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Chapter 10 Optical Communication Systems
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OPTICAL COMMUNICATION SYSTEM
Elements of an optical communication system In
optical communication systems, electrical signals
are first converted into optical or light signals
by modulating an optical source, such as light
emitting diodes (LED) or laser diodes (LD). Then
the optical signal is transmitted over long
distances via optical fiber. At the receiving
end, the optical signal is converted to
electrical signal by avalanche or PIN
photodetector followed by the receiver circuits.

• The main components of an optical communication
• system are
• Optical source
• Modulator
• Transmission media
• Repeaters/Amplifiers
• Optical detector
• Demodulator

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OPTICAL COMMUNICATION SYSTEM
System Design

• Power Budget
• Each component introduces a loss.
• Thus, while designing an optical communication
  system, we must ensure that the components of the
  links do not cause a cumulative loss higher than
  PS PR PS (dBm) is the amount of output power
  from the light source and PR (dBm) is the minimum
  detectable optical power of the receiver.
• The process is called link power budgeting
  procedure.
• Rise Time Budget
• Similarly, the slowest component in the system
  will ultimately control the system bandwidth
  since the system response time cannot be faster
  than the response time of the slowest component.
• Each element of the link is fast enough to meet
  the given bit rate.
• The process is called link rise time budgeting
  procedure.

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OPTICAL COMMUNICATION SYSTEM
Power budget
Each component in the optical link has a specific
loss in dB. If Pi and Po are the power in and out
to the component respectively, the loss Li of the
component is given by Li 10 log(Po/Pi)
Apart from the component losses, a certain amount
of power margin Psm, called as system margin, is
required for unexpected losses. Thus, the power
budget equation can be written as P PS ? PR
Ls Ld NLj ?L Psm
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Optical power-loss model
OPTICAL COMMUNICATION SYSTEM
Total optical power loss allowed between the
light source and the photodetector
where PS source power n no. of splices
?f fiber attenuation PR received power lc
connector loss (dB/km) m no. of
connectors lsp splice loss and L
transmission distance
Try Examples 8.1 8.2 (in the book by Gerd
Keiser)
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OPTICAL COMMUNICATION SYSTEM
Rise time budget
In a system with N cascaded components, each of
which has a rise time ti, the total rise time
tsys of the system is
Try Example 8.3 (in the book by Gerd Keiser)

• where
• tt transmitter rise time
• tmat material dispersion rise time of the fibre
• tmod modal dispersion (broadening in time) of
  the fiber
• tr receiver rise time
• Hence the system speed is affected by the
  parameters as stated above.

• Total rise time of a digital link should not
  exceed
• 70 for a NRZ bit period
• 35 of a RZ bit period

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OPTICAL COMMUNICATION SYSTEM
DETECTION AND MODULATION SCHEMES IN OPTICAL
COMMUNICATIONS
DETECTION SCHEMES

• There are two principal types of detection
  schemes
• Direct detection
• Coherent detection

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OPTICAL COMMUNICATION SYSTEM

• Direct detection
• The optical signal is directly converted to base
  band by the photo detector
• Coherent detection
• The incoming light is combined with a local light
  (local oscillator laser) and the combined beam is
  detected by the photo detector
• The output current is a base band signal if the
  local oscillator frequency is equal to the
  optical carrier frequency which is called
  homodyne reception
• If the local oscillator frequency differs from
  the incoming optical frequency (heterodyne), then
  the output of the photo detector is an IF
  (intermediate frequency) signal.

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OPTICAL COMMUNICATION SYSTEM

• The IF signal is then filtered by a band pass
  filter (BPF) and demodulated by an IF
  demodulator. Finally the output of the
  demodulator is passed through the decision
  circuit and finally to a low pass filter to get
  information signal.
• The generalized coherent detection scheme is
  shown in the figure below

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OPTICAL COMMUNICATION SYSTEM
MODULATION SCHEMES

• Analog modulations
• Direct Intensity modulation (D-IM)
• Sub carrier intensity modulation (SC-IM)
• Sub carrier phase modulation (SC-PM)
• Sub carrier frequency modulation (SC-FM)
• Pulse frequency modulation(PFM)-intensity
  modulation (PFM-IM)
• Frequency modulation (FM), Phase modulation (PM).

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OPTICAL COMMUNICATION SYSTEM
DIRECT INTENSITY MODULATION It is the process of
modulating the laser source directly by the
analog modulating signal. The intensity of the
optical signal is varied in accordance with the
amplitude of the modulating signal. The receiver
consists of a photo detector to convert the
optical signal to electrical form and then passed
through a low-pass filter to get the modulating
signal.
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OPTICAL COMMUNICATION SYSTEM
SUB CARRIER INTENSITY MODULATION (SC-IM) In this
scheme, the modulating signal is used to modulate
a microwave (MW) sub carrier with AM, PM or FM.
The modulated MW signal is then used to modulate
the laser using intensity modulation. In the
receiver, the output of the detector is a MW
signal with AM, PM or FM. Demodulation is then
done by using a demodulator of similar type to
get the information signal.
SUBCARRIER PHASE/FREQUENCY MODULATION These are
similar to SC-IM. Instead of Intensity modulation
(IM) here the laser is frequency or phase
modulated by the sub carrier signal.
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OPTICAL COMMUNICATION SYSTEM
PULSE FREQUENCY MODULATION (PFM) / INTENSITY
MODULATION (PFM-IM) The modulating signal is
used to frequency modulate a pulse carrier of
microwave frequency or RF frequency. This signal
is then used to intensity modulate the laser. In
the receiver, the output of the photo detector is
pass through a limiter and a low-pas filter for
PFM demodulation.
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OPTICAL COMMUNICATION SYSTEM
DIGITAL MODULATION SCHEMES The digital
modulation schemes used in optical communication
are similar to those used in conventional radio
frequency communications like ASK, PSK, FSK,
Differential PSK (DPSK), Quadrature PSK (QPSK),
pulse position modulation (PPM) etc.
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OPTICAL COMMUNICATION SYSTEM

• MULTIPLEXING SCHEMES
• There are three main multiplexing schemes used in
  optical communications
• Optical time division multiplexing (OTDM)
• Optical frequency division multiplexing (OFDM) or
  Wavelength division multiplexing (WDM)
• Sub carrier multiplexing (SCM)

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OPTICAL COMMUNICATION SYSTEM
OPTICAL TIME DIVISION MULTIPLEXING (OTDM) In this
scheme, the optical transmitters are separately
modulated by the signals from the different
channels. The type of modulation may be IM, ASK,
PSK or FSK. The transmitting laser have the same
wavelength. The optical pulses from the
transmitters are time multiplexed by sending
clock signals to the transmitters.
The time multiplexed optical pulses are then
transmitted through the optical fiber. At the
receiving end, the optical pulses are
de-multiplexed by an optical TDM de-multiplexer.
The output of the de-multiplexers are then
received by separate photo detectors followed by
receivers. The block diagram is shown in
following figure
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OPTICAL COMMUNICATION SYSTEM
Block diagram of OTDM
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OPTICAL COMMUNICATION SYSTEM
OPTICAL FREQUENCY DIVISION OR WAVELENGTH DIVISION
MULTIPLEXING In this schemes, different signals
from different channels are used to modulate
laser sources separately. The laser sources have
different frequency or wavelength. The output
signals from the different sources are then
combined by a star coupler (for FDM) or a WDM
multiplexer for WDM.
The combined signal is passed through the fiber.
At the receiving end, the different frequency
signals are separated by optical filters in case
of FDM. In case of WDM, a WDM de-multiplexer is
used to separate the different wavelengths. The
separated signals are then detected by separated
photo detectors and received by the receivers.
The block diagram is shown in the following
figure.
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OPTICAL COMMUNICATION SYSTEM
Block diagram of a WDM system
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OPTICAL COMMUNICATION SYSTEM
OFDM If the separation between the wavelengths is
large, the frequency separation is small. Then
the scheme is called Frequency division
multiplexing (FDM). In this case as wavelength
separation is large, it is not suitable to use
grating WDM multiplexers or de-multiplexers for
separating the frequencies. The frequencies can
be separated by using filters.
WDM If the separation between the wavelengths is
very small like 1 nm or less, then frequency
separation is very large such as 125 GHz.
corresponding to wavelength separation of 1 nm.
In this case it is possible to use the grating
multiplexers to multiplex or de-multiplex the
wavelengths. Then the scheme is called WDM.
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OPTICAL COMMUNICATION SYSTEM
SUB CARRIER MULTIPLEXING (SCM) In this scheme,
the signals from the different channels are used
to modulate microwave (MW) sub carriers
separately with some separation between the sub
carrier frequencies. The output of the sub
carrier modulators are then combined by a
microwave (MW) power combiner. The output of the
combiner is an electrical FDM (frequency division
multiplexed) signal. This FDM signal is then used
to modulate the laser source using analog or
digital modulations. The output of the laser is
fed to the fiber.
At the receiving end of the fiber, the optical
signal is detected by a photo detector. The
output of the PD is the electrical FDM signal
which is amplified by a low noise amplifier (LNA)
and is received by heterodyne microwave receivers.
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OPTICAL COMMUNICATION SYSTEM
Any particular channel may be selected by tuning
the local oscillator which may be a voltage
controlled oscillator (VCO). The block diagram of
the SCM scheme is shown in the figure below
Widely used in CATV distribution
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OPTICAL COMMUNICATION SYSTEM

• DEMODULATION SCHEMES IN COHERENT DETECTION
• There are two basic types of demodulation in
  coherent detection of optical signals
• Synchronous demodulation
• Non-Synchronous demodulation.

Synchronous demodulation
In synchronous demodulation, the IF modulated
signal is mixed with an IF carrier recovered from
the IF signal. At the output of the mixer the
base band signal is received which is filtered by
a low pass filter and fed to the decision
circuit. Synchronous demodulation can be used for
ASK, PSK or FSK.
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OPTICAL COMMUNICATION SYSTEM
Non-Synchronous demodulation
Non-synchronous demodulation can be applied only
for ASK and FSK. In this scheme, the demodulation
is carried out by envelope detection. The block
diagrams of ASK and FSK envelope detection
receiver is shown below
ASK
FSK