AutomaticWavelength Tuning of Series-Coupled Vernier Racetrack Resonators on SOI

Abstract

Using in-resonator photoconductive heaters to both sense and control the intra-cavity light intensity of microring resonators, automatic tuning of a silicon-on-insulator two-ring Vernier filter is demonstrated across the entire C-band. IntroductionBy coupling resonators that have different optical path lengths, the Vernier effect can be used to increase the freespectral-ranges (FSRs) and wavelength tuning ranges achievable in microring resonator-based devices [1]. Widely tunable lasers [2], wavelength selective switches [3], and reconfigurable filters [4,5], capable of meeting numerous telecom-grade filter specifications [1], have been demonstrated on silicon photonics platforms using the Vernier effect. Tunable Vernier devices allow one to adjust the performance of the devices to address variations in fabrication, operating temperatures, and wavelength of the input laser, as well as to completely reconfigure the devices to operate at various channel wavelengths [1,5]. Therefore, the ability to automatically tune these devices is an essential requirement for their practical deployment.Recently, we showed that automatic wavelength tuning and stabilization of silicon microring-based filters can be achieved using in-resonator photoconductive heaters (IRPHs) to both sense and control the light intensity inside the resonators [6]. The IRPHs are n-doped waveguide sections that can be integrated into each microring in a siliconon-insulator (SOI) filter. The photodetection in IRPHs occurs due to defect-state-absorption [7]. Nevertheless, IRPHs show high responsivities without requiring dedicated defect implantations [6]. The n-doped waveguides also act as resistive heaters, allowing for thermo-optic tuning of the microring resonators.In this work, we show how a Vernier filter fabricated with two racetrack resonators with incorporated IRPHs in a series-coupled configuration [ Fig. 1(a)] can be automatically tuned to center its response to any input laser wavelength. The automatic tuning is achieved using simple maximum-search algorithms, which are used to find the heater voltage settings that maximize the light intensity in each of the resonators. The light intensity in a resonator is determined by measuring the photocurrent generated by its IRPH. As compared to previous work [7][8][9], the automatic tuning described here is achieved without requiring dedicated defect implantations, additional material depositions, dedicated photodetectors, thermal sensors, or optical power taps. In this work, we present the first demonstration of automatic wavelength tuning of a Vernier ring resonator filter. Figure 1(a) shows a microscope image of a SOI two-ring Vernier filter which was fabricated at the A*STAR IME foundry. The IRPHs, formed by n-doping the silicon rib waveguides (rib height = 220 nm, slab height = 90 nm), are shown as resistors in the circuit. The doping was achieved by ion-implantation. The two racetrack resonators, labeled "Ring 1" and "Ring 2" had lengths L 1 = 66.83 µm and L 2 = 83.54 µm, respectively. ...

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