1. Chapter 10 Optical Communication Systems

2. 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. System Design Power Budget Each component introduces a loss.
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. P = PS  PR = Ls + Ld + NLj + L + Psm
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
OPTICAL COMMUNICATION SYSTEM
Optical power-loss model
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) 2

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
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

7. DETECTION AND MODULATION SCHEMES IN OPTICAL COMMUNICATIONS
OPTICAL COMMUNICATION SYSTEM
DETECTION AND MODULATION SCHEMES IN OPTICAL COMMUNICATIONS
DETECTION SCHEMES
There are two principal types of detection schemes
Direct detection
Coherent detection

8. 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.

9. 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

10. MODULATION SCHEMES Analog modulations :
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).

11. DIRECT INTENSITY MODULATION
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.

12. SUB CARRIER INTENSITY MODULATION (SC-IM)
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.

13. PULSE FREQUENCY MODULATION (PFM) / INTENSITY MODULATION (PFM-IM)
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.

14. DIGITAL MODULATION SCHEMES :
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.

15. 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)

16. OPTICAL TIME DIVISION MULTIPLEXING (OTDM)
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

17. OPTICAL COMMUNICATION SYSTEM
Block diagram of OTDM

18. OPTICAL FREQUENCY DIVISION OR WAVELENGTH DIVISION MULTIPLEXING
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.

19. Block diagram of a WDM system
OPTICAL COMMUNICATION SYSTEM
Block diagram of a WDM system

20. 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.

21. SUB CARRIER MULTIPLEXING (SCM)
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.

22. 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

23. DEMODULATION SCHEMES IN COHERENT DETECTION
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.

24. Non-Synchronous demodulation
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