Ultrashort Pulses in the Infrared from Synchronously Pumped Optical Parametric Oscillators

Two subjects are presented in this dissertation, the common theme being the development of ultrashort optical pulse-generation techniques: a continuous wave synchronously pumped and mode-locked optical parametric oscillator; and the use of the intracavity optical Kerr effect to enhance the performance of an actively mode-locked homogeneously broadened laser. There is very large continuing interest in generating ultrashort optical pulses in the picosecond to femtosecond time range, tunable in the mid-infrared spectral region (1-5 μm). The synchronously pumped mode-locked optical parametric oscillator, because of its extremely broad tuning range and wide gain bandwidth, offers one of the most attractive methods to generate such pulses over wavelength regions not readily available from existing tunable lasers. We have developed such an oscillator- -a truly continuous wave synchronously pumped mode-locked singly resonant optical parametric oscillator--based on KTiOPO_4. The oscillator is synchronously pumped using amplitude-stabilized and temporally compressed pulses from a lamp-pumped actively mode-locked Nd:YAG laser. Output pulses 2-3 picosecond long have been obtained with hundreds of milliwatts of average power from 1.57 to 1.59 μm in the signal branch and from 3.21 to 3.30 μm in the idler branch. The primary design considerations for achieving continuous wave oscillation and high conversion efficiency are first discussed, followed by the presentation of the performance and various characteristics of the oscillator. The penultimate chapter details the use of the optical Kerr effect to enhance the performance of a conventional lamp-pumped and actively mode-locked Nd:YAG laser. Theories of mode locking in the presense of intracavity self-phase modulation are presented, from which a pulsewidth-reduction limit is predicted. Experimentally, this technique is applied to a commercial lamp-pumped and actively mode-locked Nd:YAG laser, and the reduction of pulsewidth by approximately a factor of two, with no loss of average power, is demonstrated; these results are reported in detail. Also illustrated is the bifurcation of the mode-locked pulses which occurs when an excessive amount of intracavity self-phase modulation is present.