The LaRC Fiber Draw Tower

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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
lb center reflection wavelength of Bragg grating
(lb)-1 center reflection wavenumber
0.1
reflection()
0
0.1 nm
wavelength(nm)
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Bragg Grating as 2nd Reflector in an
Interferometer
L
Dw -(2nL)-1
L -(2nDw)-1
power
decreasing wavenumber (increasing wavelength)
(lb)-1
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Multiple Gratings, Multiple Interferometers
L1 L2 L3
reflector 1 7
output
Bragg gratings
Dw1 -(2nL1)-1
Dw2 -(2nL2)-1
Dw3 -(2nL3)-1
power
decreasing wavenumber (increasing wavelength)
(cm)-1
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FFT To Separate Gratings
FFT
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Typical Laser Sweep
wavelength
time
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FOSS Required Configuration
reference signal
data signal
Electronics
reference signal
data signal sampled according to reference length
fringes
c
data signal
A/D
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FFT To Separate Gratings
sampled set with Dw-(2nLref)-1 spacing between
samples number of samples Nsam
power
(cm)-1
FFT
Spatial set with ((Nsam-1)Dw)-1 between points
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FFT-1 To Find Grating Spectra
Spatial set with Dspat((Nsam-1)Dw)-1 point
spacing
power
L1 L2 L3
cm
FFT-1
Spectral set with Dspec((Nspec-1)Dspat)-1 point
spacing
(lb)-1
(cm)-1
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Bragg Grating Reflection Spectrum
lb center reflection wavelength of Bragg grating
(lb)-1 center reflection wavenumber
0.1
reflection()
0
0.1 nm
wavelength(nm)
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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, cont
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Theory of photonic crystals, cont
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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 fibersBy 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 Construction
Tubes 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 PCF
Preform of packed tubes is drawn in a draw tower
furnace
Single tube drawing characteristics
Preform 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