Chapter Twenty-Five:Optical Communication Systems

Introduction• Fiber-optic systems are becoming very important

in communication systems• Applications for fiber-optics include:

– 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 pulse-spreading 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 10-

and 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 fiber- optic lines to determine losses, breaks, and attenuations within a system