es (OADMs) are used to deliver signals to their proper destinations. OADMs can add or drop an individual wavelength or a set of wavelengths without electronic conversion. They can consist of a preset fixed wavelength or be dynamically configurable for one or more wavelengths (see Figure 2-49). The filtering effect is achieved via a number of technologies, notably thin-film and FBG technology, as shown in Figure 2-50. One factor most often associated with OADMs, however, is loss. Circulators access or trans- port (add or drop) signals of different wavelengths traveling on the fiber; these are passive devices that guide wavelengths from port to port in one direction only. The purpose of circulators is to guide the wavelengths that are filtered in and out of the OADM without interrupting any of the light signals. For example, in Figure 2-51, the input light signal is carrying mul- tiple (11, A2, A3, 1x) channels. As it moves through the OADM, FBG tech- nology and the circulators enable channel 1x to be dropped and channel My to be added. OADMs bring planners a step closer to the all-optical network. They're useful network elements in the context of metro networks. OADMs placed Optics Primer Figure 2-49 Fixed/configurable OADMs. Transmission Thin film add/drop filters Fiber Bragg gratings (FBG) + circulator Mach-Zehnder + FBG (a) Fixed-A OADM Transmission Fixed add/drop (mux/demux, thin-film, FBG...) coupled with switches (opto-mechanical, thermo-optic, MEMS,...) Thermally-tunable fiber Bragg gratings Acousto-optic switches Liquid crystal switching Planar lightwave circuits (b) Dynamic (Configurable) OADM Amplifier with midstage access Figure 2-50 Optical Optical Recirculator OADM technologies Recirculator X1 12. .13...14 12...13...14 13...14 (example). Input Output Input Output IIIIIII fiber fiber fiber Fiber fiber Grating Local Local fiber fiber Low loss Higher loss for low channel counts Service disruptive when adding/changing 1 Service disruptive when adding/changing 1 (a) Fixed thin-film OADM (b) Fiber gratings 