Refractive index sensitivity of fibre optic long period gratings with SiO2 nanoparticle based mesoporous coatings

A fibre optic long period grating (LPG) with a nano-assembled mesoporous coating of alternate layers of poly (allylamine hydrochloride) (PAH) and SiO2 nanospheres was used for the development of a fibre-optic refractometer. PAH/SiO2 films of different thickness have been deposited onto LPG in order to study the effect of the film thickness on sensor performance. The device showed a sensitivity of 1927 nm/RIU over a RI range of 1.3233–1.4906. INTRODUCTION Precise determination and control of the refractive index (RI) in real time continues to be an important task in a wide range of applications, including bio-chemical and food analysis, and environmental and industrial monitoring. The use of optical fibre gratings, both fibre Bragg gratings (FBGs) and long period gratings (LPGs), has been extensively investigated for refractive index measurements since they offer wavelength-encoded information which overcomes the referencing issues associated with intensity based approaches. The refractive index sensitivity of LPGs arises from the dependence of the phase matching condition upon the effective refractive index of the cladding modes, which in turn are dependent on the RI of the medium surrounding the cladding. The central wavelengths of the attenuation bands thus show a dependence upon the refractive index of the medium surrounding the cladding, with the highest sensitivity being shown for surrounding refractive indices close to that of the cladding of the optical fibre. For surrounding refractive indices higher than that of the cladding, the centre wavelengths of the attenuation bands show a considerably reduced sensitivity. The RI sensitivity of an LPG is dependent on the order of the cladding mode to which it is coupled, which allows the tuning of the sensitivity by appropriate choice of grating period, with 427.72, 203.18, 53.45, and 32.10 nm/RIU being reported for LPGs of period 159, 238, 400, and 556 μm, respectively fabricated in SMF 28. A further consideration is the geometry and composition of the fibre, with the sensitivity being shown to differ for step index and W profile fibres and a progressive three layered fibre. In order to improve sensitivity of the LPGs written in standard optical fibre configurations to the surrounding RI, approaches such as tapering the fibre, or etching the cladding have been investigated. Tapering the fibre to a diameter of 25 μm allowed the demonstration of a sensitivity of 715 nm/RIU. Etching the cladding of an arc induced LPG to a diameter of 37 μm allowed the demonstration of a sensitivity of order 20000 nm/RIU. Approaches that involve processing of the fibre, e.g. polishing, etching and tapering, produce significant enhancements in sensitivity, but at the cost of requiring careful packaging to compensate for the reduction in the mechanical integrity of the device. The deposition of thin film overlays, of thickness on the order of 200 nm, of materials of refractive index higher than that of the cladding has also been investigated for the enhancement of refractive index sensitivity . It has been shown previously theoretically and experimentally that the effective indices of the cladding modes, and thus the central wavelengths of the core-cladding mode coupling bands of LPGs, show a high sensitivity to the optical thickness of high refractive index coatings when the coating’s optical thickness is such that one of the low order cladding modes is phase matched to a mode of the waveguide formed by the coating, a condition termed the mode transition region. To optimize the sensitivity of such devices to the coating properties, the LPG period should be chosen such that, when the coating thickness corresponds with the mode transition condition, the phase matching condition for the cladding mode of interest is satisfied at the phase matching turning point. A theoretical analysis explored the optimization of the refractive index sensitivity by selecting grating period, coating thickness and refractive index, predicting a sensitivity of 5980 nm/RIU. There has recently been interest in the deposition of porous coatings onto LPGs for sensing applications. Sol-gel coatings of SiO2 and TiO2 have been deposited onto LPGs, revealing a gain of approximately 2 in the sensitivity to external RI, with the higher index TiO2 coating offering the larger response (up to 1067.15 nm/RIU). The authors noted that the 21st International Conference on Optical Fiber Sensors, edited by Wojtek J. Bock, Jacques Albert, Xiaoyi Bao, Proc. of SPIE Vol. 7753, 77539A · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.884777 Proc. of SPIE Vol. 7753 77539A-1 Downloaded from SPIE Digital Library on 29 Mar 2012 to 138.250.13.135. Terms of Use: http://spiedl.org/terms higher the ind equivalent un infusion of a deposited usin significantly b sensing of am sensor. The required to o mesoporous c