Sensitivity enhanced NIR photoacoustic CO detection with SF6 promoting vibrational to translational relaxation process

Photoacoustic trace gas detection of OCS using a 2.45 mL Helmholtz resonator and a 4823.3 nm ICL light source

A miniaturized photoacoustic spectroscopy-based gas sensor is proposed for the purpose of detecting sub-ppm-level carbonyl sulfide (OCS) using a tunable mid-infrared interband cascade laser (ICL) and a Helmholtz photoacoustic cell. The tuning characteristics of the tunable ICL with a center wavelength of 4823.3 nm were investigated to achieve the optimal driving parameters. A Helmholtz photoacoustic cell with a volume of ∼2.45 mL was designed and optimized to miniaturize the measurement system. By optimizing the modulation parameters and signal processing, the system was verified to have a good linear response to OCS concentration. With a lock-in amplifier integration time of 10 s, the 1σ noise standard deviation in differential mode was 0.84 mV and a minimum detection limit (MDL) of 409.2 ppbV was achieved at atmospheric pressure and room temperature.

High-sensitivity trace gas detection based on differential Helmholtz photoacoustic cell with dense spot pattern

A high-sensitivity photoacoustic spectroscopy (PAS) sensor based on differential Helmholtz photoacoustic cell (DHPAC) with dense spot pattern is reported in this paper for the first time. A multi-pass cell based on two concave mirrors was designed to achieve a dense spot pattern, which realized 212 times excitation of incident laser. A finite element analysis was utilized to simulate the sound field distribution and frequency response of the designed DHPAC. An erbium-doped fiber amplifier (EDFA) was employed to amplify the output optical power of the laser to achieve strong excitation. In order to assess the designed sensor's performance, an acetylene (C2H2) detection system was established using a near infrared diode laser with a central wavelength 1530.3 nm. According to experimental results, the differential characteristics of DHPAC was verified. Compared to the sensor without dense spot pattern, the photoacoustic signal with dense spot pattern had a 44.73 times improvement. The minimum detection limit (MDL) of the designed C2H2-PAS sensor can be improved to 5 ppb when the average time of the sensor system is 200 s.

High-Sensitivity Differential Helmholtz Photoacoustic System Combined with the Herriott Multipass Cell and Its Application in Seed Respiration

A highly sensitive photoacoustic detection system using a differential Helmholtz resonator (DHR) combined with a Herriott multipass cell is presented, and its implementation to sub-ppm level carbon dioxide (CO2) detection is demonstrated. Through the utilization of erbium-doped optical fiber amplifier (EDFA), the laser power was amplified to 150 mW. Within the multipass cell, a total of 22 reflections occurred, contributing to an impressive 33.6 times improvement in the system sensitivity. The normalized noise equivalent absorption coefficient (NNEA) was 8.64 × 10-11 cm-1·W·Hz-1/2 [signal-to-noise ratio, (SNR) = 1] and according to the Allan variance analysis, a minimum detection limit of 500 ppb could be achieved for CO2 at 1204 s, which demonstrates the long-term stability of the system. The system was applied to detect the respiration of rice and upland rice seeds. It is demonstrated that the system can monitor and distinguish the respiration intensity and respiration rate of different seeds in real time.

Fiber-Optic Photoacoustic CO Sensor for Gas Insulation Equipment Monitoring Based on Cantilever Differential Lock-In Amplification and Optical Excitation Enhancement

A fiber-optic photoacoustic CO sensor for gas insulation equipment is proposed, which relies on F-P interferometric cantilever-based differential lock-in amplification and optical multipass excitation enhancement. The sensor has excellent characteristics of high sensitivity, antielectromagnetic interference, fast response, and long-distance detection. The photoacoustic pressure waves in the two resonators of the differential photoacoustic cell (DPAC) are simultaneously detected by two fiber-optic interferometric cantilevers and processed differentially; thereby, the gas flow noise is effectively suppressed. Based on the comprehensive analysis of the superposition of photoacoustic excitation and multipass absorption, the diameter of the resonator is determined to be 6 mm. The optical power emitted by the 1566.6 nm distributed feedback laser is increased to 500 mW by an erbium-doped fiber amplifier. The near-infrared light is reflected 30 times in the multipass cell, which improves the order of magnitude of optical effective excitation. Due to the low sound velocity of SF6 gas, the resonant frequency of the DPAC with a resonator length of 80 mm is 760 Hz. The response time to CO/SF6 gas is 93 s with a flow rate of 500 sccm. The detection limit of the CO sensor is 53 ppb, which realizes the accurate and timely perception of the SF6 decomposition derivative CO and provides technical support for trouble-free operation of gas insulation equipment.

Trace gas sensor based on a multi-pass-retro-reflection-enhanced differential Helmholtz photoacoustic cell and a power amplified diode laser

A high-sensitive photoacoustic spectroscopy (PAS) sensor, which is based on a multi-pass-retro-reflection-enhanced differential Helmholtz photoacoustic cell (DHPAC) and a high power diode laser amplified by erbium-doped fiber amplifier (EDFA), is presented in this work for the first time. In order to improve the interaction length between the light and target gas, the incident light was reflected four times through a multi-pass-retro-reflection-cell constructed by two right-angle prisms. A 1.53 µm distributed feedback (DFB) diode laser was selected to excite photoacoustic signal. Moreover, its power was amplified by an EDFA to 1000 mW to improve the amplitude of photoacoustic signal. Acetylene (C2H2) was chosen as the target analysis to verify the reported sensor performance. Compared to double channel without multiple reflections, the 2f signal of double channel with four reflections was improved by 3.71 times. In addition, when the output optical power of EDFA was 1000 mW, the 2f signal has a 70.57-fold improvement compared with the multi-pass-retro-reflection-cell without EDFA. An Allan deviation analysis was carried out to evaluate the long-term stability of such PAS sensor. When the averaging time was 400 s, the minimum detection limit (MDL) of such PAS sensor was 14 ppb.

Sensitivity-enhanced Fabry-Perot interferometric fiber-optic microphone using hollow cantilever

Transducer components are crucial in optimizing the sensitivity of microphones. Cantilever structure is commonly used as a structural optimization technique. Here, we present a novel Fabry-Perot (F-P) interferometric fiber-optic microphone (FOM) using a hollow cantilever structure. The proposed hollow cantilever aims to reduce the effective mass and spring constant of the cantilever, thereby enhancing the sensitivity of the FOM. Experimental results demonstrate that the proposed structure outperforms the original cantilever design in terms of sensitivity. The sensitivity and minimum detectable acoustic pressure level (MDP) can reach 91.40 mV/Pa and 6.20 µPa/Hz at 1.7 kHz, respectively. Notably, the hollow cantilever provides an optimization framework for highly sensitive FOMs.

Multi-pass Differential Photoacoustic Sensor for Real-Time Measurement of SF6 Decomposition Component H2S at the ppb Level

We designed and implemented a photoacoustic (PA) sensor for H₂S detection in SF₆ background gas based on a multi-pass differential photoacoustic cell (MDPC) and a near-infrared distributed feedback (DFB) laser. In the MDPC apparatus, two resonators with identical geometric parameters were vertically and symmetrically embedded. The differential processing algorithm of two phase-reversed signals realized the effective enhancement of the PA signal and suppressed the flow noise in the dynamic sampling process. In addition, the λ/4 buffer chamber in the MDPC was utilized as a muffler to further reduce the flow noise and realize the dynamic detection of H₂S. The collimated excitation light was reflected 30 times in a multi-pass structure constituted of two gold-plated concave mirrors, and an absorption path length of 4.92 m was achieved. Due to the high gas density of SF₆, the relationship between the signal-to-noise ratio (SNR) and the gas flow was different between SF₆ and N₂ background gases. The maximum flow rate of the characteristic gas components detected in the SF₆ background is 150 standard cubic centimeters per minute (SCCM), which is lower than 350 SCCM in N₂. The linearity property was analyzed, and the results show that the sensitivity of the sensor to H₂S in the SF₆ background was 27.3 μV/ppm. With the structure, parameters, temperature, gas flow, and natural frequency of the MDPC been optimized, a minimum detection limit (MDL) of 11 ppb was reached with an averaging time of 1000 s, which furnished an effective preventive implement for the safe operation of gas insulation equipment.

Near-infrared sensitive differential Helmholtz-based hydrogen sulfide photoacoustic sensors

A near-infrared (NIR) sub-ppm level photoacoustic sensor for hydrogen sulfide (H₂S) using a differential Helmholtz resonator (DHR) as the photoacoustic cell (PAC) was presented. The core detection system was composed of a NIR diode laser with a center wavelength of 1578.13 nm, an Erbium-doped optical fiber amplifier (EDFA) with an output power of ∼120 mW, and a DHR. Finite element simulation software was used to analyze the influence of the DHR parameters on the resonant frequency and acoustic pressure distribution of the system. Through simulation and comparison, the volume of the DHR was 1/16 that of the conventional H-type PAC for a similar resonant frequency. The performance of the photoacoustic sensor was evaluated after optimizing the DHR structure and modulation frequency. The experimental results showed that the sensor had an excellent linear response to the gas concentration and the minimum detection limit (MDL) for H₂S detection in differential mode can reach 460.8 ppb.