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Design and fabrication of adiabatic vertical couplers for hybrid integration by flip-chip bonding

Rare-earth ion doped crystalline potassium double tungstates, such as KY(WO4)2, KLu(WO4)2 and KY(WO4)2, exhibit many properties that make them promising candidates for the realization of lasers and amplifiers in integrated photonics. One of the key challenges for the hybrid integration of different photonic platforms remains the design and fabrication of low-loss and fabrication tolerant couplers for transferring light between different waveguides. In this paper, adiabatic vertical couplers realized by flip-chip bonding of polymer waveguides to Si3N4 devices are designed, fabricated and tested. An efficient design flow combining 2D and 3D simulations was proposed and its validity was demonstrated. The vertical couplers will ultimately be used for the integration of erbium doped KY(WO4)2 waveguides with passive platforms. The designed couplers exhibit less than 0.5 dB losses at adiabatic angles and below 1 dB loss for ±0.5 μm lateral misalignment. The fabricated vertical couplers show less than 1dB losses in average for different adiabatic angles of Si3N4 tapers, which is in good quantitative agreement with the simulations.