Rotationally symmetric dual reflection optical beam scanner and system

Abstract

A dual reflection unpolarizing monofacet (DRUM) scanner or beam deflector is made up of two essentially identical 45.degree. right angle prism elements with their hypotenuse faces together to form a body which may be rectangular or cylindrical and may be contained in a housing which provides for aerodynamic stability when the scanner rotates at high speed. The hypotenuse faces have a partially reflective surface on which an incident beam, collinear with the rotational axis, is incident. This beam is transmitted and reflected to provide dual beams, one of which is absorbed by light absorbing material, preferably a coating, on a surface of the body and the transmitted beam is retroreflected by a mirror on another surface of the body, back to the partially reflective surface. An output beam is provided by the retroreflected beam. The DRUM deflector may be part of a system which provides a tracking beam of a wavelength different from the scanning beam which is reflected from the partially reflecting surface, then acting as a dichroic beam splitter. A dichroic beam combiner combines light of both wavelengths and directs the light to be incident on the dichroic, partially reflecting surface of the DRUM deflector. A spherical mirror with a clock grating reflects the tracking beam to a photodectector to provide an output signal from which in-scan tracking errors can be derived.