Ultrabroadband vernier cascaded microring filters based on dispersive Si3N4 waveguides

Abstract

A viable candidate for use in silicon photonics and microwave photonics is the hybrid external cavity laser (ECL) chip, which offers a high extinction ratio, ultrabroadband mode-hopping-free tuning range, and small linewidth. It requires a photonic filter device with an ultrabroadband operating bandwidth and adjustable frequency selection capability. The Vernier cascaded microring filter is a prevailing filter technique that usually ignores chromatic dispersion and will result in noticeable frequency variations, particularly over a large frequency range. Based on dispersive Si3N4 waveguides, we develop ultrabroadband Vernier cascaded microring filters and examine the impact of chromatic dispersion. For the same waveguide geometry, the filter’s effective free spectral range (FSR) varies by more than 400 GHz with and without the dispersion. Furthermore, these Vernier filters, which are made of anomalous and normal dispersive waveguides respectively, exhibit mode hopping at the opposite frequency side. It leads to a sudden mode number leap and, consequently, a diversified dispersion condition for the convoluted filtering frequency. We show that this phenomenon is caused by the interplay between half of the FSR difference, and the accumulated frequency difference caused by the chromatic dispersion. Finally, the use of thermal-optical tuning enables accurate frequency tuning. Our findings offer a valuable resource for the engineering of hybrid ECLs at the chip scale.