ransmission systems and takes on many forms depending on the specific application within which it will be used. Most single-mode fiber has an extremely narrow core diameter, on the order of seven to nine microns, and a cladding diameter of 125 microns. The advantage of this design is that it only allows a single mode to prop- agate; the downside, however, is the difficulty involved in working with it. The core must be coupled directly to the light source and the receiver in order to make the system as effective as possible; given that the core is approximately one-sixth the diameter of a human hair, the mechanical process through which this coupling takes place becomes Herculean. Single-Mode Fiber Designs The reader will recall that we spent a considerable amount of time dis- cussing the many different forms of transmission impairments (nonlin- earities) that challenge optical systems. Loss and dispersion are the key contributing factors in most cases and do, in fact, cause serious problems in high-speed systems. The good news is that optical engineers have done yeoman's work creating a wide variety of single-mode fibers that address most of the nonlinearities. Since its introduction in the early 1980s, single-mode fiber has under- gone a series of evolutionary phases in concert with the changing demands of the bandwidth marketplace. The first variety of single-mode fiber to enter the market was called nondispersion-shifted fiber (NDSF). Designed to operate in the 1,310 nm second window, dispersion in these fibers was close to zero at that wavelength. As a result, it offered high bandwidth and low dispersion. Unfortunately, it was soon the victim of its own success. As demand for high-bandwidth transport grew, a third window was created at 1,550 nm for single-mode fiber transmission. It provided attenuation levels that were less than half those measured at 1,310 nm, but unfortunately was plagued with significant dispersion. Since the bulk of all installed fiber was NDSF, the only solution available to transmissio