Abstract

Spatial frequency response as predicted by Fraunhofer optics is not the ultimate optical resolution limit. Very near-field diffraction fields, within even Fresnel zones, offer the means of exceeding the Rayleigh limit by an order of magnitude or more using no imaging optics but placing and maintaining sending and receiving elements in very near-field zones. Optical diffraction in the very near-field zone is discussed, and successfully operating the 18-bit absolute optical encoder working at a 0.65-μm wavelength light which easily resolves a 0.1-μm displacement is described in detail. The LSB length on the 3.28-in. Gray codes track is in fact 1 μm and thus requires at least 0.25-μm resolution, which is beyond the capability of even high numerical aperture optics.

© 1987 Optical Society of America