Unidirectional travelling wave operation of a mode-locked Nd:glass ring laser
Abstract The development of unidirectional mode-locked pulses in a Nd:glass ring laser is studied for various combinations of active and passive mode-locking and pulse direction discrimination elements.
Nonlinear loss in Nd‐doped laser glass
The amplifier gain and the energy transmission in Nd‐glass were investigated as functions of light intensity using single picosecond pulses. The observed reductions of gain and transmission are well accounted for by a two‐photon absorption process in the Nd3+ system. There is good agreement between the measured and calculated two‐photon absorption cross section of σ′ = (1.3 ± 0.3) × 10−32 cm4/W. The two‐photon absorption limits the obtainable output intensity in a Nd‐glass amplifier and reduces the self‐focusing action in the material.
Study of the Nd: Glass laser radiation
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 Q -switched train and expands to ∼80 cm-1in 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} m2/V2(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/cm2, presenting a serious limitation on the energy density (J/cm2), which one may hope to extract from Nd: glass laser systems.