Instabilities and chaos in a multimode, standing-wave, cw dye laser

Abstract

Experimental and theoretical investigations of a multimode, standing-wave, cw dye laser have been carried out. The optical spectrum of the dye laser was found to evolve by discrete transitions with increasing pump power, between spectra composed of three stable modes and spectra composed of many modes whose amplitudes fluctuate strongly. The variation of the intensities versus time of individual modes of the multimode laser was measured. For pump powers where the spectrum consisted of modes with strongly fluctuating amplitudes, we found that the intensities of individual modes have a correlation time that decreases, and a variance that increases, with increasing pump power. In addition, the fluctuations were found to be deterministically chaotic. This rules out quantum noise as the origin of the fluctuations, a mechanism that had been assumed in many previous studies. We also have carried out numerical simulations of this laser, using a semiclassical model based on third-order, coupled-mode laser theory, which includes population oscillations, four-wave mixing in the gain medium, spatial hole burning, and spontaneous-emission noise. The behavior of the optical spectrum and the fluctuations of single-mode intensities are well modeled for low pump powers. The modeling indicates that four-wave mixing and spatial hole burning are responsible for the discrete transitions in the optical spectrum, and that four-wave mixing drives the strong fluctuations of the individual mode intensities.