Optical fiber communication —An overview
Department of Physics, Anna University, Chennai 600 025, India
This paper deals with the historical development of optical communication systems and their failures initially. Then the different generations in optical fiber communication along with their features are discussed. Some aspects of total internal reflection, different types of fibers along with their size and refractive index profile, dispersion and loss mechanisms are also mentioned. Finally the general system of optical fiber communication is briefly mentioned along with its advantages and limitations. Future soliton based optical fiber communication is also highlighted. Keywords. Bandwidth; optical fiber; group index; group velocity; soliton v-number; dispersion. PACS Nos 42.65 Tg; 47.81 Dp; 42.79 Sz; 05.45 Yv 1. Introduction Now we are in the twenty first century, the era of ‘Information technology’ [1-6]. There is no doubt that information technology has had an exponential growth through the modern telecommunication systems. Particularly, optical fiber communication plays a vital role in the development of high quality and high-speed telecommunication systems. Today, optical fibers are not only used in telecommunication links but also used in the Internet and local area networks (LAN) to achieve high signaling rates. 1.1 Historical perspective of optical communication The use of light for transmitting information from one place to another place is a very old technique. In 800 BC., the Greeks used fire and smoke signals for sending information like victory in a war, alertting against enemy, call for help, etc. Mostly only one type of signal was conveyed. During the second century B.C. optical signals were encoded using signaling lamps so that any message could be sent. There was no development in optical communication till the end of the 18th century. The speed of the optical communication link was limited due to the requirement of line of sight transmission paths, the human eye as the receiver and unreliable nature of transmission paths affected by atmospheric effects such as fog and rain. In 1791, Chappe from France developed the semaphore for telecommunication on land. But that was also with limited information transfer.
optical fiber communication
By: Bibek Thapa
BscIT (IInd sem)
Core – thin glass center of the fiber where light travels.
Cladding – outer optical material surrounding the core
Buffer Coating – plastic
coating that protects
Type of Fibers
Optical fibers come in two types:
Single-mode fibers – used to transmit one signal per fiber (used in telephone and cable TV).
Multi-mode fibers – used to transmit many signals per fiber (used in computer networks).
Evolution of Fiber
1880 – Alexander Graham Bell
1930 – Patents on tubing
1950 – Patent for two-layer glass wave-guide
1960 – Laser first used as light source
1965 – High loss of light discovered
1970s – Refining of manufacturing process
1980s – OF technology becomes backbone of long distance telephone networks in NA.
is a method of transmitting information from one place to another by sending light through an optical fiber.
The light forms an electromagnetic carrier wave that is modulated to carry information.
There are two types of Detectors
1. P-I-N Photodiodes
2. AVALANCHE Photodiodes
Splices are the permanent between two fibers.
1. Adhesive bonding or Glue splicing
2. Mechanical splicing
3. Fusion splicing
Fiber Optic Advantages
Large bandwidth ( >5.0 GHz for 1km length
smaller size and lighter weight
immunity to environmental interference
highly secure due to tap difficulty and lack of signal radiation
DisAdvantages of fiber optics
Disadvantages include the cost of interfacing equipment necessary to convert electrical signals to optical signals. (optical transmitters, receivers) Splicing fiber optic cable is also more difficult.
Fiber Optic Disadvantages
expensive over short distance
requires highly skilled installers
adding additional nodes is difficult
optical fiber communication
General overview of optical fiber
Like all other communication system, the primary objective of optical fiber communication system also is to transfer the signal containing information (voice, data, video) from the source to the destination. The general block diagram of optical fiber communication system is shown in the figure9.
The source provides information in the form of electrical signal to the transmitter. The electrical stage of the transmitter drives an optical source to produce modulated light wave carrier. Semiconductor LASERs or LEDs are usually used as optical source here. The information carrying light wave then passes through the transmission medium i.e. optical fiber cables in this system. Now it reaches to the receiver stage where the optical detector demodulates the optical carrier and gives an electrical output signal to the electrical stage. The common types of optical detectors used are photodiodes (p-i-n, avalanche), phototransistors, photoconductors etc. Finally the electrical stage gets the real information back and gives it to the concerned destination.
Definition of optical fiber
Optical fiber is a dielectric waveguide or medium in which information (voice, data or video) is transmitted through a glass or plastic fiber, in the form of light. The basic structure of an optical fiber is shown in figure 1. It consists of a transparent core with a refractive index n1 surrounded by a transparent cladding of a slightly less refractive index n2. The refractive index of cladding is less than 1%, lower than that of core. Typical values for example are a core refractive index of 1.47 and a cladding index of 1.46. The cladding supports the waveguide structure, protects the core from absorbing surface contaminants and when adequately thick, substantially reduces the radiation loss to the surrounding air. Glass core fibers tend to have low loss in comparison with plastic core fibers. Additionally, most of the fibers are encapsulated in an elastic, abrasion-resistant plastic material which mechanically isolates the fibers from small geometrical irregularities and distortions. A set of guided electromagnetic waves, also called the modes of the waveguide, can describe the propagation of light along the waveguide. Only a certain number of modes are capable of propagating through the waveguide.
History of optical fiber
The visible optical carrier waves or light has been commonly used for communication purpose for many years. Alexander Graham Bell transmitted a speech information using a light beam for the first time in 1880. Just after four years of the invention of the telephone Bell proposed his photophone which was capable of providing a speech transmission over a distance of 200m. In the year 1910 Hondros and Debye carried out a theoretical study and in 1920 Schriever reported an experimental work. Although in the early part of twentieth century optical communication was going through some research work but it was being used only in the low capacity communication links due to severe affect of disturbances in the atmosphere and lack of suitable optical sources. However, low frequency (longer wavelength) electromagnetic waves like radio and microwaves proved to be much more useful for information transfer in atmosphere, being far less affected by the atmospheric disturbances. The relative frequencies and their corresponding wavelengths can be known from the electromagnetic spectrum and it is understandable that optical frequencies offer an increase in the potential usable bandwidth by a factor of around 10000 over high frequency microwave transmission.