Design, construction, and performance of a real-time holographic refractometry prototype for liquid analysis

Abstract

The development and the performance of a portable holographic refractometer prototype for liquid measurement employing multimode diode lasers with emission centered at 662 nm as light sources is reported. Due to the multiwavelength character of the holographic recording, a synthetic wavelength was generated, and the diffracted wave intensity was thus modulated as a function of the optical path difference between the reference and the object beams. The transparent test cell containing the liquid was placed at the reference-beam arm of the optical setup, while the contour interferogram generated on the holographic image of a flat object was used for fringe counting. A change Delta L on the liquid column length is proportional to the Delta p running fringes on the object image, and from this relation the refractive index of the test liquid was obtained. The holograms were recorded on a photorefractive Bi(12)TiO(20) crystal whether using a single multimode diode laser or by combining two diode lasers. In the latter configuration the synthetic wavelength can be varied in order to enhance the measurement sensitivity and/or to allow the analysis of turbid liquids. The size of the whole prototype is 54 x 22 x 14 cm(3). The refractive indexes of ethanol/water mixtures with different concentrations were measured, as well as the NaCl concentrations in aqueous solutions were determined upon comparison with an empirical curve. In both cases the results were compared with the ones obtained through an Abbe refractometer. (C) 2011 American Institute of Physics. [doi:10.1063/1.3523049]