Spectral characterization of differential group delay in uniform fiber Bragg gratings

Distribution profiling of a transverse load using the DGD spectrum of chirped FBGs

In this paper, we propose a new method to determine the longitudinal distribution of a non-uniform transverse force applied to an optical fiber. For that purpose, we use a chirped fiber Bragg grating (CFBG) for which we monitor the polarization parameters in reflection. In particular, we demonstrate that the differential group delay (DGD) spectrum of the CFBG is an imprint of the load profile so that it can be used for the shape determination of an applied load. Thereafter, we discuss the influence of the CFBG parameters on the achievable accuracy and resolution of our technique. An experimental validation is finally reported with two 48 mm long CFBGs subject to step transverse load profiles.

Polarization effects in polymer FBGs: study and use for transverse force sensing

Bragg gratings photo-inscribed in polymer optical fibers (POFs) are more sensitive to temperature and pressure than their silica counterparts, because of their larger thermo-optic coefficient and smaller Young's modulus. Polymer optical fiber Bragg gratings (POFBGs) are most often photo-written in poly(methylmethacrylate) (PMMA) based materials using a continuous-wave 325 nm HeCd laser. In this work, we present the first study about birefringence effects in POFBGs manufactured in different types of fiber. To achieve this, highly reflective (> 90%) gratings were produced with the phase mask technique. Their spectral response was then monitored in transmission with polarized light. Polarization dependent loss (PDL) and differential group delay (DGD) were computed from the Jones matrix eigenanalysis using an optical vector analyzer. Maximum values exceeding several dB and a few picoseconds were obtained for the PDL and DGD, respectively. An inverse scattering technique applied to the experimental data provided an estimate of the photo-induced birefringence value arising from the side fabrication process. The response to lateral force was finally investigated for various incident directions using the PDL response of FBGs manufactured in step-index POFs. As the force induced birefringence adds to the photo-induced one, a force dependent evolution of the PDL maximum value was noticed, with a good temperature-insensitivity.

Fiber Bragg grating sensors toward structural health monitoring in composite materials: challenges and solutions

Nowadays, smart composite materials embed miniaturized sensors for structural health monitoring (SHM) in order to mitigate the risk of failure due to an overload or to unwanted inhomogeneity resulting from the fabrication process. Optical fiber sensors, and more particularly fiber Bragg grating (FBG) sensors, outperform traditional sensor technologies, as they are lightweight, small in size and offer convenient multiplexing capabilities with remote operation. They have thus been extensively associated to composite materials to study their behavior for further SHM purposes. This paper reviews the main challenges arising from the use of FBGs in composite materials. The focus will be made on issues related to temperature-strain discrimination, demodulation of the amplitude spectrum during and after the curing process as well as connection between the embedded optical fibers and the surroundings. The main strategies developed in each of these three topics will be summarized and compared, demonstrating the large progress that has been made in this field in the past few years.

In fiber Bragg grating twist sensor based on analysis of polarization dependent loss

In this paper, a new technique exploiting the polarization properties of normal fiber Bragg grating (FBG) for twist sensing is firstly proposed and experimentally demonstrated. The evolution of the polarization dependent loss (PDL) response of the FBG with respect to the twist is studied. The physical model is presented and a numerical simulation based on the transfer matrix method is used to calculate the PDL spectrum of the twisted FBG. The theoretical and experimental results suggest that the PDL response of the FBG have higher twist sensitivities than that of the reflected or transmitted amplitude spectra. Based on comprehensive analysis of the resonant wavelength and the amplitude of the two main lobes of the PDL spectrum, this sensor can measure twist angle and determine twist direction simultaneously. Moreover, the performance of the sensor is not affected by strain and temperature variations.

Development of a multi-point polarization-based vibration sensor

In this paper we propose a novel kind of multi-point vibration sensor based on the polarization properties of light. Its principle relies on the combination of mechanical transducers with fiber Bragg gratings. When subject to vibrations, the mechanical transducers induce birefringence variations within the fiber and in turn modify the state of polarization, which appears as a power variation after going through a polarizer. The FBGs reflect light from different positions of the sensing fiber and provide wavelength multiplexing. We show that this sensor can provide the vibration frequencies in a quasi-distributed manner.

Spectral characterization of polarization dependent loss of locally pressed fiber Bragg grating

In this paper, the spectral characterization of polarization dependent loss (PDL) of locally pressed fiber Bragg grating (FBG) is analyzed. The evolution of the PDL response of a FBG as functions of the load magnitude the loaded length of the grating and the position of the load are studied. The physical model is presented and a numerical simulation based on the modified transfer matrix method is also used to calculate the PDL response of the FBG. The theoretical analysis and numerical simulation suggest that the PDL response of the FBG has potential applications for distributed diametric load sensor. Good agreements between experimental results and numerical simulations have been obtained.

Improved slow-light delay performance of a broadband stimulated Brillouin scattering system using fiber Bragg gratings

We present a technique for improving the pulse-delay performance of a stimulated Brillouin scattering (SBS) based broadband slow-light system by combining it with fiber Bragg gratings (FBG). We optimize the physical device parameters of three systems: (1) broadband SBS, (2) broadband SBS + a single FBG, and (3) broadband SBS + a double FBG for maximizing the delay performance. The optimization is performed under distortion and system resource constraints for a range of bit rates from 0.5 to 8.5 Gbps. We find that an optimized broadband SBS + a double FBG system improves the fractional delay 1.8 times that of the broadband SBS system at an optimum bit rate of 3 Gbps. Also, pump power consumption is reduced by 15% as compared to the broadband SBS system at the same bit rate.