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

Bertling, K

Davies, AG

Dean, P

Han, S

Indjin, D

Khanna, SP

Lim, YL

Linfield, EH

Rakić, AD

Taimre, T

Wilson, SJ

Wu, T

Zhao, C

White Rose Research Online

VOL. 2, NO. 3, SEPTEMBER 2018 3501604Electromagnetic wave sensorsDetermining Ethanol Content of Liquid Solutions Using Laser FeedbackInterferometry with a Terahertz Quantum Cascade LaserKarl Bertling1∗ , She Han1, Tengfei Wu2, Chen Zhao3, Yah Leng Lim1∗, Paul Dean4,Suraj P Khanna4, Dragan Indjin4, Edmund H. Linfield4, A. Giles Davies4, Stephen J. Wilson1,Thomas Taimre5∗∗, and Aleksandar D. Rakić1†1School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia2State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, Tianjin 300072, China3Center for Theoretical Physics, Beijing Key Laboratory of Metamaterials and Devices, Capital Normal University, Beijing 100048, China4School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, U.K.5School of Mathematics and Physics, The University of Queensland, Brisbane, QLD 4072, Australia∗Member, IEEE∗∗Associate Member, IEEE†Senior Member, IEEEManuscript received June 22, 2018; revised July 12, 2018; accepted July 20, 2018. Date of publication July 24, 2018; date of current version August 1, 2018.Abstract—Over the last decade, terahertz (THz) time-domain spectroscopy has been investigated as a technique forassaying the ethanol content of liquid solutions—indeed, operating at THz frequencies addresses some of the challengesthat traditional optical refraction measurements face, such as delineation between sugar–ethanol content, florescence, andproblems arising from carbonation or other dissolved gasses. In this article, we propose an alternative system and methodfor assaying ethanol content of liquid solutions at THz frequencies, which employs a laser feedback interferometer builtaround a 2.6-THz quantum cascade laser. The system is tested against a series of controlled water–ethanol solutions, aswell as a series of commercially available beverages. The accuracy of the estimated ethanol content compares favorablyto THz time-domain spectroscopy techniques.Index Terms—Electromagnetic wave sensors, laser feedback interferometry, terahertz (THz) sensing.I. INTRODUCTIONMeasurement of ethanol content of various liquid solutions (in-cluding alcoholic beverages) is of significant interest, notably froma process-control viewpoint during manufacture, as well as to en-sure final products meet regulatory guidelines. Traditional opticalrefraction-based measurements of alcohol (ethanol) content are af-fected by the difficulty in distinguishing changes in signal arising fromalcohol content and sugar content [1]; typically, an additional densitymeasurement (using a hydrometer) is also required to assay the finalalcohol content. More sophisticated optical techniques exist, includ-ing those operating at near- and midinfrared wavelengths and thoseutilizing Raman spectroscopy [2]–[4], as well as recent technologicaldevelopments that could be applied to the determination of ethanolcontent in liquid solutions, notably the use of nanoparticles to createlocalized surface plasmon resonance detectors [5], [6]. However, eventhese suffer from interference due to fluorescence, challenges arisingfrom carbonation (requiring liquids to be degassed prior to measure-ment), and difficulty separating sugar and ethanol contributions fromobserved spectral signatures [2].Terahertz (THz) time-domain spectroscopy (TDS) has been utilizedin the literature to measure various normal alcohols (such as methanol,ethanol, n-propanol) [7]–[9], alcohol–water mixtures [10]–[14],alcohol–fuel mixtures [15], as well as determining ethanol contentCorresponding author: Karl Bertling (rakic@itee.uq.edu.au).Associate Editor: D. Uttamchandani.Digital Object Identifier 10.1109/LSENS.2018.2858927in commercially available alcoholic beverages [10], [16]. There isalso a potential for the use of THz metamaterials to facilitate thedetection of ethanol content in liquid solutions in a similar way tonanoparticles [17]. However, at the lower end of the THz spectrum(<2 THz), the separation of signal contributions sugar and ethanolremains a challenge and can affect the accuracy of measured ethanolcontent [10]. Fortunately, since the contributions to the signal fromsugar in solution decrease faster with increasing THz frequency thanthose attributed to ethanol (see [10]), using a higher frequency THzsource, such as a THz quantum cascade laser (QCL), would mitigatethis effect.In this article, we propose a simple system for assaying ethanol–water mixtures based on a THz QCL utilizing laser feedback inter-ferometry (LFI). Laser feedback interferometry is a useful tool forlaser sensing and utilizing the so-called self-mixing effect [18], [19].Due to the possibility of extracting the LFI signal from across thelaser terminals without the need for an external or separate detec-tor, LFI is particularly applicable in the THz frequency range, whichsuffers from a lack of convenient, fast, and sensitive detector tech-nologies [20], [21]. Previously, we have developed a method for ma-terial characterization [22], [23] utilizing this effect. However, severalcomplicated and time-consuming calibration, sample preparation, andsignal-processing techniques were required to produce accurate re-sults. The method proposed here is substantially simpler, and oncecalibrated, the system only requires a small sample of the liquid forcharacterization. This approach could be adapted to investigate otherfluid mixtures at THz frequencies.This work is licensed under a Creative Commons Attribution 3.0 License. For more information, see http://creativecommons.org/licenses/by/3.0/3501604 VOL. 2, NO. 3, SEPTEMBER 2018Fig. 1. Experimental setup of the system. (Inset) Compound nature ofthe target comprised of the cuvettes R (reference) and T (liquid undertest).II. EXPERIMENTAL SETUP AND MODELINGThe experimental setup (see Fig. 1) used a 2.59-THz QCL mountedin a free-flow cryostat operating at a temperature of 15 K. TheTHz QCL consisted of a 11.6–μm-thick GaAs/AlGaAs bound-to-continuum active region [24] that was processed into a semi-insulating surface-plasmon ridge waveguide with dimensions 1.78 mm× 140 μm. The laser was biased by a laser driver (Thorlabs LDC500C)with a dc current of 0.42 A, superimposed with a 1-kHz sawtooth-wavemodulation with an amplitude of 75 mA (producing a 900-MHz fre-quency sweep). The signal was monitored by measuring the voltageacross the THz QCL’s terminals after passing through an ac-coupled20× amplifier (SRS SR560). This amplified signal was then digitizedusing a 16-b PC-DAQ with a 1 MS/s acquisition rate (National Instru-ments NI PCI-6251). Two 2-in parabolic mirrors ( f = 4 in) were usedto collect, collimate, and focus the emitted beam onto a target area.The signal was processed by negatizing the current-induced powerramp leaving only the interferometric self-mixing fringes (which werefer to as the self-mixing signal) [25].Two 4.5-ml (10 × 10 × 45 mm3) cuvettes were used to hold thefluids under test. The two identical cuvettes were modified so thatthey shared a common 3-mm thick z-cut quartz window and weremounted on a three-axis (x-y-z) motorized stage. One cuvette wasfilled with deionized water as a reference material, with the othercontaining the liquid being assayed. Measurements of both the waterreference and the liquid under test were taken in rapid succession,with a motorized stage shifting the two cuvettes (x-axis) between themeasurement point.It is well known that the amplitude of the self-mixing fringes (β) isdirectly proportional to the reflectivity of the external target [26], [27].In this setup, the external target is a compound target consisting of aquartz window and the liquid under test. Due to the long focal lengthof the mirrors used and the large resulting depth of field, the effectivereflectivity of the compound target depends on the reflectivity of twointerfaces: 1) the air–quartz and 2) quartz–liquid interfaces (see insetin Fig. 1). The reflectivity at the quartz–liquid interface will changewith the liquid under test, leading to a corresponding change in β.We exploit this relationship to create a calibrated system for assayingethanol content.In order to calibrate the system, we extract β for both the waterreference (βwater ref.) and a series of liquids under test with knownwater–ethanol solutions (βx% ethanol). This is achieved by fitting theFig. 2. Self-mixing signals corresponding to test water–ethanol solu-tions with ABW of 0% (green line), 25% (purple line), 45% (yellow line),75% (orange line), and 100% (blue line).standard model for self-mixing signals (via the excess phase equation[28]) to the measured self-mixing signals. The longitudinal (z−axis)position of the target was set to be that at which the difference betweenthe β of the reference water and a 100% ethanol solution, (βwater ref −β100% ethanol) was maximal. For this longitudinal position the beam waistis located just beyond the quartz–liquid interface.At this position, the system was calibrated by mapping the ratioof β0% ethanol/βwater ref. to 1.0, and the β100% ethanol/βwater ref. to the (arbi-trary) value of 1.5. This normalization to βwater ref. mitigates againsttemporal changes in the self-mixing signal amplitude resulting fromsmall changes in the external cavity path (such as atmospheric changeslike humidity, temperature, and minor mechanical changes).III. RESULTS AND DISCUSSIONTwo experiments were performed, with the first using a series ofethanol–water mixtures ranging from 0% to 100% ethanol content[in alcohol by weight (ABW)] to produce a known test curve for thesystem. The second was a test of samples of various alcoholic drinks,whose ethanol content was estimated using the calibrated system.Fig. 2 shows the typical self-mixing signals obtained for severalethanol concentrations. It can be clearly seen that the amplitude (andthus ultimately β) of the signals changes significantly with differentwater–ethanol mixtures. Four separate measurements of water–ethanolmixtures ranging from 0% to 100% were performed. Fig. 3 showsthe calibrated signal strength for the known test solutions (blue dotsshow mean and error bars show ±1 standard deviation from the fourmeasurements), demonstrating a linear relationship (red line) withABW.To test the calibrated system, we estimated the ABW % of ninedifferent alcoholic beverages (shown as the colored circles in Fig. 4).The estimated ABW [and alcohol by volume (ABV)] together withthe label ABW (and ABV) and of the beverages under test is shownin Table 1.The triple sec, port, and beer samples contain significantly moresugar (in solution) than the other beverages under test. Despite thelow sensitivity of THz radiation to sugar content, its impact is stillVOL. 2, NO. 3, SEPTEMBER 2018 3501604Fig. 3. Calibrated signal strength versus known ABW % water–ethanolsolutions.Fig. 4. ABW (%) (label) versus ABW (%) (estimate) for beveragesunder test.TABLE 1. ABW (%) [ABV (%)] on the Label of the Beverages UnderTest, Compared With the ABW (%) [ABV (%)] Estimated From theCalibrated System.evident here. Indeed, it is clear from Fig. 4 and Table I that the esti-mated ABW is slightly larger than that stated by the manufacturer onthe label for these beverages. Nevertheless, due to the relatively highoperating frequency of the THz QCL used here (2.6 THz), this im-pact is much reduced when compared with THz TDS measurements,which operate over a lower frequency range [10]. For the remainingbeverages under test, estimated ABW aligns well with the expectedlinear relationship—deviations are on the same order as for THz TDSmeasurements.For all these measurements, only a small volume of liquid wasused for testing (∼1 mL). However, the minimum volume of liquidrequired is much less. This is a consequence of the high attenuationof the THz signal for both ethanol and water at the measurementfrequency (2.6 THz)—the longitudinal depth of either liquid requiredto act as a half-space for THz waves is <500 μm. As such, with onlyminor changes to the geometry of the external optical components, thesame system could be implemented using a quartz window on whicha single drop of liquid under test is placed for measurement.IV. CONCLUSIONIn this article, we proposed and demonstrated a system and a methodof assaying the ethanol content of liquid solutions. The system isbuilt around a THz QCL operating at 2.6 THz as a laser feedbackinterferometer, employs a reference to mitigate environmental effectsduring measurement, and only requires a small volume of liquid fortesting. The method compares well with existing THz TDS ethanol-assaying techniques.ACKNOWLEDGMENTThis work was supported in part by the Australian Research Council’s DiscoveryProjects funding scheme under Grant DP 160 103910; in part by the Queensland Gov-ernment’s Advance Queensland programme; in part by the EPSRC, U.K., under GrantEP/P021859/1 and DTG Award; in part by the Royal Society (Wolfson Research MeritAwards WM110032 and WM150029); and in part by the European Cooperation in Scienceand Technology (COST) (Action BM1205).REFERENCES[1] J. J. Palmer, How to Brew: Everything you Need to Know to Brew Beer Right theFirst Time, 3rd ed. Boulder, CO, USA: Brewers, 2006.[2] A. Nordon, A. Mills, R. T. Burn, F. M. Cusick, and D. Littlejohn, “Comparison ofnon-invasive NIR and Raman spectrometries for determination of alcohol contentof spirits,” Analytica Chimica Acta, vol. 548, no. 1, pp. 148–158, 2005.[3] P. C. Ashok, B. B. Praveen, and K. 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Determining Ethanol Content of Liquid Solutions Using Laser Feedback Interferometry with a Terahertz Quantum Cascade Laser

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

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