Abstract: A multi-port tunable fiber-optic etalon filter (MTFET) having two spaced partially reflective mirrors, has three or more ports, with at least two on one side and at least one on the other side of the etalon. A single signal can be filtered with the reflected signal being received, forming a wavelength division multiplexor, or a plurality of signals can be filtered, with or without the reflected signals being received. A series of filters can be arranged in a cascade for sequential filtering a series of wave bands. A signal can be processed in two directions sequentially for a higher filter efficiency. A first signal can be monitored and controlled by a second signal. Various other arrangements can be provided. Integral optical processing functions can be incorporated, for example a blocking filter.