Optical Communications, 2nd Edition

7.1 FIBER POWER FLOW AND DISPERSION

In Section 1.5, it was stated that the optical fields that were launched into fibers cores propagated at ray line angles θ (Figure 7.1) within the critical angle θ_p determined by the fiber indices of refraction. This critical angle was given as

where n₁ and n₂ are the indices of the core and cladding. A closely related parameter is the numerical aperature, N A, that could be written in any of the equivalent forms,

where Δ = (n₁ − n₂)/n₁ is the fractional index difference. The number of field modes within the fiber was given in Eq. (1.5.8) as

Figure 7.1. Fiber construction with field flow.

where d is the core diameter. A single mode fiber occurs only if θ_p 0, or if Eq. (7.1.3) is equal to or smaller than one, which places a limit on the core size. Otherwise, it is a multimode fiber.

The power P_f coupled into a fiber depends on the coupling mismatch losses between the emitting area and angles of the source and the fiber core, as in Eq. (1.5.12). The ...