Fiber Optic Communication tutorial

This page on Fiber Optic Communication tutorial covers basics,benefits of fiber optic system, fiber optic cables/connectors,optical transmitter,optical receiver and optical components.

The fiber optic communication systems are mainly used for long distance telephone communication across large seas and now-a-days even for transmitting internet data from one part of the world to the other part. The other vital application of fiber optic communication system is for TV signal transmission/reception due to its large bandwidth.

As we know that increase in demand of bandwidths across the world for carrying the enormous high bit rate data leads to the transmission media which can carry huge bandwidth. fiber optic has total bandwidth of about 11 THz. If we consider bandwidth of telephone signal to be about 3KHz effectively this gives us 36 billion total number of conversations to be carried by optical fiber simultaneously.

Total No.of conversations = 1.1 x 10¹⁴ / 3 x 10³
= 36 billion

Figure describes typical fiber optic communication system comprising coder, light source transmitter, fiber optic cable, light detector and decoder. As depicted here information signals can be voice or video or data or image etc. In the first step, analog information signal need to be converted into series of digital pulses suitable to be transmitted over the medium. This is achieved with the use of ADC and coder(if required). This digital pulses are applied to the light source to make the light emission on and off as per the information(pulses). Usually LED and solid state laser devices are used as light source transmitter. This light beam pulses are coupled with care to the fiber optic cable to avoid coupling loss. This beams are transmitted to the long distances.

At the other receiving end devices such as photocell or light detectors are used to convert light pulses into electrical signals which then amplified and converted back to its original form. If required DAC is used if original signal sent was either audio or video.

In the case of very long distance communication system, repeater units are incorporated to avoid complete attenuation of the signal. Without repeater it is not possible to re-cover the signal back to its original form.

This fiber optic relay or repeater stations pick up the light beam convert it to electrical signal. This converted electrical signal is amplified before again being transformed back to the light beams. This amplified light beams are again transmitted over the fiber optic cable.

The latest development in fiber optic amplifiers have provided EDFA (Erbium Doped Fiber Amplifier). This EDFA amplifier does not convert the signal into electrical form and does the amplification in the optical domain itself.

Benefits of fiber optic system

Following are benefits of the fiber optic system:
• Fiber optic cable has large information carrying capacity due to large bandwidth.
• Electrical cables are becoming costlier due to copper price rise,this makes fiber optic less costlier comparative to the copper based cable.
• Fiber optic connectors are used for better coupling of the fiber optic cables to have less loss.
•  Abundant cheaper materials such as silica and plastic made it possible to have less cost per km for the fiber cable. • Optical fiber probes are used for medical imaging applications (endoscope), used in microscopes as light sources and in spectroscopy.

Optical Transmitter and Receiver

As mentioned above LED and semiconductor lasers are used as optical transmitter due to its fast on/off application compare to conventional incandescent lamp. This incandescent lamp can not be switched on and off fast due to thermal delay in the filament.

Photodiode is used as optical receiver device or detector to convert light source into voltage pulses.

Optical components

There are wide components used in a optical fiber chain from trasmitting end to the receiving end. They include optical coupler,isolator,circulator,amplifier,repeater,regenerator,ADM(Add and Drop Multiplexer,demultiplexer),filter, equalizer,switches, wavelength converters and more. Each has different functions in the optical chain.These components have been described in the links provided on the left panel.

These components work based on wavelengths of interest. There are various optical wavelength bands such as L band, S band, C band etc. Pls. check wavelength bands page for all the optical bands available.

Losses in Optical Fiber

Following are the losses in optical fiber system:
• Absorption Losses
-Intrinsic(Caused due to interaction of one or major component of glass)
-Extrinsic(Caused due to OH^- ion or due to transition element impurity)

• Linear scattering losses
-Rayleigh
-Mie
-Nonlinear scattering losses
-Fiber bend losses

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