ge associated with Raman amplification is the fact that an existing optical fiber can be used as a medium when properly pumped. This means that amplification is not dependent on doping of the fiber and permits the use of a wider range of existing fiber. SIGNAL SWITCHING Another evolving optical area that warrants attention concerns signal switching. Today the switching of an optical signal is primarily an all-or-nothing affair. That is, if Joe Backhoe digs where he shouldn't and breaks a SONET ring, within a few milliseconds the break will be detected and the full optical flow will commence in an opposite direction to provide a restored communications capability. To be truly effective requires a higher level of optical switching. For example, assume that a DWDM fiber is routed from Boston to Washing- ton, DC through New York and Philadelphia. Instead of using optical- to-electrical converters at each city to feed an electronic switch, a far sim- pler mechanism and obviously less expensive one would be to drop wavelengths at the intermediate locations. While some equipment per- mits communications carriers to drop a wavelength, most carriers would like the ability to both drop and add wavelengths at intermediate loca- tions. The key to obtaining this capability is a true optical switch, which would direct signals from any input port to any output port as illustrat- ed in Figure 6.3. Optical cross-connections currently occur using digital electronics, requiring the incoming optical signals to be converted into electrical pulses. One emerging type of optical switch being developed consists of arrays of miniature mirrors that move back and forth to redirect light. This type of system is referred to as a microelectromechanical system (MEMS). A relatively recent MEMS development includes the etching of planar arrays of tiny components to form silicon substrates. Through the etching process mirror elements become suspended on tiny posts Fiber in the WAN Figure 6.3 A true optical switch will direct wave- lengths from wit