Practical fibre optics for engineers & technicians

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Practical Fibre Optics for Engineers & Technicians


Data Communications

• Information is transmitted between two points in the form of data.

• Binary digital data uses a code constructed of ‘0’s and ‘1’s– In copper systems represented as

a high and low voltage levels.– In fibre systems represented as

the presence or not of a light pulse.


TRANSMITTERS, RECEIVERS ANDCOMMUNICATIONS CHANNELS

• A communications process requires – A source– A transmitter– A communications channel– A receiver– A destination


Communications Process

Transmitter ReceiverCommunicationsChannelSource Destination


Important Concepts• Interface standards

– Electrical signal characteristics– Mechanical characteristics– Functional characteristics

• Coding – The code symbols used for transmission– Example : ASCII Code


Communication Channels• Analog

– Varying amplitude, phase and frequency

• Digital– High - low voltages levels; on - off signals.


Analog Signals

Velocity ofsignal movementthough channel

VaryingAmplitude

0

-

+Wavelength

Time


Digital Signals

Velocity of signalthough channel

AMPLITUDE

0

-

+

T = Pulse Period

Time

OFFSPACE

ONMARK


• SIGNAL ATTENUATION• Natural resistive cable properties absorb the

electrical energy and turn it into heat• Limits the length of the communications channel• Digital signals have fast rising edges which

represent high frequency components. Signal attenuation increases with increasing frequency.

• Use repeater, amplifiers and equalisers• Natural resistive properties of glass absorb

electromagnetic energy


Signal Attenuation

1 1 1 10 000 TransmittedSignal

Signal atDistance d

Signal atDistance 2d

Time

Time

Time


Signal Repeaters

Transmitter Repeater Receiver


• CHANNEL BANDWIDTH• The difference between the highest and the lowest

frequencies that can pass though a channel.• Where the highest and lowest frequencies have

dropped to half power i.e. 3 dB drop in power.• Digital signals are constructed of many frequencies

but their transmission is limited by the channel analog bandwidth.

• The higher the bandwidth the higher the frequency that can be transmitted.


Channel Bandwidth

3dBBandwidth

0dB

-3dB

BANDWIDTHMinimumFrequency

MaximumFrequency

FREQUENCY

AMPLITUDE


Effect Of Channel Bandwidth on a Digital Signal

1 0 100OriginalDigitalData

1200 HzBandwidthSignal

4000 HzBandwidthSignal


Digital Regenerator

Line Driver Digital Repeater Line Receiver

Channel Channel

DegradedSignal

Received

RegeneratedSignal


TRANSMISSION MODES

• Simplex

• Half duplex

• Full duplex


Simplex

Transmitter ReceiverData Flow

In One Direction Only

Station A Station B


Half Duplex

Transmitter

Transmitter

Receiver

Receiver

Data Flow

Alternate Flow

Station A Station B


Full Duplex

Transmitter

Transmitter

Receiver

SimultaneousData Flow

Station A Station B

Receiver


LIGHT• Is represented by :

– Electromagnetic waves– Photons (particles)

• Travels at a speed of 3 x 108

m/s

• Reflect, refract and diffract off optical surfaces.


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Engineering

Practical fibre optics for engineers & technicians