Fiber 101
There are two different types of optical fiber: multimode and single-mode. Both are used in a broad range of telecommunications and data networking applications. These fiber types have dominated the commercial fiber market since the 1970�s. The distinguishing difference, and the basis for the naming of the fibers, is in the number of modes allowed to propagate in the core of a fiber. A �mode� is an allowable path for the light to travel down a fiber. A multimode fiber allows many light propagation paths, while a single-mode fiber allows only one light path.
In multimode fiber, the time it takes for light to travel through a fiber is different for each mode resulting in a spreading of the pulse at the output of the fiber referred to as intermodal dispersion. The difference in the time delay between the modes is called Differential Mode Delay (DMD). Intermodal dispersion limits multimode fiber bandwidth. This is significant because a fiber�s bandwidth determines its information carrying capacity, i.e., how far a transmission system can operate at a specified bit error rate.
The optical fiber guides the light launched into the fiber core (Figure 1). The cladding is a layer of material that surrounds the core. The cladding is designed so that the light launched into the core is contained in the core. When the light launched into the core strikes the cladding, the light is reflected from the core-to-cladding interface. The condition of total internal reflection (when all of the light launched into the core remains in the core) is a function of both the angle at which the light strikes the core-to-cladding interface and the index of refraction of the materials. The index of refraction (n) is a dimensionless number that characterizes the speed of light in a specific media relative to the speed of light in a vacuum. To confine light within the core of an optical fiber, the index of refraction for the cladding (n1) must be less than the index of refraction for the core (n2).
Fibers are classified in part by their core and cladding dimensions. Single-mode fibers have a much smaller core diameter than multimode fibers. However, the Mode Field Diameter (MFD) rather than the core diameter is used in single-mode fiber specifications. The MFD describes the distribution of the optical power in the fiber by providing an �equivalent� diameter, sometimes referred to as the spot size. The MFD is always larger than the core diameter with nominal values ranging between 8-10 microns, while single-mode fiber core diameters are approximately 8 microns or less. Unlike single-mode fiber, multimode fiber is usually referred to by its core and cladding diameters. For example, fiber with a core of 62.5 microns and a cladding diameter of 125 microns is referred to as a 62.5/125 micron fiber. Popular multimode product offerings have core diameters of 50 microns or 62.5 microns with a cladding diameter of 125 microns. Single-mode fibers also have125 micron cladding diameters.
A single-mode fiber, having a single propagation mode and therefore no intermodal dispersion, has higher bandwidth than multimode fiber. This allows for higher data rates over much longer distances than achievable with multimode fiber. Consequently, long haul telecommunications applications only use single-mode fiber, and it is deployed in nearly all metropolitan and regional configurations. Long distance carriers, local Bells, and government agencies transmit traffic over single-mode fiber laid beneath city streets, under rural cornfields, and strung from telephone poles. Although single-mode fiber has higher bandwidth, multimode fiber supports high data rates at short distances. The smaller core diameter of single-mode fiber also increases the difficulty in coupling sufficient optical power into the fiber. Relaxed tolerances on optical coupling requirements afforded by multimode fiber enable the use of transmitter packaging tolerances that are less precise, thereby allowing lower cost transceivers or lasers. As a result, multimode fiber has dominated in shorter distance and cost sensitive LAN applications.