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 functions optimized individually. The add function exhibits a conventional passband, 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. The 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 technology that permits small bending radii of 1.5 mm. Thermo-optic heaters are used to tailor and tune the filter characteristic. The on-chip losses are about 3.5 dB, the wavelength channel isolation is better than 27 dB, and the TE-TM polarization shift is smaller than 0.1 nm.