Precision non-contact displacement sensor based on the near-field characteristics of fiber specklegrams

In this paper, we present a precision non-contact displacement sensor based on the near-field property of specklegrams, achieving a precision of 10 nm and a detection range of 3-4 mu m. The compact device envelope enables the application of the new fiber-optic sensor in many space-limited applications, e.g., probe-based nanofabrication and micro-/mesoscale positioning systems. In the sensor system, the laser is first coupled into a 50/50 fiber coupler; the flexible fiber transmits the laser beam to the remote sensor head that is a few micrometers away from the sample surface; the reflected light is collected by the sensor head. The interference among the different eigenmodes propagating in the multimode fiber form unique specklegram images for different sample positions in space. By recording the varying specklegrams along the scanning path, highly precise and repeatable non-contact displacement detection can be realized by comparing the recorded specklegrams with the calibrated specklegram database through imaging processing techniques. In the experiments, we demonstrate the fiber-optic sensor can perform one-axis displacement sensing with a precision of 10 nm over a range of 3-4 mu m at 100 Hz, where the specklegram acquisition rate is limited only by the speed of the camera. The new fiber-optic sensor is flexible, precise, and low-cost, which may find important applications in optical metrology and nanomanufacturing. (C) 2019 Elsevier B.V. All rights reserved.