New Concept of DPSSL - Tuning laser parameters by controlling temperature - Junji Kawanaka
Contributors ILS/UEC Tokyo S. Tokita, T. Norimatsu, N. Miyanaga, Y. Izawa H. Nishioka, K. Ueda M. Fujita Institute for Laser Technology PHOTON IS OUR BUSINESS T. Kawashima, T. Ikegawa
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
1. IFE Driver Development and Laser Materials
Diode-Pumped Solid-State Lasers (DPSSL) Requirements Pulse Energy : 1MJ Repetition Rate : 16Hz Electrical-Optical Eff. : 10% Diode-pumped solid-state lasers
Laser Programs for IFE Single Shot Repeatable
Module Developments and Technical Issues 1 kj Segment 10 kj Module 10 kj(351nm) Laser output Laser-diode modules 1 kj Laser output 1053nm Laser output Water cooled zig-zag slab Frequency conversion optics 100 kj 1 MJ Cooling water Amplifier Optics Laser Material Laser Diode Cooling Technique 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
Critical Factors for IFE Driver Materials Emission Cross Section σ Thermal Shock Parameter R T Large Material Size Glass, Ceramics
IFE Laser Materials in the World Thermal Shock Parameter (W/m) 10000 1000 100 Glass(Polaris) Yb:S-FAP(s) Yb:YAG Preferable HAP4(HALNA) Glass (GEKKO XII,NIF,LMJ) Saturation fluence limit J<10J/cm 2 10 Parastic oscillation limit g 0 L < 4 0.5 1.0 5 10 50 Nd Yb:S-FAP(p) (Mercury) Yb Nd Yb Emission Cross Section (x 10-20 cm 2 ) Yb:YAG Thermal Low fracture Emission limit t < 2 cm, E st Cross > 0.1 J/cm Section 3 High Thermal Shock Parameter
2. Basic Researches of Cooled Yb:YAG crystal
Yb-Doped Laser Materials Absorption Spectral Region in NIR Long Fluorecence Life Time (900~1000 nm) (~ ms) Diode-Pump High Saturation Fluence (> 10 J/cm 2 ) Low Quantum Defect (< 10%) High Pulse Energy High Average Power Diode-Pumped High-Power Lasers
Yb:YAG Crystal Host λ ab (nm) λ ab (FWHM) (nm) λ em (nm) λ em (FWHM) (nm) σ abs (10-20 cm 2 ) σ em (10-20 cm 2 ) κ (Wm -1 K -1 ) R T (ms) YAG 941 17 1030 12 0.8 2.03 13 800 S-FAP 899 4 1047 4 8.6 7.3 2.0 - YLF 960 57 1018 47 0.46 0.75 GdCOB 900 11 1030 44 0.5 0.35 2.4 - KYW 950 47 1000 76 3.5 3.0 3.3 - KGW 2.2 3.3 - glass 950 86 1003 77 0.12 0.37 0.85 200 6.2 180 Yb:YAG High emission cross section High thermal conductivity High thermal shock parameter Diode-Pumped ns Lasers with High Pulse Energy High Average Power
IFE Laser Materials in the World Thermal Shock Parameter (W/m) 10000 1000 100 T=300K Glass(Polari s) Yb:S-FAP(s) Yb:YAG Preferable T=150K T=70K 150K~270K Glass (GEKKO XII,NIF,LMJ) Yb:S-FAP(p) (Mercury) Nd Yb Tuning the emission cross section (saturation fluence) 10 0.5 1.0 5 10 50 by cooling the crystal Emission Cross Section (x 10-20 cm 2 )
Absorption and Emission Spectra Absorption Cross Section cm2) 1.5E-19 1E-19 5E-20 10K 70K 130K 180K 240K 293K Emission 0 900 950 1000 1050 1100 Wavelength nm) Absorption spectral width is kept wide. Emission cross section can be changed within a factor of 7.
4-Level Laser System at Low Temperature Room Temperature Low Temperature 2 F 5/2 Laser Diode Low Brightness Pump Laser Re-absorption No Re-absorption 2 F 7/2 400~800cm -1 Quasi-3-Level 4-Level Efficient laser operation in diode-pump
Thermal Conductivity of Crystals Thermal conductivity (W/mK) 10000 1000 100 10 1 Sapphire YAG YLF 0 50 100 150 200 250 300 350 400 Temperature (K)
Why Cool the Materials? Because there are dramatic Improvements. 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
135 W-Pumped CW Oscillator at 77K 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 Sapphire (t = 1.6mm) Yb:YAG LN Dewar 10mm 10m m Yb:YAG Crystal Sapphire-Sandwiched Conductive cooling with a LN Dewar Concentration:25 at. % Thickness:0.6 mm Cupper Plate
High Output Power for TEM 00 S. Tokita et al., accepted for Appl. Phys. B 80 P out = 75 W. Output power [W] 60 40 20 η opt = 71% η slope = 80% TEM 00 0 0 20 40 60 80 100 Absorbed pump power [W]
Mode-Lock Oscillator with SESAM at 77K Output coupler (95% reflection) SESAM Chirped mirror (-400 fs 2 ) Cryo-cooled Yb:YAG Focusing lens assembly Autocorrelation 1 0.5 τ p = 6.8 ps (sech 2 ) Spectrum 1 0.5 λ FWHM = 0.26 nm 0 20 0 20 Delay time (ps) 0 1028.5 1029 1029.5 Wavelength (nm)
Small Signal Gain Coefficient g 0 8 g 0 = 8 cm -1 at 1.3 kw/cm 2 6 Dope : 25 at.% g 0 (cm -1 ) 4 Calculation Thickness: 1 mm Using the observed σ em and σ ab 2 0 0 20 40 60 80 100 120 140 160 Crystal Temperature (K) We can calculate the laser gain accurately at any temperature. any pump intensity.
How cold should we cool the crystal? 1 g eff = g 0 exp(-e in /E s ) α η ex = 1 (1 + log γ)/γ Extraction efficiency η e 0.8 0.6 0.4 0.2 pump duration : 200 µs 100 kw/cm 2 50 kw/cm 2 10 kw/cm 2 1 kw/cm 2 η e > 90% I LD =2.5 kw/cm 2 T < 200 K 0 0 50 100 150 200 250 300 Temperature (K)
Yb:YAG Active Mirror with a Large Disk at 200K Crystal Temperature (T = 200K) σ e = 4 x 10-20 cm 2 E s = 4.8 J/cm 2 Laser Beam 2 at. % L Disk-Form Efficient Cooling Efficient Beam Coupling Active Mirror 2-Pass Amplification Pump Intensity I pump = 2.5 kw/cm 2 @ 600µs Pump 53 cm Conductive cooling AR HR Parasitic Oscillation (2g 0 r < 4) g 0 = 0.038 cm -1 2r = 53 cm
Calculated Output Energy with a Single Disk Extraction energy fluence (J/cm 2 ) 4 3 2 1 0 Maximum extraction energy (kj) L = 7.5 cm 10 5 f = 16 Hz 250 240 230 220 210 0 10 20 200 L (cm) Crystal Temperature (K) Assuming η ext = 90% 2.7 kj/disk 1.3 J/cm 2 T = 4 K Pump Intensity I pump = 2.5 kw/cm 2 @ 600µs
Yb:YAG Module Yb:YAG Active Mirror LD Pump 9 MJ 300 kj 10 kj
Can We Make All Efficiencies Higher Than 90%? Optical Transfer Absorption Upper State Stokes Storage Extraction Beam Overlap η T η abs η U η stoke η st η ex η OL = η O-O 95% 95% 100% 91% 90% Depend on Pump Duration High-Brightness LD 70% (tp = 1 ms) 80% (0. ms) 90% (0.2ms) 90% =53% = 60%
How Electrical-Refrigerate Efficiency of Cryostat should be? 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 > 0.1 X < 2 Total Electrical Power 1+X Electrical-Refrigerate Efficiency 0.05 2 > 0.025 @200K
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. Amplifier Developments kj/disk in calculation Laser Materials κ Laser Diode ~ µ Cooling >