Design and characterisation of ultrafast semiconductor disk lasers

Abstract

In this thesis I present my work on improving the design of semiconductor gain and saturable absorber structures towards the goal of a Watt-level, sub-200 fs mode-locked vertical-external cavity surface-emitting laser (ML-VECSEL). The performance of MLVECSELs has increased significantly in recent years with sub-500 fs pulse durations demonstrated at Watt-level average output powers. However, ML-VECSELs with sub-200 fs pulse durations have only been reported with milliwatt average output powers.

Absence of detailed knowledge of the dynamic response of both gain and saturable absorber structures makes it difficult to develop new designs for improved laser performance. It is, therefore, critical to fully characterise the macroscopic parameters of the semiconductor laser structures used in ML-VECSELs. Here, I present the characterisation of ultrafast surface recombination semiconductor saturable absorbers, with a design that has given the shortest pulse durations from a ML-VECSEL to date. I demonstrate a ML-VECSEL utilising a cavity design based on the extracted absorber parameters that would be suitable for high average mode-locked output powers using surface-recombination absorbers.

An understanding of fundamental mode-locking mechanisms present in ML-VECSELs is vital for optimising structures as pulse durations approach 100 fs. The evolution of the spectral components present from lasing onset and during pulse formation in a ML-VECSEL is measured; identifying three distinct regimes in the spectral evolution. I interpret the measured transients to extract a value for pulse-shortening per cavity round-trip, allowing a qualitative description of pulse formation in ML-VECSELs to be developed.

I present construction and characterisation of a ML-VECSEL containing new "shortmicrocavity" gain structures that have demonstrated FWHM pulse durations of 193 fs with 1 KW peak output powers; a benchmark result for peak output power from a ML-VECSEL with sub-200 fs pulse duration..

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