Holographic Optical Elements as Conically Scanned Lidar Telescopes

Abstract

Recent developments in holographic optical elements (HOE) using volume phase holograms enables the development of a new class of light weight telescopes with specific advantages for lidar remote sensing instruments. One such device, a staring mode receiver has been fabricated for a laser range finder.1 In particular, a large aperture, narrow field of view telescope used in a conical scanning configuration can be constructed with a much smaller rotating mass compared to conventional designs which require a large flat scanning mirror or scanning the entire telescope assembly including the primary mirror, secondary, baffling, and the detector package or beam steering optics. Being a diffractive optic an HOE has spectral dispersion which can be utilized to advantage in a lidar system in which only one or more particular laser wavelengths are being observed. The spectral bandpass of the HOE can be made as narrow as 10 nm, replacing an optical filter in many applications. Other, unwanted wavelengths pass through the HOE undiffracted to be absorbed by a black backing. Thus, an HOE can be used to replace three separate components in a lidar telescope: the scanning mirror, the focusing mirror, and a narrowband interference filter. We also describe a method with which a multiplexed HOE can also be used simultaneously as a dichroic beamsplitter.