Compact and fast optical vibration sensor

Abstract

Compact and sensitive vibration sensors are essential for many strategic market sectors such as structural health monitoring for automobiles, aerospace, transportation, and civil structures. Optical sensors offer many distinct advantages in comparison to their electronic counterparts: high sensitivity, remote and distributed sensing, use in harsh environments, and immune to electromagnetic interference. The functionality of many optical sensors relies on a change in the spectral wavelength response upon an external stimulus (e.g., temperature or strain). A prominent example is the Fiber Bragg Grating (FBG) sensor, in which the reflection spectrum changes in response to the stimulus.

A major hurdled to the deployment of FBG sensors is the ability to record the requisite sub-picometer wavelength shifts with a compact, robust, and low-cost interrogation unit. In this presentation we describe a compact and fast-read-out optical vibration sensor that combines a FBG strain sensor with a low-cost, chip-size wavelength detector to resolve sub-pm wavelength shifts. The wavelength-shift detector combines a light sensitive element (e.g., photodiode array or position sensitive device) with a linear variable filter. The laterally varying transmission/reflection properties of the filter induce a spatially dependent signal from the photo-detector that characterizes the wavelength of the incident light. The complete detection system can monitor micro-strain vibrations in the 100 KHz range. High frequency vibrations with sub micro-strain amplitude are even clearly observable in the real-time signal. Therefore this system is well suited to record signature of fast impulse loads on structures that create strain responses with amplitudes smaller than 2 micro-strain.