[3] A. Candiani, M. Konstantaki, W. Margulis, and S. Pissadakis, "A spectrally tunable microstructured optical fibre Bragg grating utilizing an infiltrated ferrofluid," ...
A vectorial magnetometer utilising a microstructured optical fibre Bragg grating infiltrated by a ferrofluid A. Candiani1, W. Margulis2, C. Sterner2, M. Konstantaki1, P. Childs1, S. Pissadakis1 1. Foundation for Research and Technology-Hellas, Institute of Electronic Structure and Laser, P.O. Box 1385, Heraklion, 71 110 Greece 2. Department of Fiber Photonics, Acreo AB, Electrum 236, 16440 Stockholm, Sweden
There has been an increased interest in the infiltration of microstructured optical fibres (MOFs) using liquid or solid matrices of nano-tailored physical properties, for the development of photonic devices which exhibit new sensing and switching functionalities [1, 2]. Recently, we infiltrated ferrofluids inside MOF Bragg gratings demonstrating a novel in-fibre actuator [3]. Herein, we present a compact and fully functional, optofluidic in-fibre magnetometer, based on a MOF Bragg grating, which has been infiltrated using a ferrofluid. The demonstrated MOF device is designed for operating at the 1.5µm band and is interrogated in reflection using a 50/50 fibre coupler; the output being connected to a pigtailed photodiode and either a broadband source or a fibre pigtailed diode. The MOF used is a grape-fruit geometry Ge-doped core fibre [3], while the Bragg grating was fabricated using 193nm excimer laser radiation and a phase mask interferometer. This in-fibre device utilises a short length of oil based ferrofluid (Ferrotec EMG905) placed at a specific point within the length of the MOF Bragg grating; forming a lossy Fabry-Perot cavity [4]. The ferrofluidic defect is immobilised at a specific location along the Bragg grating so as to maximise the depth of the parasitic spectral notch observed in reflection. Defect immobilisation is achieved by sealing one end of the microstructured fibre and creating a closed air cavity, allowing the ferrofluid partial movement about its optimum immobilisation point, while returning to its initial position due to hydrostatic pressure (Fig.1a). Under the application of magnetic field along the axis of the MOF, the ferrofluidic defect moves along the Bragg grating length, while inducing significant changes at the parasitic Fabry-Perot spectral notch observed in reflection (Fig.1b). The translation of the ferrofluidic defect along the Bragg grating length introduces a significant change to the parasitic mode strength (Fig.1b).
Fig. 1 (a) Schematic of the ferrofluid infiltrated MOF-Bragg grating magnetometer. (b) Reflection spectra of the defected Bragg grating with (dashed line) and without (solid line) magnetic field perturbation. The position of the spectral defect indicates that the Bragg grating is slightly chirped. (c) Parasitic notch visibility versus magnetic field along the fibre length.
The MOF device presented here contains a ~20dB strong and 9.4mm long Bragg grating, that has being infiltrated using a ~1mm of EMG905 ferrofluid (9mPas viscosity). Prior to infiltration the MOF grating was functionalised using Polyvinylpyrrolidone [3]. The Bragg grating is located 9cm away from the sealed fibre end (see Fig.1a). A strong magnet (~1Tesla) was used at room temperature for testing the device response, along the fibre axis. The specific device presented herein has approximate onset and saturation thresholds of 740 and 2200Gauss, respectively (Fig.1c). The onset threshold is dependent upon the effective grating length, the closed cavity length, the length and optical properties of the ferrofluid, as well as its viscosity which in turn defines its mobility in the MOF capillaries. Other designs realised exhibit different sensitivities and dynamic ranges by tailoring of the aforementioned design parameters. The translation of the ferrofluid along the fibre axis under magnetic field stimulus imparts to the device vectorial sensing capabilities, rendering the MOF probe insensitive for magnetic fields applied perpendicular to the fibre axis. Further results related to this optofluidic MOF magnetic field probe's sensitivity, vectorial and directional sensing capabilities, as well as a detailed theoretical analysis, will be presented on-site. References [1] B. Eggleton, C. Kerbage, P. Westbrook, R. Windeler, and A. Hale, “Microstructured optical fiber devices,” Opt. Express 9 (13), 698–713 (2001). [2] E. Coscelli, M. Sozzi, F. Poli, D. Passaro, A. Cucinotta, S. Selleri, R. Corradini and R. Marchelli, Towards highly specific DNA biosensor: PNA-modified suspended core photonic crystal fiber, IEEE Journal of Selected Topics in Quantum Electronics 16, 967972 (2010). [3] A. Candiani, M. Konstantaki, W. Margulis, and S. Pissadakis, "A spectrally tunable microstructured optical fibre Bragg grating utilizing an infiltrated ferrofluid," Opt. Express 18, 24654-24660 (2010). [4] A. Candiani, M. Konstantaki, W. Margulis, and S. Pissadakis, "Spectral Tuning Of A Microstructured Fibre Bragg Grating Utilizing An Infiltrated Ferrofluidic Defect," in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides, OSA Technical Digest (CD) (Optical Society of America, 2010), paper BTuC2.
