Trace moisture detection using continuous-wave cavity ring-down spectroscopy

We have developed an instrument to measure trace concentrations of small hydride species in gases using continuous-wave cavity ring-down spectroscopy with near-infrared diode laser excitation. An rms baseline equivalent absorbance of 9.2 x 10(-11) cm(-1)/square root(n) is found, where n is the number of ring-down transients. When the 1396.376-nm absorption line of water is used, this corresponds to a noise equivalent moisture concentration in nitrogen gas of 68 pptv/square root(n). Water vapor concentration is detected over a range extending from 3 to 1000 ppbv and found to depend linearly on the concentration as determined by a calibrated commercial moisture sensor.     

Micro-air-gap based intrinsic Fabry-Perot interferometric fiber-optic sensor

A simple intrinsic Fabry-Perot interferometric (IFPI) sensor is developed. The sensor is fabricated by two micro air gaps as reflective mirrors in a fiber to form a Fabry-Perot cavity. Theoretical and experimental studies of the sensor are described. Experimental results show that high resolution and high sensitivity can be achieved. Two structures of micro-air-gap-based IFPI sensors offer more applications than other IFPI sensors.     

Miniaturized cavity ring-down detection in a liquid flow cell

A novel method for applying cavity ring-down spectroscopy in the liquid phase, compatible with LC analyses, is presented. The core of the setup is a home-built cavity ring-down flow cell (cell volume 12 microL) that is constructed using a silicon rubber spacer, which is clamped leak-tight between two high-reflectivity mirrors. The mirrors are in direct contact with the liquid flow, which provides for a small path length and short ring-down times. Inside the cavity there are no windows, reflection losses, or Brewster angles to be considered. Due to the small size of the presented cavity geometry, the setup can be implemented in conventional-size LC apparatuses. With a flow injection setup, a detection limit of 2.5 nM was obtained for Crystal Violet in ethanol, and the linear dynamic range of the system is at least 2 orders of magnitude. The method has the potential to become a powerful alternative for commercial LC UV/visible absorbance detectors.     

Analytical method to find the optimal parameters for gas detectors based on correlation spectroscopy using a Fabry-Perot interferometer

Several designs of infrared sensors use a Fabry-Perot interferometer (FPI) to modulate the incident light. In this work we analyze the particular case where the FPI fringes are matched with very well defined rovibrational absorption lines of a target molecule such as CO(2), CO, N(2)O, or CH(4). In this kind of sensor, modulation is induced by scanning the FPI cavity length over one half of the reference wavelength. Here we present an analytical method based on the Fourier transform, which simplifies the procedure to determine the sensor response. Furthermore, this method provides a simple solution to finding the optimal FPI cavity length and mirror reflectivity. It is shown that FPI mirrors with surprisingly low reflectivity (<50%) are generally the optimum choice for target gases at atmospheric pressure. Finally, experimental measurements and simulation results are presented.     

Detection of trace water vapor in high-purity phosphine using cavity ring-down spectroscopy

The presence of trace water vapor in process gases such as phosphine, used for compound semiconductor epitaxial growth, can negatively affect the optical and electrical properties of the final device. Therefore, sensitive H2O measurement techniques are required to monitor precursor purity and detect unacceptable contamination levels. A commercial cavity ring-down spectrometer that monitors an H2O absorption line at a wavelength of 1392.53 nm was investigated for service in high purity PH3. Spectral parameters such as the line shape of water vapor in the presence of PH3 as well as background features due to PH3 were measured at different pressures and incorporated into the data analysis software for accurate moisture readings. Test concentrations generated with a diffusion vialbased H2O source and dilution manifold were used to verify instrument accuracy, sensitivity, linearity, and response time. H2O readings at 13.2 kPa corresponded well to added concentrations (slope=0.990+/-0.01) and were linear in the tested range (0-52.7 nmol mol-1). The analyzer was sensitive to changes in H2O concentration of 1.3 nmol mol-1 based on 3sigma of the calibration curve intercept for a weighted linear fit. Local PH3 absorption features that could not be distinguished from the H2O line were present in the purified PH3 spectra and resulted in an additional systematic uncertainty of 9.0 nmol mol-1. Equilibration to changing H2O levels at a flow rate of 80 std cm3 min-1 PH3 occurred in 10-30 minutes. The results indicate that cavity ring-down spectroscopy (CRDS) at 1392.53 nm may be useful for applications such as on-line monitoring (and dry-down) of phosphine gas delivery lines or the quality control of cylinder sources.     

A developed optical-feedback cavity ring-down spectrometer and its application

A developed spectrometer based on optical-feedback cavity ring-down spectroscopy (OF-CRDS) has been demonstrated with a distributed feedback laser diode and a V-shaped glass ceramic cavity. The laser is coupled to the V-shaped cavity, which creates an absorption path length greater than 2.8 km, and resonance between the laser frequency and the cavity modes is realized by modulating the cavity length instead of tuning the laser wavelength to obtain a higher resolution. A noise-equivalent absorption coefficient of ~2.6 × 10(-8) cm(-1)Hz(-1/2) (1σ) is determined with spectral resolution of ~0.003 cm(-1) and spectral range of 1.2 cm(-1). As an application example, the absorption spectrum measurement of water vapor in the spectral range of 6590.3~6591.5 cm(-1) is demonstrated with this spectrometer.     

End-to-end sensor simulation for spectral band selection and optimization with application to the Sentinel-2 mission

An end-to-end sensor simulation is a proper tool for the prediction of the sensor's performance over a range of conditions that cannot be easily measured. In this study, such a tool has been developed that enables the assessment of the optimum spectral resolution configuration of a sensor based on key applications. It employs the spectral molecular absorption and scattering properties of materials that are used for the identification and determination of the abundances of surface and atmospheric constituents and their interdependence on spatial resolution and signal-to-noise ratio as a basis for the detailed design and consolidation of spectral bands for the future Sentinel-2 sensor. The developed tools allow the computation of synthetic Sentinel-2 spectra that form the frame for the subsequent twofold analysis of bands in the atmospheric absorption and window regions. One part of the study comprises the assessment of optimal spatial and spectral resolution configurations for those bands used for atmospheric correction, optimized with regard to the retrieval of aerosols, water vapor, and the detection of cirrus clouds. The second part of the study presents the optimization of thematic bands, mainly driven by the spectral characteristics of vegetation constituents and minerals. The investigation is performed for different wavelength ranges because most remote sensing applications require the use of specific band combinations rather than single bands. The results from the important "red-edge" and the "short-wave infrared" domains are presented. The recommended optimum spectral design predominantly confirms the sensor parameters given by the European Space Agency. The system is capable of retrieving atmospheric and geobiophysical parameters with enhanced quality compared to existing multispectral sensors. Minor spectral changes of single bands are discussed in the context of typical remote sensing applications, supplemented by the recommendation of a few new bands for the next generation of optical Sentinel sensors.     

Development of broadband cavity ring-down spectroscopy for biomedical diagnostics of liquid analytes

We present a spectrometer for sensitive absorption measurements in liquids across broad spectral bandwidths. The spectrometer combines the unique spectral properties of incoherent supercontinuum light sources with the advantages of cavity ring-down spectroscopy, which is a self-calibrating technique. A custom-built avalanche photodiode array is used for detection, permitting the simultaneous measurement of ring-down times for up to 64 different spectral components at nanosecond temporal resolution. The minimum detectable absorption coefficient was measured to be 3.2 × 10(-6) cm(-1) Hz(-1/2) at 527 nm. We show that the spectrometer is capable of recording spectral differences in trace levels of blood before and after hemolysis.     

External cavity tunable quantum cascade lasers and their applications to trace gas monitoring

Since the first quantum cascade laser (QCL) was demonstrated approximately 16 years ago, we have witnessed an explosion of interesting developments in QCL technology and QCL-based trace gas sensors. QCLs operate in the mid-IR region (3-24?μm) and can directly access the rotational vibrational bands of most molecular species and, therefore, are ideally suited for trace gas detection with high specificity and sensitivity. These sensors have applications in a wide range of fields, including environmental monitoring, atmospheric chemistry, medical diagnostics, homeland security, detection of explosive compounds, and industrial process control, to name a few. Tunable external cavity (EC)-QCLs in particular offer narrow linewidths, wide ranges of tunability, and stable power outputs, which open up new possibilities for sensor development. These features allow for the simultaneous detection of multiple species and the study of large molecules, free radicals, ions, and reaction kinetics. In this article, we review the current status of EC-QCLs and sensor developments based on them and speculate on possible future developments.     

温室气体浓度监测的光腔衰荡光谱研究进展

全球气候变化给人类生活带来的影响受到世界各国的普遍关注,温室气体是影响和改变全球气候的关键因素之一,限制和降低温室气体排放量成为人类发展的重要议题.温室气体大多都在10"(每百万个气体分子中所含该种气体分子的个数)级别,且气体分子结构差异大,因此传统方法很难获得较高的精度,而光腔衰荡光谱法是能解决该难题的关键技术之一....     

Development and application of millimeter wave radar sensors for underground mining

This paper defines the issues that are required for the development of a successful underground range measurement sensor. It considers various options, including laser and sonar implementations, before focusing on a millimeter-wave frequency modulated continuous wave radar. The implementations of radar sensors for simple ranging and three-dimensional cavity profiling are then discussed before some data obtained in underground mines is presented to verify the radar performance through thick dust and vapor.     

Back-surface gold mirrors for vibrationally resonant sum-frequency (VR-SFG) spectroscopy using 3-mercaptopropyltrimethoxysilane as an adhesion promoter

Back-surface mirrors are needed as reference materials for vibrationally resonant sum-frequency generation (VR-SFG) probing of liquid-solid interfaces. Conventional noble metal mirrors are not suitable for back-surface applications due to the presence of a metal adhesion layer (chromium or titanium) between the window substrate and the reflective metal surface. Using vapor deposited 3-mercaptopropyltrimethoxysilane (MPTMS) as a bi-functional adhesion promoter, gold mirrors were fabricated on fused silica substrates. These mirrors exhibit excellent gold adhesion as determined by the Scotch(?) tape test. They also produce minimal spectroscopic interference in the C-H stretching region (2800-3000 cm(-1)), as characterized by VR-SFG. These mirrors are thus robust and can be used as back-surface mirrors for a variety of applications, including reference mirrors for VR-SFG.     

Automated target detection system for hyperspectral imaging sensors

Over the past several years, hyperspectral sensor technology has evolved to the point where real-time processing for operational applications is achievable. Algorithms supporting such sensors must be fully automated and robust. Our approach, for target detection applications, is to select signatures from a target reflectance library database and project them to the at-sensor and collection-specific radiance domain using the weather forecast or radiosonde data. This enables platform-based detection immediately following data acquisition without the need for further atmospheric compensation. One advantage of this method for reflective hyperspectral sensors is the ability to predict the radiance signatures of targets under multiple illumination conditions. A three-phase approach is implemented, where the library generation and data acquisition phases provide the necessary input for the automated detection phase. In addition to employing the target detector itself, this final phase includes a series of automated filters, adaptive thresholding, and confidence assignments to extract the optimal information from the detection scores for each spectral class. Our prototype software is applied to 50 reflective hyperspectral datacubes to measure detection performance over a range of targets, backgrounds, and environmental conditions.     

微量气体定量分析的新方法:光腔衰荡光谱

包括半导体制造业在内的许多工业应用要求高纯度气体具有最少的杂质。微量水分的分析特别富有挑战性。光腔衰荡光谱法不但可以快速、准确地分析包括水分在内的微量杂质,而且不需要标准样气。对光腔衰荡光谱法作了简要介绍。     

基于光腔衰荡光谱法的水汽线形强度测量

随着半导体芯片行业的迅速发展,对电子气体的要求也逐渐提高。半导体加工环境中的痕量水分会严重影响芯片的良率和可靠性。光腔衰荡光谱法是近年来发展的一种具有高灵敏度和准确性的痕量气体测量方法,线形强度是光谱法测量的重要参数。为测量痕量水分,建立了一套光腔衰荡光谱系统,测量了中心频率在 7171.10491cm-1和7177.6565cm-1的吸收光谱,通过HTP(Hartmann-Tran profile)线形拟合得到线形强度,测量结果的相对标准不确定度优于1.8%,与HITRAN、HITEMP和GEISA数据库比较,相对偏差小于6%。     

基于量子级联激光器的中远红外高反射率测量

利用光腔衰荡技术开展了中远红外波段的高反镜反射率测量研究.以中心波长9.7μm附近的脉冲量子级联激光器为光源,构建了高反射率测量实验装置.利用该装置对不同腔长下腔镜的反射率进行了测量,最终测定其反射率为99.9464%,测量重复性误差优于0.0014%.实验表明该测量装置重复性精度高,可用于中远红外高反镜反射率的精确测量.     

Embedded fiber-optic Fabry-Perot ultrasound sensor

A fiber-optic ultrasound sensor is presented. The sensor consists of a continuous length of single-mode optical fiber with a built-in Fabry-Perot interferometer. The acoustic pressure produces changes in the index of refraction along the interferometer cavity through the strain-optic effect, thus modulating the reflected power of the light propagating in the fiber. The dielectric internal mirrors that form the interferometer are fabricated by joining a fiber coating with a TiO(2) film at one end to an uncoated fiber by electric arc fusion splicing. Experimental results have been obtained for sensors embedded in plastic and graphite composite materials, using ultrasound waves in the range from 100 kHz to 5 MHz. Values for the optical phase shift amplitude as large as 0.5 rad were obtained at an acoustic frequency of 200 kHz for a 1.1-cm-long interferometer embedded in plastic.     

Effect of ultraviolet illumination and ambient gases on the photoluminescence and electrical properties of nanoporous silicon layer for organic vapor sensor

The purpose of this research, the nanoporous silicon layer were fabricated and investigated the physical properties such as photoluminescence and the electrical properties in order to develop organic vapor sensor by using nanoporous silicon. The Changes in the photoluminescence intensity of nanoporous silicon samples are studied during ultraviolet illumination in various ambient gases such as nitrogen, oxigen and vacuum. In this paper, the nanoporous silicon layer was used as organic vapor adsorption and sensing element. The advantage of this device are simple process compatible in silicon technology and usable in room temperature. The structure of this device consists of nanoporous silicon layer which is formed by anodization of silicon wafer in hydrofluoric acid solution and aluminum electrode which deposited on the top of nanoporous silicon layer by evaporator. The nanoporous silicon sensors were placed in a gas chamber with various organic vapor such as ethanol, methanol and isopropyl alcohol. From studying on electrical characteristics of this device, it is found that the nanoporous silicon layer can detect the different organic vapor. Therefore, the nanoporous silicon is important material for organic vapor sensor and it can develop to other applications about gas sensors in the future.     

Chemiresistive Sensor Arrays for Gas/Volatile Organic Compounds Monitoring: A Review

Gas detection and monitoring are essential due to their direct impact on human health, environment, and ecosystem. Chemiresistive sensors are one of the most used classes of sensors for monitoring and measurement of gases thanks to their ease of fabrication, customizability, mechanical flexibility, and fast response time. While chemiresistive sensors can offer good sensitivity and selectivity to a particular gas in a controlled environment with known interferences, they may not be able to differentiate between various gases having similar physiochemical properties under uncontrolled conditions. To address this shortcoming of chemiresistive gas sensors, sensor arrays have been the subject of recent studies. Gas sensor arrays are a group of individual gas sensors that are arranged to simultaneously detect and differentiate multiple cross-reactive gases. In this regard, various sensor array technologies have been developed to differentiate a given set of gases using multivariate algorithms. This review provides an insight into the different algorithms that are used to extract the data from the sensor arrays, highlighting the fabrication techniques used for developing the sensor array prototypes, and different applications in which these arrays are used.