Infrared laser stark shift spectroscopy in ammonia

We demonstrate what is, to our knowledge, the first use of mid-infrared laser absorption spectroscopy for trace-gas measurements of cyclotron target outputs used for the generation of radioactive carbon-11 in positron emission tomography (PET). The spectrometer was based upon a liquid-nitrogen-cooled lead salt diode laser generating single-mode radiation in the wavenumber range of 2230–2240 cm−1. The sample flowed to a multiple-pass optical cell with a total path length of 15.23 m and the laser radiation was detected by two liquid-nitrogen-cooled InSb photodetectors. We present the results of CO, N2O and CO2 measurements on PET trace cyclotron output and discuss future work on 11CO and 11CO2 detection.

An optical method for measuring soot volume fraction, flame temperature, and concentration of combustion products in a diffusion flame based on time-division multiplexed tunable diode laser absorption spectroscopy technique is presented in this article. By moving the slot burner with an electric platform, two-dimensional profile of soot volume fraction, flame temperature, and gas concentrations along the flame sheet with 1-mm spatial resolution were obtained simultaneously. The flame temperature and soot volume fraction were simultaneously on-line monitored using one distributed feedback diode laser with a center wavelength at 1397.83 nm. Tunable diode laser absorption spectroscopy based temperature measurements in the flame were carried out by two-line thermometry of H2O and compared with the thermocouple technique. The attenuation of tunable diode laser intensity caused by soot extinction and gas absorption after transmitting through the diffusion flame was detected. And the soot volume fraction was determined quantitatively with laser extinction based on Rayleigh limit assumption. Two tunable distributed feedback diode lasers with center wavelength at 2001.6 and 2302 nm were used to determine the CO2 concentration and CO concentration, respectively. The experimental results of soot volume fraction, flame temperature, and the mole fraction of combustion products by varying concentrations of CO2 added in the diffusion flame were analyzed.

Most aircrafts is driven by chemic energy which is released in the combustion process. For improving the capability of engine and controlling the running on-time, the processes of fuel physics and chemistry need to be analysis by kinds of high quality sensor. In the research of designing and improving the processes of fuel physics and chemistry, the concentration, temperature and velocity of kinds of gas in the combustor need to be detected and measured. In addition, these engines and research equipments are always in the harsh environment of high temperature, high pressure and high speed. The harsh environment needs the sensor to be high reliability, well repetition, no cross- sensitivity between gases, and the traditional measurement system can't satisfy the metrical requirement well. Tunable diode laser absorption spectroscopy (TDLAS) analytic measurement technology can well satisfy the measurement in the harsh environment, which can support the whole measurement plan and high quality measurement system. Because the TDLAS sensor has the excellence of small bulk, light weight, high reliability and well specifically measurement, the TDLAS measurement technology has wide prospects. Different from most measurements, only a beam of laser can be pass through the measured environment by TDLAS, and the measurement equipment needn't be set in the harsh environment. So, the TDLAS equipment can't be interrupted by the measured equipment. The ability of subsistence in the harsh environment is very valuable, especially in the measurement on the subject of aerospace with environment of high speed, combustion and plasma. This paper focuses on the collecting the articles on the subject of oxygen detection of TDLAS. By analyzing the research and results of the articles, we conclude the central issues, difficulties and results. And we can get some instructive conclusions.

A compact single-ended optical sensor for temperature measurements via laser absorption spectroscopy

Within the last decade mid-infrared absorption spectroscopy over a region from 3 to 17µm and based on tuneable lead salt diode lasers, often called tuneable diode laser absorption spectroscopy or TDLAS, has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry in molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, organo-silicon and boron compounds has led to further applications of TDLAS because most of these compounds and their decomposition products are infrared active. TDLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetic phenomena. Information about gas temperature and population densities can also be derived from TDLAS measurements. A variety of free radicals and molecular ions have been detected by TDLAS. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of quantum cascade lasers (QCLs) offers an attractive new option for the monitoring and control of industrial plasma processes. The aim of the present paper is threefold: (i) to review recent achievements in our understanding of molecular phenomena in plasmas, (ii) to report on selected studies of the spectroscopic properties and kinetic behaviour of radicals and (iii) to describe the current status of advanced instrumentation for TDLAS in the mid-infrared.

The point measurement laser absorption spectroscopy (PMLAS) based on saturated absorption theory could surpass the defect of 'line-of-sight' measurement in traditional tunable diode laser absorption spectroscopy (TDLAS) and achieve the 'point' measurement with millimeter spatial resolution. It is realized by crossing with two frequency synchronized laser beams: one named probe beam as in traditional TDLAS and the other named saturated beam with higher power. In this paper, the theory of PMLAS was firstly analyzed by the theoretical deduction of saturated absorption coefficients with arbitrary cross angles and the numerical calculations of point absorbance under different saturation parameters. Next, a weak signal detection method based on high-frequency sinusoidal modulation of the saturated beam intensity was proposed, in which the first-order harmonic signal was theoretically deduced and verified by numerical demonstration. Furthermore, it is found that the FWHMs (full width at half maximum) of different order harmonics are all the same and equal to the width of the absorption signal without modulation, which implied that the superposition of multi-harmonics could enhance the signal-to-noise ratio (SNR) in measuring the spectrum line-width.

Tunable multi-mode diode laser absorption spectroscopy for methane detection

Magnetic resonance imaging (MRI) and tunable diode laser absorption spectroscopy (TDLAS) are focused as novel diagnostic techniques for in-situ measurement of water in operating polymer electrolyte membrane fuel cells (PEMFCs). Basic principle and practical applications of MRI to visualize water content distribution in polymer electrolyte membranes (PEMs) are described. It is shown that MRI visualization gives us fundamental understandings on water transport processes involved in PEMFCs. In-situ TDLAS measurement technique is also presented as a diagnostic tool to evaluate variation of water vapor concentration in gas flow channels in PEMFCs. TDLAS measurement in combination with numerical analysis can be used to obtain local current density distribution and local water transfer numbers across the membrane. Recent advances and future prospects on MRI and TDLAS as in-situ diagnostic technique for water transport in operational PEMFCs are summarized.

The H (n = 2) atomic density determination by means of vacuum ultra-violet (VUV) emission spectroscopy of the Lyman-α line in laboratory low pressure plasmas is strongly affected by self-absorption of emitted photons inside the plasma leading to an underestimation of this density. A correction of the densities obtained from VUV emission spectroscopy measurements is performed by using the escape factor method. The corrections applied can reach orders of magnitude even in low pressure plasmas. Assumptions on the spatial distribution of emitting and absorbing particles as well as on the corresponding line profiles have to be made. Consequently, additional measurements are performed, which raises the requirement of a benchmark of the applied correction procedure. In contrast, tunable diode laser absorption spectroscopy (TDLAS) on the Balmer-α line is a direct measurement of the H (n = 2) density and is thus suitable for a benchmark. For the ICP under consideration, H (n = 2) densities obtained via TDLAS are near 3 × 1015 m−3 whereas uncorrected VUV emission spectroscopy gives values in the range of roughly 1013 m−3 depending on pressure and applied RF power. The calculated escape factors are on the order of 2–8 × 10−3. An excellent agreement with TDLAS is observed by applying them to the results of the VUV emission spectroscopy.

Dissolved inorganic carbon (DIC) - including aqueous CO2, carbonic acid, bicarbonate, and carbonate - is often the largest pool of carbon in aquatic systems. Biogeochemical processes result in exchanges of carbon between the various DIC components and may act to move carbon into or out of the DIC pool. The isotopic composition of carbon is a product of both its source and mass-dependent fractionation as carbon changes form through the processes acting on it. Consequently, measurement of the stable carbon isotope composition of DIC is a valuable tool for understanding biogeochemical processes in aquatic systems. However, differences in isotopic composition are small, and separating source contributions requires precise measurement.Measurement of DIC can be done by conversion to CO2 in the presence of a strong acid and quantification of liberated CO2 by gas analysis. To determine isotopic composition of the liberated CO2 (δ13C) historical methods used isotope ratio mass spectrometry (IRMS). More recently, tunable diode laser absorption spectrometry (TDLAS) based gas analyzers have been adopted for these measurements but have continued to base methodological considerations on those developed for IRMS. While IRMS and TDLAS can both be used to determine δ13C, there are fundamental differences in the technology, which should be considered during application. In particular, this has meant δ13C - DIC measurements have been unable to take full advantage of TDLAS performance characteristics. Here we describe methodological advancements from integration of a TDLAS (LI-7825 carbon isotope analyzer) with a DIC measurement system (LI-5370A), that include changes to the pneumatic and analytical approach used in the DIC system. Pneumatic modifications allow the TDLAS to operate at an independent flow rate from the DIC system and serve to manipulate the residence time for CO2 along the flow path. We describe use of a non-CO2 free carrier gas, which allows the DIC measurement to take full advantage of analyzer precision and minimize errors intrinsic to δ13C as determined by TDLAS. We present data demonstrating measurement precision over a range of conditions and show that under similar conditions, these methodological changes result in precision exceeding that published previously for TDLAS-DIC measurements.

Modified laser scanning technique in wavelength modulation spectroscopy for advanced TDLAS gas sensing

We report the effects of self-mixing interference on gas detection using tunable diode laser spectroscopy. For very weak feedback, the laser diode output intensity gains a sinusoidal modulation analogous to that caused by low finesse etalons in the optical path. Our experiments show that self-mixing interference can arise from both specular reflections (e.g. cell windows) and diffuse reflections (e.g. Spectralon™ and retroreflective tape), potentially in a wider range of circumstances than etalon-induced interference. The form and magnitude of the modulation is shown to agree with theory. We have quantified the effect of these spurious signals on methane detection using wavelength modulation spectroscopy and discuss the implications for real gas detectors.

We designed a tunable diode laser absorption spectroscopy (TDLAS) sensor for the online monitoring of CO2 and H2O concentrations. It comprised a small self-design multi-pass cell, home-made laser drive circuits, and a data acquisition circuit. The optical and electrical parts and the gas circuit were integrated into a portable carrying case (height = 134 mm, length = 388 mm, and width = 290 mm). A TDLAS drive module (size: 90 mm × 45 mm) was designed to realize the function of laser current and temperature control with a temperature control accuracy of ±1.4 mK and a current control accuracy of ±0.5 μA, and signal acquisition and demodulation. The weight and power consumption of the TDLAS system were only 5 kg and 10 W, respectively. Distributed feedback lasers (2004 nm and 1392 nm) were employed to target CO2 and H2O absorption lines, respectively. According to Allan analysis, the detection limits of CO2 and H2O were 0.13 ppm and 3.7 ppm at an average time of 18 s and 35 s, respectively. The system response time was approximately 10 s. Sensor performance was verified by measuring atmospheric CO2 and H2O concentrations for 240 h. Experimental results were compared with those obtained using a commercial instrument LI-7500, which uses non-dispersive infrared technology. Measurements of the developed gas analyzer were in good agreement with those of the commercial instrument, and its accuracy was comparable. Therefore, the TDLAS sensor has strong application prospects in atmospheric CO2 and H2O concentration detection and ecological soil flux monitoring.

Applications of the Tunable Diode Laser Absorption Spectroscopy: In-Process Estimation of Primary Drying Heterogeneity and Product Temperature During Lyophilization

Temperature and OH concentration measurements by ultraviolet broadband absorption of OH(X) in laminar methane/air premixed flames