Semiconductor seeded fibre amplified sources of ultra short pulses

Abstract

This thesis reports upon an experimental investigation of passively mode-locked optically pumped vertical-external-cavity surface-emitting semiconductor lasers (VECSEL). Mode-locked VECSELs are a compact source of ultra-short pulses at GHz repetition rates, with pulse lengths as short as 190 fs being generated directly from the laser. The VECSEL is a power scalable device offering spectral versatility through band gap engineering of semiconductor gain material.

Here, for the first time the technique of frequency resolved optical gating (FROG) has been used to record a second harmonic spectrogram of the VECSEL pulse train, from which the phase information of non-transform limited sub-picosecond pulses has been retrieved. I also report the characterisation of a single stage VECSEL seeded ytterbium-doped fibre amplifier, capable of increasing the average power of a VECSEL from 20 mW to over 1.5 W while maintaining the sub-picosecond duration of the pulse train. The amplifier is capable of operating at any repetition rate obtainable with a VECSEL, amplification is demonstrated here with 1 GHz and 6 GHz seeds.

Finally, the nonlinear evolution of VECSEL pulses inside a single stage fibre amplifier has been investigated. Computer modelling of the linear gain and nonlinear pulse propagation within a single fibre has been used to design an amplifier capable of producing pulses with a parabolic profile. The modelling reveals that a parabolic amplifier would produce spectrally broader linearly chirped pulses which could be compressed to below 100 fs, with average powers > 3 W. An experimental realisation of the parabolic amplifier will require a seed with average power greater than 100 mW, this could be achieved with a re-growth of an existing sample, QT1544.

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