Dual-Passive Mode Locking of High Average Power, Solid-State Lasers
thesisposted on 07.09.2012, 00:00 by Stephanie L. Schieffer
Laser oscillators with average output powers of multiple watts and pulse durations less than 100 picoseconds (ps) have many uses. For instance, precise machining of metals takes advantage of the reduced thermal effect from laser pulses between 100 ps and 10 femtoseconds. Biologists and chemists use lasers in time-resolved spectroscopy of biochemical reactions. Ultrafast lasers are also used in chemistry, physics and material science to probe the electronic and vibrational states of various materials including semiconductors. These applications often require specific photon color and in the cases where this cannot be generated directly from a solid-state oscillator, it may be generated through nonlinear effects in optical parametric oscillators (OPO) and optical parametric amplifiers (OPA) -- techniques that typically require watt-level pump lasers. It is the focus of my dissertation to develop a stable, high average power, ultrafast laser suitable for direct use of for pumping an OPO/OPA. The laser oscillator presented in this dissertation employs the thermal-lens-shaping (TLS) concept, the basic idea of which is to actively shape and collimate the pump radiation from unlensed diode bars such that the resulting thermal lens in the gain media, which is experienced by the laser resonator, is stigmatic irrespective of the angle between the laser and the gain medium. This laser oscillator is mode locked using a novel, dual-passive technique in which a saturable Bragg reflector (SBR) provides amplitude modulation while a phase mis-matched second harmonic crystal generates phase-locking resulting in a required threshold energy that is less than half of that for SBR-only mode locking. Of course, the saturable and non-saturable absorption of the laser by the SBR results in thermally-induced stress and strain and thus bowing; an analysis of this thermal effect is conducted. Finally, the design and characterization of a high-resolution, aberration-corrected, flat-field, plane-grating spectrometer suitable for use in making ps-FROG (frequency resolved optical gating) measurements is presented. In this instrument, the grating is placed so its dispersion plane is orthogonal to tangential plane of the off-axis parabolas. This arrangement effectively disperses the Petzval field curvature and the image distortion along two directions thereby improving the overall resolution and image quality of the instrument.