Education

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 200m) 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 digital data 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.
The light-guiding principles behind optical fibers was first demonstrated in Victorian times, but modern optical fibers were only developed in the early 1950s. 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 further and carry more information), and lower the cost of fiber communications systems

Single-mode optical fiber

In fiber-optic communication, a single-mode optical fiber is an optical fiber designed to carry only a single ray of light (mode). This ray of light often contains of a variety of different wavelengths. Although the ray travels parallel to the length of the fiber, it is often called the transverse mode since its electromagnetic vibrations occur perpendicular (transverse) to the length of the fiber. Single-mode optical fibers are also called monomode optical fibers, single-mode optical waveguides, or unimode fibers.
Unlike multi-mode optical fibers, single mode fibers do not exhibit dispersion resulting from multiple spatial modes. Single mode fibers are also better at retaining the fidelity of each light pulse over long distances than are multi-mode fibers. For these reasons, single-mode fibers can have a higher bandwidth than multi-mode fibers. Equipment for single mode fiber is more expensive than equipment for multi-mode optical fiber, but the single mode fiber itself is usually cheaper in bulk
A typical single mode optical fiber has a core radius of 5 to 10 µm and a cladding radius of 120 µm. There are a number of special types of single-mode optical fiber which have been chemically or physically altered to give special properties, such as dispersion-shifted fiber. Data rates are limited by polarization mode dispersion and chromatic dispersion. In 2005, data rates of up to 10 gigabits per second were possible at distances of over 60 km with commercially available transceivers.

The lowest order bound mode is ascertained for the wavelength of interest by solving Maxwell's equations for the boundary conditions imposed by the fiber, e.g., core (spot) size and the refractive indices of the core and cladding. The solution of Maxwell's equations for the lowest order bound mode will permit a pair of orthogonally polarized fields in the fiber, and this is the usual case in a communication fiber. In step-index guides, single-mode operation occurs when the normalized frequency, V, is less than 2.405. For power-law profiles, single-mode operation occurs for a normalized frequency, V, less than approximately where g is the profile parameter. In practice, the orthogonal polarizations may not be associated with degenerate modes.

Multi-mode optical fiber

Multi-mode optical fiber (multimode fiber or MM fiber) is a type of optical fiber mostly used for communication over shorter distances, such as within a building. It can carry 1 Gbit/s for typical building distances; the actual maximum data rate (given the right electronics) depends upon the distance. Multi-mode fiber has a higher light-gathering capacity than single-mode optical fiber, making splicing less difficult, but its limit on speed × distance is lower. Because multi-mode fiber has a larger numerical aperture than single-mode fiber, it supports more than one propagation mode, resulting in larger modal dispersion and consequently higher pulse spreading rates, limiting information transmission capacity.
Multimode fibers are more useful for carrying larger amounts of power very short distances than single mode fibers. In such fibers, mode-filling becomes important, and mode scrambling attempts to fill the fiber to capacity, achieving an equilibrium mode distribution that utilizes all available fiber modes and has a more uniform energy density. These fibers are used when an intense beam is needed, as in optical pumping, laser welding, cutting, and marking. For data applications, however, multi-mode fiber's higher attenuation is another factor limiting the length of multimode fiber links (in addition to dispersion).
). The earliest fiber optic cables used a technique termed multi-mode transmission. This is where the light signals from the laser are broken up into a number of paths along the length of the fibre and are reflected off the fiber wall. The amount of reflection that occurs dictates the quality of the signal. The equipment used for communications over multi-mode optical fiber is less expensive than that for single-mode optical fiber. Typical transmission speeds/distances limits are 100 Mbit/s up to 2 km (100BASE-FX), 1 Gbit/s for distances up to 500-600 meters (1000BASE-LX - single-mode and less often multi-mode, 1000BASE-SX - multi-mode), and 10 Gbit/s for distances up to 300 meters (10GBASE-SR).

LinkWay

Broadband VSAT Network Topologies

STAR

Hub-and-spoke networks

VIRTUAL STAR

Hybrid (multi-hub)

MESH

Full one-hop connectivity

What is LINKWAY?

LINKWAY is a hub-less “VSAT-like” platform for a single-hop, STAR or MESH network connectivity; supporting broadband multi-media, intranet, and internet applications; utilizing the latest telecom standards and the most efficient space segment access.

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LINKWAY 2000

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Multiprotocol, Mesh/Hybrid, 70 MHZ, Stackable

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Multiprotocol, Mesh/Hybrid, L-Band Interface

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Broadband integrated VSAT Applications

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Features:

IP efficient, multicasting 10/100BaseT, Virtual Star, L-Band Interface

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MULTIPLEXING

To combine multiple signals (analog or digital) for transmission over a single line or media. A common type of multiplexing combines several low-speed signals for transmission over a single high-speed connection. The following are several examples of different multiplexing methods:

Frequency Division Multiplexing

A multiplexing technique that uses different frequencies to combine multiple streams of data for transmission over a communications medium. FDM assigns a discrete carrier frequency to each data stream and then combines many modulated carrier frequencies for transmission. For example, television transmitters use FDM to broadcast several channels at once.

TIME DIVISION MULTIPLEXING

A type of multiplexing that combines data streams by assigning each stream a different time slot in a set. TDM repeatedly transmits a fixed sequence of time slots over a single transmission channel. Within T-Carrier systems, such as T-1 and T-3, TDM combines Pulse Code Modulated (PCM) streams created for each conversation or data stream.

Pulse Code Mudulation

A sampling technique for digitizing analog signals, especially audio signals. PCM samples the signal 8000 times a second; each sample is represented by 8 bits for a total of 64 Kbps. There are two standards for coding the sample level. The Mu-Law standard is used in North America and Japan while the A-Law standard is use in most other countries. PCM is used with T-1 and T-3 carrier systems. These carrier systems combine the PCM signals from many lines and transmit them over a single cable or other medium. PCM is also the usual digital method used for music audio playback of music CDs. While supported by DVDs, DVDs have a greater volume so they use Linear PCM, which has a higher sampling rate — up to 24-bit at a sampling rate of 96 kHz.