h waveguide is a different length; this enables the light signals to travel at different speeds across the AWG. This difference in speed gives rise to inter- ference and diffraction when the signals enter the receive coupler. Diffrac- tion separates the wavelengths and enables the individual light signals to exit the AWG at the same time, but on different fibers. These fibers are then connected to receive-SONET terminals. As you might expect, the opposite process maps the signal from multiple transmit-SONET terminals onto a single fiber. Using AWGs as an alternative to thin-film filters and FBGs reduces the overall cost and insertion loss of the network. Therefore, AWGs have become the technology of choice for high-channel-count DWDM systems. Only one grating is needed to meet the channel count and spacing require- Optics Primer ment of the network, whereas thin-film filters require interleaving tech- nology and additional filters to meet higher-channel-count needs. 2.9.4 Regenerators and Optical Amplifiers As we've mentioned repeatedly, the signal attenuates as it travels through the fiber waveguide. Mechanisms to restore the signal to its pristine condi- tion must be put in place, particularly for long-haul applications. The fol- lowing list defines key elements in this process. Figure 2-42 displays the key functions of a regenerator. SONET regenerator a) Full duplex (dual simplex). b) Operates on one wavelength in each direction. c) Input wavelength equals the output wavelength in each direction. d) Forward wavelength may or may not equal the reverse wavelength. e) Couples SONET Sections within a Line. f) Bufferless. Pipeline fixed delay. Transmit clock is the recovered receive clock. g) Examines and/or writes SONET section overhead. h) Payload, Line, and Path information are passed through unmodified. i) Unscrambles and rescrambles to support item g. j) Keeps output Section active even if input Section fails (fault isolation). k) Reamplify, reshape, and retime (3R function). 1) Comprises section-terminating e