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Terahertz time-domain spectroscopy as well as all optical pump-probe techniques with ultrashort pulses relies on the exact knowledge of an optical delay between related laser pulses. Classical realizations of the measurement principle vary the optical path length for one of the pulses by mechanical translation of optical components. Most commonly, only an indirect measurement of the translation is carried out, which introduces inaccuracies due to imprecise mechanics or harsh environment. We present a comprehensive study on the effect of delay inaccuracies on the quality of terahertz spectra acquired with time-domain spectroscopy systems and present an interferometric technique to directly acquire the optical delay simultaneously to the terahertz measurement data. This measurement principle enables high-precision terahertz spectroscopy even in harsh environment with non-systematic disruptions.

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Conventional Abbe refractometers are used to determine the refractive index (RI) of liquid samples placed on a prism surface by critical angle evaluation. However, the use of this method is limited to the investigation of fluids in a laboratory environment. With the purpose of monitoring fluids attaching to extended planar surfaces, a different method is required. We present a RI monitoring device for plates of any given geometry. The proposed method can easily be integrated into a variety of applications using the area of interest as a waveguide for optical radiation. The developed refractometer is tested with sucrose solutions of varying RIs ranging from 1.355 68 to 1.430 80 with an LED source at 589 nm. By implementing a calibration method using standardized solutions, the average uncertainty for the RI determination is 1.6 × 10−3. Compared to the values measured by a state-of-the-art refractometer, the maximum deviation is 0.13%.