<title>Measurement of transverse strains with fiber Bragg gratings</title>

Abstract

In this paper, we present a method to measure two components of transverse strain in an optical fiber using a single Bragg grating written into high-birefringent, polarization-maintaining (PM) fiber. The reflected spectrum from this grating contains two peaks corresponding to the two orthogonal polarization modes of the fiber. If the axial strain and temperature in the fiber is known, then two components of transverse strain can be computed from the changes in wavelength of the two peaks. A Bragg grating written near 1300 nm in PM fiber was loaded in diametrical compression, and the changes in wavelength of the Bragg peaks were monitored using an optical spectrum analyzer. Transverse strains were computed from the changes in wavelength using available strain-optic coefficients for low-birefringent optical fiber. These strains are compared to finite element analysis predictions, and it is shown that the observed sensor response is greater than the response predicted by the low-birefringent analysis. A calibration factor is developed for the sensor to allow the determination of transverse strains from the measured wavelength shifts.Fiber Bragg grating sensors generally consist of a single grating written in low-birefringent optical fiber. The wavelength shift of the peak in the reflected spectrum from these sensors can be used to measure a single component of strain (usually axial) or a change in temperature, but not both. For axial strain measurements, changes in temperature or transverse strains can result in measurement errors. These errors can be significant when the sensor is embedded [1]. Many of the proposed applications for embedded fiber optic sensors, such as smart structures, manufacturing process monitoring, and health monitoring could benefit from a sensor capable of simultaneously measuring temperature and multiple components of strain. In general, it is likely that the complete state of strain and temperature in these embedded applications will be unknown, and several individual sensors oriented in different directions would be required to measure all of the unknown parameters [2]. A multi-parameter fiber optic sensor would greatly reduce the number of sensors required, making the technology potentially less expensive and more reliable. The goal of multi-parameter sensing with fiber optics has been addressed by several researchers. Xu et a! [3] demonstrated a sensor to simultaneously determine temperature and axial strain using two Bragg gratings at widely spaced wavelengths overlaid in single-mode fiber. Sirkis and Lo [4] describe a sensor to simultaneously measure axial and transverse strains in a surface mounted optical fiber using combined Michelson and polarimetric fiber optic sensors. James et al [5] developed a sensor to measure axial strain and temperature using two Bragg gratings written on either side of a splice between fibers of different diameters. Additionally, Singh and Sirkis [6] describe an optical fiber sensor which uses an intrinsic Fabry-Perot and an in-line fiber etalon sensor to...

An experimental–numerical investigation of hydrothermal response in adhesively bonded composite structures

“…In this case, two planepolarized waves propagate in the fiber core, with the principal directions of polarization oriented parallel to the directions of the principal strains. The wavelength changes for each axis are explicitly given by [22][23][24] 2…”

<title>Development of a three-axis strain and temperature fiber optic grating sensor</title>

“…To avoid ambiguity due to coatings bare fiber gratings were used for these tests. The results of these tests showed transverse strain sensitivity of about 30 microstrain for a 0.01 nm resolution demodulator [6].…”

“…This information which is contained in the spreading of the profiles of the fiber grating is a new tool that could be ofconsiderable value for understanding manufacturing processes. (6) Geometries other than circular of polarization preserving fiber need to be studied. Varying the geometry by implementing 3 axis fiber grating sensors into polarization preserving fibers that are D shaped (such as those pioneered by Dyott of Andrew Corporations) or flattened on both sides (developed by Roger Stolen at Bell Labs) have two potential advantages.…”

Effects of distributed birefringence on fiber Bragg grating under non-uniform transverse load

“…However, the past studies have shown that the characteristics of a FBG under transverse load are quite different from axial load [7][8][9][10][11]. In the case of transverse load, the stress distribution along the two perpendicular directions across the cross-section of the fiber is different, which causes that the propagation constants of the two polarization modes x HE 11 and y HE 11 to be different.…”