Fiber optic Bragg grating sensors for high-precision structural deformation control in optical systems

Abstract

Fiber optic Bragg grating (FBG) sensors show promising capabilities in the measurement of strain and temperatures in structures at many locations. In this work, the potential of FBG sensors for high-precision deformation control in opto-mechanical applications is investigated. This requires a strain resolution of < 1 um/m. A test rig with a simply supported steel beam was developed which should represent the geometry of a lightweight optical mirror with a ribbed support structure. The deformation of this beam is controlled by a piezo actuator. The reference deformation measurement is done using six capacitive displacement sensors with a resolution < 0.5 nm. It is being investigated to what level of accuracy FBG sensors can be used to reconstruct the displacement information. Different methods to increase the accuracy are discussed: decreasing the sensor noise by oversampling and increasing the number of sensors. Tests were performed using different diffraction-based interrogation techniques for the wavelength detection: a CCD-based FBG sensor system and a PSD (Position Sensitive Detector)-based high-speed FBG sensor system which - to our knowledge - has not been used for an application of this kind yet. A comparison of both systems discussing the weaknesses and strengths is given for the recording of mechanical strain < 1 um/m. The results showed that a resolution of < 0.3 um/m for the strain measurement using FBG sensors can be achieved. This study shows an interesting application potential for FBG sensors in structural deformation control for various fields such as optics or high-precision machine tools.