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 quantum cascade laser for quartz tuning fork photoacoustic spectroscopy of broad absorption features

We demonstrate mid-infrared spectroscopy of large molecules with broad absorption features using a tunable external cavity quantum cascade laser. Absorption spectra for two different Freons are measured over the range 1130-1185 cm(-1) with 0.2 cm(-1) resolution via laser photoacoustic spectroscopy with quartz tuning forks as acoustic transducers. The measured spectra are in excellent agreement with published reference absorption spectra.     

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.     

Stimulated Raman spectroscopy using chirped pulses

We present a detailed theoretical and experimental characterization of a new methodology for stimulated Raman spectroscopy using two duplicates of a chirped, broadband laser pulse. Because of the linear variation of laser frequency with time (‘chirp’), when the pulses are delayed relative to one another, there exists a narrow bandwidth, instantaneous frequency difference between them, which, when resonant with a Raman‐active vibration in the sample, generates stimulated Raman gain in one pulse and inverse Raman loss in the other. This method has previously been used for coherent Raman imaging and termed ‘spectral focusing’. Here, gain and loss signals are spectrally resolved, and the spectrally integrated signals are used to determine the spectral resolution of the measured Raman spectrum. Material dispersion is used to generate a range of pulse durations, and it is shown that there is only a small change in the magnitude of the signal and the spectral resolution as the pulse is stretched from 800 to 1800 fs in duration. A quantitative theory of the technique is developed, which reproduces both the magnitude and linewidth of the experimental signals when third‐order dispersion and phase‐matching efficiency are included. The theoretical calculations show that both spectral resolution and signal magnitude are severely hampered by the third‐order dispersion in the laser pulse, and hence, a minimal amount of chirp produces the most signal with only a slight loss of spectral resolution. Copyright © 2014 John Wiley & Sons, Ltd.     

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。     

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。     

基于旋转相干光谱的整形飞秒激光转动动力学调控

我们实验上发展了基于飞秒激光旋转相干光谱的整形飞秒激光转动动力学调控方法,通过脉冲整形技术调控激发脉冲的光谱相位,从而实现对飞秒激光作用下转动态相干激发过程中复原信号及转动布居数的调控. 研究了飞秒激光旋转相干光谱对激光频谱相位的响应机制,突出了飞秒激光频谱相位在气相分子转动态相干激发中的重要作用. 为飞秒激光作用下生物大分子和团簇鉴别及结构探测研究提供了新的参考.     

Stimulated Raman scattering and four-wave Raman mixing seeded by a supercontinuum generated in dibromomethane using picosecond and femtosecond laser sources

Stimulated Raman emission from liquid dibromomethane (vibrational Raman shift frequency, 588 cm⁻¹) is introduced into hydrogen gas (rotational Raman shift frequency, 587 cm⁻¹) as a seed beam, in order to generate numerous rotational lines by four-wave Raman mixing. Unexpectedly, a supercontinuum, which is generated by self-phase modulation in dibromomethane, acted as a seed beam to exclusively generate vibrational lines; the rotational lines are generated only when the supercontinuum is minimal. The former is explained by a competition between the high-gain vibrational and low-gain rotational Raman effects when strongly seeded by a supercontinuum. The latter is explained by stimulated Raman gain under the seed effect exclusively to the first-Stokes rotational line.     

Selective excitation of molecular mode in a mixture by femtosecond resonance-enhanced coherent anti-Stokes Raman scattering spectroscopy

Femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy is used to investigate gaseous molecular dynamics. Due to the spectrally broad laser pulses, usually poorly resolved spectra result from this broad spectroscopy. However, it can be demonstrated that by the electronic resonance enhancement optimization control a selective excitation of specific vibrational mode is possible. Using an electronically resonance-enhanced effect, iodine molecule specific CARS spectroscopy can be obtained from a mixture of iodine-air at room temperature and a pressure of 1 atm (corresponding to a saturation iodine vapour as low as about 35 Pa). The dynamics on either the electronically excited state or the ground state of iodine molecules obtained is consistent with previous studies (vacuum, heated and pure iodine) in the femtosecond time resolved CARS spectroscopy, showing that an effective method of suppressing the non-resonant CARS background and other interferences is demonstrated.     

Multipoint temperature and oxygen-concentration measurements using rotational coherent anti-Stokes Raman spectroscopy

A novel technique for coherent anti-Stokes Raman spectroscopy (CARS) measurements in multiple points is presented. With a system of cylindrical lenses, each laser beam is split into several focused beams, yielding separate planar boxcars configurations. Spectrally resolved CARS signals are detected at different heights on the CCD chip. With dual-broadband rotational CARS the setup is demonstrated for quantitative measurements of temperature- and oxygen-concentration profiles. The technique was demonstrated for three points only, but it can be extended to more points by use of special optics; this choice must be based on a sufficient signal-to-noise ratio in all points for the actual measurement condition.     

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     

微型石英音叉陀螺模态特性的有限元分析

MEMS石英音叉陀螺是近年来发展起来的新型惯性器件。在石英音叉的早期设计阶段,为了提供可靠的设计方案,并缩短设计周期,应该对其进行分析模拟。音叉的结构参数至关重要,不仅会影响陀螺的敏感性,还会影响其检测精度和稳定性。本文以U形音叉为例,利用大型有限元分析软件ANSYS建立了微型石英音叉的实体模型,对其模态特性进行了分析和计算,分析了音叉几何尺寸对其谐振频率的影响,并提供了一组可供实际制作的音叉结构参数,大大缩短了设计周期。     

发散光长程空气传输受激转动拉曼散射效应

研究强激光长程空气传输受激转动拉曼散射(SRRS)效应,寻求抑制措施是高功率固体激光驱动器建造中需要解决的关键问题之一。采用数值模拟的方法研究了发散光长程空气传输SRRS效应的演化规律,通过几何近似理论推导了该效应有效建立时的强度距离乘积阈值和抑制判据。结果表明:发散光长程空气传输SRRS效应的建立速度小于平行光,二者阈值有理论关系;能够有效抑制SRRS效应的发散角设计判据由光束口径、激光强度及安全传输距离决定。     

发散光长程空气传输受激转动拉曼散射效应

研究强激光长程空气传输受激转动拉曼散射(SRRS)效应,寻求抑制措施是高功率固体激光驱动器建造中需要解决的关键问题之一。采用数值模拟的方法研究了发散光长程空气传输SRRS效应的演化规律,通过几何近似理论推导了该效应有效建立时的强度距离乘积阈值和抑制判据。结果表明：发散光长程空气传输SRRS效应的建立速度小于平行光,二者阈值有理论关系;能够有效抑制SRRS效应的发散角设计判据由光束口径、激光强度及安全传输距离决定。     

High-spatial-resolution one-dimensional rotational coherent anti-Stokes Raman spectroscopy imaging using counterpropagating beams

A counterpropagating phase-matching geometry is employed for high-spatial-resolution one-dimensional (1D) imaging of temperature and O₂-to-N₂ concentration ratio using picosecond pure-rotational coherent anti-Stokes Raman spectroscopy (RCARS) over a large field (20 mm). A single-shot 1D RCARS image of more than 20 mm in length is thus acquired at 300 K in air. High-resolution 1D RCARS flame measurements are demonstrated using a custom-built burner and a premixed methane/air flame (Φ=0.6). This phase-matching scheme improves the spatial resolution by approximately 1 order of magnitude when compared to the standard small-angle BOXCARS phase-matching schemes typically employed in CARS measurements. Additionally, for a 20 mm 1D image, signal levels are increased by 10² because of the higher irradiance provided in the current scheme.     

Raman spectroscopy with LED excitation source

We propose and experimentally demonstrate a novel instrument arrangement, which allows for the collection of Raman spectra with a broadband light source. This is achieved by spatially dispersing the optical spectrum in the focal plane and confocally reimaging the Raman signal, which originates from different locations, onto the entrance slit of an imaging spectrometer. Using this approach and broadband radiation derived from a commercially available LED, we acquired high signal‐to‐noise spectra with a spectral resolution limited by the spectral resolution of a spectrometer. Copyright © 2013 John Wiley & Sons, Ltd.     

Tuning fork enhanced interferometric photoacoustic spectroscopy: a new method for trace gas analysis

A photoacoustic trace gas sensor based on an optical read-out method of a quartz tuning fork is shown. Instead of conventional piezoelectric signal read-out, as applied in well-known quartz-enhanced photoacoustic spectroscopy (QEPAS), an interferometric read-out method for measurement of the tuning fork’s oscillation is presented. To demonstrate the potential of the optical read-out of tuning forks in photoacoustics, a comparison between the performances of a sensor with interferometric read-out and conventional QEPAS with piezoelectric read-out is reported. The two sensors show similar characteristics. The detection limit (L) for the optical read-out is determined to be L _opt=(2598±84) ppm (1σ) compared to L _elec=(2579±78) ppm (1σ) for piezoelectric read-out. In both cases the detection limit is defined by the thermal noise of the tuning fork.     

凝聚态物质中的光声喇曼效应理论

用热弹性理论系统地分析了凝聚态物质的光声喇曼效应,分别导出了连续和脉冲激光泵浦下的光声喇曼信号表达式,并做了数值估算.