OFC Unit 2 A
OFC Unit 2 A
OFC Unit 2 A
OPTICAL SOURCE
1. CHARACTERISTICS OF A GOOD OPTICAL SOURCE
1. Be compatible in size to low-loss optical fibers by having a small light-emitting
area capable of launching light into fiber
2. Launch sufficient optical power into the optical fiber to overcome fiber attenuation
and connection losses allowing for signal detection at the receiver
3. Emit light at wavelengths that minimize optical fiber loss and dispersion.
4. Optical sources should have a narrow spectral width to minimize dispersion
5. Allow for direct modulation of optical output power
6. Maintain stable operation in changing environmental conditions (such as
temperature).
7. Cost less and be more reliable than electrical devices.
8. Permitting fiber optic communication systems to compete with conventional
systems
Diagram showing:-Energy vs. crystal momentum for a semiconductor with a direct band
gap, showing that an electron can shift from the lowest-energy state in the conduction
band (green) to the highest-energy state in the valence band (red) without a change in
crystal momentum. Depicted is a transition in which a photon excites an electron from
the valence band to the conduction band
1.LED-
Introduction
A light emitting diode (LED) is a device which converts electrical energy to light
energy. LEDs are preferred light sources for short distance (local area) optical fiber
network because they: are
1. inexpensive,
2. robust
3. have long life (the long life of an LED is primarily due to its being a cold
device, i.e. its operating temperature being much lower than that of, say, an
incandescent lamp)
4. can be modulated (i.e. switched on and off) at high speeds (this property of an
LED is also due to its being a cold device as it does not have to overcome
thermal inertia)
5. couple enough output power over a small area to couple to fibers (though the
output spectrum is wider than other sources such as laser diodes).
LED STRUCTURE
The two basic LED configuration being used for fiber optics are
1.SURFACE EMITTER LED:-Surface-emitting LEDs (SLEDs) have a thin active layer
parallel to the surface from which the light extracted. In a simple flat-diode
configuration (as shown in below figure ), the active layer is just below the emitting
surface, and the current is applied with a ring electrode. The light emitted in the
“wrong” direction is absorbed by the substrate. There are also devices where the
substrate is made transparent, and the back electrode reflects back that light, so that
at least some part of it can be used.
A variant is the Burrus-type LED, where the active layer can be deeper within the
originally grown semiconductor structure, and a trench etched into the structure
allows for more efficient light extraction. It is also possible to introduce a fiber into the
trench in order to obtain a fiber-coupled LED.
2.EDGE EMITTER LED:-Edge-emitting LEDs have a structure similar to that of edge-emitting
semiconductor lasers: they are emitting from the edge of a cleaved wafer, where the active
region meets the cleaved surface. Such devices allow more efficient fiber coupling than surface-
emitting LEDs. Applied in optical fiber communications, they allow higher data rates.
A variant of the edge-emitting LED is the super luminescent diode (SLD), where the
spontaneous emission is substantially amplified within a waveguide. Here, the emission is much
more directional, and as a consequence the brightness is much higher, even for SLDs with quite
low output power