Reconfigurable add/drop for optical fiber communication systems

#This invention relates to optical fiber communication systems and, in particular, to reconfigurable add/drop devices for adding or dropping communication channels at intermediate points within such systems.

#Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a light source, a modulator for impressing information on the light, an optical fiber transmission line for carrying the optical signals and a receiver for detecting the signals and demodulating the information they carry. Increasingly the optical signals are wavelength division multiplexed signals (WDM signals) comprising a plurality of distinct wavelength signal channels.

#Add/drop devices are important components of WDM fiber communication systems. Such devices are typically disposed at various intermediate points along the transmission fiber (called nodes) to permit adding or dropping of signal channels at the nodes. Thus, for illustration, an add/drop device would permit a transmission line from New York to Los Angeles to drop off at Chicago signal channels intended for Chicago and to add at Chicago signal channels intended for New York and Los Angeles. As the number of nodes increases, the number of add/drop devices increases, and their cost and effect on the system becomes appreciable.

#The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings. In the drawings:

#FIG. 1 is a schematic diagram of a first embodiment of an add/drop device in accordance with the invention;

#FIGS. 2A and 2B illustrate add/drop filters suitable for use in the device of FIG. 1; and

#It is to be understood that these drawings are for purposes of illustrating the concepts of the invention and are not to scale.

#Referring to the drawings, FIG. 1 illustrates an add/drop device 10 for an N channel WDM optical fiber communication system. The device 10 comprises a 1×M demultiplexer 11 for receiving an N channel WDM signal as from transmission fiber 9A and distributing the N channels among M paths 12 where M and N are integers and M<N. Also, M divides N (i.e. N/M=P where P is an integer). Each of the paths 12 includes P add/drop filters 13, each filter able to add one channel Aij; and drop one channel Dij. The filtered paths are recombined in a M×1 multiplexer 14 for further transmission in the next segment of transmission fiber, e.g. 9B. In this embodiment, the add/drop filters 13 separate the drop from the input.

#In an advantageous embodiment, the demultiplexer 11 is a periodic response demultiplexer such as described in C. Madsen, “A Multiport Band Selector with Inherently Low Loss, Flat Passbands and Low Crosstalk, ” IEEE Photon. Technol. Lett., vol. 10, no. 12, pp. 1766-1768, 1998. If we denote successive paths 12 by a subscript j, the periodic response demultiplexer distributes the N channels among the paths 12i as follows: