Gas sensors with high sensitivity, wide dynamic range, high selectivity, fast response, and small footprint are desirable across a broad range of applications in energy, environment, safety, and public health. However, designing a compact gas sensor with ultra-high sensitivity and ultra-wide dynamic range remains a challenge. Laser-based photoacoustic spectroscopy (PAS) is a promising candidate to fill this gap. Herein, we report a novel method to simultaneously enhance the acoustic and light waves for PAS using integrated optical and acoustic resonators. This increases sensitivity by more than two orders of magnitude and extends the dynamic range by more than three orders of magnitude, compared with the state-of-the-art photoacoustic gas sensors. We demonstrate the concept by exploiting a near-infrared absorption line of acetylene (C2H2) at 1531.59 nm, achieving a detection limit of 0.5 parts-per-trillion (ppt), a noise equivalent absorption (NEA) of 5.7×10-13 cm-1 and a linear dynamic range of eight orders of magnitude. This study enables the realization of compact ultra-sensitive and ultra-wide-dynamic-range gas sensors in a number of different fields.
Article
Doubly resonant sub-ppt photoacoustic gas detection with eight decades dynamic range
https://doi.org/10.21203/rs.3.rs-431688/v1
This work is licensed under a CC BY 4.0 License
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posted 18 May, 2021
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gas detection
sensors
dynamic range
Gas detection plays a significant role in almost all aspects of our modern society such as energy, environment, transportation, agriculture, safety, and security. Among many different types of gas sensors, laser-based ones are best suited to provide several combined key features: accuracy, sensitivity, selectivity, portability, fast response, and safety. By measuring light attenuation through a gas sample, quantitative gas analysis can be performed using the Beer-Lambert law. These optical sensors are increasingly required in a number of fields, including environmental monitoring [1,2], marine science [3,4], biological studies [5,6], and breath analysis [7,8]. State-of-the-art laser absorption sensors using a multipass cell can achieve a noise equivalent absorption (NEA) as low as 10-10 cm-1 with an effective path length of hundred meters [9]. To further enhance sensitivity, the absorption path length can be increased to kilometers by using a high-finesse optical cavity. Among the most sensitive laser-based gas sensing techniques, noise-immune cavity-enhanced optical heterodyne spectroscopy (NICE-OHMS) achieved a NEA of 10-14 cm-1 when operating at an extremely low pressure of 1.8 mTorr [10]; the minimum detectable concentration of a specific molecule has been achieved by saturated-absorption cavity ring-down (SCAR) with a limit of a few parts-per-quadrillion (ppq) [6, 11]. However, the linear dynamic range of this class of optical gas sensors is generally limited to 4-5 orders of magnitude and the overall footprint and weight are still quite large for many applications requiring very compact analyzers. Therefore, numerous applications require versatile gas sensors that can detect sub-part-per-billion (ppb) background concentration in ambient and hundred parts-per-million (ppm) concentration in the target emission, as well.
To get a combined high sensitivity and wide dynamic range, photoacoustic spectroscopy (PAS) is a promising candidate. Indeed, the photoacoustic signal linearly increases with the absorbed laser power, rather than undergoing significant power attenuation after a long absorption path. Instead of directly measuring light attenuation, PAS relies on the detection of acoustic waves generated in the vibrational relaxation process of molecules excited by photon absorption. Photoacoustic gas sensors also inherit the advantages of high selectivity, high sensitivity, and fast response, like other laser-based spectroscopic methods. In particular, the use of acoustic transducers instead of photodetectors makes PAS sensors more compatible with various laser wavelengths, as sensitive photodetectors are not always available over a wide wavelength range. To date, numerous studies have been focused on advancing PAS by designing new photoacoustic cells, inventing new acoustic transducers, and using different laser sources [12-19]. State-of-the-art PAS-based gas sensors have detection sensitivity between 10-8 and 10-11 cm-1 in NEA and a linear dynamic range of five orders of magnitude [20-24].
In this work, we report a PAS sensor with opto-acoustic resonance enhancement for ultra-sensitive and ultra-wide-dynamic-range gas detection. The novel photoacoustic detection module consists of a high-Q-factor acoustic resonator placed inside an optical resonator for the significant amplification of the photoacoustic signal (Fig. 1). A novel photoacoustic design, leveraging on a double standing wave effect, achieves a combined acoustic amplification factor of 175 times and laser power enhancement of almost three orders of magnitude. Using a near-infrared diode laser at 1531.59 nm as a proof-of-principle, we show that our acetylene (C2H2) sensor reaches a record sensitivity of 10-13 cm-1 (NEA) and an unprecedented linear dynamic range of eight orders of magnitude. Moreover, the C2H2 sensor is validated in a relevant environment to measure ambient background concentration.
Theory. Figure 1 shows the basics of opto-acoustic resonance for PAS. The core gas sensing element consists of an optical resonator, an acoustic resonator, and an acoustic transducer, which are arranged in a coupled configuration. Indeed, when the optical frequency of the incident laser is in resonance with a longitudinal cavity mode of the optical resonator, a standing optical wave is formed between the resonator mirrors. A high-finesse optical resonator can significantly build up the laser power, by several orders of magnitude [25], directly enhancing the photoacoustic signal which scales linearly with the laser power. The laser intensity is modulated at the same resonance frequency as the acoustic resonator. A specifically designed one-dimensional longitudinal tube can be used to amplify the acoustic signal by forming a standing acoustic wave inside it. Any types of acoustic transducers can be used to detect the amplified acoustic wave. In this work, we use both a quartz tuning fork (QTF), resonant at the same frequency as the acoustic resonator, and an electret microphone for demonstration. As shown in Fig. 1, the QTF locates nearby the antinode of the standing acoustic wave, which is generated by the acoustic resonator composed of two stainless-steel tubes placed at the opposite sides of the QTF [26]. The other photoacoustic sensor configuration using an electret microphone is described in the Supplementary Note 3.
The frequency-dependent photoacoustic signal (S) at its resonant frequency f is given by:
where b is the laser power buildup factor, g is the acoustic wave enhancement factor, K is the sensor constant, Win is the incident laser power, λ is the laser wavelength, αeff(λ) is the effective absorbance by the analyte, τ(P) is the relaxation time at the gas pressure P, and ε is the radiation-to-sound conversion efficiency [27], which depends on f and τ. Note that the factor g is independent of laser power but is related to the geometry, to the material, and to the Q-factor of the acoustic resonator, as well as to the frequency of sound waves [28]. The power buildup factor b is determined by the finesse of the optical resonator, which needs to be properly selected so that a wide linear dynamic range and a high sensitivity can be simultaneously obtained (see Methods).
Experimental setup. The schematic of the PAS sensor is shown in Fig. 2. An external cavity diode laser (ECDL) is used to detect the P(11) line of C2H2 at 1531.59 nm. The ECDL is phase modulated by an electro-optic modulator (EOM) at 20 MHz and locked to the optical resonator using the Pound-Drever-Hall (PDH) method [29]. Details of the application of the PDH method to PAS can be found in [24]. In this work, the current and piezo transducer (PZT) feedback loops are both used for a more robust locking performance. Each mirror of the optical resonator has a radius of curvature of 150 mm and reflectivity of 99.923% (finesse 4078) at the laser wavelength, as measured by cavity ring-down (see Supplementary Note 1). Compared with other cavity-enhanced absorption spectroscopic methods [6,10], the optical resonator used here for the opto-acoustic resonance features a much shorter length (60 mm in this work). Two mode matching lenses (f1 and f2) are used to maximize the coupling efficiency (84%) between the laser and the optical resonator (see Supplementary Note 2). With a maximum incident laser power of 300 mW, the intracavity optical power is boosted to 264 W in this work (see Methods).
The intracavity laser beam passes through the acoustic resonator, consisting of two stainless-steel tubes (inner diameter 1.3 mm, length 23 mm), and does not touch any surface. The central axis of the acoustic resonator is about 1.2 mm below the top of the QTF prongs, thus optimizing the piezoelectrical conversion efficiency [15]. The two tubes are placed at a distance of ~60 μm from the QTF, so that it lies near the antinode of the acoustic wave and leaves its Q-factor unaffected. The beam waist (340 μm in diameter) is located between the two prongs of the QTF, which has a resonant frequency of 7.2 kHz and a Q-factor ~8000 (gas pressure 760 Torr) [15]. Photoacoustic gas sensors using QTF as an acoustic transducer have been previously developed for detecting many different gas species [15-17]. The optical resonator, the acoustic resonator and the QTF are all enclosed inside a chamber. A high-speed lithium niobate optical switch is used to chop the laser beam at the same frequency as the resonant frequency of the QTF. The piezoelectric current from the QTF is collected by a trans-impedance amplifier and then amplified by a low-noise voltage preamplifier. Finally, a lock-in amplifier with a detection bandwidth of 1 Hz is used to demodulate the first harmonic signal (1f) at the sensor output.
Double standing wave enhancement. To evaluate the enhancement effects due to the integrated acoustic and optical resonators, the PAS-1f signal of the C2H2 line at 1531.59 nm is measured under three different configurations. Figure 3 compares the typical PAS-1f signal measured using a bare QTF (2% C2H2), a QTF with the mere acoustic resonator (0.1% C2H2), and a QTF with the complete opto-acoustic resonator (1 ppm C2H2). Note that different C2H2 concentrations are used for these three configurations because of their quite different sensitivity. All the experiments are performed at the same incident laser power of 12 mW, lock-in detection bandwidth of 1 Hz, and gas pressure of 760 Torr. After normalization by the gas concentration, a comparison of Fig. 3(a) and Fig. 3(b) shows that the acoustic resonator enhances the PAS-1f signal by 175 times. Besides, the optical resonator provides another enhancement factor of 980, as emerging from a comparison of Fig. 3(b) and Fig. 3(c). Hence, the combined opto-acoustic amplification provides an overall enhancement of the PAS signal by a factor of 105 via the double standing wave effect.
Ultra-sensitive gas detection. Figure 4 shows the PAS-1f signal measured by the photoacoustic sensor for an incident optical power of 12 mW. The measurement is performed at 760 Torr for different C2H2 concentrations (100 ppb and 10 ppb in nitrogen balance) and high-purity nitrogen (99.999% purity) when the laser wavelength is tuned from 1531.32 nm to 1531.75 nm. The peak values at 1531.59 nm are 4.05 mV (100 ppb C2H2) and 0.68 mV (10 ppb C2H2), respectively. Note that the background signal is contributed by the thermoelastic effect due to unwanted absorption at the optical window and resonator mirror [30]. This background signal has not been subtracted from the PAS-1f signal shown in Fig. 4. Interestingly, due to the excellent signal-to-noise ratio, a neighboring water line near 1531.37 nm emerges, as shown in Fig. 4(b). This is probably due to residual water in the gas chamber after the desiccation process (see Methods). Our hypothesis is confirmed by repeating the measurement with pure nitrogen (water vapor < 0.3 ppm) shown in Fig. 4(c). To reduce the measurement uncertainty, a multi-spectral fitting method, with prior knowledge of infrared spectra from the HITRAN database [31], is implemented. Hence, the background signal is automatically eliminated during the fitting procedure.
The relationship between the PAS-1f signal and incident laser power is also investigated. Figure 5(a) compares the PAS-1f signal at 1 ppb C2H2 under two different incident power levels (12 mW and 300 mW). The peak value of the PAS-1f signal is increased by a factor of about 22.5 when the incident laser power is increased from 12 mW to 300 mW (a factor of 25). The slight deviation of the enhancement factor between laser power and PAS signal is due to the variation of the optical coupling efficiency. By repeating the measurement at 100 ppb C2H2, Fig. 5(b) shows the variation of the PAS-1f amplitude with the incident optical power. The sensor signal increases almost linearly (0.25 mV/mW) with the incident power. In contrast, the noise level remains almost unchanged over the entire power range, as shown in Fig. 5(b) (1-σ standard deviation of N2 over 120 s). This makes this set-up very promising to further increase detection sensitivity by simply increasing the incident laser power.
Test of dynamic range and detection limit. The linear response of the sensor is tested at the pressure of 760 Torr by filling the gas chamber with different C2H2/N2 mixtures. Fig. 6 shows the background-subtracted PAS-1f amplitude as a function of C2H2 concentration, varying in a 1 ppb-500 ppm interval. The sensor shows a very good linear response with a slope of 36.4 μV/ppb and an R-square value of 0.99 from 1 ppb to 50 ppm. However, the sensor deviates from the linear response at higher C2H2 concentrations due to the apparent degradation of optical finesse of the optical resonator.
To evaluate the long-term stability and the minimum detection limit, the Allan–Werle deviation analysis is conducted by measuring nitrogen with the results shown in Fig. 7. The noise equivalent concentration (NEC) is determined to be 5.1 ppt (unity for signal-to-noise ratio) at an integration time of 1 s. At an incident optical power of 300 mW and a detection bandwidth of 1 Hz, we obtain a normalized noise equivalent absorption coefficient (NNEA) of 1.7×10-12 Wcm-1Hz-1/2 (see Methods). The NEC can be improved to 0.5 ppt at a longer integration time of 300 s, leading to a noise equivalent absorption (NEA) of 5.7×10-13 cm-1. As a result, the proposed photoacoustic gas sensor achieves a linear dynamic range of 1.0×108.
To verify the versatility of this technique, we also develop another photoacoustic sensor by using a conventional longitudinal acoustic resonator with buffering volumes (Q-factor 25), an electret microphone, and a longer optical resonator (80 mm) with the same finesse (see Supplementary Note 3). The microphone-based sensor can be easily aligned along the optical path and a similar performance in terms of sensitivity and dynamic range is demonstrated, as compared to the previous configuration.
The dynamic stability of the sensor is also important for applications requiring continuous gas sampling. This is evaluated by operating the gas sensor when continuously filling C2H2 gas samples into the gas chamber. Our sensor behaves well, without any interruption, during the gas filling and flow rate changing processes (see Supplementary Note 4).
The key performance parameters (e.g., linear dynamic range, NEC, and NEA) of some state-of-the-art photoacoustic gas sensors are summarized in Table 1. The photoacoustic gas sensor developed in this work shows an unprecedented linear dynamic range and NEA, superior to the state-of-the-art photoacoustic sensors using different types of laser sources and acoustic transducers. Most remarkably, the current linear dynamic range is three orders of magnitude larger than the state-of-the-art (105) [20], and the current NEA is two orders of magnitude better than the state-of-the-art (10-11 cm-1) [23]. Besides, our sensor operates at atmospheric pressure, which is more suitable for field applications.
|
Table 1 Key performance parameters of state-of-the-art photoacoustic gas sensors. |
|||||||
|
Gas |
Method |
λ (µm) |
Linear dynamic range |
NEC (ppt) |
NEA (cm-1) |
Integration time |
Pressure (Torr) |
|
NO2 |
PAS[20] |
0.447 |
1×105 |
54 |
2.3×10-10 |
1 s |
640 |
|
SF6 |
QEPAS[21] |
10.54 |
3.2×103 |
50 |
6.1×10-9 |
1 s |
75 |
|
CO2 |
I-QEPAS[22] |
4.33 |
2.8×103 |
300 |
1.4×10-8 |
20 s |
38 |
|
C2H2 |
CECEPAS[23] |
1.531 |
6.7×103 |
24 |
2.4×10-11 |
100 s |
150 |
|
C2H2 |
CE-PAS[24] |
1.531 |
1×104 |
300 |
1.9×10-10 |
300 s |
50 |
|
C2H2 |
This work |
1.531 |
1×108 |
0.5 |
5.7×10-13 |
300 s |
760 |
|
Note: The linear dynamic range, NEC, and NEA are all the best parameters reported in the relevant studies. |
|||||||
The field performance of this sensor is evaluated by measuring atmospheric C2H2 concentration in Changchun, Jilin, China. The outdoor air samples are collected in aluminum foil bags and then dehumidified before each measurement inside the gas chamber. By comparing the measured spectra with the HITRAN database, we are able to identify the C2H2 line at 1531.59 nm and the neighboring spectra of CO2 and H2O centered at 1531.65 nm and 1531.37 nm, respectively, shown in Fig. 8(a). The C2H2 gas concentration is determined from the calibration curve based on the measured PAS-1f signal shown in Fig. 6. The measured C2H2 concentration at the same location from 7:00 am to 4:00 am - next day is plotted in Fig. 8(b), showing a variation between 1 ppb and 2 ppb at 23℃ and 760 Torr. The shaded area denotes the relative systematic uncertainty, which is evaluated to be 31% by comparing the gas concentration values extracted from the fitting equation with the calibrated gas samples using the gas mixer. Our measurement result is in very good agreement with the previous ambient C2H2 studies using cavity ring-down spectroscopy, gas chromatographic separation and flame ionization detection (GC-FID) [32,33], but with a better (70-240 times) signal-to-noise ratio.
In summary, we have developed a novel photoacoustic gas sensor using an integrated optical-acoustic resonator. Taking advantage of a double standing wave enhancement, the photoacoustic signal can be improved by five orders of magnitude, thus providing ultra-sensitive gas detection with low-power semiconductor lasers. The moderate cavity finesse maintains the high coupling efficiency of the incident laser and enables a wide linear dynamic range by using a short optical resonator. As a result, we have demonstrated a photoacoustic sensor with a NEC of 0.5 ppt, a NEA of 5.7×10-13 cm-1, and a linear dynamic range of eight orders of magnitude. Furthermore, we have demonstrated it can work, for atmospheric C2H2 measurement, in a field application. Although the demonstration is performed in the near-infrared region, the concept can be extended to the mid-infrared range, where many molecules have much stronger absorption bands. Importantly, combined with other light sources such as frequency combs, the new photoacoustic gas sensing technique developed in this work would lead to a powerful analytical tool for rapid and ultra-sensitive detection of multi-species [18].
Selection of finesse for the optical resonator. The finesse of the optical resonator needs to be properly selected in this work. First, the selection of a higher finesse theoretically provides a larger power enhancement factor. However, the linewidth of the cavity mode decreases with the increased finesse, making it more challenging to achieve a high coupling efficiency of the laser into the optical resonator. Second, the linear dynamic range of the sensor is affected by the finesse. The finesse is determined by the round-trip loss inside the optical resonator which includes the mirror transmission and absorption, gas absorption, and loss induced by the acoustic module. The finesse increases with the larger reflectivity of cavity mirrors, but it decreases apparently when the loss induced by gas absorption becomes comparable. Hence, the physical length of the optical resonator should be selected as short as possible to minimize the gas-absorption loss inside the cavity. In this work, a moderate finesse of about 4000 (mirror reflectivity, 99.923%) and a physical length of 60 mm is used.
Calibration of intracavity laser power. According to the main text, the optical resonator enhances the photoacoustic signal by a factor of 980. Considering the 84% coupling efficiency, the intracavity power enhancement factor is 1166, which is slightly lower than the theoretical buildup factor of 1300. This is probably due to the extra cavity loss caused by the QTF. As the empty cavity has a finesse of 4078, an additional loss of 0.017% can deteriorate the buildup factor from 1300 to 1166. When the incident power is adjusted from 12 mW to 300 mW, there is an extra 10% degradation of coupling efficiency. Hence, the maximum of the intracavity power is determined to be 264 W.
Gas chamber dehumidification. To mitigate the interference of the residual water molecules inside the vacuum cavity, the gas chamber is heated to 60oC and purged continuously by dry nitrogen for several hours. To protect the QTF which is quite close to the acoustic resonator, the heating temperature is controlled not quite high. It is possible to have some residual water inside the chamber.
NNEA calculation. The NNEA coefficient can be calculated by the following equation:
where αmin is the noise equivalent absorption (NEA) coefficient, Win is the incident power and BW is the detection bandwidth. With the NEC of 5.1 ppt in Fig. 7, NEA can be easily obtained by referencing to the HITRAN database [31]. The incident power is 300 mW. Together with the detection bandwidth of 1 Hz, the NNEA of 1.7×10-12 Wcm-1Hz-1/2 can be calculated.
Data availability
The data that support the findings within this paper are available from the corresponding authors upon reasonable request.
Acknowledgements
This research was supported by the Strategic Priority Research Program of Chinese Academy of Sciences (XDA17040513, XDA22020502), National Natural Science Foundation of China (NSFC) (51776179, 62005267), General Research Fund (14209220) from the University Grants Committee, Seed Project (ITS/242/19) from the Innovation and Technology Commission. We thank Professor Vincenzo Spagnolo from the Technical University of Bari for providing us the custom QTF.
Author contributions
Z.W., Q.W. and W.R. conceived the idea, designed the experiments, discussed the results and prepared the manuscript. Z.W. built the systems and conducted the experiments. H.Z. and Q.W. assisted in building the systems and conducting the experiments. S.B., I.G. and P.D.N. assisted in the PDH locking and optical coupling. W.R. and Q.W. supervised and coordinated the project.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information is available for this paper.
Correspondence and requests for materials should be addressed to Q.W. or W.R.
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Supplementary Information