Disordered, strongly scattering porous materials as miniature multipass gas cells

Spectroscopic gas sensing is both a commercial success and a rapidly advancing scientific field. Throughout the years, massive efforts have been directed towards improving detection limits by achieving long interaction pathlengths. Prominent examples include the use of conventional multipass gas cells, sophisticated high-finesse cavities, gas-filled holey fibers, integrating spheres, and diffusive reflectors. Despite this rich flora of approaches, there is a continuous struggle to reduce size, gas volume, cost and alignment complexity. Here, we show that extreme light scattering in porous materials can be used to realise miniature gas cells. Near-infrared transmission through a 7 mm zirconia (ZrO2) sample with a 49% porosity and subwavelength pore structure (on the order of 100 nm) gives rise to an effective gas interaction pathlength above 5 meters, an enhancement corresponding to 750 passes through a conventional multipass cell. This essentially different approach to pathlength enhancement opens a new route to compact, alignment-free and low-cost optical sensor systems

Clinical system for non-invasive in situ monitoring of gases in the human paranasal sinuses

We present a portable system for non-invasive, simultaneous sensing of molecular oxygen (O-2) and water vapor (H2O) in the human paranasal cavities. The system is based on high-resolution tunable diode laser spectroscopy (TDLAS) and digital wavelength modulation spectroscopy (dWMS). Since optical interference and non-ideal tuning of the diode lasers render signal processing complex, we focus on Fourier analysis of dWMS signals and procedures for removal of background signals. Clinical data are presented, and exhibit a significant improvement in signal-to-noise with respect to earlier work. The in situ detection limit, in terms of absorption fraction, is about 5 x 10(-5) for oxygen and 5 x 10(-4) for water vapor, but varies between patients due to differences in light attenuation. In addition, we discuss the use of water vapor as a reference in quantification of in situ oxygen concentration in detail. In particular, light propagation aspects are investigated by employing photon time-of-flight spectroscopy. (C) 2009 Optical Society of Americ

Wall-collision line broadening of molecular oxygen within nanoporous materials

Wall-collision broadening of near-infrared absorption lines of molecular oxygen confined in nanoporous zirconia is studied by employing high-resolution diode-laser spectroscopy. The broadening is studied for pores of different sizes under a range of pressures, providing new insights on how wall collisions and intermolecular collisions influence the total spectroscopic line profile. The pressure series show that wall-collision broadening is relatively more prominent under reduced pressures, enabling sensitive means to probe pore sizes of porous materials. In addition, we show that the total wall-collision-broadened profile strongly deviates from a Voigt profile and that wall-collision broadening exhibits an additive-like behavior to the pressure and Doppler broadening

Laser Absorption Spectroscopy of Gas in Scattering Media

Absorption spectroscopy constitutes a chemical analysis tool which can be applied to various samples and application fields. This thesis focuses on gas absorption spectroscopy with the means of diode lasers - tunable diode laser absorption spectroscopy, TDLAS. In particular, the absorption of gases inside porous scattering solids and liquids, referred to as GASMAS - gas in scattering media absorption spectroscopy, has been studied. The spectrally sharp wavelength from a diode laser is scanned over the absorption fingerprint of the studied gas, and the transmitted intensity is detected. Although the light is heavily absorbed and scattered by the bulk material, the gas absorption imprint can still be distinguished. The free gas, in contrast to the bulk material which is made out of perturbed molecules, exhibits narrow absorption lines and its absorption can thus be isolated although the light has been heavily absorbed by the solids and liquids. The diffuse light propagation demands alternative solutions to extract gas concentration, which otherwise is obtained through the Beer-Lambert law using the interaction distance. Water vapor sensing in samples with liquid water present has shown to be a feasible way to achieve information about the effective distance through gas inside the sample even though the light is heavily scattered. Applications studied in this thesis work include gas sensing within the human body for medical diagnostics, gas monitoring inside food packages for quality assurance, and fundamental studied of gas in nanoporous ceramics. Investigations of GASMAS as a diagnostic tool for the paranasal sinuses, subject to the common rhinosinusitis have been carried out. Correlation between obstruction and ventilation of the sinuses diagnosed by computer tomography and GASMAS data has been demonstrated. Diagnostic useful data from the air cell system in the mastoid bone, located behind the ears, have also been obtained. Furthermore, possibilities of gas sensing in the lungs of premature born babies have been demonstrated in a feasibility study on a realistic model made out of animal lung tissue and gelatin based phantoms. Sensing of the gas non-intrusively by GASMAS has been demonstrated on packages of minced meat, bread as well as on the headspace of translucent containers with milk or orange juice. Food packages are to an increasing extent filled with a modified atmosphere where the O2 concentration is suppressed. Traditional gas detectors for food packages are intrusive or demand direct optical access. GASMAS constitutes an alternative interesting approach with high potential. Gas detection in nano-porous samples allows fundamental studies of the gas molecules. Broadening of absorption lines of O2 and H2O due to tight confinement in nano pores in ceramics has been studied, as well as diffusion of the gases. In addition to fundamental physical interest the study of broadening and diffusion allows for assessment of material parameters

Quasi zero-background tunable diode laser absorption spectroscopy employing a balanced Michelson interferometer.

Tunable diode laser spectroscopy (TDLS) normally observes small fractional absorptive reductions in the light flux. We show, that instead a signal increase on a zero background can be obtained. A Michelson interferometer, which is initially balanced out in destructive interference, is perturbed by gas absorption in one of its arms. Both theoretical analysis and experimental demonstration show that the proposed zero-background TDLS can improve the achievable signal-to-noise ratio

Laser absorption spectroscopy of water vapor confined in nanoporous alumina: wall collision line broadening and gas diffusion dynamics.

We demonstrate high-resolution tunable diode laser absorption spectroscopy (TDLAS) of water vapor confined in nanoporous alumina. Strong multiple light scattering results in long photon pathlengths (1 m through a 6 mm sample). We report on strong line broadening due to frequent wall collisions (gas-surface interactions). For the water vapor line at 935.685 nm, the HWHM of confined molecules are about 4.3 GHz as compared to 2.9 GHz for free molecules (atmospheric pressure). Gas diffusion is also investigated, and in contrast to molecular oxygen (that moves rapidly in and out of the alumina), the exchange of water vapor is found very slow

LIDAR technique for Remote Gas Analysis in Solid Scattering Media

LIDAR techniques are used to measure gases in solid scattering media remotely, by analyzing the differential absorption observed in the multiple scattering light. The gas exchange (O-2/N-2) in polystyrene foam is monitored. (C)2008 Optical Society of Americ

Simultaneous detection of molecular oxygen and water vapor in the tissue optical window using tunable diode laser spectroscopy

We report on a newly developed diode-laser spectroscopic system for gas detection in scattering media. Two pigtailed diode lasers are used, operating in a wavelength modulation scheme, to simultaneously detect molecular oxygen at 760 nm and water vapour at 935 nm within the tissue optical window (600 nm to 1.3 μm). Different modulation frequencies are used to distinguish between the two wavelengths. No crosstalk can be observed between the gas contents of the two gas channels. The system is made compact by using a computer board and performing software based lock-in detection. The noise floor obtained corresponds to an absorption fraction of about 6 · 10−5 for both oxygen and water vapour. The power of the technique is illustrated by preliminary results from a clinical trial investigating the human sinuses

Gas monitoring in human sinuses using tunable diode laser spectroscopy

We demonstrate a novel nonintrusive technique based on tunable diode laser absorption spectroscopy to investigate human sinuses in vivo. The technique relies on the fact that free gases have spectral imprints that are about 10.000 times sharper than spectral structures of the surrounding tissue. Two gases are detected; molecular oxygen at 760 nm and water vapor at 935 nm. Light is launched fiber optically into the tissue in close proximity to the particular maxillary sinus under study. When investigating the frontal sinuses, the fiber is positioned onto the caudal part of the frontal bone. Multiply scattered light in both cases is detected externally by a handheld probe. Molecular oxygen is detected in the maxillary sinuses on 11 volunteers, of which one had constantly recurring sinus problems. Significant oxygen absorption imprint differences can be observed between different volunteers and also left-right asymmetries. Water vapor can also be detected, and by normalizing the oxygen signal on the water vapor signal, the sinus oxygen concentration can be assessed. Gas exchange between the sinuses and the nasal cavity is also successfully demonstrated by flushing nitrogen through the nostril. Advantages over current ventilation assessment methods using ionizing radiation are pointed out