Ultrasensitive response of a CW dye laser to selective extinction

The sensitivity of a broad-band dye laser to selective intracavity absorption is proportional to the number of oscillating modes. This result is obtained from a rate equation model, taking into account the spectral homogeneity, but spatial inhomogeneity of the laser saturation. An increase of absorption sensitivity by a factor of 10⁵, compared to a single-pass measurement, is observed in an experiment with an iodine vapor absorption cell inside the cavity of a CW dye laser. The fluorescence of an external iodine vapor cell provides a convenient selective absorption monitor.