Transcript elshort

Light rays bend when traversing boundaries between media
with different refractive index:
in
n1
n2


n1

out
Snell’s Law
n1 sin   n2 sin 
See http://micro.magnet.fsu.edu/primer/java/scienceopticsu/refraction/index.html
n > n’ always
Optical fiber
size
n’
n’
Cladding
n
Core
~ 100 μm
1 – 10 μm
n’
n’
n’
n
n
n’
n’
Optical fibers are cylindrical waveguides,
providing light confinement by total
internal reflection along all directions
which are perpendicular to the
propagation direction. These are
essentially bendable “light pipes”.
Cross-section
Optical loss in fiber-quality fused silica. (circa 1995)
Fibers are made of ultrapure
SiO2 glass (silica). Different
dopants are added both to the
core and cladding, such that the
refractive index of the core is
slightly larger than that of the
cladding.
Communications
window
Optical loss in fiber-quality fused silica. (circa 2001)
To optimize fibers for
telecommunications applications it
was necessary to purify them to a
very high degree and remove all
traces of water. This eliminated the
high absorption losses in the
“communications window”.
Fiber-Optic Communications Systems
Laser
Light pulses travel in fiber (short or long)
Input electric pulses ~10Gb/sec
Output electric pulses
Example of fiber-optical communication link. Electrical current pulses
representing digital data drive a semiconductor laser. The emitted light
pulses pass through a fiber and are detected by a photo-detector at the
far end.
Amplifying optical signals
How far can an optical signal (light) travel in fiber before absorption causes
significant losses and signal deterioration?
Fibers can typically transmit
information over a distance of
80km, after which signals require
amplification and/or regeneration.
Fibers also have a very large
bandwidth – the communications
window where absorption losses in
the fiber are small is broad. This
allows transmitting many
wavelengths (frequencies)
simultaneously.
Communications window
Connecting fibers – optical communications systems
Different frequency
for each channel
MUX = Multiplexing
DEMUX = Demultiplexing
SCL = semiconductor laser
Mod = modulator
Det = detector