Märta Lewander

Disordered, strongly scattering porous materials as miniature multipass gas cells.

We investigate the interaction of light and gas in strongly scattering nano- and macroporous media. Manufacturing and structural characterization of ZrO(2), Al(2)O(3) and TiO(2) ceramics with different pore sizes, measurements of optical properties using photon time-of-flight spectroscopy, and high-resolution laser spectroscopy of O(2) at 760 nm are reported. We show that extreme light scattering can be utilized to realize miniature spectroscopic gas cells. Path length enhancement factors up to 750 are reached (5.4 m path through gas for light transmitted through a 7 mm ZrO(2) with 49% porosity and 115 nm pores).

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.

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

We report on a dual-diode laser spectroscopic system for simultaneous detection of two gases. The technique is demonstrated by performing gas measurements on absorbing samples such as an air distance, and on absorbing and scattering porous samples such as human tissue. In the latter it is possible to derive the concentration of one gas by normalizing to a second gas of known concentration. This is possible if the scattering and absorption of the bulk material is equal or similar for the two wavelengths used, resulting in a common effective pathlength. Two pigtailed diode lasers are operated in a wavelength modulation scheme to detect molecular oxygen ~760 nm and water vapor ~935 nm within the tissue optical window (600 nm to 1.3 mum). Different modulation frequencies are used to distinguish between the two wavelengths. No crosstalk can be observed between the gas contents measured in 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 approximately 6x10(-5) for both oxygen and water vapor, yielding a minimum detection limit of ~2 mm for both gases in ambient air. The power of the technique is illustrated by the preliminary results of a clinical trial, nonintrusively investigating gas in human sinuses.

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 (H(2)O) 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 5x10(-5) for oxygen and 5x10(-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.

Method for Studying Gas Composition in the Human Mastoid Cavity by Use of Laser Spectroscopy

Objectives: We evaluated a method for gas monitoring in the mastoid cavity using tunable diode laser spectroscopy by comparing it to simultaneously obtained computed tomographic (CT) scans. Methods: The presented optical technique measures free gases, oxygen (O2), and water vapor (H2O) within human tissue by use of low-power diode lasers. Laser light was sent into the tip of the mastoid process, and the emerging light at the level of the antrum was captured with a detector placed on the skin. The absorption of H2O was used to monitor the probed gas volume of the mastoid cavity, and it was compared to the CT scan–measured volume. The ratio between O2 absorption and H2O absorption estimated the O2 content in the mastoid cavity and thus the ventilation. The parameters were compared to the grading of mastoid cavities based on the CT scans (n = 31). The reproducibility of the technique was investigated by measuring each mastoid cavity 4 times. Results: Both O2 and H2O were detected with good reproducibility. The H2O absorption and the CT volume correlated (r = 0.69). The average ratio between the normalized O2 absorption and the H2O absorption signals was 0.7, indicating a lower O2 content than in surrounding air (expected ratio, 1.0), which is consistent with previous findings made by invasive techniques. All mastoid cavities with radiologic signs of disease were detected. Conclusions: Laser spectroscopy monitoring appears to be a usable tool for noninvasive investigations of gas composition in the mastoid cavity, providing important clinical information regarding size and ventilation.

Non-invasive gas monitoring in newborn infants using diode laser absorption spectroscopy: a case study

Non-invasive diode laser spectroscopy was, for the first time, used to assess gas content in the intestines and the lungs of a new-born, 4 kg, baby. Two gases, water vapor and oxygen, were studied with two low-power tunable diode lasers, illuminating the surface skin tissue and detecting the diffusely emerging light a few centimeters away. The light, having penetrated into the tissue, had experienced absorption by gas located in the lungs and in the intestines. Very distinct water vapor signals were obtained from the intestines while imprint from oxygen was lacking, as expected. Detectable, but minor, signals of water vapor were also obtained from the lungs, illuminating the armpit area and detecting below the collar bone. Water vapor signals were seen but again oxygen signals were lacking, now due to the difficulties of penetration of the oxygen probing light into the lungs of this full-term baby. Ultra-sound images were obtained both from the lungs and from the stomach of the baby. Based on dimensions and our experimental findings, we conclude, that for early pre-term babies, also oxygen should be detectable in the lungs, in addition to intestine and lung detection of water vapor. The present paper focuses on the studies of the intestines while the lung studies will be covered in a forthcoming paper.

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.

Wall-collision broadening of Gas absorption lines in nanoporous materials

The main broadening mechanisms of absorption lines in free gas are known to be the pressure-and-Doppler broadening. However, with gas trapped within nanoporous materials, wall-collision broadening may become non-negligible. We present and discuss experimental data which show the effect of wall-collision broadening and its applications.

Diagnostics of human gas cavities with diode laser absorption spectroscopy

Noninvasive sensing of gas in human cavities with diode laser absorption spectroscopy is possible although the light is heavily absorbed and scattered by the tissue. We present data from the human paranasal sinuses and mastoid bone as well as monitoring of gas in lung tissue.

Zero-background tunable diode laser absorption spectroscopies using balanced interferometers

Tunable diode laser spectroscopy (TDLS) is one of the most successful techniques for trace-gas environmental monitoring [1–3]. By modulating the light source at high frequency for suppressing the noise [2], frequency modulation or wavelength modulation TDLS can measure optical absorption in atomic or molecular samples with high resolution and sensitivity. Apart from these absorptive techniques, there exists a group of zero-background spectroscopic methods [3], e.g. laser-induced fluorescence spectroscopy, photo-acoustic spectroscopy, and polarization spectroscopy. Here a spectroscopic signal rises from a zero or low background with little noise and these methods therefore can achieve good signal-to-noise ratio (SNR) even without any modulation on the light source.