a 5 fs high average power opcpa laser system for attosecond pulse production philip bates, yunxin...
TRANSCRIPT
A 5 fs high average power OPCPA laser system for
attosecond pulse production
Philip Bates, Yunxin Tang, Emma Springate and Ian RossCentral 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– ћωseed + ћωidler = ћω pump
– nsωseed + niωidler = npω pump
• Very broad bandwidth esp. in non-collinear geometry• Pump beam aberrations and phase errors transferred to
idler– φidler = φseed - φpump
• No thermal deposition in gain medium, no thermal loading• High Gain with efficient extraction of pump energy
Pump
Non-linear crystal
Idlerћωp
ћωI
ћωs
Amplified Seed
Depleted Pump
Seed pulse
ksignal 1kidler 1
ksignal 2kidler 2
kpumpα = 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
OscillatorFibre -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 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
600 800 1000 1200-50
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400 HiBi 750-3 7mm 177mW with Iris Oscillator (both corrected)
Inte
nsity
(arb
)
600 800 1000 1200-50
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150
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300 NL 2.4 140mW with iris Oscillator (both corrected)
Inte
nsi
ty (arb
)
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
Broadened Spectra (Corrected) from SC-5.0-1040 7mm long
-50
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600 700 800 900 1000 1100
Wavelength
Inte
ns
ity
(a
rb)
154mW corrected 194mW corr
Diode-pumped Nd:YLF pump laser
• Pump with 25% duty cycle• Synchronised to Ti:Sa oscillator
1mJ, 1 kHz 40ps1047 nm regen
Diodes
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
3-pass amplification
0
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40 60 80 100 120Diode Current (A)
Out
put
Ene
rgy
(mJ)
0
0.5
1
1.5
2
Flue
nce
(J/c
m2)
Output Energyfluence
Rod coating damaged
Amplified beam profile3rd 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