AN INTERNALLY REFLECTING OPTICAL RESONATOR WITH CONFOCAL PROPERTIES,

A confocal system is defined as an optical resonator using spherical mirrors advantages of the confocal system over the planar mirror are that the diffraaction losses for the former can be much lower and its alignment is less critical. The confocal resonator has the following basic properties 1 a ray leaving one mirror at the axis is reflected back to that point by the opposing mirror after a single reflection 2 a ray parallel to the axis leaves one mirror and returns to that point after three reflections and 3 the path length for a complete transit of any ray leaving the axis at one mirror is independent of the ray angle. A configuration for such a confocal resonator is derived mathematically with it confocal properties may be obtained without reflecting coatings. A semi-confocal optical maser was fabricated from a 50-mm-long, 6-mm-diameter cylindrical rod with one ground end and one coated to give a few percent transmission. The material was Schott barium crown glass doped with neodymium. Stimulated emission was obtained, and threshold input energy at room temperature was less than for a planar ruby of the same dimensions.