Tunable optical filters - US 5701371 A
The tunable optical filter comprises a pair of optical switches and an arrayed-waveguide grating router that has sixteen input waveguides and sixteen output waveguides. The frequency-division-multiplexed optical signal is directed through the first 1×4 optical switch to a selected input waveguide of the router so as to be demultiplexed into output waveguides of the router. The second 1×4 optical switch selects one of four output waveguides of the router so as to obtain a selected optical frequency. By appropriately connecting the switches with the input and output waveguides of the router, sixteen combinations of switch states correspond respectively to sixteen frequencies to be passed by the tunable optical filter. As a result, the filter can be tuned digitally to one of sixteen frequencies with small-size (1×4) optical switches. It is also possible to reduce the loss in the tunable optical filter because the optical signal is only required to make a single passage through the arrayed-waveguide grating router.