Single-mode fiber allows for a higher capacity to transmit information because it can retain the fidelity of each light pulse over longer distances, and exhibits no dispersion caused by multiple modes. Single-mode fiber also enjoys lower fiber attenuation than multimode fiber. Thus, more information can be transmitted per unit of time. Similar to multimode fiber, early single-mode fiber was generally characterized as step-index fiber, meaning that the refractive index of the fiber core is a step above that of the cladding, rather than graduated as it is in graded-index fiber. Modern single-mode fibers have evolved into more complex designs such as matched clad, depressed clad, and other exotic structures [2].

Single-mode fiber has some disadvantages. The smaller core diameter makes coupling light into the core more difficult (see Fig. 4.5) [2]. The tolerances for singlemode connectors and splices are also much more demanding.

Single-mode fiber has gone through a continuing evolution for several decades now. As a result, there are three basic classes of single-mode fiber used in modern telecommunications systems. The oldest and most widely deployed type is nondispersion- shifted fiber (NDSF). These fibers were initially intended for use near 1310 nm. Later, 1550-nm systems made NDSF undesirable due to its very high dispersion at the 1550-nm wavelength. To address this shortcoming, fiber manufacturers developed dispersion-shifted fiber (DSF), which moved the zero-dispersion point to the 1550-nm region. Years later, scientists discovered that while DSF worked extremely well with a single 1550-nm wavelength, it exhibits serious nonlinearities when multiple, closely spaced wavelengths in the 1550-nm wavelength were transmitted in DWDM systems. Recently, to address the problem of nonlinearities, a new class of fibers was introduced, the non-zero-dispersion-shifted fibers (NZ-DSF). The fiber is available in both positive and negative dispersion varieties and is rapidly becoming the fiber of choice in new fiber deployment. See [2] for more information on this loss mechanism.

Figure 4.6 Cross section of PM fiber.

One additional important variety of single-mode fiber is polarization-maintaining (PM) fiber (see Fig. 4.6) [2]. All other single-mode fibers discussed so far have been capable of carrying randomly polarized light. PM fiber is designed to propagate only one polarization of the input light. This is important for components such as external modulators that require a polarized light input.

Finally, the cross section of a type of PM fiber is shown in Figure 4.6 [2]. This fiber contains a feature not seen in other fiber types. Besides the core, there are two additional circles called stress rods. As their name implies, these stress rods create stress in the core of the fiber such that the transmission of only one polarization plane of light is favored [2].²

2. Single-mode fibers experience nonlinearities that can greatly affect system performance.