Picosecond Phase-Conjugation Reflection and Gain in Saturable Absorbers by Degenerate Four-Wave Mixing
More reliable mode-locking and bandwidth-limited pulses of shorter durations are obtained when the high-reflectivity mirror is immersed in the saturable absorber solutions used to passively mode-lock pulsed and CW dye lasers, Neodymium and ruby lasers. This result had been previously explained [2,3] by preferential saturation of the absorber at the antinodes of the standing waves in the neighbourhood of the mirror, particularly when the dye cell length is comparable to the laser pulse length. With the recent demonstration of phase conjugation reflection in many materials by degenerate nonlinear mixing  involving local saturation of absorption  it seemed timely to investigate picosecond phase-conjugation in DODCI and other saturable absorbers commonly used for mode-locking dye lasers. Our results indicate that, in addition to preferential saturation, phase conjugation effects contribute to the improved performance of the immersed mirror mode-locking dye cell, even when the laser beam is focussed tin to the mirror as in the case of CW dye lasers , Photoisomer effects  are also shown to be important in the phase-conjugation process, and play a dominating role as the laser is tuned to longer wavelengths.