Diffractive refractometer for liquid characterization and transient processes monitoring.

A simple refractometer using a reflective diffraction grating immersed in the test liquid is developed and its performance is studied. Due to the dependence of the light wavelength on the refractive index, determining the angle of the diffracted beam provides the refractive index of the liquid. The glass cell containing the test liquid is cylindrical, and the grating plane is parallel to the cylinder symmetry axis. The light beam normally impinges on the cell front wall and reaches the center of the grating so that the diffracted beam leaves the cell without being deviated by refraction. It is demonstrated that this characteristic of the optical setup minimizes important error sources due to undesired beam deviations and enables real-time refractive index measurement of liquids in transient processes. Moreover, the performances of the diffractive refractometer and of a commercial Abbe refractometer are compared in the measurement of the refractive indexes of aqueous NaCl solutions. A He-Ne laser at 632.8 nm is used as a light source, and the diffraction grating has 1200 lines/mm. Measurement precisions of the order of 8 × 10-4 are achieved.

Immersed diffraction grating refractometers of liquids.

In this work, we report the development, construction, and the performance of two liquid refractometers that use a reflective diffraction grating immersed in a test liquid. The liquid is contained in a transparent glass cell with a rectangular cross section. The grating is oriented in such a way that the propagation directions of the incident beam and the beam diffracted by the lower part of the grating immersed in the liquid are perpendicular. In this configuration, the refractive index is determined by measuring the angle of the zeroth-order diffraction beam coming from the upper part of the grating, which is in contact with air. The diffractive refractometers (DR-1 and DR-2) have different angle measurement procedures and different light detection systems, and their advantages and drawbacks are pointed out. In the experiments, precisions of the order of 10-5 and 10-4 for DR-1 and DR-2 are achieved, respectively. The performances of both systems are compared with the performance of a commercial Abbe refractometer in the measurement of sugar and NaCl aqueous solutions.