New Concept of DPSSL - Tuning laser parameters by controlling temperature -
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Transcript of New Concept of DPSSL - Tuning laser parameters by controlling temperature -
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
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Contributors
S. Tokita, T. Norimatsu, N. Miyanaga, Y. Izawa
H. Nishioka, K. Ueda
M. Fujita
T. Kawashima, T. Ikegawa
Institute for Laser Technology
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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
1 0 5 3 n m L a s e r o u t p u t
1 0 k J (3 5 1 n m ) L a s e r o u t p u t
F r e q u e n c y c o n v e rs io n o p t ic s
W a te r c o o le d z ig - z a g s la b
1 k J L a s e r o u t p u t
L a s e r - d io d e m o d u le s
C o o lin g w a te r
1 kJ
10 kJ
100 kJ
1 MJ
Module
Segment
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Critical Factors for IFE Driver Materials
Emission Cross Section
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)
The
rmal
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 Thermal
Shock Parameter
△ Low Emission
Cross 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
Host
ab
(nm)
ab (FWHM)(nm)
em
(nm)
em (FWHM)(nm)
abs
(10-20 cm2)
em
(10-20 cm2)
RT
(ms)
(Wm-1K-1)
YAG 941 17 1030 12 0.8 2.03 13
S-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 Lasers with 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)
The
rmal
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 1100Wavelength (nm)
Cro
ss S
ectio
ncm
2)(
10k70k130k180k240k293k
0
5E-20
1E-19
1.5E-19
900 950 1000 1050 1100
Wavelength (nm)
Cro
ss S
ectio
n (cm
2)
10K
70K130K
180K240K
293K
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)
Th
erm
al c
on
duct
ivity
(W
/mK
)Sapphire
YAGYLF
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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 dam
ages
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 OSAKACavity Cavity Length : 910 mm TEM00 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 top Pump Power (max.) : 135 W Pump Intensity (max.) : 7.6 kW/cm2
Yb:YAG Crystal Sapphire-Sandwiched Conductive cooling with a LN Dewar Concentration : 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
slope = 80%
Absorbed pump power [W]
Ou
tpu
t po
we
r [W
]
Pout = 75 W.
opt = 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 20
0
0.5
1
1028.5 1029 1029.50
0.5
1
Wavelength (nm)
Spe
ctru
m
p = 6.8 ps(sech2)
FWHM = 0.26 nm
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0
2
4
6
8
0 20 40 60 80 100 120 140 160
g 0 (
cm-1)
Crystal Temperature (K)
Small Signal Gain Coefficient g0
g0 = 8 cm-1
at 1.3 kW/cm2
Calculation
Using the observed em and ab
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) – ex = 1 – (1 + log )/
T < 200 K
e x > 90%
ILD=2.5 kW/cm2
pump duration : 200 s
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
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)
e = 4 x 10-20 cm2
Es = 4.8 J/cm2
Laser Beam
HRAR
Pump
Pump Intensity
Ipump = 2.5 kW/cm2
@ 600s
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Calculated Output Energy with a Single Disk
2.7 kJ/diskCrysta
l Te
mp
eratu
re (K
)
L (cm)
Ma
xim
um
ext
ract
ion
en
erg
y (k
J)
Ext
ract
ion
en
erg
y flu
en
ce (
J/cm
2)
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 Intensity
Ipump = 2.5 kW/cm2
@ 600s
L = 7.5 cmAssuming ext = 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% ?
T abs U stoke st ex OL = O-O
95% 95% 100% 91% 90%
70% (tp = 1 ms)
80% (0. 6 ms)
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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Laser Electric 1 LD emission 0.5 Yb:YAG Laser 0.5x0.6=0.3
Optical Loss 0.5x0.3=0.15 LD Heat 0.5 Crystal Heat 0.5x0.1=0.05
Cryostat Electric X Refrigerate 0.05
Requirement of Electrical-Optical Efficiency Laser Output 0.3 Total Electrical Power 1+X
Electrical-Refrigerate Efficiency
> 0.1 X < 2
0.05
2> 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, )・ Thick Ceramics・ ns-pulse Demonstration(Q-switch)・ ps-pulse Amplification for Fast Ignition
Laser DiodeLaser Diode・ High Brightness (~10 kW/cm2 @ 200s)
Cooling Cooling ・ High Electrical-Refrigerate Efficiency of Cryostat ( > 2.5% @200K)