Colliding Pulse Femtosecond Lasers and Applications to the Measurement of Optical Parameters

Propagation of short coherent pulses through media poses new constraints on materials, as well as on the design of experimental set ups, in order to maintain the best possible temporal resolution. Hence the need for accurate and sensitive techniques to measure the change in pulse parameters transmitted through (or reflected off) optical samples. We show that the mode locked ring laser is in itself an accurate tool to investigate linear and nonlinear optical properties of its components. Measurements of transmission and reflection can be performed intra or extracavity. Intracavity measurements are more accurate because of the multiple passages through the sample. As in cw intracavity spectroscopy the enhancement of sensitivity when operating in the stationary mode-locking regime is of the order of the mean number of cycles in the cavity. The absorber jet is in ideal configuration to study the phase relaxation time of the dye by Degenerate Four Wave Mixing (DFWM). Because of the colliding pulse mode locking process, the two “pump pulses” for the DFWM interaction are simultaneously present in the jet, while the laser output itself can be used for the probe pulse. The “thin sample” condition required to obtain high temporal resolution in DFWM | l | is also met in such a laser.