Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Other Interrogation Techniques <s> In this paper, we completely study the wavelength dependency of differential group delay (DGD) and polarization dependent loss (PDL) for Bragg gratings written in birefringent fibers. Based on the coupled mode theory, we present analytical expressions for the evolution with wavelength of the transmission coefficient, the DGD and the PDL. The wavelength dependencies of these evolutions on the birefringence are then discussed. Experimental results are finally presented for an apodized FBG written in a bow-tie fiber. A very good agreement between theory and experience is reported, confirming the validity of the theoretical analysis. <s> BIB001 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Other Interrogation Techniques <s> The generation of surface plasmon resonances (SPRs) in gold-coated weakly tilted fiber Bragg gratings (TFBGs) strongly depends on the state of polarization of the core guided light. Recently, it was demonstrated that rotating the linear state of polarization of the guided light by 90° with respect to the grating tilt allows to turn the SPR on and off. In this work, we measure the Jones matrix associated to the TFBG transmission properties in order to be able to analyze different polarization-related parameters (i.e. dependency on wavelength of polarization dependent loss and first Stokes parameter). As they contain the information about the SPR, they can be used as a robust and accurate demodulation technique for refractometry purposes. Unlike other methods reported so far, a tight control of the input state of polarization is not required. The maximum error on refractive index measurement has been determined to be ~1 10(-5) refractive index unit (RIU), 5 times better than intensity-based measurements on the same sensors. <s> BIB002 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Other Interrogation Techniques <s> We present a novel method to prepare optimized metal coatings for infrared Surface Plasmon Resonance (SPR) sensors by electroless plating. We show that Tilted Fiber Bragg grating sensors can be used to monitor in real-time the growth of gold nano-films up to 70 nm in thickness and to stop the deposition of the gold at a thickness that maximizes the SPR (near 55 nm for sensors operating in the near infrared at wavelengths around 1550 nm). The deposited films are highly uniform around the fiber circumference and in spite of some nanoscale roughness (RMS surface roughness of 5.17 nm) the underlying gratings show high quality SPR responses in water. <s> BIB003 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Other Interrogation Techniques <s> Silver nanowires 1–3 μm in length and diameters of 0.04–0.05 μm were synthesized by a polyol process and deposited on a single mode optical fiber with the Langmuir–Blodgett technique. For nanowire surface coverage of ∼40% and partial orientation of their long axis obtained by controlling the deposition parameters, the optical properties of the nanowire coating become identical to those of a uniform metal coating obtained by sputtering or evaporation. Excitation of the nanowires by the polarized evanescent field of fiber cladding modes at near-infrared wavelengths near 1.5 μm results in surface plasmon-like resonances in the transmission spectrum of the optical fiber. The polarization-dependent loss (PDL) spectrum of the tilted fiber Bragg grating used to excite the cladding modes shows a pronounced characteristic dip indicative of a plasmon resonance for radially polarized light waves and complete shielding of light for azimuthally polarized light. The PDL dip shifts at a rate of 650 nm/(refractive index ... <s> BIB004 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Other Interrogation Techniques <s> A novel fiber optical refractive index sensor based on gold nanoshells immobilized on the surface of an etched single-mode fiber including a Bragg grating is demonstrated. The nanoparticle coating induces refractive index dependent waveguide losses, because of the variation of the evanescently guided part of the light. Hence the amplitude of the Bragg reflection is highly sensitive to refractive index changes of the surrounding medium. The nanoshell functionalized fiber optical refractive index sensor works in reflectance mode, is suitable for chemical and biochemical sensing, and shows an intensity dependency of 4400% per refractive index unit in the refractive index range between 1.333 and 1.346. Furthermore, the physical length of the sensor is smaller than 3 mm with a diameter of 6 μm, and therefore offers the possibility of a localized refractive index measurement. <s> BIB005 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Other Interrogation Techniques <s> Optical fiber optrodes are attractive sensing devices due to their ability to perform point measurement in remote locations. Mostly, they are oriented to biochemical sensing, quite often supported by fluorescent and spectroscopic techniques, but with the refractometric approach considered as well when the objective is of high measurement performance, particularly when the focus is on enhancing the measurand resolution. In this work, we address this subject, proposing and analyzing the characteristics of a fiber optic optrode relying on plasmonic interaction. A linearly tapered optical fiber tip is covered by a double overlay: the inner one–a silver thin film and over it–a dielectric layer, with this combination allowing to achieve, at a specific wavelength range, surface plasmonic resonance (SPR) interaction sensitive to the refractive index of the surrounding medium. Typically, the interrogation of the SPR sensing structures is performed, considering spectroscopic techniques, but in principle, a far better performance can be obtained, considering the reading of the phase of the light at a specific wavelength located within the spectral plasmonic resonance. This is the approach which is studied here in the context of the proposed optical fiber optrode configuration. The analysis performed shows the combination of a silver inner layer with a dielectric titanium oxide layer with tuned thicknesses enables sensitive phase reading and allows the operation of the fiber optic optrode sensor in the third telecommunication wavelength window. <s> BIB006 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Other Interrogation Techniques <s> Fiber Bragg gratings (FBGs) are inherently sensitive to temperature, axial strain, and pressure, which can be easily measured by a shift of the Bragg wavelength in their reflected/transmitted power spectrum. FBG sensors acquire many more additional sensing modalities and applications when the polarization of the interrogating light is controlled. For the polarization to have an effect, the cylindrical symmetry of the fiber must be broken, either by the structure of the fiber itself, by that of the FBG, or by the perturbation to be measured. Polarization control allows for sensing parameters that are spatially oriented, such as lateral force, bending or twist, and also for measurements of the properties of anisotropic media. Most importantly, polarization control enables high quality all-fiber surface plasmon resonance (SPR) FBG sensors and localized SPR-assisted sensing. This tutorial will cover the theory of polarized measurements in fiber gratings, their experimental implementation, and review a selection of the most important applications. <s> BIB007 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Other Interrogation Techniques <s> The Karhunen–Loeve transform (KLT) is used to retrieve the wavelength information of several fiber Bragg gratings (FBGs) that are acting as a multiplexed sensor. The modulated light of a broadband source is launched to the FBG cascade in order to capture the electrical frequency response of the system. Thanks to dispersive media, the wavelengths of the FBGs are mapped in radio-frequency delays. Wavelength changes are determined by the amplitude change of the samples in the impulse response, a change which is followed by the eigenvalue calculated by the KLT routine. The use of the KLT routine reduces by three orders of magnitude the amount of points needed to have a subdegree resolution in temperature sensing, while keeping the accuracy almost intact. <s> BIB008 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Other Interrogation Techniques <s> This work presents the development of an innovative plasmonic optical fiber (OF) immunosensor for the detection of cytokeratin 17 (CK17), a biomarker of interest for lung cancer diagnosis. The development of this sensing platform is such that it can be assessed in non-liquid environments, demonstrating that a surface plasmon resonance (SPR) can be excited in this case. For this purpose, detections have been first carried out on CK17 encapsulated in gel matrix in the aim of mimicking tissue samples. Gold-coated OF immunosensors were embedded in a specifically designed packaging providing enough stiffness to penetrate into soft matters. Resulting reflected spectra have revealed, for the first time, the presence of a stable SPR signal recorded in soft matters. Experiments conducted to detect CK17 trapped in a porous polyacrylamide gel matrix have highlighted the specific and selective biosensor response towards the target protein. Finally, the packaged OF immunosensor has been validated by a preliminary test on human lung biopsy, which has confirmed the ex-vivo CK17 detection. Consequently, this work represents an important milestone towards the detection of biomarkers in tissues, which is still a clinical challenge for minimally-invasive in vivo medical diagnosis. <s> BIB009
When working with plasmonic fiber-grating sensors, light needs to be properly polarized in order to obtain a good performance BIB007 . Additional interrogation techniques have been reported making use of polarization analysis to evaluate the sensor response with respect to the interactions produced in the surrounding medium. The measurement of the phase of the light at a wavelength matching the SPR BIB006 or the use of the polarization-dependent loss (PDL) BIB002 are two techniques that have been proven to exhibit a good performance, especially when associated with nanoparticles BIB003 BIB004 . In the case of fiber gratings, the PDL spectrum relates to the difference between orthogonally polarized spectra BIB001 , which therefore contains information about (L)SPR. For sensor interrogation in reflection mode, an optical fiber circulator is located between the source and the sensor. The OSA is also connected so that both source and detector remain located at the same side of the optical path. For etched FBGs no additional element is required due to the nature of their reflection response BIB005 and certain algorithms can be used for increasing the resolution of the measurements BIB008 . For the rest of the summarized alternatives, a mirror is deposited on the fiber tip in order to allow back-reflection of the light modes that are sensitive to the external medium BIB009 . Practically, some FBG interrogators integrate the source, circulator and detector as one, so that the sensor can be connected to a single port of the device. In this precise case, the source can be a tunable laser, offering a very high wavelength resolution (1 pm typically) and a fast acquisition rate (most often in the range 1-10 Hz with currently available commercial FBG interrogators) over the C + L wavelength bands. When working with plasmonic fiber-grating sensors, light needs to be properly polarized in order to obtain a good performance BIB007 . Additional interrogation techniques have been reported making use of polarization analysis to evaluate the sensor response with respect to the interactions produced in the surrounding medium. The measurement of the phase of the light at a wavelength matching the SPR BIB006 or the use of the polarization-dependent loss (PDL) BIB002 are two techniques that have been proven to exhibit a good performance, especially when associated with nanoparticles BIB003 BIB004 . In the case of fiber gratings, the PDL spectrum relates to the difference between orthogonally polarized spectra BIB001 , which therefore contains information about (L)SPR.