Camille Sonnenfeld

Highly birefringent microstructured fibers with enhanced sensitivity to hydrostatic pressure

We designed, manufactured and characterized two birefringent microstructured fibers that feature a 5-fold increase in polarimetric sensitivity to hydrostatic pressure compared to the earlier reported values for microstructured fibers. We demonstrate a good agreement between the finite element simulations and the experimental values for the polarimetric sensitivity to pressure and to temperature. The sensitivity to hydrostatic pressure has a negative sign and exceeds -43 rad/MPa x m at 1.55 microm for both fibers. In combination with the very low sensitivity to temperature, this makes our fibers the candidates of choice for the development of microstructured fiber based hydrostatic pressure measurement systems.

Microstructured Optical Fiber Sensors Embedded in a Laminate Composite for Smart Material Applications

Fiber Bragg gratings written in highly birefringent microstructured optical fiber with a dedicated design are embedded in a composite fiber-reinforced polymer. The Bragg peak wavelength shifts are measured under controlled axial and transversal strain and during thermal cycling of the composite sample. We obtain a sensitivity to transversal strain that exceeds values reported earlier in literature by one order of magnitude. Our results evidence the relevance of using microstructured optical fibers for structural integrity monitoring of composite material structures.

Control Over the Pressure Sensitivity of Bragg Grating-Based Sensors in Highly Birefringent Microstructured Optical Fibers

We present fiber Bragg grating (FBG)-based hydrostatic pressure sensing with highly birefringent microstructured optical fibers. Since small deformations of the microstructure can have a large influence on the material birefringence and pressure sensitivity of the fiber, we have evaluated two microstructured fibers that were made from comparable fiber preforms, but fabricated using different temperature and pressure conditions. The magnitude and sign of the pressure sensitivity are found to be different for both fibers. We have simulated the corresponding change of the Bragg peak separation with finite-element models and experimentally verified our results. We achieve very high experimental sensitivities of -15 and 33 pm/MPa for both sensors. To our knowledge, these are the highest sensitivities ever reported for birefringent FBG-based hydrostatic pressure sensing.

Shear stress sensing with Bragg grating-based sensors in microstructured optical fibers.

We demonstrate shear stress sensing with a Bragg grating-based microstructured optical fiber sensor embedded in a single lap adhesive joint. We achieved an unprecedented shear stress sensitivity of 59.8 pm/MPa when the joint is loaded in tension. This corresponds to a shear strain sensitivity of 0.01 pm/µε. We verified these results with 2D and 3D finite element modeling. A comparative FEM study with conventional highly birefringent side-hole and bow-tie fibers shows that our dedicated fiber design yields a fourfold sensitivity improvement.

Disbond monitoring in adhesive joints using shear stress optical fiber sensors

We present dedicated shear stress optical fiber sensors for in situ disbond monitoring of adhesive bonds. The shear stress sensitivity of these sensors is about 60 pm MPa−1, which corresponds to a shear strain sensing resolution of 50 μϵ. By integrating a combination of three such sensors in the adhesive bond line of a single lap joint, we can assess the internal shear stress distribution when the joint is tensile loaded. Disbonding of this joint was initiated by cyclic tensile loading, and the sensor responses were monitored during this process. Our results show that this sensing system can detect disbonds as small as 100 μm.

Towards micro-structured optical fiber sensors for transverse strain sensing in smart composite materials

We developed a highly birefringent micro-structured optical fiber that, in combination with a fiber Bragg grating sensor, allows measuring transverse strains in reinforced composites. The first generation of this dedicated fiber sensor featured a hydrostatic pressure sensitivity of −15 pm/MPa and yielded a transverse strain sensitivity of −0.16 pm/µε when embedded in a carbon fiber reinforced polymer. The second generation of this sensor has now been fabricated and hydrostatic pressure experiments and FEM simulations show that this generation returns a sensitivity of more than twice that of the first generation. FEM simulations additionally show an increased sensitivity when this sensor is embedded in a reinforced composite, achieving an unprecedented transverse strain sensitivity of 0.29 pm/µε. We explain how the optimized micro-structure yields this record-high sensitivity. In addition we demonstrate the selectivity of the bare fiber sensor, which remains insensitive to temperature changes or axial strain. This sensor can therefore play an important role in the domain of structural health monitoring.

Microstructured optical fiber Bragg grating as an internal three-dimensional strain sensor for composite laminates

In this article, we study the possibility to use a pair of specifically designed microstructured optical fiber Bragg gratings (MOFBGs) as a multi-component strain sensor when embedded within composite materials. The dependence on the orientation of the transverse sensitivity of the MOFBGs is exploited to build a sensing device able to measure the strain field along the three principal mechanical directions of a laminate composite. We developed an analytical and numerical model of such a sensor and benchmarked it with experiments performed on laminated composite coupons equipped with this sensor. We report on a theoretical strain resolution of about 5 μ in the transverse directions of the composite material, which is a six-fold improvement over results reported in literature.

Applying optical design methods to the development of application specific photonic crystal fibres

Photonic Crystal Fibres (PCFs) are well known for allowing the implementation of specific waveguiding features that cannot be achieved with conventional optical fibres. This results from the design flexibility of the holey structure in the PCF cladding and/or core regions. Today PCFs have found applications for example in supercontinuum generation, optical sensing and fibre lasers. They are now also being combined with fibre Bragg gratings, more specifically in the fields of optical fibre sensing and all-fibre laser applications. In this contribution we discuss how we applied micro-optical design methods based on commercially available software such as MODE Solutions and FDTD Solutions from Lumerical Solutions, Inc. and COMSOL Multiphysics® combined with MATLAB® scripting and additional optimization methods to develop microstructured fibres for three different purposes, i.e. PCF structures that facilitate Bragg grating inscription, PCF structures that enable temperature insensitive pressure measurements and bendable PCFs with a very large mode area for high power short pulse fibre lasers. For the three cases we describe the fibre design methods and property simulations as well as the tolerance studies that take into account manufacturing imperfections as well as possible variations in material parameters.

A micro-computed tomography technique to study the quality of fibre optics embedded in composite materials.

Quality of embedment of optical fibre sensors in carbon fibre-reinforced polymers plays an important role in the resultant properties of the composite, as well as for the correct monitoring of the structure. Therefore, availability of a tool able to check the optical fibre sensor-composite interaction becomes essential. High-resolution 3D X-ray Micro-Computed Tomography, or Micro-CT, is a relatively new non-destructive inspection technique which enables investigations of the internal structure of a sample without actually compromising its integrity. In this work the feasibility of inspecting the position, the orientation and, more generally, the quality of the embedment of an optical fibre sensor in a carbon fibre reinforced laminate at unit cell level have been proven.

Microstructured optical fiber Bragg grating-based strain and temperature sensing in the concrete buffer of the Belgian supercontainer concept

We present the use of microstructured optical fiber Bragg grating-based sensors for strain and temperature monitoring inside the concrete buffer of the Belgian supercontainer concept, demonstrated in a half-scale test in 2013. This test incorporated several optical fiber sensors inside the concrete buffer for production and condition monitoring. The optical fiber sensors presented here consist of small carbon-reinforced composite plates in which highly birefringent Butterfly microstructured optical fibers, equipped with fiber Bragg gratings, were embedded. The double reflection spectrum of these MOFGBs allows to simultaneously monitor strain and temperature, as confirmed by comparison with data obtained from thermocouples and vibrating-wire sensors installed near the MOFBGs.