Wave-Based Technology: Fiber Optics

Part 1- Background

Categorized as a wave-based technology because of its primary source as a light ray, fibre optics is a technology that deals with the transmission of light through hair-thin transparent fibres. It is formed by a very sophisticated system of fibres in a wire that works by reflecting light from mirror to mirror.

A fibre optic works by sending light wave signal from one end and transmitting it to the other end with low loss of light even when fibres are curved. The principal of this technology is based on what is called the total internal reflection which allows optical fibres to restrain the light they carry. This is done so through light striking at an angle greater than the critical angle which is created when light passes from a dense to a less dense substance. Total internal reflection occurs when light strikes at an angle of incidence which is greater than the critical angle which will result in reflection of all light without loss. Furthermore, this is very useful because the wave travelling inside cannot strike less than the critical angle unless the fibre is curved too sharply. Because fibre optic technology is so dependent on preventing light from escaping, which is also translated as wave dependent, the total internal reflections is a vital method in the survival of this technology.

Fibre optic's technology uses a wave type of an optical waveguide. Optical waveguide is typically formed of a circular cross-section of a dielectric waveguide consisting of a dielectric material. The fibre has high permittivity and high index of refraction, which is surrounded by a material with a lower permittivity. Its structure is designed to guide these optical waves by total internal reflection. As mentioned above, total internal reflection occurs when the light is incident on a dielectric interface at an angle greater than the critical angle. the light is trapped by a waveguide by surrounding a guiding region, called the core, which is made from a material with index of refraction ncore, and also with a material called the cladding, made from a material with index of refraction ncladding < ncore. Any light that enters is trapped as long as the following equation is met: sin&#61553;&#61472;> ncladding/nncore.

The size of the cable of an optical fibre varies in diameters of anywhere from above 8&#956;m and above depending on the number of fibres used. Inside the cable, the cable itself constantly reflects light into fibre to transmit signals. These reflected lights cannot escape the strand which allows in carriage of more information with less interference and signal repeaters over long distances. In addition, the wavelengths of these light waves are extremely short resulting in high frequencies.

Part 2- Technology

The capacity of optical fibres has increased dramatically over the past few years. With increased demands for faster mode of transporting information, this technology fit in our society naturally. Used in many applications, fibre optic works with the following three basic systems; transmitting device (generates light signal), optic-fibre cable (carries the light), and a receiver (accepts transmitted light signal and converts it into an electrical signal). It is very dependent on light therefore the main idea to this technology is to prevent light from escaping the fibre to reduce loss. This is done so by making the beams of light reflect inside the optic cable which is only done under the condition of these beams entering at in incident angle greater than the critical angle. Because of its high speed and available capacity to carry information, it has taken part as the backbone technology of many industries with one of the major ones being in the communications industry.

An optical fibre consists of three key elements, the core, the cladding and the outer coating, often called the buffer. The core is usually made of glass or plastic but because if the impurity level differences, glass is usually preferred over plastic when it comes to qualities. The core is the light-carrying portion of the fibre which is surrounded by the cladding. The cladding is made of a material with a slightly lower index of refraction than the core. This difference in the indices causes total internal reflection to occur at the core-cladding boundary along the length of the fibre. Because of this principal, Light is transmitted down the fibre and does not escape through the sides of the fibre.

There are several types of fibre optic cables used today. These cables are mainly classified into two types, multimode and single mode. Also, the multimode can further be split into different types of the step-index multimode fibre and the graded-index fibre. These cables are used mainly in communications industries for different purposes and cables that suits best for a certain condition.

The propagation of light in optical fibres is governed by the laws of Maxwell's equations. When information about the material constants, such as the refractive indices, and the boundary conditions of the core and the cladding is given, they can be incorporated into the equations to produce a wave equation that can be solved for the electromagnetic field distributions propagating through the fibre. These distributions of the electromagnetic field across the fibre are referred as the modes. When the diameter of the core is large compared to the wavelength of the light propagating through the fibre, the allowed number of modes becomes large and thus shoot multiples of rays simultaneously. Those fibres are called multimode fibres

In a step-index multimode fibre, the refractive index of the core is a constant and changes abruptly at the core-cladding interface.

The above diagram is the basic structure of a step-index fibre named so because it forms a shape of a stair between the core and the cladding. For such fibers, the fractional refractive index difference is given by

D = (ncore-ncladding)/ncore

Each mode in a step-index multimode fiber is associated with a different entrance angle. This is the primary reason why several light rays may travel along a different path through the fiber. Different propagating modes have different group velocities. As an optical pulse travels down a multimode fiber, the pulse begins to spread. Pulses entering separated from each other will eventually overlap each other resulting in the limits both the bandwidth and the distance over which it can transport data.

Bandwidth is the measurement of the data-carrying