NO trace gas sensor based on quartz-enhanced photoacoustic spectroscopy and external cavity quantum cascade laser

A gas sensor based on quartz-enhanced photoacoustic detection and an external cavity quantum cascade laser was realized and characterized for trace nitric oxide monitoring using the NO R(6.5) absorption doublet at 1900.075 cm⁻¹. Signal and noise dependence on gas pressure were studied to optimize sensor performance. The NO concentration resulting in a noise-equivalent signal was found to be 15 parts per billion by volume, with 100 mW optical excitation power and a data acquisition time of 5 s.     

Stimulated Raman rotational photoacoustic spectroscopy using a quartz tuning fork and femtosecond excitation

Molecular alignment of linear molecules (O₂, N₂, CO₂ and CO) is measured photoacoustically in the gas phase. The rotational excitation is accomplished using a simple femtosecond stimulated Raman excitation scheme, employing two femtosecond pulses with variable delay between the pulses. Molecular alignment is determined directly by measuring the energy dumped into the gas by quartz-enhanced photoacoustic spectroscopy (QEPAS), utilizing a quartz tuning fork as a sensitive photoacoustic transducer. The experimental results demonstrate for the first time the use of a tuning fork for resonant photoacoustic detection of Raman spectra excited by femtosecond double pulses and match both simulation and literature values.     

光声光谱气体检测系统中气体湿度的影响研究

工作环境是光声光谱气体检测系统在工业现场应用的重要影响因素.实验发现,待测气体湿度对电容式微音器灵敏度影响显著,导致现有光声光谱气体榆测系统测试结果漂移.文章提出一种气体湿度影响消除方法,在光声腔中安装扬声器,以扬声器信号幅值作为声感应器件灵敏度的自适应表征,对光声信号幅值作自行修正,有效克服电容式微音器声信号榆测中灵...     

Resonant photoacoustic detection of trace gas with DFB diode laser

A resonant photoacoustic detection system based on a low-power distributed feedback diode laser is developed. This sensor has been applied to the detection of acetylene (C₂H₂) using a specifically designed photoacoustic cell operating on its second longitudinal mode. The minimum detectable limit of about 10 parts-per-million volume (SNR=1) is achieved with an average laser power of 3.5 mW at atmospheric pressure, and an integration time constant of 3 ms; thus, the minimum detectable absorption coefficient normalised by power and bandwidth is 4.0×10⁻⁸ W cm⁻¹/√Hz. The optimum operating pressure buffered with N₂ is also investigated. The realisation of our system is described and experimental results are compared with different modulation techniques and other results reported in the literature. A number of issues arising from the conventional use of mechanical chopping of the beam can be effectively suppressed in wavelength modulation PA spectroscopy (WM-PAS) and second harmonic detection.     

光声光谱技术在多组分气体浓度探测中的应用

CO和CH_4气体作为判断变压器运行状态的故障气体,对其浓度的探测在变压器维护中具有重要意义.为了准确探测变压器运行过程中产生的CH_4和CO气体浓度,本文利用光声光谱技术,设计了一套基于宽带光源的多组分气体探测系统,和共振型光声系统相比,该系统中所用的非共振型光声池体积小,易加工,池内各处信号强度相同,降低了对声学信号探测器的安装要求.系统的性能通过对CO和CH_4气体的探测进行评估.首先,从理论上分析了信号强度与调制频率呈反比,然后根据宽带光声系统在不同调制频率下的响应,确定系统的最佳调制频率为22 Hz.在最佳调制频率下,根据温度与待测气体光声信号的关系,对光声信号进行温度补偿,消除温度变化对光声信号的影响,进一步提高了系统的稳定性.最后,通过不同浓度的CH_4和CO气体对系统进行标定.实验表明,温度补偿前后,光声信号随温度的漂移分别为0.023 23V/℃和8.383 48×10~(-5) V/℃,通过对不同浓度CH_4和CO气体的探测,系统的线性度分别达到0.995和0.998 4.在一个大气压下,积分时间为1s时,宽带光声探测系统对CO和CH_4气体的探测极限浓度能够达到1μL/L.该系统成本低,线性度好,探测灵敏度符合国标对变压器维护过程中CO和CH_4气体的探测要求.     

基于共振型高灵敏度光声光谱技术探测痕量乙炔气体浓度

乙炔气体作为判断变压器运行状态的一种故障气体,其浓度的高低反映了变压器的运行状况,因此对其浓度的探测在变压器的维护中具有重要意义。为了准确探测变压器运行过程中产生的乙炔气体浓度,为变压器的维护提供技术参数,针对基于DFB激光器的共振型光声光谱技术痕量乙炔气体检测技术开展研究,对传统的光声光谱探测系统进行改进。根据光声光谱技术的理论可知,光声信号的强度与入射激光的功率成正比,所以在光声池的出射窗口采用一个平面反射镜将红外光再次反射到光声池中以增加入射光功率,增强光声信号强度,进一步提高了光声系统的探测灵敏度。通过一定浓度的乙炔气体在不同调制频率和不同调制深度下光声信号强度的变化,确定光声探测系统的最佳调制频率和最佳调制深度为767 Hz和0.3 mV。利用不同浓度乙炔气体对系统进行标定,然后采用最小二乘法对光声信号与气体浓度进行拟合,二者具有很好的线性度。通过Allan方差计算可知,系统在平均时间达到200 s时,能够达到最低探测极限浓度。实验表明,在一个大气压下,积分时间为10 ms时,改进后的共振型光声光谱探测系统对乙炔气体的最低探测极限浓度达到了0.3 μL·L-1。还将小波去噪技术引入到低浓度下乙炔气体的光声信号处理中,有效消除了低浓度气体光声信号中的噪声,提高了信噪比。设计的共振型光声光谱探测系统操作简单,最低探测浓度符合国标中对变压器维护过程中对乙炔气体的探测需求,在变压器维护领域具有广阔的应用前景。     

Performance evaluation of a near infrared QEPAS based ethylene sensor

A sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) was evaluated for the detection of trace levels of ethylene at atmospheric pressure using a fiber coupled DFB diode laser emitting in the 1.62 μm spectral range. A noise-equivalent QEPAS signal of ∼4 ppm C₂H₄ was achieved for a 0.7 s data acquisition time using wavelength-modulation with a second-harmonic detection scheme on the strongest C₂H₄ absorption peak at 6177.14 cm⁻¹ with an average optical power of ∼15 mW. Improved detection sensitivity of 0.5 and 0.3 ppm C₂H₄ (1σ) was demonstrated using longer averaging time of 70 and 700 s, respectively. Important characteristics for the QEPAS based sensor operation in real-world conditions are presented, particularly the influence of external temperature variations. Furthermore, the response time of the ethylene sensor was measured in different configurations and it is shown that the QEPAS technique can provide a response time in a few seconds range even without active gas flow.     

Fast simultaneous measurement of multi-gases using quantum cascade laser photoacoustic spectroscopy

The authors developed a fast simultaneous method in detecting multi-gases using quantum cascade laser (QCL) based photoacoustic (PA) spectroscopy. We demonstrated the simultaneous measurement of CO and SO₂ concentrations using two QCLs working at 4.56 and 7.38 μm, corresponding to the absorption bands of CO and SO₂, respectively. The modulation frequencies of the two QCLs were 234 and 244 Hz. The response time was 0.6 seconds. A computer sound card was used to process the PA signals. Fast Fourier transform was an essential step to get the amplitudes of the PA signals at different frequencies. The concentration of each gas can be obtained from the PA signal amplitude at the corresponding modulation frequency.     

基于电学调制相消法和高功率蓝光LD的离轴石英增强光声光谱NO2传感器设计和优化

使用中心波长为450 nm的高功率多模蓝光激光管(LD)作为激励光源, 结合电学调制相消法和离轴石英增强光声光谱(QEPAS)配置, 设计了一款高灵敏二氧化氮传感器. 电学调制相消法使离轴QEPAS传感器的背景噪声降低至1/269, 在标准大气压和1 s积分时间下, 获得的探测灵敏度为4.5 ppb, 对应的归一化噪声等效吸收系数(1σ )为2.2×10^-8 cm^-1·W/Hz^1/2. 延长积分时间到46 s, 灵敏度能够进一步下降到0.34 ppb. 气体流速对该传感器的影响也被研究.     

Dual-high-frequency from single-piezoelectric crystal for ACE degradation by hybrid advanced oxidation UV-sonochemistry process

This study investigates the combination of two waves emitted from a single-piezoelectric crystal by use of a dual-frequency generator in a sonochemical reactor. The dual-frequency configurations analyzed were the double-modulated fundamental frequency (376–376 kHz), resonant and second harmonic, termed 376D, 376R and 376H respectively. The effect of the phase shift (Φ) and the percentage of modulation between added waves were described by the total acoustic power distribution (P_t) measured inside the sonoreactor. Moreover, optimal angle alignment and modulation between dual-frequency waves for 376D, 376R and 376H cases were selected in order to evaluate the ultrasonic synergy by sonochemical reactivity in production of H₂O₂, in degradation rate of a model emerging pollutant ACE, and in the TOC and biodegradability evolution in the treated effluent. Phase shift and percentage of modulation had strong effect on the resulted waveform and on the sonochemical efficiency for all, harmonic and non-harmonic, dual-frequency combinations created. In the 376D case, the best reinforcement conditions are founded at 0° and 360°. In the 376H the maximum power distribution presents a 90° period. Shift phase does not determines any cyclic pattern in the total power distribution for the 376R case. The highest H₂O₂ production rate was observed for the 376H case followed for 376D and 376R configurations with 1.61, 1.12 and 0.58 μM/min by angle alignment in 105, 0 and 110° respectively. The highest initial degradation rate of ACE was observed for the 376D case followed for 376H and 376R with 0.56, 0.42 and 0.33 μM/min at 100% modulation. Reduced mineralization was observed in all dual-frequency configurations (8.54% for 376D and approximately 4.5% for 376R and 3756H modes). Contrasting results are observed regard to biodegradability ratio following the next sequence 376D < 376H≈376R with 0.9, 2.30 and 2.33 respectively. Relevant intensification in hydroxyl radicals production is observed by the UV-US system increasing up three folds the ACE removal and mineralization and two folds higher biodegradability of effluent in particular for 376R and 376H cases at optimal operation condition of dual-frequency signal.     

Modulated resonant versus pulsed resonant photoacoustics in trace gas detection

Modulated resonant photoacoustics is a sensitive technique widely used for trace gas sensing. Generally, a continuous-wave laser is modulated at a frequency corresponding to an acoustic resonance of a photoacoustic cell. Another mode of operation—which we propose to call the pulsed resonant mode—consists in matching the frequency repetition rate of a pulsed laser to an acoustic resonance of the cell. We present a theoretical model to compare the performance of these two configurations. For a given average power of the incoming light inside the cell, the pulsed resonant mode of operation (nanosecond pulses or shorter) produces π/2 times higher photoacoustic signals than the modulated resonant scheme (the latter is optimized for a 50% duty cycle). This result agrees with experiments during which both cases were investigated at 532 nm using the same photoacoustic cell containing trace concentrations of NO₂.     

Experimental and theoretical investigation of photoacoustic-signal generation by wavelength-modulated diode lasers

The dependence of photoacoustic spectra on different experimental parameters was investigated by both theoretical and experimental means. The experiments were carried out with an inexpensive resonant optoacoustic system based on near-infrared laser diodes, which allowed photoacoustic and direct absorption spectra to be recorded simultaneously. The experimental observations were compared to theoretical predictions. It was also demonstrated that source-frequency (wavelength) modulation at the resonance frequency of the cell provides superior signal to noise ratio compared to amplitude modulation and eliminates background drifts and fluctuations.     

基于中红外量子级联激光器和石英增强光声光谱的CO超高灵敏度检测研究

采用石英增强光声光谱(QEPAS)技术对CO痕量气体展开检测研究. 为了实现超高灵敏度探测, 采用输出波长为4.6 μm的新颖中红外高功率分布反馈量子级联激光器为光源, 实现了对CO气体基频吸收带的激发与测量. 在优化了调制深度、气体压强和提高了CO分子的振动-转动弛豫速率后, 获得了1.95 ppbv的优异探测极限. 在分析检测结果的过程中, 讨论了能级寿命对信号强度的影响, 并对QEPAS信号强度的表达式进行了修正.     

Effect of multi-mode dual-frequency ultrasound pretreatment on the vacuum freeze-drying process and quality attributes of the strawberry slices

The effects of osmotic pretreatment assisted by ultrasound in different frequency modes before vacuum freeze-drying (VFD) on moisture migration and quality characteristics of strawberry slices were investigated. The frequency modes are single-frequency modes under 20, 40 kHz (SM-20, SM-40), and dual-frequency under 20/40 kHz including sequential mode (SeDM) and simultaneous mode (SiDM). The quality characteristics of dried strawberry products including rehydration, hardness, color, flavor, total anthocyanins, total phenols, vitamin C content, and active antioxidant components (DPPH and –OH) were determined. Results showed that drying time of the strawberry slices irradiated by ultrasound was reduced by 15.25%–50.00%, compared to the control samples. Besides, dual-frequency ultrasound shortened the drying time more than single-frequency ultrasound. The drying time of SeDM was the shortest. In addition to vitamin C content, the quality characteristics including rehydration, hardness, color, flavor, total anthocyanins, total phenols, and antioxidant activity of dried strawberry products pretreated by SeDM were significantly (p < 0.05) better than those of control and other pretreated samples. It can be concluded that the SeDM was an effective pretreatment method to produce high-quality vacuum freeze-dried strawberry products.     

A QEPAS based methane sensor with a 2.35 μm antimonide laser

A methane sensor based on quartz-enhanced photoacoustic spectroscopy was developed. An antimonide quantum-well diode laser was used as an excitation source. The GaInAsSb/AlGaAsSb laser was fabricated by molecular beam epitaxy on GaSb substrate. This diode laser emits in the 2.35 μm range at room temperature in the continuous wave regime. A spectrophone constituted of a quartz tuning fork and two steel microresonators was used. The analysis of the sensor response with one, two or without microresonators is presented. Second derivative wavelength modulation detection was used to perform low concentrations measurements, thus we obtained a CH₄ detection limit of 1 ppmv.     

共光声池腔芯轴型空气衬底光纤麦克风

声传感器和光声池是激光光声光谱技术的核心组件。结合光纤迈克耳孙干涉仪、相位载波解调技术和纵向共振光声池,提出一种共光声池腔的芯轴型空气衬底光纤麦克风。光纤麦克风中的铜毛细管被用作光声池的共振腔,传感臂由10 m长的超细光纤缠绕在铜毛细管上构成,参考臂为5 cm长的短臂且已进行隔声隔振处理。基于结构共振频率稳定的特点,优化光纤麦克风的共振频率,使其略低于光声池的一阶纵向共振频率,以实现准双共振。实验结果表明,麦克风在共振频率为1443 Hz处的最小可检测声压为0.69μPa√Hz。在1 kHz处,声压电压响应线性度为99.98%(5 mPa~3 Pa),动态范围为112.52 dB。该光纤麦克风适用于高温、易爆和高电磁干扰等特殊环境下痕量气体的高精度检测。     

Absorption line profile recovery based on residual amplitude modulation and first harmonic integration methods in photoacoustic gas sensing

Two methods for recovery of gas absorption line profiles are presented in this paper using photoacoustic spectroscopy and tunable diode laser spectroscopy (TDLS) with wavelength modulation (WM). A theoretical analysis based on Fourier coefficients is given in order to describe the various components that arise under simultaneous intensity and frequency modulation. The first method makes use of the residual amplitude modulation (RAM) signal which is always present in current modulation of distributed feedback (DFB) tunable diode lasers. The second method involves integration of a near-pure first harmonic derivative signal, separated from other distorting components by appropriate choice of the lock-in detection phase in the case of low modulation index. Good agreement is obtained with both methods between the experimental results and the theoretical simulation for the P17 absorption line of acetylene at 1535.39 nm but the second method gives a much improved accuracy and signal-to-noise ratio in lineshape recovery with photoacoustic spectroscopy.     

Photoacoustic detection of nitric oxide with a Helmholtz resonant quantum cascade laser sensor

Nitric oxide detection has been performed with a Helmholtz resonant photoacoustic sensor previously developed in Reims. The cell was coupled with a continuous wave distributed feedback quantum cascade laser working at cryogenic temperature in continuous wave mode in the 5.4 μm region. A detection limit of 20 ppb of nitric oxide in nitrogen with a laser power of 3 mW can be predicted. Possible applications and improvements are discussed.     

双光程光声光谱甲烷传感器

利用甲烷(CH₄)气体分子在1.6 μm的吸收特性,使用中心波数为6 046.96 cm-1的蝶形分布反馈式(DFB)激光器和自制的大内径光声池,设计了一款紧凑高灵敏的CH₄气体传感器。为了进一步增强输出光声信号强度,一个具有高反射率的平面镜放置在光声池后,使透射光束被反射后,二次通过光声池,增强了光与被测气体的作用距离,使光声信号提高了1.9倍。传感器各项参数,包括调制频率、调制深度及气体流速被优化。在标准大气压和1 s的积分时间下,该传感器最终获得的探测灵敏度为0.21 ppm,1σ归一化等效噪声系数(NNEA)为2.1×10-8 cm-1·W·Hz-1/2。该甲烷传感器使用性价比高的DFB近红外激光二极管作为激发光源,装置简单,成本低廉可以满足大气环境检测、矿井瓦斯监测、工业过程控制及无创伤医疗诊断等领域的需求。     

长光程共振式二氧化碳气体光声传感器研究

二氧化碳(CO₂)是植物光合作用的原材料,也是一种温室气体,其过量地排放会影响动植物的生态环境。在碳达峰、碳中和的背景下,研制高灵敏度的CO₂检测装置具有重要意义。为了监测大气环境中CO₂含量的变化,设计了一种长光程共振式CO₂气体光声传感器,并以此搭建了光声检测装置。以中心波长为2 004 nm的分布式反馈激光器(DFB)作为激发光源,激光射入由漫反射材料制成的球型吸收腔,在腔内多次反射以增加气体的吸收路径。吸收腔外部被两个高热传导率的铝制半球包裹,降低由池体吸收光能后产生的热噪声。吸收腔上耦合一根声学管,当其工作在一阶纵向共振模态时,光声信号被放大,在管子末端达到极大值。为了进一步增大光声信号,通过饱和加湿样品的方式来加快CO₂气体的弛豫速率,加湿后的样品产生的光声信号是干燥样品的2.1倍左右。使用一系列浓度的湿润CO₂样品标定光声检测装置,结果表明,光声信号与浓度之间呈现良好的线性关系。在此基础上,通过对标准气体的检测实验,验证了装置的准确性与稳定性...