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Fiber Optic Cable Tutorial – Specifications, Size

Fiber Specifications

Bandwidth, attenuation, and size are the common fiber specifications that you are likely to use in any system design. Although there are other types of specifications that are also taken into account by manufacturers (such as ovality - roundness of the fiber-, numerical aperture –the maximum acceptance angle to admit and transmit light on a fiber, cladding and core concentricity ) that usually do not have a direct impact on you.

More On Fiber Cable

Fiber optics sends signals through very thin strands of plastic or glass fiber. The light is directed towards the center, also called “core”, of the fiber. The cladding, which is an optical material, surrounds the center of the fiber and catches the light by means of an optical technique usually referred to as “total internal reflection.” The material with which the center of the fiber (core) and the cladding are usually made of is ultra-pure glass but there are certain fibers that are completely made of glass or plastic, or they may have a plastic cladding and a glass core. A protective plastic coat known as the "primary buffer coating" covers the fiber, defending it from wetness and other types of damage. The “cable” provides further protection because the fibers and strength member (aramid) are covered by an outer coat which is called the “jacket”

The two types of optical fibers are Multimode and Singlemode. The outside diameter of both fibers core plus cladding is 125 micron. The size of a micron is one-millionth of a meter; therefore the size of a fiber’s core plus cladding is 0.0005 inches. In comparison to a human hair (100 microns) they are a bit larger.

On the one hand, the light moving through the core of a Multimode fiber travels in many rays, named modes. The core of a Multimode fiber is large, generally 62.5 microns, but it can also be of 50 microns. It is used with LED’s or lasers at 850, 1310 and 1500 nm for networks which run at gigabits per second or more and usually with LED sources for LANS. Slower Local Area Networks utilize wavelengths of 850 and 1310

The core of a Singlemode fiber, on the other hand is smaller in comparison to the Multimode fiber: it is only around 9 microns. Light travels in just a single ray and this occurs because this type of fiber shrinks the core. As it travels in one ray, the cable may carry 100 or more gigabits. The bandwidth and transmission distance of Singlemode fiber is much greater than Multimode fiber. CATV and telephony make use of Singlemode fiber for the core of their networks, with laser sources at 1310 and 1550 nm.

POF, or Plastic Optical Fiber, is a type of large core of around 1mm which can exclusively be used for short distances and low network speeds. The first fiber design was the Step Index Multimode, but its problem was that it was extremely slow when it was used, because the various path lengths of the different modes caused dispersion. Nowadays it is only plastic optical fiber that uses a step index design. In order to compensate for the various path lengths that the modes have, Graded Index Multimode fiber makes use of different glass compositions in the core, and much more bandwidth than the step index fiber is offered: it almost reaches 2 gigahertz.

Fiber Cable Size

As it has been stated above, there are two types of fiber: Singlemode fiber and Multimode fiber. Apart from fibers which are employed in specialty applications, Singlemode can be thought of as one type and size. So, if you are working with submarine cables or long haul telecom, you may have the need to use specialty Singlemode fibers.

At the beginning, there were several sizes of Multimode Fibers, which were made as effective as possible for different sources; but in the eighties the 62.5 core fiber was standardized (the 62.5/125 fiber contains a core of 62.5 micron and with its cladding is 125 microns). In recent times, due to the popularity of gigabit and 10 gigabit networks, the 50/125 has been brought back. This type of fiber was first used during the last years of the 1970s, before the appearance of Singlemode fiber, and it was applied with lasers for telecom applications. Not only does it offer a higher bandwidth with laser sources used in gigabit networks, but it can also go longer distances, so it is replacing 62.5/125 micron fiber cables for most new applications.