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New Concept of DPSSL - Tuning laser parameters by controlling temperature -

New Concept of DPSSL - Tuning laser parameters by controlling temperature -. ILE OSAKA. Junji Kawanaka. US-Japan Workshop on Laser-IFE 21-22 March 2005 General Atomics, San Diego. Contributors. Institute for Laser Technology. PHOTON IS OUR BUSINESS. ILE OSAKA. ILS/UEC. Tokyo.

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New Concept of DPSSL - Tuning laser parameters by controlling temperature -

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  1. New Concept of DPSSL- Tuning laser parameters by controlling temperature - ILE OSAKA Junji Kawanaka US-Japan Workshop on Laser-IFE 21-22 March 2005 General Atomics, San Diego

  2. Contributors Institute for Laser Technology PHOTON IS OUR BUSINESS ILE OSAKA ILS/UEC Tokyo S. Tokita, T. Norimatsu, N. Miyanaga, Y. Izawa H. Nishioka, K. Ueda M. Fujita T. Kawashima, T. Ikegawa

  3. Outline • IFE Laser Development and Laser Materials • ・ Nd:glassand 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

  4. 1. IFE Driver Development and Laser Materials

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

  6. Laser Programs for IFE Single Shot Repeatable

  7. 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 Segment 10 kJ Module 100 kJ 1 MJ

  8. Critical Factors for IFE Driver Materials Emission Cross Section s Thermal Shock Parameter RT Large Material Size Glass, Ceramics

  9. IFE Laser Materials in the World Nd Yb Yb:YAG Glass(Polaris) Glass (GEKKO XII,NIF,LMJ) Nd Thermal fracture limit t < 2 cm, DEst > 0.1 J/cm3 Yb:S-FAP(p) (Mercury) Yb:S-FAP(s) Yb Parastic oscillation limit g0L < 4 Saturation fluence limit J<10J/cm2 10000 Preferable Yb:YAG ○High Thermal Shock Parameter △ Low Emission Cross Section 1000 Thermal Shock Parameter (W/m) HAP4(HALNA) 100 10 0.5 5 1.0 10 50 Emission Cross Section (x 10-20 cm2)

  10. 2. Basic Researches of Cooled Yb:YAG crystal

  11. Yb-Doped Laser Materials ・ 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

  12. Yb:YAG Crystal RT (ms) k (Wm-1K-1) Host lab (nm) Dlab(FWHM) (nm) lem (nm) Dlem(FWHM) (nm) sabs (10-20 cm2) sem (10-20 cm2) 800 13 YAG 941 17 1030 12 0.8 2.03 - 2.0 S-FAP 899 4 1047 4 8.6 7.3 180 6.2 YLF 960 57 1018 47 0.46 0.75 - 2.4 GdCOB 900 11 1030 44 0.5 0.35 - 3.3 KYW 950 47 1000 76 3.5 3.0 - 3.3 KGW ↑ ↑ ↑ ↑ ↑ 2.2 glass 950 86 1003 77 0.12 0.37 0.85 200 Yb:YAG ・ High emission cross section ・High thermal conductivity 　・ High thermal shock parameter ☞ Diode-Pumped ns Lasers with High Pulse Energy High Average Power

  13. IFE Laser Materials in the World T=70K Yb:YAG T=150K 150K~270K T=300K 10000 Preferable Nd Yb 1000 Thermal Shock Parameter (W/m) Glass (GEKKO XII,NIF,LMJ) Glass(Polaris) 100 Yb:S-FAP(p) (Mercury) Yb:S-FAP(s) Tuning the emission cross section (saturation fluence) by cooling the crystal 10 0.5 5 1.0 10 50 Emission Cross Section (x 10-20 cm2)

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

  15. 4-Level Laser System at Low Temperature Low Temperature No Re-absorption 4-Level Room Temperature 2F5/2 Laser Diode ・Low Brightness Laser Re-absorption Pump 2F7/2 400~800cm-1 Efficient laser operation in diode-pump Quasi-3-Level

  16. Thermal Conductivity of Crystals 10000 1000 100 10 1 0 50 100 150 200 250 300 350 400 Sapphire YAG YLF Thermal conductivity (W/mK) Temperature (K)

  17. 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

  18. 135 W-Pumped CW Oscillator at 77K Cavity 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 LN Dewar Yb:YAG Sapphire (t = 1.6mm) 10mm 10mm Cupper Plate

  19. High Output Power for TEM00 80 60 40 20 0 S. Tokita et al., accepted for Appl. Phys. B Pout = 75 W. hopt = 71% TEM00 hslope = 80% Output power [W] 0 20 40 60 80 100 Absorbed pump power [W]

  20. Mode-Lock Oscillator with SESAM at 77K Output coupler (95% reflection) SESAM Chirped mirror (-400 fs2) LD Focusing lens assembly Cryo-cooled Yb:YAG 1 tp = 6.8 ps (sech2) 1 DlFWHM = 0.26 nm Autocorrelation 0.5 Spectrum 0.5 0 0 1028.5 1029 1029.5 –20 0 20 Wavelength (nm) Delay time (ps)

  21. Small Signal Gain Coefficientg0 g0 = 8 cm-1 at 1.3 kW/cm2 8 6 Calculation Using the observed sem and sab 4 2 0 0 20 40 60 80 100 120 140 160 Dope : 25 at.% Thickness : 1 mm g0 (cm-1) Crystal Temperature (K) We can calculate the laser gain accurately at any temperature. any pump intensity.

  22. How cold should we cool the crystal ? 1 0.8 0.6 Extraction efficiencyheｘ pump duration : 200 ms 0.4 100 kW/cm2 50 kW/cm2 10 kW/cm2 1 kW/cm2 0.2 0 0 50 100 150 200 250 300 Temperature (K) geff = g0exp(-Ein/Es) – a hex = 1 – (1 + log g)/g heｘ > 90% ILD=2.5 kW/cm2 T < 200 K

  23. Yb:YAG Active Mirror with a Large Disk at 200K Crystal Temperature (T = 200K) se = 4 x 10-20 cm2 Es = 4.8 J/cm2 Disk-Form ・Efficient Cooling ・Efficient Beam Coupling Active Mirror ・2-Pass Amplification 2 at. % L Laser Beam Pump Intensity Ipump = 2.5 kW/cm2 @ 600ms Conductive cooling Pump 53 cm AR HR Parasitic Oscillation (2g0r < 4) g0 = 0.038 cm-1 2r = 53 cm

  24. Calculated Output Energy with a Single Disk 250 4 240 3 230 2 220 1 210 0 200 Assuming hext = 90% L = 7.5 cm 10 2.7 kJ/disk 1.3 J/cm2 Extraction energy fluence (J/cm2) Maximum extraction energy (kJ) DT = 4 K 5 Crystal Temperature (K) f = 16 Hz Pump Intensity Ipump = 2.5 kW/cm2 @ 600ms 10 20 0 L (cm)

  25. Yb:YAG Module LD Pump 9 MJ 300 kJ 10 kJ Yb:YAG Active Mirror

  26. Can We Make All Efficiencies Higher Than 90% ? Upper State Optical Transfer Storage Beam Overlap Absorption Stokes Extraction hThabshUhstokehsthexhOL = hO-O 95% 95% 100% 91% 90% 70% (tp = 1 ms) 80% (0.６ms) 90% (0.2ms)90% =53% =60% Depend on Pump Duration →High-Brightness LD

  27. How Electrical-Refrigerate Efficiency of Cryostat should be ?ー 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 Output0.3 Total Electrical Power1+X Electrical-Refrigerate Efficiency > 0.1 X < 2 0.05 2 > 0.025 @200K

  28. 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 (n2, dn/dt,k) ・ Thick Ceramics ・ ns-pulse Demonstration(Q-switch) ・ ps-pulse Amplification for Fast Ignition Laser Diode ・High Brightness (~10 kW/cm2 @200ms) Cooling ・High Electrical-Refrigerate Efficiency of Cryostat ( > 2.5% @200K)

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