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Fiber-Optic Communications
James N. Downing
Chapter 5
Optical Sources and Transmitters
5.1 Source Considerations
• Fiber must match:
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Power
Size
Modal characteristics
Numerical aperture
Linewidth
Fiber window
Wavelength
Data type
5.2 Electronic Considerations
• Conductors
– Flow of electrons
• Insulators
– Block current flow
• Semiconductors
– Require more energy than conductors but less
than insulators for current to flow
5.2 Electronic Considerations
• The PN Junction
– Two junctions—one highly doped with negative charge
carriers and the other doped with positive charge carriers—
are fabricated next to each other.
– When an external voltage is applied in forward bias (positive
terminal attached to the positively doped region), current will
flow through the p-n junction.
– When the external voltage is applied in reverse, no current
will flow through the p-n junction.
5.3 The Light-Emitting Diode (LED)
• LED Operation
– When a p-n junction is forward biased, electrons
obtain enough energy (bandgap energy) to jump
to a higher energy level where they begin to lose
their energy .
– When those electrons lose the energy needed to
keep them in the higher level, they drop back to
the valence level (recombination)..
5.3 The Light-Emitting Diode (LED)
• LED Operation
– When the electrons recombine, a photon of light is
emitted.
– This is called spontaneous emission.
– The light is emitted in all directions (coherent).
– This light can be focused through a lens to be
used for displays.
5.3 The Light-Emitting Diode (LED)
• Linewidth
– Defined by the difference between the energy of
photon and the band gap energy
– Internal quantum efficiency: Efficiency of the photo
producing process
5.3 The Light-Emitting Diode (LED)
• LED Physical Structure
– Homojunction
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Both p and n sides are same base material
Surface-emitting LED
Light comes out all sides
Much light is wasted
5.3 The Light-Emitting Diode (LED)
• LED Structure
– Heterojunction
• Has different base materials
• Edge-emitting LED
5.3 The Light-Emitting Diode (LED)
• LED Performance
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Voltage: 1.5 to 2.5 volts
Current: 50 to 300 mA
Couples 10 to 100 μW of power into a fiber
Fiber window: 850 to 1550 nm
Linewidth: 15 to 60 nm
Data rates: 100 Mbps
Inexpensive
Rugged
Used in LANS
5.4 The Laser Diode
• LASER
– Light Amplification by Stimulated Emission of
Radiation
• Stimulated Emission
– An external photon hits an excited electron forcing
another photon to be emitted at the same
wavelength. That created photon excites another,
etc.
5.4 The Laser Diode
• Population Inversion
– A necessary condition for laser action
– The number (population) of the excited electrons
or photons are much greater than those in the
ground state.
5.4 The Laser Diode
• Positive Feedback
– Turns the amplifier into an oscillator
– Accomplished by fabricating mirrors at each end
of the medium causing the photons to bounce
back and forth from one end to the other
5.4 The Laser Diode
• Laser Output Mode Structure
– The range of optical frequencies is finite
– Mode-suppression ratio (MSR)
• Measure of how the physical structure of the device can
be tuned to a single mode
5.4 The Laser Diode
• Laser Diode Physical Structure
– Similar to edge-emitting LEDs but with a thinner
active region
– Broad-area-semiconductor laser
• No light confinement at the faces parallel to junction
plane
• Elliptical pattern
• Unsuitable for communications
5.4 The Laser Diode
• Laser Diode Physical Structure
– Buried heterostructure laser
• Single mode output
• Bandwidth and thickness of active layer control
5.4 The Laser Diode
• Quantum Well Lasers
– Better conversion efficiency, confinement, and
wavelength availability
• Distributed Feedback
– Selectively reflects only one wavelength due to the
Bragg grating inside the structure
5.4 The Laser Diode
• External Cavity Lasers
– Implemented by moving one mirror outside of the
active region resulting in a single longitudinal
mode output with a high MSR
• Vertical Cavity Surface Emitting Lasers
– Single-mode, narrow linewidth, circular output for
easy coupling
5.4 The Laser Diode
• Tunable Lasers
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High power
Stable
Single mode
Narrow linewidth
Long-haul and ultra-long-haul communications
5.5 Transmitters
• The transmitter is a device that converts an
electrical communication signal into an optical
one, modulates the signal, and couples the
modulated signal back into a fiber.
• Consists of
– Source, modulator, driver, and coupling devices
5.5 Transmitters
• Modulator
– Amplitude modulation primary method
– AM produces changes in the population of the
charge carriers of the LED.
– The change in population will also produce a
change in the refractive index of the fiber, which in
turn creates a “chirp.”
5.5 Transmitters
• Electrical Driving Circuit
– Provides appropriate current and voltage
– Consists of
• LED: A single transistor and a few resistors
• LASER: More complex. The laser is a current driven
device and requires precise current and temperature
control to maintain a stable output.
5.5 Transmitters
• Source to Fiber Coupling
– Efficiencies vary from 1% for LEDs to 80% for VCSEL
transmitters
– Direct Coupling
• Fiber is epoxied to the source
– Lens Coupling
• A lens is used to optimize the process
• May have tapered fiber
• Efficiencies of near 100%
5.5 Transmitters
• Transmitter Packaging
– Provides protection form environment and weather
– Provides mechanical stability