Novel sources of near- and mid-infrared femtosecond pulses for applications in gas sensing, pulse shaping and material processing

Abstract

In this thesis the design, construction process and the performance of two femtosecond optical parametric oscillators and one second–harmonic generation femtosecond pulse shaper is described. One oscillator was applied to gas sensing while potential applications of other devices are outlined. ATi:sapphire oscillator was used to pump a periodically–poled lithium niobate– based optical parametric oscillator. This signal–resonant device was configured to produce broadband idler pulses tunable in the range of 2.7–3.4 μm. This wavelength coverage was matched to the ν3 optical absorption band of methane, and Fourier–transform spectroscopy of a CH4:N2 mixture was implemented by employing a mid–IR silica photonic bandgap fibre simultaneously as a gas cell and an optical waveguide. Methane sensing below a 1% concentration was demonstrated and the main limiting factors were identified and improvements suggested. Another optical parametric oscillator was demonstrated which was pumped by a commercial Yb:fibre master oscillator/power amplifier system and was based on a periodically–poled lithium niobate crystal. The signal was tunable between 1.42–1.57 μm and was intended as a source for a subsequent project for waveguide writing in silicon. The oscillator was a novel long–cavity device operating at 15 MHz. The 130 nJ pump pulse energies allowed for 21 nJ signal pulses at a pump power of 2 W. The performance of the oscillator was characterised via temporal and spectral measurements and the next steps of its development are outlined. Finally a pulse shaper based on second harmonic generation in a grating– engineered periodically–poled lithium niobate crystal was demonstrated. Pulses from a 1.53 μm femtosecond Er:fibre laser were compressed and then used as the input to the shaper. The performance of the shaper was tested by performing cross–correlation frequency–resolved optical gating measurements on the output second harmonic pulses and this confirmed the successful creation of multiple pulses and other tailored shapes including square and chirped pulses, agreeing well with theoretical calculations.