Investigating Mode Characteristics of an Ultra-High Q Silicon Nitride Micro-Disk Resonator and High Resolution Microwave Photonic Filtering

A low loss silicon nitride micro-disk resonator with ultra-high intrinsic quality factor up to ∼2.4 × 10⁷ is fabricated and investigated. By adopting the optical vector network analyzing technology, both the amplitude and phase spectra of the WGMs in the resonator were measured with high resolution, which were used to differentiate different coupling states of the WGMs and obtain their intrinsic Q factors. In addition, the well-known mode splitting phenomenon occurs for a series of WGM modes and a threshold of loaded quality factor ∼5 × 10⁶ for mode splitting was estimated statistically according to the measurements. Furthermore, by adopting one of the WGMs, tunable band-pass and band-stop MPFs with narrow bandwidth of 63.1 MHz and 61.1 MHz and high RF rejection ratio of 37.5 dB and 52.1 dB were obtained, respectively. Also frequency tuning from 0.4∼15.7 GHz and 0.1∼16.3 GHz were achieved for the band-pass and band-stop MPF, respectively. In which an unprecedented lowest filtering frequency down to hundreds of MHz has been achieved benefiting from the ultra-high Q WGM of the fabricated low loss Si₃N₄ MDR.