The LaRC Fiber Draw Tower
Presented by
Stan DeHaven
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
Fiber Optic Strain Sensor (FOSS)
Bragg Gratings
Bragg Grating Reflection Spectrum
lb = center reflection wavelength of Bragg grating
(lb) = center reflection wavenumber
-1
reflection(%)
0.1
0
0.1 nm
wavelength(nm)
Bragg Grating as 2nd Reflector in an Interferometer
L
output
reflector 1
7%
Bragg grating
< 0.1 %
power
Dw = -(2nL)-1
L = -(2nDw)-1
(lb)
-1
decreasing wavenumber
(increasing wavelength)
Multiple Gratings, Multiple Interferometers
L1
output
reflector 1
7%
L2
L3
Bragg gratings
Dw1 = -(2nL1)
-1
Dw2 = -(2nL2)
-1
Dw3 = -(2nL3)
power
-1
-1
(cm)
decreasing wavenumber
(increasing wavelength)
power
FFT To Separate Gratings
-1
(cm)
power
FFT
cm
L1
L2
L3
wavelength
Typical Laser Sweep
time
FOSS Required Configuration
reference signal
Lref
L1
data signal
reflector
(distance 0)
L2
L3
Bragg gratings
Electronics:
reference signal
c
data signal
A/D
data signal sampled
according to reference
length fringes
FFT To Separate Gratings
power
sampled set with
-1
Dw=-(2nLref) spacing
between samples
number of samples = Nsam
-1
(cm)
FFT
power
“Spatial” set with
-1
((Nsam-1)Dw) between points
cm
L1
L2
L3
-1
FFT To Find Grating Spectra
power
“Spatial” set with
-1
Dspat=((Nsam-1)Dw)
point spacing
L1
L2
L3
cm
-1
FFT
power
“Spectral” set with
-1
Dspec=((Nspec-1)Dspat)
point spacing
(lb)
-1
-1
(cm)
Bragg Grating Reflection Spectrum
lb = center reflection wavelength of Bragg grating
(lb) = center reflection wavenumber
-1
reflection(%)
0.1
0
0.1 nm
wavelength(nm)
FOSS Draw Tower Operation
Draw Tower Preform in Furnace
Bragg Grating Optics
Draw Tower View
Excimer Laser (248 nm, 500mJ)
Photonic Crystals & Fibers
• 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
What 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.
Theory of photonic crystals
Theory of photonic crystals, con’t
Theory of photonic crystals, con’t
In 2D photonic crystal structures it is possible to confine
light within a cavity.
Photonic band gaps appear in the plane of periodicity and
in
2D 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 xyplane because of the band gap but extends in the zdirection
Photonic crystal fibers
By making a suitable geometry of the periodic dielectric
medium, a phonic crystal can be used as an optical fiber.
PCF Fabrication
•PCF Preform Construction
•PCF Drawing
-similar to drawing capillary tubes
Preform Construction
Tubes are packed in a hexagonal shape with hollow, solid,
birefringent, doped or tubular core elements.
Multi-Core Preform
Drawing PCF
Preform of packed tubes is drawn in a draw tower furnace
Preform drawing down in furnace
Single tube drawing characteristics
SEM Image of Multicore Fiber
Multi-core Drop with drawn fiber
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
Draw Tower Capabilities
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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