Core-based optical fiber sensors

The pH optical fiber sensor without any pH-sensitive dye was also described70. Porous silica layer made by the sol-gel method was cladded onto optical fibre core and was exploited as the optical transducer. Acid-base properties of silica surface caused that the surface charge of silica changed with pH of the solution. For example saturation of the sol-gel layer with cations leads to an increase of the electron density of the film, hence, the refractive index of the film. Since the surface charge of silica depends on pH, the refractive index of silica film varies also with pH. Thus, changes of... 

Due to their high sensitivity to strain, temperature variation, vibration, and acoustic waves, embedded extrinsic Fabry-Perot interferometric optical fiber sensors have been developed to detect delamination, based on changes in the acoustic properties of the materials before and after delamination [41]. Impact events and corrosion cracking generate ultrasonic waves, which can be characterized using elliptical core fiber sensors [40]. The in-line Fabry-Perot interferometer seems well suited for the local detection of shear waves and the characterization of impact-induced damage. 

Villatoro, J. Monzon Hernandez, D., Low cost optical fiber refractive index sensor based on core diameter mismatch, J. Lightwave Technol. 2006, 24, 1409... 

The first aptasensor reported was particularly based on optical detection [66]. The 58-mer RNA aptamer selective to L-adenosine was immobilized onto the core of multimode fiber using avidin-biotin method. The detection was based on competitive binding of FITC-labeled L-adenosine with unlabeled analyte. This sensor also allowed to study the kinetics of binding and determine equilibrium constants. 

Absorbance evanescent-based sensors are based on the absorption or dispersion of light outside the core. They rely on light attenuation in the evanescent field following the Beer-Lambert law (ATR sensors), but owing to the low intensity of the field, they offer poor sensitivity. This can be improved because the effective optical path length can be increased, especially when using optical fibers, capillary [62] or planar waveguides [114]. 

Fig. 5.4. Optical Fiber biosensor, (a) Extrinsic optical fiber is used for the guiding the light to and from the sensor area, (b) Intrinsic the receptor molecules are immobilized on the fiber core after decladding of the fiber. The detection is based on fluorescence labels.

As an example of sensor applications, for an optical fiber with a core diameter 20 pm, ciad = 1.45, and core = 1.46, the field of the modal power forLP/ modes is shown in Fig. 28a,b for the modal orders I = 6 and /= 13, at an optical wavelength X = 0.75 pm. In the presence of external perturbations applied to the fiber, resulted in changing the cladding index to ciad = 1.455, the modal power redistribution of Z = 6 and / = 13 modes is shown in Fig. 28c, d. This theoretical analysis is based on the use of a single frequency (laser) light source. 

Luna-Moreno D, Monzon-Hemandez D (2007) Effect of the Pd-Au thin film thickness uniformity on the performance of an optical fiber hydrogen sensor. Rev Appl Surf Sci 253 8615-8619 Luna-Moreno D, Monzon-Hemandez D, VUlatoro J, Badenes G (2007) Optical fiber hydrogen sensor based on core diameter mismatch and annealed Pd-Au thin films. Sens Actuators B 125 66-71 Lundstrom 1, Shivaraman MS, Svensson C, Lundkvist L (1975) A hydrogen-sensitive MOS field-effect transistor. J Appl Phys Lett 26 55-57... 

Other devices that can be fabricated with NLO polymers are for example sensors for electric field. A number of sensors fabricated with EO polymers to detect electric field signals and map the electric field distribution have been implemented in the last years based on Mach-Zehnder intensity and polarization modulators. Other sensors are based on asymmetric Fabry-Perot microcavities to convert phase modulation into amplitude modulation and enhance the samphng signals. Very recently Sun et al. [172] presented a novel broadband electrooptic electric field sensor fabricated with an EO polymer microring resonator coupled to the core of a side-polished optical fiber. A sensitivity of 100 mV/m has been achieved at frequencies up to 550 MHz. 

A fiber-optic pH sensor based on fluorescence energy transfer can be construeted by coimmobilizing a pH-sensitive fluorophore and apH-sensitive absorber. For example, eosin (donor) and phenol red (acceptor) were coimmobilized in a polymer on the distal end of a silanized single-core optical fiber. Eosin s emission spectrum overlaps with the absorption of the basic form of phenol red. The concentration of the basic form of phenol red increases with an increase in pH. As a result, energy transfer from eosin to phenol red increases and the fluorescence intensity of eosin decreases. Thus, the pH-dependent absorption change of phenol red can be detected as changes in the fluorescence signal of eosin. 

Zhang, Y, et al., 2015b. A SPR sensor based on twin-core fiber. In Proc. SPIE 9634, 24th International Conference on Optical Fibre Sensors, 96347P, Available at http //dx.doi. org/10.1117/12.2193871. 

The optical flber SPR is the smaller device of this technology, allowing the use of this technique in distant locations. Sensors based on monomode and multimode optical flbers have been reported [23,24]. The cladding of the flber is partially removed and a gold layer is deposited symmetrically around the exposed fiber core. This type of fabrication limits the interaction area to a... 

Fiber-optic biosensor can be potentially used for clinical sample analysis. As described above, by using enzymes and antibodies immobilized on the fiber tip or around the fiber core (evanescent field), different fiber-optic biosensors have been developed. In most cases, the measurement is based on changes in fluorescence intensity. The target analytes include glucose, cholesterol, enzymes, antibodies, bacteria, and viruses. Measurements are usually fast and simple, and in many devices, the probe is disposable however, the instability of the biological recognition materials reduces the sensor lifetime. 

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