Transcript Fiber Optic Light Sources - Electrical and Computer

Fiber Optic Light Sources
Fiber Optic Communications
ECEE – 641 Dr. Kurzweg
By: Antonios Boulos
Overview
What is an Optical Source
 LEDs
 SLEDs – Surface Emitting LEDs
 ELEDs – Edge Emitting LEDs
 LDs – Laser Diodes
 Tunable Lasers
 Conclusion
 Questions
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What is an Optic Source?
The heart of a fiber optical data system
 A Hybrid Device
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Converts electrical signals into optical signals
 Launches these optical signals into an optical
fiber for data transmission.
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Device consists of an interface circuit,
drive circuit, and components for optical
source. (LEDs, ELEDs, SLEDs, LDs, etc)
LEDs – Light Emitting Diode
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Emits incoherent light through
spontaneous emission.
Used for Multimode systems w/
100-200 Mb/s rates.
Broad spectral width and wide
output pattern.
850nm region: GaAs and AlGaAs
1300–1550nm region: InGaAsP
and InP
Two commonly used types:
ELEDs and SLEDs
SLEDs – Surface Emitting LEDs
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Primary active region is a small circular area
located below the surface of the semiconductor
substrate, 20-50µm diameter and up to 2.5µm
thick.
Emission is isotropic and in lambertian pattern.
A well is etched in the substrate to allow the direct
coupling of emitted light to the optical fiber
Emission area of substrate is perpendicular to
axis of optical fiber
Coupling efficiency optimized by binding fiber to
the substrate surface by epoxy resin with
matching refractive index
Surface Emitting LED
ELEDs – Edge Emitting LEDs
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Primary active region is a narrow strip that lies beneath
the semiconductor substrate
Semiconductor is cut and polished so emission strip
region runs between front and back.
Rear face of semiconductor is polished so it is highly
reflective while front face is coated with anti-reflective,
light will reflect from rear and emit through front face
Active Regions are usually 100-150µm long and the
strips are 50-70µm wide which are designed to match
typical core fibers of 50-100µm.
Emit light at narrower angle which allows for better
coupling and efficiency than SLEDs
Edge Emitting LED
LDs – Laser Diodes
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Emit coherent light through
stimulated emission
Mainly used in Single Mode
Systems
Light Emission range: 5 to
10 degrees
Require Higher complex
driver circuitry than LEDs
Laser action occurs from
three main processes:
photon absorption,
spontaneous emission, and
stimulated emission.
Laser Diode Optical Cavity
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One reflecting mirror is at one end while the other end
has a partially reflecting mirror for partial emission
Remaining power reflects through cavity for amplification
of certain wavelengths, a process known as optical
feedback.
Construction very similar to the ELEDs.
Lasing Characteristics
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Lasing threshold is
minimum current that must
occur for stimulated
emission
Any current produced below
threshold will result in
spontaneous emission only
At currents below threshold
LDs operate as ELEDs
LDs need more current to
operate and more current
means more complex drive
circuitry with higher heat
dissipation
Laser diodes are much
more temperature sensitive
than LEDs
Tunable Laser
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Tunable Laser
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Employed in broad-band interconnections and
broadcast networks where the need for high power,
narrow line width, and a tunable single-frequency
emission is a must.
Laser that is able to produce controllable multiple
wavelengths within single cavity.
Able to switch transmission of different wavelengths
without using multiplexer for transmission to many
different channels at by tuning the output frequency to
its designated channel.
Tunable Laser Cavity
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Consists of an Active Region, and two passive regions:
Phase Control and Grating
Active region is a double heterostructure of a low
bandgap between two high gap low index claddings.
Two passive regions made from semiconductor with
intermediate bandgap between active and cladding.
Tunable Laser Operation
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Current is injected into the Active Region causing the
entire optical cavity to oscillate in a single longitudinal
mode.
A current is then injected into the grating control region
causing a refractive index decrease which induces a shift
of the Bragg wavelength and variation in the mode.
The phase region with the injected phase current allows
for recovery in Bragg wavelength in order to keep the
same mode in the center of the filter band.
This results in an output with variable wavelength.
Summary
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Optical light sources convert electrical signals into optical signals
and launch them.
Commonly used light sources include LEDs, ELEDs, SLEDs, and
LDs.
LEDs produce nonlinear incoherent light whereas a Laser Diode
produces linear coherent light.
Incoherent light sources used in multimode systems as where Laser
Diodes/Tunable Lasers in single mode systems
Laser diodes must operate above their threshold region to produce
coherent light, otherwise operating as ELED.
Laser diodes are much faster in switching response than LEDs
Tunable laser is able to produce coherent light output with controlled
variable wavelength
Tunable laser is used in multi wavelength systems by replacing a
system where many sources are coupled into a multiplexing device
system