2. 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
3. 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.
4. 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 margin
PS = source power
PR = received power
N = no. of joints
Ls = source coupling loss
Ld = detector coupling loss
Lj = joint loss
= fibre attenuation
Psm = system margin
L = total fiber length
5. Optical power-loss model
ystem Margin
T s R c sp f
P P P ml nl L S
Try Examples 8.1 & 8.2 (in the book by Gerd Keiser)
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
6. 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
2 2 2 2 2
mod
1
N
sys i t mat r
i
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 0.35
Max. allowed rise time ,
and is the bit rate for NRZ and RZ signals respectively
RZ
max max
NRZ RZ
ts OR ts
B B
NRZ
B B
Try Example 8.3 (in
the book by Gerd
Keiser)
7. DETECTION AND MODULATION SCHEMES IN
OPTICAL COMMUNICATIONS
DETECTION SCHEMES
There are two principal types of detection schemes
• Direct detection
• Coherent detection
OPTICAL COMMUNICATION SYSTEM
8. 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
9. 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
OPTICAL COMMUNICATION SYSTEM
10. 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
11. 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
12. 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.
13. 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.
OPTICAL COMMUNICATION SYSTEM
14. 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.
OPTICAL COMMUNICATION SYSTEM
15. 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
16. 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.
OPTICAL COMMUNICATION SYSTEM
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
18. 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.
20. 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.
OPTICAL COMMUNICATION SYSTEM
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.
21. 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.
OPTICAL COMMUNICATION SYSTEM
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.
22. 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
OPTICAL COMMUNICATION SYSTEM
Widely used in CATV distribution
23. DEMODULATION SCHEMES IN COHERENT DETECTION
There are two basic types of demodulation in coherent detection of
optical signals
Synchronous demodulation
Non-Synchronous demodulation.
OPTICAL COMMUNICATION SYSTEM
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.
24. 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
OPTICAL COMMUNICATION SYSTEM
Non-Synchronous demodulation
ASK
FSK
