Ultrashort pulse generation by synchronous pulsed pumping of a dye laser with a Sagnac-interferometer cavity

A systematic study was made of ultrashort pulse generation in dye lasers pumped synchronously with short (up to 100 ns) or long (up to 600 ns) trains of picosecond (30 ps) pulses. The main task was to determine the feasibility of generation of subpicosecond pulses under transient pumping conditions. A cavity with a modified Sagnac interferometer, eliminating the influence of a thermal lens in the case of a long pump train, was used as the basic optical system. In addition to the known ultrashort pulse compression methods by an increase in the duration of synchronous pumping (by the use of a longer pulse train) or by hybrid mode locking (with the aid of a nonlinear absorber in contact with a mirror), a compression effect specific to a cavity with a modified Sagnac interferometer was found: it was due to relative delay of counterpropagating ultrashort pulses at a beam-splitting mirror where constructive interference took place between these pulses. The strongest compression (relative to the ultrashort pump pulses) corresponded to the generation of 2 ps pulses.