Silicon micro-ring resonators with tunable Q-factor for ultra-low power parametric signal generation

Micro-ring resonators in silicon photonic circuits have been widely studied and implemented in systems for biosensing [1], telecommunications [2] and generation of photon pairs for quantum optics experiments [3]. The resonator Q-factor, defined by the roundtrip loss and coupling coefficient, is typically a static property of the device realised in the design stage. In this work a method by which to exert active control over both the resonator Q-factor and resonant wavelengths is presented, allowing modulation of resonance enhanced processes such as Four Wave Mixing (FWM) in devices with much smaller footprint than Mach-Zehnder based designs. Low loss waveguide technology and optimised fabrication processes have allowed resonators with loaded Q-factors in the 105 range with ring diameters of only a few tens of microns [4,5]. However, the critical coupling condition in high Q resonators exhibits a high sensitivity to group index, losses and coupling coefficient variations. The tight optical confinement to the silicon waveguide core also leads to a strong dependence of these parameters on the fabricated waveguide geometry. Fabrication variations of only a few percent in waveguide dimensions can detune the resonator from its designed condition. A relatively simple method by which microring resonant wavelengths and Q-factors can be tuned is presented in this work. Two resistive heater elements are applied over the uppercladding of the silicon resonator. One heater is situated colinearly to the ring and can exert control, through the thermo-optic effect, on the optical path length of the resonator and hence its resonant wavelengths. The second heater was designed as an asymmetrically placed strip, co-propagating with the evanescent coupler section. The thermal gradient induced across the two waveguides of the evanescent field coupler produces a detuning of their effective refractive index difference, tuning the power coupled into the ring, and th- refore its Q-factor. An optical micrograph of the device is shown in Fig1.(a). Using these two controls, resonators were realised that showed tuning of their coupling coefficient between 0.7 and 0.1, leading to a Q-factor tuning between 2x103 and 2x104 as shown in Fig.1(b). By tuning an optimised coupled ring to its critical coupling point, a resonant Q-factor of 7.3x104 was achieved. FWM experiments, Fig.1(c), were carried out and a conversion efficiency of -20dB was measured with an input power on-chip of only 0.2mW.