Experimental Studies on the Response of the Fiber Optic Evanescent Field Absorption Sensor

In the present study the response of the fiber optic evanescent field absorption sensor has been analyzed experimentally. The dependence of evanescent absorbance on sample (dye) concentration has been found to be nonlinear, in contradiction to theoretical predictions. The amount of nonlinearity as well as evanescent absorbance has been found to depend on the pH of the sample. As the pH decreases, the amount of nonlinearity and evanescent absorbance decrease. It has been shown that the nonlinearity and high evanescent absorbance at high pH value occur because of the adsorption of the positively charged dye molecules on the surface of the silica core of the plastic-clad silica (PCS) fiber, which is generally used for the sensor. A linear response has been obtained at pH close to 2.0, at which the presence of a tremendous amount of H ions in the sample restricts the adsorption of the dye molecules. The following empirical relation has been found to fit the experimental data: gamma 1.1C 1.6(pH)32C 1 3, where gamma is the evanescent absorption coefficient and C is the concentration of the dye. The first term is due to the simple Beer's law while the second term is the contribution of the pH dependent dye adsorption. In addition, we have found that the increase in core diameter increases the contribution of adsorption.     

Fiber optic evanescent field absorption sensor: Effect of fiber parameters and geometry of the probe

A comparative experimental study of the fiber-optic evanescent field absorption sensor based on straight and U-shaped probes is presented. The effects of numerical aperture and the core radius of the fiber on the sensitivity of the sensor are experimentally investigated. Increase in the numerical aperture of the fiber increases the sensitivity of the sensor in the case of both the probes. The effect of core radius on the sensitivity depends on the bending radius of the probe. In the case of straight probe (i.e. infinite bending radius) the fiber with smaller core radius has high sensitivity while in the case of U-shaped probe with 0.17 cm bending radius, the fiber with larger core radius has high sensitivity. Thus, which fiber (with small or large core radius) has maximum sensitivity depends on the bending radius of the probe. For a given fiber, decrease in the bending radius increases the sensitivity of the U-shaped probe. An inverse power law relationship between the bending radius and the evanescent absorbance and hence the sensitivity is established.     

Effect of Launching Condition on Modal Power Characteristics of Multi-Mode Step-Index Optical Fiber: A Theoretical and Experimental Investigation

Abstract

This article investigates optical power transmission characteristics with varying launching conditions concentrating on weakly guiding, step-index highly multi-moded optical fibers. A single analytical expression for the fractional power transmitted per mode in a highly multi-moded fiber is obtained. This expression is used to numerically obtain the fractional power per mode, power per modal order, and finally, the total power carried by the fiber. Experiments are carried out to investigate the effect of launching conditions on the total power carried by the fiber and to establish a possible correlation with the theoretical results. Reasons for the observed experimental discrepancies are also discussed.