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     

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.     

Intracavity photoacoustic gas detection with an external cavity diode laser

The first use of an external cavity diode-laser light source in combination with a photoacoustic detector for high-sensitivity gas detection is described. This combined system is applicable for detecting gases with absorption coefficients as low as 5 x 10^–8 cm^–1 by operating the photoacoustic cell in an intracavity mode. Measurements were made on the 1.13 m absorption lines of water vapour. For quantitative measurements, it was found to be necessary to introduce a reference cell into the system.     

Application of a broadband blue laser diode to trace NO2 detection using off-beam quartz-enhanced photoacoustic spectroscopy

We applied for the first time, to our knowledge, broadband off-beam quartz-enhanced photoacoustic spectroscopy (BB-OB-QEPAS) to trace NO2 detection using a broadband blue laser diode centered at 450?nm. A detection limit of 18?ppbv (parts in 10(9) by volume) for NO2 in N2 at atmospheric pressure was achieved with an average laser power of 7?mW at a 1?s integration time, which corresponds to a 1?σ normalized noise equivalent absorption coefficient of 4.1×10(-9) cm(-1)?W=Hz(1=2). An Allan variance analysis was performed to investigate the long-term stability of the BB-OB-QEPAS-based NO2 sensor.     

Cavity-enhanced absorption: detection of nitrogen dioxide and iodine monoxide using a violet laser diode

We present an application of cavity-enhanced absorption spectroscopy with an off-axis alignment of the cavity formed by two spherical mirrors and with time integration of the cavity-output intensity for detection of nitrogen dioxide (NO₂) and iodine monoxide (IO) radicals using a violet laser diode at λ=404.278 nm. A noise-equivalent (1σ≡ root-mean-square variation of the signal) fractional absorption for one optical pass of 4.5×10^-8 was demonstrated with a mirror reflectivity of ∼0.99925, a cavity length of 0.22 m and a lock-in-amplifier time constant of 3 s. Noise-equivalent detection sensitivities towards nitrogen dioxide of 1.8×10¹⁰ molecule cm^-3 and towards the IO radical of 3.3×10⁹ molecule cm^-3 were achieved in flow tubes with an inner diameter of 4 cm for a lock-in-amplifier time constant of 3 s. Alkyl peroxy radicals were detected using chemical titration with excess nitric oxide (RO₂+NO→RO+NO₂). Measurement of oxygen-atom concentrations was accomplished by determining the depletion of NO₂ in the reaction NO₂+O→NO+O₂. Noise-equivalent concentrations of alkyl peroxy radicals and oxygen atoms were 3×10¹⁰ molecule cm^-3 in the discharge-flow-tube experiments. Received: 4 February 2003 / Revised version: 10 March 2003 / Published online: 12 May 2003 RID="*" ID="*"Corresponding author. Fax: +44-1865/275-410, E-mail: vlk@physchem.ox.ac.uk     

Sensitive harmonic detection of ammonia trace using a compact photoacoustic resonator at double-pass configuration and a wavelength-modulated distributed feedback diode laser

A sensitive detection of ammonia in parts per billion by volume is described. The system based on photoacoustic spectroscopy (PAS) consists of distributed feedback laser diode emitting near 1,531.7 nm and a compact PA cell at double-pass configuration. In order to optimize the signal background ratio of the system, two types of modulations were tested, amplitude and wavelength modulations (WM). Using a digital lock-in amplifier, the 1f and 2f detection in WM could be investigated. A detection limit of 470 parts per million by volume could be achieved at WM-2f. In the sense of quantifing the adsorption–desorption process, the response time of the system and detection accuracy was performed in different flows. Response times between 10 and 49 s, depending on the flow rate, were obtained which enables the PA system to measure low concentrations of ammonia with high accuracy in real time.     

Quartz-enhanced photoacoustic spectroscopy-based sensor system for sulfur dioxide detection using a CW DFB-QCL

Sulfur dioxide (SO₂) trace gas detection based on quartz-enhanced photoacoustic spectroscopy (QEPAS) using a continuous wave, distributed feedback quantum cascade laser operating at 7.24 μm was performed. Influence of water vapor addition on monitored QEPAS SO₂ signal was also investigated. A normalized noise equivalent absorption coefficient of NNEA (1σ) = 1.21 × 10^?8 cm^?1 W Hz^?1/2 was obtained for the ν ₃ SO₂ line centered at 1,380.93 cm^?1 when the gas sample was moisturized with 2.3 % H₂O. This corresponds to a minimum detection limit (1σ) of 63 parts per billion by volume for a 1 s lock-in time constant.     

灵敏的基于分布反馈式半导体激光波长调制光声光谱

利用室温下单模运行的近红外半导体二极管激光,报导了波长调制共振光声光谱结合二次谐波探测技术.实验系统应用到乙炔探测,在1个标准大气压和3毫瓦平均光功率以及3毫秒锁相积分时间条件下其探测灵敏度可达10ppm(体积比),归一化到激光功率和系统带宽最小可探测吸收为4.0×10-8Wcm-1/Hz,并且实验中发现系统最佳压力响应值在2.66×104Pa附近.本实验装置可有效的消除光声光谱系统中常见的窗片和光声腔壁吸收入射光而引起的背景噪声.此外,相对于其他方法我们描述的基于半导体激光共振光声光谱具有很大的优点,为进一步发展便利、实用、便携式环境监测仪器奠定了坚实的基础.     

用发光二极管实现的双通道、双色红外CO2分析仪

介绍了一种采用发光二极管InAsSbP作光源、以PbSe探测器作光电转换器件实现的红外CO2浓度分析仪。分析仪由探测器系统、信号放大与处理系统及显示输出系统三部分组成。探测器系统主要包括发光二极管InAsS bP、红外窗口、气路、取样泵、球面反射镜、带通滤光片及PbSe探测器等;信号放大与处理系统主要包括电压跟随放大电路、采样保持电路、多路模拟开关、模 数转换电路及8031单片机等;显示输出系统主要包括显示器、打印机及声报警器等。叙述了分析仪的工作原理、基本结构及主要技术指标,讨论了其中的技术难点及其相应的解决方法,并给出了其所能达到的技术指标。     

OH sensor based on ultraviolet, continuous-wave absorption spectroscopy utilizing a frequency-quadrupled, fiber-amplified external-cavity diode laser

The development of an all-solid-state cw laser system for optical absorption measurements of the OH radical in the UV spectral range is described. The tunable output of a 1064-nm external-cavity diode laser is amplified by use of a Nd:doped, double-clad fiber amplifier. The amplified near-IR radiation is frequency doubled by a periodically poled lithium niobate crystal and then quadrupled in a beta-barium borate crystal. The design and operation of the system and measurements of OH absorption in the (2, 0) band of the A(2)?(+)- X(2)? electronic transition are discussed.     

Sensitive detection of methane with a 1.65 μm diode laser by photoacoustic and absorption spectroscopy

Received: 18 September 1997/Revised version: 11 November 1997     

High-sensitivity detection of NO2 using a 740 nm semiconductor diode laser

 An AlGaAs diode laser was used to detect NO₂ absorption lines belonging to the (0 0 0)–(2 13 1) vibrational band, within the X˜² A ₁ electronic ground state, at 739 nm. A simple absorption spectrometer based on wavelength-modulation spectroscopy with second-harmonic detection was developed. The minimum detectable pressure of pure NO₂ was 0.1 μbar with 2 m absorption path-length, corresponding to an absorbance of 10^-6. High-sensitivity detection of NO₂ was also performed in the presence of N₂ and air at different total pressures: The effects on the detection limit of our apparatus were accurately investigated. The minimum NO₂ concentration at 500 mbar of air was measured to be 2 ppm. Received: 11 June 1996 / Revised version: 11 October 1996     

可调谐半导体激光吸收光谱法监测二氧化碳的通量

含碳温室气体浓度增加所加剧的温室效应是气候变化的重要原因,大面积范围内二氧化碳气体通量的测量对于评价各类陆地生态系统对大气中主要温室气体浓度的贡献具有重要的意义。可调谐半导体激光吸收(TDLAS)光谱技术具有高分辨率、高灵敏度以及快速响应等特点,是痕量气体高灵敏快速监测的新方法。文章以可调谐分布反馈半导体激光器作为光源,通过波长调制方法对1.573μm附近二氧化碳气体某一吸收线的二次谐波信号测量,结合激光分束技术,实现对不同高度层面700多米光程范围内二氧化碳气体浓度的快速在线检测。结合大口径闪烁仪测量出来的莫宁-奥布霍夫长度和特征速度,通过公式计算得到一天内二氧化碳气体的通量在-1.5～2.5mg·(m2·s)-1范围内的波动,突破了目前对近地面痕量气体通量的监测只能提供局地结果的状况,使大面积范围内痕量气体通量的测量成为可能。     

Simultaneous detection of trace gases using multiplexed tunable diode lasers and a photoacoustic cell containing a cantilever microphone

We report what we believe to be a novel demonstration of simultaneous detection of multiple trace gases by near-IR tunable diode laser photoacoustic spectroscopy using a cell containing a cantilever microphone. Simultaneous detection of carbon monoxide (CO), ethyne (C₂H₂), methane (CH₄) and combined carbon monoxide/carbon dioxide (CO+CO₂) in nitrogen-based gas mixtures was achieved by modulation frequency division multiplexing the outputs of four near-IR tunable diode lasers. Normalized noise-equivalent absorption coefficients of 3.4×10^?9, 3.6×10^?9 and 1.4×10^?9 cm^?1?W?Hz^?1/2 were obtained for the simultaneous detection of CO, C₂H₂ and CH₄ at atmospheric pressure. These corresponded to noise-equivalent detection limits of 249.6 ppmv (CO), 1.5 ppmv (C₂H₂) and 293.7 ppmv (CH₄) respectively over a measurement period of 2.6 s at the relevant laser power. The performance of the system was not influenced by the number of lasers deployed, the main source of noise arising from ambient acoustic effects. The results confirm that small-volume photoacoustic cells can be used with low optical power tunable diode lasers for rapid simultaneous detection of trace gases with high sensitivity and specificity.     

Resonant photoacoustic simultaneous detection of methane and ethylene by means. of a 1.63-μm diode laser

A compact multi-component trace-gas detector based on the resonant photoacoustic technique and a NIR external cavity diode laser has been developed. It has been characterized using a mixture of ethylene and methane diluted in ambient air. A spectroscopic investigation of combination bands and overtones between 5900 and 6250 cm^-1, obtained with an IR pulsed laser photoacoustic spectrometer, allowed us to find a wavelength region where the 2ν₃ overtone of CH₄ and the ν₅+ν₉ combination band of C₂H₄ show uncongested rotational lines. Using a single-mode scan of the diode laser in this region, around 6150 cm^-1, the sensitivity for the simultaneous detection of ethylene and methane is 8 ppm/mW and 40 ppm/mW respectively. Factors affecting the sensitivity and selectivity of the detection system and possible improvements suitable to reach the sub-ppm detection limit are discussed. Received: 1 August 2001 / Revised version: 28 November 2001 / Published online: 7 February 2002 An erratum to this article is available at .     

Ultra-sensitive carbon monoxide detection by using EC-QCL based quartz-enhanced photoacoustic spectroscopy

A quartz-enhanced photoacoustic spectroscopy (QEPAS) based sensor for carbon monoxide detection at ppbv levels was developed with a 4.65???m external-cavity quantum cascade laser operating both in continuous wave (cw) and pulsed modes. A?23-fold enhancement of the measured CO signal amplitude was obtained when water vapor, acting as a catalyst for vibrational energy transfer, was added to the targeted analyte mixture. In the cw mode, a noise-equivalent sensitivity (NES, 1??) of 2 ppbv was achieved at a gas pressure of 100?Torr, for 1-s lock-in amplifier (LIA) time constant (TC), which corresponds to a normalized noise equivalent absorption coefficient (NNEA) of $1.48\times 10^{-8}~\mathrm{cm}^{-1}\,\mathrm{W}/\sqrt{\mathrm{Hz}}$ . In the pulsed mode, the determined NES and NNEA were 46?ppbv and $1.07\times 10^{-8}~\mathrm{cm}^{-1}\,\mathrm{W}/\sqrt{\mathrm{Hz}}$ , respectively, for a 3-ms LIA TC at atmospheric pressure with a laser scan rate of 18?cm^-1/s and a 50?% duty cycle. An intercomparison between cw and pulsed QEPAS-based CO detection is also reported.     

Room-temperature diode laser photoacoustic spectroscopy near 2.3 μm

We describe results obtained with a new type of GaSb-based semiconductor distributed feedback multiple quantum well lasers operating in continuous-wave mode at room temperature and above. The lasers show good tunability both by the excitation current and the temperature from -1 °C to +60 °C (4170 to 4360 cm^-1). They operate in a high-power (more than 1 mW) single-mode regime and can be tuned without hopping in the methane and ammonia absorption spectral range of up to 15 cm^-1. Application to photoacoustic detection is also described. Potential use lies in the areas of analytical chemistry of gases and in atmospheric pollution research. PACS 82.80.Kq; 42.62.Fi; 42.55.Px     

Pulsed near-infrared laser diode excitation system for biomedical photoacoustic imaging

A pulsed laser diode system operating at 905 nm has been developed for the generation of photoacoustic signals in tissue. It was evaluated by measuring the photoacoustic waveforms generated in a blood vessel phantom comprising three dye-filled (mu(a)=1 mm(-1)) tubes of diameters 120-580 microm immersed to a maximum depth of 9 mm in a turbid liquid (mu'(s)=1 mm(-1)). The system was then combined with a cylindrical scanning system to obtain two-dimensional images of a tissue phantom. The signal-to-noise ratio of the detected signals in both cases and the image contrast in the latter suggest that such a system could provide a compact and inexpensive alternative to current excitation sources for superficial imaging applications.