Design and Analysis of a Single Humidity Sensor Based on TDLAS for Water Vapor and Heavy Oxygen Water Vapor Detection

Ping Gong; Jian Zhou; Zhixuan Er; Yu Ju; Liang Xie

doi:10.3390/photonics9030175

Design and Analysis of a Single Humidity Sensor Based on TDLAS for Water Vapor and Heavy Oxygen Water Vapor Detection

Gong, Ping;Zhou, Jian;Er, Zhixuan;Ju, Yu;Xie, Liang 2022-03-11 00:00:00 hv photonics Article Design and Analysis of a Single Humidity Sensor Based on TDLAS for Water Vapor and Heavy Oxygen Water Vapor Detection 1 1 , 2 1 , 2 1 1 , 2 , Ping Gong , Jian Zhou , Zhixuan Er , Yu Ju and Liang Xie * State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China; pgong@semi.ac.cn (P.G.); zhoujian@semi.ac.cn (J.Z.); erzhixuan@semi.ac.cn (Z.E.); juyu@semi.ac.cn (Y.J.) College of Materials Science and Opto-Electronic Technology, Chinese Academy of Sciences, Beijing 100049, China * Correspondence: xiel@semi.ac.cn Abstract: In this paper, a single humidity sensor for water vapor and heavy oxygen water vapor detection is presented. The sensor is based on tunable diode laser absorption spectroscopy (TDLAS) and thus has high sensitivity, good selectivity, and a short response time. A 1372 nm distributed feedback (DFB) diode laser is utilized as the light source, the wavelength tuning range of which covers the absorption lines of water vapor and heavy oxygen water vapor. A Herriott gas cell with 12 m optical length is designed for signal-to-noise ratio (SNR) enhancement. The sensor can distinguish between water vapor and heavy oxygen water vapor effectively. The accuracy of water detection is within 0.5% RH. The accuracy of heavy oxygen water vapor detection is within 1.0% RH. Keywords: humidity; heavy oxygen water; TDLAS Citation: Gong, P.; Zhou, J.; Er, Z.; Ju, 1. Introduction Y.; Xie, L. Design and Analysis of a Heavy oxygen water is an isotope of water, which is widely used in diverse ﬁelds Single Humidity Sensor Based on such as disease diagnoses, organ function research, and new drug developments [1–3]. TDLAS for Water Vapor and Heavy Conventional humidity sensors such as wet and dry bulb hygrometers, humidity-sensitive Oxygen Water Vapor Detection. capacitors, and chilled mirror dew point meters are used to detect heavy oxygen water Photonics 2022, 9, 175. https:// vapor [4,5]. However, these methods are unable to distinguish between water vapor and doi.org/10.3390/photonics9030175 heavy oxygen water vapor due to their similar physical and chemical characteristics. They Received: 25 January 2022 can only provide the total concentration of water vapor and heavy oxygen water vapor Accepted: 9 March 2022 rather than their respective concentrations. Heavy oxygen water vapor is always coexistent Published: 11 March 2022 with water vapor in practice. Hence, a single humidity sensor with good selectivity is an attractive alternative to conventional humidity sensors. Publisher’s Note: MDPI stays neutral Optical gas sensing is drawing increasing attention with regards to application in with regard to jurisdictional claims in gas detection. Gas sensors based on non-dispersive infrared (NDIR), photoacoustic spec- published maps and institutional afﬁl- troscopy (PAS), cavity ring-down spectroscopy (CRDS), and tunable diode laser absorption iations. spectroscopy (TDLAS) are well-established [6]. Furthermore, some novel high-sensitivity and miniaturized spectroscopic gas sensors have been developed. Kassa-Baghdouche et al. proposed slotted photonic crystal waveguides as refractive index sensing devices, which Copyright: © 2022 by the authors. obtained a sensitivity of more than 1150 per refractive index unit (RIU) with an insertion Licensee MDPI, Basel, Switzerland. loss level of 0.3 dB [7]. In Reference [8], an optimized photonic crystal microcavity for a This article is an open access article 4 spectroscopic gas sensor was realized. The device has a Q of 10 and a detection limit as low distributed under the terms and 4 as 10 RIU. NDIR sensors are composed of a broadband light source, light path, optical conditions of the Creative Commons ﬁlters, and detectors. They have high sensitivity, low cost, and ease of implementation [9]. Attribution (CC BY) license (https:// The selectivity of NDIR sensors is not adequate to distinguish between water vapor and creativecommons.org/licenses/by/ heavy oxygen water vapor because of the broadband light source and the limited ﬁlter 4.0/). Photonics 2022, 9, 175. https://doi.org/10.3390/photonics9030175 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 175 2 of 9 width. The wavelength spacing of the water vapor absorption line and the heavy oxygen water vapor absorption line is within 1 nm, which is an order smaller than the ﬁlter width. In PAS, periodic optical absorption will excite the periodic acoustic pressure propagating in the sample due to the photoacoustic effect. The propagating pressure results in a sound wave, which can be detected with an acoustic transducer. PAS is an indirect absorption spectroscopy technology with immunity to the background, thus leading to high sensitivity. PAS sensors suffer from the vibrations of the surrounding environment [10]. The cavity of a CRDS sensor has two reﬂectors with ultra-high reﬂectivity (~99.999%) to achieve an optical length of tens or even hundreds of kilometers. The minimum detection limit of CRDS sensors can reach the ppb or even ppt levels. Given their high cost and suscepti- bility to shaking, CRDS sensors are not under consideration in this paper [11]. A TDLAS sensor is equipped with a diode laser in which the linewidth is two orders of magnitude lower than the absorption lines. The narrowband light source, as well as the ﬁngerprint characteristics of infrared absorption spectroscopy contribute to the outstanding selectivity of TDLAS [12–14]. Consequently, TDLAS has a natural advantage in the detection of isotopes. In References [15,16], Kireev et al. achieved high CO measuring accuracy 13 12 and the simultaneous boundary CO / CO ratio was consistent in the expiratory air. 2 2 Ghorbani R proposed a TDLAS sensor with an ICL light source. The sensor was applied to 12 13 real-time detection of CO and CO in the air and exhaled breath [17]. In this paper, we propose a humidity sensor based on TDLAS. The motivation for this work is to improve the sensing dynamic range so that it can meet the requirements of both water vapor detection and heavy oxygen water vapor detection. The absorption amplitude of heavy oxygen water vapor is two orders lower than that of water vapor in the near-infrared region. As a result, a large dynamic of 40 dB is necessary. We adopt direct absorption spectroscopy (DAS) for water vapor detection and wavelength modulation spectroscopy (WMS) for heavy oxygen water vapor detection, which extends the dynamic range to 40 dB [15,18]. In addition, a Herriott gas cell is designed for the signal-to-noise ratio (SNR) enhancement. 2. Experimental Setup and Theoretical Analysis The experimental setup for the proposed humidity sensor is depicted in Figure 1a. A distributed feedback (DFB) diode laser (Sichuan light source optoelectronic, model: LT-LD-1372-20-1.5-FA) is employed as the light source, in which the wavelength covers the absorption lines of both water vapor and heavy oxygen water vapor. The diode laser is controlled by a temperature controller and a current controller. The temperature controller is designed on the basis of a MAX1978 for the Peltier thermoelectric cooler (TEC) module in the diode laser. The current controller consists of a DC current source, a triangle signal generator, and a sinusoidal signal generator. The diode laser has a tail ﬁber, of which the output light is coupled to a collimator (OPEAK, model: FC37). The collimated light acts as an incident light for the Herriott gas cell. The transmitted light through the gas cell is detected by a photodetector (Lightsensing, model: LSIPD-1S), which transforms the light signal into a current signal. A trans-impedance ampliﬁer (TIA) with low noise changes the current signal to a voltage signal. The TIA contains multiple channels for feedback resistor selection according to the ampliﬁer gain setting. The voltage signal is ampliﬁed and then acquired by a 12-bit analog-to-digital conversion (ADC) (Analog Devices, model: AD7366). A lock-in ampliﬁer is used to improve the SNR of the system. The function of the microcontroller unit (MCU) is to process signals, to set parameters, and to communicate with a computer via RS232. The temperature controller, the current controller, the TIA, the ADC, the lock-in ampliﬁer, and the MCU are integrated into the homemade circuit, as shown in Figure 1b. Photonics 2022, 9, x FOR PEER REVIEW 3 of 10 Photonics 2022, 9, x FOR PEER REVIEW 3 of 10 Photonics 2022, 9, 175 3 of 9 (a) (b) Figure 1. Experimental setup for the proposed humidity sensor. (a) Schematic diagram of the exper- imental setup; (b) picture of the homemade circuit. (a) (b) 2.1. Light Source Pro Figure 1. perty Exper imental setup for the proposed humidity sensor. (a) Schematic diagram of the exper- Figure 1. Experimental setup for the proposed humidity sensor. (a) Schematic diagram of the imental setup; (b) picture of the homemade circuit. experimental setup; (b) picture of the homemade circuit. The central wavelength of the DFB diode laser depends on its temperature and driv- ing current. A proportional-integral-derivative (PID) is carried out on the temperature 2.1. Light Source Property 2.1. Light Source Property controller. The current controller generates a triangle signal and a sinusoidal signal as the The central wavelength of the DFB diode laser depends on its temperature and driv- The central wavelength of the DFB diode laser depends on its temperature and driv- ing current. A proportional-integral-derivative (PID) is carried out on the temperature modulation signal, for which the frequency and amplitude vary with parameters set by ing current. A proportional-integral-derivative (PID) is carried out on the temperature controller. The current controller generates a triangle signal and a sinusoidal signal as the controller. The current controller generates a triangle signal and a sinusoidal signal as the the MCU. As is shown in Figure 2, for wavelength tuning, the diode laser has a tempera- modulation signal, for which the frequency and amplitude vary with parameters set by modulation signal, for which the frequency and amplitude vary with parameters set by ture tuning coefficient of 0.080 nm/°C and a current tuning coefficient of 0.015 nm/mA. the MCU. As is shown in Figure 2, for wavelength tuning, the diode laser has a tempera- the MCU. As is shown in Figure 2, for wavelength tuning, the diode laser has a tempera- Therefore, we process with temperature tuning for coarse tuning and current tuning for ture tur tu eni tuning ng coef coef fici ﬁcient ent of 0.08 of 0.080 0 nm/°C nm/ and a C andca urren currtent tun tuning ing coef coef ficie ﬁcient nt of 0. of01 0.015 5 nm nm/mA. /mA. fine tuning. Therefore, we process with temperature tuning for coarse tuning and current tuning for Therefore, we process with temperature tuning for coarse tuning and current tuning for fine tuning. ﬁne tuning. (a) ( b) Figure 2. Wavelength tuning property of the diode laser: (a) wavelength varies with temperature; (a) (b) (b) wavelength varies with driving current. Figure 2. Wavelength tuning property of the diode laser: (a) wavelength varies with temperature; Figure 2. Wavelength tuning property of the diode laser: (a) wavelength varies with temperature; The light wavelength should be fixed at the strong absorption line of the target gas (b) wavelength varies ( b with driving ) wavelength varies current. with driving current. without interference. Absorption lines at 1372.10 nm for water vapor detection and The light wavelength should be ﬁxed at the strong absorption line of the target 1372.53 nm for heavy oxygen water vapor detection are chosen according to the HITRAN The light wavelength should be fixed at the strong absorption line of the target gas gas without interference. Absorption lines at 1372.10 nm for water vapor detection and database [19]. The property of the selected absorption lines is described in Figure 3. Tem- without interference. Absorption lines at 1372.10 nm for water vapor detection and 1372.53 nm for heavy oxygen water vapor detection are chosen according to the HITRAN perature is set according to the specified wavelength of the target absorption line. Water 1372.53 nm for heavy oxy database ge[n wat 19]. The er vap property or detec of the tion selected are chosen acc absorption ording to the HITRA lines is described in Figur N e 3. vapor detection is set by tuning the laser temperature to T . Heavy oxygen water vapor Temperature is set according to the speciﬁed wavelength of the target absorption line. database [19]. The property of the selected absorption lines is described in Figure 3. Tem- detection is set by tuning the laser temperature to T The central wavelength of the diode Water vapor detection is set by tuning the laser temperatur 2 e to T . Heavy oxygen water perature is set according to the specified wavelength of the target absorption line. Water laser sc vapor ans detection across the is set tar by get tuning absorption line so the laser temperatur that it is e with to T in The the measured central wavelength range. The of the vapor detection is set by tuning the laser temperature to T . Heavy oxygen water vapor madiode ximum laser light scans power acris 20 oss the mW. target absorption line so that it is within the measured range. The maximum light power is 20 mW. detection is set by tuning the laser temperature to T The central wavelength of the diode laser scans across the target absorption line so that it is within the measured range. The maximum light power is 20 mW. Photonics 2022, 9, x FOR PEER REVIEW 4 of 10 Photonics 2022, 9, x FOR PEER REVIEW 4 of 10 Photonics 2022, 9, 175 4 of 9 Figure 3. Target absorption lines of water vapor and heavy oxygen water vapor. 2.2. Optical Length Enhancement Light with a specified wavelength can be absorbed when passing through the gas to be measured, which is a uniform medium. Light with 1372.10 nm can be absorbed by wa- ter vapor. Light with 1372.53 nm can be absorbed by heavy oxygen water vapor. The Beer– Lambert law expresses the relationship between the transmitted intensity I and the in- Figure 3. Target absorption lines of water vapor and heavy oxygen water vapor. Figure 3. Target absorption lines of water vapor and heavy oxygen water vapor. cident intensity I , as follows [20,21]: 2.2. Optical Length Enhancement − α CL 2.2. Optical Length Enhancement Light with a speciﬁed wavelength , can be absorbed when passing through the gas II = e t 0 (1) to be measured, which is a uniform medium. Light with 1372.10 nm can be absorbed by Light with a specified wavelength can be absorbed when passing through the gas to water vapor. Light with 1372.53 nm can be absorbed by heavy oxygen water vapor. The be measured, which is a uniform medium. Light with 1372.10 nm can be absorbed by wa- where α , , and L are the absorption coefficient, the gas concentration, and the op- Beer–Lambert law expresses the relationship between the transmitted intensity I and the ter vapor. Light with 1372.53 nm can be absorbed by heavy oxygen water vapor. Thte Beer– tical length, respectively. In the sensing system, α CL 1 , therefore I is given by the incident intensity I , as follows [20,21]: t Lambert law expresses the relationship between the transmitted intensity I and the in- following: aCL cident intensity I , as follows [20,21]: I = I e , (1) t 0 − α CL II≈− 1 αCL () II = e , t 0 (2) t 0 (1) where a, C, and L are the absorption coefﬁcient, the gas concentration, and the optical length, respectively. In the sensing system, aCL 1, therefore I , is given by the following: where α , , and L are the absorption coefficient, the gas concentration, and the op- The absorbed intensity is represented by the following: tical length, respectively. In the sensing system, α CL 1 , therefore I is given by the I I (1 aCL). (2) t t II≈−I ≈I αCL . (3) 00 t following: The absorbed intensity I is represented by the following: II≈− 1 αCL The absorbed intensity is proportional to the concen() tration of target gas when the t 0 (2) I I I I aCL. (3) 0 0 absorption line is fixed. For this reason, the minimum detection limit of the sensor in- The absorbed intensity is represented by the following: creases as the optical length enhances. Two concave mirrors (Thorlabs, model: CM508- The absorbed intensity is proportional to the concentration of target gas when the ab- II≈−I ≈I αCL . 00 t (3) 200EH4-M02) are utsorption ilized fline or th is e H ﬁxed. errio For this tt ce reason, ll. The thest minimum ructuraldetection parame limit tersof of the the sensor gas c incr eleases l as the optical length enhances. Two concave mirrors (Thorlabs, model: CM508-200EH4-M02) are determined by optimizing the optical path model, which is sketched in Figure 4a. A The absorbed intensity is proportional to the concentration of target gas when the are utilized for the Herriott cell. The structural parameters of the gas cell are determined by absorption line is fixed. For this reason, the minimum detection limit of the sensor in- Gaussian light source serves as the incident light in the model, in which the diameter is optimizing the optical path model, which is sketched in Figure 4a. A Gaussian light source creases as the optical length enhances. Two concave mirrors (Thorlabs, model: CM508- set to 3 mm. The optical length reaches the maximum when the distance between two serves as the incident light in the model, in which the diameter is set to 3 mm. The optical 200EH4-M02) are utilized for the Herriott cell. The structural parameters of the gas cell mirrors is set to 223 mm. The number of reflections in each mirror is 28. length reaches the maximum when the distance between two mirrors is set to 223 mm. The are determined by optimizing the optical path model, which is sketched in Figure 4a. A number of reﬂections in each mirror is 28. Gaussian light source serves as the incident light in the model, in which the diameter is set to 3 mm. The optical length reaches the maximum when the distance between two mirrors is set to 223 mm. The number of reflections in each mirror is 28. (a) (b) (a) (b) Figure 4. The designed Herriott gas cell. (a) The optical path model; (b) light spots in the mirror. Figure 4. The designed Herriott gas cell. (a) The optical path model; (b) light spots in the mirror. Figure 4. The designed Herriott gas cell. (a) The optical path model; (b) light spots in the mirror. The output light of the diode laser is collimated and then coupled to the designed Herriott cell. The light is reﬂected by the two mirrors tens of times, for optical length Photonics 2022, 9, x FOR PEER REVIEW 5 of 10 The output light of the diode laser is collimated and then coupled to the designed Herriott cell. The light is reflected by the two mirrors tens of times, for optical length en- Photonics 2022, 9, 175 5 of 9 hancement. Red laser light is inputted into the gas cell as an indicating light. The length of the gas cell is 220 mm and the optical length, obtained by multiple reflections, is 12 m. An image of light spots in one mirror is shown in Figure 4b. enhancement. Red laser light is inputted into the gas cell as an indicating light. The length of the gas cell is 220 mm and the optical length, obtained by multiple reﬂections, is 12 m. 2.3. Direct Detection and Harmonic Detection An image of light spots in one mirror is shown in Figure 4b. A 1372 nm DFB diode laser is commercially available, which offers the light source 2.3. Direct Detection and Harmonic Detection for the proposed humidity sensor. A DFB diode laser has a narrow linewidth and high A op 1372 tical nm po DFB wer diode density. laser Nev is erth commer eless, cially the outp available, ut inten which sity of of fers the the diolight de laser, sour wh ce for ich is the incident intensity of the Herriott cell, drifts due to variations in environmental tempera- the proposed humidity sensor. A DFB diode laser has a narrow linewidth and high optical ture, driving current, and mechanical fluctuations [22]. Figure 5a shows the light power power density. Nevertheless, the output intensity of the diode laser, which is the incident intensity of of the the diode Herriott laser cell, vary drifts ing with due to the variations driving current, in environmental where the temp linear eratur fitting e, driving is carried out current, and mechanical ﬂuctuations [22]. Figure 5a shows the light power of the diode laser as illustrated by the red solid line and the coefficient of determination is R  0.9992 . In varying with the driving current, where the linear ﬁtting is carried out as illustrated by the DAS-TDLAS mode, the incident intensity I is acquired by linear fitting of the transmit- red solid line and the coefﬁcient of determination is R = 0.9992. In DAS-TDLAS mode, the ted intensity when the light is absorbed by the target gas. Therefore, the influence of the incident intensity I is acquired by linear ﬁtting of the transmitted intensity when the light incident intensity fluctuation is effectively eliminated due to the good linearity of the laser is absorbed by the target gas. Therefore, the inﬂuence of the incident intensity ﬂuctuation power. In Figure 5b, the blue solid line shows the transmitted intensity. The red dotted is effectively eliminated due to the good linearity of the laser power. In Figure 5b, the blue line depicts the linear fitting of the blue line. The absorption intensity is obtained when solid line shows the transmitted intensity. The red dotted line depicts the linear ﬁtting the fitting line subtracts the absorption intensity, as is illustrated in Figure 5c. of the blue line. The absorption intensity is obtained when the ﬁtting line subtracts the absorption intensity, as is illustrated in Figure 5c. (a) (b) (c) Figure 5. DAS-TDLAS detection: (a) the light power of the diode laser varies with driving current; Figure 5. DAS-TDLAS detection: (a) the light power of the diode laser varies with driving current; (b) the transmitted optical intensity varies with laser wavelength and its linear fitting; (c) the ab- (b) the transmitted optical intensity varies with laser wavelength and its linear ﬁtting; (c) the absorbed sorbed optical intensity varies with laser wavelength. optical intensity varies with laser wavelength. As for the equivalent concentration, heavy oxygen water vapor absorption is two As for the equivalent concentration, heavy oxygen water vapor absorption is two orders weaker than water vapor absorption. DAS is incapable of detecting absorptions as orders weaker than water vapor absorption. DAS is incapable of detecting absorptions weak as the system noise. A sinusoidal modulation is added for harmonic detection to as weak as the system noise. A sinusoidal modulation is added for harmonic detection extract the weak signal induced by the absorption of heavy oxygen water vapor. A home- to extract the weak signal induced by the absorption of heavy oxygen water vapor. A made lock-in amplifier is used for harmonic detection, which enhances the SNR effec- homemade lock-in ampliﬁer is used for harmonic detection, which enhances the SNR tively. In WMS-TDLAS, the laser wavelength  modulated by a sinusoidal signal is ex- effectively. In WMS-TDLAS, the laser wavelength l modulated by a sinusoidal signal is pressed as follows: expressed as follows: l = l + a sin 2p f t, (4) (4)   a sin 2 ft , where l is the center wavelength of the diode laser, a is the wavelength modulation depth, and f is the modulation frequency. where  is the center wavelength of the diode laser, a is the wavelength modulation The absorption line can be approximated as a Voigt proﬁle at room temperature and depth, and f is the modulation frequency. atmospheric pressure. The proﬁle is represented by the following: The absorption line can be approximated as a Voigt profile at room temperature and +¥ y exp t atmospheric pressure. The profile is represented by the following: g(x, y) = A dt, (5) (x t) + y p p ll g ln 2/p 0 C where A = , x = ln 2 , y = ln 2 , g , and g are the half width at half D C g g g D D D maximum (HWHM) of the Doppler and Lorentz proﬁles, respectively [23]. Photonics 2022, 9, 175 6 of 9 According to Fourier analysis, the second harmonic signal of the transmitted light can be given by the following: 2 2 + m 2 1 + m I = I G p CL, (6) 2 f 0 2 f 2 2 m 1 + m where m and G are constants. 2 f The ﬁrst harmonic signal of the transmitted light can be given by the following: I = I G a, (7) f 0 f where G is a constant. It is indicated that the ratio of the second harmonic signal and the ﬁrst harmonic signal is proportional to the gas concentration, which is free from the inﬂuence of the light output intensity [24]. The ratio is given by the following: 2 2 + m 2 1 + m I G 2 f 2 f = p CL. (8) 2 2 I a G f f m 1 + m 3. Experiments and Results 3.1. Water Vapor Detection The humidity sensor operates in DAS-TDLAS mode for water vapor detection. The central wavelength of the diode laser scans across the water vapor absorption line by scanning the driving current with a triangle signal. The voltage signal varying with the driving current is logged, shown in Figure 6. A commercial humidity sensor (Rotronic, HP32) is utilized to acquire relative humidity of detected gas, in which the precision is 0.5% RH. The measured relative humidity of the water vapor samples is 1.95% RH, 2.60% RH, 3.25% RH, 3.90% RH, 4.55% RH, 5.20% RH, 5.85% RH, 6.50% RH, 18.26% RH, 58% RH, 76% RH, and 88% RH, respectively. Curves with different colors present the results of the different relative humidity mentioned above. It is noted that the signal proﬁles are Photonics 2022, 9, x FOR PEER REVIEW 7 of 10 consistent with each other and that the absorption is more obvious when a higher relative humidity sample is detected. Figure 6. Transmitted amplitude in the DAS-TDLAS mode. Figure 6. Transmitted amplitude in the DAS-TDLAS mode. Figure 7 presents the maximum absorption signal as a function of the relative humidity, Figure 7 presents the maximum absorption signal as a function of the relative humid- which is proportional to the water vapor concentration. Linear ﬁtting of the curves is ity, which is proportional to the water vapor concentration. Linear fitting of the curves is performed. The residual sum of squares is 19.59, and the coefﬁcient of determination is performed. The residual sum of squares is 19.59, and the coefficient of determination is R = 0.9913. The sensitivity for water vapor detection is 9.64 mV/%RH. It is concluded R = 0.9913. The sensitivity for water vapor detection is 9.64 mV/%RH. It is concluded that the proposed sensor has good linearity and that the measuring accuracy is within ±0.5%RH for water vapor measurement. Figure 7. Relative humidity measured in DAS-TDLAS. 3.2. Heavy Oxygen Water Vapor Detection The performance of the proposed humidity sensor for heavy oxygen water vapor detection is evaluated in the WMS-TDLAS mode. The driving current scans so that the central wavelength scans across the absorption line. A triangle signal and a sinusoidal signal modulate the driving current simultaneously. The frequency of the triangle signal is 100 Hz, and the frequency of the sinusoidal signal is 10 kHz. The modulation depth is optimized. The second harmonic signals measured at different relative humidity are shown in Figure 8. The same humidity sensor (Rotronic, HP32) used for water vapor de- tection is used to acquire relative humidity. The measured relative humidity of the heavy oxygen water vapor samples is 3.4% RH, 6.8% RH, 10.2% RH, 13.6% RH, and 17% RH, respectively. The profiles are consistent with each other and reach their maximums at the center of the absorption lines. Photonics 2022, 9, x FOR PEER REVIEW 7 of 10 Figure 6. Transmitted amplitude in the DAS-TDLAS mode. Figure 7 presents the maximum absorption signal as a function of the relative humid- ity, which is proportional to the water vapor concentration. Linear fitting of the curves is performed. The residual sum of squares is 19.59, and the coefficient of determination is Photonics 2022, 9, 175 7 of 9 R  0.9913 . The sensitivity for water vapor detection is 9.64 mV/%RH. It is concluded that the proposed sensor has good linearity and that the measuring accuracy is within ±0.5%RH for water vapor measurement. that the proposed sensor has good linearity and that the measuring accuracy is within 0.5% RH for water vapor measurement. Figure 7. Relative humidity measured in DAS-TDLAS. Figure 7. Relative humidity measured in DAS-TDLAS. 3.2. Heavy Oxygen Water Vapor Detection 3.2. Heavy Oxygen Water Vapor Detection The performance of the proposed humidity sensor for heavy oxygen water vapor detection is evaluated in the WMS-TDLAS mode. The driving current scans so that the The performance of the proposed humidity sensor for heavy oxygen water vapor central wavelength scans across the absorption line. A triangle signal and a sinusoidal detection is evaluated in the WMS-TDLAS mode. The driving current scans so that the signal modulate the driving current simultaneously. The frequency of the triangle signal central wavelength scans across the absorption line. A triangle signal and a sinusoidal is 100 Hz, and the frequency of the sinusoidal signal is 10 kHz. The modulation depth is signal modulate the driving current simultaneously. The frequency of the triangle signal optimized. The second harmonic signals measured at different relative humidity are shown is 100 Hz, and the frequency of the sinusoidal signal is 10 kHz. The modulation depth is in Figure 8. The same humidity sensor (Rotronic, HP32) used for water vapor detection is optimized. The second harmonic signals measured at different relative humidity are used to acquire relative humidity. The measured relative humidity of the heavy oxygen shown in Figure 8. The same humidity sensor (Rotronic, HP32) used for water vapor de- water vapor samples is 3.4% RH, 6.8% RH, 10.2% RH, 13.6% RH, and 17% RH, respectively. tection is used to acquire relative humidity. The measured relative humidity of the heavy Photonics 2022, 9, x FOR PEER REVIEW 8 of 10 The proﬁles are consistent with each other and reach their maximums at the center of the oxygen water vapor samples is 3.4% RH, 6.8% RH, 10.2% RH, 13.6% RH, and 17% RH, absorption lines. respectively. The profiles are consistent with each other and reach their maximums at the center of the absorption lines. Figure Figure 8. 8. Second Second harmonic detectio harmonic detection n in in the W the WMS-TDLAS MS-TDLASmode. mode. The concentration of heavy oxygen water vapor is proportional to the second harmonic The concentration of heavy oxygen water vapor is proportional to the second har- of the transmitted intensity in the WMS-TDLAS mode. The relative humidity is a function monic of the transmitted intensity in the WMS-TDLAS mode. The relative humidity is a of the maximum of the second harmonic. Figure 9 represents the linear ﬁtting. The residual function of the maximum of the second harmonic. Figure 9 represents the linear fitting. sum of squares is 11.03, and the coefﬁcient of determination is R = 0.9850. The sensitivity The residual sum of squares is 11.03, and the coefficient of determination is R = 0.9850 for heavy oxygen water vapor detection is 30.91 mV/%RH. The proposed sensor has . The sensitivity for heavy oxygen water vapor detection is 30.91 mV/%RH. The proposed good linearity, and the measuring accuracy is within 1.0% RH for heavy oxygen water sensor has good linearity, and the measuring accuracy is within ±1.0%RH for heavy oxy- vapor detection. gen water vapor detection. Figure 9. Relative humidity measured in WMS-TDLAS. 4. Conclusions A single humidity sensor for water vapor detection and heavy oxygen water vapor detection is described in this paper. The sensor is based on TDLAS, which is an attractive alternative for the distinguishment of isotopes. A single 1372 nm DFB diode laser is uti- lized as the light source, for which the wavelength tuning range covers the absorption lines of both water vapor and heavy oxygen water vapor. A Herriott gas cell with 12 m optical length is designed for SNR enhancement, which increases the optical length by ~60 times. Water vapor is detected in the DAS-TDLAS mode, and the accuracy of water vapor detection is within ±0.5%RH. Heavy oxygen water vapor is detected in the WMS-TDLAS mode, and the accuracy of heavy oxygen water vapor detection is within ±1.0%RH. Author Contributions: Conceptualization, L.X.; methodology, P.G.; validation, P.G., J.Z., Z.E. and Y.J.; formal analysis, J.Z.; investigation, P.G.; data curation, J.Z.; writing—original draft preparation, P.G.; writing—review and editing, P.G.; project administration, L.X.; All authors have read and agreed to the published version of the manuscript. Photonics 2022, 9, x FOR PEER REVIEW 8 of 10 Figure 8. Second harmonic detection in the WMS-TDLAS mode. The concentration of heavy oxygen water vapor is proportional to the second har- monic of the transmitted intensity in the WMS-TDLAS mode. The relative humidity is a function of the maximum of the second harmonic. Figure 9 represents the linear fitting. The residual sum of squares is 11.03, and the coefficient of determination is R  0.9850 . The sensitivity for heavy oxygen water vapor detection is 30.91 mV/%RH. The proposed sensor has good linearity, and the measuring accuracy is within ±1.0%RH for heavy oxy- Photonics 2022, 9, 175 8 of 9 gen water vapor detection. Figure 9. Relative humidity measured in WMS-TDLAS. Figure 9. Relative humidity measured in WMS-TDLAS. 4. Conclusions 4. Conclusions A single humidity sensor for water vapor detection and heavy oxygen water vapor detection A single is hum described idity sensor in this for paper water . The vapor sensor detec is tion based an on d heavy TDLAS, oxwhich ygen water is an attractiv vapor e detec alternative tion is described for the distinguishment in this paper. The of isotopes. sensor is b Aas single ed on1372 TDLA nm S, DFB which diode is an laser attractive is utilized as the light source, for which the wavelength tuning range covers the absorption lines of alternative for the distinguishment of isotopes. A single 1372 nm DFB diode laser is uti- lized both as th water e light vapor sourc and e, fheavy or which oxygen the wavelen water vapor gth tuning . A Herriott range gas covcell ers th with e ab 12 sorpt m optical ion length is designed for SNR enhancement, which increases the optical length by ~60 times. lines of both water vapor and heavy oxygen water vapor. A Herriott gas cell with 12 m Water vapor is detected in the DAS-TDLAS mode, and the accuracy of water vapor detection optical length is designed for SNR enhancement, which increases the optical length by ~60 is within 0.5% RH. Heavy oxygen water vapor is detected in the WMS-TDLAS mode, times. Water vapor is detected in the DAS-TDLAS mode, and the accuracy of water vapor and the accuracy of heavy oxygen water vapor detection is within 1.0% RH. detection is within ±0.5%RH. Heavy oxygen water vapor is detected in the WMS-TDLAS mode, and the accuracy of heavy oxygen water vapor detection is within ±1.0%RH. Author Contributions: Conceptualization, L.X.; methodology, P.G.; validation, P.G., J.Z., Z.E. and Y.J.; formal analysis, J.Z.; investigation, P.G.; data curation, J.Z.; writing—original draft preparation, Author Contributions: Conceptualization, L.X.; methodology, P.G.; validation, P.G., J.Z., Z.E. and P.G.; writing—review and editing, P.G.; project administration, L.X. All authors have read and agreed Y.J.; formal analysis, J.Z.; investigation, P.G.; data curation, J.Z.; writing—original draft preparation, to the published version of the manuscript. P.G.; writing—review and editing, P.G.; project administration, L.X.; All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number U1930205. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest. References 1. Magomedbekov, E.P.; Selivanenko, I.L.; Kulov, N.N.; Veretennikova, G.V. Conditioning Heavy-Oxygen Water by Rectiﬁcation under Vacuum. Theor. Found. Chem. Eng. 2019, 53, 719–724. [CrossRef] 2. Kutus, B.; Shalit, A.; Hamm, P.; Hunger, J. Dielectric response of light, heavy and heavy-oxygen water: Isotope effects on the hydrogen-bonding network’s collective relaxation dynamics. Phys. Chem. Chem. Phys. 2021, 23, 5467–5473. [CrossRef] [PubMed] 3. Nagano, Y.; Miyazaki, Y.; Matsuo, T.; Suga, H. Heat capacities and enthalpy of fusion of heavy oxygen water. J. Phys. Chem. 1993, 97, 6897–6901. [CrossRef] 4. Whipple, F. The wet-and-dry-bulb hygrometer: The relation to theory of the experimental researches of Awbery and Grifﬁths. Proc. Phys. Soc. 1933, 45, 307–319. [CrossRef] 5. Khelifa, N. Effect of Measurement of Dew Point Temperature in Moist Air on the Absorption Line at 1392.53 nm of Water Vapor. Int. J. Opt. 2019, 2019, 3424172. [CrossRef] 6. Hodgkinson, J.; Tatam, R.P. Optical gas sensing: A review. Meas. Sci. Technol. 2013, 24, 012004. [CrossRef] 7. Kassa-Baghdouche, L.; Cassan, E. Mid-infrared refractive index sensing using optimized slotted photonic crystal waveguides. Photonics Nanostruct.-Fundam. Appl. 2018, 28, 32–36. [CrossRef] 8. Kassa-Baghdouche, L. High-sensitivity spectroscopic gas sensor using optimized H1 photonic crystal microcavities. J. Opt. Soc. Am. B 2020, 37, A227–A284. [CrossRef] 9. Jha, R.K. Non-Dispersive infrared gas sensing technology: A review. IEEE Sens. J. 2022, 22, 6–15. [CrossRef] 10. Sgobba, F.; Sampaolo, A.; Patimisco, P.; Giglio, M.; Menduni, G.; Ranieri, A.C.; Hoelzl, C.; Rossmadl, H.; Brehm, C.; Mackowiak, V.; et al. Compact and portable quartz-enhanced photoacoustic spectroscopy sensor for carbon monoxide environmental monitoring in urban areas. Photoacoustics 2022, 25, 100318. [CrossRef] Photonics 2022, 9, 175 9 of 9 11. Song, Z.; Xu, L.; Xie, H.; Cao, Z. Random vibration-driven continuous-wave CRDS system for calibration-free gas concentration measurement. Opt. Lett. 2020, 45, 746–749. [CrossRef] [PubMed] 12. Zhao, W.S.; Xu, L.J.; Huang, A. A WMS Based TDLAS Tomographic System for Distribution Retrievals of both Gas Concentration and Temperature in Dynamic Flames. IEEE Sens. J. 2020, 20, 4179–4188. [CrossRef] 13. Liang, W.; Wei, G.; He, A.; Shen, H. A novel wavelength modulation spectroscopy in TDLAS. Infrared Phys. Technol. 2021, 114, 103661. [CrossRef] 14. Xie, Y.; Chang, J.; Chen, X.; Sun, J.; Zhang, Q.; Wang, F.; Zhang, Z.; Feng, Y. A DFB-LD Internal Temperature Fluctuation Analysis in a TDLAS System for Gas Detection. IEEE Photonics J. 2019, 11, 1–8. [CrossRef] 15. Kireev, S.V.; Kondrashov, A.A.; Shnyrev, S.L. Application of the Wiener ﬁltering algorithm for processing the signal obtained by the TDLAS method using the synchronous detection technique for the measurement problem of CO concentration in exhaled air. Laser Phys. Lett. 2019, 16, 085701. [CrossRef] 16. Kireev, S.V.; Kondrashov, A.A.; Shnyrev, S.L. Improving the accuracy and sensitivity of C online detection in expiratory air using the TDLAS method in the spectral range of 4860–4880 cm . Laser Phys. Lett. 2018, 15, 105701. [CrossRef] 17. Ghorbani, R.; Schmidt, F.M. ICL-based TDLAS sensor for real-time breath gas analysis of carbon monoxide isotopes. Opt. Express 2017, 11, 12743–12752. [CrossRef] 18. Li, J.; Peng, Z.; Ding, Y. Wavelength modulation-direct absorption spectroscopy combined with improved experimental strategy for measuring spectroscopic parameters of H O transitions near 1.39 m. Opt. Lasers Eng. 2020, 126, 105875. [CrossRef] 19. The Hitran Molecular Spectroscopic Database. Available online: https://www.spectralcalc.com (accessed on 10 July 2021). 20. Swinehart, S.F. The Beer-Lambert law. J. Chem. Educ. 1962, 39, 333. [CrossRef] 21. Žitnik, M.; Krušic, ˇ Š.; Bucar ˇ , K.; Mihelic, ˇ A. Beer-Lambert law in the time domain. Phys. Rev. A 2018, 97, 063424. [CrossRef] 22. Xia, H.; Kan, R.; Xu, Z.; He, Y.; Liu, J.; Chen, B.; Yang, C.; Yao, L.; Wei, M.; Zhang, G. Two-step tomographic reconstructions of temperature and species concentration in a ﬂame based on laser absorption measurements with a rotation platform. Opt. Lasers Eng. 2017, 90, 10–18. [CrossRef] 23. De Labachelerie, M.; Nakagawa, K.; Awaji, Y.; Ohtsu, M. High frequency-stability laser at 1.5 m using Doppler-free molecular lines. Opt. Lett. 1995, 20, 572–574. [CrossRef] [PubMed] 24. Sun, K.; Xie, L.; Ju, Y.; Wu, X.; Hou, J.; Han, W.; Wang, X.; Man, J.; Liu, Y.; Yuan, H.; et al. Compact ﬁber-optic diode-laser sensor system for wide-dynamic-rang relative humidity measurement. Chin. Sci. Bull. 2011, 56, 3486–3492. 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References (11)

F. Sgobba, A. Sampaolo, P. Patimisco, M. Giglio, G. Menduni, A. Ranieri, C. Hoelzl, H. Rossmadl, Christian Brehm, V. Mackowiak, Dario Assante, E. Ranieri, V. Spagnolo (2021)
Compact and portable quartz-enhanced photoacoustic spectroscopy sensor for carbon monoxide environmental monitoring in urban areas
Photoacoustics, 25
N. Khelifa (2019)
Effect of Measurement of Dew Point Temperature in Moist Air on the Absorption Line at 1392.53 nm of Water Vapor
International Journal of Optics
Wenke Liang, Guangfen Wei, Aixiang He, Hui Shen (2021)
A novel wavelength modulation spectroscopy in TDLAS
Infrared Physics & Technology, 114
Jidong Li, Zhimin Peng, Yanjun Ding (2020)
Wavelength modulation-direct absorption spectroscopy combined with improved experimental strategy for measuring spectroscopic parameters of H2O transitions near 1.39 μm
Optics and Lasers in Engineering, 126
L. Kassa-Baghdouche, E. Cassan (2018)
Mid-infrared refractive index sensing using optimized slotted photonic crystal waveguides
Photonics and Nanostructures: Fundamentals and Applications, 28
D. Swinehart (1962)
The Beer-Lambert Law
Journal of Chemical Education, 39
Y. Nagano, Y. Miyazaki, T. Matsuo, H. Suga (1993)
Heat capacities and enthalpy of fusion of heavy oxygen water
The Journal of Physical Chemistry, 97
F. Whipple (1933)
The wet-and-dry-bulb hygrometer: the relation to theory of the experimental researches of Awbery and Griffiths
, 45
Ke Sun, Liang Xie, Yung-chul Ju, X. Wu, J. Hou, W. Han, Xin Wang, J. Man, Yu Liu, Haiqing Yuan, Z. Luo, S. Xiong, N. Zhu (2011)
Compact fiber-optic diode-laser sensor system for wide-dynamicrange relative humidity measurement
Chinese Science Bulletin, 56
J. Hodgkinson, R. Tatam (2012)
Optical gas sensing: a review
Measurement Science and Technology, 24
Huihui Xia, Ruifeng Kan, Zhenyu Xu, Yabai He, Jianguo Liu, Bing Chen, Chenguang Yang, Lu Yao, M. Wei, Guang-Le Zhang (2017)
Two-step tomographic reconstructions of temperature and species concentration in a flame based on laser absorption measurements with a rotation platform
Optics and Lasers in Engineering, 90

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DOI: 10.3390/photonics9030175
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Abstract

hv photonics Article Design and Analysis of a Single Humidity Sensor Based on TDLAS for Water Vapor and Heavy Oxygen Water Vapor Detection 1 1 , 2 1 , 2 1 1 , 2 , Ping Gong , Jian Zhou , Zhixuan Er , Yu Ju and Liang Xie * State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China; pgong@semi.ac.cn (P.G.); zhoujian@semi.ac.cn (J.Z.); erzhixuan@semi.ac.cn (Z.E.); juyu@semi.ac.cn (Y.J.) College of Materials Science and Opto-Electronic Technology, Chinese Academy of Sciences, Beijing 100049, China * Correspondence: xiel@semi.ac.cn Abstract: In this paper, a single humidity sensor for water vapor and heavy oxygen water vapor detection is presented. The sensor is based on tunable diode laser absorption spectroscopy (TDLAS) and thus has high sensitivity, good selectivity, and a short response time. A 1372 nm distributed feedback (DFB) diode laser is utilized as the light source, the wavelength tuning range of which covers the absorption lines of water vapor and heavy oxygen water vapor. A Herriott gas cell with 12 m optical length is designed for signal-to-noise ratio (SNR) enhancement. The sensor can distinguish between water vapor and heavy oxygen water vapor effectively. The accuracy of water detection is within 0.5% RH. The accuracy of heavy oxygen water vapor detection is within 1.0% RH. Keywords: humidity; heavy oxygen water; TDLAS Citation: Gong, P.; Zhou, J.; Er, Z.; Ju, 1. Introduction Y.; Xie, L. Design and Analysis of a Heavy oxygen water is an isotope of water, which is widely used in diverse ﬁelds Single Humidity Sensor Based on such as disease diagnoses, organ function research, and new drug developments [1–3]. TDLAS for Water Vapor and Heavy Conventional humidity sensors such as wet and dry bulb hygrometers, humidity-sensitive Oxygen Water Vapor Detection. capacitors, and chilled mirror dew point meters are used to detect heavy oxygen water Photonics 2022, 9, 175. https:// vapor [4,5]. However, these methods are unable to distinguish between water vapor and doi.org/10.3390/photonics9030175 heavy oxygen water vapor due to their similar physical and chemical characteristics. They Received: 25 January 2022 can only provide the total concentration of water vapor and heavy oxygen water vapor Accepted: 9 March 2022 rather than their respective concentrations. Heavy oxygen water vapor is always coexistent Published: 11 March 2022 with water vapor in practice. Hence, a single humidity sensor with good selectivity is an attractive alternative to conventional humidity sensors. Publisher’s Note: MDPI stays neutral Optical gas sensing is drawing increasing attention with regards to application in with regard to jurisdictional claims in gas detection. Gas sensors based on non-dispersive infrared (NDIR), photoacoustic spec- published maps and institutional afﬁl- troscopy (PAS), cavity ring-down spectroscopy (CRDS), and tunable diode laser absorption iations. spectroscopy (TDLAS) are well-established [6]. Furthermore, some novel high-sensitivity and miniaturized spectroscopic gas sensors have been developed. Kassa-Baghdouche et al. proposed slotted photonic crystal waveguides as refractive index sensing devices, which Copyright: © 2022 by the authors. obtained a sensitivity of more than 1150 per refractive index unit (RIU) with an insertion Licensee MDPI, Basel, Switzerland. loss level of 0.3 dB [7]. In Reference [8], an optimized photonic crystal microcavity for a This article is an open access article 4 spectroscopic gas sensor was realized. The device has a Q of 10 and a detection limit as low distributed under the terms and 4 as 10 RIU. NDIR sensors are composed of a broadband light source, light path, optical conditions of the Creative Commons ﬁlters, and detectors. They have high sensitivity, low cost, and ease of implementation [9]. Attribution (CC BY) license (https:// The selectivity of NDIR sensors is not adequate to distinguish between water vapor and creativecommons.org/licenses/by/ heavy oxygen water vapor because of the broadband light source and the limited ﬁlter 4.0/). Photonics 2022, 9, 175. https://doi.org/10.3390/photonics9030175 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 175 2 of 9 width. The wavelength spacing of the water vapor absorption line and the heavy oxygen water vapor absorption line is within 1 nm, which is an order smaller than the ﬁlter width. In PAS, periodic optical absorption will excite the periodic acoustic pressure propagating in the sample due to the photoacoustic effect. The propagating pressure results in a sound wave, which can be detected with an acoustic transducer. PAS is an indirect absorption spectroscopy technology with immunity to the background, thus leading to high sensitivity. PAS sensors suffer from the vibrations of the surrounding environment [10]. The cavity of a CRDS sensor has two reﬂectors with ultra-high reﬂectivity (~99.999%) to achieve an optical length of tens or even hundreds of kilometers. The minimum detection limit of CRDS sensors can reach the ppb or even ppt levels. Given their high cost and suscepti- bility to shaking, CRDS sensors are not under consideration in this paper [11]. A TDLAS sensor is equipped with a diode laser in which the linewidth is two orders of magnitude lower than the absorption lines. The narrowband light source, as well as the ﬁngerprint characteristics of infrared absorption spectroscopy contribute to the outstanding selectivity of TDLAS [12–14]. Consequently, TDLAS has a natural advantage in the detection of isotopes. In References [15,16], Kireev et al. achieved high CO measuring accuracy 13 12 and the simultaneous boundary CO / CO ratio was consistent in the expiratory air. 2 2 Ghorbani R proposed a TDLAS sensor with an ICL light source. The sensor was applied to 12 13 real-time detection of CO and CO in the air and exhaled breath [17]. In this paper, we propose a humidity sensor based on TDLAS. The motivation for this work is to improve the sensing dynamic range so that it can meet the requirements of both water vapor detection and heavy oxygen water vapor detection. The absorption amplitude of heavy oxygen water vapor is two orders lower than that of water vapor in the near-infrared region. As a result, a large dynamic of 40 dB is necessary. We adopt direct absorption spectroscopy (DAS) for water vapor detection and wavelength modulation spectroscopy (WMS) for heavy oxygen water vapor detection, which extends the dynamic range to 40 dB [15,18]. In addition, a Herriott gas cell is designed for the signal-to-noise ratio (SNR) enhancement. 2. Experimental Setup and Theoretical Analysis The experimental setup for the proposed humidity sensor is depicted in Figure 1a. A distributed feedback (DFB) diode laser (Sichuan light source optoelectronic, model: LT-LD-1372-20-1.5-FA) is employed as the light source, in which the wavelength covers the absorption lines of both water vapor and heavy oxygen water vapor. The diode laser is controlled by a temperature controller and a current controller. The temperature controller is designed on the basis of a MAX1978 for the Peltier thermoelectric cooler (TEC) module in the diode laser. The current controller consists of a DC current source, a triangle signal generator, and a sinusoidal signal generator. The diode laser has a tail ﬁber, of which the output light is coupled to a collimator (OPEAK, model: FC37). The collimated light acts as an incident light for the Herriott gas cell. The transmitted light through the gas cell is detected by a photodetector (Lightsensing, model: LSIPD-1S), which transforms the light signal into a current signal. A trans-impedance ampliﬁer (TIA) with low noise changes the current signal to a voltage signal. The TIA contains multiple channels for feedback resistor selection according to the ampliﬁer gain setting. The voltage signal is ampliﬁed and then acquired by a 12-bit analog-to-digital conversion (ADC) (Analog Devices, model: AD7366). A lock-in ampliﬁer is used to improve the SNR of the system. The function of the microcontroller unit (MCU) is to process signals, to set parameters, and to communicate with a computer via RS232. The temperature controller, the current controller, the TIA, the ADC, the lock-in ampliﬁer, and the MCU are integrated into the homemade circuit, as shown in Figure 1b. Photonics 2022, 9, x FOR PEER REVIEW 3 of 10 Photonics 2022, 9, x FOR PEER REVIEW 3 of 10 Photonics 2022, 9, 175 3 of 9 (a) (b) Figure 1. Experimental setup for the proposed humidity sensor. (a) Schematic diagram of the exper- imental setup; (b) picture of the homemade circuit. (a) (b) 2.1. Light Source Pro Figure 1. perty Exper imental setup for the proposed humidity sensor. (a) Schematic diagram of the exper- Figure 1. Experimental setup for the proposed humidity sensor. (a) Schematic diagram of the imental setup; (b) picture of the homemade circuit. experimental setup; (b) picture of the homemade circuit. The central wavelength of the DFB diode laser depends on its temperature and driv- ing current. A proportional-integral-derivative (PID) is carried out on the temperature 2.1. Light Source Property 2.1. Light Source Property controller. The current controller generates a triangle signal and a sinusoidal signal as the The central wavelength of the DFB diode laser depends on its temperature and driv- The central wavelength of the DFB diode laser depends on its temperature and driv- ing current. A proportional-integral-derivative (PID) is carried out on the temperature modulation signal, for which the frequency and amplitude vary with parameters set by ing current. A proportional-integral-derivative (PID) is carried out on the temperature controller. The current controller generates a triangle signal and a sinusoidal signal as the controller. The current controller generates a triangle signal and a sinusoidal signal as the the MCU. As is shown in Figure 2, for wavelength tuning, the diode laser has a tempera- modulation signal, for which the frequency and amplitude vary with parameters set by modulation signal, for which the frequency and amplitude vary with parameters set by ture tuning coefficient of 0.080 nm/°C and a current tuning coefficient of 0.015 nm/mA. the MCU. As is shown in Figure 2, for wavelength tuning, the diode laser has a tempera- the MCU. As is shown in Figure 2, for wavelength tuning, the diode laser has a tempera- Therefore, we process with temperature tuning for coarse tuning and current tuning for ture tur tu eni tuning ng coef coef fici ﬁcient ent of 0.08 of 0.080 0 nm/°C nm/ and a C andca urren currtent tun tuning ing coef coef ficie ﬁcient nt of 0. of01 0.015 5 nm nm/mA. /mA. fine tuning. Therefore, we process with temperature tuning for coarse tuning and current tuning for Therefore, we process with temperature tuning for coarse tuning and current tuning for fine tuning. ﬁne tuning. (a) ( b) Figure 2. Wavelength tuning property of the diode laser: (a) wavelength varies with temperature; (a) (b) (b) wavelength varies with driving current. Figure 2. Wavelength tuning property of the diode laser: (a) wavelength varies with temperature; Figure 2. Wavelength tuning property of the diode laser: (a) wavelength varies with temperature; The light wavelength should be fixed at the strong absorption line of the target gas (b) wavelength varies ( b with driving ) wavelength varies current. with driving current. without interference. Absorption lines at 1372.10 nm for water vapor detection and The light wavelength should be ﬁxed at the strong absorption line of the target 1372.53 nm for heavy oxygen water vapor detection are chosen according to the HITRAN The light wavelength should be fixed at the strong absorption line of the target gas gas without interference. Absorption lines at 1372.10 nm for water vapor detection and database [19]. The property of the selected absorption lines is described in Figure 3. Tem- without interference. Absorption lines at 1372.10 nm for water vapor detection and 1372.53 nm for heavy oxygen water vapor detection are chosen according to the HITRAN perature is set according to the specified wavelength of the target absorption line. Water 1372.53 nm for heavy oxy database ge[n wat 19]. The er vap property or detec of the tion selected are chosen acc absorption ording to the HITRA lines is described in Figur N e 3. vapor detection is set by tuning the laser temperature to T . Heavy oxygen water vapor Temperature is set according to the speciﬁed wavelength of the target absorption line. database [19]. The property of the selected absorption lines is described in Figure 3. Tem- detection is set by tuning the laser temperature to T The central wavelength of the diode Water vapor detection is set by tuning the laser temperatur 2 e to T . Heavy oxygen water perature is set according to the specified wavelength of the target absorption line. Water laser sc vapor ans detection across the is set tar by get tuning absorption line so the laser temperatur that it is e with to T in The the measured central wavelength range. The of the vapor detection is set by tuning the laser temperature to T . Heavy oxygen water vapor madiode ximum laser light scans power acris 20 oss the mW. target absorption line so that it is within the measured range. The maximum light power is 20 mW. detection is set by tuning the laser temperature to T The central wavelength of the diode laser scans across the target absorption line so that it is within the measured range. The maximum light power is 20 mW. Photonics 2022, 9, x FOR PEER REVIEW 4 of 10 Photonics 2022, 9, x FOR PEER REVIEW 4 of 10 Photonics 2022, 9, 175 4 of 9 Figure 3. Target absorption lines of water vapor and heavy oxygen water vapor. 2.2. Optical Length Enhancement Light with a specified wavelength can be absorbed when passing through the gas to be measured, which is a uniform medium. Light with 1372.10 nm can be absorbed by wa- ter vapor. Light with 1372.53 nm can be absorbed by heavy oxygen water vapor. The Beer– Lambert law expresses the relationship between the transmitted intensity I and the in- Figure 3. Target absorption lines of water vapor and heavy oxygen water vapor. Figure 3. Target absorption lines of water vapor and heavy oxygen water vapor. cident intensity I , as follows [20,21]: 2.2. Optical Length Enhancement − α CL 2.2. Optical Length Enhancement Light with a speciﬁed wavelength , can be absorbed when passing through the gas II = e t 0 (1) to be measured, which is a uniform medium. Light with 1372.10 nm can be absorbed by Light with a specified wavelength can be absorbed when passing through the gas to water vapor. Light with 1372.53 nm can be absorbed by heavy oxygen water vapor. The be measured, which is a uniform medium. Light with 1372.10 nm can be absorbed by wa- where α , , and L are the absorption coefficient, the gas concentration, and the op- Beer–Lambert law expresses the relationship between the transmitted intensity I and the ter vapor. Light with 1372.53 nm can be absorbed by heavy oxygen water vapor. Thte Beer– tical length, respectively. In the sensing system, α CL 1 , therefore I is given by the incident intensity I , as follows [20,21]: t Lambert law expresses the relationship between the transmitted intensity I and the in- following: aCL cident intensity I , as follows [20,21]: I = I e , (1) t 0 − α CL II≈− 1 αCL () II = e , t 0 (2) t 0 (1) where a, C, and L are the absorption coefﬁcient, the gas concentration, and the optical length, respectively. In the sensing system, aCL 1, therefore I , is given by the following: where α , , and L are the absorption coefficient, the gas concentration, and the op- The absorbed intensity is represented by the following: tical length, respectively. In the sensing system, α CL 1 , therefore I is given by the I I (1 aCL). (2) t t II≈−I ≈I αCL . (3) 00 t following: The absorbed intensity I is represented by the following: II≈− 1 αCL The absorbed intensity is proportional to the concen() tration of target gas when the t 0 (2) I I I I aCL. (3) 0 0 absorption line is fixed. For this reason, the minimum detection limit of the sensor in- The absorbed intensity is represented by the following: creases as the optical length enhances. Two concave mirrors (Thorlabs, model: CM508- The absorbed intensity is proportional to the concentration of target gas when the ab- II≈−I ≈I αCL . 00 t (3) 200EH4-M02) are utsorption ilized fline or th is e H ﬁxed. errio For this tt ce reason, ll. The thest minimum ructuraldetection parame limit tersof of the the sensor gas c incr eleases l as the optical length enhances. Two concave mirrors (Thorlabs, model: CM508-200EH4-M02) are determined by optimizing the optical path model, which is sketched in Figure 4a. A The absorbed intensity is proportional to the concentration of target gas when the are utilized for the Herriott cell. The structural parameters of the gas cell are determined by absorption line is fixed. For this reason, the minimum detection limit of the sensor in- Gaussian light source serves as the incident light in the model, in which the diameter is optimizing the optical path model, which is sketched in Figure 4a. A Gaussian light source creases as the optical length enhances. Two concave mirrors (Thorlabs, model: CM508- set to 3 mm. The optical length reaches the maximum when the distance between two serves as the incident light in the model, in which the diameter is set to 3 mm. The optical 200EH4-M02) are utilized for the Herriott cell. The structural parameters of the gas cell mirrors is set to 223 mm. The number of reflections in each mirror is 28. length reaches the maximum when the distance between two mirrors is set to 223 mm. The are determined by optimizing the optical path model, which is sketched in Figure 4a. A number of reﬂections in each mirror is 28. Gaussian light source serves as the incident light in the model, in which the diameter is set to 3 mm. The optical length reaches the maximum when the distance between two mirrors is set to 223 mm. The number of reflections in each mirror is 28. (a) (b) (a) (b) Figure 4. The designed Herriott gas cell. (a) The optical path model; (b) light spots in the mirror. Figure 4. The designed Herriott gas cell. (a) The optical path model; (b) light spots in the mirror. Figure 4. The designed Herriott gas cell. (a) The optical path model; (b) light spots in the mirror. The output light of the diode laser is collimated and then coupled to the designed Herriott cell. The light is reﬂected by the two mirrors tens of times, for optical length Photonics 2022, 9, x FOR PEER REVIEW 5 of 10 The output light of the diode laser is collimated and then coupled to the designed Herriott cell. The light is reflected by the two mirrors tens of times, for optical length en- Photonics 2022, 9, 175 5 of 9 hancement. Red laser light is inputted into the gas cell as an indicating light. The length of the gas cell is 220 mm and the optical length, obtained by multiple reflections, is 12 m. An image of light spots in one mirror is shown in Figure 4b. enhancement. Red laser light is inputted into the gas cell as an indicating light. The length of the gas cell is 220 mm and the optical length, obtained by multiple reﬂections, is 12 m. 2.3. Direct Detection and Harmonic Detection An image of light spots in one mirror is shown in Figure 4b. A 1372 nm DFB diode laser is commercially available, which offers the light source 2.3. Direct Detection and Harmonic Detection for the proposed humidity sensor. A DFB diode laser has a narrow linewidth and high A op 1372 tical nm po DFB wer diode density. laser Nev is erth commer eless, cially the outp available, ut inten which sity of of fers the the diolight de laser, sour wh ce for ich is the incident intensity of the Herriott cell, drifts due to variations in environmental tempera- the proposed humidity sensor. A DFB diode laser has a narrow linewidth and high optical ture, driving current, and mechanical fluctuations [22]. Figure 5a shows the light power power density. Nevertheless, the output intensity of the diode laser, which is the incident intensity of of the the diode Herriott laser cell, vary drifts ing with due to the variations driving current, in environmental where the temp linear eratur fitting e, driving is carried out current, and mechanical ﬂuctuations [22]. Figure 5a shows the light power of the diode laser as illustrated by the red solid line and the coefficient of determination is R  0.9992 . In varying with the driving current, where the linear ﬁtting is carried out as illustrated by the DAS-TDLAS mode, the incident intensity I is acquired by linear fitting of the transmit- red solid line and the coefﬁcient of determination is R = 0.9992. In DAS-TDLAS mode, the ted intensity when the light is absorbed by the target gas. Therefore, the influence of the incident intensity I is acquired by linear ﬁtting of the transmitted intensity when the light incident intensity fluctuation is effectively eliminated due to the good linearity of the laser is absorbed by the target gas. Therefore, the inﬂuence of the incident intensity ﬂuctuation power. In Figure 5b, the blue solid line shows the transmitted intensity. The red dotted is effectively eliminated due to the good linearity of the laser power. In Figure 5b, the blue line depicts the linear fitting of the blue line. The absorption intensity is obtained when solid line shows the transmitted intensity. The red dotted line depicts the linear ﬁtting the fitting line subtracts the absorption intensity, as is illustrated in Figure 5c. of the blue line. The absorption intensity is obtained when the ﬁtting line subtracts the absorption intensity, as is illustrated in Figure 5c. (a) (b) (c) Figure 5. DAS-TDLAS detection: (a) the light power of the diode laser varies with driving current; Figure 5. DAS-TDLAS detection: (a) the light power of the diode laser varies with driving current; (b) the transmitted optical intensity varies with laser wavelength and its linear fitting; (c) the ab- (b) the transmitted optical intensity varies with laser wavelength and its linear ﬁtting; (c) the absorbed sorbed optical intensity varies with laser wavelength. optical intensity varies with laser wavelength. As for the equivalent concentration, heavy oxygen water vapor absorption is two As for the equivalent concentration, heavy oxygen water vapor absorption is two orders weaker than water vapor absorption. DAS is incapable of detecting absorptions as orders weaker than water vapor absorption. DAS is incapable of detecting absorptions weak as the system noise. A sinusoidal modulation is added for harmonic detection to as weak as the system noise. A sinusoidal modulation is added for harmonic detection extract the weak signal induced by the absorption of heavy oxygen water vapor. A home- to extract the weak signal induced by the absorption of heavy oxygen water vapor. A made lock-in amplifier is used for harmonic detection, which enhances the SNR effec- homemade lock-in ampliﬁer is used for harmonic detection, which enhances the SNR tively. In WMS-TDLAS, the laser wavelength  modulated by a sinusoidal signal is ex- effectively. In WMS-TDLAS, the laser wavelength l modulated by a sinusoidal signal is pressed as follows: expressed as follows: l = l + a sin 2p f t, (4) (4)   a sin 2 ft , where l is the center wavelength of the diode laser, a is the wavelength modulation depth, and f is the modulation frequency. where  is the center wavelength of the diode laser, a is the wavelength modulation The absorption line can be approximated as a Voigt proﬁle at room temperature and depth, and f is the modulation frequency. atmospheric pressure. The proﬁle is represented by the following: The absorption line can be approximated as a Voigt profile at room temperature and +¥ y exp t atmospheric pressure. The profile is represented by the following: g(x, y) = A dt, (5) (x t) + y p p ll g ln 2/p 0 C where A = , x = ln 2 , y = ln 2 , g , and g are the half width at half D C g g g D D D maximum (HWHM) of the Doppler and Lorentz proﬁles, respectively [23]. Photonics 2022, 9, 175 6 of 9 According to Fourier analysis, the second harmonic signal of the transmitted light can be given by the following: 2 2 + m 2 1 + m I = I G p CL, (6) 2 f 0 2 f 2 2 m 1 + m where m and G are constants. 2 f The ﬁrst harmonic signal of the transmitted light can be given by the following: I = I G a, (7) f 0 f where G is a constant. It is indicated that the ratio of the second harmonic signal and the ﬁrst harmonic signal is proportional to the gas concentration, which is free from the inﬂuence of the light output intensity [24]. The ratio is given by the following: 2 2 + m 2 1 + m I G 2 f 2 f = p CL. (8) 2 2 I a G f f m 1 + m 3. Experiments and Results 3.1. Water Vapor Detection The humidity sensor operates in DAS-TDLAS mode for water vapor detection. The central wavelength of the diode laser scans across the water vapor absorption line by scanning the driving current with a triangle signal. The voltage signal varying with the driving current is logged, shown in Figure 6. A commercial humidity sensor (Rotronic, HP32) is utilized to acquire relative humidity of detected gas, in which the precision is 0.5% RH. The measured relative humidity of the water vapor samples is 1.95% RH, 2.60% RH, 3.25% RH, 3.90% RH, 4.55% RH, 5.20% RH, 5.85% RH, 6.50% RH, 18.26% RH, 58% RH, 76% RH, and 88% RH, respectively. Curves with different colors present the results of the different relative humidity mentioned above. It is noted that the signal proﬁles are Photonics 2022, 9, x FOR PEER REVIEW 7 of 10 consistent with each other and that the absorption is more obvious when a higher relative humidity sample is detected. Figure 6. Transmitted amplitude in the DAS-TDLAS mode. Figure 6. Transmitted amplitude in the DAS-TDLAS mode. Figure 7 presents the maximum absorption signal as a function of the relative humidity, Figure 7 presents the maximum absorption signal as a function of the relative humid- which is proportional to the water vapor concentration. Linear ﬁtting of the curves is ity, which is proportional to the water vapor concentration. Linear fitting of the curves is performed. The residual sum of squares is 19.59, and the coefﬁcient of determination is performed. The residual sum of squares is 19.59, and the coefficient of determination is R = 0.9913. The sensitivity for water vapor detection is 9.64 mV/%RH. It is concluded R = 0.9913. The sensitivity for water vapor detection is 9.64 mV/%RH. It is concluded that the proposed sensor has good linearity and that the measuring accuracy is within ±0.5%RH for water vapor measurement. Figure 7. Relative humidity measured in DAS-TDLAS. 3.2. Heavy Oxygen Water Vapor Detection The performance of the proposed humidity sensor for heavy oxygen water vapor detection is evaluated in the WMS-TDLAS mode. The driving current scans so that the central wavelength scans across the absorption line. A triangle signal and a sinusoidal signal modulate the driving current simultaneously. The frequency of the triangle signal is 100 Hz, and the frequency of the sinusoidal signal is 10 kHz. The modulation depth is optimized. The second harmonic signals measured at different relative humidity are shown in Figure 8. The same humidity sensor (Rotronic, HP32) used for water vapor de- tection is used to acquire relative humidity. The measured relative humidity of the heavy oxygen water vapor samples is 3.4% RH, 6.8% RH, 10.2% RH, 13.6% RH, and 17% RH, respectively. The profiles are consistent with each other and reach their maximums at the center of the absorption lines. Photonics 2022, 9, x FOR PEER REVIEW 7 of 10 Figure 6. Transmitted amplitude in the DAS-TDLAS mode. Figure 7 presents the maximum absorption signal as a function of the relative humid- ity, which is proportional to the water vapor concentration. Linear fitting of the curves is performed. The residual sum of squares is 19.59, and the coefficient of determination is Photonics 2022, 9, 175 7 of 9 R  0.9913 . The sensitivity for water vapor detection is 9.64 mV/%RH. It is concluded that the proposed sensor has good linearity and that the measuring accuracy is within ±0.5%RH for water vapor measurement. that the proposed sensor has good linearity and that the measuring accuracy is within 0.5% RH for water vapor measurement. Figure 7. Relative humidity measured in DAS-TDLAS. Figure 7. Relative humidity measured in DAS-TDLAS. 3.2. Heavy Oxygen Water Vapor Detection 3.2. Heavy Oxygen Water Vapor Detection The performance of the proposed humidity sensor for heavy oxygen water vapor detection is evaluated in the WMS-TDLAS mode. The driving current scans so that the The performance of the proposed humidity sensor for heavy oxygen water vapor central wavelength scans across the absorption line. A triangle signal and a sinusoidal detection is evaluated in the WMS-TDLAS mode. The driving current scans so that the signal modulate the driving current simultaneously. The frequency of the triangle signal central wavelength scans across the absorption line. A triangle signal and a sinusoidal is 100 Hz, and the frequency of the sinusoidal signal is 10 kHz. The modulation depth is signal modulate the driving current simultaneously. The frequency of the triangle signal optimized. The second harmonic signals measured at different relative humidity are shown is 100 Hz, and the frequency of the sinusoidal signal is 10 kHz. The modulation depth is in Figure 8. The same humidity sensor (Rotronic, HP32) used for water vapor detection is optimized. The second harmonic signals measured at different relative humidity are used to acquire relative humidity. The measured relative humidity of the heavy oxygen shown in Figure 8. The same humidity sensor (Rotronic, HP32) used for water vapor de- water vapor samples is 3.4% RH, 6.8% RH, 10.2% RH, 13.6% RH, and 17% RH, respectively. tection is used to acquire relative humidity. The measured relative humidity of the heavy Photonics 2022, 9, x FOR PEER REVIEW 8 of 10 The proﬁles are consistent with each other and reach their maximums at the center of the oxygen water vapor samples is 3.4% RH, 6.8% RH, 10.2% RH, 13.6% RH, and 17% RH, absorption lines. respectively. The profiles are consistent with each other and reach their maximums at the center of the absorption lines. Figure Figure 8. 8. Second Second harmonic detectio harmonic detection n in in the W the WMS-TDLAS MS-TDLASmode. mode. The concentration of heavy oxygen water vapor is proportional to the second harmonic The concentration of heavy oxygen water vapor is proportional to the second har- of the transmitted intensity in the WMS-TDLAS mode. The relative humidity is a function monic of the transmitted intensity in the WMS-TDLAS mode. The relative humidity is a of the maximum of the second harmonic. Figure 9 represents the linear ﬁtting. The residual function of the maximum of the second harmonic. Figure 9 represents the linear fitting. sum of squares is 11.03, and the coefﬁcient of determination is R = 0.9850. The sensitivity The residual sum of squares is 11.03, and the coefficient of determination is R = 0.9850 for heavy oxygen water vapor detection is 30.91 mV/%RH. The proposed sensor has . The sensitivity for heavy oxygen water vapor detection is 30.91 mV/%RH. The proposed good linearity, and the measuring accuracy is within 1.0% RH for heavy oxygen water sensor has good linearity, and the measuring accuracy is within ±1.0%RH for heavy oxy- vapor detection. gen water vapor detection. Figure 9. Relative humidity measured in WMS-TDLAS. 4. Conclusions A single humidity sensor for water vapor detection and heavy oxygen water vapor detection is described in this paper. The sensor is based on TDLAS, which is an attractive alternative for the distinguishment of isotopes. A single 1372 nm DFB diode laser is uti- lized as the light source, for which the wavelength tuning range covers the absorption lines of both water vapor and heavy oxygen water vapor. A Herriott gas cell with 12 m optical length is designed for SNR enhancement, which increases the optical length by ~60 times. Water vapor is detected in the DAS-TDLAS mode, and the accuracy of water vapor detection is within ±0.5%RH. Heavy oxygen water vapor is detected in the WMS-TDLAS mode, and the accuracy of heavy oxygen water vapor detection is within ±1.0%RH. Author Contributions: Conceptualization, L.X.; methodology, P.G.; validation, P.G., J.Z., Z.E. and Y.J.; formal analysis, J.Z.; investigation, P.G.; data curation, J.Z.; writing—original draft preparation, P.G.; writing—review and editing, P.G.; project administration, L.X.; All authors have read and agreed to the published version of the manuscript. Photonics 2022, 9, x FOR PEER REVIEW 8 of 10 Figure 8. Second harmonic detection in the WMS-TDLAS mode. The concentration of heavy oxygen water vapor is proportional to the second har- monic of the transmitted intensity in the WMS-TDLAS mode. The relative humidity is a function of the maximum of the second harmonic. Figure 9 represents the linear fitting. The residual sum of squares is 11.03, and the coefficient of determination is R  0.9850 . The sensitivity for heavy oxygen water vapor detection is 30.91 mV/%RH. The proposed sensor has good linearity, and the measuring accuracy is within ±1.0%RH for heavy oxy- Photonics 2022, 9, 175 8 of 9 gen water vapor detection. Figure 9. Relative humidity measured in WMS-TDLAS. Figure 9. Relative humidity measured in WMS-TDLAS. 4. Conclusions 4. Conclusions A single humidity sensor for water vapor detection and heavy oxygen water vapor detection A single is hum described idity sensor in this for paper water . The vapor sensor detec is tion based an on d heavy TDLAS, oxwhich ygen water is an attractiv vapor e detec alternative tion is described for the distinguishment in this paper. The of isotopes. sensor is b Aas single ed on1372 TDLA nm S, DFB which diode is an laser attractive is utilized as the light source, for which the wavelength tuning range covers the absorption lines of alternative for the distinguishment of isotopes. A single 1372 nm DFB diode laser is uti- lized both as th water e light vapor sourc and e, fheavy or which oxygen the wavelen water vapor gth tuning . A Herriott range gas covcell ers th with e ab 12 sorpt m optical ion length is designed for SNR enhancement, which increases the optical length by ~60 times. lines of both water vapor and heavy oxygen water vapor. A Herriott gas cell with 12 m Water vapor is detected in the DAS-TDLAS mode, and the accuracy of water vapor detection optical length is designed for SNR enhancement, which increases the optical length by ~60 is within 0.5% RH. Heavy oxygen water vapor is detected in the WMS-TDLAS mode, times. Water vapor is detected in the DAS-TDLAS mode, and the accuracy of water vapor and the accuracy of heavy oxygen water vapor detection is within 1.0% RH. detection is within ±0.5%RH. Heavy oxygen water vapor is detected in the WMS-TDLAS mode, and the accuracy of heavy oxygen water vapor detection is within ±1.0%RH. Author Contributions: Conceptualization, L.X.; methodology, P.G.; validation, P.G., J.Z., Z.E. and Y.J.; formal analysis, J.Z.; investigation, P.G.; data curation, J.Z.; writing—original draft preparation, Author Contributions: Conceptualization, L.X.; methodology, P.G.; validation, P.G., J.Z., Z.E. and P.G.; writing—review and editing, P.G.; project administration, L.X. All authors have read and agreed Y.J.; formal analysis, J.Z.; investigation, P.G.; data curation, J.Z.; writing—original draft preparation, to the published version of the manuscript. P.G.; writing—review and editing, P.G.; project administration, L.X.; All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number U1930205. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest. References 1. Magomedbekov, E.P.; Selivanenko, I.L.; Kulov, N.N.; Veretennikova, G.V. Conditioning Heavy-Oxygen Water by Rectiﬁcation under Vacuum. Theor. Found. Chem. Eng. 2019, 53, 719–724. [CrossRef] 2. Kutus, B.; Shalit, A.; Hamm, P.; Hunger, J. Dielectric response of light, heavy and heavy-oxygen water: Isotope effects on the hydrogen-bonding network’s collective relaxation dynamics. Phys. Chem. Chem. Phys. 2021, 23, 5467–5473. [CrossRef] [PubMed] 3. Nagano, Y.; Miyazaki, Y.; Matsuo, T.; Suga, H. Heat capacities and enthalpy of fusion of heavy oxygen water. J. Phys. Chem. 1993, 97, 6897–6901. [CrossRef] 4. Whipple, F. The wet-and-dry-bulb hygrometer: The relation to theory of the experimental researches of Awbery and Grifﬁths. Proc. Phys. Soc. 1933, 45, 307–319. [CrossRef] 5. Khelifa, N. Effect of Measurement of Dew Point Temperature in Moist Air on the Absorption Line at 1392.53 nm of Water Vapor. Int. J. Opt. 2019, 2019, 3424172. [CrossRef] 6. Hodgkinson, J.; Tatam, R.P. Optical gas sensing: A review. Meas. Sci. Technol. 2013, 24, 012004. [CrossRef] 7. Kassa-Baghdouche, L.; Cassan, E. Mid-infrared refractive index sensing using optimized slotted photonic crystal waveguides. Photonics Nanostruct.-Fundam. Appl. 2018, 28, 32–36. [CrossRef] 8. Kassa-Baghdouche, L. High-sensitivity spectroscopic gas sensor using optimized H1 photonic crystal microcavities. J. Opt. Soc. Am. B 2020, 37, A227–A284. [CrossRef] 9. Jha, R.K. Non-Dispersive infrared gas sensing technology: A review. IEEE Sens. J. 2022, 22, 6–15. [CrossRef] 10. Sgobba, F.; Sampaolo, A.; Patimisco, P.; Giglio, M.; Menduni, G.; Ranieri, A.C.; Hoelzl, C.; Rossmadl, H.; Brehm, C.; Mackowiak, V.; et al. Compact and portable quartz-enhanced photoacoustic spectroscopy sensor for carbon monoxide environmental monitoring in urban areas. Photoacoustics 2022, 25, 100318. [CrossRef] Photonics 2022, 9, 175 9 of 9 11. Song, Z.; Xu, L.; Xie, H.; Cao, Z. Random vibration-driven continuous-wave CRDS system for calibration-free gas concentration measurement. Opt. Lett. 2020, 45, 746–749. [CrossRef] [PubMed] 12. Zhao, W.S.; Xu, L.J.; Huang, A. A WMS Based TDLAS Tomographic System for Distribution Retrievals of both Gas Concentration and Temperature in Dynamic Flames. IEEE Sens. J. 2020, 20, 4179–4188. [CrossRef] 13. Liang, W.; Wei, G.; He, A.; Shen, H. A novel wavelength modulation spectroscopy in TDLAS. Infrared Phys. Technol. 2021, 114, 103661. [CrossRef] 14. Xie, Y.; Chang, J.; Chen, X.; Sun, J.; Zhang, Q.; Wang, F.; Zhang, Z.; Feng, Y. A DFB-LD Internal Temperature Fluctuation Analysis in a TDLAS System for Gas Detection. IEEE Photonics J. 2019, 11, 1–8. [CrossRef] 15. Kireev, S.V.; Kondrashov, A.A.; Shnyrev, S.L. Application of the Wiener ﬁltering algorithm for processing the signal obtained by the TDLAS method using the synchronous detection technique for the measurement problem of CO concentration in exhaled air. Laser Phys. Lett. 2019, 16, 085701. [CrossRef] 16. Kireev, S.V.; Kondrashov, A.A.; Shnyrev, S.L. Improving the accuracy and sensitivity of C online detection in expiratory air using the TDLAS method in the spectral range of 4860–4880 cm . Laser Phys. Lett. 2018, 15, 105701. [CrossRef] 17. Ghorbani, R.; Schmidt, F.M. ICL-based TDLAS sensor for real-time breath gas analysis of carbon monoxide isotopes. Opt. Express 2017, 11, 12743–12752. [CrossRef] 18. Li, J.; Peng, Z.; Ding, Y. Wavelength modulation-direct absorption spectroscopy combined with improved experimental strategy for measuring spectroscopic parameters of H O transitions near 1.39 m. Opt. Lasers Eng. 2020, 126, 105875. [CrossRef] 19. The Hitran Molecular Spectroscopic Database. Available online: https://www.spectralcalc.com (accessed on 10 July 2021). 20. Swinehart, S.F. The Beer-Lambert law. J. Chem. Educ. 1962, 39, 333. [CrossRef] 21. Žitnik, M.; Krušic, ˇ Š.; Bucar ˇ , K.; Mihelic, ˇ A. Beer-Lambert law in the time domain. Phys. Rev. A 2018, 97, 063424. [CrossRef] 22. Xia, H.; Kan, R.; Xu, Z.; He, Y.; Liu, J.; Chen, B.; Yang, C.; Yao, L.; Wei, M.; Zhang, G. Two-step tomographic reconstructions of temperature and species concentration in a ﬂame based on laser absorption measurements with a rotation platform. Opt. Lasers Eng. 2017, 90, 10–18. [CrossRef] 23. De Labachelerie, M.; Nakagawa, K.; Awaji, Y.; Ohtsu, M. High frequency-stability laser at 1.5 m using Doppler-free molecular lines. Opt. Lett. 1995, 20, 572–574. [CrossRef] [PubMed] 24. Sun, K.; Xie, L.; Ju, Y.; Wu, X.; Hou, J.; Han, W.; Wang, X.; Man, J.; Liu, Y.; Yuan, H.; et al. Compact ﬁber-optic diode-laser sensor system for wide-dynamic-rang relative humidity measurement. Chin. Sci. Bull. 2011, 56, 3486–3492. [CrossRef]

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Photonics – Multidisciplinary Digital Publishing Institute

Published: Mar 11, 2022

Keywords: humidity; heavy oxygen water; TDLAS

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Design and Analysis of a Single Humidity Sensor Based on TDLAS for Water Vapor and Heavy Oxygen Water Vapor Detection

Design and Analysis of a Single Humidity Sensor Based on TDLAS for Water Vapor and Heavy Oxygen Water Vapor Detection

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Design and Analysis of a Single Humidity Sensor Based on TDLAS for Water Vapor and Heavy Oxygen Water Vapor Detection

Design and Analysis of a Single Humidity Sensor Based on TDLAS for Water Vapor and Heavy Oxygen Water Vapor Detection

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