Spectral behavior and pulse train instabilities of a synchronously pumped mode-locked dye laser

The pulse-train envelope of a synchronously pumped mode-locked dye laser has been experimentally studied as a function of cavity length detuning. When a bandwidth-limiting birefringent tuning filter is included and the cavity is adjusted to the length that yields optimally short (approx.2 psec) pulses, there is a slow (less than or equal to800 kHz), erratic amplitude modulation that provides a 20% variation in pulse energies. As the cavity length is detuned on the order of one part in 10/sup 4/, the modulation vanishes, but the individual pulse lengths increase dramatically. Further detuning results in a clean periodic envelope with a 20% modulation depth and a approx.5-..mu..sec period. With the birefringent filter removed, the dye-laser wavelength and bandwidth are studied as functions of the cavity length. Temporally, the clean periodic modulation persists at large detunings, but there is no length at which any erratic modulation can be observed without the bandwidth-limiting filter.