Determining Ethanol Content of Liquid Solutions Using Laser Feedback Interferometry with a Terahertz Quantum Cascade Laser

Over the last decade, terahertz (THz) time-domain spectroscopy has been investigated as a technique for assaying the ethanol content of liquid solutions-indeed, operating at THz frequencies addresses some of the challenges that traditional optical refraction measurements face, such as delineation between sugar-ethanol content, florescence, and problems arising from carbonation or other dissolved gasses. In this article, we propose an alternative system and method for assaying ethanol content of liquid solutions at THz frequencies, which employs a laser feedback interferometer built around a 2.6-THz quantum cascade laser. The system is tested against a series of controlled water-ethanol solutions, as well as a series of commercially available beverages. The accuracy of the estimated ethanol content compares favorably to THz time-domain spectroscopy techniques.