OADM devices

The basic function of an Optical Add/Drop Multiplexer (OADM) device is to add and drop one wavelength in a multi-wavelength network. All the wavelengths enter the input port. They exit the output port except the one which exits at the drop port. The same wavelength, but carrying another signal, may then enter the add port and exits the output port. Hence the signal carried by one channel is dropped and a new channel can be added to the network.
Gratings define the filter function of the OADM device. Using an excimer laser 10 mm long Bragg gratings were simultaneously written in both arms of a Mach-Zehnder interferometer. After fabrication, part of the signal at the Bragg wavelength is back-reflected to the input port indicating that the signals reflected by the two gratings are experiencing different optical paths. This phase difference can be cancelled by UV-trimming i.e. by inducing a permanent increase of the refractive index in one arm of the Mach-Zehnder interferometer. UV trimming is realised with a continuous wave frequency doubled argon ion laser delivering 244 nm light. The back-reflected light at the input port is recorded while trimming. The tuning is stopped when minimum power is reached. In order to keep the phase balance, another UV-trimming is realised on the other arm of the Mach-Zehnder interferometer.
MIC has fabricated and characterised two fibre pigtailed OADM prototypes for the EU project METON. For 400 GHz spacing applications, the isolation of the adjacent channel is better than 30 dB. The transmission dip is better than 50 dB, representing a world record for OADM devices! Its strength determines the isolation between the drop and the add channels, the so-called intra-channel crosstalk. The insertion loss is below 2 dB.