<title>Optical refractometer for complex refractive index measurement in UV-NIR range</title>

Abstract

ABSTRACT An optical system for measuring the spectrum ofthe complex refractive index ofliquids in the UV-NIR range is described.The presented optical fiber measurement method allows remote control ofthe index. The developed refractometer has beentested with samples from different sections ofpaper mill and chemical pulp mill production processes. The obtainedaccuracy of the real part refractive index measurements was 1 O with resolution 2 1O (for the wavelength in range 300—920nm with spectral resolution better than 0.5 urn). The absorption corresponding to the imaginary part ofthe refractive index ismeasured in 0÷20 dB/cm range with 0.01 dB/cm resolution.Keywords: refractive index, fiber optic sensor, spectroscopy, chemical pulp mill, paper mill. 1. DffRODUCTION Increasing demands placed are on industry processes to produce high quality, economically efficient products and requirethe application ofthe latest technological developments to process control and measurement instruments. Particularly, due toenvironmental concerns, the trend of closing up pulp and paper mill water systems calls for new on-line measurements forthe analysis of pulping liquors and for wet-end control. The fiber optic technique allows the remote multipoint monitoring ofthe production process. The chemical constitution of liquors and waters can be determined by optical spectroscopy methodsapplying fiber optics [1]. The spectral measurement of the complex refractive index seems to be useful in this case. The realpart ofthe index depends on light velocity in the medium while its imaginary part depends on the absorption ofthis medium.Various measurement techniques of the real part of the index can be applied, for example, the reflection of light on theboundary between the two media with different refractive indices [2], the change of the light propagation in a wave-guidedue to a change of conditions of the total internal reflection [3], the modification of the coupling condition of the opticalfiber modes in a coupler [4]. The absorption can be determined by measurement ofthe light attenuation in the medium [5}