ABSTRACT
The unique properties of lasers have made them valuable tools in applications ranging from medicine to metrology. In applications such as holography and coherent optical communication, high spectral and spatial coherence are of primary importance. This is achieved by designing the laser to operate in a single longitudinal and single transverse mode. Other applications such as optical recording and playback simply require high spatial coherence from a single transverse mode. The high spatial coherence (accompanied by good phase uniformity) permits focusing of the beam to a spot size that is limited entirely by diffraction from the focusing optic. Still other applications such as material processing, welding and laser pumping place a premium on power, often at the expense of spatial coherence. Lasers for these applications may operate in several spatial modes simultaneously, and the ability to concentrate their output light is limited by the radiance theorem (Boyd, 1983).
