Microlens integration on photonic integrated circuits: a platform for achieving relaxed packaging tolerances and hybrid integration of external functionality

In this paper, we discuss 3 examples in which microlenses can be a useful tool to address challenges in coupling between a fiber array and photonic integrated circuit (PIC). The (arrays of) microlenses used in this work were realized by the photoresist reflow method and can either be monolithically integrated on the back side of a PIC or separately fabricated microlens blocks can be mounted on the device side of a PIC. The first example involves the implementation of silicon microlenses etched at the back side of a sensing PIC (operating in C-band) aiming at relaxed alignment tolerances and keeping the device side free of interfacing fibers. The second example involves the implementation of a 4-mm long working distance expanded beam (30 μm mode field diameter, C-band) interface for telecom/datacom applications which also greatly relaxes lateral and longitudinal alignment tolerances between grating couplers on PIC and a fiber array. The final example involves the integration of an isolator in this long working distance expanded beam interface. The isolator stack consisted of a polarizer (0.55 mm thick), a non-reciprocal Faraday Rotator (485 μm thick film latching Faraday Rotator) and half-wave plate (HWP, 91 μm quartz) glued on top of each other. We obtained broadband operation exhibiting a very low (between 1 and 1.5 dB) insertion loss and good extinction ratio (between 17 and 20 dB) in C-band (around 1550 nm).