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Hybrid integration of multiple optical chips in three dimensions is an important technology for realizing highly functional, compact optoelectronic microsystems. In this paper, we report successful 3-D integration of optical chips performed using Au-Au surface-activated bonding (SAB) at a relatively low bonding temperature of 150degC. A glass substrate with the flip-chip-bonded photodiode chips was vertically stacked on a Si substrate with the bonded laser diode chip using Au-Au SAB. By applying this technique, compact and thin optical microsensors (2.8 mm times 2.8 mm times 1 mm thick) were fabricated. The feasibility of measuring velocity was demonstrated using prototype microsensors.