Using the same principle as used for passive optical filter, we can make a an active optical filter.
The principle is similar to that of the optical switch except now the coupling is wavelength dependent. So if we design a coupler such that when no voltage is applied to the device, the coupler length is even multiple of coupling lengths at both wavelengths, whereas when the voltage is applied, the coupler length becomes even multiple of the coupling length of one wavelength but odd multiple of coupling length for the other wavelength. The two wavelengths will then appear at the two ports at the output.
