1: Evanescent field in the core/cladding interface of an optical fiber. 

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... e m is the real part of the dielectric constant of the metal, e d the dielectric constant of the medium and l the radiation wavelength. Plasmon excitation strongly depends on the refractive index of the surrounding medium, thereby, all these sensors can be considered to be refractometers. Most of the proposed SPR configurations are made up of bulk optic components. The first optical fiber SPR device was proposed by Jorgenson and Yee [108] in 1993. In this work the sensing element was built by removing a section of the fiber cladding and depositing an SPR active thin metal layer symmetrically around the fiber core. This work was based on a resonant wave- length interrogation, and this sensing structure was capable of detecting variations in the SRI within the operating range between 1.2-1.4 RIU with a resolution up to ± 5×10 -5 RIU in the high refractive index range (corresponding to a resonant wave- length resolution of 0.5 nm). This work represented the beginning of a new concept of compact and robust SPR devices. In particular, side polished fibers [109], tapered fibers [110][111][112] and D-type [113] fibers have constituted useful alternatives to the classical configurations [114]. Figure 2.15 shows the scheme of a singlemode tapered optical fiber as a coupling mechanism to excite the SPR. ...

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... the cladding modes to variations of the SRI increases with mode order, since the penetration depth of the evanescent field increases for higher-order modes. With the increment of the SRI, the center wavelength of the resonances experiences a shift to higher wavelengths. In addition to their spectral shift, the intensity drops progres- sively, to fit a smooth loss curve. Thus, monitoring the shifts of the cladding modes relative to the Bragg resonance or measuring the normalized envelope of the cladding mode resonances in the transmission spectrum can yield an accurate measurement of the SRI. Figure 2.5 shows a conceptual representation of a TFBG. The TFBGs used in the experiment of Laffont et al. were written in a standard single mode fiber using a Lloyd mirror interferometer. The measurement of SRI was based on the normalized envelope of the cladding mode resonances spectrum observed in transmission. It was also shown that this parameter was relatively insensitive to temperature. Another reason for using the envelope of the resonance spectrum is that, choosing the proper tilt angle, this parameter can change monotonically and smoothly for RI values between 1.32-1.42, with a small change in sensitivity. Using the normalized area parameter and a 16° TFBG, a resolution of ± 10 -4 RIU was achieved. In 2007 Chan et al. [16] proposed a relative measurement of refractive index, based on the separation distance between certain cladding modes, that were dependent on the refractive index and temperature, and the core mode, which is refractive index independent. A 4° TFBG was used and a RI sensitivity of 10 nm/RIU was obtained, achieving a resolution of ± 10 -4 RIU. TFBGs are a suitable option for refractometric sensing in terms of performance and robustness of the fiber structure. However, a TFBG couples the core mode to a number of cladding modes in a large wavelength bandwidth, which renders difficult the signal readout and multiplexing. In addition, the fact that the measurement must be made in transmission, requiring access to the sensor from both sides, can represent a difficulty in some ...

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... FBG based refractometers have been described that are based on the measure- ment of the refractive index changes for the measurement of sucrose, salt, ethylene glycol, isopropyl alcohol among others [4,9,21,22]. Just few works were presented using functional layers. The first demonstration of the concept of biosensor based of the target molecules. Thus, bovine serum albumin (BSA) (antigen) and anti-BSA (antibody) were used to study the reaction kinetics of the antigen-antibody recogni- tion by changing the antibody concentration in different configurations for the antigen immobilization. In 2009, Jesus et al. [25] showed an FBG based Fabry-Perot cavity for determination of acetic acid. The sensing probe was based on a FBG Fabry-Perot cavity, formed between the reflection of a low reflectivity FBG and the Fresnel re- flection from the cleaved fiber tip [20]. The distal end of the fiber was coated with a thin film of sol-gel-PVP (polyvinylpyrrolidone) composite material. Figure 2.6 shows the scheme of the sensing probe. The polymeric thin film renders the interferometric output sensitive to the presence of carboxylic acid species. Results showed that the interferometric peaks change with acetic acid concentration, enabling its quantifica- ...

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... light is totally reflected inside the core, no electromagnetic field is propagating into the cladding. Nevertheless, the electromagnetic field actually penetrates a short distance into the lower refractive index medium, propagating parallel to the core- cladding interface and decaying exponentially with the distance from the interface (see Figure 2.1). The physical explanation for this phenomenon is that, when ...

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... this case, high selectivities can be achieved, depending on the affinity between the biorecognition element and the target. Figure 2.2 conceptually shows an example of a label-free fiber optic biosensor. A functional coating is used to support and enhance the attachment of the bioreceptor molecules, which bind to the analyte [6]. ...

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... full width at the half maximum (FWHM) of the resonant peak of the Bragg grating is typically a few hundred picometers. It depends on the physical length of the grating, which is usually few millimeters. Figure 2.3 illustrates the principle of operation of an FBG. When a broadband optical signal reaches the grating, a narrow spectral fraction is reflected and the remaining is transmitted. The peak wavelength of the reflected signal is defined by the Bragg resonance wavelength. FBG sensors have been widely used for strain and temperature measurement ...

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... sensitivity of an LPG is then typically defined as a shift of the resonant wave- length induced by a measurand. Figure 2.8 shows the typical behavior of resonance wavelength (a) and its optical power (b) to refractive index changes, respectively. The sensitivity characteristic of a bare LPG to SRI changes has an increasing (in modulus) non-linear monotone trend. The result is that the maximum sensitivity is achieved when the external index is close to the cladding index while for lower refractive indices (around 1.33) the LPG is scarcely sensitive. This standard behavior can be changed when a thin layer of sub-wavelength thickness (few hundreds of nanometers) and with higher refractive index than the cladding is deposited thereon. The use of HRI over- lays in fiber optic refractometers based on evanescent wave was explored initially by Schroeder et al. [9] for a polished FBG. The use of coated LPGs with thin HRI layers was firstly proposed by Rees et al. [32] and since then, several authors have explored its use for LPG RI sensitivity enhancement [32][33][34][35][36][37] and to develop highly sensitive biochemical sensing devices [38,39]. The HRI overlay draws the optical field towards the external medium extending its evanescent wave. As a result there is an increased sensitivity of the device to the surrounding RI. Due to the refractive-reflective regime at the cladding-overlay interface, the cladding modes in a HRI coated LPG are bounded within the structure comprising the core, the cladding and the overlay. This means that a relevant part of the optical power carried by the cladding modes is radiated within the overlay. The field enhancement in the overlay depends strongly on the overlay features (thickness (T h) and refractive index (n) -nT h) and the SRI. For a fixed overlay thickness and refractive index, by increasing the SRI, the transition from cladding to overlay modes occurs: the lowest order cladding mode (cladding mode with highest effective refractive index) becomes guided into the overlay. At the same time, the higher order modes move to recover the previous effective indices distribution. This is reflected through the phase matching condition in the shift of each attenuation band toward the next lower one [35]. It results from this modal transition that the attenuation bands can exhibit a sensitivity of thousands of nanometers per refractive index unit for a particular RI operation range. For a 5 th order resonance, sensitivities of ⇠ 5000 nm/RIU (near 1.41) and ⇠ 2500 nm/RIU (near 1.38) were achieved for coating thicknesses of 270 nm and 320 nm, respectively. The reported data showed how by changing the overlay thickness it is 24 2.5. Long Period Gratings possible to tune the sensitivity characteristic, for a considered cladding mode, in the desired refractive index range. High order cladding modes that strongly penetrate the external medium, on the other hand, offer higher sensitivity, and obviously these are the most desirable for sensing purposes. An increase in the order of the coupled cladding mode can be obtained by decreasing the grating period [40]. Pilla et al. [41] reported recently in 2012 a PS coated LPG with a period of 200 mm. The coating thickness was approximately 245 nm. For an 11 th order resonance, sensitivity over 9000 nm/RIU near 1.347 was achieved, which is so far the best sensitivity obtained for a fiber device for this range of RI. This result shows HRI coated LPGs as a promising technology for high-performance label-free sensing ...

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... on the wavelength and geometrical length, the light into the MMF can in- terfere constructively or destructively resulting, at the end, in a device with different spectral characteristics [89]. Therefore the length of the MMF determines the spectral features of the MMI device. Depending where the interference pattern is 'intersected', constructive or destructive interference results, at different wavelengths yielding the transmission of resonant peaks or resonant losses, respectively. The transmitted spec- tral power distribution is, therefore, highly sensitive to the optical path length of the MMF section. It is important to refer that in MMI devices based on standard MMF, the optical signal does not access the external medium. Therefore, they are insensi- tive to the SRI. MMI based refractometers usually relies on etched cladding MMF, tapered MMF or coreless multimode fiber (CMF). Figure 2.13 shows conceptually a SMS device based in a CMF, where constructive interference is present resulting in a resonant peak in the transmitted spectrum. MMI fiber devices are very attractive due to their high potential for refractive index sensing. [92] studied a tapered SMS structures for high sensitivity index sensing. The device relies on a coreless MMF, part of which was tapered down by flame brushing technique. For a 55 mm MMF taper waist diameter the results showed that in the lower index range of 1.30-1.33, a sensitivity of 148 nm/RIU was achieved, while in the high sensitivity index region of 1.42-1.43, a value of 2946 nm/RIU was ...

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... in the same range; for an LPG with period of 320 mm written by UV radiation exposure in a Corning standard 1310 nm fiber. Figure 2.7 illustrates the principle of operation of LPGs. Shu et al. [30] reported in 2002 a LPG written in B-Ge co-doped fiber by UV laser irradiation technique, with a period of 202 mm. For the 11 th cladding order mode, a RI sensitivity of 1481 nm/RIU was shown in the range between 1-1.36 RIU, which is according with our knowledge the best sensitivity for a bare LPG reported for this range. Electric-arc induced LPGs are attractive due to its simplicity and flexibility, as well as the low cost of the fabrication process and its applicability not only to commonly used photosensitive fibers, but also to photonic crystal fibers, which are made of pure silica. In 2011 Smietana et al. [31] published a work on gratings with periods of 345 mm and 221 mm, respectively, for LPGs based on the SMF-28 and PS1250/1500 fibers. There are the shortest periods achieved for this type of fibers using the electric-arc manufacturing technique. Results showed RI sensitivities of 302 nm/RIU and 483 nm/RIU in the range between 1.33-1.41, representing also the highest sensitivity reported for a bare LPG made by electric-arc technique for the specified measuring ...

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... Figure 2.4 illustrates an FBG based refractometer, when the cladding of the op- tical fiber was partially etched. Thus, the wavelength of the reflected signal depends on the external refractive index. Regarding sensitivity enhancement and temperature compensation, Schroeder et al. [9] presented in 2001 a configuration with two in-line FBGs written on a single- mode depressed-cladding optical fiber with a cut-off wavelength of 750 nm. One of the gratings was side-polished to become sensitive to the surrounding refractive index (SRI) and the second one was a standard FBG used for thermal compensation. The effect of high refractive index (HRI) overlays was studied in order to shift the mode field to the surface of the sensor and to enhance the sensitivity for low refractive index analytes. Operation in wavelengths far above the cut-off wavelength was also explored resulting in an improvement of the sensitivity of the sensor. The response of the sensor to the SRI was obtained by immersing the sensing element in different solutions with different refractive indices in a range between 1.30-1.46 RIU. The max- imum sensitivity for an external refractive index close to 1.45 was found to be 300 nm/RIU and the measured resolution was ± 2×10 -6 RIU. It is worth to note that due to the asymmetry induced by the polishing process, the SRI sensitivity becomes po- larization dependent. A simpler solution for thermal compensation was published by , where a single grating was half-etched and used for simultane- ous measurement of refractive index and temperature. The operation principle relies on the splitting of the original grating spectral response in two different peaks due to a selective etching over the grating length, where one of them becomes sensitive to the external refractive index and the other one is just sensitive to temperature [10]. Concerning enhancements in sensitivity, in 2005 Chryssis et al. [11] has shown that an effective solution is provided by etching the core of a FBG. A maximum sensitivity of 1394 nm/RIU is achieved as the surrounding index approaches the core index when the residual diameter was reduced to 3.4 ...

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... the other hand, the measurement of the refractive index is strongly dependent of LPG based interferometers have shown that, it is possible to attain higher reso- lution in the RI measurement when used instead of a single LPG. The advantage of using those structures lies on their interferometric nature and its principle of opera- tion, where the coupled core and cladding modes from one LPG combine again at a second matched LPG to form interference fringes. The core and cladding optical paths constitute the arms of an all fiber Mach-Zehnder interferometer [49]. In 2002, Allsop More recently, in 2010 Mosquera et al. [52], presented an optical fiber refrac- tometer based on a Fabry-Perot resonator that incorporates an intracavity LPG that couples and recovers energy to the fiber cladding after being phase shifted by the SRI. Figure 2.10 shows the sensing head configuration. The resonator is formed by two high reflectivity (⇠ 95%) FBGs separated by 47.5 mm. The SRI is monitored by the resonant frequencies of the Fabry-Perot interferometer, which can be measured either in transmission or in reflection. Results give a detection limit of ± 2.1×10 −5 RIU at n=1.33. Figure 2.10: Intracavity LPG Fabry-Perot ...

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... the other hand, the measurement of the refractive index is strongly dependent of LPG based interferometers have shown that, it is possible to attain higher reso- lution in the RI measurement when used instead of a single LPG. The advantage of using those structures lies on their interferometric nature and its principle of opera- tion, where the coupled core and cladding modes from one LPG combine again at a second matched LPG to form interference fringes. The core and cladding optical paths constitute the arms of an all fiber Mach-Zehnder interferometer [49]. In 2002, Allsop More recently, in 2010 Mosquera et al. [52], presented an optical fiber refrac- tometer based on a Fabry-Perot resonator that incorporates an intracavity LPG that couples and recovers energy to the fiber cladding after being phase shifted by the SRI. Figure 2.10 shows the sensing head configuration. The resonator is formed by two high reflectivity (⇠ 95%) FBGs separated by 47.5 mm. The SRI is monitored by the resonant frequencies of the Fabry-Perot interferometer, which can be measured either in transmission or in reflection. Results give a detection limit of ± 2.1×10 −5 RIU at n=1.33. Figure 2.10: Intracavity LPG Fabry-Perot ...