In an article published in Chemosensors, researchers have proposed a novel continuous-wave distributed feedback (CW-DFB) laser diode for hydrogen (H2) detection.
Study: Highly Sensitive Hydrogen Sensing Based on Tunable Diode Laser Absorption Spectroscopy with a 2.1 mm Diode Laser. Image Credit: Kelly Marken/Shutterstock.com
The device employs a Herriott multipass gas cell (HMPC) with an optical length of 10.13 m and a tunable diode laser absorption spectroscopy (TDLAS) sensor. The TDLAS sensor utilizes wavelength modulation spectroscopy (WMS) and the Daubechies (DB) wavelet denoising methods to accomplish sensitive hydrogen detection.
The wavelength modulation spectroscopy (WMS) method and the second harmonic (2ƒ) demodulation approach reduces system noise and optimizes data processing.
When the laser wavelength modulation depth reached the ideal value of 0.016 cm-1, the 2ƒ signal of the H2-tunable diode laser absorption spectroscopy (TDLAS) sensor was evaluated for various H2 concentrations. Daubechies (DB) wavelet denoising was used to enhance the system's minimum detection limit (MDL) and signal-to-noise ratio (SNR), which resulted in 10 vanishing moments.
In recent decades, there has been much discussion concerning hydrogen detection. H2 is an innovative source of energy with clean and environmentally friendly features. However, research into hydrogen detection is restricted due to the absence of adequate laser sources and the poor optical absorption of H2. The tunable diode laser absorption spectroscopy (TDLAS) technique proposed in the present work can significantly increase sensitive hydrogen detection.
Improving the Commercial Viability of Hydrogen
Global energy consumption has expanded tremendously due to the modern economy's expeditious expansion and the accelerated growth of the world population. Natural gas and oil as sources of energy have been over-exploited, and the excessive use of these fossil fuels has, in turn, increased carbon dioxide emissions and aggravated the greenhouse effect. Most nations now view using novel clean and renewable energy sources as a critical strategic objective.
H2 is one such source with optimistic prospects as a potential energy source as it is clean, ecological, pollution-free, abundant, and has remarkable combustion efficiency. However, H2 is a highly explosive and flammable gas.
The manufacturing, transportation, and storage of H2 pose significant dangers since it might cause an explosion upon contact with naked fire. Therefore, to eliminate the potential safety risks brought on by H2 leakage, real-time monitoring of H2 concentration levels in the air is essential.
Numerous hydrogen detection techniques have been rapidly evolving in recent years. While using electrochemical H2 detectors, the H2 concentration can be determined by observing the device's electromotive force or current. However, the applicability of these approaches is restricted by their subpar detection performance and long-term stability.
Standard technologies used in optical gas sensing include:
- Tunable diode laser absorption spectroscopy (TDLAS)
- Light-induced thermoelastic spectroscopy (LITES)
- Quartz-enhanced photoacoustic spectroscopy (QEPAS).
Due to benefits such as in situ detection, excellent selectivity, non-contact measurement, rapid response, low price, and multi-parameter, multi-component measurement, the tuneable diode laser absorption spectroscopy (TDLAS) technology is frequently utilized in the detection of numerous gases, including hydrogen.
The present paper demonstrated a tuneable diode laser absorption spectroscopy (TDLAS)-based hydrogen detection sensor. The strongest H2 absorption line at 2121.83 nm was covered using a continuous-wave distributed feedback (CW-DFB) diode laser. The optical absorption path of H2 in the gas chamber was extended to laser energy using a 10.13 m Herriott multipass gas cell (HMPC), which increased the signal amplitude because the H2 absorption line had a low line strength.
The second harmonic (2ƒ) demodulation and the wavelength modulation spectroscopy (WMS) techniques were employed to minimize the noise level and enhance the hydrogen detection performance. Signal denoising was accomplished using the Daubechies (DB) wavelet denoising.
Experimental Setup to Achieve Sensitive Hydrogen Detection
As a spectroscopic technology, tuneable diode laser absorption spectroscopy (TDLAS) is severely impacted by laser sources. A single mode laser can be produced via a distributed feedback (DFB) laser with an integrated Bragg grating. The broad gas detection range of the DFB laser results from its superior monochromaticity and side-mode suppression ratio. Therefore, a DFB diode laser was used in the current paper.
The Herriott multipass gas cell (HMPC) has distinct reflection points that do not overlap, preventing interference fringes from being produced. The tuneable diode laser absorption spectroscopy (TDLAS) technology extensively utilized the Herriott multipass gas cell (HMPC) for sensitive hydrogen detection.
Pure nitrogen (N2) and H2 with a concentration of 100% were sent into the gas dilution system, respectively, to assess the response performance of the wavelength modulation spectroscopy (WMS) and Herriott multipass gas cell (HMPC)-based H2-TDLAS sensing method. Mixed gases with various H2 concentrations were then obtained by varying the flow rate of two gas flowmeters in the dilution system.
The findings of the 2ƒ H2-TDLAS signal analysis revealed the existence of noise. Therefore, the Daubechies (DB) wavelet denoising technique was used to enhance the sensor's performance further. For a six times denoising of a 2ƒ signal, a Daubechies (DB) wavelet denoising with 10 vanishing moments was used. When the H2 concentration was 100%, the 2ƒ signal was observed and recorded, both with and without Daubechies (DB) wavelet denoising.
The data in the 2ƒ signal beyond the absorption peak were considered system noise. While the 2ƒ signal amplitude was nearly equivalent, the 1σ noise variance with the Daubechies (DB) wavelet denoising was reduced by 50% compared to the original 2ƒ signal.
The findings of the current work demonstrated that the wavelength modulation spectroscopy (WMS) and Herriott multipass gas cell (HMPC)-based H2-TDLAS sensing system accomplished significantly better sensitive hydrogen detection at weak line strength of H2 absorption.
Novel Laser Makes Sensitive Hydrogen Detection a Reality
H2 detection has become a prominent research topic in recent years due to its clean and environmentally friendly qualities as a novel energy source. The current work presented a tunable diode laser absorption spectroscopy (TDLAS)-based H2 sensor, which employed a CW-DFB laser diode, the wavelength modulation spectroscopy (WMS) technique, and a Herriott multipass gas cell (HMPC) for highly sensitive hydrogen detection.
The strength of H2's absorption was increased using a Herriott multipass gas cell (HMPC) with an optical length of 10.13 m. The TDLAS signal was modulated and demodulated using the wavelength modulation spectroscopy (WMS) approach and the second harmonic detection. A strong H2 absorption line at 4712.90 cm-1 generated a high H2-TDLAS signal level.
The wavelength modulation spectroscopy (WMS) and Herriott multipass gas cell (HMPC)-based H2-TDLAS sensing system's optimal modulation depth was 0.016 cm-1.
The present study's findings showed that the chosen H2 absorption line width was exceptionally narrow and, therefore, not easily vulnerable to interference from other gases. Using the Daubechies (DB) wavelet denoising, the resultant H2-TDLAS 2ƒ signal was further denoised.
The researchers believe that the novel wavelength modulation spectroscopy (WMS) and Herriott multipass gas cell (HMPC)-based H2 tunable diode laser absorption spectroscopy (TDLAS) sensor proposed in the paper will make H2 a dominant renewable energy source in the future.
Reference
T. Liang, S. Qiao, X. Liu, Y. Ma, Highly Sensitive Hydrogen Sensing Based on Tunable Diode Laser Absorption Spectroscopy with a 2.1 mm Diode Laser. 2022. Chemosensors. https://www.mdpi.com/2227-9040/10/8/321/htm
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