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

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)