Resonant coupler-based tunable add-after-drop filter in silicon-oxynitridetechnology 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 oper ates in the 1550-nm window with a wavelength spacing of 1.6 nm (200 GHz) an d a periodicity of 6.4 nm (800 GHz), The splitting of the add and drop func tions in an add-after-drop configuration results in extremely low homowavel ength crosstalk and allows the individual design of the add and drop subfun ctions to be optimized, The design of the flat-passband drop function is pr esented in detail. The compact filter is realized in high-index-contrast si licon-oxynitride technology, which permits small bending radii of 1.5 mm, P hase errors in the delay lines of the RC devices are compensated using the thermooptic effect, which is also used to tune the device, Measurement resu lts on packaged devices show fiber-to-fiber losses of between 5 and 7 dB. T he isolation of the transit WDM channels to the drop port is greater than 2 7 dB, and the rejection of the drop channel is 33 dB, The components are po larization insensitive and have been integrated in an add-drop WDM subsyste m.