Dr. Vijay H. Pithadia
Section: Engineering – Electronics
Abstract: Technological changes based on electrons are reaching their limitations and thus there is increased focus on technologies that use photons. There are vast opportunities to carrying information in the field of fiber optics for communication purpose. Basically there are two types of fiber optics cables are used i.e. step graded and graded index. FOSAPP objectives to develop a high-speed fiber optic data link, future prospects such as FTT/FISL and HFC are the challenging opportunities in the status of fiber optics in India.
Key Words: Fiber Optic, Optical Fiber, Wave-Guide, Telecom Communication, FOSAPP, FTTC and HFC
Availability of Data: Quoted data and information are available from public sources, which are mentioned in references.
Prologue: Optical communication systems date back two centuries, to the "optical telegraph" that French engineer Claude Chappe invented in the 1790s. His system was a series of semaphores mounted on towers, where human operators relayed messages from one tower to the next. It beat hand-carried messages hands down, but by the mid-19th century was replaced by the electric telegraph, leaving a scattering of "Telegraph Hills" as its most visible legacy.
Fiber Optic is one of the branches of Opto-electronics. Presently Opto-electronics becomes an established as a subject in its own right and with developmental future Fiber optic and optical communication provides good example of systems. Opto-electronics incorporate a wide range of devices including those based on semiconductor and those based on the behavior of light in crystals subject to external fields. Photonics is declared as on of the twelve emerging technologies, which are being tracked closely between USA and Japan, while Department of Commerce, USA, recognize prominently in the list of ten technologies
As a system point of view an optical system that uses one or more glass or Perspex fibers as a light guide or for transmitting optical images. The fiber has polished surfaces coated with a material of suitable refractive index. Light entering one end within a certain solid angle undergoes total refraction at the surface and is transmitted through fiber. The technical point of view there are main two types of optical fiber wave-guide is used at the present time. There are two basic types of fiber used today and many different types of Fiber Optic Cable. The two types of fiber are called Single Mode (SM) and Multimode (MM), and SM fiber is more expensive but more efficient than MM fiber. Single Mode fiber is generally used where the distances to be covered are greater. Cables come in a variety of configurations determined by a variety of factors. With the major research and development work being done in the field of fiber optics and its diversified applications in telecom industry and submarine communications.
Advantages of Fiber Optic Systems:
For many years it has been appreciated that the use of optical (light) waves as a carrier wave provides an enormous potential bandwidth. Optical carriers are in the region of Hz to Hz, i.e. three to six orders of magnitude higher than microwave frequencies. However, the atmosphere is a poor transmission medium for light waves. Optical communication only became a widespread option with the development of low-loss dielectric waveguide. In addition to the potential bandwidth, optical fiber communication offers a number of benefits:
· Size, weight, flexibility. Optical fibers have very small diameters. A very large number of fibers can be carried in a cable the thickness of a coaxial cable.
· Electrical isolation. Optical fibers are almost completely immune from external fields. They do not suffer from cross-talk, radio interference, etc.
· Security. It is difficult to tap into an optical line. It is extremely difficult to tap into an optical line unnoticed.
· Low transmission loss. Modern optical fiber now has better loss characteristics than coaxial cable. Fibers have been fabricated with losses as low as.
Fiber optic transmission systems – a fiber optic transmitter and receiver, connected by fiber optic cable – offer a wide range of benefits not offered by traditional copper wire or coaxial cable. These include:
1. The ability to carry much more information and deliver it with greater fidelity than either copper wire or coaxial cable.
2. Fiber optic cable can support much higher data rates, and at greater distances, than coaxial cable, making it ideal for transmission of serial digital data.
3. The fiber is totally immune to virtually all kinds of interference, including lightning, and will not conduct electricity. It can therefore come in direct contact with high voltage electrical equipment and power lines. It will also not create ground loops of any kind.
4. As the basic fiber is made of glass, it will not corrode and is unaffected by most chemicals. It can be buried directly in most kinds of soil or exposed to most corrosive atmospheres in chemical plants without significant concern.
5. Since the only carrier in the fiber is light, there is no possibility of a spark from a broken fiber. Even in the most explosive of atmospheres, there is no fire hazard, and no danger of electrical shock to personnel repairing broken fibers.
6. Fiber optic cables are virtually unaffected by outdoor atmospheric conditions, allowing them to be lashed directly to telephone poles or existing electrical cables without concern for extraneous signal pickup.
7. A fiber optic cable, even one that contains many fibers, is usually much smaller and lighter in weight than a wire or coaxial cable with similar information carrying capacity. It is easier to handle and install, and uses less duct space. (It can frequently be installed without ducts.)
8. Fiber optic cable is ideal for secure communications systems because it is very difficult to tap but very easy to monitor. In addition, there is absolutely no electrical radiation from a fiber.
An appreciation of the underlying technology will provide a useful framework for understanding the reasons behind its many benefits.
The primary disadvantage of optical fiber is the technical difficulties associated with reliable and cheap connections, and the development of an optical circuit technology that can match the potential data-rates of the cables. The speed of these circuits, which are electronically controlled, is usually the limiting factor on the bit-rate. The difficulty of connection and high-cost of associated circuitry result in optical fibers being used only in very high bit-rate communication. There is considerable current debate as to whether optics will ever completely replace electronic technology. In addition, good phase control of an optical signal is extremely difficult. Optical communications are forced to use the comparatively crude method of ASK modulation.
Numerical aperture (NA) of the fiber defines which light will be propagated and which will not. NA defines the light-gathering ability of the fiber. Imagine a cone coming from the core. Light entering the core from within this cone will be propagated by total internal reflection. Light entering from outside the cone will not be propagated.
A high NA gathers more light, but lowers the bandwidth. A lower NA increases bandwidth.
NA has an important consequence. A large NA makes it easier to inject more light into a fiber, while a small NA tends to give the fiber a higher bandwidth. A large NA allows greater modal dispersion by allowing more modes in which light can travel. A smaller NA reduces dispersion by limiting the number of modes
Bandwidth: Fiber bandwidth is given in MHz-km. A product of frequency and distance, bandwidth scales with distance: if you half the distance, you double the frequency. If you double the distance, you half the frequency what does this mean in premises cabling? For a 100-meter run (as allowed for twisted pair cable), the bandwidth for 62.5/125-micrometer fiber is 1600 MHz at 850 nm and 5000 MHz at 1300 nm. For the 2-km spans allowed for most fiber networks, bandwidth is 80 MHz at 850 nm and 250 MHz at 1300 nm. With single mode fibers, the bandwidth for a 100-meter run is about 888 GHz.
Attenuation: Attenuation is loss of power. During transit, light pulses lose some of their energy. Attenuation for a fiber is specified in decibels per kilometer (dB/km). For commercially available fibers, attenuation ranges from approximately 0.5 dB/km for single mode fiber to 1000 dB/km for large-core plastic fibers.
Attenuation varies with the wavelength of light. There are three low-loss "windows" of interest: 850 nm, 1300 nm, and 1550 nm. The 850-nm window is perhaps the most widely used because 850-nm devices are inexpensive. The 1300-nm window offers lower loss, but at a modest increase in cost for LEDs. The 1550-nm window today is mainly of interest to long-distance telecommunications applications.
Fiber Optics System and Products Project [FOSAPP]
The objectives of FOSAPP are the development of high-speed fiber optic data links, Inter connects and FDDI products. The implementing agencies are C-DAC, Pune, ECIL- Hyderabad and IIT – Madras. Under the FOSAPP program a FOSAPP center has been created at ECIL –Hyderabad which is capable of taking commercial orders for supplying fiber optic LAN optic and other network and carrying out turnkey installation projects also. A wide range of fiber optic products is useful for transmission of analog; digital mixed and imaging signals over short and minimum ranges have already been developed and supplied. Another projects on fiber optic remote signaling and communication system for railways has been jointly funded by Department of Electronics, Government of India, New Delhi and Indian Railways, where as development work has been completed jointly by ECIL – Hyderabad and IIT – Madras.
An optical fiber (or fiber in British English) is a transparent thin fiber for transmitting light. Fiber optics is the branch of science and engineering concerned with optical fibers. The optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. Although fibers can be made out of either plastic or glass, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical absorption of glass. The light transmitted through the fiber is confined due to total internal reflectionwithin the material. This is an important property that eliminates signal crosstalk between fibers within the cable and allows the routing of the cable with twists and turns. In telecommunications applications, the light used is typically infrared light, at wavelengths near to the minimum absorption wavelength of the fiber in use.
Fiber optic is emerging field in the Indian context and there are vast opportunities to information carrying capacity in the fiber optics panorama. The biggest challenge remaining for fiber optics is economic. Today telephone and cable television companies can cost-justify installing fiber links to remote sites serving tens to a few hundreds of customers. However, terminal equipment remains too expensive to justify installing fibers all the way to homes, at least for present services. Instead, cable and phone companies run twisted wire pairs or coaxial cables from optical network units to individual homes. Time will see how long that lasts.
# E. C. Young, " The new penguin Dictionary of Electronics", Low Price Edition, ELBS and Penguin Books, England, 1979 P. No. 165
# A. K. Chakravarti & N. Shroff, " Photonics and Opto-electronics activities in India: Present and future directions", Electronics Information and Planning, New Delhi, 1998, P. No. 1 to 6
# J. Wilson et. al, "Opto-electronics – An Introduction", Prentice Hall of India Pvt. Ltd., New Delhi, 1996 Chapter No. 9
# EFY Correspondent, "Fiber Optics: The Indian Scenario", EFY, New Delhi, December, 1995 P. No. 71 – 74
# A. K. Pipal et. al., "Technology watch on HFC Network", ELP, New Delhi 1996
# G. Keiser, "Optical Fiber Communications", McGraw Hill, London, 1983, Section 9.4
# G. Mahlke and P. Gosling, "Fiber Optic cables, fundamentals", Cable Engineering Systems Planning, Wiley – Chi Chester, 1987
# M. J. Howes and D. V. Morgan [Eds.], "Optical Fiber Communications", Wiley, New York, 1980, Chapter No. 3