Using Narrow-Linewidth Lasers for Rapidly Tunable Microwave Signal Generators

Narrow-linewidth lasers are a key component of photonic microwave signal generators, as the width of the generated RF signal is equal to the beat note of used lasers. Heterogeneous silicon photonics platform opens up a possibility of improving the coherence of fully integrated photonic microwave generators by providing means to separate the photon resonator and absorbing active medium; improving the total Q factor of the laser cavity and providing the control of the spontaneous emission into the lasing mode. Further improvement in the laser linewidth is possible by using ring resonators inside the laser cavity. Using the rings inside the cavity benefits the linewidth in two ways: (1) resonance cavity length enhancement and (2) negative optical feedback. The combined effect allows for record linewidth performance as was recently demonstrated: widely-tunable fully monolithically-integrated semiconductor lasers with 50 kHz integrated linewidths. We further theoretically predict that at least an order of magnitude better performance is achievable and that sub-kHz linewidths should be obtainable using low-loss silicon waveguide platform with ~0.5 dB/cm of loss. Heterogeneous platform further complements the microwave signal generator with demonstrated highspeed modulators with 74 GHz bandwidth and detectors with 12 dBm output power at 40 GHz. The InP-based modified uni-traveling carrier photodiodes on SOI waveguides have the highest reported output power levels at multi-GHz frequencies for any waveguide photodiode technology including native InP, Ge/Si, and heterogeneously integrated photodiodes.