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-P érot interferometer. When a low-coherence source is used, interference is only attained when the cavity length of this Fabry-P érot 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-P érot 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.

Heat-flux measurement using fibre-Bragg-grating Fabry-Pérot sensors

“…This was achieved by careful selection of a suitable laser diode (LTO24, 784 nm, 30 mW) which was operated at a controlled temperature and injection current to allow the laser wavelength to be tuned to match the grating Bragg wavelength. Equation (5) shows that the transfer function is periodic. However, the expected phase changes are very much less than 2π rad over the time period of interest.…”

“…If the gratings' reflectivities are low, typically a few per cent, then the Fabry-Pérot cavity has a low finesse, resulting in a near-sinusoidal transfer function, the phase of which is related to the applied measurand. Fan et al [5] describe a long-gauge-length fibre-strain sensor based on this principle, comprising two BGs 0.5 mm long separated by about 1 m, illuminated by a tunable laser diode. Demodulation of the cavity phase, and hence fibre-strain measurement, was achieved by scanning the laser wavelength across the reflection spectrum of the BGs.…”

Damage detection of rotating wind turbine blades using local flexibility method and long-gauge fiber Bragg grating sensors

“…However, the application of the rotation of mode shape is restricted to purely numerical cases, possibly due to the difficulty in measuring dynamic rotational vibration signals. On the other hand, because longgauge FBG strain sensors can measure average strain (macro strain) over a certain gauge length up to several centimeters or meters, a larger area of structures could be monitored when measuring structural responses for applications to SHM (Fan et al 1998, Ansari 2005. Li and Wu (2007) further illustrated the feasibility of damage detection algorithms on the basis of dynamic macro-strain measurements from long-gauge FBG sensors.…”

Dual-fibre Bragg grating sensor for barometric pressure measurement

“…Bragg gratings are a sensing element of choice in many applications, because of the convenient wavelength encoding of the measurand that they confer and their suitability for wavelength division multiplexing (WDM) [3]. However, in situations where the applied stress is limited, their sensitivity is small in comparison with, for example, interferometric sensing elements [4]. The dual grating system we describe here mechanically amplifies the measured stress induced in one of the sensing elements by typically one order of magnitude, thus increasing the range of applications of the Bragg grating sensing element.…”