Optical Fibres

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Optical Fibres
MCS.end-1
Maj JW Paul
Refs lecture notes LFTSP 2003 Dr Smain Amari
lecture notes ACS 2003 Mr Josh Dore
lecture notes CELE Ph III 2001 Capt Christian Rene
2
Education teaches you what is possibleExperience
teaches you what is not...
3
Review

• What are the sources of loss in metallic cable?

4
The Electromagnetic Spectrum
5
Outline

• Fiber vs. Metallic Cables
• Light Propagation in Fiber
• Types
• Acceptance Cone
• Losses
• Sources and Detectors

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Introduction

• 1960 Laser invented
• 1966 Concept of transmission in
  an optical fiber first proposed
• 1970 First low-loss (20 dB/km)
  optical fiber ( coaxial cables)
• Today Attenuations around 0.15 dB/km are
  achievable (theory 0.13)
• Plastics and silica are common materials

7
Some properties of light...

• light travels more slowly in an optically dense
  medium than it does in a less dense medium
• A measure of this effect is the refractive index
  (index of refraction)
• Gives us refraction and reflection

in vacuum
light

of

Speed
h gt 1

h
material
in
light

of

Speed
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Reflected Rays
Exit Ray
h2
h1
? a
Partial Reflection
? c
Incident Ray
Reflected Ray
? a acceptance angle
? c critical angle
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Fiber Cables Design

• Note glass-air interface would provide
  sufficient change in h
• Cladding protects glass from
• surface scratches,
• surface contaminants..
• Which would let light escape

Typical ? 860 nm 1300 nm 1550 nm
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Fiber Types

• Plastic core and cladding
• Glass core with plastic cladding
• Also called PCS fiber (plastic-clad silica)
• Glass core and glass cladding
• Also called SCS fiber (silica-clad silica)
• Under development
• Non-silicate (Zinc chloride) which could be 1000
  times as efficient as glass

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Fiber vs. Metallic Cables

• Advantages
• Larger bandwidth
• Immune to cross-talk
• Immune to static interference
• Do not radiate RF
• spark free
• No corrosion, more environment resistive

• Disadvantages
• Initial cost of installation high
• Brittle
• Maintenance and repair more difficult and more
  expensive

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BW comparisons
1 THz (TeraHerz) 1012 Hz 109 Mhz
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How to change h
step index
gradient index
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Modes
55km range
single mode
cladding
10?
core
20km range
50?
multi mode - step index
125?
multi mode - gradient index
note high and low order modes
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Single-Mode Step Index

• Advantages
• Minimum dispersion (one path only)
• Larger bandwidth
• Disadvantages
• Difficult to couple light (small core)
• Small source needed
• Expensive and difficult to manufacture

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Multi-Mode Step Index

• ADV
• Inexpensive to manufacture, and simple
• Easy to couple light into

• DIS
• Different paths, more dispersion
• Info rate and BW is less

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Losses in Fibers
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Attenuation

• Absorption
• interaction of light with electrons molecule
  vibration
• Rayleigh Scattering
• caused by compositional fluctuations in glass
  material. Energy escapes not converted
• Material Fabrication
• caused impurities (transition metal ions)
• Fiber Fabrication
• caused by fiber imperfections (defects/stresses)
  Leads to Mie scattering which is ?
  independent
• Deployment/Environmental
• caused by bends and microbends
  Leads to mode conversions

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Splicing Loss
Coupling Surface
Separation Distance
Alignment Angle
Surface Roughness
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Splicing Methods

• Fusion cut, polish, align and fuse together
  by passing current (0.1 dB loss)
• Mechanical cut, polish, align then place
  mechanical (glued) support sheath around
  junction (0.2 dB loss)

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Modal Dispersion

• Different paths take different times to propagate
• Cause by light beam width not tightly focused
  (LED vs Laser beam)
• Can occur only in multi-mode fibers

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Chromatic Dispersion

• Pulse spreading due to the fact that different
  wavelengths travel at different speeds in the
  fiber.
• Major limiting factor in single-mode fibres.
• Not as significant as modal dispersion in
  multi-mode fibers.

Lowest for 1300 nm region
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Pulse-Width Dispersion

• Pulse stretching may cause ISI in digital
  transmission
• Also caused in multi-mode fibers because of
  different paths taken by the rays of light

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Sources

• LED (light emitting diodes)
• ILD (injection laser diodes

• Pros of ILD
• Easier to couple (more directive beam)
• Output power greater
• Higher bit rate
• Monochromatic (less chromatic dispersion)

• Cons of ILD
• 10 x more expensive
• Shorter lifetime due to high power
• More temperature dependant (performance varies)

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Detectors

• PIN Diode (most common, a.k.a. Photodiode)
• Avalanche photodiode (different junction than
  PIN)
• more sensitive than PIN but longer transit time
• Characteristics of Detectors
• Responsivity (measure of conversion efficiency)
• Dark current (leakage current, when no light
  present)
• Transit time (time to travel across junction,
  affects bps)
• Spectral response (range of detectable wavelength)

26
Review

• Name the 3 types of fibre cables
• What are the differences between step-index and
  graded-index fibers?
• Name and describe the main impairments to optical
  cables

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Review

• What are the sources of loss in metallic cable?
• Resisitance
• Inductance
• Capacitance
• Impedence
• Attenuation Distortion
• Delay Distortion
• Crosstalk noise