On the generation of short light pulses with reference to the ruby laser

The results are presented of an experimental and theoretical study of short light pulse generation in a ruby laser. Initial work on the construction of a 'mode-locked' ruby laser led to the invention of a new type of laser reflector based on the non-resonant interference of components of the laser beam. An accurate measurement was made of pulse lengths using the two-photon fluorescence technique. An automated apparatus then allowed a wide range of accurate measurements to be made under various conditions of general-ion, showing that a compound solution of saturable absorbers, or a suitable etalon, could lead to a substantial reduction in pulse length. The shortest pulse generated had a duration or 15ps. A threshold intensity for pulse substructure was observed. Below this intensity, the shape and duration of the pulse were accurately described by a computational theory of pulse generation from spontaneous emission noise. The important role of the laser linewidth was brought out, and further evidence adduced for excited state absorption in the saturable absorber, DDI. The effects of the fast nonlinear refractive index of sapphire were included, at first in a model for plane waves, and finally, a model which included a limited degree of self-focussing. It was shown that the spectrum rapidly evolved until its width was of the order of the laser linewidth, when parts of the pulse became strongly attenuated and substructure formed in the pulse on the scale of the inverse of the linewidth. It was found that the model with self-focussing gave a sharp 'threshold' for the formation of substructure, and also gave an integrated pulse spectrum in excellent agreement with experiment. The value of nonlinear index which best fitted the experimental threshold was n2 = 0.7 x 10-13 e.s.u., on the assumption of an electronic distortion mechanism for the O-ray polarisation in sapphire. Some general guidelines for the construction of short pulse lasers are given