappointments and undesired surprises.
Diamond turning is an ultraprecision machining process especially suitable for generating aspheric and diffractive surfaces on most infrared lens materials such as germanium, zinc sulfide, zinc selenide, silicon, gallium arsenide, calcium fluoride, arsenic trisulfide, Amtir, and some chalcogenide glasses. Diamond turning is also the preferred method for producing high-precision reflective surfaces on metals. Heat-treated aluminum alloy 6061 is a favored substrate material for such mirrors. Since aluminum is relatively soft, the preshaped substrate is nickel plated prior to the final cut. The harder nickel surface also permits postpolishing if the required surface finish cannot be achieved by diamond turning.
10.2 Overview
Flat and spherical surfaces as well as cylinders have been generated by single-point diamond turning more than a half century ago. The technology was used primarily to produce precision mechanical components. To achieve the accuracy and precision demanded for optical surfaces, better control techniques and high-precision bearings were required. When laser distance-measuring interferometers and air bearing spindles were developed in the early 1970s, diamond turning became an acceptable process for generating a wide variety of rotationally symmetric surfaces such as conic sections and other aspheres.
Over the past decades, progress continued and besides the advancements made in better controls and improved general machine features, diamond-turning machines have become more affordable. While there were only a few organizations and institutions in the early days that had such machines, today, many smaller optical companies consider these machines standard equipment. Figure 10.1 shows a modern two-axis contouring diamond-turning machine.
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