re is also a further layer known as the secondary cladding that does not participate in the propagation but gives the fiber a minimum level of protection. This second layer is referred to as a coating. Chapter 5 Optical Fibers Secondary Secondary Cladding Cladding Cladding n1 > n2 Cladding Fig. 5.1 (a) Cross section and (b) longitudinal cross section of a typical optical fiber. Transmitted Ray tr=90° Transmitted n1>n2 Partially Incident Reflected Incident Ocrit Fig. 5.2 Snell's law. The basics of light propagation can be discussed with the use of geometric optics. The basic law of light guidance is Snell's law (Figure 5.2a). Consider two dielectric media with different refractive indices and with N1 > n2 and that are in perfect contact, as shown in Figure 5.1. At the interface between the two dielectrics, the incident and refracted rays satisfy Snell's law of refraction-that is, (5.1) sing1__n2 (5.2) In addition to the refracted ray there is a small amount of reflected light in the medium with refractive index N1. Because n1 >n2 then always O2 >01. As the angle of the incident ray increases there is an angle at which the refracted ray emerges parallel to the interface between the two dielectrics (Figure 5.2(b)). This angle is referred to as the critical angle, Ocrit, and from Snell's law is given by sinderit= (5.3) The Structure and Physics of an Optical Fiber Refractive Index Geometric Axis of the Structure Fig. 5.3 Light guidance using Snell's law. Cladding Acceptance Cone Fiber Core Cladding Fig. 5.4 Geometry for the derivation of the acceptance angle. If the angle of the incident ray is greater than the critical angle, the ray is reflected back into the medium with refractive index N1. This basic idea can be used to propagate a light ray in a structure with N1 > N2, and Figure 5.3 illustrates this idea. The light ray incident at an angle greater than the critical angle can propagate down the waveguide through a series of total reflections at the interface between the two dielec- trics. The ray shown in Figure