Transcript FibreOpticCommSys

CE00038-2
Communications
Optical fibre communication
Dr Mohammad N Patwary
Room C336 (Beacon)
Email: [email protected]
Phone: 353 557
1
Introduction

Transmission via beams of light traveling over thin glass fibers is a relative newcomer
to communications technology,
 beginning in the 1970s,
 reaching full acceptance in the early 1980s,
 and continuing to evolve since then

Fibers now form a major part of the infrastructure for telecommunications information
highways around the globe and serve as the transmission media of choice for numerous
local area networks.

In addition, short lengths of fiber serve as transmission paths for the control of
manufacturing processes and for sensor applications.

The steadily increasing demand for information capacity has driven the search for
transmission media capable of delivering the required bandwidths.

Optical carrier transmission has been able to meet the demand and should continue to
do so for many years.
2
Fundamentals

Optical communications refers to the transmission of information signals over carrier
waves that oscillate at optical frequencies.

Optical fields oscillate at frequencies much higher than radio waves or microwaves, as
indicated on the abbreviated chart of the electromagnetic spectrum in following figure.

Frequencies and wavelengths are indicated on the figure.
3
Fundamentals

For historical reasons, optical oscillations are usually described by their wavelengths
rather than their frequencies. The two are related by
c
f
where f is the frequency in hertz,  is the wavelength, and c is the velocity of light in
empty space (3×108 m/s).


A frequency of 3×1014 Hz corresponds to a wavelength of 10-6m (a millionth of a
meter is often called a micrometer).

Wavelengths of interest for optical communications are on the order of a micrometer.
4
Fundamentals
 Glass fibers have low loss in the three regions illustrated in the
following figure, covering a range from 0.8 to 1.6 μm (800 to 1600 nm).
5
Fundamentals
 This corresponds to a total bandwidth of almost 21014Hz. The loss is
specified in decibels, defined by

Where P1 and P2 are the input and output powers.
 Typically, fiber transmission components are characterized by their loss
or gain in decibels.
 The beauty of the decibel scale is that the total decibel value for a series
of components is simply the sum of their individual decibel gains and
losses.
6
Fundamentals

Losses in the fiber and in other components limit the length over which
transmission can occur.

Optical amplification and regeneration are needed to boost the power levels of
weak signals for very long paths.

The characteristically high frequencies of optical waves (on the order of 2×1014
Hz) allow vast amounts of information to be carried.

A single optical channel utilizing a bandwidth of just 1% of this center
frequency would have an enormous bandwidth of 2×1012 Hz.

As an example of this capacity, consider frequency division multiplexing of
commercial television programs. Since each TV channel occupies 6 MHz, over
300,000 television programs could be transmitted over a single optical channel.
7
Multiplexing

In addition to electronic multiplexing schemes, such as frequency-division
multiplexing of analog signals and time-division multiplexing of digital signals,
numerous optical multiplexing techniques exist for taking advantage of the
large bandwidths available in the optical spectrum.

These include


wavelength division multiplexing (WDM) and
optical frequency-division multiplexing (OFDM).

These technologies allow the use of large portions of the optical spectrum.

The total available bandwidth for fibers approaches 2×1014 Hz (corresponding
to the 0.8-1.6mm range).

Although atmospheric propagation is possible, the vast majority of optical
communications utilizes the waveguiding glass fiber.
8
Optical Communications Systems History

A key element for optical communications, a coherent source of light, became available in
1960 with the demonstration of the first laser.

This discovery was quickly followed by plans for numerous laser applications, including
atmospheric optical communications.

Developments on empty space optical systems in the 1960s laid the groundwork for fiber
communications in the 1970s.

The first low-loss optical waveguide, the glass fiber, was fabricated in 1970. Soon after,
fiber transmission systems were being designed, tested, and installed.

Fibers have proven to be practical for path lengths of under a meter to distances as long as
needed on the Earth’s surface and under its oceans (for example, almost 10,000 km for
transpacific links).
9
Optical Communications Systems History

Fiber communications are now common for telephone, local area, and cable
television networks.

Fibers are also found in short data links (such as required in manufacturing
plants), closed-circuit video links, and sensor information generation and
transmission.
10
Optical Communications Systems
 A block diagram of a point-to-point fiber optical communications
system shown in the figure below. This is the structure typical of the
telephone network.
11
Optical Communications Systems

The fiber telephone network is digital, operating at data rates from a few
megabits per second up to 2.5 Gb/s and beyond.

At the 2.5-Gb/s rate, several thousand digitized voice channels (each operating
at 64 kb/s) can be transmitted along a single fiber using time-division
multiplexing (TDM).

Because cables may contain more than one fiber (in fact, some cables contain
hundreds of fibers), a single cable may be carrying hundreds of thousands of
voice channels.

Rates in the tens of gigabit per second are attainable, further increasing the
potential capacity of a single fiber.
12
Optical Communications Systems

Telephone applications may be broken down into several distinctly different areas:
transmission between telephone exchanges, long-distance links, undersea links, and
distribution in the local loop (that is, to subscribers).

Although similarities exist among these systems, the requirements are somewhat
different.

Between telephone exchanges, large numbers of calls must be transferred over
moderate distances.

Because of the moderate path lengths, optical amplifiers or regenerators are not
required.

On the other hand, long-distance links (such as between major cities) require signal
boosting of some sort (either regenerators or optical amplifiers).

Undersea links (such as transatlantic or transpacific) require multiple boosts in the
signal because of the long path lengths involved
13
Optical Communication Networks

One architecture for the subscriber distribution network, called fiber-to-the-curb (FTTC), is
depicted in following figure. Signals are transmitted over fibers through distribution hubs into
the neighborhoods.
14
Optical Communication Networks

The fibers terminate at optical network units (ONUs) located close to the
subscriber.

The ONU converts the optical signal into an electrical one for transmission over
copper cables for the remaining short distance to the subscriber.

Because of the power division at the hubs, optical amplifiers are needed to keep
the signal levels high enough for proper signal reception.

Cable television distribution remained totally conducting for many years. This
was due to the distortion produced by optical analog transmitters.

Production of highly linear laser diodes [such as the distributed feedback (DFB)
laser diode] in the late 1980s allowed the design of practical analog television
fiber distribution links.
15
Optical Communication Networks

Conversion from analog to digital cable television transmission is facilitated by
the vast bandwidths that fibers make available and by signal compression
techniques that reduce the required bandwidths for digital video signals.

Applications such as local area networks (LANs) require distribution of the
signals over shared transmission fiber.

Possible topologies include



the passive star,
the active star, and
the ring network
16
Passive star network
FIGURE :Passive star network:T represents an optical transmitter and R represents an optical receiver
17
Active star Network
18
Ring Network
Fibers connect the nodes together, while the
terminals and nodes are connected electronically
Ring network: T represents an optical transmitter and R represents an optical receiver.
The nodes act as optical regenerators.
19
Components for Optical Communications Systems

The major components found in optical communications systems are:








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Modulators,
Light sources,
Fibers,
Photo-detectors,
Connectors,
Splices,
Directional couplers, Star couplers,
Regenerators,
Optical amplifiers.
They are briefly described in the remainder of this Lecture.
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Fibers

Fiber links spanning more than a kilometer typically use silica glass fibers, as
they have lower losses than either plastic or plastic cladded silica fibers.

The loss properties of silica fibers were indicated in slide 5 .

Material and waveguide dispersion cause pulse spreading, leading to intersymbol interference.

This limits the fiber’s bandwidth and, subsequently, its data-carrying capability.
The amount of pulse spreading is given by
Dt  ( M  M g ) LD
21
Fibers

Where M is the material dispersion factor and Mg is the waveguide dispersion
factor, L is the fiber length, and Dλ is the spectral width of the emitting light
source.

Because dispersion is wavelength dependent, the spreading depends on the
chosen wavelength (λ) and on the spectral width (D) of the light source.

The total dispersion (M+Mg) has values near 120, 0, and 15 ps/(nm  km) at
wavelengths 850, 1300, and 1550 nm, respectively.
22
Multimode fibers

Allow more than one mode to simultaneously traverse the fiber. This produces
distortion in the form of widened pulses because the energy in different modes
travels at different velocities. Again, intersymbol interference occurs. For this
reason, multimode fibers are only used for applications where the bandwidth (or
data rate) and path length are not large.
23
Single-mode fibers
 Limit the propagation to a single mode, thus eliminating multimode
spreading.
 Since they suffer only material and waveguide dispersive pulse
spreading, these fibers (when operating close to the zero dispersion
wavelength) have greater bandwidths than multimode fibers and are
used for the longest and highest data rate systems.
24
Fibers and Bandwidth

Tables given below list bandwidth limits for several types of fibers and
illustrates typical fiber sizes.
25
Fiber’s Properties

Step index fibers (SI) have a core having one value of refractive index and a cladding
of another value.

Graded-index (GRIN) fibers have a core index whose refractive index decreases
with distance from the axis and is constant in the cladding.

As noted, single-mode fibers have the greatest bandwidths. To limit the number of
modes to just one, the cores of single-mode fibers must be much smaller than those of
multimode fibers.

Because of the relatively high loss and large dispersion in the 800-nm first window,
applications there are restricted to moderately short path lengths (typically less than a
kilometer). Because of the limited length, multimode fiber is practical in the first
window.

Light sources and photo detectors operating in this window tend to be cheaper than
those operating at the longer wavelength second and third windows
26
Fiber’s Properties

The 1300-nm second window, having moderately low losses and nearly zero
dispersion, is utilized for moderate to long path lengths.

Non-repeatered paths up to 70 km or so are attainable in this window. In this
window, both single-mode and multimode applications exist.

Multimode is feasible for short lengths required by LANs (up to a few
kilometer) and single-mode for longer point-to-point links.

Fiber systems operating in the 1550-nm third window cover the highest rates
and longest unamplified, unrepeated distances.

Lengths on the order of 200 km are possible. Single-mode fibers are typically
used in this window. Erbium-doped optical amplifiers operate in the third
window, boosting the signal levels for very long systems (such as those
traversing the oceans).
27
Other Components

Semiconductor laser diodes (LD) or light-emitting diodes (LED) serve as the
light sources for most fiber systems. These sources are typically modulated by
electronic driving circuits. The conversion from signal current I to optical
power P is given by

Where a0 and a1 are constants. Thus, the optical power waveform is a replica of
the modulation current.

For very high-rate modulation, external integrated optic devices are available
to modulate the light beam after its generation by the source.
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Other Components

Laser diodes are more coherent (they have smaller spectral widths) than LEDs
and thus produce less dispersive pulse spreading, according to Eq. :
Dt  ( M  M g ) LD

In addition, laser diodes can be modulated at higher rates (tens of gigabit per
second) than LEDs (which are limited to rates of just a few hundred megabit
per second).

LEDs have the advantage of lower cost and simpler driving electronics.

Photodetectors convert the optical beam back into an electrical current.
Semiconductor PIN photodiodes and avalanche photodiodes (APD) are
normally used. The conversion for the PIN diode is given by the linear equation
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Other Components

The conversion for the PIN diode is given by the linear equation

Where I is the detected current, P is the incident optical power, and ρ is the
photodetector’s responsivity.
 Typical values of the responsivity are on the order of 0.5 mA/mW.
 The receiver current is a replica of the optical power waveform (which
is itself a replica of the modulating current). Thus, the receiver current
is a replica of the original modulating signal current, as desired.
30
An optical regenerator

An optical regenerator (or repeater) consists of an optical receiver, electronic
processor, and an optical transmitter.

Regenerators detect (that is, convert to electrical signals) pulse streams that
have weakened because of travel over long fiber paths, electronically determine
the value of each binary pulse, and transmit a new optical pulse stream
replicating the one originally transmitted.

Using a series of regenerators spaced at distances of tens to hundreds of
kilometers, total link lengths of thousands of kilometers are produced.
Regenerators can only be used in digital systems.

Optical amplifiers simply boost the optical signal level without conversion to
the electrical domain. This simplifies the system compared to the use of
regenerators.

In addition, optical amplifiers work with both analog and digital signals.
31
Splices and connectors

Splices and connectors are required in all fiber systems. Many types are
available. Losses tend to be less than 0.1 dB for good splices and just a few
tenths of a decibel for good connectors.

Fibers are spliced either mechanically or by actually fusing the fibers together.

Directional couplers split an optical beam traveling along a single fiber into
two parts, each traveling along a separate fiber.

The splitting ratio is determined by the coupler design. In a star coupler the
beam entering the star is evenly divided among all of the output ports of the
star.

Typical stars operate as 88, 1616, or 3232 couplers. As an example, a 3232
port star can accommodate 32 terminals on a LAN.
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Signal Quality

Signal quality is measured by the signal-to-noise ratio (S/N) in analog systems and
by the bit error rate (BER) in digital links. The signal-to-noise ratio in a digital
network determines the error rate and is given by:

Where P is the received optical power, ρ is the detector’s un-amplified responsivity,
M is the detector gain if an APD is used, n (usually between 2 and 3) accounts for
the excess noise of the APD, B is the receiver’s bandwidth, k is the Boltzmann
constant (k= 1.381023 J/K), e is the magnitude of the charge on an electron
(1.61019 C), T is the receiver’s temperature in kelvin, ID is the detector’s dark
current, and RL is the resistance of the load resistor that follows the photodetector
33
.
Signal Quality

The first term in the denominator of Eq. (previous slide) is caused by shot
noise, and the second term is attributed to thermal noise in the receiver. If the
shot noise term dominates (and the APD excess loss and dark current are
negligible), the system is shot-noise limited. If the second term dominates, the
system is thermal-noise limited In a thermal-noise limited system, the
probability of error Pe (which is the same as the bit error rate) is

where erf is the error function, tabulated in many references

An error rate of 10-9 requires a signal-to-noise ratio of nearly 22 dB (S/N =
158.5).
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System Design

System design involves ensuring that the signal level at the receiver is sufficient
to produce the desired signal quality.

The difference between the power available from the transmitting light source
(e.g., Pt in dBm) and the receiver’s sensitivity (e.g., Pr in dBm) defines the
system power budget L.

Thus, the power budget is the allowed accumulated loss for all system
components and is given (in decibels) by
35
System Design

In addition to ensuring sufficient available power, the system must meet the
bandwidth requirements for the given information rate.

This requires that the bandwidths of the transmitter, the fiber, and the receiver
are sufficient for transmission of the message
36
Defining Terms

Avalanche photodiode: Semiconductor photodetector that has internal gain
caused by avalanche breakdown.

Bit error rate: Fractional rate at which errors occur in the detection of a digital
pulse stream. It is equal to the probability of error.

Dispersion: Wavelength-dependent phase velocity commonly caused by the
glass material and the structure of the fiber. It leads to pulse spreading because
all available sources emit light covering a (small) range of wavelengths. That is,
the emissions have a nonzero spectral width.

Integrated optics: Technology for constructing one or more optical devices on a
common waveguiding substrate.

Laser: A source of coherent light, that is, a source of light having a small
spectral width.

Laser diode: A semiconductor laser. Typical spectral widths are on the order of
1–5 nm.
37
Defining Terms

Light-emitting diode: A semiconductor emitter whose radiation typically is not as
coherent as that of a laser. Typical spectral widths are on the order of 20–100 nm.

Multimode fiber: A fiber that allows the propagation of many modes.

Optical frequency-division multiplexing: Multiplexing many closely spaced
optical carriers onto a single fiber. Theoretically, hundreds (and even thousands) of
channels can be simultaneously transmitted using this technology.

PIN photodiode: Semiconductor photodetector converting the optical radiation into
an electrical current.

Receiver sensitivity: The optical power required at the receiver to obtain the desired
performance (either the desired signal-to-noise ratio or bit error rate).

Responsivity: The current produced per unit of incident optical power by a
photodetector.
38
Defining Terms

Signal-to-noise ratio: Ratio of signal power to noise power.

Single-mode fiber: Fiber that restricts propagation to a single mode. This
eliminates modal pulse spreading, increasing the fiber’s bandwidth.

Wavelength-division multiplexing: Multiplexing several optical channels onto
a single fiber. The channels tend to be widely spaced (e.g., a two-channel
system operating at 1300 nm and 1550 n
39