Widely Tunable Microwave Photonic Oscillator With a High-Q Silicon Nitride Microring Resonator

A compact microring resonator in all-pass configuration using a low-loss bimodal SiN waveguide design has been demonstrated and subsequently it was successfully used for the demonstration of high-performance tunable microwave photonic oscillator in dual-loop configuration. The microring resonator design was optimized through a detailed theoretical analysis (for multimode coupling interactions) such that its Q-value can be maximized for any given fabrication process parameters (CMOS fabrication process compatible). We have recorded an experimental Q-value as high as$\sim 2\times 10^{6}$from such a device design by exciting fundamental mode into the bimodal bus and ring waveguides. Our experimental results also show that an equivalent device design with a singlemode waveguide and fabricated with same fabrication process technology exhibit nearly 7 times lower Q-value ($\sim 2.5\times 10^{5})$with an FSR of$\sim$120 GHz. The cavity loss of the fundamental guided mode in a bimodal waveguide is reduced to$\sim$0.1 dB/cm due to reduced scattering from fabrication-induced sidewall roughness and low bend-induced radiation losses. These results are matching well with our theoretical predictions. Such a device has been used further to demonstrate a widely tunable narrow-bandpass microwave photonic filter. The experimental results exhibit a narrowband filter with a 3-dB bandwidth as low as 290 MHz with broad tunability from 5-40 GHz and the side-band rejection of$\sim$30 dB has been recorded. Furthermore, the device was used to demonstrate a widely tunable microwave oscillator (up to 20 GHz) with a signal-to-sidemode suppression ratio of$\sim$70 dB and phase noise of -125 dBc/Hz at 10 kHz offset. The typical 3-dB bandwidth of the generated microwave signal (at 10 GHz) is$\sim$4 kHz. Though both filter and oscillator can be tuned up to$\sim$60 GHz, we are limited in characterizing it due to the phase modulator bandwidth (40 GHz) and the RF amplifier bandwidth (20 GHz). The experimental results closely matches with the theoretical model prediction.