Analysis of a mode-locked ring laser: chirped solitary pulse solutions

Measurements are presented on mode-locked ring dye lasers that demonstrate the role of self-phase modulation in the saturable absorber dye. The resulting bandwidth increase plays an essential role in ultrashort-pulse generation, because it leads to intracavity-pulse compression in the presence of normally dispersive components. The role of various intracavity components (glass, coatings) in intracavity compression is analyzed. As opposed to extracavity compression, measurements of pulse amplitude and phase show that the nonlinear-phase modulation can be completely compensated. The interaction of the light pulse with the absorbers is analyzed using Maxwell-Bloch equations. Next, the steady-state solution for the problem of pulse propagation is found through an infinite periodic medium containing all the essential intracavity elements (bandwidth-limiting filter, amplifier, absorber, dispersive component). The condition that the solitary pulses be unchirped enables the determination of the optimum intracavity dispersion, which, in agreement with the experimental data, leads to the shortest pulses.