External photoacoustic detection of a trace vapor inside a multimode laser.

We combine highly sensitive intracavity absorption spectroscopy in a multimode laser with external photoacoustic detection, providing high detectivity. Photoacoustic measurements of the intracavity iodine concentration in a Rhodamine 6G laser are compared with simultaneous spectral recordings. They demonstrate slightly improved overall sensitivity and a greatly enhanced dynamic range of 5 orders of magnitude that may be shifted along the absolute scale of absorber density.