ong the waveguide, that is, along the fiber strand. Energy is lost due to impurities, imperfections, and physical phenomena. Figure 2-11 traces the improvement of attenuation figures of merit over the years. 2.1.2 Dispersion Phenomena What are the distortion issues? Light not only becomes weaker when it goes through a long segment of fiber, but individual pulses of light in a digital signal may also become broadened or blurred, so that they overlap. Both effects make a signal more difficult to decipher at the remote end; in turn, Optics Primer Fiber- Fiber- Figure 2-10 distributing distributing Losses and design frame frame issues for fiber links. Regenerator Regenerator DC PWR DC PWR Jumper Connector Splice Connector Jumper cable loss losses cable loss Transmitter Fiber Receiver Pulse rate Loss of single- PIN or APD sensitivity LED or laser or multimode Pulse broadening Light power and Bending loss Error rate wavelength Mode field diameter Emitted spectra cutoff Chirp Dispersion this results in a higher bit error rate (BER). The principle cause of wave- form distortion is dispersion. Optical fibers are assigned dispersion ratings expressed either in bandwidth in megahertz per kilometer or in time spread in picoseconds per kilometer. In the latter case, the dispersion of a fiber is measured in picoseconds of pulse spreading per kilometer of fiber per nanometer of source spectral width (ps/km/nm). Bandwidth is limited by the amount of pulse spreading and the response of source and detector. The total dispersion (pulse spreading or broadening) and the related fiber bandwidth are characterized by two key effects: modal dispersion (also called intermode or multimode dispersion) for multimode transmission sys- tems and chromatic dispersion (also called intramode or spectral disper- sion) for single-mode transmission systems. When a very short pulse of light is injected into an optical fiber, the opti- cal energy does not all reach the far end at the same time. This causes the exit pulse to be broadened or dispersed. I