Tracking changes in a photonic integrated circuit is an essential task for many applications, such sensing or telecommunication systems. In particular, locking of laser to a microring resonator and tracking resonance shifts over time with high accuracy can improve several applications such as sensing and biosensing. In this work, we present a novel system to lock a laser to a silicon photonics microring resonance and track the changes in wavelength over time. An electronic digital feedback loop balances the power at outputs of the microring (at the through and the drop ports) by tuning finely the wavelength of the input laser. The silicon photonics chip is equipped with integrated photodiodes at each port of the microring. The low noise of photodiodes, together with the resolution of the tuning of the laser, allows achieving locking with less than 7 femtometers as residual noise at 1550 nm. The digital implementation of the feedback loop permits to reach bandwidth up to 1 kHz. Demonstration of the locking has been made with several different microring resonators, with Q-factor varying from 5000 to 60000.
A new technological platform aimed at making prototypes and feasibility studies has been setup at STMicroelectronics using 300mm wafer foundry facilities. The technology, called DAPHNE (Datacom Advanced PHotonic Nanoscale Environment), is devoted at developing and evaluating new devices and sub-systems in particular for wavelength division multiplexing (WDM) applications and ring resonator based applications. Developed in the course of PLAT4MFP7 European project, DAPHNE is a flexible platform that fits perfectly R&D needs. The fabrication flow enables the processing of photonic integrated circuits using a silicon-on-insulator (SOI) of 300nm, partial etches of 150nm and 50nm and a total silicon etching. Consequently, two varieties of rib waveguides and one strip waveguide can be fabricated simultaneously with auto-alignment properties. The process variability on the 150nm partially etched silicon and the thin 50nm slab region are both less than 6 nm. Using a variety of different implantation configurations and a back-end of line of 5 metal layers, active devices are fabricated both in germanium and silicon. An available far back-end of line process consists of making 20 μm diameter copper posts on top of the electrical pads so that an electronic integrated circuit can be bonded on top the photonic die by 3D integration. Besides having those fabrication process options, DAPHNE is equipped with a library of standard cells for optical routing and multiplexing. Moreover, typical Mach-Zehnder modulators based on silicon pn junctions are also available for optical signal modulation. To achieve signal detection, germanium photodetectors also exist as standard cells. The measured single-mode propagation losses are 3.5 dB/cm for strip, 3.7 dB/cm for deep-rib (50nm slab) and 1.4 dB/cm for standard rib (150nm slab) waveguides. Transition tapers between different waveguide structures are as low as 0.006 dB.