Evanescent optical fiber sensors

Schwotzer G. et. al., Optical sensing of hydrocarbons in air or in water using UV absorption in the evanescent field of fibers, Sensors Actuators B 1997 38 150-153. 

Matejec V., Chomat M., Pospisilova M., Hayer M., Kasik I. Optical fiber with novel geometry for evanescent-wave sensing, Sensors Actuators (1995) B 29, pp. 416-422. 

Doupovec J., Brunner R., Suchy F., Evanescent fiberoptic sensors based on capillary optical fibers, Proc. 3rd Europtrode (1996), Zurich, p.150. 

Gupta B.D., Sharma A., Singh C.D., Fiber optic evanescent-wave absorption sensors based on uniform and tapered fibers, Proc. 2nd Europtrode (1994), Firenze, p.189. 

Sol-gel coating technique for optical chemical sensors and biosensors is now in extensive research phase. For example, the side-coating of optical fibers or waveguides in evanescent-wave sensors it is particularly important to control precisely the sensitivity determining parameters, such as the coating thickness and length45. 

The fabrication and characterization of a fiber optic pH sensor based on evanescent wave absorption was presented by Lee63. The unclad portion of a multi-mode optical fibre was coated with the sol-gel doped with pH sensitive dye. The sensitivity of the device increased when the multiple sol-gel coatings were used in the sensing region. The dynamic range and the temporal response of the sensor were investigated for two different dyes -bromocresol purple and bromocresol green. 

Fig. 13.1 Photonic sensors that are either based on MNF sensing ((a),(b), (d), (e), (f)) or use MNF for the input and output connection (c), (g) (j). (a) straight MNF sensor with surrounding evanescent field (b) straight MNF sensor coated with bio or chemical layer and surrounding evanescent field (c) generic structure of MNF based optical sensor with surrounding evanescent field (d) straight MNF sensor (e) MNF loop resonator (MLR) sensor (f) MNF coil resonator (MCR) sensor (g) MNF/microsphere sensor (h) MNF/microdisk sensor (i) MNF/micro cylinder sensor (j) MNF/microcapillary sensor (k) a sensor composed of an MNF coupled to a series of microcylinders (optical fibers)...

Fig. 13.2 Different types of evanescent sensors, (a) a planar waveguide (b) a polished optical fiber (c) an MNF taper...

Paul, P. H. Kychakoff, G., Fiber optic evanescent field absorption sensor, Appl. Phys. Lett. 1987,51,12 14... 

B.D. Gupta and D.K. Sharma, Evanescent wave absorption based fiber optic pH sensor prepared by dye doped sol-gel immobilization technique, Opt. Commun., 140(1-3) (1997) 32-35. 

The second possibility is to make use of the continuous evanescent field that exists at the surface of an optical fiber. The cladding layer is replaced by a layer containing the fluorescent molecular sensor in a short portion of the optical fiber (Figure 10.45). At each reflection at the surface of the fiber core, the eva-... 

S. Zhao and W. M. Reichert, Protein adsorption using an evanescent chemical sensor with a fused optical fiber coupler, J. Colloid Interface Sci. 140, 294-297 (1990). 

A fiber-optic device has been described that can monitor chlorinated hydrocarbons in water (Gobel et al. 1994). The sensor is based on the diffusion of chlorinated hydrocarbons into a polymeric layer surrounding a silver halide optical fiber through which is passed broad-band mid-infrared radiation. The chlorinated compounds concentrated in the polymer absorb some of the radiation that escapes the liber (evanescent wave) this technique is a variant of attenuated total reflection (ATR) spectroscopy. A LOD for chloroform was stated to be 5 mg/L (5 ppm). This sensor does not have a high degree of selectivity for chloroform over other chlorinated aliphatic hydrocarbons, but appears to be useful for continuous monitoring purposes. 

Fluorescence from labeled adsorbed protein has also been excited with the evanescent surface wave created by integrated optics. Both optical fiber I60) and flat rectangular waveguides193) have been used. Interesting use of optical fiber as a remote protein sensor was demonstated the excitation light was sent down the fiber whose tip was immersed in protein solution, evanescently excited fluorescence was collected by the same fiber and delivered to a scanning monochromator 160). 

OF optical fibers, IOS integrated optical sensors, A absorbance, R reflectance, F fluorescence, ev evanescent wave, ISP isopropyl alcohol, DOS bis(2-ethylhexyl)sebacate, o - NPOE ortho-nitrophenyl octyl ether, TOP tris(2-ethylhexyl)phosphate... 

Luminescent evanescent wave-based sensors use optical fibers and planar waveguides [105,106] as fight-guiding structures, and they are more complex than the absorbance ones. However, such optodes have been satisfactorily applied to measure fluorescence of indicators or labels for the measurement of gas molecules, proteins or labeled antigen-antibody interactions as well as directly in solution [24,107] when immobilized in matrices [23,109]. 

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]. 

Modified fiber-optic-based sensors can be used for sensing pollutants, explosives, drugs, pharmaceuticals, and miscellaneous organics (Yeh et al. 2006). Optical fibers coated with porous silica can be used to detect the presence of chlorinated hydrocarbons. Alternatively, these compounds can also be detected using fiber-optic-coupled surface plasmon resonance methods. Aromatic compounds were detected by evanescent wave absorption spectroscopy. Suitably modified fiber-optic array tips can be used to detect presence of explosive materials (Wolfbeis 2000). 

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