Optical fiber elements and optical cable

The fiber element within an optical cable usually consists of a core and a cladding Figure 1. The core provides the light path, the cladding surrounds the core, and the optical properties of the core and cladding junction cause the light to remain within the core.

Although the core and the cladding diameters, expressed in micrometers (μm), are often used to describe an optical cable, they actually indicate the physical size of the fiber element. For example, a fiber element having a core diameter of 62.5 μm and a cladding diameter of 125 μm is called 62.5/125-μm fiber.

In an optical cable, the core and cladding are typically surrounded by other layers (such as a primary and secondary buffer), a strength member, and an outer jacket (Figure 1) that provide strength and environmental protection. It is possible to transmit multiple data channels over a single optical fiber by sending each data stream at a different optical wavelength. This is known as Wavelength Division Multiplexing (WDM). For more information, see the following red papers that describe several products offered by IBM® strategic partners:

Because information transfer usually occurs in only one direction through an optical fiber, various fiber types have been developed for different applications. The optical properties and specifications of a fiber determine many characteristics. For example, single mode fiber (nominally about 9.0 μm) provides one high-bandwidth information "path". Single Mode fiber is usually used to transfer information over greater distances compared to multimode fiber (50.0 μm, for example), which provides multiple paths and has a lower bandwidth. The terms single mode and multimode are often used to describe the optical fiber and optical cable types.

Generally, long wavelength (LX or 1300 nm) laser diodes use single mode fiber to transmit information, while long wavelength light-emitting diodes (LEDs) or short wavelength (SX or 850 nm) lasers use multimode fiber. In a data processing environment using optical fiber, product, distance, and right-of-way considerations usually determine if single mode or multimode fiber is used.