What is Fiber Optic?

Optical fiber (or "fiber optic") refers to the medium and the technology associated with the transmission of information as light pulses along a glass or plastic wire or fiber. Optical fiber carries much more information than conventional copper wire and is in general not subject to electromagnetic interference and the need to retransmit signals. Most telephone company long-distance lines are now of optical fiber. Transmission on optical fiber wire requires repeating at distance intervals. The glass fiber requires more protection within an outer cable than copper. For these reasons and because the installation of any new wiring is labor-intensive, few communities yet have optical fiber wires or cables from the phone company's branch office to local customers (known as local loop). Single mode fiber is used for longer distances; multimode fiber is used for shorter distances. In optical fiber technology, single mode fiber is optical fiber that is designed for the transmission of a single ray or mode of light as a carrier and is used for long-distance signal transmission. For short distances, multimode fiber is used.

A little history:
In 1961 Elias Snitzer at American Optical demonstrated that it was possible to draw fibers with cores so small that they carried light in only one waveguide mode. Fiber optic functions as a "light guide," guiding the light introduced at one end of the cable through to the other end. In 1966 it was shown that the optical waveguide has an information-carrying capacity of at least one gigacycle, or the equivalent of about 200 tv channels or more than 200,000 telephone channels. In 1970 a single-mode fiber with loss of 17 dB/km at 633 nanometers and in 1972 a multimode germania-doped fiber with 4 dB/km loss and much greater strength was developed. The invention of the first optical fibers was the catalyst to ignite the communications revolution we witness today. Today’s low-loss glass fiber optic cable offers almost unlimited bandwidth and unique advantages over all previously developed transmission media. The basic point-to-point fiber optic transmission system consists of three basic elements: the optical transmitter, the fiber optic cable and the optical receiver.

How it works:
There are basically three parts of a fiber optics cable: the Core, the thin glass center of the fiber where the light travels, the Cladding, the outer optical material surrounding the core that reflects the light back into the core and the Buffer Coating, the plastic coating that protects the fiber from damage and moisture.

The light in a fiber optic cable travels through the core by constantly bouncing from the outer cladding, a principle called total internal reflection. Because the cladding does not absorb light from the core, the light wave can travel long distances without large attenuation.

The transmitter converts an electrical analog or digital signal into a corresponding optical signal. The source of the optical signal can be either a light emitting diode, or a solid state laser diode. The most popular wavelengths of operation for optical transmitters are 850, 1300, or 1550 nanometers. In a fiber optic system, these devices are mounted in a package that enables an optical fiber to be placed in very close proximity to the light emitting region in order to couple as much light as possible into the fiber. The receiver converts the optical signal back into a replica of the original electrical signal. The detector of the optical signal is either a PIN-type photodiode or avalanche-type photodiode.

All basic attenuation factors in fiber optics result in simple attenuation that is independent of bandwidth: a 3 dB loss means that 50% of the light will be lost whether it is being modulated at 10 Hz or at 100 MHz. But there is also a bandwidth limitation that depends on frequency and length of the fiber optics cable. Typical bandwidths for common fibers range from a few MHz per km for very large core fibers, to hundreds of MHz per km for standard multimode fiber, to thousands of MHz per km for single-mode fibers.

There are two basic groups of fiber optics: Single Mode and Multimode:

Single Mode Fiber
Single Mode cable is a single stand of glass fiber with a diameter of 8.3 to 10 microns that has one mode of transmission. Single mode fiber has a relatively narrow diameter through which only one mode will propagate. It can carry higher bandwidth and enables up to 50 times more distance than multimode fiber, but it requires a light source with a narrow spectral width. The costs for single mode fiber are also higher. The small core and single light wave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fiber cable type. Wavelengths are typically 1300nm to 1550nm.

Multimode Fiber
In optical fiber technology, multimode fiber is optical fiber that is designed to carry multiple light rays or modes concurrently, each at a slightly different reflection angle within the optical fiber core. Multimode fiber transmission is used for relatively short distances because the modes tend to disperse over longer lengths (this is called modal dispersion). For longer distances, single mode (sometimes called monomode) fiber is used. Multimode fiber has a larger core than single mode. Multimode fiber optics enables high bandwidth at high speeds over medium distances. Light waves are dispersed into numerous paths, or modes, as they travel through the cable's core. The Wavelength is typically 850 or 1300nm. Multimode fiber optics has larger cores (typically 50, 62.5, and 100 micrometers). Wavelengths are typically 850nm to 1300nm. With multimode cables in long cable runs greater than 3000 feet (1000 meter), multiple paths of light can cause signal distortion.

Gigabit Ethernet
By now you may have heard of this new standard. Gigabit Ethernet is a local area network (LAN) transmission standard that provides a data rate of 1 billion bits per second (one gigabit). Gigabit Ethernet is defined in the IEEE 802.3 standard and the first product versions of it are now available. Gigabit Ethernet is used as an enterprise backbone. Gigabit Ethernet is carried primarily on optical fiber (with very short distances possible on copper media). Existing Ethernet LANs with 10 and 100 Mbps cards can feed into a Gigabit Ethernet backbone. An alternative technology that competes with Gigabit Ethernet is ATM.