Systematic errors in laser gain, saturation irradiance, and cavity loss measurements and comparison with a HCN laser

A pair of laser parameters of considerable practical interest are the small signal gain and saturation irradiance of the gain medium. These are commonly measured by observing the dependence of the output power on some adjustable cavity loss parameter and comparing the measured data with the predictions of a suitable laser model. Because of the inevitable approximations in this model the resulting estimates of gain and saturation irradiance are always affected to some extent by systematic errors. The small-gain, plane-wave, mean-field, and pure homogeneous or inhomogeneous line-broadening approximations are considered, with estimates of the magnitudes of these errors being presented for the case in which the gain, the saturation irradiance, and the cavity loss are fitted to the data. It is shown that these errors can be quite substantial, and therefore accurate absolute measurements of the three laser parameters can be quite difficult to obtain using the variable loss method. As an illustration of these errors, a comparison between the measured output power from a HCN laser and the power predicted using experimentally measured gain and saturation irradiance values is shown. The poor quality of these predictions illustrates the serious effects that the systematic errors can have, although an alternative analysis in which the cavity loss is supplied and only the gain and saturation irradiance fitted is also shown and gives good predictions despite inaccuracies in the model.