Experimental verification of response of embedded optical fiber Bragg grating sensors in non-homogeneous strain fields

One of the advantages of optical fiber sensors is their ease of embedment within a structure for non-destructive strain monitoring. In particular, Bragg grating sensors are written directly into an optical fiber hence remaining unobtrusive. In addition, several gratings can be written in series along a single fiber, permitting sensing at discrete points throughout the strain field. However, in regions of strong strain gradients, measuring the strain at discrete points may not be sufficient. One solution is to write a Bragg grating longer than the strain region of interest and use the change in its spectral response to determine the applied strain field as a function of position along the fiber. This paper presents an experimental verification of the response of an embedded optical fiber Bragg grating (OFBG) to applied non-homogeneous strain fields. Optical fiber Bragg grating sensors were embedded in four epoxy specimens of different forms so as to apply known strain functions along the gauge length when the specimen is under uniaxial tension. The complete spectral response of the Bragg gratings was then measured as a function of increasing load. The results are compared with analytical calculations, based on the piecewise-uniform period assumption for chirped gratings. Finally, the use of these spectra is discussed as possible basis functions for the resolution of an arbitrary applied strain distribution.     

Modelling of guided-wave Bragg gratings and grating sensors using the collocation method

We have used the collocation method to model the characteristics of guided-wave Bragg gratings. The collocation method being a total field method, takes into account all modes, guided as well as radiation. We have first studied the effect of the grating structural parameters such as the grating profile and the duty cycle of periodic variation and have shown that these can have significant effect on the Bragg wavelength and the reflection spectrum. We have then obtained the response characteristics of gratings for their use in strain, temperature and pressure sensing. Our results compare very well with available experimental results. Comparisons with the coupled mode theory have also been included.     

A planar lightwave circuit based micro interrogator and its applications to the interrogation of multiplexed optical fiber Bragg grating sensors

Optical fiber Bragg grating sensors have found potential applications in many fields, but the lack of a simple, field deployable and low cost interrogation system is hindering their deployment. To tackle this, we have developed a micro optical sensor interrogator using a monolithically integrated planar lightwave circuit based echelle diffractive grating demultiplexer and a detector array. The design and development of this device are presented in this paper. It has been found that the measurement range of this micro interrogator is more than 25 nm with better than 1 pm resolution. This paper also reports the applications of the micro interrogator developed to the monitoring of commercial optical fiber Bragg grating (FBG) temperature sensors and mechanical sensors. The results obtained are very satisfactory and in some cases, they are better than those obtained using commercial bench top lab equipment.     

Simultaneous measurement of strain and temperature based on a fiber Bragg grating combined with a high-birefringence fiber loop mirror

We present an all-fiber sensor for simultaneous measurement of temperature and strain. The sensing head is formed by introducing a fiber Bragg grating into a high-birefringence fiber loop mirror that acts as a Mach-Zehnder interferometer for temperature and strain discrimination. A sensing resolution of ±1 °C in temperature and ±21 με in strain has been experimentally achieved over a temperature range of 60 °C and strain range of 600 με.     

Propagation characteristics of annular laser beams passing through the reflection Bragg grating with deformation

When high-power annular laser beams produced by the unstable resonator pass through the volume Bragg grating (VBG), absorption of light in the VBG will induce a temperature increment, resulting in changes in surface distortion. Considering that the surface distortion of the grating induces index and period differences, the scalar wave equations for the annular laser beams propagating in the VBG have been solved numerically and iteratively using finite-difference and sparse matrix methods. The variation in intensity distributions, the total power reflection coefficient, and the power in the bucket (PIB) for the annular laser beams passing through the reflection VBG with deformation have been analyzed quantitatively. It can be shown that the surface distortion of the VBG and the beam orders of the annular beams affect evidently the intensity distributions, the power reflection coefficient, and the PIB of the output beam. The peak intensity decreases as the deformation of the VBG increases. The total power reflection efficiency decreases significantly with the increase in deformations of the VBG. The PIB of the output beam decreases as the obscuration ratio β and the deformation of the VBG increase. For the given obscuration ratio β, the influence of deformation of reflection VBG on the PIB of the annular beams is more sensitive with increase in distortion of the VBG and decrease in beam order.