The role of wavelength division multiplexing (WDM) in telecommunication networks can be expanded well beyond providing high capacity on point-to-point transmission links. WDM can be used to perform network functions such as routing, switching and add/drop multiplexing. The optical add/drop multiplexer (OADM) is a key component for WDM networks. An OADM adds and drops one or more of the signals in a wavelength division multiplex of optical signals without interfering with other channels on the fiber. Different approaches are available for implementing OADMs. These include thin film filters, arrayed waveguide gratings, circulators with fiber Bragg gratings (FBGs), and FBGs in the arms of a Mach-Zehnder interferometer (MZI). Fig. 1 shows configurations of add/drop multiplexers that use fiber Bragg gratings with two different resonant wavelengths'. One configuration is based on an MZI and FBGs, and the other is based on circulators and FBGs. In order to add/drop more than one channel, a multiple number of narrowband gratings are used. Imperfect reflection of the FBGs at the resonant wavelengths will introduce multipath interference from the reflections at discrete points. This is referred to as interferometric noise. 201 and 202 are the selected wavelengths for add/drop and correspond to the resonant wavelengths of the gratings.
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