Investigation and Optimization of a Line-Locked Quartz Enhanced Spectrophone for Rapid Carbon Dioxide Measurement

Abstract

We have developed a rapid quartz enhanced spectrophone for carbon dioxide (CO₂) measurement, in which the laser wavelength was tightly locked to a CO₂ absorption line and a custom quartz tuning fork (QTF) operating at 12.5 kHz was employed. The intrinsic QTF oscillation-limited response time, as well as the optimal feedback interval, was experimentally investigated. By tightly locking the laser to the R(16) transition of CO₂, we obtained a stable laser operation with its center wavelength variation kept within 0.0002 cm⁻¹, merely three times the laser linewidth. The reported CO₂ sensor achieved a detection limit of 7 ppm, corresponding to a normalized noise equivalent absorption coefficient (NNEA) of 4.7 × 10⁻⁹ W·cm⁻¹·Hz^−1/2, at a response time of 0.5 s. The detection limit can be further improved to 0.45 ppm at an integration time of 270 s, illustrating a good system stability. This spectrophone enables the realization of compact and fast-response gas sensors for many scenarios, where CO₂ concentration from sub-ppm to hundreds of thousands of ppm is expected.

1. Introduction

Carbon dioxide (CO₂) is an important component of the atmosphere that protects life on Earth via photosynthesis and heat preservation. However, its adverse impact, mainly the greenhouse effect, is emerging with massive emissions from increasingly extensive and intensive human activities in industrial, agricultural, and ecological fields [1]. In addition to our living environment, biological CO₂ functions as a valuable biomarker for human disease predictions such as Helicobacter pylori infection, lung lesions, and liver malfunctions [2]. Moreover, CO₂ abundance and its variation tendency are critical in many other applications, including combustion analysis, industrial process control, and deep-sea exploration [3,4,5]. Therefore, CO₂ sensors with fast response, high sensitivity, and wide dynamic range are desirable across a broad range of fields.

Currently, commercially available instruments for CO₂ detection include gas chromatography systems, electrochemical sensors, non-dispersing infrared analyzers, and infrared spectrometers. Suffering from bulky size, short service life and even poor selectivity, most of them are not well suited to scenarios where real-time and in situ CO₂ measurement is highly needed. In contrast, the development of photoacoustic spectroscopy (PAS) is filling this gap [6,7,8]. PAS relies on the detection of acoustic waves induced by the process of optical heat deposition and thermal expansion. In particular, quartz-enhanced photoacoustic spectroscopy (QEPAS), exploiting a tiny quartz tuning fork (QTF) as an acoustic transducer, is one of the most attractive techniques, with high selectivity, high sensitivity, good immunity to environmental acoustic noise, and ultra-low gas consumption [9,10]. Since its invention by Kosterev et al. in 2002 [11], QEPAS has been demonstrated for detection of numerous organic and inorganic trace gases, including precise CO₂ measurements by spectroscopy analysis at trace level, molecular relaxation investigations and interference studies on other gaseous species [12,13,14].

The commonly used standard QTFs were optimized for timing purposes, operating at ~32.7 kHz, instead of spectroscopic applications. On the other side, the energy transfer of CO₂ cannot follow the fast molecular vibration excitation due to its relatively long relaxation time constant [13]. Thus, an inadequately generated acoustic wave results in a weaker signal. To date, most QEPAS CO₂ sensors have a concentration detection limit of tens to hundreds of ppm, especially when the in situ excitation power is limited to merely mW level [15,16,17,18]. Following the radiation-to-sound conversion efficiency ε of photoacoustic process, i.e., ε = [1 + (2πf₀τ)²]^–1/2, where τ is the relaxation time [19], the CO₂ QEPAS signal can benefit from a QTF with lower resonant frequency f₀. Since the breakthrough of custom-made QTFs in 2013, QEPAS sensors have been capable of operating at a frequency much more suitable to low-relaxation-rate gas species [20,21].

Generally, QEPAS spectrophones are based on 2f wavelength modulation spectroscopy. A sub-Hertz wavelength scanning, along with a modulation at half of the QTF resonance frequency, is used to plot the 2f spectrum, whose maximum value always appears at the absorption line center, leading to a simple model to diagnose the analyte. However, the response time of scanning-assisted QEPAS can be extended up to tens of seconds due to its high-quality factor Q (10,000 to 13,000) [22]. That is still too slow to perform rapid gas measurements, especially when the information needs to be captured in real time. This issue could be mitigated if the absorption feature can be targeted without scanning. To prevent the laser frequency drift from the fluctuation of laser operation temperature and current, several rugged methods have been commonly applied to gas sensing, such as real-time wavelength stabilization using odd harmonics as the error signal [23,24] and quasi wavelength locking with a regular calibration by referring to the second harmonic maximum [25].

In this work, a LabVIEW-based servo loop was developed to perform the line-locked process, in which the laser wavelength was tightly locked to the target transition line of CO₂. A sensitive and rapid CO₂ sensor, combining the line-locked process and a custom QTF with a low resonant frequency of 12.5 kHz, was demonstrated. Averaging filtering was then fully exploited on data processing for lower system noise without sacrificing the response time. Response time limitation and the optimal feedback interval were investigated by experimental measurements. Ultimately, the sensor was validated to measure the ambient background and exhaled CO₂ concentration, in a field application.

2. Sensor Configuration and Line-Locked Process

A line-locked quartz enhanced spectrophone system was built with the configuration shown in Figure 1. The sensing system consists of a wavelength locking part and a gas sensing part. A 2.004 μm DFB laser diode (KELD1G5BAAA, NTT Electronics, Yokohama-shi, Japan) is driven by a LabVIEW-based electrical control unit and a low noise laser driver (LDC501, Stanford Research Systems, Sunnyvale, CA, USA). After careful laser characterization, the driving temperature and bias current are selected as 19 °C and 120 mA, respectively. Laser radiation thus can be obtained to exploit the R(16) line of CO₂ with less spectral overlap interference and high laser power (see Appendix A Figure A1). An acoustic detection module (ADM01, Thorlabs, Newton, NJ, USA) is used the perform CO₂ measurement, this ADM consists of a custom QTF as the acoustic transducer, two acoustic resonators for acoustic amplification, and two wedged BaF₂ windows for optical access. A small fraction of the DFB laser radiation, separated by a 1:9 fiber splitter, interrogates a pigtailed CO₂ chamber (length 6 cm; pressure 50 Torr) for wavelength locking. The rest of the laser radiation (calibrated as 5 mW) propagates through the on-beam acoustic resonators (inner diameter: 1.6 mm; length: 12.4 mm) and the custom QTF (prong space: 800 μm) successively. Two mode-matching lenses, L₁ and L₂, allow adequate focalization of the excitation beam down to ~200 μm between QTF prongs without hitting them. With the laser modulated at half of the QTF resonance frequency f₀, a photoacoustic wave is excited at the presence of the analyte. A trans-impedance preamplifier with a feedback resistance of 10 MΩ is used for signal amplification prior to demodulation by a lock-in amplifier (MFLI, Zurich Instruments, Zurich, Switzerland).

To perform tight wavelength locking to the absorption line, we developed a LabVIEW-based software platform with the flow chart shown in Figure 2. Usually, for wavelength modulation spectroscopy, the odd harmonics have centrosymmetric profiles about the absorption line center [23,24,25]. Hence, the third harmonic (3f signal) is used to generate an error signal for the following line-locked process. Firstly, a preset bias current, as well as the modulation signal at f₀/2, is applied via a digital-analog converter (i.e., AO0) of the DAQ card (USB-6356, National Instruments, Austin, TX, USA), generating a radiant wavelength near the absorption line. Secondly, the CO₂ absorption information inside the reference chamber is captured via the analog-digital converter (i.e., AI1) and then is demodulated as the 3f signal using a digital lock-in amplifier. Thirdly, a PID subprogram is employed to make the 3f signal closer to the set value (zero in this paper) by iterating the bias current. Thus, the CO₂ measurement is conducted with the photoacoustic signal captured (via AI0) and averaged during the feedback interval.

Note that, the feedback interval (ΔT) cannot be discretionarily selected as it would have a significant influence on the tuning fork response. As shown in Figure 2, if the ending of modulation period I is not connected with the beginning of the successive modulation period II, the phase mismatch will break the energy accumulation process of QTF, leading to a weakened photoacoustic signal. Our hypothesis is confirmed by a detailed feedback interval investigation, which can be found in Experimental Results and Discussion section below. Thus, in the implementation of line-locked QEPAS, phase matching between successive modulation periods should be considered in selecting the feedback interval, which should be an integer multiple (n) of the QTF oscillation period as:

$$∆T=n/f_0$$

(1)

3. Experimental Results and Discussion

3.1. Response Time of the QTF Spectrophone

QEPAS acoustic wave detection strongly relies on sufficient energy accumulation in a QTF, which, in turn, limits the response time of the spectrophone. The effective QEPAS response to CO₂ was experimentally determined by real-time monitoring of the QTF oscillation curve, in which acoustic stimulation was generated after sufficient QTF oscillation release (2.5 s). CO₂ (1%) was filled into the ADM and the radiation wavelength was turned to the absorption line prior to the response time investigation. Figure 3 depicts the measured raw data of QTF oscillation response to the periodic stimulation. Based on the time it takes for signal amplitude increases from zero to a plateau level, the response time is determined to be 0.5 s, while the time it takes to reach 90% of the final level is 0.365 s, that is consistent with the theoretical model of τ = Q/f₀ [11,26] using the calibrated quality factor Q and resonant frequency f₀ (see Appendix B Figure A2). Note that Q and f₀ depend on the operation temperature and gas pressure [27], all the following experiments are performed under room temperature and 1 bar.

3.2. Feedback Interval Determination

The photoacoustic signal response to feedback interval was investigated to optimize the CO₂ measurement with maximum amplitude. Sample gas with 1000-ppm CO₂ diluted in N₂ was sealed in the ADM and a modulation current (frequency: 6226 Hz; amplitude: 9.4 mA) was added on an iterated bias current. Figure 4 depicts the QEPAS signals with periodic variations around typical feedback intervals of 0.05 s, 0.1 s, 0.5 s, 1.3 s, and 3.3 s. As expected, the variation frequency, which is obtained by averaging the five oscillation periods, equals the resonant frequency of the custom QTF.

Interestingly, stronger variation amplitude exists around a shorter feedback interval. This is probably due to the short energy accumulate and limited accumulated energy can be easily counteracted down to almost zero by the successive phase-mismatched acoustic source. This issue can be mitigated with a longer feedback interval, e.g., oscillation amplitudes around 1.3 s and 3.3 s are alleviated and will become imperceptible over an interval over 8 s. Again, the hypothesis on phase matching in reasonable feedback interval selection has been confirmed. Although similar maxima, compared with feedback intervals of 0.5 s or longer, can be obtained at about 0.05 s and 0.1 s without interrupting energy accumulation, sharper oscillations would make the spectrophone need a rather stable electronic control unit. Any small fluctuations (e.g., clock jitter, time difference of data processing for each feedback interval etc.) would lead to a misinformation. Considering the response time of the QTF spectrophone shown in Figure 3 as well as the feedback interval investigation, we selected a feedback interval of 0.50514 s to perform tight wavelength locking for the following experiments, while maintaining sufficient energy accumulation.

3.3. Line-Locked Wavelength Stabilization

The DFB laser wavelength was firstly scanned across the CO₂ spectrum by superposing a 0.05 Hz ramp on a 6226 Hz sinusoidal modulation. The preset bias current, corresponding to the absorption line center ν₀, was then figured out for the following line-locked process. The 3f component of the absorption feature was used as the error signal to compensate for the laser wavelength drift. With the PID feedback control started, as shown in Figure 5, the 3f reaches an approximately constant value with a small variation around zero for hours, meaning real-time wavelength locking to the absorption line during continuous operation. One might argue that when working at atmospheric pressure (ADM, 760 Torr) there is a shift of the CO₂ line, with respect to the reference chamber (50 Torr). In this case, the shift is about 150 MHz [28], only 3% of the CO₂ linewidth (4.4 GHz at 760 Torr). Its influence on the signal amplitude is evaluated to be within 0.5%. Besides, a narrower linewidth at low pressure can benefit the wavelength locking because the error signal is more sensitive to the laser frequency drift. The wavelength stabilization performance is evaluated by comparing the 3f amplitude fluctuation under line-locked operation with the scanning spectrum (i.e., wavelength scanned part in Figure 5a). The 3f amplitude over 6000 s is shown in Figure 5b with the wavenumber deviations evaluated to be within 0.0002 cm⁻¹. Therefore, an improvement in stability, accuracy, and response time can be expected for QEPAS spectrophones.

3.4. Sensor Performance

After completing the construction of the experimental system, we first optimized the laser wavelength modulation depth coefficient (i.e., m = α/Δν, where α is modulation depth and Δν is the HWHM of the absorption spectrum) to improve the 2f QEPAS signal amplitude (see Appendix C Figure A3). With the laser wavelength tightly locked to the absorption line, the sensor performance was further investigated at 1 bar and 296 ± 2 K. CO₂/N₂ mixtures were uniformly pumped through the ADM at a flow rate of 200 mL/min. The mixtures were prepared by diluting certified 1% and 100% CO₂ with pure nitrogen (purity 99.999%) using a commercial gas mixer. The sensor response is illustrated in Figure 6 by plotting the photoacoustic 2f amplitude as a function of CO₂ concentration, ranging from 10 ppm to 50%. The vertical error bars take into account the uncertainty of the measured signal amplitude (i.e., the standard deviation of measurement over 30 s). A linear fitting is performed to the measured data and an R-square value of 0.9985 is obtained, indicating a good linear response to the CO₂ concentration. Above a CO₂ concentration of 30%, the acoustic signal begins to decline. It is probably caused by the significant variation of gas components, which could change the QTF resonant frequency by a higher gas density, the optimal resonant tube length by a lower sound velocity, or the modulation depth coefficient by a broadened HWHM.

To evaluate the achievable minimum detection limit of the current QEPAS spectrophone, we performed an Allan deviation analysis, measuring and averaging the photoacoustic signal of continuous dry nitrogen at a flow rate of 200 mL/min. Meanwhile, the laser wavelength was locked to the absorption line of CO₂. The Allan deviation plotted in Figure 7 reports a detection limit of 7 ppm at an integration time of ~0.5 s. The corresponding normalized noise equivalent absorption coefficient (NNEA) is calculated to be 4.7 × 10⁻⁹ W·cm⁻¹·Hz^−1/2. Furthermore, the detection limit can be improved to 0.45 ppm at an integration time of 270 s. This minimum detection limit together with the upper detection concentration of 30% in Figure 6 determines the linear dynamic range, i.e., 6.7 × 10⁵.

3.5. Field Applications for Atmospheric and Exhaled CO₂ Measurement

The field performance of the spectrophone was evaluated by measuring the CO₂ level in air (both indoors and outdoors) and exhalations of three researchers, successively. The gas samples were fully dehumidified before each measurement inside the ADM, otherwise CO₂ molecular relaxation would be accelerated by the presence of humidity in the gas [13,29], leading to enhanced photoacoustic amplitude, however, at the expense of sensing reliability under an unstable water vapor concentration. The CO₂ concentrations are determined using the calibration curve in Figure 6 with the results depicted in Figure 8. The observed outdoor CO₂ concentration, 410 ± 4.7 ppm, is in good agreement with global greenhouse gas monitoring [30]. While the indoor CO₂ concentration, 547 ± 4.6 ppm is a bit higher, mainly due to the contribution by people working in the laboratory. The observed exhaled CO₂ concentrations of three people vary from 4.3 to 4.6%, which lie in a healthy and common range according to a number of breath analysis studies, reviewed in [31].

4. Conclusions

In summary, we have realized a quartz-enhanced spectrophone for rapid CO₂ measurement. A custom QTF with a low resonant frequency of 12.5 kHz was used to increase the QEPAS response to the slow-relaxing CO₂. A LabVIEW-based servo loop was developed for stable laser operation with its center wavelength variation kept within 0.0002 cm⁻¹. Phase match between successive feedback intervals, as well as the response time limitation of the QTF, was experimentally investigated. Thus, the response time of the QEPAS sensor was enhanced to ~0.5 s for continuous CO₂ measurements while maintaining sufficient energy accumulation. Compared with several typical QEPAS-based CO₂ measurements, which achieved a detection limit of tens to hundreds ppm [16,18,26], a better detection limit of 7 ppm was achieved, notably with a much faster response time. The system stability was evaluated by performing Allan deviation analysis, achieving a minimum detection limit of 0.45 ppm at an integration time of 270 s. Besides, a linear dynamic range of 6 × 10⁵ was obtained and the implementations for atmospheric and exhaled CO₂ measurements, in a field application, were demonstrated. Future work to investigate the details of H₂O influence on CO₂ measurement with a humidity range from 0 to saturation is planned. The real-time correction will be performed by monitoring H₂O concentrations to fully explore the capacity of this integrated quartz enhanced spectrophone for in situ and rapid CO₂ measurements in many different fields.