cted along a predefined linear polarization (the polarization of the only LP fundamen- tal mode of the fiber), this polarization is maintained during propagation [28]. Several different designs of PM fiber are used. Most work by inducing stress in the core via a noncircular cladding cross section, or via rods of another material included within TABLE 4.5 A Few Key Requirements from G.653 G.653 Cladding diameter (um) 125 + 1.0 Core-cladding concentricity (um) Cladding on-circularity (%) Cutoff wavelength in cable (nm) <1270 Power attenuation maximum in cable (dB/km) <0.35 (at 1550 mm) Chromatic dispersion parameter (ps/nm/km) <3.5 at 1550 nm Mode field diameter at 1550 nm (7.8-8.5) + 0.8 D3 (ps/nm2/km) <0.093 Zero dispersion wavelength (nm) 1525 < do 1575 PMD (ps/km / 2 Telecommunication Networks TABLE 4.6 A Few Key Requirements from G.655 G.655 (C) Cladding diameter (um) 125 + 1 Core-cladding concentricity (um) <0.80 Cladding on-circularity (%) Cutoff wavelength in cable (nm) <1450 Power attenuation in cable (dB/km) <0.35 (at 1550 nm) <0.40 (at 1625 nm) Chromatic dispersion parameter <1.0 at 1530 nm (ps/nm/km) <10 at 1565 nm Chromatic dispersion coefficient sign Positive or negative Mode field diameter at 1550 nm (8.0-11) + 0.7 <0.20 the cladding. Several different shapes of rods are used, and the resulting fiber design has the standardized name such as "PANDA" and "Bow-tie," as it is shown in Figure 4.11. There are small differences between different types of PM fiber design, more in the manu- facturing process than in the final product performance. It is to underline that a PM fiber does not polarize light as a polarizer does. Rather, PM fiber maintains the input linear polarization if it is launched into the fiber along the correct direction. If the polarization of the input light is not aligned with the stress direction in the fiber, the output will vary between linear and circular polarization (and generally will be elliptically polarized). Elliptical core Elliptical cladding Bow tie PANDA single polarizatio