Fiber Converter

Fiber Optics

The field of fiber optics communications has exploded over the past two decades. Fiber is an integral part of modern day communication infrastructure and can be found along roads, in buildings, hospitals and machinery.

The fiber itself is a strand of silica based glass, it's dimensions similar to those of a human hair, surrounded by a transparent cladding. Light can be transmitted along the fiber over great distances at very high data rates providing an ideal medium for the transport of information. This section will provide explanation's for some of the terms associated with the field of fiber optic engineering for telecommunications.

Fiber Basics

Fiber Structure
The diagram shows the typical structure of a fiber used for communication links. It has an inner glass core with an outer cladding. This is covered with a protective buffer and outer jacket. This design of fiber is light and has a very low loss , making it ideal for the transmission of information over long distances.
Light in a fiber
The light propagates along the fiber by the process of total internal reflection. The light is contained within the glass core and cladding by careful design of their refractive indices. The loss along the fiber is low and the signal is not subject to electromagnetic interference which plagues other methods of signal transmission, such as radio or copper wire links.
The signal is, however, degraded by other means particular to the fiber such as dispersion (described below) and non linear effects (caused by a high power density in the fiber core)

Transmission Characteristics of Fiber

Attenuation
The loss, or attenuation in fiber depends on the wavelength of the light propagating within it. The image shows the attenuation spectrum of a typical single mode fiber used within the telecommunications industry. There are three main bandwidth 'windows' of interest in the attenuation spectrum of fiber. The 1st window is at 800-900nm, here there is a good source of cheap silicon based sources & detectors. The 2nd window is at 1260-1360nm, here there is low fiber attenuation coupled with zero material dispersion (see dispersion ) . The 3rd window of interest is at 1430-1580nm where fiber has it's attenuation minimum. Typically the telecommunications industry use wavelengths in the 3rd window which coincides with the gain bandwidth of Fiber Amplifiers (see EDFAs ) In the future the search for greater bandwidth is likely to open up other windows for fiber transmission.

Jargon Buster

EDFA - Erbium Doped Fiber Amplifier
Otherwise known as a fiber or optical amplifier, the EDFA is an important component in long distance fiber links. Fiber and component attenuation in modern telecommunications links degrade the transmitted signal. When the signal power becomes too low errors will occur at the optical receiver as it struggles to recognise the transmitted signal from received noise.

Before the introduction of EFDAs, in order to transmit signals over long distances the signal would be detected and retransmitted at regular intervals, this process was called regeneration. EDFAs provide the telecommunications engineer with the means to optically amplify the signal en-route without converting the signal from the optical back to the electrical domain. The component works by the principle of stimulated emission. A piece of fiber doped with Erbium irons is pumped by a laser at high powers. The excited erbium irons release their energy when the data signal is passed through the fiber. The process is such, that the energy they release matches the signal exactly, thus amplifying the signal.

TDM - Time Division Multiplexing
The diagram below illustrates a method of incorporating many signals into one. Many slower speed signals are sampled onto one high speed signal.

DWDM - Dense Wavelength Division Multiplex
Dense Wavelength Division Multiplexing is a method of expanding the bandwidth of fiber. Many high speed signals are multiplexed together using different wavelength (or colours) for transmission over one fiber. The diagram below illustrates the concept.

Three high speed signals transmitted over the same fiber using different wavelengths.