Optimization of a compact photoacoustic quantum cascade laser spectrometer for atmospheric flux measurements: application to the detection of methane and nitrous oxide

Room temperature (RT) quantum cascade lasers (QCL) are now available even in continuous wave (cw) mode, which is very promising for in situ gas detectors. Ambient air monitoring requires high sensitivity with robust and simple apparatus. For that purpose, a compact photoacoustic setup was combined with two cw QCLs to measure ambient methane and nitrous oxide in the 8 μm range. The first laser had already been used to calibrate the sensitivity of the photoacoustic cell and a detection limit of 3 ppb of CH₄ with a 1s integration time per point was demonstrated. In situ monitoring with this laser was difficult because of liquid nitrogen cooling. The second laser is a new RT cw QCL with lower power, which enabled one to reach a detection limit of 34 ppb of methane in flow. The loss in sensitivity is mainly due to the weaker power as photoacoustic signal is proportional to light power. The calibration for methane detection leads to an estimated detection limit of 14 ppb for N₂O flux measurements. Various ways of modulation have been tested. The possibility to monitor ambient air CH₄ and N₂O at ground level with this PA spectrometer was demonstrated in flux with these QCLs. PACS 07.88; 92.60.Sz     

Photoacoustic phase shift as a chemically selective spectroscopic parameter

The phase information obtained in photoacoustic experiments can be used to separate the signals originating from chemical species with overlapping absorption spectra. This approach was applied to quantify parts per million CO levels in propylene using quartz-enhanced photoacoustic spectroscopy and a quantum cascade laser as an excitation source. The experimental data were used to evaluate V–T relaxation rates of CO and N₂O in propylene. PACS 42.62.Fi; 43.35.Sx     

Simultaneous determination of the spatial profile of the laser beam and vibrational-to-translational relaxation time by pulsed photoacoustics

Measuring the vibrational-to-translational relaxation time τ_V-T in gases is one of the applications of pulsed photoacoustic spectroscopy. Unfortunately, the spatial profile of the laser beam can significantly influence the accuracy of these measurements. Namely, minor changes in the spatial profile of the laser beam cause significant errors in the τ_V-T measuring. We present a method for simultaneous determination of the spatial profile of the laser beam and vibrational-to-translational relaxation time. It is based on the temporal shape of the photoacoustic pulse and utilizes a mathematical algorithm developed for photoacoustic tomography. PACS 39.30.+w; 82.80.Kq; 07.57.Ty     

Crystallization of glassy metal surfaces in Mg–Zn alloy determined by resonant photoacoustic detection

Amorphous layers in a Mg-based alloy are studied by a resonant photoacoustic technique. The technique is shown to provide information on the crystallization temperature of a thin amorphous layer when the sample is heated. This determination provides crucial information regarding thermal stability of the treated surface, not accessible by standard calorimetric techniques. The layer analyzed is tens of micrometers thick, produced by rapid melting by a pulsed electron gun and subsequent rapid cooling towards the substrate. It is shown that the signal from the photoacoustic detection arises mainly from the volume change during crystallization at about 390 K. The volume change due to the structural relaxation of the glass before crystallization is also detected. PACS 64.70.Pf; 78.20.Nv; 81.70.Cv     

Optical diagnostics for particle-cleaning process utilizing laser-induced shockwave

The dynamics of laser-induced plasma/shockwave and the interaction with a surface in the laser shock cleaning process are analyzed by optical diagnostics. Shockwaves are generated by a Q-switched Nd:YAG laser in air or with N₂, Ar, and He injection into the focal spot. The shock velocity is measured by monitoring the photoacoustic probe–beam deflection signal under different conditions. In addition, nanosecond time-resolved images of shockwave propagation and interaction with the substrate are obtained by the laser-flash shadowgraphy. The results reveal the effect of various operation parameters of the laser shock cleaning process on shockwave intensity, energy-conversion efficiency, and flow characteristics. Discussions are made on the cleaning mechanisms based on the experimental observations. PACS 81.65.Cf; 42.62.-b; 47.40.Nm     

Relaxation effects and high sensitivity photoacoustic detection of NO<Subscript>2</Subscript> with a blue laser diode

A detection limit of 200 ppt of NO₂ in N₂ at atmospheric pressure was obtained with a photoacoustic detector and a high power blue laser diode. This corresponds to a normalized noise (1σ) equivalent absorption coefficient of 2×10^-9 cm^-1W/Hz^0.5. Measurements at different laser modulation frequencies showed no frequency dependence of the photoacoustic signal, indicating a relaxation time τ < 4 μs. Mixing O₂ into the NO₂ containing gas results in a decrease of the photoacoustic signal. A simple model shows that this effect can be attributed to an increased vibrational-vibrational relaxation of NO₂ to O₂. PACS 31.70.Hq; 34.50.Ez; 42.55.Px     

阶梯复合形光声池声-流特性计算与评估

为了提高光声池的检测性能,提出并分析了一种阶梯复合形光声池。以传统圆柱共振型光声池为基准模型,通过对比解析与模拟计算结果,验证了所采用模拟方法的可靠性,基于模拟方法求解并获得了阶梯腔半径、阶梯腔长度和阶梯腔数量对阶梯复合形光声池声-流特性的影响规律。结果表明:减小阶梯复合形光声池中阶梯腔半径,光声信号相对增强,阶梯腔长度存在最佳尺寸使得光声信号达到最强,阶梯腔数目应选1为宜;流场方面,阶梯复合形光声池构型特征改善了腔内气体涡漩回流的情况,若进一步对其腔内过渡处进行圆角或倒角处理,腔内流速梯度将变得更为平稳。选择一组设计参数进行构型与计算,得到阶梯复合形光声池腔体容积降低为对应圆柱共振型光声池的39.7%,光声信号相对提升约18.7%,同时其频响带宽变窄,品质因数相对得到提升,整体结果显示阶梯复合形光声池声-流特性要优于对应的圆柱共振型光声池。研究内容可为光声光谱光声池的结构优化与改进提供参考。     

改进的同步迭代算法在光声血管成像中的应用

光声成像结合了光学成像和声学成像的优点，是一种高分辨率，高对比度的无损伤医学成像技术.一种改进的同步迭代算法应用于光声图像重建.仿真和模拟结果表明，与传统的代数迭代算法相比，在90°，135°，180°的有限场光声成像中，此算法对测量误差的校正和迭代次数的收敛上具有较大的优势，图像重建的速度和成像质量都有了明显的提高.实验中，一种圆形扫描结构的光声成像装置，用于180°的有限场扫描，利用改进的同步迭代算法，重建出了高对比度和高分辨率（60μm）的鸡胚胎光声血管图像.实验证明，这种算法的应用，大幅度减少了数据采集时间，为光声成像技术运用于实时监测血流灌注和肿瘤光动力治疗的血管损伤效应提供了潜在的应用价值. 关键词： 光声成像 有限角度 代数迭代算法 光声血管成像     

Photoacoustic spectroscopy of formaldehyde with tunable laser radiation at the parts per billion level

Selective and sensitive detection of formaldehyde (CH₂O) was achieved using a grazing-incidence optical parametric oscillator (GIOPO), pumped at high repetition rate, as a light source for photoacoustic spectroscopy. The photoacoustic spectrum of formaldehyde was measured in the range from 2785 to 2840 cm^-1 and an absorption line at 2805.0 cm^-1 was selected for detection. Concentrations down to 20 ppbv (parts per billion by volume) formaldehyde in nitrogen were recorded. The detection limit determined by background fluctuations was 3 ppbv (S/N=1) for 3 s lock-in time constant and 3 min acquisition time. The 2805.0 cm^-1 absorption line of the ν₁ vibrational mode was chosen because of the absence of interference with water and carbon-dioxide bands. This allowed the direct detection of formaldehyde in laboratory air without filtering. PACS 42.62.Fi; 42.68.Ca; 42.65.Yj; 07.07.Df     

Photoacoustic detection of heat generation in proton-implanted Nd:YAG waveguides with and without laser operation

In this work the heat generation in proton-implanted planar Nd:YAG waveguides with and without laser action is determined by using photoacoustic techniques. From the analysis of the photoacoustic signal in the two emission regimes (stimulated and spontaneous) as a function of the absorbed power, it was found that the heat generated is substantially reduced during laser action. The quotient of the heat generated in both emission channels (stimulated and spontaneous) was found to be S_stimulated/S_spontaneous=0.65±0.03. This value is in agreement with the theoretical estimation considering a Nd³⁺ quantum efficiency _waveguide=0.78, extracted from the lifetime measured in the waveguide, slightly lower than the value in the bulk. This work demonstrates that photoacoustic measurements provide a sensitive method to characterize the performance of waveguide lasers. PACS 42.79.Gn; 42.82.Et; 42.60.Lh; 43.60.+d     

Cantilever enhanced photoacoustic detection of carbon dioxide using a tunable diode laser source

Recently introduced cantilever enhanced photoacoustic sensing has been applied to the tunable diode laser-based trace gas detection. The pressure variations due to the photoacoustic signal are detected with a miniature silicon cantilever, whose displacement is measured with a compact Michelson type laser interferometer. The system has been used to detect carbon dioxide (CO₂) at 1572 nm with a distributed feedback diode laser. With a new photoacoustic cell, that was optimized for the laser sources, a normalized noise equivalent sensitivity of 1.7×10^-10 cm^-1W/ at atmospheric pressure was realized. The result obtained in the non-resonant operation mode is at least 10 times better than in previous reports. The future improvements of the technique are also discussed. PACS 07.07.Df; 42.55.Px; 82.80.Kq     

Optical-resolution photoacoustic microscopy for in vivo imaging of single capillaries

Capillaries, the smallest blood vessels, are the distal end of the vasculature where oxygen and nutrients are exchanged between blood and tissue. Hence, noninvasive imaging of capillaries and function in vivo has long been desired as a window to studying fundamental physiology, such as neurovascular coupling. Existing imaging modalities cannot provide the required sensitivity and spatial resolution. We present in vivo imaging of the microvasculature including single capillaries in mice using optical-resolution photoacoustic microscopy (OR-PAM) developed in our laboratory. OR-PAM provides a lateral resolution of 5 microm and an imaging depth >0.7 mm.     

Design and characteristics of a differential Helmholtz resonant photoacoustic cell for infrared gas detection

The Helmholtz resonant photoacoustic (PA) cell is a very convenient design of PA system for air pollution monitoring based on infrared molecular absorption. A simple differential Helmholtz resonator designed for flow measurements is presented in this work. The investigation of the PA systems characteristics based on this design includes experimental study of the responsitivity both of the separate photoacoustic cell and the whole photoacoustic system applied to trace gases detection. The experimental observations are compared to the theoretical predictions. A simple arrangement to enhance the photoacoustic signal of the whole system by a factor of 2 is presented.     

Cantilever-based photoacoustic detection of carbon dioxide using a fiber-amplified diode laser

A compact and sensitive photoacoustic setup has been developed based on a recently demonstrated cantilever technique. A micromechanical cantilever transducer is attached to a cylindrical photoacoustic cell and the cantilever’s deflection is monitored with a compact Michelson interferometer. A commercial 1-Watt optical fiber amplifier was used to enhance the performance of the system. A normalized sensitivity of 1.4×10^-10 cm^-1 W Hz^-1/2 was achieved in the detection of carbon dioxide at 1572 nm wavelength. Using 34 mW optical power from a DFB diode laser, the noise-equivalent detection limit for carbon dioxide at this wavelength is 4.0 ppm. Employing the fiber amplifier, we improved the sensitivity to yield measurement of sub-ppm concentrations. PACS 42.62.Fi; 42.55.Px; 82.80.Ch     

Near-infrared laser photoacoustic detection of methane: the impact of molecular relaxation

A photoacoustic sensor has been developed for trace-gas monitoring using a near-infrared semiconductor laser emitting in the 2ν₃ band of methane at 1.65 μm. The apparatus was designed for on-line process control in the manufacturing of the novel low-water-peak fibres developed for optical telecommunications. The importance of collisional relaxation processes in the generation of the photoacoustic signal is reported in the particular case of CH₄ detection in dry O₂ and O₂–N₂ mixtures. The negative influence of these effects results in a strongly reduced and phase-shifted photoacoustic signal, induced by a fast resonant coupling between the vibrational states of methane and oxygen, associated with the slow relaxation of the excited oxygen molecules. An unusual parabolic response of the sensor with respect to the methane concentration has been observed and is discussed. Finally, the beneficial effect of several species, including water vapour and helium, acting as a catalyst to hasten the relaxation of the CH₄–O₂ system, is demonstrated. PACS 42.62.Fi; 33.20.Ea; 34.50.Ez     

Photoacoustic system for on-line process monitoring of hydrogen sulfide (H2S) concentration in natural gas streams

A dual-channel hydrogen sulfide (H₂S) concentration measuring system based on photoacoustic spectroscopy is described. The system uses a single-mode, fiber-coupled, room-temperature-operated, telecommunication-type diode laser with a wavelength of 1574.5 nm and an output optical power of 40 mW and two identical resonant photoacoustic cells to achieve minimum detectable H₂S concentration at 0.5 ppm (3σ) in both measured natural gas streams. The instrument features excellent long-term stability and unattended automatic on-line monitoring even when operated in harsh industrial environments. The potentially deteriorating effect of temporal variation in the natural gas composition was successfully suppressed by applying a spectral baseline correction method and by introducing an additional measurement phase with measurement of a reference gas from which the H₂S has been removed. Various tests of the instrument demonstrate its reliable performance and suitability for process-control application. PACS 82.80.Kq; 42.62.Fi; 07.07.Df     

Investigation and optimisation of a multipass resonant photoacoustic cell at high absorption levels

A theoretical and experimental investigation of photoacoustic (PA) signals in a resonant multipass PA cell with high background absorption (up to 29 m^-1) is presented. An analogous electric transmission line model including discontinuity inductances at cross section changes was used to model the first longitudinal acoustic mode of the multipass PA cell equipped with two buffer volumes. This model was validated with experimentally obtained results and used to predict the behaviour of the PA cell for different multipass arrangements and different buffer volume diameters. The highest PA signal is obtained for high pass number and large buffer radius. Increasing the absorption coefficient of the medium enhances the PA signal until a maximum is reached, leading to a minimum for the PA signal sensitivity. For a given background absorption, the number of passes required to maximise the sensitivity depends on the absorption coefficient. The model allows the determination of the best-suited number of passes for a given absorption coefficient and cell geometry. PACS 82.80.Kd; 42.62.Fi; 82.80.Gk     

Influence of molecular relaxation dynamics on quartz-enhanced photoacoustic detection of CO2 at λ =2 μm

Carbon dioxide (CO₂) trace gas detection based on quartz enhanced photoacoustic spectroscopy (QEPAS) using a distributed feedback diode laser operating at λ=2 μm is performed, with a primary purpose of studying vibrational relaxation processes in the CO₂-N₂-H₂O system. A simple model is developed and used to explain the experimentally observed dependence of amplitude and phase of the photoacoustic signal on pressure and gas humidity. A (1σ) sensitivity of 110 parts-per-million (with a 1 s lock-in time constant) was obtained for CO₂ concentrations measured in humid gas samples. PACS 07.07.Df; 42.62.Fi; 82.80.Kq; 42.55.Px     

The potential of mid-infrared photoacoustic spectroscopy for the detection of various doping agents used by athletes

The feasibility of laser-photoacoustic measurements for the detection and the analysis of different isolated doping agents in the vapour phase is discussed. To the best of our knowledge, this is the first time that photoacoustic vapour-phase measurements of doping substances have been presented. Spectra of different doping classes (stimulants, anabolica, diuretica, and beta blockers) are shown and discussed in terms of their detection sensitivity and selectivity. The potential of laser spectroscopy for detecting the intake of prohibited substances by athletes is explored. PACS 07.57.Ty; 33.20.Ea; 82.80.Kq     

Study of optical absorption differences of doped polyaniline films by photothermal spectroscopies

In this work, we studied the optical absorption spectral differences between doped and slightly doped polyaniline films and a derivative by photopyroelectric and photoacoustic spectroscopies. There exist great spectral differences between doped and slightly doped samples as shown by conventional optical absorption spectroscopy, but not greatly evidenced by photopyroelectric spectroscopy. The latter was applied to obtain thermal parameters such as thermal diffusivity and thermal conductivity, and as a result it showed that these thermal properties of polyaniline films are very similar for doped and slightly doped samples. Also it showed that for thicker films (about 20 μm), there are no significant optical absorption differences between them. However, for thin films both techniques showed greater optical absorption differences for doped and slightly doped samples, mainly detected by photoacoustic spectroscopy. These behaviors are in accordance with published results which is the granular metal model for polyaniline. This model explains the polyaniline polymeric matrix as formed by conductive islands in the insulating bulk material. PACS 61.82.Pv; 82.35.Cd; 82.35.Lr