Abstract
We present a refractometer with main- and vernier-scale to measure the refractive index (RI) of liquids with high precision by using the fine spectrum structure of a tilted fiber Bragg grating (TFBG). The absolute RI values are determined by the accurate wavelength of cut-off mode resonances. The main- and vernier-scale are calibrated by measuring large groups of fine spectra at different cut-off mode resonances in a small RI range, and the use of vernier-scale certainly reduces the RI measurement uncertainty resulted from the discrete cladding mode resonances. The performance of the TFBG-based vernier refractometer is experimentally verified by exploring the temperature dependence of RI of anhydrous ethanol in a near infrared region, showing an enhanced accuracy to the order of 10-4, high repeatability and temperature self-calibration capability.
Highlights
The refractive index (RI) is an important physical property to be measured in laboratories and field tests, and it can be obtained with a refractometer
Among them based on fiber gratings, tilted fiber Bragg gratings (TFBGs) have demonstrated outstanding performance in high-sensitivity RI measurement by taking advantages of well-established FBG technology and resonant excitations of cladding modes [12]
In the work reported here, we introduce a new concept TFBG based on refractometer which has main- and vernier- scale capable of RI measurement of high precision and large dynamic range
Summary
The refractive index (RI) is an important physical property to be measured in laboratories and field tests, and it can be obtained with a refractometer. Abbe refractometers based on Snell’s law have been mostly used through measurement of the critical angle of total reflection, which is dependent on the known, relatively high RI of a glass prism and the RI of the sample to be tested [1,2,3]. Utilizing different spectral features of a TFBG with regards to the surrounding RI, several methods have been developed to measure the RIs of liquid samples: i) the envelope area of the cladding mode spectrum [13, 14]; ii) the wavelengths of cladding modes [15]; iii) the transmission or reflection power [16, 17]; iv) the wavelength of cut-off mode [18, 19], which can be regarded as a “true” refractometer and achieve the absolute measurement of RI, completely analogous to an Abbe refractometer. A small continuous RI change of ethanol in the temperature range of 20°C ~ 50°C has led to a covering of a few cut-off mode resonances, and more accurate RI and temperature measurement are simultaneously presented
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