Degradation monitor for laser optics

Abstract

Refractive elements typically have ground glass edges in areas where the perly-transmitted laser beam does not impinge. Defects on either incident optical surface or the exit optical will produce wide-angle scattering. Some of this scattered radiation will impinge directly upon the ground glass; other scattered radiation will reach it after one or more internal reflections in the optical element: An optical fiber abutted to an edge transfers scattered light to a detector. If a threshold is exceeded, a visual or auditory signal may be generated to alert the operator and/or an interlock may be activated to disable the laser. One photodetector is used to detect returns from several elements. Fibers of different lengths are used to couple the different components into the detector. In this way, a pulsed laser with a pulse length short compared with the delay difference between fibers may be used to identify the individual element which contains the defect. The time taken between the laser pulse and a threshold exceedence can be used to identify the optical element which is the source of the stray light scattering. The surface of other optical elements, such as front-surfaced mirrors and stops, may also be monitored by locating the fiber off the main beam path at a point where stray light from the surface will be coupled into its numerical aperture.