Resonant coupler-based tunable add after-drop filter in silicon-oxynitride technology for WDM networks

We present a tunable add-drop filter for wavelength division multiplexing (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 homowavelength 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 thermooptic 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.