Microdisplacement sensor using an optically trapped microprobe based on the interference scale

Positioning technology is one of the most important technologies for developing microsystems. In particular, displacement sensors are necessary for positioning devices with nanoscale accuracy. In this study, we propose a new displacement sensor that uses an interference scale as a linear scale and a laser-trapped microsphere as a sensing probe. This sensor has a wide measuring range, high resolution, and accessibility for narrow target areas. A glass microsphere was optically trapped by means of the laser trapping technique. Between the target surface and the probe, an interference scale was generated along the optical axis. The scale origin was fixed on the target surface. The distance between the probe and the target surface could be measured in terms of the shift in the interference scale. This study investigated the fundamental performance of the sensor. The resolution and accuracy of the sensor were 10 and ±50 nm, respectively; these values could be improved by using trapping lasers having shorter wavelengths. The measurable range was 250 μm. This sensor can provide useful displacement information from a target area having dimensions smaller than 15 μm. In addition, the displacement sensor can measure the distance even for surfaces inclined at angles less than 15°; thus, a flexible arrangement can be used to carry out measurements. In addition, the direction of displacement can be identified.