Optical displacement detection using multi-level diffraction gratings having no zeroth-order reflection

Displacement detection using optical interferometric techniques allows for low minimum detectable displacements which are unmatched by other displacement measurement methods. However, diffraction gratings commonly used in such applications can generate zeroth-order reflected beams which result in reduced sensor performance, packaging limitations, and laser instability. A new grating concept has been fabricated and tested which has the effect of eliminating the zeroth-order component by imparting a half-wavelength phase shift to a portion of the reflected light. The design criteria for zeroth-order beam elimination is illustrated using a simple model based on phasor arithmetic. Fabrication methods for producing these optical gratings, as well as comparisons between experimental test results and models, will be reported.