Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> Bragg gratings have been produced in germanosilicate optical fibers by exposing the core, through the side of the cladding, to a coherent UV two-beam interference pattern with a wavelength selected to lie in the oxygen-vacancy defect band of germania, near 244 nm. Fractional index perturbations of approximately 3 x 10(-5) have been written in a 4.4-mm length of the core with a 5-min exposure. The Bragg filters formed by this new technique had reflectivities of 50-55% and spectral widths, at half-maximum, of 42 GHz. <s> BIB001 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> Optical fibre Bragg reflectors have been fabricated using a single pulse of high power 249 nm excimer laser light to photoinduce point-by-point each individual index element forming the grating. Bragg reflectors with a length of 360 μm and reflectivity of 70% have been made. <s> BIB002 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> A photolithographic method is described for fabricating refractive index Bragg gratings in photosensitive optical fiber by using a special phase mask grating made of silica glass. A KrF excimer laser beam (249 nm) at normal incidence is modulated spatially by the phase mask grating. The diffracted light, which forms a periodic, high‐contrast intensity pattern with half the phase mask grating pitch, photoimprints a refractive index modulation into the core of photosensitive fiber placed behind, in proximity, and parallel, to the mask; the phase mask grating striations are oriented normal to the fiber axis. This method of fabricating in‐fiber Bragg gratings is flexible, simple to use, results in reduced mechanical sensitivity of the grating writing apparatus and is functional even with low spatial and temporal coherence laser sources. <s> BIB003 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> Keywords: [OFD] Note: Ouellette, FConference on Photosensitivity and Self-Organization in Optical Fibers and WaveguidesAUG 17-18, 1993QUEBEC, CANADA0-8194-1303-8 Reference LOA-ARTICLE-1993-003 Record created on 2009-01-20, modified on 2017-05-10 <s> BIB004 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> In this paper, we describe the spectral characteristics that can be achieved in fiber reflection (Bragg) and transmission gratings. Both principles for understanding and tools for designing fiber gratings are emphasized. Examples are given to illustrate the wide variety of optical properties that are possible in fiber gratings. The types of gratings considered include uniform, apodized, chirped, discrete phase-shifted, and superstructure gratings; short-period and long-period gratings; symmetric and tilted gratings; and cladding-mode and radiation-mode coupling gratings. <s> BIB005 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> The historical beginnings of photosensitivity and fiber Bragg grating (FBG) technology are recounted. The basic techniques for fiber grating fabrication, their characteristics, and the fundamental properties of fiber gratings are described. The many applications of fiber grating technology are tabulated, and some selected applications are briefly described. <s> BIB006 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> A novel tunable fiber Bragg grating (FBG) filter with a chemically etched cladding has been demonstrated. By reducing the cladding diameter of a FBG through etching with HF, the tension tuning force can be lowered by over an order of magnitude. Wavelength tuning up to a failure point of 21.2 nm has been demonstrated along with repeated cycling over a 9 nm tuning range. Observations of increased coupling to resonant cladding modes in the etched FBG's are also noted. <s> BIB007 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> Photosensitivity in optical fibres properties of Fibre Bragg gratings inscribing Bragg gratings in optical fibres Fibre Bragg grating theory applications of Bragg gratings in communications Fibre Bragg grating sensors impact of Fibre Bragg gratings. <s> BIB008 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> An opto-chemical in-fibre Bragg grating (FBG) sensor for refractive index measurement in liquids has been developed using fibre side-polishing technology. At a polished site where the fibre cladding has partly been removed, a FBG is exposed to a liquid analyte via evanescent field interaction of the guided fibre mode. The Bragg wavelength of the FBG is obtained in terms of its dependence on the refractive index of the analyte. Modal and wavelength dependences have been investigated both theoretically and experimentally in order to optimize the structure of the sensor. Using working wavelengths far above the cut-off wavelength results in an enhancement of the sensitivity of the sensor. Measurements with different mode configurations lead to the separation of cross sensitivities. Besides this, a second FBG located in the unpolished part can be used to compensate for temperature effects. Application examples for monitoring fuels of varying quality as well as salt concentrations under deep borehole conditions are presented. <s> BIB009 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> A novel hydrogel-coated single-mode fiber Bragg grating (FBG) sensor is investigated. The sensor uses a swellable polymer material, hydrogel, as the active sensing component. The sensing mechanism is based on the stress that is induced in the chemically sensitive swellable hydrogel coating. The stress shifts the Bragg wavelength of the FBG. By reducing the cladding diameter of FBG through etching with hydrofluoric acid, the tension tuning force of FBG can be lowered. Therefore, the stress induced in hydrogel can shift the wavelength of FBG more and the sensitivity of the sensor is improved. When the grating fiber is etched 37.5 μm in diameter, the sensitivity of sensor is approximately 10-fold larger than that without fiber etching. <s> BIB010 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> In this work, the numerical and experimental analysis on the use of thinned fiber Bragg gratings as refractive index sensors have been carried out. Wet chemical etching in a buffered hydrofluoric acid solution was used for sensor fabrication. Experimental characterization for an almost full etched cladding sensor is presented demonstrating good agreement with numerical results and resolutions of /spl ap/10/sup -5/ and /spl ap/10/sup -4/ for outer refractive index around 1.45 and 1.333, respectively. <s> BIB011 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> The theoretical model of etched fiber Bragg grating (FBG) backward cladding-mode resonances for ambient refractive index sensing is presented. The dependent behaviors of the mode resonances have been analyzed in the etching process and the ambient refractive index changed. The analysis is based on the classical coupling-mode theory while considering interactions among multiple modes and developed on a three-layer step-index fiber geometry. Experimental data match the theoretical model wonderfully. This model not only describes the relationship between the FBG backward cladding-mode resonances and the ambient index but also is valuable for the design of a flexible highly sensitive ambient index sensor. <s> BIB012 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> A refractive index sensor based on the thinned and microstructure fiber Bragg grating (ThMs-FBG) was proposed and realized as a chemical sensing. The numerical simulation for the reflectance spectrum of the ThMs-FBG was calculated and the phase shift down-peak could be observed from the reflectance spectrum. Many factors influencing the reflectance spectrum were considered in detail for simulation, including the etched depth, length, and position. The sandwich-solution etching method was utilized to realize the microstructure of the ThMs-FBG, and the photographs of the microstructure were obtained. Experimental results demonstrated that the reflectance spectrum, phase shift down-peak wavelength, and reflected optical intensity of the ThMs-FBG all depended on the surrounding refractive index. However, only the down-peak wavelength of the ThMs-FBG changed with the surrounding temperature. Under the condition that the length and cladding diameter of the ThMs-FBG microstructure were 800 and 14 mum, respectively, and the position of the microstructure of the ThMs-FBG is in the middle of grating region, the refractive index sensitivity of the ThMs-FBG was 0.79 nm/refractive index unit with the wide range of 1.33-1.457 and a high resolution of 1.2 x 10(-3). The temperature sensitivity was 0.0103 nm/ degrees C, which was approximately equal to that of common FBG. <s> BIB013 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> A new type of hydrogen sensor based on a side-polished fiber Bragg grating (FBG) coated with thin palladium film was demonstrated experimentally. The used FBG with the reflectivity of 90% is fabricated in a hydrogen-loaded single-mode fiber (SMF-28) by using the phase mask writing technique of a KrF excimer laser. The experimental results show that proposed sensor can be applied for hydrogen concentration measurements. <s> BIB014 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> Spectral data for each pixel in a confocal spatial scan are acquired by mapping spectral slices into the time domain with an array of visible fiber Bragg gratings. Multispectral images of biomedical tissue can be generated in real time. <s> BIB015 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> The point-by-point technique of fabricating fibre-Bragg gratings using an ultrafast laser enables complete control of the position of each index modification that comprises the grating. By tailoring the local phase, amplitude and spacing of the grating’s refractive index modulations it is possible to create gratings with complex transmission and reflection spectra. We report a series of grating structures that were realized by exploiting these flexibilities. Such structures include gratings with controlled bandwidth, and amplitude- and phase-modulated sampled (or superstructured) gratings. A model based on coupled-mode theory provides important insights into the manufacture of such gratings. Our approach offers a quick and easy method of producing complex, non-uniform grating structures in both fibres and other mono-mode waveguiding structures. <s> BIB016 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> Fiber Bragg grating (FBG) is side-polished to enable interaction with sensitive materials around FBG fiber core. Using TbFeCo magneto-optic thin film deposited onto FBG fiber core as transducer, a FBG refractive index senor for magnetic field/current characterization is first proposed and demonstrated in this paper. Magnetic field sensing experiments show 19 pm of wavelength shift at a magnetic field intensity of 50 mT, the average linearity of magnetic field response is 0.9877. <s> BIB017 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> WO3-Pd composite films were deposited on the side-face of side-polished fiber Bragg grating as sensing elements by magnetron sputtering process. XRD result indicates that the WO3-Pd composite films are mainly amorphous. Compared to standard FBG coated with same hydrogen sensitive film, side-polished FBG significantly increase the sensor’s sensitivity. When hydrogen concentrations are 4% and 8% in volume percentage, maximum wavelength shifts of side-polished FBG are 25 and 55 pm respectively. The experimental results show the sensor’s hydrogen response is reversible, and side-polished FBG hydrogen sensor has great potential in hydrogen’s measurement. <s> BIB018 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> To compensate for the temperature dependency of a standard FBG, a cladding-etched FBG immersed with a liquid mixture having a negative thermo-optic coefficient is presented, and its characteristics are investigated. The Bragg wavelength of the cladding-etched FBG is shifted counter to the direction of the Bragg wavelength shift of a conventional FBG according to the mixing ratio of glycerin to water; thus, the temperature-dependent Bragg wavelength shift was almost compensated by using a liquid mixture of water (50%) and glycerin (50%) having the negative thermo-optic coefficient of −5 × 10−4 °C−1. <s> BIB019 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> Abstract We present a refractive index (RI) distribution sensor based on an etched fiber Bragg grating (FBG). The FBG is etched by hydrofluoric acid solution until the residual diameter of its core is about 5.73 μm, and then it is installed in a micro-tube where the RI of the liquid medium decreases linearly along its axis. The reflection spectrum of the etched FBG changes from a single resonant peak into many micro-resonant peaks under the inhomogeneous liquid medium. The difference of the wavelengths between the maximum micro-resonant peak and the minimum one changes linearly with the RI gradient, and the average change of the wavelengths of them changes linearly with the average change of the RI as well. Experimental results show that the sensitivity of RI gradient and the average sensitivity of RI for this etched FBG sensor is about 336.2 nm mm/RIU and 49.44 nm/RIU, respectively, when the surrounding RI ranges from 1.330 to 1.3586. Associated with the sensitivity of the RI gradient and the average sensitivity of the RI, the linear RI distribution of inhomogeneous liquid medium surrounding the etched FBG can be obtained. <s> BIB020 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> In this letter, we investigate the inscription of fiber Bragg gratings in the visible spectral range using deep ultraviolet femtosecond laser exposure and two-beam interferometry. The properties of first-order reflection gratings and third-order gratings for use in the visible wavelength range are compared. Stronger gratings have been achieved for first-order reflecting Bragg gratings compared with third-order gratings. We demonstrate a fiber Bragg grating with a grating period of 226 nm and a filtering efficiency of more than 30 dB. <s> BIB021 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> The fabrication of fiber Bragg gratings was here demonstrated using ultrashort pulse laser point-by-point inscription. This is a very convenient means of creating fiber Bragg gratings with different grating periods and works by changing the translation speed of the fiber. The laser energy was first optimized in order to improve the spectral properties of the fiber gratings. Then, fiber Bragg gratings were formed into D-shaped fibers for use as refractive index sensors. A nonlinear relationship was observed between the Bragg wavelength and liquid refractive index, and a sensitivity of ∼30 nm/RIU was observed at 1.450. This shows that D-shaped fiber Bragg gratings might be used to develop promising biochemical sensors. <s> BIB022 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Unclad Uniform Fiber Bragg Gratings <s> Abstract The review summarizes recent studies on D-shaped optical fibers and their recent applications in optical sensors. The configurations and working principles of D-shaped optical fibers are introduced. For each optical fiber sensor, the structure principles and measuring methods are all discussed in detail, with their optimal characteristics and performances being compared. Results from various studies show that it is possible to realize a high-sensitivity optical fiber sensor with a simple structure, good mechanical properties, and strong anti-interference ability. This may be due to the excellent structural design of the D-shaped optical fiber. Finally, key issues and new challenges on the D-shaped optical fiber are discussed. <s> BIB023
A uniform fiber Bragg grating (FBG) is a periodic and permanent refractive index modulation of the fiber core with fringes perpendicular to the propagation axis BIB008 BIB005 . It behaves as a selective mirror in wavelength for the light propagating in the core, reflecting a narrow spectral band centered on the so-called Bragg wavelength, as depicted in Figure 2 . According to the phase-matching condition, the latter is given by λ Bragg = 2n eff,core Λ where n eff,core is the effective refractive index of the core mode (close to the refractive index of silica, 1.45 at 1550 nm), and Λ is the grating period. Most often, Λ is~530 nm to ensure that the Bragg wavelength falls in the band of minimum attenuation of the optical fiber centered on 1550 nm, but gratings can be made at almost any wavelength also in the visible domain BIB015 BIB021 . This is especially true relying on the point-by-point (or line-by-line) inscription process with a femtosecond pulses laser BIB002 BIB016 . The most common means of production remain the phase-mask technique BIB003 and interferometric methods (both Talbot interferometer BIB001 and Lloyd mirror BIB004 configurations), most often used with a continuous frequency-doubled Argon laser emitting at 244 nm or a pulsed excimer laser emitting at 248 nm (ArF) or 193 nm (KrF). Gratings are usually manufactured in single-mode optical fibers, made of an 8 µ m thick core surrounded by a 125 µ m cladding. Such fibers are widely available at low cost (less than 100 US$ per km) and are telecommunication-grade. Overall, gratings preserve the fiber integrity (only the polymer jacket is removed at the grating location) while providing a strong coupling between the core-guided light and the cladding. Different grating configurations in single-mode optical fibers can be used for SPR excitation. Their characteristics and operating principle will be summarized hereafter. A uniform fiber Bragg grating (FBG) is a periodic and permanent refractive index modulation of the fiber core with fringes perpendicular to the propagation axis BIB008 BIB005 . It behaves as a selective mirror in wavelength for the light propagating in the core, reflecting a narrow spectral band centered on the so-called Bragg wavelength, as depicted in Figure 2 . According to the phase-matching condition, the latter is given by λBragg = 2neff,coreΛ where neff,core is the effective refractive index of the core mode (close to the refractive index of silica, 1.45 at 1550 nm), and Λ is the grating period. Most often, Λ is ~530 nm to ensure that the Bragg wavelength falls in the band of minimum attenuation of the optical fiber centered on 1550 nm, but gratings can be made at almost any wavelength also in the visible domain BIB015 BIB021 . This is especially true relying on the point-by-point (or line-by-line) inscription process with a femtosecond pulses laser BIB002 BIB016 . The most common means of production remain the phase-mask technique BIB003 and interferometric methods (both Talbot interferometer BIB001 and Lloyd mirror BIB004 configurations), most often used with a continuous frequency-doubled Argon laser emitting at 244 nm or a pulsed excimer laser emitting at 248 nm (ArF) or 193 nm (KrF). The Bragg wavelength is inherently sensitive to temperature and axial strain, through a change of both neff and Λ BIB006 . In practice, a change of temperature of +1 °C yields a Bragg wavelength shift of ~10 pm at 1550 nm. The sensitivity to axial strain is of the order of 1.2 pm/µε, also at 1550 nm. These sensitivities decrease with the operating wavelength. Such changes are easily measured with standard telecommunication instruments since the full spectral width of the main reflection band from a typical 1 cm-long grating is of the order of 100 pm. As it corresponds to light confined in the fiber core, the Bragg resonance is not directly suited to excite a surface plasmon wave at the metal-surrounding medium interface. Hence, in practice, unclad optical fibers are used, with the core exposed to the surrounding medium so that an evanescent wave can extend in this medium. This is usually obtained through a chemical etching process, most often with hydrofluoric acid (HF) BIB007 BIB010 BIB011  BIB012 BIB013 BIB019 BIB020 or through a side-polishing process BIB009 BIB014 BIB017 BIB018 BIB022 . The latter configuration has strong similarities with the use of Bragg gratings produced in D-shaped optical fibers BIB023 . Unclad FBGs can be used to sense SRI changes in the range [1.30-1.45] . The maximum SRI sensitivity is obtained for values above 1.40, since the effective refractive index of the core mode is The Bragg wavelength is inherently sensitive to temperature and axial strain, through a change of both n eff and Λ BIB006 . In practice, a change of temperature of +1 • C yields a Bragg wavelength shift of 10 pm at 1550 nm. The sensitivity to axial strain is of the order of 1.2 pm/µε, also at 1550 nm. These sensitivities decrease with the operating wavelength. Such changes are easily measured with standard telecommunication instruments since the full spectral width of the main reflection band from a typical 1 cm-long grating is of the order of 100 pm. As it corresponds to light confined in the fiber core, the Bragg resonance is not directly suited to excite a surface plasmon wave at the metal-surrounding medium interface. Hence, in practice, unclad optical fibers are used, with the core exposed to the surrounding medium so that an evanescent wave can extend in this medium. This is usually obtained through a chemical etching process, most often with hydrofluoric acid (HF) BIB007 BIB010 BIB011  BIB012 BIB013 BIB019 BIB020 or through a side-polishing process BIB009 BIB014 BIB017 BIB018 BIB022 . The latter configuration has strong similarities with the use of Bragg gratings produced in D-shaped optical fibers BIB023 . Unclad FBGs can be used to sense SRI changes in the range [1.30-1.45] . The maximum SRI sensitivity is obtained for values above 1.40, since the effective refractive index of the core mode is 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.