Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Excessively Tilted Fiber Gratings <s> For the first time to the authors' knowledge, fiber Bragg gratings (FBGs) with >80 degrees tilted structures have been fabricated and characterized. Their performance in sensing temperature, strain, and the surrounding medium's refractive index was investigated. In comparison with normal FBGs and long-period gratings (LPGs), >80 degrees tilted FBGs exhibit significantly higher refractive-index responsivity and lower thermal cross sensitivity. When the grating sensor was used to detect changes in refractive index, a responsivity as high as 340 nm/refractive-index unit near an index of 1.33 was demonstrated, which is three times higher than that of conventional LPGs. <s> BIB001 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Excessively Tilted Fiber Gratings <s> We report a distinctive polarization mode coupling behaviour of tilted fibre Bragg gratings (TFBGs) with a tilted angle exceeding 45°. The ex-45° TFBGs exhibit pronounced polarization mode splitting resulted from the birefringence induced by the grating structure asymmetry. We have fabricated TFBGs with a tilted structure at 81° and studied their properties under transverse load applied to their equivalent fast and slow axes. The results show that the light coupling to the orthogonally polarized modes of the 81°-TFBGs changes only when the load is applied to their slow axis, giving a prominent directional loading response. For the view of real applications, we further investigated the possibility of interrogating such a TFBG-based load sensor using low-cost and compact-size single wavelength source and power detector. The experimental results clearly show that the 81°-TFBGs plus the proposed power-measurement interrogation scheme may be developed to an optical fibre vector sensor system capable of not just measuring the magnitude but also recognizing the direction of the applied transverse load. Using such an 81°-TFBG based load sensor, a load change as small as 1.6 × 10-2 g may be detected by employing a standard photodiode detector. <s> BIB002 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Excessively Tilted Fiber Gratings <s> By exploiting the polarization-sensitive coupling effect of graphene with the optical mode, we investigate the polarization modulation properties of a hybrid waveguide of graphene-integrated excessively tilted fiber grating (Ex-TFG). The theoretical analysis and experimental results demonstrate that the real and imaginary parts of complex refractive index of few-layer graphene exhibit different effects on transverse electric (TE) and transverse magnetic (TM) cladding modes of the Ex-TFG, enabling stronger absorption in the TE mode and more wavelength shift in the TM mode. Furthermore, the surrounding refractive index can modulate the complex optical constant of graphene and then the polarization properties of the hybrid waveguide, such as resonant wavelength and peak intensity. Therefore, the unique polarization tuning property induced by the integration of the graphene layer with Ex-TFG may endow potential applications in all-in-one fiber modulators, fiber lasers, and biochemical sensors. <s> BIB003 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Excessively Tilted Fiber 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> BIB004 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Excessively Tilted Fiber 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> BIB005
Excessively tilted fiber gratings (ETFGs) are characterized by a tilt angle θ > 45 •  BIB002 BIB001 BIB003 . Given the very high angle value, they couple light to the cladding in the forward direction and present a hybrid behavior between the aforementioned weakly tilted FBGs and the long-period fiber gratings introduced in Section 2.5. As depicted in Figure 4 , their transmitted amplitude spectrum is composed of multiple cladding mode resonances (with FWHM of a few nanometers, typically) spread over a wavelength range of a few hundred nanometers. They are usually produced with a continuous-wave laser using custom phase masks with a period higher than standard ones used for uniform and weakly tilted FBGs photo-inscription. Amplitude masks can also be used. In  , theoretical considerations about light coupling in these structures are presented. 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 BIB004 BIB005 . Excessively tilted fiber gratings (ETFGs) are characterized by a tilt angle θ > 45°  BIB002 BIB001 BIB003 . Given the very high angle value, they couple light to the cladding in the forward direction and present a hybrid behavior between the aforementioned weakly tilted FBGs and the long-period fiber gratings introduced in Section 2.5. As depicted in Figure 4 , their transmitted amplitude spectrum is composed of multiple cladding mode resonances (with FWHM of a few nanometers, typically) spread over a wavelength range of a few hundred nanometers. They are usually produced with a continuous-wave laser using custom phase masks with a period higher than standard ones used for uniform and weakly tilted FBGs photo-inscription. Amplitude masks can also be used. In  , theoretical considerations about light coupling in these structures are presented.  Excessively tilted fiber gratings (ETFGs) are characterized by a tilt angle θ > 45°  BIB002 BIB001 BIB003 . Given the very high angle value, they couple light to the cladding in the forward direction and present a hybrid behavior between the aforementioned weakly tilted FBGs and the long-period fiber gratings introduced in Section 2.5. As depicted in Figure 4 , their transmitted amplitude spectrum is composed of multiple cladding mode resonances (with FWHM of a few nanometers, typically) spread over a wavelength range of a few hundred nanometers. They are usually produced with a continuous-wave laser using custom phase masks with a period higher than standard ones used for uniform and weakly tilted FBGs photo-inscription. Amplitude masks can also be used. In  , theoretical considerations about light coupling in these structures are presented. 