Measurements of Cavity Loss in a Pulsed Ruby Laser

Abstract

IN a cavity laser system, oscillation by stimulated emission occurs when the gain is sufficient to overcome the total loss in the cavity. Cavity loss not only determines the threshold condition of laser emission, it is also related to various properties of the laser output, such as the mode structure and the mode pulling effects. It is therefore important to measure accurately the cavity loss in a laser system. Previous workers have used the threshold electric input energy of the xenon flashtube as a means of evaluating the cavity loss for a pulsed ruby laser^1–3. The non-linear behaviour of the xenon flashtube can cause measurement error, especially when the measurement was made over a wide range of temperature. There are two parts to this non-linear property: the total and peak radiant outputs in the pumping band are not linearly proportional to the electric input to the flashtube⁴, and the dependence on time of the radiant output varies with the electric input. However, if the time-varying flashtube radiant output power in the pumping band of the laser is directly measured, the total threshold radiant energy can be obtained by time integration of this pumping power up to the onset of the laser emission, assuming that the fluorescence decay time is long. This results in a loss measurement independent of the non-linearity of the flashtube.