Transcript A 5 fs high average power OPCPA laser system for

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
ћωI
Pump
Depleted Pump
Idler
ћωp
ћωs
Seed pulse
•
•
•
•
•
Non-linear
crystal
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
Amplified Seed
kidler 1
ksignal 1
ksignal 2
α = 1.2°
kpump
kidler 2
OPCPA has broad gain
spectrum
250
LBO
BBO
200
Gain
• 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
150
100
50
0
600
800
1000
1200
Wavelength (nm)
1
LBO 4.6 fs
BBO 5.3fs
0.5
0
-15
-10
-5
0
Time (fs)
5
10
15
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
OPA stages
Grating stretcher
Verdi
Oscillator
Prism
stretcher
AOM
C-E phase
stabilisation
Fibre -broadening
Dazzler
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
prisms
grating stretcher
Group delay (ps)
25
glass
20
grating compressor
15
residual
• Compressor uses large out of
plane angle to balance some
high order phase
10
5
0
• Residual chirp designed to
optimise Dazzler correction for
high efficiency
-5
-10
-15
-20
-25
650
750
850
950
Wavelength (nm)
1050
Fibre Broadening
400
HiBi 750-3 7mm 177mW with Iris
350
Oscillator (both corrected)
300
Intensity (arb)
250
200
150
100
50
0
-50
600
300
800
1000
1200
NL 2.4 140mW with iris
Oscillator (both corrected)
250
200
Intensity (arb)
• 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
150
100
50
0
-50
600
800
1000
1200
Recompression of fibrebroadened pulses
Broadened Spectra (Corrected) from SC-5.0-1040 7mm long
• Recompressed output of fibre
with Dazzler and prism system
154mW corrected
194mW corr
300
Intensity (arb)
250
• 3 types of fibre tried, all single
mode:
200
150
100
50
1) PM silica
0
-50600
2) Anomalous dispersion PCF
700
800
900
1000
1100
Wavelength
3) Normal Dispersion PCF
25000
ACF of SC-5.0-1040 194mW
• Recompressed to ~18 fs but all
show signs of double pulses and
variation across beam spatially
Signal (arb)
20000
15000
10000
5000
0
-150
-100
-50
0
50
100
Time (fs) - approx.
150
200
Diode-pumped Nd:YLF pump
laser
1mJ, 1 kHz 40ps
1047 nm regen
5 x 1 kW diode bars
Diodes
Diodes
SHG
• Pump with 25% duty cycle
• Synchronised to Ti:Sa oscillator
100 X 5 mm Nd:YLF rod
Current output of Nd:YLF
pump laser
3-pass amplification
Output Energy (mJ)
Output Energy
fluence
80
1.5
60
Rod
coating
damaged
40
1
0.5
20
0
0
40
60
80
Diode Current (A)
100
• Current setup: 3 pass amplification
• Output pulse energy up to 80mJ
• Damage to rod coating at 80mJ
120
Fluence (J/cm2)
2
100
Amplified beam profile
3rd 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