Types of Fiber & Specification

Fiber Optics is the communications medium that works by sending optical signals down hair-thin strands of extremely pure glass or plastic fiber. The light is "guided" down the center of the fiber called the "core". The core is surrounded by a optical material called the "cladding" that traps the light in the core using an optical technique called "total internal reflection." The fiber itself is coated by a "buffer" as it is made to protect the fiber from moisture and physical damage. The buffer is what one strips off the fiber for termination or splicing.
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The core and cladding are usually made of ultra-pure glass, although some fibers are all plastic or a glass core and plastic cladding. The core is designed to have a higher index of refraction, an optical parameter that is a measure of the speed of light in the material, than the cladding, which causes "total internal reflection" to trap light in the core up to a certain angle, which defines the “numerical aperture” of the fiber.

Glass fiber is coated with a protective plastic covering called the "primary buffer coating" that protects it from moisture and other damage. More protection is provided by the "cable" which has the fibers and strength members inside an outer protective covering called a "jacket".

Fiber Types: Multimode & Singlemode, Core/Cladding Size

The two types of fiber are multimode and singlemode. Within these categories, fibers are identified by their core composition (MM step-index or graded-index) and core/cladding diameters expressed in microns (one millionth of a meter), e.g. 50/125 micron graded-index multimode fiber. Most glass fibers are 125 microns in outside diameter - a micron is one one-millionth of a meter and 125 microns is 0.005 inches- a bit larger than the typical human hair.

Multimode fiber has light traveling in the core in many rays, called modes. It has a larger core (almost always 50 or 62.5 microns) which supports the transmission of multiple modes (rays) of light. Multimode is generally used with LED sources at wavelengths of 850 and 1300 nm (see below!) for slower local area networks (LANs) and lasers at 850 (VCSELs) and 1310 nm (Fabry-Perot lasers) for networks running at gigabits per second or more.

Singlemode fiber has a much smaller core, only about 9 microns, so that the light travels in only one ray (mode.) It is used for telephony and CATV with laser sources at 1310 and 1550 nm because it has lower loss and virtually infinite bandwidth.

1300 or 1310nm?

Fiber is filled with jargon that is traditional and often obtuse in meaning. The 1300/1310 issue goes back to the beginning. AT&T's long-wavelength lasers were statistically centered around 1310nm (but varied from 1290-1330 or more) so they adopted the 1310nm nomenclature. LEDs with a wider and more varied spectral output (~1260-1350nm with spectral widths of 60-150nm depending on the construction) became known as 1300nm devices.

When NBS (now NIST) created a calibration standard for power meters, they used 850, 1300 and 1550nm so meter calibration is usually at those wavelengths, although some manufacturers offer both 1300 and 1310 or call it 1300/1310 because it is an irrelevant difference in calibration.

Plastic Optical Fiber (POF) is large core ( about 1mm) fiber, usually step index, that is used for short, low speed networks.
PCS/HCS (plastic or hard clad silica, plastic cladding on a glass core) has a smaller glass core (around 200 microns) and a thin plastic cladding.
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Fiber Specifications

The usual fiber specifications are size (core/cladding diameter in microns), attenuation coefficient (dB/km at appropriate wavelengths) and bandwidth (MHz-km) for multimode fiber and chromatic and polarization-mode dispersion for singlemode fiber. While manufacturers have other specs for designing and manufacturing the fiber to industry standards, like numerical aperture (the acceptance angle of light into the fiber), ovality (how round the fiber is), concentricity of the core and cladding, etc., these specs do not generally affect users who specify fibers for purchase or installation

Some fibers have been designed to be much less sensitive to bend-induced losses. These "bend-insensitive" fibers are designed for use as patchcords or in tight premises appplications where regular fibers would suffer losses.

Attenuation

The primary specification of optical fiber is the attenuation. Attenuation means a loss of optical power. The attenuation of an optical fiber is expressed by the attenuation coefficient which is defined as the loss of the fiber per unit length, in dB/km.
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The attenuation of the optical fiber is a result of two factors, absorption and scattering. The absorption is caused by the absorption of the light and conversion to heat by molecules in the glass. Primary absorbers are residual OH+ and dopants used to modify the refractive index of the glass. This absorption occurs at discrete wavelengths, determined by the elements absorbing the light. The OH+ absorption is predominant, and occurs most strongly around 1000 nm, 1400 nm and above1600 nm. Many fibers today are "low water peak" fibers where the OH+ absorption bands have been greatly reduced, allowing a version of wavelength division multiplexing to use these wavelengths.
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