Editorial Feature

Near-IR-Based H2S and CO Photoacoustic Gas Sensor

Although advancements in technology and science have significantly benefitted society, it is not without consequence. The production and eventually discharge of toxic and harmful gaseous products has led to the deterioration of the environment, as well as human health.  Developing sensitive, effective sensors to monitor the release of these products is crucial in mitigating their effects.

Image Credit: Ralf Geithe/Shutterstock.com

Gaseous pollutants can be easily diffused, are hard to treat, and can be difficult to detect. 

The continuous release of pollutant gases from vehicle and aircraft exhaust emissions, the chemical industry, and the electric power industry has led to the gradual increase of gas concentration in the atmosphere. This has resulted in severe air pollution problems, including acid rain and an increased risk for lung disease.

There are several gas detection techniques for the qualitative and quantitative analysis of trace gases: semiconductor sensors, analytical sensors, laser-based absorption sensors, and electrochemical sensors.

Laser absorption spectroscopy (LAS) can deliver a quicker response time of <1 second, a low detection limit of ppb level and even down to ppt level, as well as real-time in-situ measurements than other techniques.

Photoacoustic spectroscopy (PAS) is based on an optical absorption process and is a well-known technology for trace gas detection in several applications like non-invasive medical diagnosis, pollution gas monitoring, and industrial process control.

The PAS technology offers two unique benefits. First, the PAS-based gas sensors offer a detection limit proportional to the excitation power before a power saturation effect is noted, implying that the sensors’ performance can benefit from the constantly refreshed power records of different excitation sources.

Second, the excitation wavelength is independent of the PAS gas sensors, enabling its use to detect different gas species at the same time with different absorption wavelengths.

New research published in the journal Optics Express has developed a gas sensor that can simultaneously detect harmful gases like CO and H2S.

A two-stage commercial optical fiber amplifier featuring a full output power of 10 W was used to develop a ppb-level CO and H2S photoacoustic spectroscopy (PAS) gas sensor.

Two near-infrared diode lasers featuring the central wavenumbers of 6377.4 cm−1 and 6320.6 cm−1 were used as the excitation laser source.

A time-division multiplexing technique helped detect CO and H2S simultaneously using an optical switch. A theoretically simulated dual-resonator structural photoacoustic cell (PAC) was designed using a finite element analysis. The multi-component sensor’s performance was assessed after optimizing for pressure, frequency, and modulation depth.

Methodology

The signal-to-noise ratio (SNR) governs the detection limit of PAS-based gas sensors.

A higher SNR can be achieved in two ways: (1) by increasing the photoacoustic signal, which can be achieved by selecting a high-power excitation laser source or a strong absorption line; (2) by decreasing the background noise, specifically for a poor-quality laser beam.

As illustrated in Figure 1, a stainless-steel PAC including dual Helmholtz resonators was simulated and designed to decrease the background noise for the high-power laser radiation.

Structural diagram (a) and finite element simulation (b) of the dual-resonator photoacoustic cell. The relative PAS signal intensity values are shown in the color bar.

Figure 1. Structural diagram (a) and finite element simulation (b) of the dual-resonator photoacoustic cell. The relative PAS signal intensity values are shown in the color bar. Image Credit: Yin, et al., 2021

An Erbium-doped fiber amplifier (EDFA) allowed compensation for the loss in sensitivity caused by the selected weak absorption line strength in the near-IR region. Figure 2 shows that the EDFA included Erbium ions, optical isolators, pump laser supply, wavelength division multiplex (WDM) couplers, and optical filter.

Structural diagram of the Erbium-Doped Fiber Amplifier (EDFA).

Figure 2. Structural diagram of the Erbium-Doped Fiber Amplifier (EDFA). Image Credit: Yin, et al., 2021

Figure 3 shows a schematic of the PAS-based sensor system that enables concurrent detection of the H2S and CO pollution gases. For H2S detection, a DFB laser that emits at a central wavelength of 1582 nm was used. The other DFB laser helped target the absorption line of the CO molecule at 1568 nm.

Structural diagram of the photoacoustic pollution gas sensor for simultaneous detection of CO and H2S.

Figure 3. Structural diagram of the photoacoustic pollution gas sensor for simultaneous detection of CO and H2S. Image Credit: Yin, et al., 2021

Results

In the case of the resonant photoacoustic gas sensor, it is essential for the laser to be modulated to one of the typical frequencies, where it is viable to form standing acoustic waves in the resonator. Figure 4 illustrates that a maximum signal amplitude was realized by setting the sine frequency to 1780 Hz.

Frequency and phase response curve of the PAC within the range of 1380–2180 Hz.

Figure 4. Frequency and phase response curve of the PAC within the range of 1380–2180 Hz. Image Credit: Yin, et al., 2021

Figure 5 depicts the maximum 2f acoustic signals and quantified actual excitation powers after the PAC at various power levels. The sensor noise was maintained constantly at µV level because of the differential PAC with 0.9-mm light spot diameter and 7-mm diametral resonator.

Actual excitation powers and acoustic signals as a function of EDFA setting power in (a) 50 ppm H2S/N2 and (b) 500 ppm CO/N2.

Figure 5. Actual excitation powers and acoustic signals as a function of EDFA setting power in (a) 50 ppm H2S/N2 and (b) 500 ppm CO/N2. Image Credit: Yin, et al., 2021

Figure 6(a) illustrates that there was an increase in the maximum acoustic signal amplitude with a gas pressure increase.

(a) Acoustic signal as a function of sensor gas pressure and modulation depth. (b) 2f H2S photoacoustic signal at 300 Torr and 700 Torr ranging from 6319 to 6322 cm-1.

Figure 6. (a) Acoustic signal as a function of sensor gas pressure and modulation depth. (b) 2f H2S photoacoustic signal at 300 Torr and 700 Torr ranging from 6319 to 6322 cm−1. Image Credit: Yin, et al., 2021

Figure 7(a) illustrates that the H2S/N2 concentration gradient ranged from 0 to 7.5 ppm. The inset in Figure 7(a) shows plotting of the mean signals in 0–50 ppm H2S/N2 gas mixtures to verify the sensor response linearity under various concentrations.

Figure 7(b) shows a signal amplitude of 160.5 µV obtained from a 50-ppm CO/N2 gas mixture. A normalized noise equivalent absorption (NNEA) coefficient of 1.7 × 10−7 cm−1 W Hz−1/2 and a 342.7-ppb detection limit were obtained. The inset in Figure 7(b) shows the linearity of CO gas as indicated by the R2 of 0.9997.

Photoacoustic signal amplitude and response linearity (Inset) of the developed PAS-based gas sensor in stepwise H2S (a) and CO (b) concentration levels

Figure 7. Photoacoustic signal amplitude and response linearity (Inset) of the developed PAS-based gas sensor in stepwise H2S (a) and CO (b) concentration levels. Image Credit: Yin, et al., 2021

Various H2S sensors from near-IR, mid-IR, to THz that benefit from the wavelength independence of PAS-based gas sensors are listed in Table 1.

Table 1. Comparison of different photoacoustic H2S sensors. Source: Yin, et al., 2021

Spectral
range
Wavelength Line
strength
(cm/mol)
Power Technology Detection
limit
NNEA (cm−1
 W Hz−1/2)
Near-IR 1582 nm 1.1×10−22 1402 mW QEPAS [50] 734 ppb @ 1 s 9.8×10−9
1576 nm 1.3×10−22 1 W Fiber-optic
acoustic [15]
33 ppb @ 10 s Not
available
2639 nm 1.7×10−21 3 mW QEPAS [51] 4 ppm @ 1 s 2.4×10−9
1582 nm 1.1×10−22 9.54 W This work 31.7 ppb @ 1 s 8.4×10−8
Mid-IR 8018 nm 6.8×10−22 50 mW PAC [52] 0.3 ppm @ 30 s 1.1×10−8
8100 nm 7.8×10−22 118 mW QEPAS [38] 492 ppb @ 1 s 3.1×10−9
7893 nm 1.5×10−21 53 mW QEPAS [37] 330 ppb @ 30 s 7.3×10−9
THz 2.91 THz 1.1×10−22 0.24 mW QEPAS [53] 30 ppm @ 3 s 4.4×10−10
2.87 THz 5.5×10−20 150 mW QEPAS [39] 360 ppb @ 10 s 3.1×10−8

 

Results

Here, researchers detailed the development of a highly sensitive photoacoustic gas sensor for simultaneous detection of CO and H2S with two DFB lasers featuring the central wavelength of 1568 nm and 1582 nm. A dual-resonator PAC was developed and simulated using a finite element simulation.

An optical switch enabled the simultaneous detection of CO and H2S gas with the help of a commercial fiber amplifier. Optimization of pressure and modulation depth from 200 to 700 Torr enabled achieving the minimum detection limits of 342.7 and 31.7 ppb for CO and H2S, respectively, at ambient temperature and atmospheric pressure.

The corresponding NNEA coefficients were found to be 1.7 × 10−7 and 8.4 × 10−8 cm−1 W Hz−1/2.

The ppb-level detection limitation was analogous to the sensor designed with a mid-IR laser source, and even more for H2S by an order of magnitude. More pollutant gases can be detected by using a frequency division multiplexing method because the PAC features a higher resonance frequency.

Journal Reference:

Yin, X., Gao, M., Miao, R., Zhang, L., Zhang, X., Liu, L., Shao, X., Tittel, F. K. (2021) Near-infrared laser photoacoustic gas sensor for simultaneous detection of CO and H2S. Optics Express, 29(21), pp. 34258–34268. Available online: https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-29-21-34258&id=460206.

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Megan Craig

Written by

Megan Craig

Megan graduated from The University of Manchester with a B.Sc. in Genetics, and decided to pursue an M.Sc. in Science and Health Communication due to her passion for combining science with content creation. As part of her studies, Megan partnered with Jodrell Bank Discovery Centre as a Digital Marketing Assistant, producing content and updating sections of their website. In her spare time, she loves to travel, exploring each location's culture and history - including the local cuisine. Her other interests include embroidery, reading fiction, and practicing her Japanese language skills.

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