The bright future of Optical fiber
2008-08-21

                                                                                                    The bright future of Optical fiber       Optical fiber is a flexible optically transparent fiber, usually made of glass or plastic, through which light can be transmitted by successive internal reflections.       In 1870, John Tyndall, using a jet of water that flowed from one container to another and a beam of light, demonstrated that light used internal reflection to follow a specific path. As water poured out through the spout of the first container, Tyndall directed a beam of sunlight at the path of the water. The light, as seen by the audience, followed a zigzag path inside the curved path of the water. This simple experiment, illustrated in the right picture, marked the first research into the guided transmission of light.       From that time, through the efforts of the numbers of scientists, Fiber optic technology was discovered, and experienced a phenomenal rate of progress in the second half of the twentieth century.       For almost a century, Optical fiber has played a vital role in the society. Optical fibers have a wide use in variety areas. Optical fiber communication       Optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. It is especially advantageous for long-distance communications, because light propagates through the fiber with little attenuation compared to electrical cables. This allows long distances to be spanned with few repeaters. Additionally, the light signals propagating in the fiber can be modulated at rates as high as 40 Gb/s, and each fiber can carry many independent channels, each by a different wavelength of light (wavelength-division multiplexing). Over short distances, such as networking within a building, fiber saves space in cable ducts because a single fiber can carry much more data than a single electrical cable. Fiber is also immune to electrical interference, which prevents cross-talk between signals in different cables and pickup of environmental noise. Also, wiretapping is more difficult compared to electrical connections, and there are concentric dual core fibers that are said to be tap-proof. Because they are non-electrical, fiber cables can bridge very high electrical potential differences and can be used in environments where explosive fumes are present, without danger of ignition.       Although fibers can be made out of transparent plastic, glass, or a combination of the two, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical attenuation. Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances (up to 500 m), and single-mode fiber used for longer distance links. Because of the tighter tolerances required to couple light into and between single-mode fibers (core diameter about 10 micrometers), single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components. Fiber optic sensors       Fibers have many uses in remote sensing. In some applications, the sensor is itself an optical fiber. In other cases, fiber is used to connect a non-fiber optic sensor to a measurement system. Depending on the application, fiber may be used because of its small size, or the fact that no electrical power is needed at the remote location, or because many sensors can be multiplexed along the length of a fiber by using different wavelengths of light for each sensor, or by sensing the time delay as light passes along the fiber through each sensor. Time delay can be determined using a device such as an optical time-domain reflect meter.       Optical fibers can be used as sensors to measure strain, temperature, pressure and other quantities by modifying a fiber so that the quantity to be measured modulates the intensity, phase, polarization, wave length or transit time of light in the fiber. Sensors that vary the intensity of light are the simplest, since only a simple source and detector are required. A particularly useful feature of such fiber optic sensors is that they can, if required, provide distributed sensing over distances of up to one meter.       Extrinsic fiber optic sensors use an optical fiber cable, normally a multimode one, to transmit modulated light from either a non-fiber optical sensor, or an electronic sensor connected to an optical transmitter. A major benefit of extrinsic sensors is their ability to reach places which are otherwise inaccessible. An example is the measurement of temperature inside aircraft jet engines by using a fiber to transmit radiation into a radiation pyrometer located outside the engine. Extrinsic sensors can also be used in the same way to measure the internal temperature of electrical transformers, where the extreme electromagnetic fields present make other measurement techniques impossible. Extrinsic sensors are used to measure vibration, rotation, displacement, velocity, acceleration, torque, and twisting. Other uses of optical fibers       A Frisbee illuminated by fiber optics Fibers are widely used in illumination applications. They are used as light guides in medical and other applications where bright light needs to be shone on a target without a clear line-of-sight path. In some buildings, optical fibers are used to route sunlight from the roof to other parts of the building (see non-imaging optics). Optical fiber illumination is also used for decorative applications, including signs, art, and artificial Christmas trees.        A fiber-optic Christmas Tree Optical fiber is also used in imaging optics. A coherent bundle of fibers is used, sometimes along with lenses, for a long, thin imaging device called an endoscope, which is used to view objects through a small hole. Medical endoscopes are used for minimally invasive exploratory or surgical procedures. Industrial endoscopes (see fiberscope or bore scope) are used for inspecting anything hard to reach, such as jet engine interiors.       An optical fiber doped with certain rare-earth elements such as erbium can be used as the gain medium of a laser or optical amplifier. Rare-earth doped optical fibers can be used to provide signal amplification by splicing a short section of doped fiber into a regular optical fiber line. The doped fiber is optically pumped with a second laser wavelength that is coupled into the line in addition to the signal wave. Both wavelengths of light are transmitted through the doped fiber, which transfers energy from the second pump wavelength to the signal wave. The process that causes the amplification is stimulated emission.       Optical fibers doped with a wavelength shifter are used to collect scintillation light in physics experiments.       Optical fiber can be used to supply a low level of power (around one watt) to electronics situated in a difficult electrical environment. Examples of this are electronics in high-powered antenna elements and measurement devices used in high voltage transmission equipment.       As the widely usages, so optical fiber has a bright future.       At the end of 2007, nearly 29 million subscribers were connected with fiber infrastructure worldwide. Most of the subscribers are receiving service via FTTH (fiber-to-the-home) or FTTB (fiber-to-the-Building). The two terms together are commonly called FTTP (fiber-to-the-premise). The growth of fiber is expected to continue at a very fast pace with the number of fiber subscribers expected to grow to over 100 million by the end of 2012. Currently, fiber broadband comprises 7.5% of all broadband users and is expected to increase to 16% of all broadband users by 2012.       Two fundamental fiber architectures are being deployed in today's access networks: point-to-multipoint which is commonly referred to as PON (passive optical networks) and point-to-point or P2P, also referred to as active Ethernet.       PON networks have a single fiber that runs from the central office to deep in the network and usually terminates at a splitter cabinet. While the splitter cabinet typically contains a 1x32 splitter, split ratios of 1x16 and 1x8 are sometimes used. New standards are calling for even larger split ratios of 1x64 and 1x128. From the splitter cabinet short runs of fiber connect each of the homes.       In contrast, with the P2P architecture a single fiber runs all the way from the central office to the home.       Both architectures are being deployed, with P2P currently outpacing PON installations. By 2012 PON will catch up to P2P, and it is expected that P2P will start to decline and PON will continue to grow and will dominate.       For densely populated regions of the world, high-rise multi-dwelling units can take advantage of resource sharing through traffic aggregation with a centralized Ethernet switch or digital subscriber line access multiplexer (DSLAM) in each building. In this case, a dedicated single fiber link back to the central office makes a lot of sense. China-India and Asia-Pacific are currently the leading regions for point-to-point access due to their large number of densely populated areas. We expect Western Europe to catch up and surpass Asia-Pacific in the later years.       Worldwide, the PON market is expected to grow at a CAGR of 15% between 2005 and 2012. North America will be more aggressive in its deployment of PON networks during the forecast period, while Asia-Pacific PON deployments will remain relatively steady. The fastest-growing regions for PON sales are forecast to be Western Europe and North America.       There is also movement between different flavors of PON. Starting in 2007, the migration from broadband PON (BPON) to gigabit PON (GPON) in North America began as the pricing of GPON became more attractive. In other places, Ethernet-based PON (EPON) is more popular. Asia-Pacific and China-India will continue to favor EPON, although GPON will make its presence known in the later years of the forecast.