Development of an ytterbium fibre based chirped pulse amplification laser system for high harmonic generation

• Main Author: Driever, Steffen
• Other Authors: Zair, Amelle; Smith, Roland
• Published: Imperial College London 2015
• Subjects: 530
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.666507

In this thesis, I describe the development of a high repetition rate femtosecond fibre based chirped pulse amplification system (FCPA) for strong-field physics experiments. This project was set in a newly established sub-group of the Laser Consortium at Imperial College London with the aim to push the strong-field and attosecond science experiments to be conducted at 100s of kHz repetition rate. It was important to design and implement a compact, CEP stable, high repetition rate fibre CPA system. Custom optics and mounts were employed in order to achieve a compact stretcher and compressor design. The stretcher was designed to stretch the oscillator pulses with a bandwidth of 14nm and a duration of 90 fs to 1 ns to avoid non-linearities in the fibre amplifier. It is based on an Offner type configuration. The fast rise time RTP based Pockels cell can be adjusted to deliver 50 -350 kHz of repetition rate to the large mode area (LMA) ytterbium doped fibre with 30 um core and 250 um cladding (Yb1200-30/250DC-PM, Nlight). It was possible to generate up to 13 uJ of pulse energy at 100 kHz repetition rate before compression with 14W pump power. Higher pulse energies up to 130 uJ have been demonstrated at 38W of pumping (55W pump diode Nlight), however mechanical instabilities impaired the spectral and spatial performance at this power level. Improvements to optimise the performance of the system are suggested in the conclusion. Additionally to this experiments in the near-IR have been conducted on post compression mechanisms. The principle of filamentation was employed to generate tunable few-cycle pulses at wavelengths from 1.6 - 2 um and subsequent high harmonic generation in a proof of principle experiment. These results were published in Applied Physics Letters [1] and Journal of Physics B: At. Mol. Opt. Phys. [2].