Localized long gage fiber optic strain sensors

Abstract

Long gage length integrated strain sensing is frequently required in structural measurement applications. An optical fiber structural sensing system based on a low-coherence Michelson interferometer was built and shown to be of low cost and capable of absolute measurement and moderate accuracy for quasistatic measurement of strain or structural deformations. This type of sensor was found to be useful for monitoring the hoop-strain around structures like shells, cylinders and columns. We have also shown that localization of the sensing section of an optical fiber can be achieved through the use of one or more Bragg gratings. In effect, the sensing section of the optical fiber acts as a Fabry-Pérot interferometer. When a low-coherence source is used, interference is only attained when the cavity length of this Fabry-Pérot matches the optical path difference (OPD) between the two mirrors of a fiber optic Michelson interferometer. Changes in the sensing length are determined from the commensurate changes that must be made to the reference Michelson interferometer to maintain some fixed degree of interference. Recently, we have developed a novel single-ended localized fiber optic sensor for making absolute strain measurements over arbitrary (cm to m) long gage lengths using a tunable laser. The sensor's gage is again defined between two in-fiber broad-bandwidth Bragg gratings or one grating and the mirror coated end of the same fiber. For this sensing system the change in the OPD of the sensing Fabry-Pérot interferometer with respect to the OPD of a fixed Michelson reference interferometer is determined from the measurement of the phase change, recorded by a low-pass filtered photodetector, associated with a known sweep of the laser wavelength. This tunable laser demodulation scheme avoids the use of moving parts and lends itself to a compact, portable system. This type of sensor is particularly well suited for certain structural applications, such as monitoring the time variation in the hoop-stress in composite material wraps used to rehabilitate and strengthen corroded or earthquake damaged concrete columns.