Bichromatic emission in a homogeneously broadened ring dye laser: evidence of a new mode structure hierarchy

Abstract

We present experimental measurements on a high-Q rhodamine 6G ring dye laser which was found to exhibit bichromatic emission with splittings of the order of 15–80 Å while operating in the range of 2–6 times above threshold in the bidirectional mode. Experiments were performed with the insertion of a low loss Faraday isolator (loss < 1%) in order to examine the effects of saturation gratings on the bichromatic emission. The results showed that even though the intracavity power went up, owing to the elimination of spatial hole burning, the bichromatic emission was completely eliminated in the operating range between 1 and 10 times above threshold. In addition the sidelight fluorescence of the jet was spectrally examined in order to search for sideband splittings similar to those found in Na atoms in a strong resonant field. No evidence of splittings or fluorescence shape change was found when the laser was 1–10 times above threshold. When a weak reflection from an intracavity element was allowed to make a single pass through the active part of the jet in a direction opposite to that allowed by the Faraday rotator, bichromatic emission was recovered at high pump powers (2–5 W). This is believed to be a consequence of a Bragg grating which appears when the weak beam is sent through the active medium. We describe the experiments and the results which we obtained, as well as offer a theory which consistently explains our observations and perhaps those of others.