Download Chapter 25 - Heartland Community College

Chapter Twenty-Five:
Optical Communication Systems
Introduction
• Fiber-optic systems are becoming very important
in communication systems
• Applications for fiber-optics include:
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Cable television
Data networks
Telephone systems
Hybrid systems
Basic Fiber-Optic Systems
• A basic fiber-optic system includes:
– Transmitter - LED or laser diode
– Receiver - PIN diode or APD diode
– Length of fiber - either multimode or single-mode
• In general, short-range systems use LED emitters and
multimode fiber, while long-range systems use laser diodes
and single-mode fiber
• Bit rates of 10 Gb/s are common in high-speed systems and
even higher rates are used in the newest undersea cables
Loss Budget
• The most basic limitation on the length of the fiber-optic link is
loss in the fiber, connectors, and splices
• If the length is too great, the optical power level at the receiver
will be insufficient to produce an acceptable signal-to-noise ratio
• Given the optical power output of the transmitter and the signal
level required by the receiver, a loss budget may be drawn up
• If the losses along the line are enough to reduce the power at the
receiver below minimum requirements, then one of the following
needs to occur:
– Increase the transmitter power
– Increase receiver sensitivity
– Decrease the length of the cable
Rise Time Budget
• As noted before, dispersion in a fiber cable limits the length that
can be used
• The effect of dispersion increases with the length of the fiber
• The effect of dispersion is also proportional to the bandwidth of
the information signal
• Most of the dispersion in multimode fiber is due to the numerous
modes
• The fiber itself is not the only part of the system that limits
bandwidth and data rates; both receivers and transmitters have
finite rise times that limit their bandwidth
Pulse Spreading and Rise Times
• As a pulse of light propagates down the fiber, its duration increases
• The amount of pulse spreading is proportional to the length of the
fiber and to its dispersion per kilometer, which is known as its pulsespreading constant
• The pulse-spreading constant is given in nanoseconds or picoseconds
per kilometer
Repeaters and Optical Amplifiers
• Because of loss or dispersion, there is always a limit to the
length of a single span of fiber-optic cable
• When distances are great, some form of gain must be
provided, using one of two different ways:
– Change the signal to electrical form, amplify it, regenerate it if it is
digital, and then convert it back to an optical signal
– Simply amplify the optical signal
Regenerative Repeaters
• In its most common form, a repeater converts the signal from optical to
electrical energy, then converts it back to optical form
• One of the advantages of using digital techniques is the fact that regenerative
repeaters can be used
• As long as repeaters are spaced closely enough, they can avoid accumulation
of noise and distortion
Erbium-Doped Fiber Amplifiers
• In situations where fiber loss, not dispersion, is the limiting factor on the
length of a fiber span, it is possible to amplify the optical signal directly
• An optical amplifier can work with any type of signal, analog or digital,
whether multiplexed or not
• The construction of optical amplifiers is based on principles similar to
laser operations
Wavelength-Division Multiplexing
• Most optical systems use TDM to take advantage of the
available bandwidth using one LED or laser diode
• This bandwidth, which is limited only by dispersion, is
only a small fraction of the actual bandwidth available on
a fiber
• Several light sources, each operating at a different
wavelength, can be coupled into the same fiber
• This scheme, called wavelength-division multiplexing,
requires lasers with narrow bandwidth
Wavelength-Division
Multiplexing Operation
• WDM is really a form of frequency-division multiplexing
• One difference between WDM and FDM is that for FDM, the separation
between carriers is limited by the sidebands created by modulation, whereas
with lasers, the width of the carrier signal itself determines the the signal
bandwidth
Dense Wavelength-Division
Multiplexing
• When many wavelengths are used in an optical systems, dense
wavelength-division multiplexing technique is used
• The state of the art is to use 80 wavelengths on one fiber, but
systems using from 36 to 40 wavelengths are more common
• With each wavelength capable of carrying 10 Gb/s, the increase
in capacity of DWDM is impressive, though costly
Submarine Cables
• The use of fiber optics for underwater telephone cables is a logical
application
• Coaxial cables have traditionally been used, but these have less
bandwidth, and the number of repeaters required is greater
• Short fiber-optic cables with lengths under about 100 km are generally
built without repeaters
• The first fiber-optic transatlantic cable was completed in December of
1988, with a repeater spacing of 70 km, using a total of 109 repeaters
• The latest generation of fiber-optic cables operates at double the data
rate and the repeater spacing is more than 100 km
The Synchronous Optical
Network (SONET)
• The very high data transmission rates with fiber optics require
new standards for digital transmission
• The synchronous optical network (SONET) standard was
especially developed for fiber-optic transmission
• SONET is an American standard; the European equivalent is
called the synchronous digital hierarchy (SDH) and is very
similar to SONET
• The basic signal rate is 51.840 Mb/s and any multiple of this
rate is possible
Fiber in Local Area Networks
• Most LANs use twisted-pair or coaxial cable
• Fiber optics have started to become more popular in LANs
because of the greater bandwidth and lower losses
• Of the three common topologies used with LANs (star,
ring, bus), the ring topology lends itself best for use with
fiber optics
• Most fiber LANs use one of three technologies:
– Fiber distributed data interface (FDDI)
– High-speed Ethernet
– Gigabit Ethernet
Fiber Distributed Data Interface
(FDDI)
• FDDI systems use multimode fiber at a wavelength of 1.3
micrometers
• LEDs and PIN diodes are used for low cost
• The data rate is 100 Mb/s
• The FDDI uses two token rings that carry signals in opposite
directions. Usually only one is used and the other is for backup
• The length between nodes can be quite high, up to 2 km, with
a total length up to 200 km
Ethernet on Fiber
• Fiber can be used instead of copper for both 10and 100-Mb/s data transmission rates
• Multimode glass fiber is used and LED sources
operating at 1300 nm
• The network is a logical bus, but a physical star
• The main advantage of using fiber with Ethernet is
the longer distances that are possible
Gigabit Ethernet
• The gigabit Ethernet system was originally
designed to be implemented using fiber optics,
though it can be used with twisted-pair copper for
short distances
• For short distances, multimode fiber is used with
low-cost laser diodes operating at 850 nm and
increased up to 5 km using laser diodes operating
at 1300 nm and single-mode fiber
Local Telephone Applications
• Nearly all new trunk lines for
long-distance telephony are now
fiber
• Most fiber trunks use single-mode
fiber operating at 1.3 micrometers
• Many local loops remain on
copper because of the cost to
upgrade infrastructure and the
need to install electrical-to-optical
interfaces within the systems
• Two terms used within telephony
when referring to fiber:
– Fiber in the loop (FITL)
– Fiber to the curb (FTTC)
Cable-Television Applications
• CATV systems are switching to
fiber because of the increased
bandwidth and the decrease in
signal loss, requiring fewer
repeaters
• Fiber systems lend themselves to
compressed digital transmission
• CATV systems are also now
providing Internet services to
customers and fiber lends itself
to the high bandwidth required
Experimental Techniques
• There is still much work to be done in fiber optics
and two of the newer developments in fiber
technology are:
– Solitons- solves some of the problem of chromatic
dispersion by using a wavelength slightly greater than
the zero-dispersion value
– Heterodyne Reception - using a laser diode with PIN
diode mixer, heterodyning has been accomplished as a
transmission mode in optical systems
Optical Time-Domain Reflectometry
• Optical time-domain
reflectometry (OTDR)
is used to analyze fiberoptic lines to determine
losses, breaks, and
attenuations within a
system