Fabrication of waveguide-based vibration sensors by femtosecond laser micromachining

We report on the fabrication of vibration sensors using femtosecond laser-micromachined waveguides. We micromachine an optically connected waveguide across three pieces of glass using a nJ high-repetition rate femtosecond laser oscillator in a transverse writing geometry. The three pieces are placed in close proximity of one another such that there is low loss across the segmented waveguide. The middle piece is mounted on a flexible beam and hence is affected by external forces. Vibration and acceleration are detected by monitoring the induced change in transmission through the assembly. These sensors have a linear frequency response to input vibrations and an acceleration sensitivity below 0.01 m/s². Moreover, the sensors exhibit a negligible temperature dependence, overcoming a serious limitation encountered in the widely used fiber Bragg grating optical sensors.