Fiber Optical Communication System

LIGHT COMMUNICATION

Fiber vs. Metallic Cables 

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

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

Typical Fiber Optical Communication System

Elements of a Fiber Data Link 

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Transmitter emits light pulses (LED or Laser) Connectors and Cables passively carry the pulses Receiver detects the light pulses Transmitter

Cable

Receiver

Repeaters 

For long links, repeaters are needed to compensate for signal loss

Fiber

Repeater

Fiber

Repeater

Fiber

Repeater

Fiber

Optical Fiber 

Core • Glass or plastic with a higher index of refraction than the cladding • Carries the signal

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Cladding • Glass or plastic with a lower index of refraction than the core

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Buffer • Protects the fiber from damage and moisture

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Jacket • Holds one or more fibers in a cable

Singlemode Fiber 

Singlemode fiber has a core diameter of 8 to 9 microns, which only allows one light path or mode • Images from arcelect.com (Link Ch 2a)

Index of refraction

Multimode Step-Index Fiber 

Multimode fiber has a core diameter of 50 or 62.5 microns (sometimes even larger) • Allows several light paths or modes • This causes modal dispersion – some modes take longer to pass through the fiber than others because they travel a longer distance

• See animation at link Ch 2f

Index of refraction

Multimode Graded-Index Fiber 

The index of refraction gradually changes across the core • Modes that travel further also move faster • This reduces modal dispersion so the bandwidth is greatly increased

Index of refraction

Attenuation 

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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 Leads • caused by fiber imperfections (defects/stresses) to Mie scattering which is λ independent Deployment/Environmental • caused by bends and microbends Leads to mode conversions

Three Types of Dispersion 

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Dispersion is the spreading out of a light pulse as it travels through the fiber Three types: • Modal Dispersion • Chromatic Dispersion • Polarization Mode Dispersion (PMD)

Modal Dispersion 

Modal Dispersion • Spreading of a pulse because different modes (paths) through the fiber take different times • Only happens in multimode fiber • Reduced, but not eliminated, with graded-index fiber

Chromatic Dispersion 

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Different wavelengths travel at different speeds through the fiber This spreads a pulse in an effect named chromatic dispersion Chromatic dispersion occurs in both singlemode and multimode fiber • Larger effect with LEDs than with lasers • A far smaller effect than modal dispersion

Polarization Mode Dispersion 

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Light with different polarization can travel at different speeds, if the fiber is not perfectly symmetric at the atomic level This could come from imperfect circular geometry or stress on the cable, and there is no easy way to correct it It can affect both singlemode and multimode fiber.

Light Sources � Light Emitting Diode (LED) •� simple construction and drive circuitry •� best for short distances, modest bit rates, and low channel capacity � Semiconductor Laser Diode •� high drive currents and complex circuitry •� produce high power for higher bit rates and long distances

Light Sources: LED •� Usually a P-N junction aluminium-gallium arsenide (AlGaAs) or • Gallium-arsenide-phosphide (GaAsP) •

Spontaneous emission through recombination of electrons and holes

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Works in forward bias, energy released as a photon

• A photon = a quantum of E/M wave energy

Light Sources: Laser Diode •Light Amplification by Stimulated Emission of Radiation � A laser diode Is an LED with two important differences: � (1) The operating current is much higher in order to produce OPTICAL GAIN � (2) Two of the ends of the LD are cleaved parallel to each other. These ends act as perfectly aligned mirrors which reflect the light back and forth through the "gain medium" in order to get as much amplification as possible � The typical response time of a laser diode Is 0.5 ns. The line width is around 2 nm with a typical laser power of 10's of milliwatts. The wavelength of a laser diode can be 850 nm, 1300 nm, or 1500 nm.

Photo-Detectors � Must detect down to the order of 10-14 W � Need high conversion efficiency between light and electrical energy � Must respond fast for high bandwidth � Must have low-noise power and good light-collecting properties � Ideally, they must operate at low voltage, be easy to use, be robust and immune to changes in ambient conditions, have a long life, be reliable and inexpensive � Two devices stand out: � Positive-intrinsic-negative (PIN) diodes � Avalanche photodiodes (APD)

Detection Procedure � Photons collide with the electrons in the valence band � The electrons absorb photon energy, hv, and cross the band gap into the conduction band with charge q. � Incident optical power, P, transfers to the device with efficiency η. � The generated photocurrent is � We resort to mean values because the whole photo-detection process is stochastic.

Detectors: PIN Diode

Detectors: The APD Device

Detectors: Characteristics � Responsivity � Measure of conversion efficiency, a ratio of the output current to the input optical power (A/W) � Dark current � Leakage current flowing with no light input � Transit time � Time it takes a photo-induced carrier to cross the depletion Region � Spectral response � A relative spectral response vs. wavelength or frequency curve displays the range or system length possible for a given wavelength.