Development and characterization of a fibre laser driven high-harmonic source

Abstract

Table-top high-harmonic sources of extreme ultraviolet (XUV) light are very attractive for applications in coherent diffractive imaging, spectroscopy, and lithography. When compared with synchrotrons and free electron lasers they have greatly reduced size and cost, so beam time would be more readily available to users. Increasing the repetition rate of table-top XUV sources is of interest as it enables faster data acquisition, but this cannot be achieved easily using traditional solid state sources, which require cryogenic cooling for average powers beyond ~5 W. The demonstrated ability of fibre chirped pulse amplification (CPA) systems for average power and repetition rate scaling enabled by their inherent thermal stability and readily available, low-cost pump lasers will allow high average power high-harmonic generation systems to become widely available. A prototype fibre CPA system was developed by a previous student at the Optoelectronics Research Centre, and was subsequently demonstrated as a high-harmonic source. However, the system could not be power scaled beyond ~1 W because of a mismatch between the seed wavelength (1055 nm) and gain peak of the final amplifier (1040 nm). This thesis reports the development of a new, power scalable version with excellent stability due to a reduction in operating wavelength, an optimized amplification chain with grating stabilized pump diodes, and a realigned bulk stretcher and compressor. The system is shown to be capable of state of the art XUV generation efficiency, improving on previously reported values for equivalent systems by a factor of two.

A new seed laser was required for the upgraded fibre CPA system, and during its development a novel multi-wavelength modelocking state was observed. New vectorial simulations of the cavity dynamics definitively identify the cause of this modelocking state and represent the first detailed theoretical description of the onset of multi-wavelength cavity dynamics in breathing pulse Yb-fibre lasers operated below the threshold for multi-pulsing. A novel XUV characterization method was also developed by extending a spatially resolved spectrometry technique to include radius of curvature measurements for full spatial XUV beam profiling from a single exposure. The method was used with the fibre CPA system, allowing for the spatial properties of a fibre laser high-harmonic source to be measured for the first time.

A more exploratory, unrelated supercontinuum experiment was also performed, initially to exploit optical wavebreaking of picosecond pulses in all-normal dispersion photonic crystal fibre to generate octave-spanning coherent spectra. Unexpectedly, cascaded Raman scattering was observed for the first time under these conditions. An experimental and numerical investigation of the coherence properties of Raman-dominated supercontinuum in the normal dispersion regime was performed, and the mechanism causing supercontinua to become incoherent is conclusively identified for this dispersion regime.

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Identifiers

ORCID for Jonathan Price: orcid.org/0000-0003-0256-9172

ORCID for William Brocklesby: orcid.org/0000-0002-2123-6712

ORCID for David Richardson: orcid.org/0000-0002-7751-1058

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