Holographic Optical System For Aberration Corrections In Laser Doppler Velocimetry

Abstract

An optical system containing multi-faceted holographic optical elements (HOEs) was constructed to correct for aberrations introduced by a non-flat window in NASA Laser Doppler Velocimetry (LDV) experiments in an internal combustion engine. For this experiment, two green beams separated by 82 mm in a vertical plane, and two blue beams separated by 82 mm in a horizontal plane, are brought into coincident focus inside a thick walled sapphire cylinder. The focal power to bring these beams to a common focus and the aberration correction required by the presence of the cylinder were built into the holographic elements. A modular optical breadboard system was configured to fabricate, test, and use the custom made HOE's. Additional HOE's were also fabricated to provide aberration correction and color separation for the multispectral output waves emerging from the focal volume. Compensation for the birefringence of a thick sapphire cylindrical window in the experiments is also accomplished. A general system such as we have built is useful wherever high efficiency, localized optical correction of wavefront errors is needed in LDV or other optical experiments.