Directly interfacing a photonic integrated circuit allows at best an alignment tolerance of a few micrometer due to the small dimensions of optical (coupling) features on chip, but when using microlenses integrated on the substrate-side, alignment tolerances for interfacing the chips can greatly be relaxed. This is demonstrated on a 750 μm thick chip with standard grating couplers (operation wavelength around 1550 nm). Low roughness silicon microlenses were realized by transferring reflowed photoresist into the silicon substrate using reactive ion etching. The microlens allows interfacing the chip from the backside with an expanded beam, drastically increasing lateral alignment tolerances. A 1 dB alignment tolerance of ±8 μm and ±11 μm (along and perpendicular to the grating coupler direction, respectively) was experimentally found when a 40 μm mode field diameter beam was used at the input.
III-V/silicon photonic integrated circuits (ICs) promise to enable low cost and miniature optical sensors for trace-gas detection, bio-sensing and environmental monitoring. A lot of these applications can benefit from the availability of photonic ICs beyond the telecommunication wavelength range. The 2 μm wavelength range is of interest for spectroscopic detection of many important gases and blood constituents. In this contribution we will present 2 μmwavelength-range III-V/silicon photonic ICs consisting of tunable laser sources, photodetectors and silicon waveguide circuits. Silicon waveguides with a loss of ~0.5 dB/cm are obtained in a well-established silicon photonics platform. Based on the waveguides, low insertion loss (2-3 dB) and low crosstalk (25-30 dB) arrayed waveguide gratings (AWGs) are realized for the 2.3 μm wavelength range. Active opto-electronic components are integrated on the photonic IC by the heterogeneous integration of an InP-based type-II epitaxial layer stack on silicon. III-V-on-silicon 2.3 μm range distributed feedback (DFB) lasers can operate up to 25 °C in continuous-wave regime and shows an output power of 3 mW. By varying the silicon grating pitch, a DFB laser array with broad wavelength coverage from 2.28 μm to 2.43 μm is achieved. III-V-on-silicon photodetectors with the same epitaxial layer stack exhibit a responsivity of 1.6 A/W near 2.35 μm. In addition, we also report a 2 μm range GaSb/silicon hybrid external cavity laser using a silicon photonic IC for wavelength selective feedback. A wavelength tuning over 58 nm and side mode suppression ratio better than 60 dB is demonstrated.