Narrow bandwidth thermally tunable optical filters with higher order coupled resonator optical waveguides (CROWs) in a post-CMOS compatible silicon nitride platform

We demonstrate a narrow bandwidth thermally tunable coupled resonator optical waveguide (CROW) filters, fabricated in a post-CMOS compatible low temperature plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiN). We design these tunable CROW filters by optimizing the resonator size, and the coupling coefficients of bus-to-ring and ring-to-ring to obtain a narrow-band flat top transmission spectrum with a steep roll-off (box like transmission spectrum). These properties are particularly advantageous for applications such as filtering closely spaced spectral lines in frequency combs to use individual frequency lines as separate carriers in optical transmitters. To achieve optical tunability of the filter for selecting different wavelength inputs and to mitigate variations in spectral responses induced by fabrication imperfections, Ti/TiN heaters are included. These heaters are positioned at an optimal distance from the waveguides to balance the trade-off between excess losses associated with the proximity of the metal to the waveguides and effective thermal tuning efficiency. For a third order CROW filter (consisting of three ring resonators) designed for a free spectral range (FSR) of 55 GHz, we measure a 3 dB bandwidth of approximately 3.5 GHz and achieve an extinction ratio (ER) around 30 dB, closely matching our simulations. We achieve resonance tuning of the filter by one full FSR with 300 mW of electrical power dissipated in each of the three heaters on the three ring resonators.