Wavelength-Tunable Add/Drop Filter for Optical Networks

A wavelength-tunable optical add/drop filter with four channels 1.6 nm apart (corresponding to 200 GHz) for wavelength division multiplexing applications at 1550 nm was realized. The filter is based on a resonant coupler design in a so-called add-after-drop configuration with separate subdevices for the add and drop functions, thereby minimizing the homo-wavelength crosstalk between the add and drop ports. The device was realized using a novel high-refractive-index-contrast planar SiON waveguide technology enabling low-loss bends with a radius of curvature of only 1.5 mm. The device can be tuned over the full free spectral range of 6.4 nm by using the thermo-optic effect. The on-chip losses of the 75-mm-long filter are about -3.5 dB. The wavelength channel isolation is better than 27 dB and the polarization dependence is negligible with measured values for the TE-TM shift of less than 0.1 nm.