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06 Nov, 2003, IAEA CRP Meeting in Vienna
Feasibility study on
Scalable Self-Phase Locking of two beam
combination using stimulated Brillouin
scattering phase conjugate mirrors
Hong-Jin Kong, Seong-Ku Lee, and Dong-Won Lee
Dept. of Physics, KAIST,
Daejon 305-701, Korea.
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Motivation
• Laser IFE Program has the bottle neck mainly due to
the laser driver with high rep. rate over 10 Hz. (NIF
operates at 1 shot per several hours)
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Why bottleneck?
Cooling
Solid amp
• For high power and high energy
output,
• big size of laser amplifier to reduce
the optical damage and gain
saturation is necessary.
• But, it gives very high thermal load
so that
• it takes too much time to cool down .
Cooling
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
How to solve the thermal load?
• Use the pumping source with LD (Mercury,
LLNL)
• Use the high thermal conductivity laser
materials such as ceramic crystals (K.Ueda)
• Break down the amplifier with small size to
reduce the size of the amplifier (H.Kong)
Beam combination
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Beam combination
Phase distortion
Easy to cool down the
sliced amps
Each beam has different
phases and distorted by
the amp’s nonlinearities
such as thermal lensing
effect and thermal
birefringence, different
gain profiles and so on.
Break up the amp into
small size amp to reduce
the thermal load
Piston error
How to combine the
beams together in
phase? (to fix the
piston error)
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Phase conjugation mirror (PCM)
Conventional mirror
Phase conjugation
mirror
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Types of PCM
• 4 wave mixing PCM (complicated, phase
controllable)
• Stimulated Brillouin Scattering PCM (SBS
PCM) (most simple, random phase piston
error)
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Stimulated Brillouin Scattering PCM (SBS-PCM)
•
•
•
•
Backward scattering – high reflectivity more than 90 %
Phase conjugate wave – high fidelity
Scattering by acoustic phonon induced by laser field
Frequency down shift –negligible; ~1 GHz for liquids
< 1 GHz for gases
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Cross-type PCM amplifier :
Perfect Optical Isolator with PCMs
Proposed by Kong (will be submitted, patent pending)
 Compensate the phase
distortions experienced
passing through the
amplifier by PCM
 Perfect optical isolation
 Insensitive to misalignment:
Fixed beam pointing
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
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)
Fusion Eng. & Design 44, 407-417(1999)
•
Interference between the
beams at their boundaries
in beam combination gives
spatial spiking if their
phases are not matched.
•
Phase difference between
the beams should be less
than /4 for constructive
interference at the
boundary of the
combination not give
spatial spiking.
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Phase locking between beams
• Interference between the beams at their boundaries in
beam combination gives spatial spiking if their
phases are not matched.
• Phase difference between the beams should be less
than /4 for constructive interference at the boundary
of the combination not give spatial spiking.
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Previous works for phase locking
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.
Lens
SBS Cell
Seeding Beam
Polarizer
Faraday
Rotator
SBS Cell
SBS Cell
Beam
Splitter
Mirror
a) Overlap of two focal points
b) Back-seeding of Stokes wave
D.A.Rockwell and C.R.Giuliano, Opt. Lett. 11, 147 (1986)
T.R.Loree, et. al., Opt. Lett. 12, 178 (1987)
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Proposed “Self-Phase locking”
1. Interference between the counter-propagating beams generates a
very weak standing wave, which ignites the phonon and locks the
phase of the moving grating (phonon).
2. This phonon locks the phase of the SBS wave.
SBS Cell #1
SBS Cell#1
Concave Mirror
Lens
Concave Mirror
SBS Cell #2
(a) Self-seeding
SBS Cell #2
(b) Backward focusing SBS
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Basic concept of a new phase locking
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Experimental setup for self-phase locking of two beams
W1
M1
L3
SBS Cell
CM1
SBS Cell
CM2
Faraday
Rotator
PBS
HWP2
Isolator
L1
L2
L4
45 degree
M2
Reflection
Coated Prism
W2
P1
HWP1
CCD
Camera
Nd:YAG Laser
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
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Properties of the pumping laser
 Nd3+:YAG laser (1064 nm)
 Repetition rate: 10 Hz
 Single longitudinal mode
Line-width: ~120MHz
 Pulse width: 6-8 ns
 Beam diameter : 8 mm
Beam profile
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Properties of SBS medium (FC-75)
1.0
0.8
Refletivity
Main component
C8F18
Absorption coefficient
<10-5(cm-1)
Optical breakdown
threshold
100∼130GW/cm2
Brillouin frequency shift 1.34 GHz
Hyper sound velocity V 563 m/sec
SBS gain coefficient g
4.5∼5 cm/GW
Brillouin bandwidth
350 ~ 400 (MHz)
Hyper sound decay time 0.8 ns
0.6
0.4
FC-75
single mode, f=15cm
single mode, f=25cm
0.2
0.0
1
10
100
Energy(mJ)
Reflectivity vs. pump energy
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Average fluctuation  0.6 
Normal case: No locking
180
135
Lens(f=250mm)
SBS Cell #2
90
Phase (degree)
SBS Cell #1(l=500mm)
45
0
-45
-90
-135
-180
0
50
100
150
Number of shot (A.U.)
Intensity profile of interference
Intensity profile of interference for 160 shots
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Average fluctuation  0.27 
180
Backward focusing SBS
135
SBS Cell #1(l=500mm)
Phase (degree)
90
45
0
-45
-90
-135
SBS Cell #2
Concave Mirror
(R=500mm, 100%R)
-180
0
50
100
150
200
Number of shot (A.U)
228/238 : success (96%)
Intensity profile of interference for 238 shots
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Average fluctuation  0.33 
180
SBS Cell#1 (l=500mm)
135
90
Phase (degree)
Concentric seeding beam
case #1
45
0
-45
-90
-135
-180
0
50
100
150
200
Number of shot (A. U.)
Lens(f=250mm)
Concave Mirror
(R=300mm, uncoated)
SBS Cell #2
178/203 : success (88%)
Intensity profile of interference for 203 shots
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Average fluctuation  0.31 
180
Confocal seeding beam
135
case #2
SBS Cell#1(l=300mm)
Phase (degree)
90
45
0
-45
-90
-135
-180
Lens(f=250mm)
Concave Mirror
(R=100mm, uncoated)
0
50
100
150
200
Number of shot (A.U.)
SBS Cell #2
181/199 : success (91%)
Intensity profile of interference for 199 shots
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Average fluctuation  0.38 
180
Confocal seeding beam
135
case #3
SBS Cell#1(l=300mm)
Phase (degree)
90
45
0
-45
-90
-135
-180
0
Lens(f=250mm)
Concave Mirror
(R=90mm, AR coated)
50
100
150
200
Number of shot (A.U)
SBS Cell #2
178/216 : success (82%)
Intensity profile of interference for 216 shots
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Average fluctuation  0.09 
180
Two mirrors
135
Two mirrors
Phase(degree)
90
45
0
-45
Mirror1
-90
Mirror2
-135
-180
0
50
100
150
200
250
Number of Shot (A. U)
256 shots
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Average fluctuations for each case
180
Average Fluctuation (degree)
135
90
Normal case
Backward focusing case
Self Seeding beam case #1
Self Seeding beam case #2(uncoated)
Self Seeding beam case #3(AR coated)
45
0
-45
-90
-135
-180
0
1
2
3
4
5
A.U.
LSRL
06 Nov, 2003, IAEA CRP Meeting in Vienna
Summary
• We have proposed a new scalable phase locking method, “the self-phase
locking”.
• For the case of the backward focusing, the average fluctuation of phases
between two beams is  0.27  (96% success).
• Further works are necessary to reduce the phase locking fluctuation < 0.25
 with 100% success.
• This new method can easily scaled up to a number of beams to get a high
power and high energy laser output for IFE or other applications.
• With or without LD pumped ceramic laser materials developed by K.
Ueda, this new technique is expected to advance the Laser Fusion Program,
IFE, and also expand the high power laser application area including laser
machining.
LSRL