68 CHAPTER 2. OPTICAL FIBERS
Figure 2.20: Several index profiles used in the design of single-mode fibers. Upper and lower
rows correspond to standard and dispersion-shifted fibers, respectively.
the cladding. The major design issues are related to the refractive-index profile, the
amount of dopants, and the core and cladding dimensions [82]–[86]. The diameter of
the outermost cladding layer has the standard value of 125μm for all communication-
grade fibers.
Figure 2.20 shows typical index profiles that have been used for different kinds of
fibers. The top row corresponds to standard fibers which are designed to have minimum
dispersion near 1.3μm with a cutoff wavelength in the range 1.1–1.2μm. The simplest
design [Fig. 2.20(a)] consists of a pure-silica cladding and a core doped with GeO 2 to
obtain∆≈ 3 × 10 −^3. A commonly used variation [Fig. 2.20(b)] lowers the cladding
index over a region adjacent to the core by doping it with fluorine. It is also possible to
have an undoped core by using a design shown in Fig 2.20(c). The fibers of this kind
are referred to as doubly clad ordepressed-cladding fibers[82]. They are also called
W fibers, reflecting the shape of the index profile. The bottom row in Fig. 2.20 shows
three index profiles used for dispersion-shifted fibers for which the zero-dispersion
wavelength is chosen in the range 1.45–1.60μm (see Table 2.1). A triangular index
profile with a depressed or raised cladding is often used for this purpose [83]–[85]. The
refractive indices and the thickness of different layers are optimized to design a fiber
with desirable dispersion characteristics [86]. Sometimes as many as four cladding
layers are used for dispersion-flattened fibers (see Fig. 2.11).
2.7.2 Fabrication Methods
Fabrication of telecommunication-grade silica fibers involves two stages. In the first
stage a vapor-deposition method is used to make acylindrical preformwith the desired
refractive-index profile. The preform is typically 1 m long and 2 cm in diameter and
contains core and cladding layers with correct relative dimensions. In the second stage,
the preform is drawn into a fiber by using a precision-feed mechanism that feeds the
preform into a furnace at the proper speed.