Detection Of Water Vapor Research Articles

Wavelength Modulated Cavity Enhanced Absorption Spectroscopy of Water in the 1.37 μm Region

We describe the performance of a high-sensitivity wavelength modulated cavity enhanced infrared tunable diode laser absorption spectrometer for the detection of water vapor in the 1.37 mum region. The spectrometer can measure a fractional absorption of approximately 10(-5) for an absorption path length of a few kilometers. The instrument's sensitivity is more than sufficient to detect water isotopomers (H(2)(16)O, H(2)(18)O, HDO) at Martian atmospheric concentrations. The instrument is amenable to miniaturization, so a future compact, multiple-species version of the spectrometer will be highly suitable for in situ planetary exploration.

Water-Vapor Detection Using Asynchronous THz Sampling

The use of a fiber-coupled terahertz (THz) transmitter/receiver pair for spectroscopic detection of water vapor is investigated. Transmission signals of an alumina cylinder demonstrate that the measurement approach can be applied in a windowless ceramic combustor. First, a conventional commercial transmitter/receiver pair is used to make measurements for frequencies to 1.25 THz. Water-vapor absorption is clearly evident within the alumina transparency window and is readily modeled using existing databases. A variety of data-acquisition schemes is possible using THz instrumentation. To assess signal-collection techniques, a prototype THz transmitter/receiver pair is then used with the asynchronous optical-sampling (ASOPS) technique to obtain asynchronous THz-sampling signals to 1 THz without the need for an optomechanical delay line. Two mode-locked Ti:sapphire lasers operating at slightly different repetition rates are used for pumping the transmitter and receiver independently to permit a complete time-domain THz signal to be recorded. The resulting repetitive phase walkout is demonstrated by collecting power spectra of room air that exhibit water-vapor absorption.

Characteristics of several NIR tuneable diode lasers for spectroscopic based gas sensing: A comparison

Tuneable laser diodes were characterized and compared for use as tuneable sources in gas absorption spectroscopy. Specifically, the characteristics of monolithic widely tuneable single frequency lasers, such as sampled grating distributed Bragg reflector laser and modulated grating Y-branch laser diodes, recently developed for optical communications, with operating wavelengths in the 1520 nm ≤ λ ≤ 1570 nm are compared. The assessment also includes an external cavity laser and a distributed feedback laser for water vapour detection, both emitting at 935 nm. Characteristics investigated include side-mode suppression ratio, ease of tuning, tuning range, spectral emission linewidth, frequency stability and wavelength modulation. While some characteristics differ significantly across the range of lasers, each device has a number of useful intrinsic qualities for gas sensing. Specifically, the modulated grating Y laser and the sampled grating DBR laser have wide quasi-continuous tuneability (30–40 nm) and display relatively low residual amplitude noise when grating-modulated in a harmonic detection scheme. They are particularly suitable for multi-gas sensing. ECLs are also capable of wide quasi-continuous tuneability (100 nm) but their architecture renders them unsuitable for gas sensing application outside a controlled laboratory environment. DFB devices are by far the easiest with which to work but their modest tuneability (4 nm maximum by temperature) almost invariably limits their use to single gas sensing applications.

On the pressure dependent sensitivity of a photoacoustic water vapor detector using active laser modulation control

A detailed study on the pressure dependent sensitivity of a wavelength modulated diode laser based photoacoustic (PA) water vapor detection system is presented. It is shown that the pressure dependence of sensitivity is primarily determined by the pressure dependence of the microphone’s sensitivity and the quality factor of the excited acoustic resonance of the PA cell. Effort was made to improve the system’s sensitivity for the whole pressure range (from 200 mbar to 1000 mbar) of operation typical in atmospheric research, while maintaining the inherent fast response time of the PA system. For this purpose active control of modulation based on the continuous adjustment of the unmodulated and modulated parts of the laser current in accordance with the actual gas pressure was introduced, with which a minimum detectable water vapor concentration (3 σ) of 300 ppb at 200 mbar and 188 ppb at 1000 mbar was achieved. The system’s sensitivity improves slightly at the lower end of the pressure range and increases by a factor of more than two at the higher end, when compared with that of our PA system currently on board of a commercial aircraft within the framework of an atmospheric research project (CARIBIC—Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container). Test measurements proved the feasibility of the implementation of the developed modulation method within the framework of the CARIBIC project.

Arsine Absorption Behavior at 1392 nm and Strategy for Trace Water Vapor Detection in Arsine using Cavity Ring-down Spectroscopy

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Intracavity Cr4+ :YAG laser absorption analyzed by time-resolved Fourier transform spectroscopy

A high-resolution time-resolved Fourier transform interferometer is combined with a multimode Cr4+:YAG laser for intracavity laser absorption spectroscopy (ICLAS) experiments. Atmospheric absorption spectra are recorded in the 1.5 μm region with a minimum detectable absorption coefficient equal to 8×10-11 cm-1 Hz-1/2. The broad gain bandwidth of the crystal allows a simultaneous spectral coverage at most equal to 38 nm. The laser tunability covers the 1360–1577 nm range. Water vapor detection domain extends from the 100 ppmv down to the 0.1 ppbv level.

Detection of high-temperature water vapor at 940 nm with vertical-cavity surface-emitting lasers

A vertical-cavity surface-emitting laser was used to study the absorption of water vapor in the 940 nm region. Measurements for several absorption lines within the 2 v1 + V3 vibrational band were performed. Line strengths at room temperature and in a heated absorption cell over the temperature range of 420-970 K were obtained. The line strength values were in good agreement with simulations based on the values of the HITRAN 2004 database. The measurements also showed that water vapor transitions near 940 nm are suitable for sensitive temperature determination.

Application of laser photoacoustic spectroscopy for the detection of water vapor near 1.38 μm

Laser photoacoustic (PA) spectroscopy is applied for the determination of the trace content of water vapor using a differential Helmholtz resonant (DHR) cell and a narrow bandwidth diode laser operating near 1380 nm. The PA spectroscopy revealed a rich absorption spectra in this wavelength region and the observed result was compared with the HITRAN database. A multipass optical system was also developed by adopting one aluminum-coated flat mirror with a small uncoated spot for the laser introduction to the detection chamber and one aluminum-coated concave mirror. The multipass optical system enabled the enhancement of the PA signal up to eight times when compared to the single pass case. The calibration curve was plotted by measuring PA signals for various pressures of the water vapor. The sensitivity of the PA detection system is estimated as 7.3×10 12 molecules cm −3 with a signal-to-noise ratio (SNR) of 1.

Carboxylation multi-walled carbon nanotubes modified withLiClO4 for water vapour detection

In this paper, the humidity sensing properties of multi-walled carbon nanotubes modified withLiClO4 are investigated. The new sensing material is characterized by FESEM, FTIR, Raman, ICPand XPS. For comparison, the humidity sensing characteristics of the four conventional materialsMnWO4 (hubnerite),BaTiO3 (perovskite),NiWO4 (huberite),and ZnCr2O4 (spinel) are also discussed. The selectivity of the new humidity sensing material is studied.Finally, the response to water vapour of the new sensing material is discussed by using adynamic testing method.

VCSEL based detection of water vapor near 940 nm

A vertical-cavity surface-emitting laser (VCSEL) was used to study the absorption spectrum of water vapor in the 940 nm region. Measurements were performed in ambient air at room temperature and in a hydrogen–oxygen flame over the temperature range of 1500–1800 K. Several rotational absorption lines within the 2 ν 1+ ν 3 vibrational band were measured. The absorption spectra were well resolved, which demonstrates the feasibility of VCSEL-based spectroscopic measurements of water vapor at room and high-temperature in this spectral region. The results were in good agreement with the values obtained from the HITRAN-96 database.

Diode laser based photoacoustic water vapor detection system for atmospheric research.

A wavelength modulated, distributed feedback diode laser based photoacoustic water vapor mixing ratio measuring system for atmospheric research applications is presented. Laser modulation parameters were optimized either at 180 or 500 mbar total pressure to enhance the system's sensitivity for low or high pressures (upper troposphere/lower stratosphere or biosphere exchange layer), respectively. A wavelength locking method was developed that ensured sub-picometer absolute (5 x 10(-7) relative) wavelength stability of the laser while consuming minimum additional measurement time. At the calibration of the system, correction factors for the pressure- and temperature-dependence of the photoacoustic signal were determined, which were in turn applied to the calculation of the water vapor mixing ratio from the measured signal during the test operation of the system. The introduced features resulted in reliable, sub-ppm-level water vapor detection even under abrupt gas pressure or temperature variations typical in open atmospheric applications.

Development of a micro-balance system for dust and water vapour detection in the Mars atmosphere

Quartz crystal piezoelectric sensors are suitable for deposition analyses that need very high sensitivity. Due to the wide working ranges and high performances, micro-balances can measure the mass settling in average Mars conditions during a period of months before saturation is reached. This ensures a proper use for short and long term water and dust deposition monitoring. Micro-balances have been studied, calibrated and used for the GIADA (grain impact analyser and dust accumulator) experiment for the ESA-Rosetta space mission. Experience on micro-balance performance study by dust deposition has been acquired and water vapour deposition studies are in progress in a Martian atmosphere simulation chamber. Preliminary results show that micro-balances are capable to detect up to partial pressure values corresponding to parts per billion of the typical Martian atmosphere.

Difference-frequency-generator�??based spectrometer at 3 μm for high-sensitivity C2H2 and H2O detection

We detect acetylene and water-vapor lines by using a difference-frequency generator in the spectral region around 3 mm. Both C2H2 and H2O lines belong to fundamental vibrational bands and exhibit a line strength of the order of 10-20 cm/mol. Acetylene molecules were detected either by pure absorption or by first-derivative wavelength-modulation spectroscopy. The minimum detection sensitivity achieved for C2H2 in nitrogen was 4 ppb (parts in 109). Moreover, we discuss the effects of C2H2 pressure reduction in the presence of nitrogen in order to estimate systematic errors in the concentration measurements. Finally, we tested the accuracy of our spectrometer by detecting water vapor present as an impurity in a nitrogen cylinder at a nominal concentration of approximately 5 ppm.

Water Vapor Uptake in Photolithographic Polymers Observed by Infrared Near-Field Scanning Optical Microscopy in a Controlled Environment

An infrared near-field scanning optical microscope (NSOM) is used in a controlled environment chamber to detect water vapor uptake in thin polymer films. A chemically amplified photoresist sample composed of alternating 2 μm lines of the original 1000 nm thick poly-(tert-butylmethacrylate) (PTBMA) and the photochemically modified 500 nm thick poly(methacrylic acid) (PMAA) is studied both in low and high water vapor environments. The degree of water vapor absorption by the sample is measured using the infrared transmission of 2.85 μm light on a small spatial scale (<500 nm). The accompanying topographic swelling of the samples is measured using a shear-force feedback loop. Distortion of the topographic structure and variation in transmission contrast indicate that the PMAA zones absorb more water than the PTBMA regions in the water vapor environment. The PMAA swells 280 nm more than PTBMA when exposed to a partial pressure of water vapor of 2.1 kPa (16 Torr), whereas the infrared optical contrast is increased to a 6 ± 1% difference in the PMAA regions compared to the PTBMA.

Diode laser based photoacoustic humidity sensors

Various diode laser based photoacoustic (PA) systems applied in water vapour detection are presented. The dependence of the systems’ performance on the physical and chemical properties of the measured gas is discussed. It was found that the pressure dependence of the measured PA signal is different for amplitude and wavelength modulated lasers. This difference is explained. Linearity in a concentration range of more than four orders of magnitude in combination with typical accuracy of a few percentages was proved.

Intracavity absorption spectroscopy with thulium-doped fibre laser

Abstract For applications to intracavity laser absorption spectroscopy, we study the spectral properties of a multimode Tm 3+ -doped silica fibre laser pumped by a diode laser. In cw operation, the emission spectrum of the fibre laser is typically 4 nm wide and can be tuned from 1.70 to 1.98 μm. It shows many absorption lines of H 2 O and CO 2 due to the presence of the atmosphere in the cavity. The strength of the lines corresponds to a 7 km effective absorption path length. With repetitively pulsed operation of the fibre laser, measurements of the spectral dynamics are carried out by a specially designed rapid-scan Fourier spectrometer. Highly sensitive detection of water vapour in the laser cavity is possible when tuning the laser to the strongest absorption near 1.9 μm, and by using photoacoustic detection combined with wavelength modulation.

Submillimeter Wave Astronomy Satelliteand Arecibo Observations of H2O and OH in a Diffuse Cloud along the Line of Sight to W51

Observations of W51 with the Submillimeter Wave Astronomy Satellite have yielded the first detection of water vapor in a diffuse molecular cloud. The water vapor lies in a foreground cloud that gives rise to an absorption feature at an LSR velocity of 6 km s-1. The inferred water column density is 2.5 × 1013 cm-2. Observations with the Arecibo radio telescope of hydroxyl molecules at 10 positions in W51 imply an OH column density of 8 × 1013 cm- 2 in the same diffuse cloud. The observed H2O/OH ratio of ~0.3 is significantly larger than an upper limit derived previously from ultraviolet observations of the similar diffuse molecular cloud lying in front of HD 154368. The observed variation in H2O/OH likely points to the presence in one or both of these clouds of a warm (T 400 K) gas component in which neutral-neutral reactions are important sources of OH and/or H2O.

Detection of Water Vapor in the Photosphere of Arcturus

We report detections of pure rotation lines of OH and H2O in the K1.5 III red giant star Arcturus (α Bootis) using high-resolution, infrared spectra covering the regions 806-822 cm-1 (12.2-12.4 μm) and 884-923 cm-1 (10.8-11.3 μm). Arcturus is the hottest star yet to show water vapor features in its disk-averaged spectrum. We argue that the water vapor lines originate from the photosphere, albeit in the outer layers. We are able to predict the observed strengths of OH and H2O lines satisfactorily after lowering the temperature structure of the outermost parts of the photosphere (log τ500 = -3.8 and beyond) compared to a flux-constant, hydrostatic, standard MARCS model photosphere. Our new model is consistently calculated including chemical equilibrium and radiative transfer from the given temperature structure. Possible reasons for a temperature decrease in the outermost parts of the photosphere and the assumed breakdown of the assumptions made in classical model atmosphere codes are discussed.

Deuterium in comets—and AGB stars?

We discuss the deuterium chemistry of cometary comae, and show how the fractionation observed in coma molecules relates to the D/H ratios in the nuclear ices. For most molecules, we find that the fractionation is unchanged in the coma. In addition, we calculate the potential effects of degradation of D-rich polymers/organic refractory material on selected gas-phase fractionation ratios. We demonstrate that for molecules known to have an extended coma source, the observed D/H ratios may not necessarily reflect those in the nuclear ice. The recent detection of water vapour in the stellar wind of the carbon-rich red giant IRC+10°216 has been attributed to the vaporization of a remnant population of comets surrounding this evolved star. This theory would be confirmed by the detection of deuterated molecules in this object: as deuterium is destroyed in stars, any D-bearing molecules must originate from cometary or planetary objects. We quantitatively assess the possibility of detecting HDO in this source if such comets contain similar D/H ratios to those in our solar system, and show that the 1 11–0 00 transition at 894 GHz is the most promising candidate for detection.

Containerless photothermal spectroscopy using photorefractive interferometric detection

Photothermal spectroscopy of gases is explored using optical detection of the thermal expansion of the gas through the use of photorefractive dynamic holography. The photorefractive effect in bismuth silicon oxide is exploited to demodulate the optical phase shift of a signal beam traversing a gaseous environment and coincident with a tunable chopped excitation beam. Molecular absorption at the excitation wavelength produces heat that causes local expansion and subsequent acoustic wave radiation. A test cell, although unnecessary for this technique that can perform measurements in a containerless or open environment, was used to provide known gas mixtures to be tested. A model of the photoacoustic absorption and the optical phase detection process has been developed at low frequencies, where the process can be approximated as photothermal expansion. Measurement and modeling results are presented that illustrate the ability of the method to detect water vapor and hydrogen fluoride concentrations in nitrogen atmosphere backgrounds near 800 nm, currently producing sensitivities in the 20 to 1000 ppm range. The effects of buffer gas concentrations, excitation frequencies, and the ability to measure temporal changes of trace gas concentrations are illustrated. Limitations of the technique and methods for extending to ppb sensitivities with infrared excitation are also discussed.