High-Power solid-state Slab laser with optimized diode pumping design

Transcription

1 High-Power solid-state Slab laser with optimized diode pumping design Antonio Lapucci & Marco Ciofini Istituto Nazionale di Ottica Applicata, Largo E. Fermi 6 I Firenze Italy Tel: Fax: lapo@ino.it 1

2 Summary: Background & Motivations The slab laser architecture» Zig-zag scheme & pumping geometry» Thermal loading considerations» Resonator schemes Preliminary experimental results:» Pumping optimization» Extraction efficiency» Beam Quality & Thermal lens effects Conclusions 2

3 Motivation and background: DRIVERS: Industrial high power DPSSL Combination of high power extraction and beam quality Ruggedness and compactness TRADE-OFFS: Slab geometry vs. rod or disk geometry Side-pumping vs. end-pumping schemes * 3

4 The Ceramic Nd:YAG slab Dimensions: 3mm x 10mm x 100 mm brewster angle cut Manufactured by Konoshima Chemical / Baikowski Group Parameter Single crystal YAG Typical value Ceramic YAG Typical value Refractive index Young Module 2.82 x [N/m 2 ] 2.82 x [N/m 2 ] Poisson Coefficient Thermal Expansion Coefficient 8.0 x 10-6 [1/ C] 8.0 x 10-6 [1/ C] Specific heat 5.9 x 10 2 [J/Kg C] 5.9 x 10 2 [J/Kg C] Mass density 4.55 x 10 3 [Kg/m 3 ] 4.55 x 10 3 [Kg/m 3 ] Thermal conductivity 10.7 [W/m C] 10.7 [W/m C] Maximum Doping (uniform) ~ 1.2 % >4 % Fracture limit Stress x 10 8 [N/m 2 ] 1 x 10 9 [N/m 2 ] 4

5 The zig-zag geometry beam aperturing α angle θ angle θ B 1 = tan ( n2 / n1) sin( θ ') = ( n 1 / n 2 ) sin( θ ) α B 90 θ = B θ = 90 θ ' Slab n α (cut angle) θ (bounce angle) Nd:YAG Nd:Glass α > θ partial active medium filling α < θ beam aperturing (complete filling) 5

6 The zig-zag propagation 22 internal bounces Large a.m. filling with small beam aperturing 6

7 Pumping geometry: Slab-side illumination Parallel cooling and pumping Simple direct coupling Horizontal diode stacks 7

8 Face Pumping Scheme: pump radiation coupling (slab laser front view) Parallel duct coupling Optimized duct coupling Free space propagation (matched distance) 8

9 Face Pumping Scheme: diode radiation distributions Skewed duct Parallel duct Free Propagaton Ray tracing simulations Fluorescence measurements 9

10 Face Pumping Scheme: comparison of extraction efficiencies (with a m.m. stable resonator) 10

11 Extraction efficiency: tests performed with two 600W QCW arrays ( 1200 W total input power) Pump coupling scheme Free propagation Parallel Duct Skewed Duct P max 160 W 350 W 350 W Optical to optical efficiency 13.3% 29% 29% Slope efficiency 20% 48% 51% 11

12 Maximum Power loading: 3 mm thick ceramic slab F.E.M. calculations 3 kw per side with uniform distribution 1.5 mm thick ceramic slab >12 kw total admitted power Higher Optical losses Rigrod-analysis gives 2.8% r.t. in our present device. Self-quenching due to higher ion concentration stress temperature 12

13 Possible resonator schemes: Symmetric stable resonator (adopted in our preliminary experiments) Hybrid stable-unstable resonator Folded stable resonator 13

14 Beam shape (stable resonator): Near field profiles Propagation caustic 14

15 Beam shape (stable-unstable resonator): Propagation caustic (focussing with a 700 mm E.F.L.) Beam profiles 15

16 Beam quality: (with present dimensions and pumping level) Resonator Type Stable nr.1 (0.5m) Stable nr.2 (1.0 m) Hybrid stable-unstable Maximum 350 W 320 W 200 W Power Conversion ~30% ~26% ~16.5% Efiiciency M 2 x (BPP x ) 15 (5 mm.mrad) 9 (3 mm.mrad) 9 (3 mm.mrad) M 2 y (BPP y ) 180 (60 mm.mrad) 120 (40 mm.mrad) 3 (1 mm.mrad) 16

17 Thermal lens effect (narrow direction): N.F. diameter unaffected Div. Slightly decreasing Thermal Lens: Diopt.Power < 0.2 (0 180 W load) 17

18 Foreseen Extraction Performance: 1000 W output power with M 2 ~ 3 from W diode pumping power (optimizing the slab thickness) ( 22-25% optical to optical efficiency) (9-10% electrical efficiency) 18

19 Parameter space location of our source: Direct diodes Lamp pump. YAG CO 2 Fiber Disk Our source (foreseen) Other slabs 19

20 Conclusions: We described the architecture of compact diode pumped ceramic slab laser. Preliminary experimental results confirm the attainment of an interesting combination of extraction efficiency, beam quality & thermal stability. Our laser head is immediately scalable to the kw power level (simply raising the number of pumping units). 20