ILE OSAKA New Concept of DPSSL - Tuning laser parameters by controlling temperature - Junji Kawanaka ILE OSAKA US-Japan Workshop on Laser-IFE 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/UEC Tokyo PHOTON IS OUR BUSINESS

ILE OSAKA 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

ILE OSAKA 1. IFE Driver Development and Laser Materials

ILE OSAKA Diode-Pumped Solid-State Lasers (DPSSL) Requirements Pulse Energy: 1MJ Repetition Rate: 16Hz Electrical-Optical Eff.: 10% Diode-pumped solid-state lasers

ILE OSAKA Laser Programs for IFE Single Shot Repeatable

ILE OSAKA 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 kJ 10 kJ 100 kJ 1 MJ Module Segment

ILE OSAKA Critical Factors for IFE Driver Materials Emission Cross Section  Thermal Shock Parameter R T Large Material Size Glass, Ceramics

ILE OSAKA Parastic oscillation limit g 0 L < 4 Saturation fluence limit J<10J/cm 2 Thermal fracture limit t 0.1 J/cm 3 Nd Yb Glass (GEKKO XII,NIF,LMJ) Glass(Polaris) Yb:S-FAP(p) (Mercury) Yb:S-FAP(s) Thermal Shock Parameter (W/m) Emission Cross Section (x cm 2 ) Preferable IFE Laser Materials in the World Nd Yb Yb:YAG HAP4(HALNA) Yb:YAG ○ High Thermal Shock Parameter △ Low Emission Cross Section

ILE OSAKA 2. Basic Researches of Cooled Yb:YAG crystal

ILE OSAKA ・ Absorption Spectral Region in NIR (900~1000 nm) ・ Long Fluorecence Life Time (~ ms) ・ High Saturation Fluence (> 10 J/cm 2 ) ・ Low Quantum Defect (< 10%) Diode-Pump High Pulse Energy High Average Power ☞ Diode-Pumped High-Power Lasers Yb-Doped Laser Materials

ILE OSAKA Yb:YAG Crystal Host ab (nm)  ab (FWHM ) (nm) em (nm)  e m (FWHM ) (nm)  abs ( cm 2 )  em ( cm 2 ) R T (ms)  (Wm -1 K -1 ) YAG S-FAP YLF KYW KGW↑↑↑↑↑ GdCOB Yb:YAG ・ High emission cross section ・ High thermal conductivity ・ High thermal shock parameter ☞ Diode-Pumped ns Lasers withHigh Pulse Energy High Average Power glass

ILE OSAKA Glass (GEKKO XII,NIF,LMJ) Yb:S-FAP(p) (Mercury) Yb:S-FAP(s) Glass(Polari s) Thermal Shock Parameter (W/m) Emission Cross Section (x cm 2 ) Preferable IFE Laser Materials in the World Nd Yb Yb:YAG T=150K T=70K T=300K 150K~270K Tuning the emission cross section (saturation fluence) by cooling the crystal

ILE OSAKA Absorption and Emission Spectra Absorption Emission Absorption spectral width is kept wide. Emission cross section can be changed within a factor of 7.

ILE OSAKA 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 2 F 7/2 2 F 5/2 Efficient laser operation in diode-pump

ILE OSAKA Thermal Conductivity of Crystals Temperature (K) Thermal conductivity (W/mK) Sapphire YAG YLF

ILE OSAKA 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 OSAKA Cavity Cavity Length ： 910 mm TEM 00 Diameter : 1.5 mm (1/e 2 ) 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/cm 2 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 Cupper Plate Sapphire (t = 1.6mm)

ILE OSAKA  slope = 80% Absorbed pump power [W] Output power [W] P out = 75 W.  opt = 71% High Output Power for TEM 00 TEM 00 S. Tokita et al., accepted for Appl. Phys. B

ILE OSAKA Mode-Lock Oscillator with SESAM at 77K LD SESAM Output coupler (95% reflection) Focusing lens assembly Cryo-cooled Yb:YAG Chirped mirror (-400 fs 2 ) Delay time (ps) Autocorrelation – Wavelength (nm) Spectrum  p = 6.8 ps (sech 2 )  FWHM = 0.26 nm

ILE OSAKA g 0 (cm -1 ) Crystal Temperature (K) Small Signal Gain Coefficient g 0 g 0 = 8 cm -1 at 1.3 kW/cm 2 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

ILE OSAKA How cold should we cool the crystal ? g eff = g 0 exp(-E in /E s ) –   ex = 1 – (1 + log  )/  T < 200 K  e ｘ > 90% I LD =2.5 kW/cm 2 pump duration : 200  s Temperature (K) Extraction efficiency  e ｘ 100 kW/cm 2 50 kW/cm 2 10 kW/cm 2 1 kW/cm 2

ILE OSAKA Yb:YAG Active Mirror with a Large Disk at 200K L 53 cm 2 at. % Conductive cooling Disk-Form ・ Efficient Cooling ・ Efficient Beam Coupling Active Mirror ・ 2-Pass Amplification Parasitic Oscillation (2g 0 r < 4) g 0 = cm -1 2r = 53 cm Crystal Temperature (T = 200K)  e = 4 x cm 2 E s = 4.8 J/cm 2 Laser Beam HRAR Pump Pump Intensity I pump = 2.5 kW/cm 600  s

ILE OSAKA Calculated Output Energy with a Single Disk 2.7 kJ/disk Crystal Temperature (K) L (cm) Maximum extraction energy (kJ) Extraction energy fluence (J/cm 2 )  T = 4 K f = 16 Hz J/cm 2 Pump Intensity I pump = 2.5 kW/cm 600  s L = 7.5 cm Assuming  ext = 90%

ILE OSAKA Yb:YAG Module LD Pump 9 MJ 300 kJ 10 kJ Yb:YAG Active Mirror

ILE OSAKA 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

ILE OSAKA Laser Electric 1 LD emission0.5Yb:YAG Laser 0.5x0.6=0.3 Optical Loss0.5x0.3=0.15 LD Heat0.5 Crystal Heat0.5x0.1=0.05 Cryostat Electric XRefrigerate0.05 Requirement of Electrical-Optical Efficiency Laser Output0.3 Total Electrical Power1+X Electrical-Refrigerate Efficiency > 0.1 X < > How Electrical-Refrigerate Efficiency of Cryostat should be ? ー

ILE OSAKA Summary and Future Plan – Yb:YAG – §Tuning of parameters by controlling the temperature has been proposed instead 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 Materials ・ Material Characteristics (n 2, dn/dt,  ) ・ Thick Ceramics ・ ns-pulse Demonstration(Q-switch) ・ ps-pulse Amplification for Fast Ignition Laser Diode ・ High Brightness (~10 kW/cm 200  s)Cooling ・ High Electrical-Refrigerate Efficiency of Cryostat ( >