Planar Optical Add/ Drop multiplexer

The capacity of optical networks can be increased significantly by propagating several wavelengths simultaneously through the optical fibres. An Optical Add/Drop Multiplexer (OADM) is one of the key components in such multi-wavelength transmission systems because of its ability to extract or add a certain wavelength at a node of the network.
One of the most promising designs of an OADM device is based on a Mach-Zehnder interferometer with Bragg gratings written in the arms.
A Mach-Zehnder interferometer is a four-port element made of two identical 3dB-couplers linked by straight waveguides. The glass deposited on the silicon is photosensitive. It means that when exposed to UV-light the refractive index of the glass increases. When positioning a phase mask between the UV-beam and the waveguide, an interference pattern can be generated resulting in the inscription of Bragg gratings in the arms of the Mach-Zehnder interferometer. These gratings reflect one wavelength and are transparent to all the other wavelengths.
Several wavelengths (usually 4) enter the device at the input port. They all exit the Mach-Zehnder interferometer at the output port except the one which is reflected by the Bragg gratings. This wavelength exit the device at the drop port. An identical wavelength, but carrying different information, may be inserted at the add port and will exit the component together with the other wavelengths.