(TFBG) covered by silver nanowires aligned perpendicularly to the fiber axis. TBFGs are a convenient way to measure surrounding refractive index, as they provide intrinsic temperature-insensitivity and preserve the optical fiber structural integrity. With bare TFBGs, sensitivity is about 60 nm/RIU (refractive index unit) while when coated with a gold thin film, surface plasmon resonance can be excited leading to a sensitivity about 600 nm/RIU. In our case, we show that localized plasmon resonances can be excited on silver nanowires. These nanowires (100 nm diameter and about 2.5 µm length) were synthetized by polyol process (ethylene glycol reducing silver nitrate in the presence of poly (vinyl pyrrolidone and sodium chloride). The nanowires were aligned and deposited perpendicularly to the fiber axis on the gratings using the Langmuir-Blodgett technique in order to maximise the coupling between azimuthally polarized light modes and the localized plasmons. Excitation of surface plasmons at wavelengths around 1.5 µm occurred, leading to a dip in the polarization dependent losses of the grating. This dip is highly dependent of the surrounding refractive index, leading to a sensitivity of 650 nm/RIU, which is a 10-fold increase compared to bare gratings. We obtain results equal or slightly higher than those obtained using a gold layer on TFBGs. In spite of the comparable bulk refractometric sensitivity, the use of these oriented nanowire layers provide significantly higher contact surface area for biochemical analysis using bioreceptors, and benefit from stronger polarization selectivity between azimuthal and radially polarized modes.
Tilted fiber Bragg gratings (TFBGs) transmitted amplitude spectra intrinsically depend on light polarization, leading to
the existence of two peak families separated by a few picometers to a few tens of picometers for the s and p-polarized
light states. In this paper, we demonstrate that, thanks to a dense thin film with high refractive index (Zinc oxide)
deposited around TFBGs, the polarization dependency can be drastically enhanced, resulting in a wavelength separation
of up to 550 picometers between the two orthogonal resonance peak families. This results in two important assets: the
sensitivity is extended to surrounding refractive index (SRI) values above 1.45 while the strong polarization dependency
makes the demodulation process easier with differential SRI sensitivities between the two modes families.
Tilted fiber Bragg gratings (TFBGs) present a refractive index modulation blazed by a few degrees in the plane
perpendicular to the optical fiber axis. This results in two kinds of couplings: self-backward coupling of the core mode
and numerous backward couplings between the core mode and the cladding modes. TFBG transmitted amplitude spectra
are therefore characterized by several tens of cladding mode resonances that possess their own sensitivity to the
surrounding refractive index (SRI). TFBGs naturally allow the realization of refractometers accurate to 10-4 RIU
(refractive index unit) in the SRI range between 1.33 and 1.45 (refractive index of silica). In this work, we demonstrate
that a dense thin film of zinc oxide, a high refractive index material (n=1.9), deposited by RF sputtering in two steps, on
the TFBGs tightly modifies their transmitted amplitude spectra. While the wavelength difference between orthogonally
polarized modes in nude TFBGs remains within a few picometers, the association of a nanoscale metal oxide coating
increases this difference up to 500 picometers. This results in two main assets: the sensitivity is extended to SRI values
above 1.45 while the strong polarization dependency makes the demodulation process easier.
This paper presents a sensor based on a Tilted Fiber Bragg Grating (TFBG) covered with a mesoporous coating
consisting of ZnO nanoparticles for the detection of volatile organic compounds.
TFBGs are Bragg gratings that are tilted by a small angle inducing a coupling between the forward-going fundamental
core mode and the backward-going cladding modes. They present a transmission amplitude spectrum consisting of
several tens of resonances, which present their own sensitivity to the surrounding refractive index. Specific sensors can
be built by using TFBGs covered with a dedicated coating that changes its refractive index when in contact with target
chemical species. This concept was illustrated with ethanol and a mesoporous ZnO coating whose refractive index
changes due to the gas adsorption on the ZnO particles.
The exposure to ethanol vapors of the covered TFBG yields important modifications of the transmission amplitude
spectrum in the range 1510-1590 nm. All cladding mode resonances show a red shift while their peak-to-peak amplitude
decreases with increasing ethanol concentration in air. The response, defined as the amplitude or the wavelength change
of a resonance peak, is fast (1s), linear, reversible and without hysteresis (red shift of 60 pm/vol% ethanol and -3.3
dB/vol% at 1550 nm).
A novel infrared radiation detector based on a pair of fiber Bragg gratings (FBGs) is described. In the proposed
configuration, the two FBGs are distant by a few centimeters and are characterized by Bragg resonances separated by a
few nanometers. One FBG of the pair is coated with an IR-absorbing layer which converts the radiation into heat.
Therefore, exposure to IR radiations will increase the temperature of the coated FBG, which in turn induces a Bragg
wavelength shift to higher values. To take into account the ambient temperature fluctuations, the second grating is
protected by an IR-reflecting tube which prevents heating of this grating that can then be used as temperature reference.
IR radiation measurements are finally obtained through the monitoring of the differential shift between both Bragg
wavelengths. This shift shows a monotonic behavior as a function of the IR radiations. This sensor shows a strong
potential for early fire detection as it detects radiation emitted during the fire instead of the temperature increase when
the fire is fully developed.
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