Assessment of vibrational-translational relaxation dynamics of in a wet-nitrogen matrix through QEPAS

A high sensitive methane QEPAS sensor based on self-designed trapezoidal-head quartz tuning fork and high power diode laser

A high sensitive methane (CH4) sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) using self-designed trapezoidal-head quartz tuning fork (QTF) and high power diode laser is reported for the first time in this paper. The trapezoidal-head QTF with low resonant frequency (f 0 ) of ∼ 9 kHz, serves as the detection element, enabling longer energy accumulation times. A diode laser with an output power of 10 mW is utilized as the excitation source. A Raman fiber amplifier (RFA) is employed to boost the diode laser power to 300 mW to increase the excitation intensity. Acoustic micro-resonators (AmRs) are designed and placed on both sides of the QTF to form an acoustic standing wave cavity, which increases the acoustic wave intensity and enhances the vibration amplitude of the QTF. Additionally, the long-term stability is analyzed by Allan deviation analysis. When the average time of the sensor system is increased to 150 s, the minimum detection limit (MDL) of the CH4-QEPAS sensor system can be improved to 15.5 ppb.

Improved T-shaped quartz tuning fork with isosceles-trapezoidal grooves optimized for quartz-enhanced photoacoustic spectroscopy

The quartz tuning fork (QTF) being the acoustic-electrical conversion element for quartz-enhanced photoacoustic spectroscopy (QEPAS) system directly affects the detection sensitivity. However, the low electromechanical conversion efficiency characteristic of standard QTF limits the further enhancement of the system. Therefore, the optimized design for QTF is becoming an important approach to improve the performance of QEPAS. In this work, 9 kHz T-shaped QTFs with isosceles-trapezoidal grooves are firstly applied to gas sensing experiments. Four types of 9 kHz QTFs are fabricated and applied to gas detection experiments. Simulation results reveal QTFs with isosceles-trapezoidal grooves are conducive to optimizing the stress distribution and enhancing electromechanical conversion efficiency. The results of the gas sensing experiment (acetylene C2H2) indicate that the signal peak and signal-to-noise ratio values of T-shaped QTF with positive isosceles-trapezoidal grooves can reach 1.44 and 1.85 times greater than the normal QTF with rectangular cross-section prongs.

Noncontact Fiber-Optic Cantilever-Enhanced Photoacoustic Spectroscopy

Conventional cantilever-enhanced photoacoustic spectroscopy (CEPAS) usually requires the cantilever to be in contact with the gas, which limits its application in dusty and corrosive gas sensing. To overcome this challenge, this paper proposes noncontact fiber-optic cantilever-enhanced photoacoustic spectroscopy (NCFO-CEPAS) for high-sensitivity trace gas analysis. A polyethylene film with corrosion-resistant properties is affixed to the end of the cantilever acoustic sensor, and the effect of the distance between the polyethylene film and the cantilever acoustic sensor on the sensitivity of the system is analyzed by finite element analysis. Compared to the conventional CEPAS, NCFO-CEPAS has a lower background noise deviation. Besides, the potential damage to the silicon cantilever beam by corrosive gases can be avoided as the gas to be measured is not in contact with the silicon cantilever beam, and the gap of the cantilever beam will not be blocked in a high dust environment. Experimental results show that the detection limit of C2H2 gas is 0.5 ppm, and the normalized noise equivalent absorption coefficient reaches 5.93 × 10-9 cm-1 W Hz-1/2. The NCFO-CEPAS technique has the advantages of noncontact remote gas monitoring, not being corroded by gases, and high resistance to electromagnetic interference.

A mini-resonant photoacoustic sensor based on a sphere-cylinder coupled acoustic resonator for high-sensitivity trace gas sensing

This paper reports a mini-resonant photoacoustic sensor for high-sensitivity trace gas sensing. The sensor primarily contains a sphere-cylinder coupled acoustic resonator, a cylindrical buffer chamber, and a fiber-optic acoustic sensor. The acoustic field distributions of this mini-resonant photoacoustic sensor and the conventional T-type resonant photoacoustic sensor have been carefully evaluated, showing that the first-order resonance frequency of the present mini-resonant photoacoustic sensor is reduced by nearly a half compared to that of the T-type resonant photoacoustic sensor. The volume of the developed photoacoustic cavity is only about 0.8 cm3. Trace methane is selected as the target analytical gas and a detection limit of 101 parts-per-billion at 100-s integration time has been achieved, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 1.04 × 10-8 W·cm-1·Hz-1/2. The developed mini-resonant photoacoustic sensor provides potential for high-sensitivity trace gas sensing in narrow spaces.

Dense Multibutterfly Spots-Enhanced Miniaturized Optical Fiber Photoacoustic Gas Sensor

A miniaturized optical fiber photoacoustic gas sensor enhanced by dense multibutterfly spots is reported for the first time. The principle of space light transmission of neglecting paraxial approximation is theoretically analyzed for designing a dense multibutterfly spots-based miniature multipass cell. In a multipass photoacoustic tube with a diameter of 16 mm, the light beam is reflected about a hundred times. The light spots on the mirror surfaces at both ends of the photoacoustic tube form a dense multibutterfly distribution. The volume of the micro multipass gas chamber is only 5.3 mL. An optical fiber cantilever based on F-P interference is utilized as a photoacoustic pressure detector. Compared with that of the single-pass structure, the gas detection ability of the photoacoustic system with dense multibutterfly spots is improved by about 50 times. The proposed miniaturized sensor realizes a detection limit of 3.4 ppb for C2H2 gas with an averaging time of 100 s. The recognized coefficients of minimum detectable absorption (αmin) and normalized noise equivalent absorption are 1.9 × 10-8 cm-1 and 8.4 × 10-10 W cm-1 Hz-1/2, respectively.

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.

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

We present an all-fiber-based laser gas analyzer (LGA) employing quartz-enhanced photoacoustic spectroscopy (QEPAS) and a side-polished fiber (SPF). The LGA comprises a custom quartz tuning fork (QTF) with 0.8 mm prong spacing, two acoustic micro-resonators (mR) located on either side of the prong spacing, and a single-mode fiber containing a 17 mm polished section passing through both mRs and QTF. The SPF polished face is positioned to enable the evanescent wave (EW) to create a photoacoustic wave and excite the fundamental flexural mode of the QTF. Sensor performance was demonstrated using methane in nitrogen gas mixtures, with CH4 mixing ratios ranging from 75 ppmv to 1% (by volume), measured with an accumulation time of 300 ms, and a minimum detection limit of 34 ppmv subsequently determined. The EW-QEPAS sensor is ideal for miniaturization, as it does not contain any free-space optics and is suitable for gas sensing in harsh environments and where mobility is required.