80476641 Chapter10 Optical Communication Systems
Transcript of 80476641 Chapter10 Optical Communication Systems
Chapter 10Chapter 10
Optical Communication Optical Communication
SystemsSystems
OPTICAL COMMUNICATION SYSTEM
Elements of an optical communication systemIn 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
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.
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
P = power marginPS = source powerPR = received powerN = no. of joints
Ls = source coupling lossLd = detector coupling lossLj = joint loss
= fibre attenuationPsm = system marginL = total fiber length
Optical power-loss model
ystem MarginT s R c sp fP P P ml nl L S
Try Examples 8.1 & 8.2 (in the book by Gerd Keiser)
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Total optical power loss allowed between the light source and the photodetector
wherePS = source power; n = no. of splices; f = fiber attenuationPR = received power; lc = connector loss; (dB/km); m = no. of connectors; lsp = splice loss; and L = transmission distance
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Rise time budgetIn a system with N cascaded components, each of which has a rise time ti, the total rise time tsys of the system is
2 2 2 2 2mod
1
N
sys i t mat ri
t t t t t t
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
0.7 0.35Max. allowed rise time ,
and is the bit rate for NRZ and RZ signals respectivelyRZ
max maxNRZ RZ
ts OR tsB B
NRZB B
Try Example 8.3 (in the book by Gerd Keiser)
DETECTION AND MODULATION SCHEMES IN OPTICAL COMMUNICATIONS
DETECTION SCHEMES
There are two principal types of detection schemes
• Direct detection
• Coherent detection
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.
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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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).
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.
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.
OPTICAL COMMUNICATION SYSTEM
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.
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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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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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)
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.
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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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Block diagram of OTDM
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.
OPTICAL COMMUNICATION SYSTEM
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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Block diagram
of a WDM 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.
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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.
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.
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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.
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
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Widely used in CATV distribution
DEMODULATION SCHEMES IN COHERENT DETECTION
There are two basic types of demodulation in coherent detection of
optical signals Synchronous demodulation
Non-Synchronous demodulation.
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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.
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
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Non-Synchronous demodulation
ASK
FSK