The Frequency Shifting of Femtosecond Laser Pulses.

Two new and widely applicable methods of shifting the frequency of femtosecond laser pulses have been developed experimentally and theoretically analyzed: (1) intracavity frequency doubling, for low energy (nanojoule) pulses, and (2) blue shifting in a laser-produced plasma, for high energy (millijoule) pulses. For the first method, we efficiently extract an ultraviolet femtosecond pulse train of milliwatt average power and 100 MHz repetition rate from a colliding pulse mode-locked (CPM), ring, dye laser by intracavity frequency doubling in KDP. The ultraviolet and visible outputs, which are comparable in power and pulse duration, are perfectly synchronized with each other. We then construct a quantitative theoretical model of an intracavity frequency doubled and passively mode-locked laser and support the results with our experimental observations. Our major findings are that, for second harmonic conversion efficiencies consistent with the continuing laser operation (<5 percent): (1) a stable mode-locking regime always exists, although it narrows somewhat with increasing conversion efficiency; (2) the duration of the fundamental pulses can always be preserved by readjusting the saturable gain and saturable loss parameters; (3) the energy of the fundamental pulses can also be preserved under the same conditions. In order to make high sensitivity low noise optical measurements with the unamplified CPM (fundamental and/or second harmonic), we developed a new data acquisition system based on fast scan measurements with signal averaging, which is shown to have a better noise rejection capability in a shorter data collection time than standard lockin detection. Reflection and transmission measurements Ge and GaP samples are presented as a comparison of the new system with the standard lockin system. For the second method, we discuss a new way to frequency shift an intense femtosecond pulse in the blue direction by tightly focusing it in an atmospheric density gas is presented and the atmospheric density plasma that it creates, which causes the frequency shift. The incorporation of new pumping geometries in the gain stages of a YAG laser pumped optical amplifier provides the high amplification of our femtosecond pulse without damaging its phase front so that the amplified pulse is able to be focused to near the diffraction limit and produce intensities ( ~10^{16} W/cm^2) never before achieved in a laser pulse with a femtosecond pulse duration.