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Fiber Optics
Sep 14, 2009
The first commercial fiber optic installation was for telephone signals in
Chicago, installed in 1976.
The copper cable has about 1000 pairs of conductors. Each pair can only
carry about 24 telephone conversations a distance of less than 3 miles.
The fiber cable carries more than 32,000 conversations hundreds or even
thousands of miles before it needs regeneration. Then each fiber can
simultaneously carry over 150 times more by transmitting at different colors
(called wavelengths) of light.
Applications of Fiber Optics
• Why use Fiber Optics?
The biggest advantage of optical fiber is the fact it can transport
more information longer distances in less time than any other
communications medium.
Fiber is unaffected by the interference of electromagnetic radiation
which makes it possible to transmit information and data with less
noise and less error.
Fiber is lighter than copper wires which makes it popular for aircraft
and automotive applications.
• Inexpensive
The cost of transmitting a single phone conversation over fiber
optics is only about 1% the cost of transmitting it over copper wire!
• Large bandwidth
Single mode fiber used in telecommunications and CATV has a
bandwidth of greater than a terahertz. Standard systems today carry up
to 64 channels of 10 gigabit signals - each at a unique wavelength.
Where is the bulk of telephone cabling?
• Only 10% is in long distance networks, which were the
first links converted to fiber - years ago.
• Another 10% is local loop (metropolitan) connecting
central offices and switches - now mostly converted to
fiber too.
• Fully 80% of all fiber cabling is subscriber loop - the “last
mile” that connects the end user to the system.
• After 20 years of fiber optic installations, virtually all long
distance and local loop connections are already fiber.
• Only the “last mile” is still copper, and much of it is very
old and incapable of carrying modern high bandwidth
digital signals.
Optical fiber is comprised of a light carrying core surrounded by a
cladding which traps the light in the core by the principle of total internal
reflection.
Most optical fibers are made of glass, although some are made of plastic.
The core and cladding are usually fused silica glass which is covered by
a plastic coating called the buffer which protects the glass fiber from
physical damage and moisture.
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Single-fiber construction.
FIGURE 24-2
(a) Basic principle of the law of refraction; (b) total internal reflection prevents any light from leaving medium 1.
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Refraction of light.
Critical angle.
Modes of propagation.
FIGURE 24-3
Light paths and dispersion in (a) step index, (b) graded index, and (c) single-mode fibers.
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Types of optical fiber.
Graded-index fiber.
Light Used in Fiber Optics
• The ultra-pure glass used in making optical fiber has less
attenuation (signal loss) at wavelengths (colors) in the
infrared, beyond the limits of the sensitivity of the human
eye. The fiber is designed to have the highest
performance at these wavelengths.
• The particular wavelengths used, 850, 1300 and 1550
nm, correspond to wavelengths where optical light
sources (lasers or LEDs) are easily manufactured.
• Some advanced fiber optic systems transmit light at
several wavelengths at once through a single optical
fiber to increase data throughput. We call this method
wavelength division multiplexing.
Upper portion of
electromagnetic spectrum
Figure 18-4 The electromagnetic wavelength spectrum.
•
The input end of a WDM system is really quite simple. It is a simple coupler that
combines or multiplexes all the signal inputs into one output fiber. The demultiplexer
separates the light at the end of the fiber. It shines the light on a grating (a mirror like
device that works like a prism and looks similar to the data side of a CD) which
separates the light into the different wavelengths by sending them off at different
angles. Optics capture each wavelength and focuses it into another fiber, creating
separate outputs for each wavelength of light.
DWDM
Demultiplexer
Output Fibers
• Cables will include strength members, typically a strong synthetic
fiber like Kevlar, which takes the stress of pulling the cable. The thin
yellow fibers in the photo are the strength members.
• The outside of the cable is called the jacket. It is the final protection
for the fibers and must withstand extremes of temperatures,
moisture and the stress of installation. Some cables even have a
layer of thin metal under the jacket to prevent rodents from chewing
through the cable.
FIGURE 24-4 Power loss versus wavelength of a typical fiber; other fiber formulations have differing wavelengths of minimum
and maximum loss.
1290 to 1350 nm
830 to 860 nm
1530 to 1600 nm
850 nm
1310 nm
1550 nm
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 24-5
Three types of fiber misalignment, causing power loss at a splice.
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Sources of connection loss.
Figure 18-20 (continued) Sources of connection loss.
FIGURE 24-6
Geometrical meaning of numerical aperture and acceptance cone (courtesy of Belden Wire and Cable Corp.).
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Total Internal Reflection
• By making the core of the fiber of a material with
a higher refractive index, we can cause the light
in the core to be totally reflected at the boundary
of the cladding for all light that strikes at greater
than a critical angle determined by the difference
in the composition of the materials used in the
core and cladding.
(a) Development of numerical aperture;
(b) acceptance cone.
FIGURE 24-8 Relative output power versus wavelength
(and bandwidth) of LED and laser diode.
William Schweber
Electronic Communication Systems, 4e
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Spectral response of a p-i-n diode.
LED modulating circuit is relatively simple to interface to data.
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 24-10 PIN diode receiver circuit includes a trans-impedance
amplifier, secondary-stage amplifier, threshold comparator, and digital circuit
at receiver.
William Schweber
Electronic Communication Systems, 4e
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
• Fiber optic transmission systems all consist of a transmitter which
takes an electrical input and converts it to an optical output from a
laser diode or LED. The light from the transmitter is coupled into the
fiber with a connector and is transmitted through the fiber optic cable
plant.
• The light is ultimately coupled to a receiver where a detector
converts the light into an electrical signal which is then conditioned
properly for use by the receiving equipment.
FIGURE 24-11 Link function schematic and budget analysis of a basic point-to-point fiber optic system shows all factors and
resultant maximum and minimum loss and signal power from source to receiver (courtesy of Belden Wire and Cable Corp.).
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 18-21 Fiber connectors.
Fiber-optic communication system.
Attenuation versus wavelength
for typical high-quality fiber.
Semiconductor laser.
Light output versus bias current
for a laser diode.
LED modulator.
P-i-n diode.
Avalanche photodiode.
Splice technique using
alignment pins.
Fiber-optic
emitter/detector/connector
assembly.
(Courtesy of General Electric Company.)
Bus topology optical LAN.
A fiber link showing emitter, detector,
connectors, and fiber cable.
Figure 18-22 System design.
Figure 18-23 A graphical view of the system design problem shown in Figure 18-22.
Figure 18-24 An alternative view of the system design problem.
Figure 18-25 An OTDR trace of an 850-nm fiber.
Figure 18-25 (continued) An OTDR trace of an 850-nm fiber.