Optimization of sensitivity in Long Period Fiber Gratings with overlay deposition

Universidad de Navarra, Iruña, Navarre, Spain
Optics Express (Impact Factor: 3.49). 02/2005; 13(1):56-69. DOI: 10.1364/OPEX.13.000056
Source: PubMed


The deposition of an overlay of higher refractive index than the cladding in a Long Period Fiber Grating (LPFG) permits to improve the sensitivity to ambient refractive index changes in a great manner. When the overlay is thick enough, one of the cladding modes is guided by the overlay. This causes important shifts in the effective index values of the cladding modes, and henceforward fast shifts of the resonance wavelength of the attenuations bands in the transmission spectrum. This could be applied for improving the sensitivity of LPFG sensors. The problem is analysed with a numerical method based on LP mode approximation and coupled mode theory, which agrees with so far published experimental results.

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Available from: Ignacio Del Villar, Feb 07, 2014
    • "However, for SRI higher than the CRI there is very low wavelength sensitivity to SRI variations and a considerable reduction in transmission optical power changes is observed[20]. This disadvantage can be overlapped by coating the bare LPFG with a material that has a RI higher than the cladding[22]. The wavelength shift of the LPFGs depends on the RI and thickness of the film[23], as reported for the case of titanium dioxide (TiO 2 ) thin film coating[24]and for Langmuir–Blodgett nanolayers[20,25]. "
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    ABSTRACT: A hybrid optical sensing scheme based on a fiber Bragg grating (FBG) combined with a titanium dioxide coated long period fiber grating (LPFG) for monitoring organic solvents in high refractive index edible oils is reported. In order to investigate and optimize the sensor performance, two different FBG/LPFG interrogation systems were investigated. The readout of the sensor was implemented using either the wavelength shift of the LPFG resonance dip or the variation in the optical power level of the reflected/transmitted light at the FBG wavelength peak, which in turn depends on the wavelength position of the LPFG resonance. Hexane concentrations up to 20%V/V, corresponding to the refractive index range from 1.451 to 1.467, were considered. For the transmission mode of operation, sensitivities of 1.41 nm/%V/V and 0.11 dB/%V/V, with resolutions of 0.58%V/V and 0.29%V/V, were achieved when using the LPFG wavelength shift and the FBG transmitted optical power, respectively. For the FBG reflection mode of operation, a sensitivity of 0.07 dB/%V/V and a resolution better than 0.16%V/V were estimated.
    No preview · Article · Feb 2016 · Talanta
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    • "Recently, the combination of this phenomenon with the dispersion turning point [8] has permitted achieving sensitivities approaching 10000 nm/RIU [9]. The mode transition consists of a reorganization of the effective indices of cladding modes when one of these modes is guided through the thin-film deposited on the cladding of an LPFG [5]. Recently, new simpler structures that do not require the inscription of a grating have been explored when a thin-film is deposited on the cladding. "
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    ABSTRACT: Two simple optical fibre structures that do not require the inscription of a grating, a cladding removed multimode optical fibre (CRMOF) and a single-mode multimode single-mode structure (SMS), are compared in terms of their adequateness for sensing once they are coated with thin-films. The thin-film deposited (TiO2/PSS) permits increasing the sensitivity to surrounding medium refractive index. The results obtained can be extrapolated to other fields such as biological or chemical sensing just by replacing the thin-film by a specific material.
    Full-text · Article · Jun 2015 · Journal of Sensors
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    • "In both cases anyway, a wavelength shift of the attenuation bands in the transmission spectrum occurs and it can be monitored in real-time. The literature up to now accounts for three different approaches for improving the performance of an LPG in terms of RIS: 1. by increasing the cladding mode order (thus by decreasing Λ) from low-order cladding modes to high-order ones (standard LPGs) [6]; 2. by coupling to a special dual-band LPG in which the high-order cladding mode shows a turn-around point (TAP) in its phase-matching curve (TAP LPGs) [6]; 3. by depositing over the grating region a nm-thick overlay characterized by a RI higher than the RI of fiber cladding (coated LPGs) [7]. If the overlay is thick enough, the cladding mode starts to be guided by the overlay and a dramatic re-distribution of neff,clad of the cladding modes occurs thus causing huge shifts of λres (LPGs in modal transition). "
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    DESCRIPTION: Long period fiber gratings (LPGs) have been proposed as label-free biosensors since a few years. LPGs are a growing, effective and alternative choice with respect to other label-free optical platforms, such as surface plasmon resonance, interferometric configurations, structures using different optical fibers and optical resonators. A chemical/biochemical interaction taking place along the grating portion induces a change in surrounding refractive index (RI), which yields a change in the transmission spectrum of the fiber. Different types of LPGs - standard, turn-around point (TAP) and sol-gel coated - were manufactured in order to assess the optical characteristics and thus to enhance the RI sensitivity of these devices. The functionalization of the fiber surface was implemented using Eudragit L100 copolymer. After that, an IgG/anti-IgG immunoassay was performed in order to analyze the antigen/antibody interaction. Following the same model assay but using different LPG types, a comparative study of the biosensor performance was carried out. The study encompassed the anti-IgG (antigen) interaction at different concentrations in order to achieve the calibration curves of the immunoassays and thus the characteristic parameters of the biosensor. Experimental results showed an enhancement in the RI sensitivity and in the biosensor performance in the case of both TAP and coated LPGs, with values of limit of detection (LOD) lower than 0.5 10-9 M. These values were also confirmed carrying out measurements on complex matrixes of human serum.
    Full-text · Research · May 2015
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