based, optical step-index fiber exist, as shown in Fig- ure 2-8: multimode and single mode. The transmission properties that are Chapter 2 Figure 2-6 Cladding How light travels in a fiber. Critical Angle Fiber Cladding Angle at which the refracted ray points along the surface (i.e. angle of refraction = 90 degrees) For angles of incidence larger than this critical angle, no refracted ray exists. Phenomenon called total internal reflection OC = sin- (n2/n1) For air/glass, this is 41.8 degrees. Important result: light can be piped from one point to another with little loss. Note: Core/cladding not shown to scale. characteristic of the two types of optical fiber arise from the differing core diameters and refraction-index profiles. Figure 2-8 depicts the typical dimensions and refraction-index profiles, showing the cross-section of the cable along the diameter. Note that both optical fibers have an outer diam- eter of 125 um, but the core size is different. Multimode fibers have various ratios of core-to-cladding diameter. The most common ratio is 62.5:125 um. (Multimode fibers are more typical of local area networks [LANs]; some intraCO wiring is also multimode, but single-mode fiber remains prevalent even in these applications.) Numerical aperture (NA) is a measure of fiber's capability to accept light. Typical fiber with a 50-um core may transmit several hundred modes. The NA of a mul- Optics Primer STEP 1 Figure 2-7 Sintered Glass Silica Pipe Basic fabrication technique. Gasses: SiCl4 POCl3 GeCl4 Reactant H202 Burner Exhaust Soot Deposit STEP 2 Preform Rod Primary Furnace Measurement Primary Coating Furnace Spool Testing timode fiber is defined as the sine of the maximum angle of acceptance for coupling optical power into the fiber: NA = sin a, 2a = Full acceptance angle Chapter 2 Outer Figure 2-8 diameter Outer diameter 10um core Characteristics of 125-um 50-um core 125-um fiber strands. Multimode Single-mode fiber fiber Diameter Diameter n = Index of refraction Nominal NAs are in the range of 0.20 to 0.29 um. 