Photon Statistics of a Dye Laser Far Below Threshold

The relative mean square fluctuations 〈(ΔI)²〉〈I〉² of the intensity I of a single-mode dye laser have been measured by a photon counting technique, in the region from about threshold to intensities 1000 times below threshold. The results show a steady increase of 〈(ΔI)²〉〈I〉² from less than 1 to about 150 as $〈I〉$ is reduced, followed by a rapid drop to zero. This behavior appears to be described fairly well by an equation of motion containing both additive spontaneous emission fluctuations and multiplicative pumping fluctuations.