the larc fiber draw tower presented by stan dehaven
TRANSCRIPT
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The LaRC Fiber Draw Tower
Presented by
Stan DeHaven
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Draw Tower Research
• FOSS (Fiber Optic Sensing System) strain sensors. Use Bragg grating in fiber core.
• Holey Fiber – Multi-core hollow fiber. Tens of Micron hole spacing. NUPERC activity.
• PCF (Photonic Crystal Fiber) – Micron scale hole spacing. NUPERC activity.
• Draw Tower Capabilities
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Fiber Optic Strain Sensor (FOSS) Bragg Gratings
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Bragg Grating Reflection Spectrum
refl
ecti
on(%
)
0
0.1
0.1 nm
wavelength(nm)
b = center reflection wavelength of Bragg grating
b)-1 = center reflection wavenumber
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Bragg Grating as 2nd Reflector in an Interferometer
reflector 17 %
Bragg grating< 0.1 %
output
pow
er
b)-1
decreasing wavenumber(increasing wavelength)
L
w = -(2nL)-1
L = -(2nw)-1
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Multiple Gratings, Multiple Interferometers
reflector 17 % Bragg
gratings
output
L1 L2 L3
decreasing wavenumber(increasing wavelength)
pow
er
w1 = -(2nL1)-1
w2 = -(2nL2)-1
w3 = -(2nL3)-1
(cm)-1
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FFT To Separate Gratings
pow
er
(cm)-1
FFT
pow
er
cm L1 L2 L3
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Typical Laser Sweep
time
wav
elen
gth
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FOSS Required Configuration
reflector(distance 0) Bragg
gratings
L1 L2 L3
Lref
reference signal
data signal
Electronics:
reference signal
data signal A/Dc data signal sampled
according to referencelength fringes
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FFT To Separate Gratings
pow
er
(cm)-1
FFT
sampled set withw=-(2nLref)
-1 spacingbetween samplesnumber of samples = Nsam
pow
er
cm L1 L2 L3
“Spatial” set with ((Nsam-1)w)-1 between points
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FFT-1 To Find Grating Spectra
pow
er
cmL1 L2 L3
“Spatial” set with spat=((Nsam-1)w)-1 point spacing
FFT-1
pow
er
(cm)-1b)-1
“Spectral” set withspec=((Nspec-1)spat)-1
point spacing
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Bragg Grating Reflection Spectrum
refl
ecti
on(%
)
0
0.1
0.1 nm
wavelength(nm)
b = center reflection wavelength of Bragg grating
b)-1 = center reflection wavenumber
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FOSS Draw Tower Operation
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Draw Tower Preform in Furnace
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Bragg Grating Optics
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Draw Tower View
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Excimer Laser (248 nm, 500mJ)
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• Fundamentals of Photonic Crystals- What are photonic crystals?- Theory of Photonic Crystals
• Photonic Crystal Fibers (PCF)• Fabrication of PCF
• Advantages and Challenges
Photonic Crystals & Fibers
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Photonic Crystals & fibersWhat are photonic crystals?Features of a photonic crystal:• Made of low-loss periodic dielectric medium• Optical analog to the electrical semiconductors• Able to localize light in specified areas by preventing light from propagating in certain directions – optical bandgap.
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Theory of photonic crystals
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Theory of photonic crystals, con’t
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Theory of photonic crystals, con’t
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In 2D photonic crystal structures it is possible to confinelight within a cavity.Photonic band gaps appear in the plane of periodicity and in2D we can achieve linear localization.
By introducing a defect, i.e. removing one column, we may obtain a peak in the density of states localized in the photonic band gap – similar to semiconductors.The defect mode cannot penetrate the crystal in the xy-plane because of the band gap but extends in the z-direction
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Photonic crystal fibers
By making a suitable geometry of the periodic dielectricmedium, a phonic crystal can be used as an optical fiber.
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PCF Fabrication
•PCF Preform Construction
•PCF Drawing-similar to drawing capillary tubes
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Preform ConstructionTubes are packed in a hexagonal shape with hollow, solid, birefringent, doped or tubular core elements.
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Multi-Core Preform
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Drawing PCFPreform of packed tubes is drawn in a draw tower furnace
Single tube drawing characteristicsPreform drawing down in furnace
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SEM Image of Multicore Fiber
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Multi-core Drop with drawn fiber
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Advantages:•PCF with high-index core is more flexible than conventional fiber:
- Possible to make very large core area to send high power - Possible to make core very small compared to conventional
fibers. Designer wavelengths possible.
•Air-guiding PCF (hollow core of fiber):
- Possible to send high power - No entrance or exit reflectance
Challenges:
•PCF is difficult to fabricate
•PCF is limited to specific frequencies
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Draw Tower Capabilities
• Graphite Furnace
• Multiple heat zone furnace elements
• Fiber diameter feedback control
• IR and UV thermal fiber coating capability
• Excimer Laser
• Ability to write gratings while drawing fiber for increased fiber strength