New Concept of DPSSL Tuning laser parameters by
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 ILE OSAKA
Contributors ILE OSAKA S. Tokita, T. Norimatsu, N. Miyanaga, Y. Izawa ILE OSAKA ILS/UEC Tokyo H. Nishioka, K. Ueda M. Fujita Institute for Laser Technology PHOTON IS OUR BUSINESS T. Kawashima, T. Ikegawa
Outline ILE OSAKA 1. 2. 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
Diode-Pumped Solid-State Lasers (DPSSL) ILE OSAKA Requirements Pulse Energy : 1 MJ Repetition Rate : 16 Hz Electrical-Optical Eff. : 10% Diode-pumped solid-state lasers
Laser Programs for IFE ILE OSAKA Single Shot Repeatable
Module Developments and Technical Issues ILE OSAKA §Amplifier 1 k. J ・Laser Material ・Laser Diode ・Cooling Technique Segment §Optics ・Wave Front Control ・Optical Switch ・High Damage Threshold Coating ・Non-Linear Optics ・Ultrashort Pulse Technique for F. I. 10 k. J Module §System Engineering 100 k. J 1 MJ ・Compact, Long-Life Power Supply ・Segment Assembly ・Spatial Beam Arrangement ・Focused Beam Profile ・Beam Steering
Critical Factors for IFE Driver Materials ILE OSAKA Emission Cross Section s Thermal Shock Parameter RT Large Material Size Glass, Ceramics
IFE Laser Materials in the World ILE OSAKA Thermal Shock Parameter (W/m) 10000 Preferable Nd Yb Yb: YAG 1000 Yb: YAG ○ High Thermal Shock Parameter HAP 4(HALNA) Glass(Polaris) 100 Yb: S-FAP(s) Saturation fluence limit J<10 J/cm 2 10 0. 5 1. 0 Glass (GEKKO XII, NIF, LMJ) Nd Thermal fracture limit △ Low Emission 3 t < 2 cm, DEst > 0. 1 J/cm Yb: S-FAP(p) (Mercury) Cross Section Yb Parastic oscillation limit g 0 L < 4 5 10 Emission Cross Section (x 10 -20 cm 2) 50
ILE OSAKA 2. Basic Researches of Cooled Yb: YAG crystal
Yb-Doped Laser Materials ILE OSAKA ・ Absorption Spectral Region in NIR (900~1000 nm) ・ Long Fluorecence Life Time (~ ms) Diode-Pump ・ High Saturation Fluence (> 10 J/cm 2) High Pulse Energy ・ Low Quantum Defect High Average Power (< 10%) ☞ Diode-Pumped High-Power Lasers
Yb: YAG Crystal lab Dlab Host (nm) (FWHM) YAG lem Dlem sabs sem k (Wm-1 K-1) ILE OSAKA RT (ms) (FWHM) (nm) (10 -20 cm 2) 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 6. 2 180 Gd. COB 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 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 ILE OSAKA Thermal Shock Parameter (W/m) 10000 Preferable Nd T=70 K Yb: YAG 1000 T=150 K T=300 K 100 Yb Glass(Polari s) Yb: S-FAP(s) 150 K~270 K Glass (GEKKO XII, NIF, LMJ) Yb: S-FAP(p) (Mercury) Tuning the 10 emission cross section (saturation fluence) 5 1. 0 10 50 by cooling 0. 5 the crystal Emission Cross Section (x 10 -20 cm 2)
Absorption and Emission Spectra ILE OSAKA Absorption Emission Absorption spectral width is kept wide. Emission cross section can be changed within a factor of 7.
4 -Level Laser System at Low Temperature ILE OSAKA Room Temperature 2 F Low Temperature 5/2 Laser Diode Laser ・Low Brightness Pump 2 F 7/2 Re-absorption No Re-absorption 400~800 cm-1 Quasi-3 -Level Efficient laser operation 4 -Level in diode-pump
Thermal Conductivity of Crystals Thermal conductivity (W/m. K) ILE OSAKA 10000 Sapphire YAG YLF 1000 10 1 0 50 100 150 200 250 300 350 400 Temperature (K)
Why Cool the Materials ? ILE OSAKA 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 77 K Cavity ILE OSAKA 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 k. W/cm 2 Yb: YAG Sapphire (t = 1. 6 mm) LN Dewar 10 mm 10 m m Cupper Plate Yb: YAG Crystal Sapphire-Sandwiched Conductive cooling with a LN Dewar Concentration: 25 at. % Thickness: 0. 6 mm
High Output Power for TEM 00 S. Tokita et al. , accepted for Appl. ILE Phys. B OSAKA 80 Output power [W] Pout = 75 W. TEM 00 hopt = 71% 60 hslope = 80% 40 20 0 0 20 40 60 80 Absorbed pump power [W] 100
Mode-Lock Oscillator with SESAM at 77 K ILE OSAKA Output coupler (95% reflection) SESAM Chirped mirror (-400 fs 2) Cryo-cooled Yb: YAG LD tp = 6. 8 ps (sech 2) 1 Spectrum Autocorrelation 1 Focusing lens assembly 0. 5 0 – 20 0 20 Delay time (ps) Dl. FWHM = 0. 26 nm 0. 5 0 1028. 5 1029. 5 Wavelength (nm)
Small Signal Gain Coefficient g 0 ILE OSAKA g 0 = 8 cm-1 8 at 1. 3 k. W/cm 2 Dope g 0 (cm-1) 6 Thickness : 25 at. % 4 : 1 m Calculation Using the observed sem and sab 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 ? geff = g 0 exp(-Ein/Es) – a hex = 1 – (1 + log g)/g Extraction efficiency hex 1 ILE OSAKA hex > 90% 0. 8 ILD=2. 5 k. W/cm 2 0. 6 pump duration : 200 ms 0. 4 100 k. W/cm 2 50 k. W/cm 2 1 k. W/cm 2 0. 2 T < 200 K 0 0 50 100 150 200 Temperature (K) 250 300
Yb: YAG Active Mirror with a Large Disk at 200 K Crystal Temperature (T = 200 K) ILE OSAKA Disk-Form ・Efficient Cooling ・Efficient Beam Coupling se = 4 x 10 -20 cm 2 Es = 4. 8 J/cm 2 2 at. % L Laser Beam Pump Intensity Ipump = 2. 5 k. W/cm 2 Pump Conductive cooling 53 cm @ 600 ms AR HR Parasitic Oscillation (2 g 0 r < 4) g 0 = 0. 038 cm-1 2 r = 53 cm Active Mirror ・ 2 -Pass Amplification
Calculated Output Energy with a Single Disk 3 2 1 0 Maximum extraction energy (k. J) 4 10 Assuming hext = 90% L = 7. 5 cm 250 240 230 5 220 f = 16 Hz 0 10 L (cm) 210 20 200 Crystal Temperature (K) Extraction energy fluence (J/cm 2) ILE OSAKA 2. 7 k. J/disk 1. 3 J/cm 2 DT = 4 K Pump Intensity Ipump = 2. 5 k. W/cm 2 @ 600 ms
Yb: YAG Module ILE OSAKA Yb: YAG Active Mirror LD Pump 9 MJ 300 k. J 10 k. J
Can We Make All Efficiencies Higher Than 90% ? Upper State Optical Transfer Absorption Storage Stokes Beam Overlap Extraction h. T habs h. U hstoke hst hex h. OL 95% 100% 91% 90% 70% (tp = 1 ms) 80% (0. 6 ms) 90% (0. 2 ms) 90% Depend on Pump Duration → High-Brightness LD = h. O-O = 53% = 60% ILE OSAKA
How Electrical-Refrigerate Efficiency of Cryostat should be ? ー ILE OSAKA Laser Electric 1 Cryostat Electric X LD emission 0. 5 LD Heat 0. 5 Refrigerate 0. 05 Yb: YAG Laser 0. 5 x 0. 6=0. 3 Optical Loss 0. 5 x 0. 3=0. 15 Crystal Heat 0. 5 x 0. 1=0. 05 Requirement of Electrical-Optical Efficiency Laser Output 0. 3 > 0. 1 Total Electrical Power 1+X Electrical-Refrigerate Efficiency 0. 05 2 > 0. 025 @200 K X<2
Summary and Future Plan – Yb: YAG – ILE OSAKA §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 (k. J/disk in calculation) ・No thermal effects such like thermal lensing ・High optical efficiency §Amplifier Developments Laser Materials ・ Material Characteristics (n 2, dn/dt, k) ・ Thick Ceramics ・ ns-pulse Demonstration(Q-switch) ・ ps-pulse Amplification for Fast Ignition Laser Diode ・High Brightness (~10 k. W/cm 2 @ 200 ms) Cooling ・High Electrical-Refrigerate Efficiency of Cryostat ( > 2. 5% @200 K)
- Slides: 28