Use of Optical Phase Conjugation for Laser Irradiation on Laser Fusion Fuel Pellets

Transcription

1 Use of Optical Phase Conjugation for Laser Irradiation on Laser Fusion Fuel Pellets Shigeaki Uchida, Oleg Kotiaev, and Takayoshi Norimatsu* Institute for Laser Technology, Osaka, Japan *Institute of Laser Engineering, Osaka University, Japan Second US/Japan Workshop on Target Fabrication, Injection and Tracking General Atomics, San Diego, California February 3-4, 2003

2 Outline Why optical phase conjugation technique? Precise and automatic targeting of laser radiation Compensate turbulence inside the fusion reactor What is Optical Phase Conjugation? Experiments Efficiency Compensation Tracking How can it be used to irradiate ICF targets

3 Wave Front Distortion Compensation Effect of Optical Phase Conjugation Turbul ence Sour ce Ordi nary mi rror Turbul ence Sour ce Phase conj ugati ng mi rror Ordi nary mi rror Phase conj ugati on mi rror Phase conjugated waves share identical wave front shape with its original wave while their wave vectors are in opposite direction Can be used for automatic compensation / targeting

4 Phase conjugated wave can restore severe distortion Initial signal Distorted signal Corrected signal Initial signal Distorted signal Corrected signal 0.5 mrad Laser beam divergence = 0.78 Laser beam divergence = 0.71

5 Making Optical Phase Conjugating Waves Use diffraction gratings to recode and recover information of optical wave fronts Ways to make gratings (modulation of refractive index) Brillouin nonlinearity (acoustic waves) Photorefractive effects (electron excitation in semiconductors) For high power applications, Brillouin effects are suitable Stimulated Brillouin Scattering (SBS) Degenerate Four Wave Mixing Brilluoin Enhanced Four Wave Mixing

6 Phase Conjugation by SBS Nonl i near opti cal medi um Scatteri ng I nci dent l i ght?i Ď?aĚ?j Stationary waves by incident and reflecting waves Need certain incident power to overcome SBS threshold

7 Phase Conjugation by Four Wave Mixing (degenerate) Forward pump Phase conjugated light Amplification with phase conjugation No high intensity focus to the medium (compared to SBS)

8 Further improvement is possible by Brillouin Enhanced Four Wave Mixing SBS cell The SBS cell provides frequency shift so that the motion of interference and acoustic wave matches resulting in the energy transfer enhancement Helps the two pump beams counter-propagate by themselves

9 What is good Phase Conjugation Material? Broad Brilluoin resonance A few hundreds of MHz Low linear & nonlinear absorption a<10-5 cm -1, a NL <10-5 cm -1 GW -1 High optical breakdown threshold >100 GWcm -2

10 Materials for Phase Conjugation Gases Inert, diatomic, heavy molecular Need to be high pressure Transparent solids Silica Damage Inorganic liquids Metal tetrachlorides, Freon-113 (C 2 Cl 3 F 3 ) Toxic, carcinogen, ozone layer hazardous

11 Fluorocarbon liquids for Phase Conjugation F 6 C 16 (FC72), F 8 C 18 (FC75) Preferred properties Low absorption from UV to IR region High damage threshold >100 GW/cm -2 at 1mm and 1-ns pulse width Chemically stable Environmentally harmless No interaction with optical surfaces

12 The phase conjugater cell 300 mm 写真 Liquid: FC72 (C6F16)

13 Experimental setup for BEFWM characteristics Equi pment: RC PD SBS cel l L1 QWP P1 Bs1 T Bs2 M4 Bs3 Ndl aser RB S M2 RPP ODL PB HWP P2 M3 M2 P3 CS BE- DH cel l CCD L2 Bs4 SP PD M1 Nd- l aser: MOPA, E = 1 J, t = 20 ns, d = 15 mm SBS cel l - Sti mul ated Br i l l oui n Scatter i ng cel BE- ldh cel l - Bri l l oui n Enhanced Dynami c Hol ogr am cel l Nonl i near medi um - l i qui d fl uor ocarbon Fl our i ner t FC- 72 r efr acti ve i ndex n = sound vel oci ty vs = 512 m/sec Br i l l oui n fr equency DnB QWP = - 1. Quar 2 GHz ter Wave Pl ate HWP - Hal f Wave Pl ate T - Tel escope, M = 0. 4 P - Pol aril zer - Lens BS - Beamspl i t- ter M - Mi r- rrc or - Refer ence Cal or i meter PD - Pi n- di CCD ode - Vi deo camer a SP - Sensi ti ve paper RPP - Random Phase Pl ate ODL - Opti cal Del ay Li ne Si gnal s and energy: S - Si gnal to be cor r ected 40 PB mj - Pr obe Beam 500 RB -mj Refer ence Beam 100 CS mj - Cor r ected Si gnal 20 mj

14 Diffraction efficiency strongly depends on probe intensity Diffraction efficiency Signal Probe Referene (Pump) Normalized Laser Intensity, I/I0 BEFWM exhibits 10 times improvement in diffraction efficiency from DFWM All intensities are normalized to their optimum intensity

15 Interaction length of FWM is 30 cm Diffraction Efficiency Diffraction Efficiency Length of BE-DH Cell, cm Length of BE-DH Cell, cm

16 The degree of wavefront distortion recovery Initial Initial signal Distorted signal Corrected signal 0.5 mrad Laser Beam Divergence = 0.78 Laser Beam Divergence = 0.71 Laser beam divergence = 0.78 Laser beam divergence = 0.71 Laser beam divergence = energy in diffraction limited solid angle / total energy Fidelity of PC = 0.71 / 0.78 = 90 %

17 Experimental Setup for targeting Pol. BS Mi r ror Laser Nd: YAG 10pps (?`100 mj)?`mj?`50 mj M Tar get M P- BS Aberr ati on HWP SF72 cel l (Four wave mi xi ng cel l ) QWP SBS cel l I mage i n PC CCD (M=0. 25)

18 Al plane target Diameter : 1 mm

19 Light from target seen through turbulent media and the compensation effect of phase conjugation Without PC With PC Turbulence compensation and tracking function

20 How will PC be implemented to ICF target irradiation system I CF Laser Dri ver Power Ampl i fi ers i nj ecti on BEFWM cel l I CF target

21 Summary Use of optical phase conjugation technique is proposed for ICF target beaming Brillouin Enhanced Four Wave Mixing is a promising technique for the purpose Preliminary experiments show the technique exhibits sufficient wavefront compensation and beaming capability Improvement for increase energy usage through the system will be one of the future tasks