A 5 fs high average power OPCPA laser system for attosecond pulse production

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A 5 fs high average power OPCPA laser system for
attosecond pulse production
Philip Bates, Yunxin Tang, Emma Springate and Ian
Ross Central Laser Facility, CCLRC Rutherford
Appleton Laboratory, Chilton, Didcot UK
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OPCPA can produce more energetic few-cycle laser
pulses

• Current system TiSa amplified system hollow
  fibre broadening
• Gain narrowing in TiSa limits amplified pulse
  duration to 20-30fs. Typical systems have
  energies 1mJ
• Hollow fibre required to broaden spectrum to 5fs
  level with limited throughput
• OPCPA allows us to maintain full spectrum
  throughout amplification
• Requires more complex stretcher compressor design
• Energy limited only by available pump laser

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Optical Parametric Amplification
Depleted Pump
Pump
Idler
Seed pulse
Non-linear crystal

• Process obeys energy conservation and
  phasematching
• ??seed ??idler ?? pump
• ns?seed ni?idler np? pump
• Very broad bandwidth esp. in non-collinear
  geometry
• Pump beam aberrations and phase errors
  transferred to idler
• fidler fseed - fpump
• No thermal deposition in gain medium, no thermal
  loading
• High Gain with efficient extraction of pump
  energy

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OPCPA has broad gain spectrum

• Monochromatic pump pulse amplifies broadband
  chirped seed pulse
• 40ps Pump pulse duration 2x chirped seed
• Phase-matching angle chosen to give largest gain
  bandwidth
• Saturated Gain bandwidths gt 400 nm
• Output energies
• 23 extraction efficiency
• Up to 20 mJ with 85 mJ pump
• Can amplify full 5fs spectrum

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Stretcher-compressor designed for 400nm BW

• TiSapphire 12 fs oscillator
• CEP stabilised
• Broadened to 400 nm in optical fibre
• 3-component stretcher prisms gratings Dazzler
• Recompressed in transmission grating compressor

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Grating stretcher includes system of 8 lenses

• Angular magnification -1 for all wavelengths
  across bandwidth
• Preserves collimation of input beam
• Minimal change in group delay across our 2mm beam
• High-order phase terms from glass can be
  compensated for in rest of stretcher system

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Stretcher compressor calculations

• 8-prism system compensates for high-order phase
  terms due to glass in Dazzler and grating
  stretcher lens
• Compressor uses large out of plane angle to
  balance some high order phase
• Residual chirp designed to optimise Dazzler
  correction for high efficiency

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Fibre Broadening

• Current oscillator bandwidth of 250 nm has
  transform limit of 11.2 fs
• Need spectrum covering 700-1100 nm to get 5 fs
  pulses and match gain in OPA
• Single-mode fibre
• not enough broadening
• Photonic crystal fibre
• Too much broadening so some energy outside gain
  bandwidth

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Recompression of fibre-broadened pulses

• Recompressed output of fibre with Dazzler and
  prism system
• 3 types of fibre tried, all single mode
• 1) PM silica
• 2) Anomalous dispersion PCF
• 3) Normal Dispersion PCF
• Recompressed to 18 fs but all show signs of
  double pulses and variation across beam spatially

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Diode-pumped NdYLF pump laser
1mJ, 1 kHz 40ps 1047 nm regen
Diodes
Diodes
SHG

• Pump with 25 duty cycle
• Synchronised to TiSa oscillator

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Current output of NdYLF pump laser
Amplified beam profile 3rd pass on rod 60mJ

• Current setup 3 pass amplification
• Output pulse energy up to 80mJ
• Damage to rod coating at 80mJ

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Summary

• OPCPA allows amplification of full 5fs pulse
  bandwidth and hence higher power pulses
• Transmission grating stretcher and compressor
  system including prism stretcher and Dazzler
• Best compression within 1-2fs of transform limit
  of oscillator spectrum without dazzler
• Fibre broadening has limited success so far
• Initial pump laser tests promising with 80mJ
  output
• Two-stage OPCPA amplification is being designed
• Gain bandwidth tests of OPA crystals to begin soon