 J1(Vc) = 0 l = 1 modes Jo(Vc) = 0 bem(Vcc) = (6.1.9) 2.4048 3.8317 Therefore, for wavelengths longer than the cutoff 5.5201 wavelength the mode cannot propagate in the optical 7.0156 8.6537 fiber. Cutoff values for the V number for a few LP modes 10.1735 11.7915 are given in Table 6.1-1. The fundamental mode can, to l=2modes J1(Vc) = 0; Vc#0 l = 3 modes J2(Vc) = 0j; Vc#0 better than 96% accuracy, be described using a Gaussian 3.8317 function 5.1356 7.0156 8.4172 E(r) = (6.1.10) 10.1735 11.6198 13.3237 14.7960 where E0 is the amplitude and 2wg is the mode field diameter (MFD) (Fig. .6.1-3). The meaning of the MFD Reprinted from Ref. [5], p. 380, courtesy of Cambridge University Press. is shown in Fig. 6.1-5. The MFD for the fundamental CHAPTER 6.1 Optical fiber, cable and connectors Table 6.1-2 CCITT recommendation G.652. e-1E0 Parameters Specifications Cladding diameter 125 um Mode field diameter 9-10 um Cutoff 1100-1280 nm wavelength 1 Aco 1550 nm bend loss <1 dB for 100 turns of 7.5 cm diameter Fig. 6.1-5 The electric field of the HE11 mode is transverse and approximately Gaussian. The mode field diameter is determined by Dispersion <3.5 ps/nm . km between 1285 and 1330 nm the points where the power is down by e-2 or where the amplitude <6 ps/nm . km between 1270 and 1340 nm is down by e-1. The MFD is not necessarily the same dimension <20 ps/nm . km at 1550 nm as the core. Reprinted from Ref. [6], p. 144, courtesy of Irwin. Dispersion slope 0.095 ps/nm2 . km 6.1.1.3 Multimode fiber Reprinted from Ref. [2], p. 126, courtesy of Irwin. The previous discussion has in principle been for a step-index MM fiber. Because of the severe differences Instead, a graded refractive index core is used for a MM in propagation time between different modes in a fiber (Fig. 6.1-7). step-index fiber, these are not commonly used in practice. The various graded-index profiles are generated by (6.1.12) n2(1-2A) = n3; r>a for different q's, q is called the profile exponent. The optimum profile is the one that gives the minimum dispers