Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Tilted-Fiber Bragg Gratings <s> A detailed theoretical treatment is presented of bound-mode to bound-mode Bragg reflection and bound-mode to radiation-mode coupling loss in a tilted optical-fiber phase grating. Numerical predictions of the effects of grating tilt on the spectral characteristics of such a grating are calculated. These predictions are compared with experimentally measured spectra of strong gratings written by ultraviolet irradiation of deuterium-sensitized fiber with grating tilt angles ranging from 0° to 15°. Good agreement is obtained between the theoretical predictions and the experimental results. <s> BIB001 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Tilted-Fiber Bragg Gratings <s> In this letter, a demodulation technique is presented in order to measure the surrounding refractive index in the range 1-1.45 by means of a weakly tilted fiber Bragg grating. This technique is based on the global monitoring of the cladding modes in the transmitted spectrum and on the computation of two statistical parameters. We report a resolution of 210/sup -4/ in terms of the refractive index as well as a temperature-insensitive behavior. <s> BIB002 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Tilted-Fiber Bragg Gratings <s> Short-period fiber Bragg gratings with weakly tilted grating planes generate multiple strong resonances in transmission. Our experimental results show that the wavelength separation between selected resonances allows the measurement of the refractive index of the medium surrounding the fiber for values between 1.25 and 1.44 with an accuracy approaching 1x10(-4). The sensor element is 10 mm long and made from standard single-mode telecommunication grade optical fiber by ultraviolet light irradiation through a phase mask. <s> BIB003 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Tilted-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> BIB004 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Tilted-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> BIB005
Tilted-fiber Bragg gratings (TFBGs) are short period (~500 nm) gratings with a refractive index modulation angled (θ < 45 • ) with respect to the perpendicular to the optical fiber axis BIB001 . In addition to the self-backward coupling of the core mode at the Bragg wavelength, TFBGs redirect some light to the cladding. There, the diameter is such that several possible cladding modes can propagate, each with its own phase velocity (and, subsequently, effective refractive index n eff,clad ) BIB001 . These possible modes of propagation correspond to different ray angles, as sketched in Figure 3 . There is a one-to-one relationship between the wavelength at which coupling occurs for a given cladding mode and its effective refractive index. This relationship is expressed by a similar phase-matching condition as for uniform FBGs: λ i clad = (n eff,core + n i eff,clad )Λ where the superscript i denotes the mode number. Figure 3 displays the transmitted amplitude spectrum of a 1 cm-long 10 • TFBG. Each resonance of the spectral comb corresponds to the coupling from the core mode to a group of backward-propagating cladding modes. As a result of phase matching, the spectral position of a resonance depends on the effective refractive index of the corresponding cladding mode, which in turn depends on the optical properties of the medium over or near the cladding surface. close to 1.447 at 1550 nm for a standard single-mode optical fiber. An SRI change yields a Bragg wavelength shift in the amplitude spectrum of the grating. Tilted-fiber Bragg gratings (TFBGs) are short period (~500 nm) gratings with a refractive index modulation angled (θ < 45°) with respect to the perpendicular to the optical fiber axis BIB001 . In addition to the self-backward coupling of the core mode at the Bragg wavelength, TFBGs redirect some light to the cladding. There, the diameter is such that several possible cladding modes can propagate, each with its own phase velocity (and, subsequently, effective refractive index neff,clad) BIB001 . These possible modes of propagation correspond to different ray angles, as sketched in Figure  3 . There is a one-to-one relationship between the wavelength at which coupling occurs for a given cladding mode and its effective refractive index. This relationship is expressed by a similar phase-matching condition as for uniform FBGs: λ i clad = (neff,core + n i eff,clad)Λ where the superscript i denotes the mode number. Figure 3 displays the transmitted amplitude spectrum of a 1 cm-long 10° TFBG. Each resonance of the spectral comb corresponds to the coupling from the core mode to a group of backward-propagating cladding modes. As a result of phase matching, the spectral position of a resonance depends on the effective refractive index of the corresponding cladding mode, which in turn depends on the optical properties of the medium over or near the cladding surface. Therefore, spectral shifts of individual resonances can be used for refractometry purposes, either in the surrounding medium or inside a thin coating deposited on the fiber outer surface. The first demonstration of SRI sensing with TFBGs dates back to 2001  . For an SRI increase between 1.30 and 1.45, a progressive smoothing of the transmitted amplitude spectrum was reported, starting from the shortest wavelengths. In practice, two main demodulation techniques can be used to correlate the spectral content with the SRI value quantitatively. The first method considers the global spectral evolution and involves monitoring the area delimited by the cladding mode resonance spectrum, through a computation of the upper and lower envelopes as resonances gradually disappear when the SRI reaches the cut-off points of each cladding mode  BIB002 . The other method is more local and tracks the wavelength shift and amplitude variation of individual cladding mode resonances as they approach the cut-off wavelength. The latter is the wavelength at which the effective refractive index of a given cladding mode resonance equals that of the surrounding medium BIB003 . Both techniques present minimum detectable SRI changes of ~10 −4 RIU when used with bare gratings. In terms of wavelength shift, this yields a sensitivity that peaks between 10 nm/RIU and 25 nm/RIU for the modes near cut-off. In all cases, the Bragg wavelength provides an absolute power and wavelength reference, which can therefore be used to remove uncertainties related to systematic fluctuations (such as unwanted power-level changes from the light source) and even ambient temperature changes. Indeed, all cladding mode resonances shift similarly to the Bragg resonance when temperature varies, as reported in  . Therefore, spectral shifts of individual resonances can be used for refractometry purposes, either in the surrounding medium or inside a thin coating deposited on the fiber outer surface. The first demonstration of SRI sensing with TFBGs dates back to 2001  . For an SRI increase between 1.30 and 1.45, a progressive smoothing of the transmitted amplitude spectrum was reported, starting from the shortest wavelengths. In practice, two main demodulation techniques can be used to correlate the spectral content with the SRI value quantitatively. The first method considers the global spectral evolution and involves monitoring the area delimited by the cladding mode resonance spectrum, through a computation of the upper and lower envelopes as resonances gradually disappear when the SRI reaches the cut-off points of each cladding mode  BIB002 . The other method is more local and tracks the wavelength shift and amplitude variation of individual cladding mode resonances as they approach the cut-off wavelength. The latter is the wavelength at which the effective refractive index of a given cladding mode resonance equals that of the surrounding medium BIB003 . Both techniques present minimum detectable SRI changes of~10 −4 RIU when used with bare gratings. In terms of wavelength shift, this yields a sensitivity that peaks between 10 nm/RIU and 25 nm/RIU for the modes near cut-off. In all cases, the Bragg wavelength provides an absolute power and wavelength reference, which can therefore be used to remove uncertainties related to systematic fluctuations (such as unwanted power-level changes from the light source) and even ambient temperature changes. Indeed, all cladding mode resonances shift similarly to the Bragg resonance when temperature varies, as reported in  . 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 .