ABSTRACT
Add/Drop filter is an optical wavelength filter that can extract one or a few wavelengths from busline waveguide and can also merge the same wavelength to busline waveguide. Basically, a 2 × 2 port device configuration is needed, that is, input port, drop port, through port, and add port. The wavelength intended to be extracted is transmitted to the drop port and other wavelengths pass straight through to the through port. In addition, the same wavelength can be incident on the add port and is transmitted to the through port, i.e. the wavelength is added to other wavelengths. Microring resonator filter and acousto-optic filter are categorized as Add/Drop filter, which should be distinguished from wavelength multi/demultiplexers, such as arrayed waveguide grating (AWG) demultiplexer. A few 1 × 1 port devices such as the fiber Bragg grating and multilayer thin film filter, can also be used as an Add/Drop filter by attaching circulators to the input and output ports, or by incorporating the grating into two arms of Mach–Zehnder interferometer.
