Optical Fiber

An optical fiber (or fibre) is a glass or plastic fiber designed to guide light along its length by confining as much light as possible in a propagating form. In fibers with large core diameter, the confinement is based on total internal reflection. In smaller diameter core fibers, (widely used for most communication links longer than 200 meters) the confinement relies on establishing a waveguide. Fiber optics is the overlap of applied science and engineering concerned with such optical fibers. Optical fibers are widely used in fiber-optic communication, which permits transmission over longer distances and at higher data rates than other forms of wired and wireless communications. They are also used to form sensors, and in a variety of other applications.

The term optical fiber covers a range of different designs including graded-index optical fibers, step-index optical fibers, birefringent polarization-maintaining fibers and more recently photonic crystal fibers, with the design and the wavelength of the light propagating in the fiber dictating whether or not it will be multi-mode optical fiber or single-mode optical fiber. Because of the mechanical properties of the more common glass optical fibers, special methods of splicing fibers and of connecting them to other equipment are needed. Manufacture of optical fibers is based on partially melting a chemically doped preform and pulling the flowing material on a draw tower. Fibers are built into different kinds of cables depending on how they will be used.

History

The light-guiding principle behind optical fibers was first demonstrated in by Daniel Collodon and Jaques Babinet in the 1840s, with Irish inventor John Tyndall offering public displays using water-fountains ten years later.[1] 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 principle was first used for internal medical examinations by Heinrich Lamm in the following decade. In 1952 physicist Narinder Singh Kapany conducted experiments that led to the invention of optical fiber, based on Tyndall's earlier studies; modern optical fibers, where the glass fiber is coated with a transparent cladding to offer a more suitable refractive index, appeared later in the decade.[1] Development then focused on the development of fiber bundles for image transmission. The first fiber optic semi-flexible gastroscope was patented by Basil Hirschowitz, C. Wilbur Peters, and Lawrence E. Curtiss, researchers at the University of Michigan, in 1956. In the process of developing the gastroscope, Curtiss produced the first glass-clad fibers; previous optical fibers had relied on air or impractical oils and waxes as the low-index cladding material. A variety of other image transmission applications soon followed. Optical fibers became practical for use in communications in the late 1970s, once the attenuation was reduced sufficiently; since then, several technical advances have been made to improve the attenuation and dispersion properties of optical fibers (i.e., allowing signals to travel farther and carry more information), and lower the cost of fiber communications systems.

Applications


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. In total, a single fiber-optic cable can carry data at rates as high as 14444 Tb/s. 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. Because they are non-electrical, fiber cables 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; single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.

Fiber optic sensors

Optical fibers can be used as sensors to measure strain, temperature, pressure and other parameters. The small size and the fact that no electrical power is needed at the remote location gives the fiber optic sensor advantages to conventional electrical sensor in certain applications.

Optical fibers are used as hydrophones for seismic or SONAR applications. Hydrophone systems with more than 100 sensors per fiber cable have been developed. Hydrophone sensor systems are used by the oil industry as well as a few countries' navies. Both bottom mounted hydrophone arrays and towed streamer systems are in use. The German company Sennheiser developed a microphone working with a laser and optical fibers[2].

Optical fiber sensors for temperature and pressure have been developed for downhole measurement in oil wells. The fiber optic sensor is well suited for this environment as it is functioning at temperatures too high for semiconductor sensors (Distributed Temperature Sensing).

Another use of the optical fiber as a sensor is the optical gyroscope which is in use in the Boeing 767 and in some car models (for navigation purposes) and the use in Hydrogen microsensors.

Fiber-optic sensors have been developed to measure co-located temperature and strain simultaneously with very high accuracy[3]. This is particularly useful to acquire information from small complex structures.

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. Swarovski boutiques use optical fibers to illuminate their crystal showcases from many different angles while only employing one light source. Optical fiber is an intrinsic part of the light-transmitting concrete building product, LiTraCon.

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 (endoscopy). Industrial endoscopes (see fiberscope or borescope) 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 (undoped) 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.

LAN - Local Area Network..

A local area network is a computer network covering a small geographic area, like a home, office, or group of buildings. Current LANs are most likely to be based on switched IEEE 802.3 Ethernet technology, running at 10, 100, 1,000 or 10,000 Mbit/s, or on IEEE 802.11 Wi-Fi technology. Each node or computer in the LAN has its own computing power but it can also access other devices on the LAN subject to the permissions it has been allowed. These could include data, processing power, and the ability to communicate or chat with other users in the network.

The defining characteristics of LANs, in contrast to WANs (wide area networks), include their much higher data transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines.

Technical aspects

Although switched Ethernet is now the most common data link layer protocol (OSI 7-Layer Model), and IP as a network layer protocol, many different options have been used (see below), and some continue to be popular in niche areas. Smaller LANs consist of a few switches typically connected to each other and with one connected to a router, cable modem, or DSL modem. A traditional model of access, distribution, and core switches was popularized by Cisco Systems and has been in use for many years.

Larger LANs are characterized by distributing Ethernet traffic roles within the network. Each layer aggregates traffic of the layer below it and will typically maintain redundant links with switches capable of quality of service and spanning tree protocol to prevent loops and the recovery of failed uplinks.

LANs may have connections with other LANs via routers and leased lines. Traditionally, the network connecting two or more LANs is referred to as the WAN (Wide Area Network). Recently, service providers have begun to offer additional services to link LANs together. These technologies, such as Metropolitan Area Networks (MANs), and MPLS/VPN services have diversified the standard model of interconnecting sites. There are also methods of connecting LANs together through the use of Internet connections, VPN software or hardware, and 'tunneling' across the Internet using VPN technologies.

Topology, protocols and media (The cables, like CAT5, or radio waves that connect devices in the LAN) are the characteristics that differentiate LANs.

Home networks

With the proliferation of computers and IT devices in the modern home has come the frequent use of LANs to connect them together. Many of these home LANs are wireless, using the 802.11g/b wireless standards

Hubs

A common connection point for devices in a network. Hubs are commonly used to connect segments of a LAN. A hub contains multiple ports. When a packet arrives at one port, it is copied to the other ports so that all segments of the LAN can see all packets.

A passive hub serves simply as a conduit for the data, enabling it to go from one device (or segment) to another. So-called intelligent hubs include additional features that enables an administrator to monitor the traffic passing through the hub and to configure each port in the hub. Intelligent hubs are also called manageable hubs.

A third type of hub, called a switching hub, actually reads the destination address of each packet and then forwards the packet to the correct port.

Router...

A router is a computer networking device that buffers and forwards data packets across an internetwork toward their destinations, through a process known as routing. Routing occurs at layer 3 (the Network layer e.g. IP) of the OSI seven-layer protocol stack.

Function

Routers are like intersections whereas switches are like streets

A router acts as a junction between two or more networks to buffer and transfer data packets among them. A router is different from a switch and a hub: a router is working on layer 3 of OSI model, a switch on layer 2 and a hub on layer 1. This makes them work for different situations: a switch connects devices to form a Local area network (LAN) (which might, in turn, be connected to another network via a router).

One easy illustration for the different functions of routers and switches is to think of switches as neighborhood streets, and the router as the intersections with the street signs. Each house on the street has an address within a range on the block. In the same way, a switch connects various devices each with their own IP address(es) on a LAN. However, the switch knows nothing about IP addresses except its own management address. Routers connect networks together the way that on-ramps or major intersections connect streets to both highways and freeways, etc. The street signs at the intersection (routing table) show which way the packets need to flow.

So for example, a router at home connects the Internet Service Provider's (ISP) network (usually on an Internet address) together with the LAN in the home (typically using a range of private IP addresses, see network address translation) and a single broadcast domain. The switch connects devices together to form the LAN. Sometimes the switch and the router are combined together in one single package sold as a multiple port router.

In order to route packets, a router communicates with other routers using routing protocols and using this information creates and maintains a routing table. The routing table stores the best routes to certain network destinations, the "routing metrics" associated with those routes, and the path to the next hop router. See the routing article for a more detailed discussion of how this works.

Routing is most commonly associated with the Internet Protocol, although other less-popular routed protocols are in use.


History

The first Internet router was developed at BBN [1] as part of their contract to build out the original Arpanet. The first multiprotocol router was created at Stanford University by a staff researcher named William Yeager [2] in January of 1980. His boss at the time told him that he was the "network guy" and to find a way to connect the computers in the computer science department, medical center and department of electrical engineering. He first wrote a network operating system and routing code to run on a DEC PDP11/05. He used Alan Snyder's Portable C compiler but it generated too much code so he modified the compiler to improve the code generators.That still wasn't good enough so he wrote an optimizer for PDP11/05 assembler that reduced the code size further.

Types of routers

In the original era of routing (from the mid-1970s through the 1980s), general-purpose mini-computers served as routers. Although general-purpose computers can perform routing, modern high-speed routers are highly specialized computers, generally with extra hardware added to accelerate both common routing functions such as packet forwarding and specialised functions such as IPsec encryption.

Other changes also improve reliability, such as using DC power rather than line power (which can be provided from batteries in data centers), and using solid-state rather than magnetic storage for program loading. Large modern routers have thus come to resemble telephone switches, with whose technology they are currently converging and may eventually replace, whilst small routers have become a common household item.

A router that connects clients to the Internet is called an edge router. A router that serves solely to transmit data between other routers, e.g. inside the network of an Internet service provider, is called a core router.

A router is normally used to connect at least two networks, but a special variety of router is the one-armed router, used to route packets in a virtual LAN environment. In the case of a one-armed router, the multiple attachments to different networks are all over the same physical link.

In mobile ad-hoc networks every host performs routing and forwarding by itself, while in wired networks there is usually just one router for a whole broadcast domain.