OPTICAL COMMUNICATIONS

                                                                                                    Introduction

                                                              One of the earliest known optical communication links was a fire signal method used by the Greeks in the eigth century BC for sending alarms.call for help or announcements of certain events. Light sources are either light emitting diodes (LED's) or laser diodes and detectors are phototransistors or photodiodes.

Fibre optic?

                                                             Dielectric waveguide of cylindrical geometry with core and cladding of suitable material.

refractive index of core > refractive index of cladding

Main Motivation

                                To meet demand of increase in the telecommunication data transmission.

Physical Principle

                                   Total internal reflection (critical angle, using Snell’s law).

Main Advantages

                                  1.Higher bandwidth (extremely high data transfer rate).

                                  2.Less signal degradation.

                                  3.Less costly per meter.

                                 4.Lighter and thinner then copper wire.

                                  5.Lower transmitter launching power.

                                6.Less susceptible to electromagnetic interference.

                                7.Flexible use in mechanical and medical imaging systems.

Main Applications

                              1.Telecommunications.

                              2.Sensors.

                             3.Fiber Lasers.

                            4.Bio-medical.

                            5.Automotive and many other industories.

                                                              The path of optical networks

    Prior to about 1980 most communication tenchnologies involved some type of electrical transmission mechanism. The rea of electrical communications started in 1873 with the invention of the telelgraph by Samuel F.B.Morse. The telegraph system used the morse code,which represents letters and numbers by a coded series of dots and dashes.The encoded symbols were conveyed by sending short and long pulses of electricity over a copper wire at a rate of tens of pulses per second. More advanced telegraph schemes,such as the baudot system invented in 1874,enabled the information speeds to increase to about 120 bits per second but required the use of skilled operators. Shortly there after in 1876 Alexander Graham Bell developed a fundamentally different device at could transmit the entire voice signal in an analog form and which did not require to use

     Both the telegraph and the analog voice signals were sent using a baseband transmission mode, baseband refers to the technology in which a signal is transmitted directly over a channel. For example this month is used toon standard twisted- pair wire links running from an analog telephone to the nearest switching interface equipment. The same base band method is used widely in optical communications, that is ,the optical output from a light source is turned on and off in response to the variations in voltage levels of an information-bearing electrical signal.

      In these developments the basic trend for advancing the link capacity was to use increasingly higher channel frequencies. The reason for this trend is that a time varying base band information-bearing signal may be transferred over a communication channel by superim posing it onto a sinusoidal electromagnetic wave,which is known as the carrier wave and processed as desired.since the amount of information that can be transmitted that can be transmitted is directly related to the frequency range over which the carrier operates,increasing the carrier frequency theortically increases the available transmission bandwidth and consequently,provides a larger information capacity

                                   Silica and Plastic as Fibre Optic Materials

Silica Fibers

        - Both core and cladding are of glass.

        - Very pure SiO2 or fused quartz.

        - Germanium or Phosphorus to increase the index of refraction.

       - Boron or Flourine to decrease the index of refraction.

       - Silica fibers mainly used due to their low intrinsic  absorption at wavelengths of operation.

       - Any other remaining impurities cause attenuation and scattering.

 Plastic Fibers

      - Plastic core and plastic cladding.

     - Polymethyl Methacrylate (most commonly used).

     - Flexible and Light.

    - Widely used in short distance applications

Plastic-clad Fibers

Glass as core and plastic as cladding.

Which is better? (Plastic or Silica)

- Plastic less expensive, flexible, lighter.

- Plastic is larger in diameter, so easy to connect across joints.

- Plastic is less efficient then Silica.

- Plastic has more attenuation, and less bandwidth making  it more suitable for shorter distances.

Plastic Fibers

Optical communication

simple block diagram

Original signal

V(t)

E.g. analog  voice

Many channels  muxed

into larger total BW

Digital encoding

V(t)

Modulation, error

correction, routing and

header info

Direct modulation

P(t)

Power,

stability

Chirp

Multiplexing

Crosstalk, loss

Transmission

P(t)

Attenuation, jitter,

noise, polarization

scrambling, dispersion

Demultiplexing

i(t)

Crosstalk, loss

Detection

Noise, bandwidth

V(t)

Decoding

Bit error rate