Fiber Bragg gratings in novel microstructured fibers dedicated to longitudinal strain sensing

Fiber Bragg gratings in novel microstructured fibers dedicated to longitudinal strain sensing
Tadeusz TENDERENDA*a,b, Lukasz SZOSTKIEWICZc, Michal MURAWSKIa, Martin BECKERd, Manfred ROTHHARDTd, Hartmut BARTELTd, Pawel MERGOe, Ryszard PIRAMIDOWICZb, Tomasz NASILOWSKIa
aInPhoTech Ltd., Slominskiego 17/31, 00-195 Warszawa, Poland
bInstitute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75/19, 00-662 Warszawa, Poland
cPolish Centre For Photonics and Fibre Optics, Rogoznica 312, 36-060 Glogow Malopolski, Poland dInstitute of Photonic Technology, Albert-Einstein-Strasse 9, D-07745 Jena, Germany eMaria Curie-Sklodowska University, Pl. Marii Curie-Sklodowskiej 5, 20-031 Lublin, Poland
ABSTRACT
Microstructured fibers (MSF), sometimes also referred to as Photonic Crystal Fibers (PCF), have been a subject of extensive research over two decades. This is mainly due to the fact that by changing the topology and distribution of the air holes, the fiber guiding (therefore also sensing) properties can be significantly modified and tailored to desired purposes [1, 2]. Furthermore different modes in a few-mode MSF can exhibit different sensing properties [3]. While pressure and temperature transducers based on novel MSF designs have been already reported, an example of fiber geometry with significantly enhanced longitudinal strain sensitivity has not yet been proposed. Therefore, in this work we focus on the influence of the microstructure geometry on longitudinal strain sensitivity of the effective refractive index (neff) of fundamental and higher order modes in novel fiber designs. As direct neff measurements may be problematic, we have written fiber Bragg gratings (FBGs) in the fibers’ core region and analyze the changes of the Bragg wavelength (λB), which is directly correlated with neff (as λB = 2neff × Λ, where Λ is the grating period).
In this work we present a series of novel highly Ge doped MSF designs (e.g. [4, 5]) dedicated for analysis of the microstructure’s geometry influence on neff longitudinal sensitivity. After numerical simulations of the propagation conditions (with effective refractive index, confinement loss and mode area calculated) in real fiber structures, the MSFs were subjected to femtosecond and nanosecond FBG inscription. We show the resulting typical FBG spectra, as well as measure the gratings’ longitudinal strain and temperature sensitivities. The experimentally measured increase of the Bragg wavelength strain sensitivity was noticed for specific air-hole geometries, which is in good agreement with the numerical simulation results. According to our knowledge this is the first report on the influence of the air-hole diameter in an MSF on the effective refractive index sensitivity to longitudinal strain. Further optimization of the MSF design which is also included in the work may lead to application of the proposed structures as highly sensitive FBG based transducers for strain monitoring.
References
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