Resonant Coupler-Based Tunable Add-after-Drop Filter in Silicon-Oxynitride Technology for WDM Networks

We present a tunable add/drop filter for WDM networks based on the resonant coupler (RC) principle. The device operates in the 1550 nm window with a wavelength spacing of 1.6 nm (200 GHz) and a periodicity of 6.4 nm (800 GHz). The splitting of the add and drop functions in an add-after-drop configuration results in extremely low homo-wavelength crosstalk and allows the individual design of the add and drop subfunctions to be optimized. The design of the flat-passband drop function is presented in detail. The compact filter is realized in high-index-contrast silicon-oxynitride technology, which permits small bending radii of 1.5 mm. Phase errors in the delay lines of the RC devices are compensated using the thermo-optic effect, which is also used to tune the device. Measurement results on packaged devices show fiber-to-fiber losses of between 5 and 7 dB. The isolation of the
transit WDM channels to the drop port is greater than 27 dB, and the rejection of the drop channel is 33 dB. The components are polarization-insensitive and have been integrated in an add/drop WDM
subsystem.