Design and analysis of a micro-optical speckle displacement sensor

The presented displacement sensor is based on the interference of a reference beam and the back-scattered speckle light from the moving surface under inspection. In contrast to incremental sensors this system works on nearly arbitrary surfaces without any special patterns. The design goal is a micro-optical system assembled with injection-molded plastic components and replicated gratings manufactured with lithographic technologies. The system design starts with ray tracing for the layout of the laser diode module, the beam splitter grating and the refocusing optics. After this geometric optics first order design the profile of the buried reflection beam splitter grating is optimized with special diffractive optics design tools to achieve maximum efficiency in the required diffraction orders. The evaluation of the design is carried out mainly under two aspects: First part of analysis deals with investigation of aberrations and system tolerancing by ray tracing. The second part concentrates on wave-optical modeling of the changing interference signals, caused by the movement of the scattering surface.