Miniaturised optical encoder for ultra precision metrology systems

We report here on a novel high resolution optical encoder, which can be employed within ultra precision metrology systems. Microsystems techniques have been used to monolithically integrate the discrete components of an encoder onto a single compound semiconductor chip, thereby radically reducing the footprint and cost of assembly. Gratings manufactured at the wafer level by standard photo-lithography, allow for the simultaneous alignment of many devices in a single process step. This development coupled with a unique photodiode configuration not only significantly increases performance but also improves the alignment tolerances in both manufacture and set-up, which substantially reduces the required set-up time. The optical encoder chip has been successfully demonstrated under test conditions on a 4 μm pitch scale: alignment tolerances have been increased with DC to AC ratios of the order of 7:1 and signal-to-noise ratios greater than 40:1.