Duration of dye laser pulses under steady-state mode locking conditions

A simple physical model is developed of passive, active, and combined mode locking in a dye laser. The model describes the dependence of the duration of the laser pulses on the degree of excitation, on the lasing wavelength, and on the mismatch between the resonators. This model can be used to find such dependences in an analytic or nearly analytic form. According to the model, the duration of the laser pulses is determined by the excess of the gain above the absorption ("pure gain") at the center of a pulse and by the spectral bandwidth of the laser. An increase in the pure gain (increase of saturation of the absorption, reduction of saturation of the gain, and increase in the rate of rise of the gain) inevitably reduces the pulse duration. It is convenient to assume that the only parameter that governs the pulse duration is the dimensionless energy density in an amplifier. The relationship between this energy density and the pulse duration determines changes in the duration in all the dependences mentioned above. A comparison of the calculations with experiments shows that they are in good agreement.