Abstract: A plug-in, optical add/drop (OAD) device includes a filter for adding and dropping a light beam of wavelength (?k) from a combined light beam having individual beams of different wavelengths (?1 . . . n). Several devices can be selectively integrated to create an optical communications system that will handle “n” different optical communications circuits. In operation, the devices are individually connected (plugged-in) to a GBIC, and they are serially connected (plugged-in) to each other. Consequently, immediately upstream from a device, the combined light beam includes wavelengths (?1 through ?n). On the other hand, the combined light beam immediately downstream includes (?1 through ?k?1 and ?k+1 through ?n).