AN INTERFEROMETRIC METHOD OF LASER-BEAM ANALYSIS

A new interferometric method is described which allows the separate an alysis of the individual transverse modes of multimode laser beams. Th e determination of the phase and the amplitude distribution of the mod es by means of a Mach-Zehnder-type interferometer is possible with a h omodyne or heterodyne measurement method. The required tunable single- frequency reference wave which serves as an optical local oscillator i s generated from the multimode beam itself by means of acousto-optical frequency shifting and mode selecting devices which are positioned in the reference branch of the interferometer. In homodyne operation the individual modes are characterized by specific stationary interferenc e patterns obtained by the synchronization of the reference wave frequ ency with the respective mode frequency. The use of conventional phase -shifting methods yields the phase distribution of the individual mode s. The corresponding mode order can be determined from the phase jumps by pi occurring in the fringe pattern. Results of the investigation o f a slow-flow cw-CO2 laser with stable resonator configuration are pre sented, proving the feasibility of on-line diagnosis provided by the s ystem. (C) 1995 American Institute of Physics.