Patent ID: 8340523

Claim:
A tunable optical filter for selecting any single optical frequency channel from a plurality of equidistantly spaced optical frequency channels of an optical signal, each said optical frequency channel having a central frequency, the filter comprising: a plurality of sequentially coupled tunable Mach-Zehnder (MZ) interferometers each having a plurality of equidistantly spaced conterminous frequency passbands and frequency stopbands, wherein the MZ interferometers are tunable so as to have one passband of each MZ interferometer centered on the central frequency of the single frequency channel being selected, and to have at least one of the stopbands of the MZ interferometers centered on the central frequency of each remaining optical frequency channel of the optical signal, so as to suppress each said remaining optical frequency channel of the optical signal, wherein the central frequencies of the optical frequency channels are substantially at an ITU frequency grid having a frequency spacing of Δf ITU , and wherein each MZ interferometer has a free spectral range (FSR), wherein, for a sub-group of M of the plurality of the MZ interferometers, the FSR of an m th MZ interferometer of the sub-group is FSR m =(2 m-1 )*Δf ITU , wherein m=1 . . . M, and M is an integer ≧2, wherein the tunable optical filter further comprises a switchable stage serially coupled to the plurality of the tunable MZ interferometers, the switchable stage having: a 1×2 optical switch having an input port for inputting the optical signal, and first and second output ports; a 2×1 optical switch having first and second input ports, and an output port for outputting the optical signal; and at least one additional tunable MZ interferometer having an input port and an output port; wherein the first output port of the 1×2 optical switch is coupled to the first input port of the 2×1 optical switch; wherein the second output port of the 1×2 optical switch is coupled to the input port of the at least one additional MZ interferometer, and wherein the output port of the at least one additional tunable MZ interferometer is coupled to the second input port of the 2×1 optical switch; whereby the at least one additional tunable MZ interferometer is switchable in and out of an optical path of the optical signal.