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