Direct determination of triplet state influence on CW dye laser action – a new technique for intracavity absorption measurements

Abstract

A new technique for intracavity absorption measurements is presented. The method is used to investigate triplet–triplet absorption of laser dyes. The triplet state population of the active medium is varied in a well-defined way. Due to the presented experimental setup, this can be done without changing any other molecular or optical parameter, which influences the laser action. In this way an increased triplet state population leads to an enhanced laser threshold which can be observed visually. Owing to the high sensitivity of the new technique, the triplet state influence on CW laser action can be studied simply by visual observation. No detection electronics is required. In order to analyse the experimental data, a refined theory of CW dye laser action is derived. The laser threshold of typical CW dye lasers can be calculated without numerical analysis. This theory can be applied even if the jet motion reduces the number of triplet molecules interacting with the dye laser beam to a dynamic equilibrium value. All experimental results are in good agreement with theoretical predictions. The triplet state properties, fundamental for laser action, can be extracted from the experiment. It turns out that the laser efficiency even of rhodamine 6G, one of the best laser dyes, is reduced significantly by triplet–triplet absorption. The triplet disturbance obtained for a new laser dye is found to be less than that of rhodamine 6G.