A Study of Transverse Modes of Ruby Lasers Using Beat Frequency Detection and Fast Photography

摘要

Intensity variations in the frequency range of 6 to 141 Mcps were detected in the output of ruby lasers. These are interpreted to be beats between simultaneously occurring transverse modes with the same axial mode number, rather than beats between two modes in regions of the ruby rod having slightly different path lengths. Evidence for this interpretation was provided primarily by two experiments: (1) the beats disappeared when the ruby laser rod was operated with small mirrors, and (2) streak photographs of a narrow strip of the end of the rod always showed a phase variation across the rod of the beat frequency. Extensive measurements of a ruby rod whose path length variations were approximately lamboa/10 showed that the transverse modes and beat frequencies were characteristic of a noncomfocal resonator rather than a plane-parallel resonator. The probability of observing transverse mode beats increases as the temperature is lowered and as the pump energy is increased. It is possible to excite only one transverse mode per axial mode number at room temperature and at pump energies 20% over threshold. Thus single frequency operation of ruby lasers is possible if axial mode selection is used. (Author)