System alignment advantages of diamond- machined optics

Abstract An important feature of the diamond-machining process is the ability to accomplish both high-precision machining and optical finishing on the same fixture and to like tolerances. Thus highly accurate mechanical relationships can be built into an optical component which, while increasing part and fixturing complexity, provide a much simpler overall system design. It is this reduction in system complexity as it relates to system assembly and alignment, that provides the subject for this paper. Types of features that can be introduced, their typical accuracies and examples of their use will be presented.IntroductionPrecision diamond-machining (also known as diamond-turning) of optical surfaces results from the combination of ultra-smooth mechanical motions of both tool and work piece and the near-ideal cutting properties of the natural diamond cutting tool. The former is achieved by air-bearing and/or precision-finished slide configurations and low vibration air-bearing spindle designs operating in the low micro-inch TIR runout regime. The latter provides a near-ideal tool cutting plane resulting from diamond's inherent physical properties: high directional hardness, high thermal conductivity, low coefficient of thermal expansion and low compressibility. When assembled into a computerized numerically controlled (CNC) lathe configuration, these features permit machined finishing of optical surfaces to accuracies of fractions of a visible wave and demonstrated diffraction-limited infrared system perfor­ mance. 1A significant feature of the process is that ultra-precise machining and optical finish­ ing can both be accomplished in the same fixture and, more importantly, to the same order of tolerance levels. Thus highly accurate mechanical relationships can be built into an optical component which while increasing part and fixturing complexity, provide a much sim­ pler overall system design. It is this reduction in complexity from the system standpoint, particularly as it relates to system assembly and alignment, that provides the subject for this paper. Types of features that can be introduced, their typical accurances and examples of their use will be presented.The features of inherent mechanical accuracy which can be built into a component using precision diamond-machining can be placed into two general categories : accurate dimension­ ing of an optical surface with respect to an axis and accurate referencing to a mechanical or other optical surface. It should be emphasized at this point that these are not features which for the most part, are unattainable by conventional fabrication techniques but which by such approaches are tedious, highly iterative in nature and therefore costly. However, once appropriate and readily achievable accuracies are built into the fabrication fixturing, precision diamond-machining permits these features to be attained more readily in prototype quantities and at significantly lower cost in high volume production.Axial RelationshipsMechanical relationships of optical surfaces to a symmetry axis can be most readily con­ ceptualized for cases where the surfaces are either parallel or perpendicular to that axis. (See Figure I). A typical example of the former involves the polygonal scanning mirror. Absolute dimension of each facet with respect to the spin axis of the mirror can typically be held to within 500 y-in. (0.5 mil). Pyramid angle with respect to the axis can generally be achieved to within 2 to 4 arc seconds depending on facet number, and size; non-cumulative facet-to-facet angle accuracies have been demonstrated down to 4 arc seconds. The case of perpendicular relationship to an axis can be considered for optical reference flats. These can readily be held normal to the rotational axis to within 2 arc seconds.In the case of more complex optical surfaces, a significant advantage is achievable for alignment purposes from axis-identification of off-axis aspheric elements. (See Figure II). For the typically sized infrared system, centering accuracies of the aspheric surface with respect to a diamond-machined interface or a precision pinning arrangment can be held to within 50 y-in. TIR. Greater ease of system alignment can be achieved for those asymmetric elements extending across the optic axis so that the surface machining marks, concentric to 5 la-in. , can be visually sighted with an alignment telescope to identify the optical axis.