Optical Fiber Long-Period Gratings for the Refractometry of Aqueous Solutions

Abstract

ABSTRACT Refractometers are of interest for measuring concentrations of solutions, since the refractive index of a solution isproportional to its concentration. On-line continuous refractometry is important in many industrial processes, as well as forchecking the concentration stability of solutions in stock. An optical fiber long-period grating is presented, which was testedfor refractometry in the 1 34-1 .42 range.Keywords: refractometry, optical fiber grating 1. INTRODUCTION The measurement of the concentration of stock solutions, dilutions, oils, and water-soluble industrial fluids is of interest inmany quality control industrial processes in order to check the concentration stability and abnormal ageing effects. Off-linerefractometers, such as the Abbe type, which offer a resolution of 0.2%, are commonly used.This paper presents an optical fiber long-period grating which has been tested for the refractometry of sodium chloride andcalcium chloride aqueous solutions for a range of density varying from that of distilled water to that of a nearly-saturatedcondition.An optical fiber long-period grating (LPG) couples the fundamental mode to discrete forward-propagating cladding modes,which attenuate on propagation. Consequently, the transmission spectrum shows loss peaks at the resonant wavelengths 14•