Fiber Optic Communication Tutorial

This page provides a tutorial on Fiber Optic Communication, covering the basics, benefits of fiber optic systems, fiber optic cables/connectors, optical transmitters, optical receivers, and optical components. Fiber optic communication systems are mainly used for long-distance telephone communication across large seas and, nowadays, even for transmitting internet data from one part of the world to another. Another vital application of fiber optic communication systems is for TV signal transmission/reception due to its large bandwidth.

As we know, the increase in demand for 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 a total bandwidth of about 11 THz. If we consider the bandwidth of a telephone signal to be about 3 KHz effectively, this gives us 36 billion total numbers of conversations to be carried by optical fiber simultaneously.

Total No. of conversations = $\frac{1.1 \times 10^{14}}{3 \times 10^3}$ = 36 billion

Figure describes a typical fiber optic communication system comprising a coder, light source transmitter, fiber optic cable, light detector, and decoder. As depicted here, information signals can be voice, video, data, or image, etc. In the first step, the analog information signal needs to be converted into a series of digital pulses suitable to be transmitted over the medium. This is achieved with the use of ADC and a coder (if required). These 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 transmitters. 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 photocells or light detectors are used to convert light pulses into electrical signals which are then amplified and converted back to their original form. If required, DAC is used if the original signal sent was either audio or video.

In the case of very long-distance communication systems, repeater units are incorporated to avoid the complete attenuation of the signal. Without a repeater, it is not possible to recover the signal back to its original form. This fiber optic relay or repeater station picks up the light beam and converts it to an electrical signal. This converted electrical signal is amplified before again being transformed back into light beams. These amplified light beams are again transmitted over the fiber optic cable. The latest development in fiber optic amplifiers has 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 the 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 rises; this makes fiber optic less costly compared to copper-based cables.
• 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 transmitters due to their fast on/off application compared to a conventional incandescent lamp. This incandescent lamp cannot be switched on and off fast due to thermal delay in the filament.

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

Optical components

There are wide components used in an optical fiber chain from the transmitting end to the receiving end. They include optical couplers, isolators, circulators, amplifiers, repeaters, regenerators, ADM (Add and Drop Multiplexer, demultiplexer), filters, equalizers, switches, wavelength converters, and more. Each has different functions in the optical chain. 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 the wavelength bands page for all the optical bands available.

Losses in Optical Fiber

Following are the losses in the optical fiber system:

• Absorption Losses
  • Intrinsic (Caused due to the interaction of one or a 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