ntramodal distortion is manifested in two ways: 1. As material dispersion resulting from slightly different indices of refraction that fiber presents to different wavelengths of light 2. Waveguide dispersion resulting from a longer path taken by longer wavelengths that reflect at more oblique angles with cladding Chapter 2 The laser was invited in the 1960s, and the concept was refined in the next Figure 2-11 few years. Some fiber optics history. Transmission through optical fiber was proposed in the literature in 1966. Low-loss optical fiber was announced in 1970. It was silica-based and it had an attenuation of 20 dB/km. By 1986, the attenuation had been reduced to 0.154 dB/km. This is close to the theoretical minimum for silica, this being 0.13 dB/km at 1.55 um. (dB/km) Silica Glass Multimode Silica Glass Single-mode 0.154 dB/km Progress of optical fiber in terms of loss, over the years 2.1.3 Intermodal Delay Distortion In simple step-index multimode fiber (which, as noted, has a large uniform core), rays of light propagate through the core at different angles. Rays that go straight through have a shorter path than those with large angles, which bounce off the inside wall of the core; hence, straight rays traverse the fiber faster. Step-index multimode fiber enables the transmission of any light ray that enters optical fiber within an angle of acceptance. High-order modes Optics Primer travel over a longer distance than low-order modes, and therefore they arrive out of phase at any given point along the fiber. This causes inter- modal delay distortion and thus limits the bandwidth capacity of the fiber. Low-order modes, such as those launched into fiber at small angles with respect to the axis, travel a shorter ray distance to reach a given point along the fiber than the distance traveled by high-order modes launched at large angles. Intermodal distortion constricts the bandwidth of the cable because waves that start at the same instant arrive out of phase at the remote end, causing distortion. The problem can