Optical Networking Best Practices Handbook

4.2. THE FIBER-OPTIC CABLE MODES

The two distinct types of fiber-optic strands are the single- (single path) and multimode (multiple paths). The practical differences between these two cable types depend on the light source used to send light down the fiber core (see Table 4.1) [1].

Figure 4.1. Fiber-optic cable construction.

Table 4.1. Multimode Versus Single Mode.

Multimode Fiber	Single-Mode Fiber
62.5+ μm in core diameter	8.3 μm in core diameter
Generally uses cheap light-emitting diode light source	Utilizes expensive laser light
Multiple paths used by light	Light travels in a single path down the core
Short distances, <5 miles	Long distances, >5 miles
Power distributed in 100% of the fiber core and into the cladding	Power in the center of the fiber core only

4.2.1. The Single Mode

The light source of the single-mode fiber is laser light that travels in a straight path down the narrow core, which makes it ideal for long-distance transmission; also the core size is so small that bouncing of light waves is almost eliminated. A single-mode cable is a single strand of glass fiber, which is about 8.3–10 μm in diameter and has only one mode of transmission [1].

When a bright monochromatic light is sent down the core of a fiber, the light attempts to travel in a straight line. However, the fiber is often bent or curved, so straight lines are not always possible. As the fiber bends, the light ...