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

OPCPA can produce more energetic few-cycle laser pulses Current system: Ti:Sa amplified system + hollow fibre broadening Gain narrowing in Ti:Sa 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

Optical Parametric Amplification Process obeys energy conservation and phasematching –ћω seed + ћω idler = ћω pump –n s ω seed + n i ω idler = n p ω pump Very broad bandwidth esp. in non-collinear geometry Pump beam aberrations and phase errors transferred to idler –φ idler = φ seed - φ pump No thermal deposition in gain medium, no thermal loading High Gain with efficient extraction of pump energy Pump Non-linear crystal Idler ћωpћωp ћωIћωI ћωsћωs Amplified Seed Depleted Pump Seed pulse k signal 1 k idler 1 k signal 2 k idler 2 k pump α = 1.2°

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 > 400 nm Output energies –23% extraction efficiency –Up to 20 mJ with 85 mJ pump Can amplify full 5fs spectrum

Stretcher-compressor designed for 400nm BW Ti:Sapphire 12 fs oscillator CEP stabilised Broadened to 400 nm in optical fibre 3-component stretcher: prisms+ gratings + Dazzler Recompressed in transmission grating compressor C-E phase stabilisation AOM Oscillator Fibre -broadening Dazzler Prism stretcher OPA stages compressor Grating stretcher Verdi

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 ~80 mm

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

Fibre Broadening Current oscillator bandwidth of 250 nm has transform limit of 11.2 fs Need spectrum covering 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

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

Diode-pumped Nd:YLF pump laser Pump with 25% duty cycle Synchronised to Ti:Sa oscillator 1mJ, 1 kHz 40ps 1047 nm regen Diodes SHG 5 x 1 kW diode bars 100 X 5 mm Nd:YLF rod

Current output of Nd:YLF pump laser Current setup: 3 pass amplification Output pulse energy up to 80mJ Damage to rod coating at 80mJ Amplified beam profile 3 rd pass on rod ~60mJ

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