Generation of high-contrast, terawatt to petawatt OPCPA laser pulses

• Main Author: Stuart, Nicholas
• Other Authors: Smith, Roland
• Published: Imperial College London 2016
• Subjects: 621.36
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.724155

Laser-matter experiments using >10^18 W cm^2 focused light intensities from terawatt to petawatt peak-power laser systems are highly sensitive to the optical noise sources that are generated and amplified in conjunction with the primary high-power laser pulse. This limits the temporal intensity contrast, the signal to noise of the primary pulse intensity in time. This thesis presents a detailed investigation into the noise sources that degrade the contrast of high-power laser pulses created for laser-matter interaction experiments. To achieve this, a high-contrast multi-terawatt peak-power laser system, named Cerberus, has been developed to deliver up to 5 J, 500 fs pulses (10 TW) using an optical parametric chirped pulse amplification (OPCPA) front-end and Nd:Glass laser rod power amplifiers. To compliment this study, >10^10 dynamic range diagnostics have been developed that measure these noise sources in the presence of the intense primary pulse. Parametric fluorescence noise from parametric amplifiers was measured with a diagnostic able to observe the real-time seeded OPCPA fluorescence noise emission and determined the fluorescence emission to be directly linked to the level of pump-depletion, with a ~500 times contrast enhancement from the undepleted to maximum pump depletion modes of operation. A combination of picosecond-pumped OPCPA and nanosecond-pumped OPCPA amplifiers effectively suppresses the longer timescale parametric fluorescence to >10^8-10^10 contrast. A scanning third-order autocorrelator diagnostic (TOAD) was developed to characterise the compressed pulses emitted from a range of mode-locked laser oscillator sources and the compressed high-power laser pulses from major laser laboratories in the UK. We found the commonly used chirped pulse amplification architecture limits the temporal contrast from formation of an incoherent noise pedestal that can extend many hundred of picoseconds before the arrival of the primary laser pulse. We also observed non-linear generation of pre-pulses from internal reflections inside parallel-faced OPCPA crystals incident with long duration pump pulses.