ABSTRACT
Because of the specific shape of the microstructure in our MOF we refer to this fiber as the butterfly MOF (Martynkien 2010). This MOF is highly birefringent (on the order of 10−3) and therefore contains two orthogonally polarized modes that can operate individually (Fig. 1). The central part
1 INTRODUCTION
1.1 Fibre Bragg gratings in micro-structured optical fibres
One of the most well known possibilities to turn an optical fiber into a strain sensor is to use a so-called Fiber Bragg Grating (FBG) (Othonos 1999). This device is a wavelength-selective filter fabricated inside the core of an optical fiber for which the reflected wavelength changes under the influence of external perturbations. Although these sensors have many advantages such as a low weight, small size, immunity to electromagnetic interference, high linearity of the response, etc., they still show a number of shortcomings. For example, a FBG in a conventional step-index optical fiber is sensitive to both temperature changes and axial strain leading to cross-sensitivity problems. Furthermore the sensitivity of FBGs in conventional optical fiber to transverse strain remains considerably lower than to axial strain, while monitoring transverse strains is most important in many applications.
