Why Laser Accelerator?
• High power: ~ petawatts• High intensity (field): 1022 W/cm2 ~
2.7x1012 V/cm• Accelerator gradient: limited by materials
damage threshold ~ 1GV/m (fused silica)• Low loss in guiding structure: 0.2dB/km
protectivepolymersheath
Optical Fibers Today
silica claddingn ~ 1.45
more complex profilesto tune dispersion
“high” indexdoped-silica core
n ~ 1.46
“LP01”confined mode
field diameter ~ 8µm
losses ~ 0.2 dB/kmat =1.55µm
(amplifiers every50–100km)
Solid-core Holey Fibers
solid core
holey cladding formseffective
low-index material
[ J. C. Knight et al., Opt. Lett. 21, 1547 (1996) ]
Can have much higher contrastthan doped silica…
strong confinement = enhancednonlinearities, birefringence, …
Hollow-core Bandgap Fibers1000x better
loss/nonlinear limits(from density)
Photonic Crystal
1dcrystal
Bragg fiber[ Yeh et al., 1978 ]
+ omnidirectional
= OmniGuides
2dcrystal
PCF[ Knight et al., 1998 ]
Experimental Air-guiding PCF[ R. F. Cregan et al., Science 285, 1537 (1999) ]
10µm
5µm
Fabrication: Air-guiding PCF
silica glass tube (cm’s)
fiberdraw
~1 mm
(outercladding)
fuse &draw
~50 µm
Fabrication: Bragg Fiber[Y. Fink et al., MIT ]
compatible materialsThermal evaporation
chalcogenide glass, n ~ 2.8+ polymer (or oxide), n ~ 1.5
1
Make pre-form(“scale model”)
2
fiber drawing
3
A Drawn Bandgap Fiber
white/grey= chalco/polymer
[Y. Fink et al., MIT ]
0 1 2 3 4 5-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
r0/
E z and
E r
beta=1k0 n1=2.6; n2=1.6; 20x
=k=k00=2=2//
a/l=1a/l=1nnhighhigh=2.6=2.6nnlowlow=1.6=1.6
EErr=-j=-jr/r/EEzz
Radiation loss: 0.2dB/kmRadiation loss: 0.2dB/km20-pair of 20-pair of
layers layers
Acceleration Mode in Bragg Fiber
Small Surface Field Structure
0 1 2 3 4-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
r0/
Ez a
nd E
r
beta=0.79253k0 n1=2.6; n2=1.6; 20x
=0.79253k=0.79253k00
a/a/=1=1nnhighhigh=2.6=2.6nnlowlow=1.6=1.6
EErr=0 at r=0 at r00=a=a
Dispersion Relation: TM01
0.0 0.1 0.2 0.3 0.4 0.50.0
0.1
0.2
0.3
0.4
0.5
fa/c
a/2No perturbation
1m
Dispersion Relation: TM011m
0.0 0.1 0.2 0.3 0.4 0.50.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
fa/c
a/2
Surface Field
4acc
surface
EE
a/
Perturbation in Core Perturbation in Cladding (Metal)
5.3acc
surface
EE
a/
Summery• Hollow core fiber is able to confine
acceleration mode (TM01, vp=c)• Uniform light wave guiding does not
reduce Er (r/aEz)• Looking for structures that can be made
for acceleration (polymer microstructured fiber, tapered fiber)
Other Challenges
• light coupling, (evanescent coupling)• mode excitation, • imperfection vs. mode mixing• …
0 2 4 6 8 100.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
=0.9981k0, r0=10um =0.5k0, r0=9um =0.5k0, r0=10um
Laye
r Thi
ckne
ss (
m)
Layer
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