Evanescent Field Absorption Detection of Co²⁺ in Aqueous Solutions in Microstructured Optical Fibers

Abstract

There is a growing interest in microstructured optical fibers (MOFs) for advanced sensors by infiltrating the air-holes with gas, liquid and polymers. The evanescent field overlap with infiltrations is enhanced compared with conventional fibers. The pure silica material is chemically and biologically inert, which well prevents the evaporation of water. Recently, liquid core waveguide cell has become widely used to minimise source light loss to the cell. However, the construction material Teflon® AF, is one of the most expensive commercial polymers. Moreover, it is highly gas permeable, causing problems of enhanced evaporation of water from the internal solution. In this paper, we demonstrate a novel and highly sensitive detection technique of Co2+ absorbance in aqueous solutions using two different structures of microstructured optical fibers (MOF). The evanescent field is numerically investigated in a systematic manner with the variation of structure parameters and wavelength. The effects of solution concentration, effective pathlength, temperature stability and bending effect on the absorption sensitivity are explored and compared experimentally. The enhanced evanescent field from the second structure can achieve absorption sensitivity up to 1.6 Mol-1. The sensitivity using longitudinal detection method is at least sixty times higher compared with that using the perpendicular measurement technique.

Keywords:

• Fiber Optics Sensor
• Microstructured Optical Fiber
• Absorption
• Sensitivity
• Evanescent Field