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Abstract
A sensitive acetylene (C2H2) sensing system based on a novel triple-row circular multi-pass cell (CMPC) was demonstrated. This CMPC has an effective optical length of 21.9 m within an extremely small volume of 100.1 mL. We utilized wavelength modulation spectroscopy (WMS) for absorption spectroscopy detection of C2H2. The distance between the two minima of the second harmonic was used to normalize the maximum value of it, which makes the time to obtain stable output for continuous detection shorten dramatically. A fiber-coupled distributed feedback (DFB) diode laser emitting at 1.5316 µm was employed as a light source. An Allan deviation analysis yielded a detection sensitivity of 76.75 ppb with a normalized noise equivalent absorption coefficient of 8.8 × 10−10 cm-1 Hz-1/2 during an average time of 340 s. With a fast stable time, reduced size and high detection sensitivity, the proposed sensing system is suitable for trace gas sensing in a weight-limited unmanned aerial vehicle (UAV) and an exhalation diagnosis for smoking test.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The formation of acetylene (C2H2), a common hydrocarbon in the troposphere, is almost entirely anthropogenic. For this reason, acetylene is often used as a marker of anthropogenic emissions and to track contaminated air masses in atmospheric studies [1]. Due to its flammability and other chemical properties, C2H2 is also an important fuel and raw material in industrial applications [2]. Recently, it has also been found that C2H2 can be used as a marker of smoking status and air pollution to human health [3,4]. The exhalation concentration of C2H2 after smoking or exposing to polluted air can reach hundreds of ppb, followed by a relatively rapid elimination (exponential decay) in about 3 hours. Therefore, the accurate measurement of C2H2 is of great significance in variety applications of scientific research, industry and medical care. Typically, in respiratory diagnosis, in addition to high sensitivity, the gas sensing system also needs to have a small inflation volume and fast response [5,6].
Methods based on laser absorption spectroscopy are effective approaches for gas sensing in environmental monitoring [7], industrial process control and medical diagnosis. The most common ones of these methods are cavity ring-down spectroscopy (CRDS) [8], quartz-enhanced photoacoustic spectroscopy [9], and tunable diode laser absorption spectroscopy (TDLAS) [10]. Among them, TDLAS is commonly used with advantages of highly sensitivity, simple structure and low cost [11,12]. Wavelength modulation spectroscopy (WMS) technology is the most frequently used kind of TDLAS [13]. To achieve a high detection sensitivity, TDLAS is usually used in combination with a multi-pass cell (MPC) which can obtain a long light path in a relatively small volume [14–17]. The most popular MPCs are Herriott cell and its improved types [16,17]. In a Herriott cell, the effective areas where laser beams are reflected on the two mirrors are relatively small, therefore a relatively high path-to-volume ratio (PVR) can be achieved [18]. Zheng et al., implemented C2H2 concentration detection based on WMS using a 480 mL Herriott cell with an effective optical length of 9.28 m [16], and the 1σ minimum detectable concentration of 85 ppb was achieved with an average time of 250 s. The astigmatic Herriott-type cell can achieve an improved PVR, but it requires extremely high accuracy for manufacture and adjustment of the mirrors. Liu and co-workers demonstrated methane (CH4) concentration measurement based on WMS using a 280 mL astigmatic Herriott cell with an effective optical length of 26.4 m [17], a detection limit of 100 ppb was obtained with a lock-in time constant of 1 ms.
Circular multi-pass cell (CMPC) is another kind of MPC in which the laser beam is reflected back and forth on a single circular mirror [19,20]. Béla et al., designed a single-row CMPC with an optical length of 4.1 m within 40 ml volume and realized a detection of the oxygen-isotope ratio (18O/16O) in 1% carbon dioxide (CO2) [19]. For this kind of CMPC, the input and out beams share the same aperture on the side wall, so it is hard to further increase the effective optical length because of interference between fore and rear optical systems. Recently, we proposed a novel kind of CMPC which consists of two spherical circular mirrors with identical radii of curvature, and multiple horizontal rows of spots can be generated on the inner surfaces of the mirrors during the reflections [21,22]. In this version, the entrance and exit are separated, and the overall optical length can be increased without degradation of PVR. Based on the parameterization design method, a triple-row CMPC with an effective optical length of 21.9 m within 100.1 mL was demonstrated, which has the smallest volume among the existing MPCs with similar overall optical path lengths [22].
In this paper, we report on an acetylene sensing system with our newly developed triple-row CMPC based on the WMS technology in TDLAS. C2H2 detection was performed to evaluate the performance of the sensing system. The distance between the two minima of second harmonic was used to normalization the peak value of second harmonic at different acquired data points to tranquilize the data fluctuation in the earliest period of air inflation. Consequently, the time to obtain stable output for each continuous measurement is significantly shortened. This kind of fast stable, low-cost, compact sensing system is very suitable for trace gas measurements in harsh environments and exhalation diagnosis, et. al. By changing probe laser to other near-infrared distributed feedback (DFB) lasers with corresponding output center wavelengths, the system can be also used to detect other gases.
2. Sensor configuration
2.1 Design of the C2H2 sensor
The schematic of the C2H2 sensor system is shown in Fig. 1(a). A fiber-coupled DFB laser (NLK1C5GAAA, NEL, Japan) emitting at 1531 nm was employed as the probe laser. Temperature and injection current of the laser was controlled with a diode laser controller (LDTC0520, Wavelength, USA). The temperature and injection current were first set to ensure the initial output wavelength of the laser was exactly at the absorption peak. A 5 Hz triangular scanning signal and a 5 kHz sine modulation signal generated by the LabVIEW controlled data acquisition (DAQ) card (USB6341, National Instruments, USA) are applied to modulate the injection current of probe laser.
Fig. 1. (a) Schematic of the C2H2 sensor system. (b) Photograph of the triple-row CMPC. DFB: distributed feedback laser; FC: fiber collimator; DM: dichroic mirror; M1, M2: convex lens; V1, V2: valves; C&T Control: Current and temperature controller for laser; DAQ: data acquisition card.
A fiber collimator (PAF-X-7-C, Thorlabs, USA) was used to collimate the output beam from the DFB laser. An indicated beam from a red laser (MLL-III-660L-180mW, Changchun New Industries Optoelectronics Tech. Co., Ltd, China) emitted at 660 nm was aligned collinearly with the collimated probe laser through a dichroic mirror (DM10-950LP, LBTEK, China) to guide the optical alignment of the sensing system. Then the combined laser beam was focused around the center of the CMPC by a convex lens (f = 100 mm, Thorlabs, USA), to maintain steady beam propagation within the cell. After reflected back and forth in the triple-row CMPC, the output laser was then collected by an identical convex lens and focused on the InGaAs amplified detector (PDA20CS, Thorlabs, USA). The output of the detector was sent to the DAQ card and processed by the LabVIEW based digital lock in amplifier (LIA) to extract the corresponding 2f signals. The sampling rate of the DAQ card was set to be 200 kHz. The low-pass filter used in the digital LIA is a 128-order Bessel infinite impulse response (IIR) digital filter with a cut-off frequency of 80 Hz.
A photograph of the triple-row CMPCT is shown in Fig. 1(b). Outline of the CMPC is a cylinder with a height of 24.33 mm, a radius of 68 mm; the height and radius of the inflation volume are about 12.74 mm and 50 mm, respectively. The cell consists of two spherical circular mirrors with identical radii of curvature of 50 mm, and three horizontal rows of reflection spots can be generated on the inner surfaces of the mirrors. Reflectivity of the internal surfaces of the circular mirrors is higher than 98.8% in 1000 nm -1700nm by coating with metal gold and multilayer medium films. An absorption mask coating with black oxide film was mounted inside the CMPC to suppress interference of stray light [22], which has three rows of holes corresponding to designated positions of reflection spots on the circular mirrors. An effective optical path of 21.9 m was achieved with 220 multiple reflections of laser beam within a volume of 100.1 mL.
2.2 Absorption line selection
We selected the absorption peak of C2H2 at 1531.59 nm (6529.17 cm-1) to measurement of C2H2 concentration. Around this wavelength, other gases in the air have little effect on acetylene absorption [23]. Absorption of 100 ppm acetylene from 6528.4 cm-1 to 6529.9 cm-1 is shown in Fig. 2, which is derived from the Hitran database [23]. This simulation is based on ambient conditions with a pressure of 1 atm and a temperature of 291.15 K. The center driver current of probe laser was set to be 114 mA with a temperature of 21 °C to make the center wavelength of output beam coincide with the selected absorption line of 6529.17 cm-1. The amplitude of the scanning triangular signal was set to be 0.045 V (500 mV/A), make a scanning of the injection current from 90 mA to 135 mA, corresponding to a wavenumber range of 6528.5 cm-1-6529.9 cm-1. Relationship between the laser injection current and its output wavenumber was also depicted in Fig. 2. It can be found that the half width at half-maximum (HWHM) of the selected absorption line is about 0.1525 cm-1.
Fig. 2. Simulated absorption spectrum of 100 ppm C2H2 and the curve of the DFB diode laser wavenumber as a function of injection current at 21 °C.
3. Measurements and results
3.1 Fluctuation elimination of the maximum of second harmonic for WMS
To obtain the performance of the C2H2 sensing system, we measured a 60.1 ppm C2H2 sample (Beijing Huatong Jingke Gas Chemical Co., Ltd, China, other gases used in the following experiments are all from this company) at a pressure of 1 atm and a temperature of 291.15 K for about 0.9 hour with a sampling interval of 2 s at first. The amplitude of sinusoidal signal was set to about 0.008 V, at which the amplitude of the 2f signal is relatively large and the waveform was decent. The obtained second harmonic in steady state is shown in Fig. 3(a). The maximum value of the 2f signal (S2fmax) is shown in Fig. 3(b). We can find that the amplitude needs a relative long time (about 25 min) to reach stability, which means that the value fluctuates in a small range without going up or down afterwards. The measurements results of acetylene with concentration of 49.9 ppm and 40.3 ppm behave similarly, which can be found in Figs. 3(d) and 3(f), where S2fmax fluctuated relatively large and irregular at first, and then become stable after 20 to 40 minutes. This is definitely undesirable for systems that require fast response. During the detection of C2H2 sample with different concentrations, the DFB laser remained on continuous operation, so this fluctuation should not be caused by light intensity fluctuations.
Fig. 3. (a) The detected second harmonic signal with 60.1 ppm C2H2, Sp: Sampling points. (b) and (c) are the detected S2fmax and D2f with 60.1 ppm C2H2. (d) and (e) are the detected S2fmax and D2f with 49.9 ppm C2H2. (f) and (g) are the detected S2fmax and D2f with 40.3 ppm C2H2.
After analyzing the detected second harmonic signal, we found that the interval between the two minima of second harmonic (D2f) changed in a very similar trend comparing with S2fmax, as depicted in Figs. 3(b)–3(g). Similar variation tendencies appear on the S2fmax and D2f at the three different concentrations: the inflexions of the two curves are almost the same, while the D2f almost remains the same when the S2fmax is steady. In order to rule out the possibility that the fluctuations are due to low concentrations of gas sample, in which the distribution of gas molecules in the measurement optical path may be uneven, a pure CO2 sample (99.999% CO2) was also detected by our system at a pressure of 1 atm, the absorption line at 6527.64 cm-1 was selected where pure CO2 and 100 ppm C2H2 have similar absorption coefficients [23]. The experimental phenomenon was similar. Therefore, we inferred that D2f can be used to normalize the detected S2fmax for eliminating fluctuations. The D2f -normalized S2fmax of C2H2 for different concentration and pure CO2 are shown in Fig. 4, the D2f-normalized S2fmax is adjusted to the same order of magnitude as S2fmax. From Fig. 4, we can find that the D2f -normalized S2fmax can be stable from the beginning time of long-term continuous detection, and changes in direct proportion to the gas concentration. The standard deviations of S2fmax and D2f -normalized S2fmax of different gases can be found in Table 1. After normalization, the standard deviation of detected signal has been significantly reduced. In experiments demonstrated below, we use the same normalization method to reduce the steady time of detections.
Fig. 4. (a) Detected S2fmax values of 40.3 ppm C2H2 (blue), 49.9 ppm C2H2 (red) and 60.1 ppm C2H2 (black), respectively. (b) The S2fmax signals normalized by D2f of 40.3 ppm C2H2 (blue), 49.9 ppm C2H2 (red) and 60.1 ppm C2H2 (black), respectively. (c) The detected values of S2fmax of pure CO2. (d) The S2fmax signals normalized by D2f of pure CO2.
Table 1. Standard deviation of S2fmax and standard deviation of D2f -normalized S2fmax
3.2 Calibration and data-fitting
To calibrate the relationship between concentrations and output signals of the system, C2H2 gas samples of seven different concentrations (0 ppm, 20.2 ppm, 30.4 ppm, 40.3 ppm, 49.9 ppm, 60.1 ppm, 80.6 ppm) was measured at pressure of 1 atm to get the D2f -normalized S2fmax signals. The triangle signal and sinusoidal signal were the same as those we used in Sec. 3.1 to ensure the amplitude of the second harmonic was relatively large and the waveform was decent. For each concentration, we took samples for 3 minutes, and the obtained values of D2f -normalized S2fmax were then averaged. The relationship between the detected concentrations and the averaged D2f -normalized S2fmax signals is shown in Fig. 5. The linear relation (R-square value: 0.9993) between the D2f -normalized S2fmax (S2fmax/ D2f, mV) and the concentrations (C, ppm) can be fitted as:
3.4 Sensor measurement stability
The noise level of the sensing system can be evaluated by detecting pure N2. The measured D2f -normalized S2fmax was transformed to C2H2 concentration based on Eq. (1). We measured the pure N2 for an hour with a sampling interval of 2 s and the obtained concentration after transformation is shown in Fig. 6(a). It can be found that the retrieved concentrations are at a range of -3 ∼ 3.5 ppm for an hour measurement. The Allan deviation is given in Fig. 6(b) to evaluate the system detection stability and limit after D2f -normalization. Allan deviation analysis shows that the system had a detection precision of 0.8302 ppm during an averaging time of 2 s, and 76.75 ppb for a long average time of 340 s. Based on the experiment results, a normalized noise equivalent absorption coefficient of 8.8 × 10−10 cm-1 Hz-1/ 2 was obtained. This result may be limited because of that the reflectivity of the coating in the internal of the triple-row CMPC is not high enough and can be improved by increasing the reflectivity of the coating.
Fig. 6. (a) Time series measurements of C2H2 for an hour. (b) Allan standard deviation curve of C2H2 detection based on Fig. 6(a).
3.5 Detection sensitivity estimation by SNR
C2H2 samples with a concentration of 100.6 ppm were tested to evaluate the detection sensitivity of the C2H2 sensing system. The obtained second harmonic signals without averaging are shown in Fig. 7. The obtained D2f -normalized S2fmax of it is about 0.7665 mV, corresponding to a retrieved concentration value of 99.2 ppm, according to Eq. (1), which is close to the nominal concentration with a measurement error of 1.4 ppm and is acceptable. If we use the standard deviation of the non-absorption region of the second harmonic to estimate the system noise (1σ = 0.0266), the signal-to-noise (SNR) of the system is about 56.
Fig. 7. The detected 2f signal with a 100.6 ppm C2H2 at pressure of 1 atm and temperature of 291.15 K.
4. Discussion
In our system, it takes a relatively long time for S2fmax to achieve stability in some cases, which is not caused by the fluctuation of laser intensity. Through the pure CO2 detection experiment, we also know that it has nothing to do with low gas concentration. The total pressure and temperature also didn’t change during the experiment, so we believe that it is mainly due to the gas movement during the detection period. As for why the interval between the two minima of second harmonic changed similarly with the fluctuation of maximum value of the second harmonic, it can be inferred with the mathematical expression of the detected second harmonic. The second harmonic obtained by digital LIA can be expressed as [24]:
5. Conclusion
We demonstrated a C2H2 sensing system with a newly developed triple-row circular multi-pass cell based on wavelength modulation spectroscopy. The CMPC has excellent PVR which obtains an optical length of 21.9 m within a tiny volume of 100.1 ml. The interval of the two minima of detected second harmonic was used to normalize the 2f signal then the precise gas concentration was obtained once the gas was passed to the CMPC and reached a pressure of 1 atm. Other TDLAS systems with problem of too long steady time may also be improved by the same method. Experiments were carried out to evaluate the performance of the sensing system. A SNR of 56 and a limit of detection of 76.75 ppb with an averaging time of 340 s were obtained. The normalized noise equivalent absorption coefficient of the system is 8.8 × 10−10 cm-1 Hz-1/ 2. The sensing system has promising potential for gas sensing applications which calls for small size, fast response and high measurement sensitivity.
Funding
National Key Research and Development Program of China (2018YFF0109600).
Disclosures
The authors declare no conflicts of interest.
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