Fiber Optics

What are fiber optics?

An optical fiber is a transparent thin fiber, usually made of glass or plastic, for transmitting light. Fiber optics is the branch of science and engineering concerned with such optical fibers.

History of fiber optics

Guiding of light by refraction, the principle that makes fiber optics possible, was first demonstrated by Daniel Colladon and Jacques Babinet in Paris in the early 1840s. John Tyndall included a demonstration of it in his public lectures in London, 12 years later.[10] Tyndall also wrote about the property of total internal reflection in an introductory book about the nature of light in 1870.

Practical applications, such as close internal illumination during dentistry, appeared early in the twentieth century. Image transmission through tubes was demonstrated independently by the radio experimenter Clarence Hansell and the television pioneer John Logie Baird in the 1920s.

The first working fiber-optical data transmission system was demonstrated by German physicist Manfred Börner at Telefunken Research Labs in Ulm in 1965, which was followed by the first patent application for this technology in 1966.

The first useful optical fiber was invented in 1970 by researchers Maurer, Keck, Schultz, and Zimar working for American glass maker Corning Glass Works. They manufactured a fiber with 17 dB optic attenuation per kilometer by doping silica glass with titanium.

The emerging field of photonic crystals led to the development in 1991 of photonic-crystal fiber, which guides light by diffraction from a periodic structure, rather than by total internal reflection. The first photonic crystal fibers became commercially available in 2000. Photonic crystal fibers can carry higher power than conventional fibers and their wavelength-dependent properties can be manipulated to improve performance.

What are fiber optics used for?

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 transparent plastic (POF = plastic optical fibers) or glass, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical absorption. The light transmitted through the fiber is confined due to total internal reflection within 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.

Fibers are generally used in pairs, with one fiber of the pair carrying a signal in each direction, however bidirectional communications is possible over one strand by using two different wavelengths (colors) and appropriate coupling/splitting devices.

Fibers, like waveguides, can have various transmission modes. The fibers used for long-distance communication are known as single mode fibers, as they have only one strong propagation mode. This results in superior performance compared to other, multi-mode fibers, where light transmitted in the different modes arrives at different times, resulting in dispersion of the transmitted signal. Typical single mode fibers can sustain transmission distances of 80 to 140 km between regenerations of the signal, whereas most multi-mode fibers have a maximum transmission distance of 300 to 500 metres. Note that single mode equipment is generally more expensive than multi-mode equipment. Fibers used in telecommunications typically have a diameter of 125 µm. The transmission core of single-mode fibers most commonly have a diameter of 9 µm, while multi-mode cores are available with 50 µm or 62.5 µm diameters. The refractive index,n, of the pure glass in the core is typically 1.5.

Because of the remarkably low loss and excellent linearity and dispersion behavior of single-mode optical fiber, data rates of up to 40 gigabits per second are available in real-world use on a single wavelength. Wavelength division multiplexing can then be used to allow many wavelengths to be used at once on a single fiber, allowing a single fiber to bear an aggregate bandwidth measured in terabits per second.

The fiber transmission loss is minimal for 1550 nm light and dispersion is minimal at 1310 nm making these the optimal wavelength regions for data transmission.

Modern fiber cables can contain up to a thousand fibers in a single cable, so the performance of optical networks easily accommodate even today's demands for bandwidth on a point-to-point basis. However, unused point-to-point potential bandwidth does not translate to operating profits, and it is estimated that no more than 1% of the optical fiber buried in recent years is actually 'lit'.

Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches, installation in conduit, lashing to aerial telephone poles, submarine installation, or insertion in paved streets. In recent years the cost of small fiber-count pole mounted cables has greatly decreased due to the high Japanese and South Korean demand for Fiber to the Home (FTTH) installations.

Recent advances in fiber technology have reduced losses so far that no amplification of the optical signal is needed over distances of hundreds of kilometers. This has greatly reduced the cost of optical networking, particularly over undersea spans where the cost reliability of amplifiers is one of the key factors determining the performance of the whole cable system. In the past few years several manufacturers of submarine cable line terminal equipment have introduced upgrades that promise to quadruple the capacity of older submarine systems installed in the early to mid 1990s.

Longer-range systems still have to use optical amplifiers.

What are the Advantages and Disadvantages of Fiber Optics?

Advantages of optical fibers over wires

• Lower cost in the long run
• Low loss of signal (typically less than 0.3 dB/km), so repeater-less transmission over long distances is possible
• Large data-carrying capacity (thousands of times greater, reaching speeds of up to 1.6 Tb/s in field deployed systems and up to 10 Tb/s in lab systems)
• Immunity to electromagnetic interference, including nuclear electromagnetic pulses (but can be damaged by alpha and beta radiation)
• No electromagnetic radiation; difficult to eavesdrop
• High electrical resistance, so safe to use near high-voltage equipment or between areas with different earth potentials
• Low weight
• Signals contain very little power
• No crosstalk between cables
• No sparks (e.g. in automobile applications)
• Difficult to place a tap or listening device on the line, providing better phyisical network security

Disadvantages of optical fibers compared to wires

• High investment cost
• Need for more expensive optical transmitters and receivers
  
• More difficult and expensive to splice than wires
  
• At higher optical powers, is susceptible to "fiber fuse" wherein a bit too much light meeting with an imperfection can destroy as much as 1.5 kilometers of wire at several metres per second . A "Fiber fuse" protection device at the transmitter can break the circuit to prevent damage, if the extreme conditions for this are deemed possible.
• Cannot carry electrical power to operate terminal devices. However, current telecommunication trends greatly reduce this concern: availability of cell phones and wireless PDAs; the routine inclusion of back-up batteries in communication devices; lack of real interest in hybrid metal-fiber cables; and increased use of fiber-based intermediate systems.
• Almost all these disadvantages have been surmounted or bypassed in contemporary telecommunications usage, and communication systems are now unthinkable without fiber optics. Their cost is much more economic than old coaxial cables because the transmitters and receivers (laser and photodiodes) turn out cheaper than electric circuitry as their capacity is much superior. The cost of regeneration in electrical long distance transmission systems is completely impractical for modern communications.

Almost all these disadvantages have been surmounted or bypassed in contemporary telecommunications usage, and communication systems are now unthinkable without fiber optics. Their cost is much more economic than old coaxial cables because the transmitters and receivers (laser and photodiodes) turn out cheaper than electric circuitry as their capacity is much superior. The cost of regeneration in electrical long distance transmission systems is completely impractical for modern communications.

Uses of optical fibres

• Fibers can be used as light guides in medical and other applications where bright light needs to be brought to bear on a target without a clear line-of-sight path.
• Optical fibers can be used as sensors to measure strain, temperature, pressure and other parameters.
• Bundles of fibers are used along with lenses for long, thin imaging devices called endoscopes, which are used to view objects through a small hole.
• Medical endoscopes are used for minimally invasive exploratory or surgical procedures (endoscopy). Industrial endoscopes (see fiberscope or borescope) are used for inspecting anything hard to reach, such as jet engine interiors.
• In some high-tech buildings, optical fibers are used to route sunlight from the roof to other parts of the building.
• Optical fibers have many decorative applications, including signs and art, artificial Christmas trees, and lighting.
• A few communities have Fiber to the Home technology which provides subscribers with Ultra High Speed Internet, Telephone, and Television services.
• The German company Sennheiser developed a microphone working with a Laser and optical fibres. German article about this microphone