Integrated micro laser Doppler velocimeter with 3-D structure

Compact and thin micro laser Doppler velocimetry (LDV) sensor with 3-D structure (2.8 mm × 2.8 mm × 1 mm thick) was developed using Au-Au surface-activated bonding method (bonding temperature: 150°C). It consists of two elements: a glass substrate with bonded photodiode chips, electrodes, and refractive microlenses; and a micromachined Si substrate with a submount, a bonded laser diode chip, micromirrors, and through-silicon vias. The feasibility of measuring velocity was demonstrated for various moving objects such as a reflective grating, an Au wire, and a light diffusing ground glass. The micro LDV sensor detected relative speeds as low as 0.4 μm/s and Doppler beat signal frequency showed good linearity to the relative velocity.