Optical fiber technology

Transmission of signals over optical fiber uses the phenomenon of total internal reflection to transmit data with rays of light through a strand of glass. This strand is called an optical fiber. Total internal reflection occurs because the cladding that surrounds the core of the optical fiber has a lower index of refraction than the core. This difference in index of refraction causes the light to be contained in the fiber as it travels from source to destination. See Figure 1.

In optical fiber, mode describes the propagation of light rays in an optical fiber. Optical fiber supports many modes of propagation, depending on the size of the fiber.

When the core has a diameter that is large compared to the wavelength of the light signal, many modes of light are propagated. Such fiber is called multimode optical fiber. The different modes of light reach the end of the fiber at different times, because each mode takes a different path through the core. This causes the pulse to spread out; this is referred to as modal dispersion.

Optical fiber is usually referred to by the diameter of its core and cladding with its numerical aperture. For example, a multimode fiber with a diameter of 62.5 microns (millionths of a meter; hereafter, microns will be abbreviated as μm, which is the scientific notation) and a cladding diameter of 125 μm with a numerical aperture of .275, is designated as 62.5/125 NA .275 fiber (hereafter referred to as 62.5/125 fiber). Based on the work of standards organizations, 62.5/125μm fiber was initially the recommended fiber cable for most multimode fiber applications. More recently, 50/125 NA .2 fiber (hereafter referred to as 50/125μm fiber) has become the multimode fiber of choice. Multimode optical fiber is available with core diameters that range from 50 to 200 μm. Although other fiber sizes are supported by IBM®, their use should be based on the requirements of the attaching products and is not recommended for new installations.

Most multimode optical fiber is graded-index fiber, which is manufactured with a gradually changing refractive index, instead of an abrupt change between the core and the cladding. This gradually changing refractive index allows for lower modal dispersion and thus a higher bandwidth. See Figure 1 for an example of how the light in multimode optical fiber is propagated through a graded-index fiber.

In single mode fiber, light is propagated as only one mode. This mode travels in both the core and the cladding, with most of the light within a diameter of 9 to 10 μm (9 to 10 μm being the mode field diameter). This fiber is called single mode optical fiber. Single Mode optical fiber eliminates modal dispersion consideration.

Multimode fiber is usually used with a short wave length laser light source, whereas single mode fiber is usually used with long wavelength laser light source. Because of the laser light source and the propagation of a single mode of light, single mode fiber provides longer transmission distances and faster data rates.