Intracavity photoacoustic sensing of water vapor with a continuously tunable external-cavity quantum-cascade laser operating near 5.5 μm

Parts-per-billion-level detection of hydrogen sulfide based on doubly resonant photoacoustic spectroscopy with line-locking

We report on the development of a highly sensitive hydrogen sulfide (H₂S) gas sensor exploiting the doubly resonant photoacoustic spectroscopy technique and using a near-infrared laser emitting at 1578.128 nm. By targeting the R(4) transition of H₂S, we achieved a minimum detection limit of 10 part per billion in concentration and a normalized noise equivalent absorption coefficient of 8.9 × 10^-12 W cm^-1 Hz^-1/2. A laser-cavity-molecule locking strategy is proposed to enhance the sensor stability for fast measurement when dealing with external disturbances. A comparison among the state-of-the-art H₂S sensors using various spectroscopic techniques confirmed the record sensitivity achieved in this work.

Photoacoustic-Based Gas Sensing: A Review

The use of the photoacoustic effect to gauge the concentration of gases is an attractive alternative in the realm of optical detection methods. Even though the effect has been applied for gas sensing for almost a century, its potential for ultra-sensitive and miniaturized devices is still not fully explored. This review article revisits two fundamentally different setups commonly used to build photoacoustic-based gas sensors and presents some distinguished results in terms of sensitivity, ultra-low detection limits, and miniaturization. The review contrasts the two setups in terms of the respective possibilities to tune the selectivity, sensitivity, and potential for miniaturization.

Highly sensitive acetylene detection based on multi-pass retro-reflection-cavity-enhanced photoacoustic spectroscopy and a fiber amplified diode laser

In this paper, a multi-pass retro-reflection-cavity-enhanced photoacoustic spectroscopy (PAS) based gas sensor is reported for the first time. The multi-pass retro-reflection-cavity consisted of two right-angle prisms and was designed to reflect the laser beam to pass through the photoacoustic (PA) cell four times, which improved the acetylene (C₂H₂)-PAS sensor signal level significantly. The optical power of a near-infrared distributed feedback (DFB) diode laser emitting a continuous wave (CW) was amplified to 1000 mW with an erbium-doped fiber amplifier. The background noise was reduced with wavelength modulation spectroscopy (WMS) and 2nd harmonic demodulation techniques. The linear optical power and concentration response of such a PAS sensor were investigated, and the experimental results showed excellent characteristics. When the integration the time of the sensor system was set to 1 s, the minimum detection limit (MDL) for C₂H₂ detection was 8.17 ppb, which corresponds to a normalized noise equivalent absorption coefficient (NNEA) of 1.84 × 10^-8 cm^-1W/√Hz. The long-term stability of such a multi-pass retro-reflection-cavity-enhanced PAS based C₂H₂ sensor was evaluated by an Allan deviation analysis. It was demonstrated that the multi-pass retro-reflection-cavity-enhanced PAS sensor had an excellent stability. An MDL of 600 ppt was achieved when the integration time was set to ~1000 s. It was verified that the method of multi-pass retro-reflection-cavity-enhanced PAS with an amplified laser source improved the sensor performance significantly. If an appropriate cavity design with increasing reflection times is used, the MDL of such a PAS-based sensor can be further improved.

Mid-infrared sensing of CO at saturated absorption conditions using intracavity quartz-enhanced photoacoustic spectroscopy

The sensitivity of quartz-enhanced photoacoustic spectroscopy (QEPAS) can be drastically increased using the power enhancement in high-finesse cavities. Here, low noise resonant power enhancement to 6.3 W was achieved in a linear Brewster window cavity by exploiting optical feedback locking of a quantum cascade laser. The high intracavity intensity of up to 73 W mm^-2 in between the prongs of a custom tuning fork resulted in strong optical saturation of CO at 4.59 µm. Saturated absorption is discussed theoretically and experimentally for photoacoustic measurements in general and intracavity QEPAS (I-QEPAS) in particular. The saturation intensity of CO's R9 transition was retrieved from power-dependent I-QEPAS signals. This allowed for sensing CO independently from varying degrees of saturation caused by absorption induced changes of intracavity power. Figures of merit of the I-QEPAS setup for sensing of CO and H₂O are compared to standard wavelength modulation QEPAS without cavity enhancement. For H₂O, the sensitivity was increased by a factor of 230, practically identical to the power enhancement, while the sensitivity gain for CO detection was limited to 57 by optical saturation.

Random Multiple Scattering Enhanced Photoacoustic Gas Spectroscopy with Disordered Porous Ceramics

Light-gas interaction can be enhanced by using disordered porous materials because multiple random scattering increases light intensity near the surface of the material. Here we report signal enhancement of photoacoustic gas spectroscopy with disordered porous ceramics. The amplitude and frequency characteristics of photoacoustic signal due to gas absorption in disordered materials are modeled theoretically. Experiment with a porous Al₂O₃ sample demonstrates photoacoustic signal enhancement of ∼4 times at 5 kHz.

Fiber-ring laser-based intracavity photoacoustic spectroscopy for trace gas sensing

We demonstrated a novel trace gas sensing method based on fiber-ring laser intracavity photoacoustic spectroscopy. This spectroscopic technique is a merging of photoacoustic spectroscopy (PAS) with a fiber-ring cavity for sensitive and all-fiber gas detection. A transmission-type PAS gas cell (resonant frequency f₀=2.68  kHz) was placed inside the fiber-ring laser to fully utilize the intracavity laser power. The PAS signal was excited by modulating the laser wavelength at f₀/2 using a custom-made fiber Bragg grating-based modulator. We used this spectroscopic technique to detect acetylene (C₂H₂) at 1531.6 nm as a proof of principle. With a low Q-factor (4.9) of the PAS cell, our sensor achieved a good linear response (R²=0.996) to C₂H₂ concentration and a minimum detection limit of 390 ppbv at 2-s response time.