Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Eccentric Fiber Bragg Gratings <s> The spectral characteristics of a fiber Bragg grating (FBG) with a transversely inhomogeneous refractive index profile, differs con- siderably from that of a transversely uniform one. Transmission spectra of inhomogeneous and asymmetric FBGs that have been inscribed with focused ultrashort pulses with the so-called point-by-point technique are investigated. The cladding mode resonances of such FBGs can span a full octave in the spectrum and are very pronounced (deeper than 20dB). Using a coupled-mode approach, we compute the strength of resonant coupling and find that coupling into cladding modes of higher azimuthal order is very sensitive to the position of the modification in the core. Exploiting these properties allows precise control of such reflections and may lead to many new sensing applications. <s> BIB001 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Eccentric Fiber Bragg Gratings <s> Highly localized fiber Bragg gratings can be inscribed point-by-point with focused ultrashort pulses. The transverse localization of the resonant grating causes strong coupling to cladding modes of high azimuthal and radial order. In this paper, we show how the reflected cladding modes can be fully analyzed, taking their vectorial nature, orientation and degeneracies into account. The observed modes’ polarization and intensity distributions are directly tied to the dispersive properties and show abrupt transitions in nature, strongly correlated with changes in the coupling strengths. <s> BIB002 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Eccentric Fiber Bragg Gratings <s> Highly localized refractive index modulations are photo-written in the core of pure silica fiber using point-by-point focused UV femtosecond pulses. These specific gratings exhibit a comb-like transmitted amplitude spectrum, with polarization-dependent narrowband cladding mode resonances. In this work, eccentric gratings are surrounded by a gold sheath, allowing the excitation of surface plasmon polaritons (SPP) for radially-polarized light modes. The spectral response is studied as a function of the surrounding refractive index and a maximum sensitivity of 50 nm/RIU (refractive index unit) is reported for a well-defined cladding-mode resonance among the spectral comb. This novel kind of plasmonic fiber grating sensor offers rapidity of production, design flexibility, and high temperature stability. <s> BIB003 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Eccentric Fiber Bragg Gratings <s> New dual temperature and strain sensor has been designed using eccentric second-order fiber Bragg gratings produced in standard single-mode optical fiber by point-by-point direct writing technique with tight focusing of 800 nm femtosecond laser pulses. With thin gold coating at the grating location, we experimentally show that such gratings exhibit a transmitted amplitude spectrum composed by the Bragg and cladding modes resonances that extend in a wide spectral range exceeding one octave. An overlapping of the first order and second order spectrum is then observed. High-order cladding modes belonging to the first order Bragg resonance coupling are close to the second order Bragg resonance, they show a negative axial strain sensitivity (-0.55 pm/με) compared to the Bragg resonance (1.20 pm/με) and the same temperature sensitivity (10.6 pm/°C). With this well conditioned system, temperature and strain can be determined independently with high sensitivity, in a wavelength range limited to a few nanometers. <s> BIB004 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Eccentric Fiber Bragg Gratings <s> Off-axis fiber Bragg gratings are inscribed by ultraviolet irradiation limited to expose only a portion of the fiber core cross section. The coupling to cladding modes is significantly increased, and the amplitude of the cladding mode resonances becomes sensitive to bending in magnitude and direction. Sensitivities ranging from +1.17 dB/m(-1) to -1.25 dB/m(-1) were obtained for bending in different directions relative to the offset direction of the grating, for curvatures from 0 to 1.1 m(-1), a range ideal for the shape sensing of large structures. The bending sensor response is also shown to be independent of temperature and the surrounding refractive index. <s> BIB005 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Eccentric Fiber Bragg Gratings <s> Fibre sensors are key to many minimally-invasive detection techniques but, owing to an index mismatch, they are often limited to aqueous environments. Here, Caucheteur et al. develop a high-resolution fibre gas sensor with a tilted in-fibre grating that allows coupling to higher-order plasmon modes. <s> BIB006 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Eccentric Fiber Bragg Gratings <s> The conventional single-angle tilted fiber Bragg grating (TFBG) can only excite a certain range of cladding modes, limiting it for refractive index (RI) measurement in a wide range. In this letter, we fabricate and demonstrate a multi-angle TFBG, in which five individual TFBGs with tilt angles ranging from 5° to 25° are sequentially inscribed along the core of a single mode fiber within a length of 20 mm. The multi-angle TFBG excites a continuous spectral comb of narrowband-cladding modes over a much wider wavelength range (>170 nm) than a single-angle TFBG, making it suitable for dynamic RI measurement over a wide range (1.15–1.45). We have experimentally measured aqueous solutions with RI ranging from 1.30 to 1.45 using the uncoated (by monitoring the cut-off mode) and gold-coated (by monitoring the surface Plasmon resonance) multi-angle TFBGs, and both methods show linear responses, with RI sensitivities about 500 nm/RIU. <s> BIB007
Eccentric fiber Bragg gratings (EFBGs) correspond to a point-by-point refractive index modulation highly localized in the core, close to the cladding region BIB001 BIB002 BIB003 BIB004 , as sketched in Figure 5 . They are usually obtained with a tight focussing of a femtosecond pulses laser and the use of an air-bearing translation stage. Their strong localization close to the core-cladding interface induces light coupling in the fiber cladding, in a manner very similar to weakly tilted FBGs. Hence, their transmitted amplitude spectrum looks like a dense spectral comb, with hundreds of cladding mode resonances. An important difference with respect to TFBGs should be noted: EFBGs continuously couple cladding mode resonances with effective refractive indices ranging from 1.45 (at the right-hand side of the spectrum) to 1.00 (at the left-hand side) where they are cut-off when the grating is surrounded by air BIB005 . In the case of TFBGs, light coupling happens in privileged wavelength ranges, depending on the tilt angle value BIB007 . Their sensitivity to surrounding refractive index changes is very similar to that of TFBGs BIB002 .  Eccentric fiber Bragg gratings (EFBGs) correspond to a point-by-point refractive index modulation highly localized in the core, close to the cladding region BIB001 BIB002 BIB003 BIB004 , as sketched in Figure 5 . They are usually obtained with a tight focussing of a femtosecond pulses laser and the use of an air-bearing translation stage. Their strong localization close to the core-cladding interface induces light coupling in the fiber cladding, in a manner very similar to weakly tilted FBGs. Hence, their transmitted amplitude spectrum looks like a dense spectral comb, with hundreds of cladding mode resonances. An important difference with respect to TFBGs should be noted: EFBGs continuously couple cladding mode resonances with effective refractive indices ranging from 1.45 (at the right-hand side of the spectrum) to 1.00 (at the left-hand side) where they are cut-off when the grating is surrounded by air BIB005 . In the case of TFBGs, light coupling happens in privileged wavelength ranges, depending on the tilt angle value BIB007 . Their sensitivity to surrounding refractive index changes is very similar to that of TFBGs BIB002 . The higher the tilt angle value, the higher the coupling to higher-order cladding modes. For tilt angle values exceeding 30°, it is possible to couple light to modes with an effective refractive index close to 1.00, which can be used for refractometry in gaseous media BIB006 BIB007 . Eccentric fiber Bragg gratings (EFBGs) correspond to a point-by-point refractive index modulation highly localized in the core, close to the cladding region BIB001 BIB002 BIB003 BIB004 , as sketched in Figure 5 . They are usually obtained with a tight focussing of a femtosecond pulses laser and the use of an air-bearing translation stage. Their strong localization close to the core-cladding interface induces light coupling in the fiber cladding, in a manner very similar to weakly tilted FBGs. Hence, their transmitted amplitude spectrum looks like a dense spectral comb, with hundreds of cladding mode resonances. An important difference with respect to TFBGs should be noted: EFBGs continuously couple cladding mode resonances with effective refractive indices ranging from 1.45 (at the right-hand side of the spectrum) to 1.00 (at the left-hand side) where they are cut-off when the grating is surrounded by air BIB005 . In the case of TFBGs, light coupling happens in privileged wavelength ranges, depending on the tilt angle value BIB007 . Their sensitivity to surrounding refractive index changes is very similar to that of TFBGs BIB002 . 