Fiber optic cables are the medium of choice in telecommunications infrastructure, enabling the transmission of high-speed voice, video, and data traffic in enterprise and service provider networks. Depending on the type of application and the reach to be achieved, various types of fiber may be considered and deployed, such as single mode fiber type and multimode duplex fiber.
Fiber optic cables come in several different configurations, each ideally suited to a different use or application. Early fiber designs that are still used today include single mode fiber type and multimode fiber. Since Bell Laboratories invented the concept of application-specific fibers in the mid-1990s, fiber designs for specific network applications have been introduced. These new fiber designs – used primarily for the transmission of communication signals – include Non-Zero Dispersion Fiber (NZDF), Zero Water Peak Fiber (ZWPF), 10-Gbps laser optimized multimode fiber, and fibers designed specifically for submarine applications. Specialty fiber designs, such as dispersion compensating fibers and erbium doped fibers, perform functions that complement the transmission fibers. The differences among the different transmission fiber types result in variations in the range and the number of different wavelengths or channels at which the light is transmitted or received, the distances those signals can travel without being regenerated or amplified, and the speeds at which those signals can travel.
There are two different types of fiber optic cable: multimode and single mode fiber type (MMF and SMF). 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. The “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 duplex fiber have a much smaller core diameter than multimode duplex fiber optic cable. 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 type, 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 have 125 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 type, 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 duplex fiber has higher bandwidth, multimode fiber supports high data rates at short distances. The smaller core diameter of single mode duplex 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 duplex fiber optic cable has dominated in shorter distance and cost sensitive LAN applications.