Optical Moulation Techniques

7/29/2019 Optical Moulation Techniques

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Presented By:-

Prachir Beriwal

08EC000325


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MODULATION

We need to transform the signal into a formsuitable for transmission through a givencommunication channel.

Modulation may be defined as the process bywhich some characteristics of a signal calledcarrier is varied in accordance with theinstantaneous value of another signal calledmodulating signal.

Characteristics of carrier varied are Amplitude,frequency, phase and polarization.


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Need for Modulation

Modulation is needed for:

Practicality of antenna.

To remove interference. Reduction of noise.


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Optical Communication

Fiber-optic communication is a method oftransmitting information from one place to anotherby sending light through an optical fiber.

The main benefits of fiber are :

Exceptionally low loss allowing long distancescommunication between amplifiers or repeaters.

Inherently high data-carrying capacity, such thatthousands of electrical links would be required toreplace a single high bandwidth fiber.

No crosstalk.


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Modulation formats

In digital communications, the modulatingsignal consists of Binary data.

This data is used to modulate carrierwave(usually sinusoidal) with fixed

frequency. The channel may be telephone channel,

microwave radio link, satellite channel oroptical fiber.

The modulation process involves switchingor keying the amplitude, frequency, phaseor polarization of the carrier in accordancewith the input data.


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The modulation formats of

optical communication are:

1. Amplitude Shift Keying (ASK).

2. Frequency Shift Keying (FSK).

3. Phase Shift Keying (PSK).

4. Polarization Shift Keying (PolSK).


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Amplitude Shift

Keying(ASK) Also known as ON-Off Keying (OOK).

Modulate the intensity of the carrier signal.

Carrier is switched between On and Off

states depending on the input binary

sequence.


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Amplitude Shift Keying

Digitalinformation

Carrier wave

ASK

modulated

signal

Carrier present Carrier absent

Amplitude varying-

frequency constant


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Advantage:

The ASK-based modulation formats arecharacterized by simple signalgeneration and detection, due to whichall currently deployed optical

transmission systems employ ASK-basedmodulation formats.

DISADVANTAGE:

It has not got a constant envelope. Thismakes its processing (e.g., poweramplification) more difficult.


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Frequency shift keying(FSK)

It is realized by switching the laser light

frequency between two frequency

values.

Carrier frequency is switched depending on

the input binary sequence.

If binary data=0; low frequency

binary data=1; high frequency


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Frequency shift keying


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Advantages:

Frequency shift keying has a better signalto noise ratio than on off keying.

High Data Rates/Long Range.

Excellent Immunity to Harsh Environments.

DISADVANTAGE:

The optical signal envelope remainsunchanged and the complexity of signalgeneration and detection increasescompared to ASK modulation.


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Phase shift keying(PSK)

It uses the phase of the signal to

encode information.

Phase of the carrier is switched

depending on the input binary

sequence.


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Advantages:

Widely used these days within a wholeraft of radio communications systemsmore efficiently than FSK.

Improved receiver sensitivity than ASK.

DISADVANTAGES:

Noise interference is more. Higher probability of error.

Regeneration of carrier is difficult.


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Polarization shift keying(PolSK)

It is the most exotic modulation

format.

The optical PolSK signals are

generated by switching the signal

polarization between two orthogonal

states of polarization.


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Advantages:

The PolSK is characterized by a Constant signalenvelope enabling an improved nonlinear tolerance,an improved sensitivity compared to ASK-basedmodulation.

Enable a better utilization of the system

bandwidth by the use of orthogonal polarizationas an additional degree of freedom.

DISADVANTAGES:

Increased complexity of signal generation and

detection, as well as the sensitivity to polarization.


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Modulators

1. Internal Modulators(On-Off keying)

2. External Modulators

i. Electro-Optic Modulator(Phase

Modulator)

ii. Mach-Zehnder modulator(Amplitude

Modulator)


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Optical Moulation Techniques