Systematic Design and Demonstration of Multipole, Coupled-Cavity Integrated Photonic Bandpass Filters with High FSR, High-Q Single-Mode Microresonators in Low-Loss Silicon Nitride Platform
Tunable, low-loss, narrowband, and frequency-stabilized filters play a critical role in the realization of transceivers for efficient signal processing in telecommunications and sensing applications in the presence of strong interference and noise. We systematically design, fabricate, and demonstrate multi-cavity, multipole integrated photonic bandpass filters with gigahertz to sub-gigahertz bandwidths. The filters feature ultra-wideband tunable center frequencies exceeding 400 GHz, ultra-low insertion loss, steep roll-off, and compact footprints, making them suitable for radio-frequency, microwave, and millimeter-wave front-end applications. Using a set of design principles for multi-cavity high-order filters together with supporting nanofabrication techniques in a silicon nitride platform, the demonstrated filters achieve record-high figures of merit and advance the state of the art in integrated photonic filtering for radio-frequency, microwave, and millimeter-wave systems. The resonator structures, which are the key building blocks enabling the reported performance, combine a large free spectral range of approximately 70 GHz with a high quality factor of 2.1 times ten to the power of seven. This performance is enabled by combining wide multimode waveguide segments with narrow single-mode regions connected through adiabatic tapers. To the best of our knowledge, the experimentally demonstrated bandwidth of 520 MHz, tuning range of one free spectral range, filter insertion loss of 2 dB, and out-of-band rejection ratios of up to 55 dB represent record performance metrics for integrated photonic filtering of radio-frequency, millimeter-wave, and terahertz signals.