Evanescent wave quartz-enhanced photoacoustic spectroscopy employing a side-polished fiber for methane sensing

Recent advances in optical fiber-based gas sensors utilizing light-induced acoustic/elastic techniques

Gas sensing detects gas properties, such as physical, molecular, optical, thermodynamic, and dynamic properties. Light-induced acoustic techniques include monitoring the optical and physical properties of the gas. Fiber-based gas sensing is important because it offers several unique advantages compared to traditional gas sensing technologies, such as high sensitivity and accuracy, a compact and lightweight design, remote sensing capabilities, multiplexing, and distributed sensing. We review the recent developments in optical fiber-based gas sensors utilizing light-induced acoustic/elastic techniques based on photoacoustic spectroscopy, Brillouin scattering, and light-induced thermoelastic spectroscopy (LITES).

Rapid ppb-Level Methane Detection Based on Quartz-Enhanced Photoacoustic Spectroscopy

In the paper, quartz-enhanced photoacoustic spectroscopy (QEPAS) and heterodyne quartz-enhanced photoacoustic spectroscopy (H-QEPAS)-based ppb-level methane (CH4) detection using a self-designed low-frequency round-head quartz tuning fork (QTF) and power-amplified diode laser is reported for the first time. Compared to the standard 32.768 kHz QTF, the novel round-head QTF, with a resonance frequency (f0) of 9.7 kHz, is utilized as the acoustic wave transducer, benefiting from a longer energy accumulation time and reduced optical noise. A Raman fiber amplifier (RFA) is adopted to amplify the optical power of the continuous wavelength distributed feedback (CW-DFB) diode laser to 300 mW. Acoustic microresonators (AmRs) at specific sizes are on both sides of the QTF for enhancement of acoustic waves. It is observed that, after the installation of AmRs, the signal level is enhanced by a factor of 107.029 compared to the bare QTF. Both CH4-QEPAS and CH4-H-QEPAS sensors show excellent linearity in response to optical power and CH4 concentration, with R-squared values exceeding 0.99 for each. The minimum detection limit (MDL) is determined to be 1.321 and 2.126 ppb for CH4-QEPAS and CH4-H-QEPAS, respectively, when the integration time of the sensor systems is extended to 1000 s. Compared to the 50 s measurement period of the CH4-QEPAS sensor, CH4-H-QEPAS can identify the f0 of QTF and finish the measurement in 3 s, demonstrating its rapid measurement capability. Furthermore, H-QEPAS technology allows for the acquisition of the f0 without interrupting the measurement, enabling real-time calibration of the f0. Finally, the sensor is utilized for continuous monitoring of CH4 concentrations in air and human-exhaled gases, demonstrating its practical measurement capabilities.

Compact step-added T-type PAC for enhanced hydrogen detection: A photoacoustic frequency shift approach

In this study, we introduce an innovative photoacoustic frequency shift (PAFS) technique for hydrogen (H2) detection, complemented by both theoretical models and practical experiments. To mitigate cross-sensitivity, we analyzed the sound speeds of six different gases, confirming minimal interference with H2 due to significant velocity disparities. Central to our approach is the design of a miniaturized step-added T-type Photoacoustic Cell (PAC), with parameters meticulously optimized for enhanced performance. Using COMSOL Multiphysics' Thermal Viscous Acoustics module, we conducted simulations to evaluate the quality factor and acoustic pressure, both crucial for the sensor's efficiency. Additionally, we assessed the system's stability, influenced by gas flow, through gas velocity distribution analyses using the Computational Fluid Dynamics module. Experimental investigations focused on the system's sensing performance, revealing a distinct frequency shift of ∼45 Hz for every 1 % change in H2 concentration, with a high linear correlation (R2 = 0.99825). The system's response and recovery times were measured at 1.09 s and 1.25 s, respectively. Long-term stability, evaluated over 3000 s using Allan deviation, indicated a minimum detection limit (MDL) of 102.47 ppm at an integration time of 375 s. These findings validate the efficacy of the step-added T-type PAC in H2 detection.

A Review on Photoacoustic Spectroscopy Techniques for Gas Sensing

The rapid growth of industry and the global drive for modernization have led to an increase in gas emissions, which present significant environmental and health risks. As a result, there is a growing need for precise and sensitive gas-monitoring technologies. This review delves into the progress made regarding photoacoustic gas sensors, with a specific focus on the vital components of acoustic cells and acoustic detectors. This review highlights photoacoustic spectroscopy (PAS) as an optical detection technique, lauding its high sensitivity, selectivity, and capability to detect a wide range of gaseous species. The principles of photoacoustic gas sensors are outlined, emphasizing the use of modulated light absorption to generate heat and subsequently detect gas pressure as acoustic pressure. Additionally, this review provides an overview of recent advancements in photoacoustic gas sensor components while also discussing the applications, challenges, and limitations of these sensors. It also includes a comparative analysis of photoacoustic gas sensors and other types of gas sensors, along with potential future research directions and opportunities. The main aim of this review is to advance the understanding and development of photoacoustic gas detection technology.

Micro-Ring Resonator Assisted Photothermal Spectroscopy of Water Vapor

We demonstrated, for the first time, micro-ring resonator assisted photothermal spectroscopy measurement of a gas phase sample. The experiment used a telecoms wavelength probe laser that was coupled to a silicon nitride photonic integrated circuit using a fibre array. We excited the photothermal effect in the water vapor above the micro-ring using a 1395 nm diode laser. We measured the 1f and 2f wavelength modulation response versus excitation laser wavelength and verified the power scaling behaviour of the signal.