Chapter 10 Optical Communication Systems. OPTICAL COMMUNICATION SYSTEM Elements of an optical communication system In optical communication systems, electrical

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


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.


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)


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


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


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.


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


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).


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.


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.

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.


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.


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 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


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.


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.


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.


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.


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


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.


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


Non-Synchronous demodulation

ASK

FSK

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Chapter 10 Optical Communication Systems. OPTICAL COMMUNICATION SYSTEM Elements of an optical communication system In optical communication systems, electrical