Wavelength tunable optical add-after-drop filter with flat passband for WDM networks

A tunable optical add-after-drop filter is presented with add and drop func tions optimized individually. The add function exhibits a conventional pass band, whereas the drop passband is flattened (less than 0.1-dB ripple over more than 0.5 nm), The design is based on the resonant coupler principle. T he free spectral range is 6.4 nm and the channel spacing 1.6 nm, The device is realized in a newly developed high-refractive-index-contrast SiON techn ology that permits small bending radii of 1.5 mm. Thermooptic heaters are u sed to tailor and tune the filter characteristic. The on-chip losses are ab out 3.5 dB, the wavelength channel isolation is better than 27 dB, and the TE-TM polarization shift is smaller than 0.1 nm.