Frequency Beats in the off-Axis Modes of Ruby Lasers

摘要

This paper describes the use of photomixing methods to study modes radiating in off-axis directions. In addition to the modes radiating axially in cylindrical ruby rods, it is possible to excite modes in rods with polished sidewalls which radiate in uniquely defined offaxis directions. The main purpose was to observe the best frequencies between off-axial modes, and see how parameters such as pump power and nonuniform reflectivity affect them. The microwave frequency beats between these modes were measured with the aid of a Philco 4501 photodiode and compared with the values predicted by ideal cavity resonator theory. The observed values agree very well with those predicted. In order to observe beats in the various off-axis 'rings', it is necessary to have a degree of control over the modes of oscillation. By selecting the proper nonuniform mirror reflectivity pattern, it was hoped that the thresholds of oscillation could be lowered for the local modes desired. Four different silvering patterns were used. The results indicate that it is possible to lower substantially the thresholds of oscillation of preselected modes with a proper choice of mirror silvering pattern, as long as the average reflectivity of the mirrors is not too drastically reduced.