The fibre laser has revolutionised modern technology, underpinning applications
as diverse as materials processing, communications, and medical imaging.
Demand has grown for fibre-based laser sources across a wide range of spectral
regions. This thesis outlines progress towards extending the spectral coverage
of fibre laser technology, through efficient nonlinear conversion techniques
such as stimulated Raman scattering, four-wave mixing, and second-harmonic
generation.
We target two distinct areas of research: nonlinear frequency conversion
in bulk crystals, and four-wave mixing in photonic crystal fibre. We begin by
introducing an architecture for generating nanosecond pulsed, MHz repetitionrate
pulses at 1240 nm based on stimulated Raman scattering in phosphosilicate
fibre. The resulting amplifier has ideal properties as a pump for subsequent
nonlinear conversion in crystals. We demonstrate the use of this phosphosilicate Raman amplifier as the pump
system for three novel nonlinear sources. The first of these utilises seeded optical
parametric generation in a cadmium silicon phosphide crystal to generate
nanosecond pulsed, MHz repetition-rate, mid-infrared radiation between
4.2–4.6 μm. The second source we present delivers watt-level average powers
at 620 nm, based on the direct frequency-doubling of the 1240 nm amplifier system,
with ideal properties for stimulated-emission depletion microscopy. The
third source extends the concept of the frequency-doubled Raman amplifier system
at 620 nm to 743 nm, by utilising cascaded Raman amplification in phosphosilicate
fibre.
Lastly, we outline an architecture for generating short, tuneable pulses at
794–828 nm based on fibre optical-parametric chirped pulse amplification, a
technique which utilises four-wave mixing in photonic crystal fibre. This architecture
aims to generate ultrashort pulses in spectral regions not traditionally
well served by existing fibre lasers. We underpin the work with both experimental
results and simulations, discussing the design, implementations and limitations
of this architecture, as well as presenting avenues for the future research
and development of similar systems.