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Title: CE000382 Communications


1
CE00038-2Communications
Optical fibre communication
  • Dr Mohammad N Patwary
  • Room C336 (Beacon)
  • Email m.n.patwary_at_staffs.ac.uk
  • Phone 353 557

2
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.

3
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.

4
Fundamentals
  • 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.

5
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).

6
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.

7
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 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.

8
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 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.

9
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 Earths surface and under its
    oceans (for example, almost 10,000 km for
    transpacific links).

10
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.

11
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.

12
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.

13
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

14
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.

15
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.

16
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

17
Passive star network
FIGURE Passive star networkT represents an
optical transmitter and R represents an optical
receiver
18
Active star Network
19
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.
20
Components 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.

21
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 inter-symbol interference.
  • This limits the fibers bandwidth and,
    subsequently, its data-carrying capability. The
    amount of pulse spreading is given by

22
Fibers
  • 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.

23
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.

24
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.

25
Fibers and Bandwidth
  • Tables given below list bandwidth limits for
    several types of fibers and illustrates typical
    fiber sizes.

26
Fibers 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

27
Fibers 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).

28
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.

29
Other 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

30
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 ? 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.

31
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.

32
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.

33
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, ? 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.

34
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).

35
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 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

36
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

37
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 15 nm.

38
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 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.

39
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 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
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