Characteristics of the Fiber

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Characteristics of the Fiber 1

• How are fiber optics made and how does this
  affect how they are used?
• Fiber Types and Optical Design Fiber
  Manufacture
• Techniques and Effects Raman effect / Bragg
  grating/ stokes

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Methods of Fiber Manufacture-Preform

• MCVD Modified Chemical Vapor Deposition
  vaporized silica and other select gases are
  deposited either in a glass tube (IVD) or on the
  outside of a glass rod (OVD).
• Variations exist for the OVD method using plasma
  (PCVD, PMCVD).
• VAD (vapor phase axial deposition is a type of
  OVD where the glass rod rotates and moves up as
  the silica soot is deposited.

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Consolidation of the Preform

• For IVD and OVD (the mandrel is removed first)
  the preform is heated to a high temperature which
  collapses the preform. In this phase the preform
  turns from a white color to a clear glass.

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Optical Fiber Manufacturing - Drawing

Glass Blank
Furnace (2000o C)
Diameter Laser Monitor
Coating Applicator
UV Curing
Drawing
Capstans
Take-up Reel
Fiber Proof Testing
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Fiber Mechanical Strength
? (pulling stress)

• Theoretically fiber is very strong (600-750 kpsi)
• All fiber has small inherent cracks in cladding
• Stress applied to fiber is concentrated to the
  largest crack flaw
• As long as load is applied the crack will grow to
  a failure
• The greater the crack size the lower the fibers
  breaking point
• Bending fiber places it under strain
• Recommend minimum bending radius for long term is
  1.5 inches
• 100 kpsi proof fiber has maximum 0.86 um crack
  depth

Stress Concentration
(crack depth)
? (pulling stress)
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Fiber Coatings

• Fiber core and cladding are covered with two
  layers of urethane Acrylate (plastic). A soft
  layer and hard layer.
• Coating acts as primary buffer.
• Provides abrasion resistance.
• Provides a hermetic seal to protect silica fiber
  from humidity and hostile environments.
• Coating has higher index of refraction than core
  and cladding.
• This allows unwanted light to refract from
  cladding.
• Stripping of coating removes protective layer and
  must be resealed with a protective splice sleeve.

Mechanical Stripping of Acrylate Coating
Fiber Splice Protective Sleeve
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Other Fiber Coatings

• High temperature acrylate 150C options
• Silicone - 200C capable, often coated with PFA
  for further protection and processability
• Carbon provides hydrogen protection and fatigue
  resistance, another coating goes over the carbon
• Polyimide 300C capable
• Copper/Aluminum/Gold beyond 300C up to 700C

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Raman Effect

• When photons interact with components in the
  glass, they can either gain, lose or stay the
  same energy state.
• Rayleigh Scattering photons are elastically
  scattered
• Raman Scattering (or Effect) photons are
  scattered at a different energy than the incident
  energy

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Raman Effect
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Raman Effect Stokes/Anti-Stokes

• Stokes Signal (lower wavelength than Rayleigh)
  doesnt vary with temperature
• Anti-Stokes Signal (higher wavelength than
  Rayleigh) varies with temperature
• Analysis of the two signals results in measuring
  the temperature along the length of the fiber

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Brillouin Scattering

• Similar to Stokes/Anti-Stokes phenomena but
  effect is on strain ratio of signals results in
  a measurement of distributed strain

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FBG Fiber Bragg Grating A FBG consists of a
periodic change of the index of refraction (IOR)
along the core of an optical fiber as presented
below. FBGs are created by exposing a
photosensitive fiber to an intense pa UV light
through a mask with precise slots. In its basic
form, the resulting grating reflects selectively
the light guided by the optical fiber at the
Bragg wavelength lB 2nL , where n and L are the
effective index of refraction of the fiber and
the pitch of the grating in the fiber. The
reflection of light from the grating results in
information on the spacing of the grating. This
information is used to estimate strain or
temperature depending upon the application.
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FBG Operation
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Questions?