Development and application of saturable absorbers to femtosecond solid-state laser mode-locking

Abstract

Semiconductor saturable absorbers have had a major impact on the field of ultrashort pulse generation by increasing the stability and reliability of ultrashort pulse lasers, making them more useful in many applications. These versatile devices can be grown both epitaxially with molecular beam epitaxy and non-epitaxially using RF sputtering. In this thesis, the development and application of both types of saturable absorbers to self-starting mode-locking in solid-state lasers was examined. The first part of this thesis describes the use of an epitaxially grown saturable Bragg reflector to mode-lock an extended cavity femtosecond Cr:LiSAF laser. Inexpensive single mode diodes were used as a pump source and a multi-pass cavity was used to lower the laser repetition rate. Pulses with durations of 39 fs and energies of 0.75 nJ were generated at an 8.6 MHz repetition rate. These pulse energies and durations are comparable to those produced from commercially available Ti:sapphire lasers that have a significantly higher cost. The second part of this thesis explored the further development and application of non-epitaxially grown semiconductor-doped silica films. A novel pump-probe system with independent pump and probe wavelength tunability from 700 to 1000 nm and a time resolution of 17 fs was developed for device characterization. The linear and nonlinear optical properties of InAs-doped silica films deposited by RF sputtering were characterized as a function of fabrication parameters, including nanocrystallite size, pump and probe wavelength relative to the absorption edge, and rapid thermal annealing temperature. Guidelines for the optimization of semiconductor-doped silica films for saturable absorber applications were extracted from the experimental data.

(cont.) Large nanocrystallites, high annealing temperatures, and an operating wavelength close to the absorption edge were found to optimize saturable absorber performance, with a low saturation fluence of 640 pJ/cm2 obtained at 1.54 grm. These saturable absorber devices were then designed to self-start mode-locking in a Cr:forsterite laser, obtaining self-starting 25 fs pulses with 91 nm bandwidth at 1.3 gm. These versatile devices can be designed for any solid-state laser system using the guidelines developed in this work and have the potential to replace epitaxially grown saturable absorbers in many applications.