Continuously tunable super-efficient microcombs

Microcombs are steadily advancing toward system-level applications. Recent progress in high-Q silicon nitride microresonators and coupled cavities allows for generating coherent microcombs with an optical conversion efficiency approaching unity. The generation of efficient soliton microcombs in this coupled cavity arrangement (photonic molecule) relies on controlling the strength and location of an avoided mode crossing to compensate for the spectral shift introduced by the temporal soliton. While resulting in exceptionally high efficiency and reliability, it is challenging to attain continuous broadband tuning of the pump, limiting deployment in some practical applications. In this work, we demonstrate offset-tunable super-efficient microcombs in photonic molecules featuring low-noise operation and a smooth, constant spectral envelope. This is achieved by thermally tuning in tandem the main and auxiliary cavities, allowing to attain the desired avoided mode crossing across multiple free spectral ranges of the main cavity. Additionally, we analyze the impact of the thermal tuning on the phase noise and repetition rate stability as well as the thermal response of heaters. The demonstrated broadband, post fabrication tunability establishes photonic molecules as a highly flexible and efficient platform for the generation of super-efficient microcombs at arbitrary pump wavelengths, making this configuration a particularly appealing one for applications in metrology and precision spectroscopy.