UC Santa Barbara

The development of narrowband resonators has far reaching applications in integrated optics. As a precise reference of wavelength, filters can be used in sensors, metrology, nonlinear optics, microwave photonics, and laser stabilization. In this work, we develop record high quality factor (Q) Si3N4 waveguide resonators, and utilize them to stabilize a heterogeneously integrated Si/III V laser.

To increase the Q factor of waveguide resonators, particular attention is given to loss mechanisms. Propagation loss of <0.1 dB/m is demonstrated on the ultra low loss waveguide platform, a low index contrast, high aspect ratio Si3N4 waveguide geometry fabricated with high quality materials and high temperature anneals. Ideality in the directional couplers used for coupling to the resonators is studied and losses are reduced such that 81 million intrinsic Q factor is achieved. Additional results include 1×16 resonant splitters, low κ narrowband gratings, and a dual layer waveguide technology for low loss and low bend radius in separate regions of the same device layer.

We then combine an ultra high Q resonator and a heterogeneous Si/III V laser in a Pound Drever Hall (PDH) frequency stabilization system to yield narrow linewidth characteristics for a stable on chip laser reference. The high frequency noise filtering is performed with Si resonant mirrors in the laser cavity. A 30 million Q factor Si3N4 resonator is used with electrical feedback to reduce close in noise and frequency walk off. The laser shows high frequency noise levels of 60×10^3 Hz^2/Hz corresponding to 160 kHz linewidth, and the low frequency noise is suppressed 33 dB to 10^3 Hz^2/Hz with the PDH system.