dices of refraction: with respect to vacuum At 1 = 5890 A Water 1.00033 Dense glass design, the index of refraction changes abruptly at the core/cladding bound- ary, the resulting fiber is called step-index fiber. There is also a fiber type that has a gradual change in the index across the cross-section of the fiber; this is called graded-index fiber. An optical fiber is manufactured by first fabricating a preform and then performing a process called drawing. The fabrication of the preform uses one of three standard processes. Lucent uses modified chemical vapor depo- sition (MCVD), which is also called inside chemical vapor deposition (ICVD). Corning uses outside chemical vapor deposition (OCVD), and NTT suppliers use a vapor axial deposition (VAD) process. The fiber is drawn by lowering the preform rod into an oven at a controlled rate; eventually, the preform reaches a high enough temperature to soften, enabling the fiber to be pulled to a specified diameter. The diameter depends on the rate of rota- Optics Primer Figure 2-5 Index of Refraction Fiber refractive index profiles. (a) Graded-index fiber, multimode (b) Step index fiber, multimode A, n2-n3 n1 n3 Core diameter Cladding diameter Cladding diameter (c) Matched-cladding fiber, single-mode (d) Depressed-cladding fiber, single-mode Claddings (e) Depressed-cladding triangular-core fiber, (f) Quadruple-cladding triangular-core fiber, single-mode single-mode tion of the take-up drum and the rate of feed of the preform into the oven (see Figure 2-7). To form cables, individual fiber strands are placed into slots, tubes, bun- dles, or ribbons. Several layers of protection may be used in the cable depending on the application. Four major cable structures exist: the slotted core or channel cable, the ribbon cable, the loose tube cable, and the tightly bound cable. In the channel or the loose tube design, one or more fibers are placed in an insulating channel or buffer tube. In the ribbon design, several fibers are combined into a flat ribbon. Two types of silica-