new concept of dpssl ile osaka - university of...
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ILE OSAKANew Concept of DPSSL- Tuning laser parameters by controlling temperature -
Junji KawanakaILE OSAKA
US-Japan Workshop on Laser-IFE21-22 March 2005
General Atomics, San Diego
ILE OSAKA
Contributors
S. Tokita, T. Norimatsu, N. Miyanaga, Y. Izawa
H. Nishioka, K. Ueda
M. Fujita
T. Kawashima, T. Ikegawa
Institute for Laser Technology
ILE OSAKA
ILS/UECTokyo
PHOTON IS OUR BUSINESS
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Outline
1. IFE Laser Development and Laser Materials・ Nd:glass and Yb:YAG
2. Basic Researches of Cooled Yb:YAG crystal ・ Advantages of Cryogenic Cooling
・ High Average Power and High Optical efficiency (CW Oscillator)
・ Mode-Lock Oscillator with SESAM
3. Summary and Future Plan
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1. IFE Driver Development and Laser Materials
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Diode-Pumped Solid-State Lasers (DPSSL)
Requirements
Pulse Energy : 1MJ
Repetition Rate : 16Hz
Electrical-Optical Eff. : 10%
Diode-pumped solid-state lasers
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Laser Programs for IFE
Single Shot
Repeatable
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Module Developments and Technical Issues
§Amplifier・Laser Material・Laser Diode・Cooling Technique
§Optics・Wave Front Control・Optical Switch・High Damage Threshold Coating・Non-Linear Optics・Ultrashort Pulse Technique for F.I.
§System Engineering・Compact, Long-Life Power Supply・Segment Assembly・Spatial Beam Arrangement・Focused Beam Profile・Beam Steering
1053nm Laser output
10 kJ(351nm) Laser output Frequency conversion optics
Water cooled zig-zag slab
1 kJ Laser output
Laser-diode modules
Cooling water
1 kJ
10 kJ
100 kJ
1 MJ
Module
Segment
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Critical Factors for IFE Driver Materials
Emission Cross Sectionσ
Thermal Shock ParameterRT
Large Material Size
Glass, Ceramics
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Parastic oscillation limitg0L < 4
Saturation fluence limitJ<10J/cm2
Thermal fracture limitt < 2 cm, ∆Est > 0.1 J/cm3
Nd
Yb
Glass(GEKKO XII,NIF,LMJ)
Glass(Polaris)
Yb:S-FAP(p) (Mercury)Yb:S-FAP(s)
Ther
mal
Sho
ck P
aram
eter
(W/m
)
1000
10
100
10000
Emission Cross Section (x 10-20 cm2)
0.5 1.0 50105
Preferable
IFE Laser Materials in the World
Nd
Yb
Yb:YAG
HAP4(HALNA)
Yb:YAG
○ High ThermalShock Parameter
△ Low EmissionCross Section
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2. Basic Researches of Cooled Yb:YAG crystal
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・ Absorption Spectral Region in NIR (900~1000 nm)
・ Long Fluorecence Life Time (~ ms)
・ High Saturation Fluence (> 10 J/cm2)
・ Low Quantum Defect (< 10%)
Diode-Pump
High Pulse Energy
High Average Power
☞ Diode-Pumped High-Power Lasers
Yb-Doped Laser Materials
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Yb:YAG Crystal
Hostλab
(nm)
∆λab(FWHM)
(nm)
λem
(nm)
∆λem(FWHM)
(nm)
σabs
(10-20 cm2)
σem
(10-20 cm2)
RT
(ms)
κ
(Wm-1K-1)
YAG 941 17 1030 12 0.8 2.03 13S-FAP 899 4 1047 4 8.6 7.3 -2.0
YLF 960 57 1018 47 0.46 0.75
800
1806.2
KYW 950 47 1000 76 3.5 3.0 -3.3
KGW ↑ ↑ ↑ ↑ ↑ 2.2 -3.3
GdCOB 900 11 1030 44 0.5 0.35 -
Yb:YAG ・ High emission cross section ・ High thermal conductivity ・ High thermal shock parameter
☞ Diode-Pumped ns Laserswith High Pulse Energy
High Average Power
glass 950 86 1003 77 0.12 0.37 200
2.4
0.85
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Glass(GEKKO XII,NIF,LMJ)
Yb:S-FAP(p) (Mercury)Yb:S-FAP(s)
Glass(Polaris)
Ther
mal
Sho
ck P
aram
eter
(W/m
)
1000
10
100
10000
Emission Cross Section (x 10-20 cm2)
0.5 1.0 50105
Preferable
IFE Laser Materials in the World
Nd
YbYb:YAG
T=150K
T=70K
T=300K 150K~270K
Tuning the emission cross section (saturation fluence)by cooling the crystal
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Absorption and Emission Spectra
0
5E-21
1E-20
1.5E-20
900 950 1000 1050 1100
Wavelength (nm)
Cro
ss S
ection(c
m2)
10k
70k
130k
180k
240k
293k
0
5E-20
1E-19
1.5E-19
900 950 1000 1050 1100
Wavelength (nm)
Cro
ss S
ectio
n(cm
2)
10K70K130K180K240K293K
Absorption Emission
Absorption spectral width is kept wide. Emission cross section can be changedwithin a factor of 7.
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Room Temperature
Pump
Laser
Re-absorption
4-Level Laser System at Low Temperature
Laser Diode・Low Brightness
400~800cm-1
Quasi-3-Level
Low Temperature
No Re-absorption
4-Level
2F7/2
2F5/2
Efficient laser operation in diode-pump
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Thermal Conductivity of Crystals
1
10
100
1000
10000
0 50 100 150 200 250 300 350 400
Temperature (K)
Ther
mal
con
duct
ivity
(W/m
K)
SapphireYAGYLF
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Why Cool the Materials ?
1. Wide Tuning Range of Emission Cross Section (Saturation Fluence)
→ Realize an efficient energy extraction without optics damages
2. 4-Level Laser System
→ Enough Laser gain even in diode-pump
3. Improved Thermal Conductivity
→ High average power operation
Because there are dramatic Improvements.
ILE OSAKACavityCavity Length : 910 mmTEM00 Diameter : 1.5 mm (1/e2) Coupler : R = 75%, r = 5000 mm
Pump (on the Crystal)Beam Dia. : 1.5 mm (FWHM)Spatial Profile : Flat topPump Power (max.) : 135 WPump Intensity (max.) : 7.6 kW/cm2
Yb:YAG CrystalSapphire-SandwichedConductive cooling with a LN DewarConcentration:25 at. %Thickness:0.6 mm
135 W-Pumped CW Oscillator at 77K
Yb:YAG LN Dewar
10mm
10mm
Cupper Plate
Sapphire(t = 1.6mm)
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0
20
40
60
80
0 20 40 60 80 100
ηslope = 80%
Absorbed pump power [W]
Out
put p
ower
[W]
Pout = 75 W.
ηopt = 71%
High Output Power for TEM00
TEM00
S. Tokita et al., accepted for Appl. Phys. B
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Mode-Lock Oscillator with SESAM at 77K
LD
SESAM
Output coupler(95% reflection)
Focusing lensassembly
Cryo-cooledYb:YAG
Chirped mirror(-400 fs2)
Delay time (ps)
Aut
ocor
rela
tion
–20 0 200
0.5
1
1028.5 1029 1029.50
0.5
1
Wavelength (nm)
Spe
ctru
m
τp = 6.8 ps(sech2)
∆λFWHM = 0.26 nm
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0
2
4
6
8
0 20 40 60 80 100 120 140 160
g 0(c
m-1
)
Crystal Temperature (K)
Small Signal Gain Coefficient g0
g0 = 8 cm-1
at 1.3 kW/cm2
CalculationUsing the observed σem and σab
We can calculate the laser gain accurately at any temperature.any pump intensity.
Dope : 25 at.%Thickness: 1 mm
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How cold should we cool the crystal ?
geff = g0exp(-Ein/Es) – αηex = 1 – (1 + log γ)/γ
T < 200 K
ηex > 90%ILD=2.5 kW/cm2
pump duration : 200 µs
0
0.2
0.4
0.6
0.8
1
0 50 100 150 200 250 300
Temperature (K)
Ext
ract
ion
effic
ienc
y η e
x
100 kW/cm2
50 kW/cm2
10 kW/cm2
1 kW/cm2
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Yb:YAG Active Mirror with a Large Disk at 200K
L
53 cm
2 at. %
Conductivecooling
Disk-Form・Efficient Cooling・Efficient Beam Coupling
Active Mirror・2-Pass Amplification
Parasitic Oscillation (2g0r < 4)g0 = 0.038 cm-1
2r = 53 cm
Crystal Temperature (T = 200K)σe = 4 x 10-20 cm2
Es = 4.8 J/cm2
Laser Beam
HRAR
PumpPump Intensity
Ipump = 2.5 kW/cm2
@ 600µs
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Calculated Output Energy with a Single Disk
2.7 kJ/diskCrystal Tem
perature (K)
L (cm)
Max
imum
ext
ract
ion
ener
gy (k
J)
Ext
ract
ion
ener
gyflu
ence
(J/c
m2 )
∆T = 4 K
0
2
3
4
1
5
10
f = 16 Hz
0 10 20200
210
220
240
250
230
1.3 J/cm2
Pump IntensityIpump = 2.5 kW/cm2
@ 600µs
L = 7.5 cmAssuming ηext = 90%
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Yb:YAG Module
LD Pump9 MJ
300 kJ
10 kJ
Yb:YAGActive Mirror
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Can We Make All Efficiencies Higher Than 90% ?
ηT ηabs ηU ηstoke ηst ηex ηOL = ηO-O
95% 95% 100% 91% 90%
70% (tp = 1 ms)
80% (0.6ms)
90% (0.2ms) 90% = 53% = 60%
Optical Transfer
Absorption
Upper State
Stokes
Storage
Extraction
Beam Overlap
Depend on Pump Duration
→ High-Brightness LD
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LaserElectric 1 LD emission 0.5 Yb:YAG Laser 0.5x0.6=0.3
Optical Loss 0.5x0.3=0.15LD Heat 0.5 Crystal Heat 0.5x0.1=0.05
CryostatElectric X Refrigerate 0.05
Requirement of Electrical-Optical EfficiencyLaser Output 0.3Total Electrical Power 1+X
Electrical-Refrigerate Efficiency
> 0.1 X < 2
0.052
> 0.025 @200K
How Electrical-Refrigerate Efficiency of Cryostat should be ?ー
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Summary and Future Plan – Yb:YAG –
§Tuning of parameters by controlling the temperature has been proposedinstead of producing new materials.
§Cooled Yb:YAG ceramics is one of the promised laser materials.
・High pulse energy (kJ/disk in calculation)
・No thermal effects such like thermal lensing・High optical efficiency
§Amplifier Developments
Laser MaterialsLaser Materials・ Material Characteristics (n2, dn/dt, κ)・ Thick Ceramics・ ns-pulse Demonstration(Q-switch)・ ps-pulse Amplification for Fast Ignition
Laser DiodeLaser Diode・High Brightness (~10 kW/cm2 @ 200µs)
Cooling Cooling ・High Electrical-Refrigerate Efficiency of Cryostat ( > 2.5% @200K)