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

refl

ecti

on(%

)

0

0.1

0.1 nm

wavelength(nm)

b = center reflection wavelength of Bragg grating

b)-1 = center reflection wavenumber

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

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

FFT To Separate Gratings

pow

er

(cm)-1

FFT

pow

er

cm L1 L2 L3

Typical Laser Sweep

time

wav

elen

gth

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

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

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

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

FOSS Draw Tower Operation

Draw Tower Preform in Furnace

Bragg Grating Optics

Draw Tower View

Excimer Laser (248 nm, 500mJ)

• 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

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.

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 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

Photonic crystal fibers

By making a suitable geometry of the periodic dielectricmedium, a phonic crystal can be used as an optical fiber.

PCF Fabrication

•PCF Preform Construction

•PCF Drawing-similar to drawing capillary tubes

Preform ConstructionTubes are packed in a hexagonal shape with hollow, solid, birefringent, doped or tubular core elements.

Multi-Core Preform

Drawing PCFPreform of packed tubes is drawn in a draw tower furnace

Single tube drawing characteristicsPreform drawing down in furnace

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

• 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