The Nd:glass laser has become one of the most useful sources of light pulses a few picoseconds in duration. In this paper, we review the results of an extensive study of the time and spectral structure of the Nd:glass laser radiation. The time structure was studied by means of two-photon fluorescence (TPF) patterns: these were scanned by a very thin (28-μ) cell containing the fluorescent dye, the fluorescence being monitored by a photomultiplier. When the Nd:glass laser is Q switched by a rotating mirror or when it is free running, we find TPF patterns fully consistent with a model where the laser emission has the character of Gaussian noise (thermal light), i.e., a model where the modes are randomly phased. When the laser was simultaneously Q switched and mode locked we made two observations : 1) the TPF patterns show that the ultra-short pulses observed previously have an overall duration of ∼8 ps, but also possess an internal substructure containing peaks 0.4-0.8 ps in duration; 2) the spectral width of these pulses is <20 cm ^-1 at the beginning of the Q -switched train and expands to ∼80 cm ^-1 in the middle of the train. This rapid spectral broadening during pulse buildup is attributed to self-phase modulation in the laser glass matrix due to a nonlinear index n 2 which we evaluate as n{2}= (2 \pm 1) \times 10^{-22} m ² /V ² (or 1.8 \times 10^{-13} esu). Gain limiting due to this effect and self-focusing become very important at power densities above 1 GW/cm ² , presenting a serious limitation on the energy density (J/cm ² ), which one may hope to extract from Nd: glass laser systems.

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