Fiber optic sensor for measurements of refractive index of liquid

Katarzyna Karpienko, Marcin Marzejon

Department of Metrology and Optoelectronics, Faculty of Electronics, Telecommunications and

Informatics, Gdańsk University of Technology, Narutowicza Street 11/12, 80-233 Gdańsk, Poland

This paper presents a study of a fiber optic sensor for measurements of refractive index of

liquid. Sensor consists of sensing head – Fabry-Pérot interferometer, two broadband light

sources with central wavelength of 1300 nm and 1550 nm, an optical spectrum analyzer and

a 2x1 single mode fiber coupler which connects all devices. One arm of the coupler is also a

part of Fabry-Pérot interferometer. Three different fiber lengths between sensing head and

coupling element has been used: direct connection, extension to 1 m and extension to 10 m.

Measurements with different lengths of the fiber were performed to determine whether the

length of the optical path affects the measurement result.

Measurements of five liquids with different refractive indices were performed. As a result,

spectral separation between maxims in registered spectra were calculated and correlated

with refractive index of substances.

Obtained results indicate that designed sensor provides satisfactory accuracy for all

examined lengths of the optical fiber. It means that it can be used for remote measurements

of refractive index of liquid over long distances.

Keywords: fiber-optic sensor, refractive index sensor, low-coherence interferometry