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A Long and Winding Road
Photonic-Crystal Fibers
Photonic Crystals:Periodic Surprises in Electromagnetism
Steven G. Johnson
MIT
1/31/02 INSPECliterature search: 14810 hits 458604 hits 87652 hits
Bloch’s theorem is more importantthan Maxwell’s equations ;^)
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protectivepolymersheath
Optical Fibers Today(not to scale)
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/km
(amplifiers every50–100km)
but this is~ as good as
it gets…[ R. Ramaswami & K. N. Sivarajan, Optical Networks: A Practical Perspective ]
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The Glass Ceiling: Limits of Silica
Long DistancesHigh Bit-Rates
Dense Wavelength Multiplexing (DWDM)
Loss: amplifiers every 50–100km
…limited by Rayleigh scattering (molecular entropy)…cannot use “exotic” wavelengths like 10.6µm
Nonlinearities: after ~100km, cause dispersion, crosstalk, power limits(limited by mode area ~ single-mode, bending loss)
also cannot be made (very) large for compact nonlinear devices
Compact Devices
Radical modifications to dispersion, polarization effects?…tunability is limited by low index contrast
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Breaking the Glass Ceiling:
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 ](You can also
put stuff in here …)
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Breaking the Glass Ceiling:
Hollow-core Bandgap FibersBragg fiber
[ Yeh et al., 1978 ]
+ omnidirectional
= OmniGuides
PCF[ Knight et al., 1998 ]
white/grey= chalco/polymer
5µm[ R. F. Cregan et al.,
Science 285, 1537 (1999) ]
[ figs courtesy Y. Fink et al., MIT ]
silica
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Breaking the Glass Ceiling:
Hollow-core Bandgap Fibers
white/grey= chalco/polymer
5µm[ R. F. Cregan et al.,
Science 285, 1537 (1999) ]
[ figs courtesy Y. Fink et al., MIT ]
silica
Guiding @ 10.6µm(high-power CO2 lasers)
loss < 1 dB/m(material loss ~ 104 dB/m)
Guiding @ 1.55µmloss ~ 13dB/km
[ Smith, et al.,Nature 424, 657 (2003) ]
[ Temelkuran et al.,Nature 420, 650 (2002) ]
OFC 2004: 1.7dB/kmBlazePhotonics
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Breaking the Glass Ceiling II:
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, …
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Breaking the Glass Ceiling II:
Solid-core Holey Fibers
[ K. Suzuki,Opt. Express 9,
676 (2001) ]
polarization-maintaining
[ Wadsworth et al.,JOSA B 19,
2148 (2002) ]
nonlinear fibers
[ T. A. Birks et al.,Opt. Lett. 22, 961 (1997) ]
endlesslysingle-mode
[ J. C. Knight et al.,Elec. Lett. 34,1347 (1998) ]
low-contrastlinear fiber(large area)
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Omnidirectional Bragg Mirrors
a 1d crystal can reflect light fromall angles and polarizations
[ Winn, Fink et al. (1998) ]
perfect metal
…it behaves
like a metal
(but at any wavelength)
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OmniGuide Fibers
omnidirectional mirrors
[ S. G. Johnson et al., Opt. Express 9, 748 (2001) ]
(also a Cambridge MAstart-up: www.omni-guide.com)
c.f. Photonic Bandgap Fibers& Devices Group @ MIT
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Hollow Metal Waveguides, RebornQuickTime™ and aGraphics decompressorare needed to see this picture.
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wavenumber
freq
uenc
y
metal waveguide modes
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OmniGuide fiber gaps
wavenumber
1970’s microwave tubes@ Bell Labs
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QuickTime™ and aGraphics decompressorare needed to see this picture.Hollow Metal Waveguides, Reborn
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wavenumber
freq
uenc
y
metal waveguide modes OmniGuide fiber modes
wavenumber
1970’s microwave tubes@ Bell Labs
modes are directly analogous to those in hollow metal waveguide
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An Old Friend: the TE01 mode
lowest-loss mode,just as in metal
r E
(near) node at interface= strong confinement
= low losses
Rfrom metal:optimal R ~ 10
Here, use R=13µm for =1.55µm … n=4.6/1.6 (any omnidirectional is similar)
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TE01 vs. PMD
r E non-degenerate mode, so
cannot be split
i.e. immuneto birefringence
i.e. PMD is zero
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Let’s Get Quantitative
Gas can havelow loss
& nonlinearity
…but what aboutthe cladding?
& may need to usevery “bad” material
to get high index contrast
…some fieldpenetrates!
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Let’s Get Quantitative
Absorption (& Rayleigh Scattering)
= small imaginary
Nonlinearity
= small ~ |E|2
Acircularity, bending, roughness, …
= small perturbations
Hard to compute directly… use Perturbation Theory
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Perturbation Theory
Given solution for ideal systemcompute approximate effect
of small changes
…solves hard problems starting with easy problems
& provides (semi) analytical insight
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Perturbation Theoryfor Hermitian eigenproblems
given eigenvectors/values: ˆ O u =uu
…find change & for smallΔu Δu Δ ˆ O
Solution:expand as power series in Δ ˆ O
Δu=0+Δu(1) +Δu(2) +…Δu =0+Δu (1) +…&
Δu(1) =uΔ ˆ O u
uu (first-order is usually enough)
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Perturbation Theoryfor electromagnetism
Δω(1) =c2
2ω
r H Δˆ Θ
r H
r H
r H
=−ω2
Δεr E
2
∫ε
r E
2
∫
Δβ(1) =Δω(1) /vg vg =dωdβ
…e.g. absorptiongives
imaginary = decay!
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Suppressing Cladding LossesQuickTime™ and aGraphics decompressorare needed to see this picture.
1x10-5
1x10-4
1x10-3
1x10-2
1.2 1.6 2 2.4 2.8
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(µm)
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Mode Losses/
Bulk Cladding Losses:
Large differential loss
TE01 cladding lossstrongly suppressed!
(like ohmic losses)
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TE01
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EH11
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QuickTime™ and aGraphics decompressorare needed to see this picture.
1x10-9
1x10-8
1x10-7
1x10-6
1.2 1.6 2 2.4 2.8
Suppressing Cladding NonlinearityQuickTime™ and aGraphics decompressorare needed to see this picture.
TE01
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(µm)
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Mode Nonlinearity/
Cladding Nonlinearity:
Will be dominated bynonlinearity of air
~10,000 times weakernonlinearity than silica
* “nonlinearity” = (1) / P
*
(includingfactor of 10 in area)
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Absorption & Nonlinearity ScalingQuickTime™ and aGraphics decompressorare needed to see this picture.
BB
BB
BB
BB
BB
B B B B B B B B B B B B B B
JJ
JJ J J J J J J J J J J J J J J J J J J J J
1x10-9
1x10-8
1x10-7
1x10-6
1x10-5
1x10-4
1x10-3
1x10-2
3 10 13core radius (µm)
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cladding absorption ~ 1/R3
(like metal ohmic loss!)QuickTime™ and aGraphics decompressorare needed to see this picture.
cladding nonlinearity ~ 1/R5
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Radiation Leakage Loss (17 layers)QuickTime™ and aGraphics decompressorare needed to see this picture.
1x10-4
1x10-3
1x10-2
1x10-1
1x100
1x101
1x102
1x103
1x104
1x105
1.2 1.6 2 2.4 2.8
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(µm)
leak
age
loss
(dB
/km
)
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Finite # layers:
modes are “leaky”
loss decreasesexponentially
with number of layers
(~ 1/R3)
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ther Losses
Acircularity & Bendingmain effect is coupling to lossier modes,but can be ~ 0.01 dB/km with enough (~50) layers
Surface Roughnesssuppressed like absorption
tricky
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Acircularity & Perturbation Theory
1
2
= 1 – 2
= 2 – 1
… just plug ’s intoperturbation formulas?
FAILS for high index contrast!
beware field discontinuity…fortunately, a simple correction exists
[ S. G. Johnson et al.,PRE 65, 066611 (2002) ]
(or any shifting-boundary problem)
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Acircularity & Perturbation Theory
1
2
= 1 – 2
= 2 – 1
[ S. G. Johnson et al.,PRE 65, 066611 (2002) ]
(or any shifting-boundary problem)
Δω(1) =−ω2
Δh ΔεE||2 −Δ
1ε
D⊥2⎡
⎣ ⎢ ⎤ ⎦ ⎥
surf.∫
εE2∫
h(continuous field components)
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Yes, but how do you make it?[ figs courtesy Y. Fink et al., MIT ]
find compatible materials(many new possibilities)
chalcogenide glass, n ~ 2.8+ polymer (or oxide), n ~ 1.5
1
Make pre-form(“scale model”)
2
fiber drawing
3
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Fiber Draw Tower @ MITbuilding 13, constructed 2000–2001
~6 meter~6 meter(20 feet)(20 feet)
research research towertower
[ figs courtesy Y. Fink et al., MIT ]
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A Drawn Bandgap Fiber
• Photonic crystal structural Photonic crystal structural uniformity, adhesion, uniformity, adhesion, physical durability through physical durability through large temperature excursionslarge temperature excursions
white/grey= chalco/polymer
[ figs courtesy Y. Fink et al., MIT ]
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Band Gap Guidance
0.0
0.4
0.8
1.2
200040006000800010000
Wavenumber (cm-1)
Wav
evec
tor
Transmissionwindow can be
shifted by scaling(different draw speed)
original (blue)& shifted (red)transmission:
[ figs courtesy Y. Fink et al., MIT ]
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High-Power Transmissionat 10.6µm (no previous dielectric waveguide)
[ figs courtesy Y. Fink et al., MIT ]
0
2
4
6
8
5 5 6 6 7 7 8 8 9 9 10 10 11 11 12 12
slope = -0.95 dB/m
-4.5
-4.0
-3.5
-3.0
2.5 3.0 3.5 4.0Length (meters)
Wavelength (m)
5 6 7 8 9 1210 11
R2 = 0.99
Polymer losses @10.6µm ~ 50,000dB/m…
…waveguide losses ~ 1dB/m
[ B. Temelkuran et al.,Nature 420, 650 (2002) ]
cool movie
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Enough about MIT already…
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2d-periodic Photonic-Crystal Fibers [R. F. Cregan et al., Science 285, 1537 (1999) ]
Not guided via2d TE bandgap:
QuickTime™ and aGraphics decompressorare needed to see this picture.
JJ
JJ
JJ
JJ
JJ J J J J J J J J J J J
JJ
JJ
JJ
J
J
J
J
J J J J JJ
JJ
JJ J J J J J J J J J J J
JJ
JJ
JJ
J J J J
J J J J J J J J J J J J J J J J J J J J J J J J J J J J J J JJ J J J J J J J J J J J J J J J J J J J J J J J J J J J J J J
J J J J J J J
E
E
E
E
E
E
EE
EE E E E E E E E E E E E E
EE
E
E
E
E
E
E
E
EE
EE
E E E E E E E E E E E E E EE
EEE
EE
E E E EE
EE
E
0
0.1
0.2
0.3
0.4
0.5
0.6
TE bandsTM bands
Γ M K Γ
air holes
silica(usually)
wavenumber breaks mirror plane,so no pure TE/TM polarizations
a
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PCF: Holey Silica Cladding 2r
a
n=1.46
(2π/a)
r = 0.1a
(2π
c/a)
light cone
= c
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PCF: Holey Silica Cladding 2r
a
n=1.46
(2π/a)
r = 0.17717a
(2π
c/a)
light cone
= c
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PCF: Holey Silica Cladding 2r
a
n=1.46
(2π/a)
r = 0.22973a
(2π
c/a)
light cone
= c
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PCF: Holey Silica Cladding 2r
a
n=1.46
(2π/a)
r = 0.30912a
(2π
c/a)
light cone
= c
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PCF: Holey Silica Cladding 2r
a
n=1.46
(2π/a)
r = 0.34197a
(2π
c/a)
light cone
= c
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PCF: Holey Silica Cladding 2r
a
n=1.46
(2π/a)
r = 0.37193a
(2π
c/a)
light cone
= c
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PCF: Holey Silica Cladding 2r
a
n=1.46
(2π/a)
r = 0.4a
(2π
c/a)
light cone
= c
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PCF: Holey Silica Cladding 2r
a
n=1.46
(2π/a)
r = 0.42557a
(2π
c/a)
light cone
= c
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PCF: Holey Silica Cladding 2r
a
n=1.46
(2π/a)
r = 0.45a
(2π
c/a)
light cone
= c index-guided modesgo here
gap-guided modesgo here
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PCF: Holey Silica Cladding 2r
a
n=1.46
(2π/a)
r = 0.45a
(2π
c/a)
light cone
air lig
ht lin
e =
c
above air line:
guiding in air coreis possible
below air line: surface states of air core[ figs: West et al,
Opt. Express 12 (8), 1485 (2004) ]
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PCF Projected Bands[ J. Broeng et al., Opt. Lett. 25, 96 (2000) ]
bulkcrystal
continuum
air light li
ne
band gap“fingers”appear!
(2π/a)
(
2πc/
a)
0.8
1.2
1.6
2.0
2.4
1.11 1.27 1.43 1.59 1.75 1.91 2.07 2.23 2.39
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PCF Guided Mode(s)
(2π/a)
(
2πc/
a)[ J. Broeng et al., Opt. Lett. 25, 96 (2000) ]
bulkcrystal
continuum
fundamental & 2nd order guided modes
air light li
ne
fundamentalair-guided
mode
0.8
1.2
1.6
2.0
2.4
1.11 1.27 1.43 1.59 1.75 1.91 2.07 2.23 2.39
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Experimental Air-guiding PCFFabrication (e.g.)
silica glass tube (cm’s)
fiberdraw
~1 mm
(outercladding)
fuse &draw
~50 µm
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Experimental Air-guiding PCF[ R. F. Cregan et al., Science 285, 1537 (1999) ]
10µm
5µm
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Experimental Air-guiding PCF[ R. F. Cregan et al., Science 285, 1537 (1999) ]
(c/a) (not 2πc/a)
transmitted intensityafter ~ 3cm
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State-of-the-art air-guiding losses
3.9µm
hollow (air) core (covers 19 holes)
guided field profile:(flux density)
[Mangan, et al., OFC 2004 PDP24 ]
1.7dB/kmBlazePhotonics
over ~ 800m @1.57µm
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State-of-the-art air-guiding losses
13dB/kmCorning
over ~ 100m @1.5µm[ Smith, et al., Nature 424, 657 (2003) ]
1.7dB/kmBlazePhotonics
over ~ 800m @1.57µm[ Mangan, et al., OFC 2004 PDP24 ]
larger core =less field penetrates
cladding
ergo,roughness etc.
produce lower loss
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State-of-the-art air-guiding losseslarger core = more surface states crossing guided mode
100nm20nm
13dB/kmCorning
over ~ 100m @1.5µm[ Smith, et al., Nature 424, 657 (2003) ]
1.7dB/kmBlazePhotonics
over ~ 800m @1.57µm[ Mangan, et al., OFC 2004 PDP24 ]
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Index-Guiding PCF & microstructured fiber:
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, …
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Holey Projected Bands, Batman!
(a–1)
(
c/a)
(not
2πc
/a)
bulkcrystal
continuum
(Schematic)
guided bandlies below
“crystal light line”
band gapsare
unused
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Guided Mode in a Solid Coresmall computation: only lowest- band!
QuickTime™ and aGraphics decompressorare needed to see this picture.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2
2r
a
n=1.46
r = 0.3a
1.46
–
c/
= 1
.46
– n ef
f
/a
holey PCF light cone
fundamentalmode
(two polarizations)
endlessly single mode: neff decreases with
flux density
(~ one minute, planewave)
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Holey Fiber PMF (Polarization-Maintaining Fiber)
no longer degenerate with
Can operate in a single polarization, PMD = 0(also, known polarization at output)
[ K. Suzuki, Opt. Express 9, 676 (2001) ]
birefringence B = c/= 0.0014
(10 times B of silica PMF)
Loss = 1.3 dB/km @ 1.55µmover 1.5km
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Nonlinear Holey Fibers:
[ W. J. Wadsworth et al., J. Opt. Soc. Am. B 19, 2148 (2002) ]
Supercontinuum Generation
e.g. 400–1600nm “white” light:from 850nm ~200 fs pulses (4 nJ)
(enhanced by strong confinement + unusual dispersion)
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Endlessly Single-Mode[ T. A. Birks et al., Opt. Lett. 22, 961 (1997) ]
at higher (smaller ),
the light is moreconcentrated in silica
http://www.bath.ac.uk/physics/groups/opto
…so the effectiveindex contrast is less
…and the fiber can staysingle mode for all
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Low Contrast Holey Fibers
The holes can also form aneffective low-contrast medium
i.e. light is only affected slightlyby small, widely-spaced holes
This yieldslarge-area, single-modefibers (low nonlinearities)
…but bending loss is worse
[ J. C. Knight et al., Elec. Lett. 34, 1347 (1998) ]
~ 10 times standard fiber mode diameter
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Holey Fiber Losses
Best reported results:
0.28 dB/km @1.55µm
[ Tajima, ECOC 2003 ]
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QuickTime™ and aGraphics decompressorare needed to see this picture.QuickTime™ and aGraphics decompressorare needed to see this picture.
The Upshot:
Potential new regimes for fiber operation,
even using very poor materials.
The Story of Photonic Crystals
Finding Materials –> Finding Structures
The Upshot