Title: CE000382 Communications
1CE00038-2Communications
Optical fibre communication
- Dr Mohammad N Patwary
- Room C336 (Beacon)
- Email m.n.patwary_at_staffs.ac.uk
- Phone 353 557
2Introduction
- 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.
3Fundamentals
- 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.
4Fundamentals
- For historical reasons, optical oscillations are
usually described by their wavelengths rather
than their frequencies. The two are related by -
- where f is the frequency in hertz, ? is the
wavelength, and c is the velocity of light in
empty space (3108 m/s). - A frequency of 31014 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.
5Fundamentals
- 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).
6Fundamentals
- 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.
7Fundamentals
- 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 21014 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 21012 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.
8Multiplexing
- 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 21014 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.
9Optical 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 Earths surface and under its
oceans (for example, almost 10,000 km for
transpacific links).
10Optical 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.
11Optical 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.
12Optical 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.
13Optical 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
14Optical 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.
15Optical 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.
16Optical 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
17Passive star network
FIGURE Passive star networkT represents an
optical transmitter and R represents an optical
receiver
18Active star Network
19Ring 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.
20Components for Optical Communications Systems
- The major components found in optical
communications systems are - 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.
21Fibers
- 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 inter-symbol interference. - This limits the fibers bandwidth and,
subsequently, its data-carrying capability. The
amount of pulse spreading is given by
22Fibers
- Where M is the material dispersion factor and Mg
is the waveguide dispersion factor, L is the
fiber length, and D? is the spectral width of the
emitting light source. - Because dispersion is wavelength dependent, the
spreading depends on the chosen wavelength (?)
and on the spectral width (D) of the light
source. - The total dispersion (MMg) has values near 120,
0, and 15 ps/(nm ? km) at wavelengths 850, 1300,
and 1550 nm, respectively.
23Multimode 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.
24Single-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.
25Fibers and Bandwidth
- Tables given below list bandwidth limits for
several types of fibers and illustrates typical
fiber sizes.
26Fibers 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
27Fibers 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).
28Other 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.
29Other Components
- Laser diodes are more coherent (they have smaller
spectral widths) than LEDs and thus produce less
dispersive pulse spreading, according to Eq. - 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
30Other 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 ? is the
photodetectors 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.
31An 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.
32Splices 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.
33Signal 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, ? is the
detectors 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 receivers 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 receivers temperature in kelvin, ID
is the detectors dark current, and RL is the
resistance of the load resistor that follows the
photodetector.
34Signal 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).
35System 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 receivers 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
36System 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
37Defining 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 15 nm.
38Defining 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 20100 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.
39Defining 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 fibers
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