Surface plasmon resonance (SPR) excitation has been widely studied in the well-known Kretschmann prism configuration, leading to a large variety of refractometric optical sensors. In recent research, this bulky optical device has found a counterpart thanks to the use of metal-coated optical fibers, mainly allowing to considerably reduce the size of the sensors. Some approaches make use of multimode, etched or unclad fibers while the grating- based alternatives are mostly focused on uniform, long period or tilted fiber Bragg gratings (TFBGs). However, plasmonic optical fiber sensing has been pretty much restricted to aqueous solutions due to the remarkable applications of these devices in (bio)chemical sensing. This work gives the roadmap through SPR excitation in air by using a 10° TFBG refractometer. With regard to aforementioned developments, the photo-inscription process is carried out with an excimer laser emitting at 193 nm, which creates the grating planes in a position close to the core-cladding interface. By doing so, it is possible to obtain a cladding mode resonance comb covering the range of the spectrum that corresponds to refractive index values around the one of the air, without the need of using highly tilted FBGs. Indeed, the coupling of cladding modes to the outer medium can be observed in the optical transmitted spectrum of a bare TFBG. In addition, the thickness of the gold thin-film deposited at the grating location is reduced to one third of the one used for SPR excitation in liquids. In this way, all the cladding modes are reflected by the metal when the TFBG is immersed in solution but when it is left in the air a SPR signature appears in the spectrum. The methods described in the present paper are intended to support further developments on plasmonic optical fiber solutions applied to refractive index sensing in gaseous atmospheres.
Gold-coated tilted fiber Bragg gratings (TFBGs) can now be considered as a mature technology for lab-on-fiber sensing based on surface plasmon resonance (SPR) excitation. This sensing architecture brings considerable assets such as easy light injection, temperature fluctuations immunity and remote operation in very small volumes of analytes. Different metal configurations have been used so far, without considerations about their relative performances in terms of surrounding refractive index (SRI) sensing. In this work, we study the impact of the coating on the cladding mode distribution in the TFBG transmitted amplitude spectrum and subsequently on its SRI sensitivity. Different configurations of gold coating are produced and tested, relying on both the sputtering and electroless deposition processes. Interesting spectral features are reported, confirming that the coating thickness and its relative disparity are important design parameters that drive the overall sensing performances.
In this paper, we present the results of a new pH sensor based on a polyaniline (PAni) coating on the surface of a tilted fiber Bragg grating. The pH-sensitive PAni was deposited by in situ chemical oxidative polymerization. The performance of the fabricated pH sensor was tested and the obtained pH values were compared with the results obtained using a pH meter device. It was found that the sensor exhibits response to pH changes in the range of 2-12, achieving a sensitivity of 46 pm/pH with a maximum error due to the hysteresis effect of ±1.14 pH. The main advantages of this PAni-TFBG pH sensor are biochemical compatibility, temperature independence, long-term stability and remote realtime multipoint sensing features. This type of sensor could be used for biochemical applications, pipeline corrosion monitoring or remote-multipoint measurements.
Two interrogation techniques for plasmonic tilted fiber Bragg grating sensors are reported and experimentally tested. Typical interrogation methods are usually based on tracking the wavelength shift of the most sensitive cladding mode, but for biosensing applications, spectrometer-based methods can be replaced by more efficient solutions. The proposed techniques thus rely on the measurement of the induced changes in optical power. The first one consists of a properly polarized tunable laser source set to emit at the wavelength of the sensor most sensitive mode and an optical power meter to measure the transmitted response. For the second method, a uniform fiber Bragg grating is photo-inscribed beyond the sensor in such a way that its central wavelength matches the sensor most sensitive mode, acting as an optical filter. Using a LED source, light reflected backwards by this grating is partially attenuated when passing through the sensor due to plasmon wave excitation and the power changes are quantified once again with an optical power meter. A performance analysis of the techniques is carried out and they both result competitive interrogation solutions. The work thus focuses on the development of cost-effective alternatives for monitoring this kind of biosensors in practical situations.
An infrared femtosecond pulses laser is used to manufacture point-by-point gratings in telecommunication-grade optical fibres. The refractive index modulations are localized close to the core-cladding interface, yielding a strong coupling to cladding mode resonances together with an important photo-induced birefringence. Such gratings have been recently used for refractrometric measurements. In this work, their transmitted amplitude spectrum is measured with polarized light while they are exposed to temperature changes up to 900 °C. Despite an overall good thermal stability of the gratings that confirms their robustness for high-temperature refractometry, we report an interesting polarization effect depending on both the cladding mode resonance family (radially- and azimuthally-polarized modes) and mode order. While the birefringence of the core mode resonance decreases with the temperature, certain cladding mode resonances show an increase of the wavelength splitting between their orthogonally-polarized components. This differential behaviour can be of high interest to develop high-resolution multiparametric sensing platforms.
We report a practical study of the thermal decay of cladding mode resonances in tilted fiber Bragg gratings, establishing
an analogy with the “power law” evolution previously observed on uniform gratings. We examine how this process
contributes to a great thermal stability, even improving it by means of a second cycle slightly increasing the annealing
temperature. In addition, we show an improvement of the grating spectrum after annealing, with respect to the one just
after inscription, which suggests the application of this method to be employed to improve saturation issues during the
photo-inscription process.
A tilted fiber Bragg grating is photo-inscribed in the core of a single-mode optical fiber, leading to the coupling of cladding mode resonances all along a wide region of the near-infrared spectrum. The grating is then coated with a thin film of gold in order to create a metal-dielectric interface. This way, light propagating through the cladding of the optical fiber is able to excite a surface plasmon wave on the outer interface. As sensitive element, a molecularly imprinted polymer is deposited by electropolymerization as a thin film around the previous gold coating. The thickness of the polymer is controlled by means of the surface plasmon resonance signature in order to preserve a correct surrounding refractive index sensitivity when used in a gaseous environment. The chosen polymer has an affinity to formaldehyde, which is a volatile organic compound worth to detect, especially because of its toxicity for the human being. We report a global wavelength shift of the grating cladding mode resonances in the presence of formaldehyde in gaseous state. This shift is due to a change in the refractive index of the polymer when it bounds to the target molecules. The sensor exhibits a linear response, together with a low limit of detection.
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