An optical fiber sensor for measurement of the refractive index of liquids is described. Three parallel fibers and a mirror are used. One fiber is used to launch the light to the mirror and the other two capture the reflected light. The operation principle is based on the output light angle change due to the refractive index of the liquid into which the optical fiber probe is dipped. The paper contains the theoretical consideration as well as the experimental results. Aqueous solutions of NaCl and LiBr are used as testing media.

Optical fiber reflection refractometer

A refractometer using low-cost components is described. A multimode optical fibre is illuminated by a collimated beam at a variable oblique incidence angle. The value of the refractive index of a liquid is deduced from the measurement of an angle and does not depend on fluctuations of the light intensity. The use of an optical fibre allows remote sensing to be performed. As predicted by a simple theoretical analysis based on geometrical optics, the accuracy of the refractometer is about 1 × 10−4 in refractive index units. Owing to its good sensitivity, this set-up can be used as a chemical sensor.

A simple intrinsic optical fibre refractometer

An overview of optical fibre sensors and instrumentation is presented. Sensors can be divided into two types: continuous transducers in which light is modulated along the whole length of the fibre,...

Principles of Optical Fibre Sensors and Instrumentation

We present the results of a theoretical numerical analysis of a novel fiber optic refractometric transducer. It consists of a hemispherical glass detection element and a pair of the multimode optical fibers attached symmetrically to the element's flat equatorial plane. The internal reflection of light from the element's spherical surface is sensitive to the refractive index of the surrounding medium. We exploit several internal reflections in series and the focusing of the beam by the element's spherical surface to achieve enhanced sensitivity to the refractive index of the surrounding medium and reduced intrinsic optical loss in the transducer. We present the data on the effect of transducer's parameters on its transmission function. We show that the transducer can operate in a relatively wide range of the refractive index of the surrounding medium (we show this for the interval n = 1.0 to n = 1.45). We also present experimental data confirming our theoretical results.

Novel optical fiber refractometric transducer employing hemispherical detection element

A new critical angle refractometer (CAR) for high accuracy refractive index measurement of liquid has been developed. The instrument improves the accuracy by two reflections in an elongated parallelogram prism, and acquires the angular reflectivity without any angle scanning parts through introduction of a point source with a divergent beam and a charge coupled device. In addition, it employs a simple and robust measurement method that gets the critical angle by differentiating the angular reflectivity. Through investigating absorbing media with absorption index κ (the imaginary part of refractive index) from 0 to 10−2.1, the theoretical calculation shows that the proposed two-reflection CAR would outperform the traditional one-reflection CAR on lowering the principal error from the differentiation method and improving the ability of getting the critical angle. By testing two typical liquids–salt-water solution and milk, the preliminary experiment indicates that this two-reflection divergent differentiati...

A two-reflection divergent differentiating critical angle refractometer.

A fiber optics refractometer has been developed based on a simple low‐volume cell (5–7 μl) which confines the liquid to be measured around a waveguide whose multiple reflections can enhance the contrast at the critical angle. The electromechanical and electrooptical design allows the fiber optics refractometer to measure absolute refractive index with manual or automatic readout, and differential operation with automatic tracking that permits its use as a universal detector for gradient elution chromatography. Present sensitivity is approximately 1×10−5 refractive index units in the static mode and 7×10−5 refractive index units in the scanning beam mode. These sensitivities can probably be improved by a factor of 5 by appropriate system changes.

F. C. Unterleitner

Papers

Design, development, and performance of a fiber optics refractometer: Application to HPLC