Trace gas sensor based on quartz tuning fork enhanced laser photoacoustic spectroscopy

A compact photoacoustic gas sensor based on a quartz tuning fork and fiber-coupled distributed feedback (DFB) diode laser for detection of trace gas at atmospheric pressure has been developed. The sensor performance was evaluated by detection of water vapor in ambient air at normal atmospheric pressure. A normalized noise equivalent absorption coefficient of 1.68×10⁻⁸ cm⁻¹ W/Hz^1/2 was achieved. Influence of different acoustic microresonators and sample pressure on the sensor performance, and the characterization of the sensor response time were investigated. Approaches to improve the current sensor performance are discussed.     

External cavity coherent quantum cascade laser array

We report on the development of a coherent quantum cascade laser array that consists in the fabrication of multi-stripes array. The main characteristic of this kind of source is that an anti-symmetrical signature with two lobes is obtained in the far field. Taking advantage of this drawback, a grating is aligned with one lobe of the source. Thus a Littrow configuration is designed that permit to obtain a wide tunability of the source. First results are presented and a preliminary test of the source is realized by measurements on acetone.     

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.     

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.     

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.     

Sub-Doppler spectroscopy with an external cavity quantum cascade laser

The linewidth of an external cavity quantum cascade laser is studied as a function of injection current and laser scan rate. The laser linewidth is inferred to be ca. 2.5 MHz from Lamb-dip spectra on a low pressure sample of NO and its variation with injection current is well modeled using literature values for the intrinsic material properties of the lasing medium. The laser linewidth measurements are corroborated by polarization spectroscopy studies as well as by analysis of hyperfine structure and cross-over resonances.     

Broadly tunable quantum cascade laser in cantilever-enhanced photoacoustic infrared spectroscopy of solids

An external cavity quantum cascade laser (EC-QCL) is applied in the photoacoustic detection of solid samples. The EC-QCL used has a broad tuning range of 676 cm^?1 (970–1,646 cm^?1) in the mid-infrared region, which enables accurate broadband spectroscopy of large molecules. The high spectral power density of the EC-QCL is combined with an extremely sensitive optical cantilever microphone of the photoacoustic detector to achieve an ultimate sensitivity. The carbon black, polyethylene, and hair fiber samples were measured with the EC-QCL photoacoustic detection using electrical amplitude modulation to demonstrate the possibilities of the setup. The same measurements were repeated with a Fourier transform infrared (FTIR) spectrometer combined with a photoacoustic detector for a comparison. The EC-QCL photoacoustic setup yielded roughly a decade better signal-to-noise ratios than the FTIR setup with the same measurement time.     

Optical-feedback cavity-enhanced absorption spectroscopy with a quantum cascade laser

Optical-feedback cavity-enhanced absorption spectroscopy is demonstrated in the mid-IR by using a quantum cascade laser (emitting at 4.46 μm). The laser linewidth reduction and frequency locking by selective optical feedback from the resonant cavity field turns out to be particularly advantageous in this spectral range: It allows strong cavity transmission, which compensates for low light sensitivity, especially when using room-temperature detectors. We obtain a noise equivalent absorption coefficient of 3 × 10(-9)/cm for 1 s averaging of spectra composed by 100 independent points. At 4.46 μm, this yields a detection limit of 35 parts in 10(12) by volume for N(2)O at 50 mbar, corresponding to 4 × 10(7) molecules/cm(3), or still to 1 fmol in the sample volume.     

Gas-phase photoacoustic sensor at 8.41 μm using quartz tuning forks and amplitude-modulated quantum cascade lasers

We demonstrate the performance of a novel infrared photoacoustic laser absorbance sensor for gas-phase species using an amplitude-modulated quantum cascade (QC) laser and a quartz tuning fork microphone. The photoacoustic signal was generated by focusing 5.3 mW of a Fabry–Pérot QC laser operating at 8.41 μm between the tines of a quartz tuning fork which served as a transducer for the transient acoustic pressure wave. The sensitivity of this sensor was calibrated using the infrared absorber Freon 134a by performing a simultaneous absorption measurement using a 31-cm absorption cell. The power and bandwidth normalized noise equivalent absorption sensitivity (NEAS) of this sensor was determined to be D=2.0×10^-8 W cm^-1/Hz^1/2. A corresponding theoretical analysis of the instrument sensitivity is presented and is capable of quantitatively reproducing the experimental NEAS, indicating that the fundamental sensitivity of this technique is limited by the noise floor of the tuning fork itself. PACS 43.60.Vx; 43.58.Wc; 43.58.Hp; 84.40.Xb     

Compensation of stray capacitance of the quartz tuning fork for a quantitative force spectroscopy

We have introduced a simple but robust approach to realize stray-capacitance compensation of a quartz tuning fork force gradient sensor by using a tied-prong of the same tuning fork instead of using a variable precision capacitor. The results of quantitative force measurements with the proposed device show excellent agreement with the numerical method of cancellation via the theoretical compensation method with the resonance response curves. Futhermore, the mechanical properties of the condensed nano-water bridges are investigated with the proposed compensator, which provides the potential promise of the intrinsic quartz tuning fork for a quantitative precision force spectroscopy.     

基于7.6μm量子级联激光的光声光谱探测N_2O气体

近年来,气候变化对地球的生态环境产生严重影响,而大气温室气体在气候变化中具有重要的作用.一氧化二氮(N_2O)作为一种重要的温室气体,其浓度变化对大气环境产生重要影响,因此对其浓度的探测在大气环境研究中具有重要意义.本文开展了基于中国自主研发的7.6μm中红外量子级联激光的共振型光声光谱探测N_2O的研究,建立了N_2O光声光谱传感实验系统.此系统在传统的光声光谱探测的基础上优化改进,采用双光束增强的方式,增加了有效光功率,进一步提高了系统的探测灵敏度.探测系统以1307.66 cm~(-1)处的N_2O吸收谱线作为探测对象,结合波长调制技术对N_2O气体进行探测研究.通过对一定浓度的N_2O气体在不同调制频率和调制振幅的光声信号的探测,确定了系统的最佳调制频率和调制振幅分别为800 Hz和90 mV.在最优实验条件下对不同浓度的N_2O气体进行了测量,获得了系统的信号浓度定标曲线.实验表明,在锁相积分时间为30 ms时,系统的浓度探测极限为150×10~(-9).通过100次平均后,系统噪声进一步降低,实现了大气N_2O的探测,浓度探测极限达到了37×10~(-9).     

Stability of widely tuneable,continuous wave external-cavity quantum cascade laser for absorption spectroscopy

The performance of widely tuneable, continuous wave (cw) external-cavity quantum cascade laser (EC-QCL) has been evaluated for direct absorption spectroscopy measurements of nitric oxide (NO) in the wavenumber range 1872–1958 cm^?1 and with a 13.5 cm long optical cell. In order to reduce the absorption measurement errors due to the large variations of laser intensity, normalisation with a reference channel was used. Wavelength stability within the scans was analysed using the Allan plot technique for the reduced wavenumber range of 1892.4–1914.5 cm^?1. The Allan variances of the NO absorption peak centres and areas were observed to increase with successive scan averaging for all absorption peaks across the wavelength scan, thus revealing short- and long-term drifts of the cw EC-QCL wavelength between successive scans. As an example application, the cw EC-QCL was used for NO measurements in the exhaust of an atmospheric pressure packed-bed plasma reactor applied to the decomposition of dichloromethane in waste gas streams. Etalon noise was reduced by subtracting a reference spectrum recorded when the plasma was off. The NO limit of detection (SNR = 1) was estimated to be ～2 ppm at atmospheric pressure in a 20.5 cm long optical cell with a double pass and a single 7 s scan over 1892.4–1914.5 cm^?1.     

Detection of chemical warfare agents based on quantum cascade laser cavity ring-down spectroscopy

Chemical warfare agents(CWAs) are recognized as serious threats of terrorist acts against the civilian population.Minimizing the impact of these threats requires early detection of the presence of CWAs.Cavity ring-down spectroscopy(CRDS) is an exquisitely sensitive technique for the detection of trace gaseous species.In this letter,the CRDS technique is employed using a pulsed quantum cascade laser for the detection of dimethyl methylphosphonate(DMMP).A limit of DMMP detection of approximately 77 ppb is achieved.The best achievable sensitivity that corresponds to noise-equivalent absorption is approximately 2×10-7cm 1.     

石英音叉温度频率特性的温度传感器（英）

提出了一种基于石英晶体温度频率特性的石英音叉微谐振式温度传感器。通过理论分析的方法对传感器进行设计,并采用有限元仿真对传感器的结构参数进行优化。采用光刻和蚀刻微加工技术制造石英音叉谐振器,对石英音叉温度传感器样机的频率温度特性进行实验研究。实验结果表明:石英音叉温度传感器的标准谐振频率为36.545 kHz,灵敏度为-1.9 Hz/℃,在-20到100 ℃的温度范围内,其非线性误差小于0.18%,迟滞为0.02%,与理论研究相吻合。该传感器具有高精度、高灵敏度、低功耗和低成本的特点,为高性能温度测量提供较好的解决方案。     

石英音叉温度频率特性的温度传感器（英）

提出了一种基于石英晶体温度频率特性的石英音叉微谐振式温度传感器。通过理论分析的方法对传感器进行设计,并采用有限元仿真对传感器的结构参数进行优化。采用光刻和蚀刻微加工技术制造石英音叉谐振器,对石英音叉温度传感器样机的频率温度特性进行实验研究。实验结果表明：石英音叉温度传感器的标准谐振频率为36.545 kHz,灵敏度为-1.9 Hz/℃,在-20到100 ℃的温度范围内,其非线性误差小于0.18%,迟滞为0.02%,与理论研究相吻合。该传感器具有高精度、高灵敏度、低功耗和低成本的特点,为高性能温度测量提供较好的解决方案。     

Formaldehyde sensor using interband cascade laser based quartz-enhanced photoacoustic spectroscopy

A novel continuous-wave mid-infrared distributed feedback interband cascade laser was utilized to detect and quantify formaldehyde (H₂CO) using quartz-enhanced photoacoustic spectroscopy. The laser was operated at liquid-nitrogen temperatures and provided single-mode output powers of up to 12 mW at 3.53 m (2832.5 cm^-1). The noise equivalent (1) detection sensitivity of the sensor was measured to be 2.2×10^-8 cm^-1W(Hz)^-1/2 for H₂CO in ambient air, which corresponds to a detection limit of 0.6 parts in 10⁶ by volume (ppmv) for a 10 s sensor time constant and 3.4 mW laser power delivered to the sensor module. PACS 42.62.Fi; 72.50.+b     

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.     

石英音叉陀螺音叉封装问题研究

封装是石英音叉陀螺的关键工艺,封装管壳的选择、封装方式及石英音叉的固定形式对音叉的性能有直接影响。封装管壳的有限元仿真分析,可以作为封装管壳选取的依据。采用单点固定封装方式有利于石英音叉工作性能的稳定。实验中通过改变封装盖板的开合状态来改变封装方式,石英音叉的阻尼和品质因数发生了明显的改变。     

Simultaneous measurement of two compounds in aqueous solution with dual quantum cascade laser absorption spectroscopy

Two pulsed Fabry–Pérot quantum cascade lasers (QCL) have been employed for the simultaneous measurement of two analytes in aqueous solutions. Two laser beams (1393 and 1080 cm^-1) were combined by an optical system of parabolic mirrors and a ZnSe-beam splitter. Measurements were made in transmission using a 41 μm CaF₂-flow cell and a mercury-cadmium-telluride (MCT) detector. Using glucose and sodium acetate as model analytes, the measurements show the potential of dual QCL absorption spectroscopy for analyte specific detection and background compensation. The use of the two lasers gives quantitative information about the analytes, even when they show overlapping absorption bands typically found in condensed phase. PACS 42.55.Px; 42.62.Fi