Bichromatic emission in a ring dye laser

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Abstract
We have performed an experimental study of a high-Q Rhodamine 6G ring dye laser and have measured bichromatic emission with wavelength spacings as large as 110 Å when the laser operated bidirectionally. The bichromatic emission vanished at all excitations when the laser was forced into unidirectional operation using a Faraday isolator. However, when a weak reflected beam was allowed to make a single pass in the direction opposite to that allowed by the Faraday device, bichromatic emission is recovered at the higher pump powers. The experiments show that no discontinuities occur in the intracavity power as a function of excitation and that the intracavity fields which are completely consistent with the measured thresholds are not capable of producing Rabi splittings larger than a few angstroms. In addition, we have observed no structure in the spontaneous emission spectrum of the dye when the bichromatic emission (80-Å splitting) was present in the laser mode. We have been able to quantitatively fit the threshold and wavelengths of the bichromatic emission using a distributed feedback mode analysis with a cavity-mode-induced susceptibility grating.