1. LSRL Go to contents IFE laser driver using independent phase control of stimulated Brillouin scattering phase conjugate mirrors by a self-generated density modulation Hong Jin Kong, Seong Ku Lee, and Dong Won KAIST

2. LSRL Go to contents Wave-front dividing method New laser fusion driver with SBS beam combination

3. LSRL Go to contents Contents 1.MotivationMotivation 2.Cross type amplifier with symmetric SBS-PCMsCross type amplifier with symmetric SBS-PCMs 3.Beam combinationBeam combination 4.Phase controlling of SBS-PCM wavePhase controlling of SBS-PCM wave 5.Pre-pulse technique for preserving the SBS pulse shapePre-pulse technique for preserving the SBS pulse shape 6.ConclusionsConclusions 7.Future workFuture work

4. LSRL Go to contents Motivation LFE has the bottle neck in its laser driver with high rep. rate over 10 Hz. (NIF operates at 1 shot per several hours)

5. LSRL Go to contents New laser fusion driver Using SBS beam combination a.Scalable easily b.Maintenance easy c.High repetition rate over 10Hz

6. LSRL Go to contents Why bottleneck in lasers? High thermal load for cooling Cooling Solid amp

7. LSRL Go to contents 1.LD pumping can reduce the thermal load. 2.Ceramic crystals with high thermal conductivity can reduce the thermal load. 3.Fast ignition can reduce the driver energy. But these technologies will not be able to realize the practically useful fusion driver technology. Beam combination using SBS-PCM can be one of the promising technologies for the laser fusion driver. How to solve the thermal load?

8. LSRL Go to contents Break up Beam combination Easy cooling Piston error Phase distortion

9. LSRL Go to contents How to solve the problems? 1.Phase distortions By phase conjugation mirror 2.Piston errors By phase locking / controlling the phase

10. LSRL Go to contents Phase conjugation mirror (PCM) Conventional mirror Phase conjugation mirror

11. LSRL Go to contents Comparison of PCM to conventional mirror

12. LSRL Go to contents Types of PCMs 4  mixing SBS MediumcrystalAny g, l, s, or p Damage thresholdlowhigh Size of the beam Limited by the crystal No limitation Complexitycomplicatedmost simple Phase controlyesNo before this work

13. LSRL Go to contents Stimulated Brillouin scattering (SBS) Principles of SBS 1.Scattering by acoustic phonon induced by laser field a.Acoustic phonon = moving grating with the acoustic velocity in the medium 2.Backward scattering ; high reflectivity a.50% for 10mJ pumping b.90 % for 100mJ pumping 3.Phase conjugated wave ; high fidelity over 90% 4.Frequency shift ; ~ 1GHz for liquid

14. LSRL Go to contents Single mode pumping (  ~120MHz,  =350MHz) (No break down) Multi mode pumping (  ~30GHz,  =350MHz) (Break down occurs) Reflectivity and breakdown probability depending on the laser mode of SBS-PCM with FC-75

15. LSRL Go to contents Cross-type amplifier with symmetric PCMs:

16. LSRL Go to contents Cross-type amplifier with symmetric PCMs: 1.Compensate the phase distortions such as birefringence, thermal lensing effect, etc. 2. Perfect optical isolation possible 3.Insensitive to misalignment

17. LSRL Go to contents Beam combination using SBS-PCM for highly repetitive high power and high energy laser Proposed by H.J.Kong, Optical Review 4, 277-283(1997) and Fusion Eng. & Design 44, 407-417(1999)

18. LSRL Go to contents Phase controlling of SBS wave 1.Interference between the beams at their boundaries in beam combination gives spatial spiking if their phases are not matched. 2.Phase difference between the beams should be less than /4 for constructive interference at the boundary of the combination not give spatial spiking. 3.It is necessary to control the phases of the SBS waves to match the phase difference between the neighboring beams less than /4

19. LSRL Go to contents Previous works for phase locking b) Back-seeding of Stokes wave T.R.Loree, et. al., Opt. Lett. 12, 178 (1987) 1. Overlapping the SBS focal points locks the phases of the beams. 2. Phase locking by back seeding the Stokes shifted beam, which locks the phase of the PC wave. PC can be broken by back seeding beam No PC anymore a)Overlap of two focal points D.A.Rockwell and C.R.Giuliano, Opt. Lett. 11, 147 (1986) Impractical for many beams > 10

20. LSRL Go to contents Proposed “Self-phase control” 1.Feed back mirror > Counter propagating beams > standing wave > density modulation 2.Standing density modulation locks the ignition position of the moving phonon 3.This phonon locks the phase of the SBS wave. 4.Phase controlling is possible by positioning the feed back mirror. (a)Concentric type(b) Confocal type

21. LSRL Go to contents Basic concept of a new phase control

22. LSRL Go to contents SBS wave reflects from the density wave And we should determine the initial position and time, z 0 and t 0, at which the density modulation occurs. z 0 is determined by the ignition position of the density wave. t 0 is determined by the ignition time when the acoustic grating occurs by the pumping pulse. Theoretical backgrounds for phase control of SBS

23. LSRL Go to contents Let us assume that Then, (Pump wave) (Stokes wave) (Density modulation) (DC) (Fast osc.) (Acoustic wave) Thus, If the acoustic wave is considered as,and..,., Let us assume that

24. LSRL Go to contents, where t 0 and z 0 are the initial time and position when and where the acoustic wave is ignited and generated. For fixing  a, t 0 and z 0 must be fixed. The density modulation by the standing wave can fix the ignition position z 0 of the acoustic wave, assuming that the nodal points of the standing wave act as an ignition position. Then, the initial phase of the acoustic wave has the form: If we assume that the acoustic wave is generated at t 0 and z 0, then the acoustic wave has the form:

25. LSRL Go to contents Determination of z 0 Nodal point of the standing wave z 0 has the uncertainty of ( p /2)N. But it does not affect the accuracy of the phase.

26. LSRL Go to contents Determination of t 0 Assume that the pump beam generates acoustic wave by its front side of the energy:,where E c and P(t) are the critical energy for generating the acoustic grating and the pump pulse profile, respectively. t c is the time when the ignition occurs. t c is determined by the pumping pulse energy and its profile.

27. LSRL Go to contents Let t 0 =t c. For fixing t c, the pump energy must be fixed.  a =  t 0 – k a z 0 The uncertainty of the phase with dz 0 =0, d  a can be expressed by E t is the total energy of the pump pulse. For getting a good beam combination, d  a, <  /4 ( /8).

28. LSRL Go to contents Pulse width ~10 ns (a=8.66) 10 ns Pump energy E t vs. Critical time t c d  a <  /4

29. LSRL Go to contents Pump energy E t vs. d  a <  /4

30. LSRL Go to contents d  a <  /4

31. LSRL Go to contents d  a <  /4 19.2% energy fluctuation for 1000mJ 15.1% ~ 500mJ 8.4% ~ 100mJ 2.8% ~ 10mJ 1.4% ~ 5mJ

32. LSRL Go to contents d  a <  /50(  1.5% energy fluctuation for 1000mJ 1.2% ~ 500mJ 0.67% ~ 100mJ 0.22% ~ 10mJ 0.11% ~ 5mJ

33. LSRL Go to contents d  a <  /50(  3.4% energy fluctuation for 10J 2.7% energy fluctuation for 5J

34. LSRL Go to contents Experimental setup for self-phase locking of two beams (wavefront dividing) M1&M2, mirror; W1&W2, wedge; L1&L2, cylindrical lens: L3&L4, focusing lens, CM1&CM2, concave mirror; HWP1&HWP2, half wave-plate; P1, polarizer; PBS, polarization beam splitter

35. LSRL Go to contents Wave-front dividing 1.Requirements: a.Phase accuracy : < /4 for constructive interference b.Image relaying from the divider to the center of the amplifier

36. LSRL Go to contents Normal case: No locking Intensity profile of interference for 160 shots Intensity profile of interference Average fluctuation ~ 0.3

37. LSRL Go to contents Intensity profile of interference for 203 shots Concentric case Average fluctuation ~ 0.165 178/203 : success (88%)

38. LSRL Go to contents Confocal case Intensity profile of interference for 238 shots Average fluctuation ~ 0.135 228/238 : success (96%)

39. LSRL Go to contents Pump energy: 9.4 mJ and 4.66 mJ 242/251 : success (96%) Experimental setup for amplitude dividing Average fluctuation ~ 0.12

40. LSRL Go to contents Experimental setup for amplitude dividing 256/256 : success (100%) Pump energy:10.56 mJ and 10.55 mJ Intensity profile of interference for 256 shots Average fluctuation ~0.08

41. LSRL Go to contents Two mirrors Average fluctuation  0.09 256 shots

42. LSRL Go to contents Comparison of control and mirror cases Mirror caseControlling case

43. LSRL Go to contents Amplitude dividing Requirements: Phase accuracy:  for intensity fluctuation :  Sin 2 ( 

44. LSRL Go to contents Pre-pulse technique Motivation: 1. Backward SBS wave has a steep rising edge. 2. This can generate undesirable effects such as optical breakdown, poor fidelity when entering next SBS cell. Purpose: Preserving a waveform of the backward SBS wave using a pre- pulse.

45. LSRL Go to contents Pulse shape of the pump and the SBS wave Transmitted and spent for generating an acoustic wave

46. LSRL Go to contents Experimental setup

47. LSRL Go to contents Waveform of the incident and reflected wave (a) incident wave (b) reflected wave

48. LSRL Go to contents E(pre)=2mJ [E(main)=10mJ, delay time=5ns] (a) with pre-pulse(b) without pre-pulse There is no significant difference between (a) and (b)

49. LSRL Go to contents E(pre)=3mJ (a) with pre-pulse (b) without pre-pulse (c) Comparison between (a) and (b)(d) Comparison with incident wave

50. LSRL Go to contents E(pre)=4mJ (a) with pre-pulse (b) without pre-pulse (c) Comparison between (a) and (b)(d) Comparison with incident wave

51. LSRL Go to contents E(pre)=5mJ (a) (b) (c) (d)

52. LSRL Go to contents E(pre)=5mJ at delay time=3ns, 5ns, 7ns, 10ns (a) 3ns(b) 5ns (c) 7ns (d)10ns

53. LSRL Go to contents Proposed pre-pulse system for preserving the wave form of SBS wave

54. LSRL Go to contents Amplitude dividing method New laser fusion driver with SBS beam combination

55. LSRL Go to contents Wave-front dividing method New laser fusion driver with SBS beam combination

56. LSRL Go to contents Comparison of WD (wavefront dividing) and AD (amplitude dividing) WDAD Spatial profile Depends on the phase difference Good Temporal fluctuation Good Depends on the phase fluctuation Requirements  Large 45 rotator (crystal) NoNecessary Image relying NecessaryNo

57. LSRL Go to contents Summary 1.We have succeeded to control the phase of the SBS wave. 2.We have developed the necessary theoretical background the phase control mechanism. 3.We have demonstrated the feasibility of the beam combination using SBS-PCM. 4.We have proposed the next generation of the laser fusion driver system with SBS beam combination. No control Phase control with WD Phase control with AD

58. LSRL Go to contents Conclusions 1.A cross type amplifier using SBS-PCM has many advantages such as the alignment insensitiveness, the stable beam pointing, and the perfect optical isolation. 2.Pre-pulse technique is expected to suppress the deformation of the SBS wave 3.Phase controlling of the SBS wave has been successfully achieved by self density modulation.

59. LSRL Go to contents Future work 1.Demonstrate the phase controlling of the SBS wave having the amplifiers 2.Stage I : Demonstrate 400J/10Hz system with 2 stage amplifier system (4 X 100J@10Hz) 3.Stage II : Demonstrate 6kJ/10Hz system (64 X 100J@10Hz) 4.Stage III : Construct a single channel of laser fusion driver, 100kJ@10Hz (1024 X 100J@10Hz) 5.Stage IV : Construct a full laser fusion driver, > 500kJ@10Hz

60. LSRL Go to contents SBS for ultra-short pulse 1.The following techniques might be used for SBS for the ultra-short pulse : a.Pre-pulse for making the acoustic grating b.Phase controlling by self density modulation

61. LSRL Go to contents Conditions for ultra-short pulse SBS For single-mode-like (  <  ), E th ~ 3mJ (FC75)    MHz  l ~ 3*10 12 Hz  10nm@ =1um) E l-pre-pulse > E th  l    ~30J E l-main-pulse > 100J for 50% R (10mJ) 500J for 80% R (50mJ) For multi-mode-like (  > , c/  > l), E th ~ 3mJ (FC75)   ` ‘  GHz  l ~ 3*10 12 Hz  10nm@ =1um) E l-pre-pulse > E th  l    ~300mJ E l-main-pulse > 3J for 75% R (30mJ) Phase locked

62. LSRL Go to contents Stimulated process of 3 waves, pumping, back scattered, and acoustic waves; an incident laser beam of frequency  L scatters from the refractive index variation associated with a sound wave of frequency  B The two optical fields are governed by the Maxwell’s equations, whereas the acoustic field is described by Navier-Stokes equation with electrostrictive driving term: Principles of SBS

63. LSRL Go to contents Basic equations (for an acoustic wave) (for Incident wave) (for Stokes wave) If we consider three waves in the form (Acoustic wave) (Incident wave) (Stokes wave) Principles of SBS

64. LSRL Go to contents Steady state SBS SBS generator Z=0 Z=L,where g B is SBS gain Principles of SBS

65. LSRL Go to contents Phase conjugation by SBS (Phase conjugate wave) (Incident wave) Thus, the phase conjugation is in fact equivalent to time reversal:., hence the direction of propagation is reversed. The polarization vector of the outgoing wave is the complex conjugate of, hence the wavefront is reversed. i) ii) iii) Principles of SBS

66. LSRL Go to contents An example of phase conjugation by SBS SBS-PCMOrdinary Mirror Principles of SBS

67. LSRL Go to contents Three physical regimes of relevance to SBS 1. Narrow band case (single mode) :  <  2. Broad band case (multi mode) :  > , c/  > l 3. Broad band case (multi mode) :  > , c/  < l  : Pumping laser line-width  : Brillouin line-width t p = 1/  : Acoustic phonon life time l: Geometrical interaction length ( ~ Rayleigh length) c: Light velocity c/  : coherence length } High gain Low gain Principles of SBS

68. LSRL Go to contents Experimental setup for reflectivity measurement Experimental setup for the measurement of the SBS reflectivity; /2, half wave plate; Pol, polarizer; M, mirror; ND, neutral density filter; /4, quarter wave plate; PBS, polarizing beam splitter; BS, beam splitter; PD, photodiode Principles of SBS

69. LSRL Go to contents Liquid  (MHz) g B (cm/GW) n 2 (10 -22 m 2 /V 2 ) Pc (MW)E b (mJ) Carbon tetrachloride (CCl 4 )5283.8 5.9 0.42 Fluorinert FC-753504.5-5 0.33 7.16 Aceton11915.8 8.6 0.281.5 Carbon disulfide (CS 2 )5068 122 0.02<0.5 Properties for liquids ( λ = 1μm) , Brillouin linewidth; g B, SBS gain; n 2, nonlinear refractive index; P C ; critical power of the self-focusing; E b, breakdown threshold. Principles of SBS

70. LSRL Go to contents Characteristics of some liquids SubstancesFrequency shift (MHz) Linewidth (spont.) (MHz) Gain Factor Calculated (max) (cm/MW) Measured (max) (cm/MW) CS 2 Aceton n-Hexane Toluene CCl 4 Methanol Benzene H 2 O Cylohexane FC-72 FC-75 5850 4600 5910 4390 4250 6470 5690 5550 1100 1340 52.3 224 222 579 520 250 289 317 774 270 350 0.197 0.017 0.027 0.013 0.0084 0.013 0.024 0.0066 0.007 0.13 0.020 0.026 0.013 0.006 0.013 0.018 0.0048 0.0068 0.006 0.005 Principles of SBS

71. LSRL Go to contents Compensation of birefringence Double-pass + Faraday rotator + SBS-PCM can compensate the optical birefringence.

72. LSRL Go to contents Compensation of birefringence Measuring the depolarization for 1-pass amplification

73. LSRL Go to contents Depolarization for 1-pass amp Compensation of birefringence

74. LSRL Go to contents Leak beam patterns for 1-pass amp depending the pumping energy (b) No pumping(c) 3.4 J (d) 9.4 J (e) 18.4 J (f) 33.8 J (a) Input beam Compensation of birefringence

75. LSRL Go to contents Experimental set-up for measuring the depolarization for (double pass amp) + FR/QW + SBS-PCM Compensation of birefringence

76. LSRL Go to contents Birefringence of the gain medium Compensation of birefringence

77. LSRL Go to contents Theoretical calculation for 4 types of doube pass amp Type 1 Type 2 Type 3 Type 4 Japanese Journal of Applied Physics - Part 2 : Letters, Vol. 34, P. 994 - 996, 1995. Compensation of birefringence

78. LSRL Go to contents Depolarization for (double pass amp) + FR + SBS-PCM Compensation of birefringence

79. LSRL Go to contents Depolarization for (double pass amp) + QW + SBS-PCM Compensation of birefringence