A laser beam is expanded by means of a beam expander device, the expanded beam striking the surface of a mirror which has an aperture formed therein. A portion of the beam passes through the aperture and is focused so that it strikes an optical detector. The major portion of the beam is reflected from...http://www.google.com/patents/US4042822?utm_source=gb-gplus-sharePatent US4042822 - Laser radar device utilizing heterodyne detection
