Marcus Wolff

Finite element calculation of photoacoustic signals

A procedure for the numerical calculation of photoacoustic signals is introduced. It is based on the finite element method and uses an expansion of the signal into acoustic eigenmodes of the measuring cell. Loss is included by the incorporation of quality factors. Surface and volume loss effects attributable to viscosity and thermal conductivity are considered. The method is verified for cylindrical cells with excellent accordance. The application to photoacoustic cells of unconventional shape yields good agreement with experimental data.

VOC breath biomarkers in lung cancer

This review provides an overview of volatile organic compounds (VOCs) which are considered lung cancer biomarkers for diagnostic breath analysis. It includes results of scientific publications from 1985 to 2015. The identified VOCs are listed and ranked according to their occurrence of nomination. The applied detection and sampling methods are specified but not evaluated. Possible reasons for the different results of the studies are stated. Among the most frequently emerging biomarkers are 2-butanone and 1-propanol as well as isoprene, ethylbenzene, styrene and hexanal. The outcome of this review may be helpful for the development of a lung cancer screening device.

Photoacoustic spectrometer based on a DFB-diode laser

We have developed a photoacoustic spectrometer based on a distributed feedback diode laser. The laser can be tuned continuously over 700 GHz enabling both the precise determinations of absorption line parameters such as the pressure broadening coefficient and pressure shift as well as sensitive concentration measurements.

Eigenmode analysis of photoacoustic sensors via finite element method

In the field of photoacoustic sensors, research has mainly been concentrating on the investigation of cylinder shaped cells. For cylinder cells, important acoustical properties can be obtained by analytical methods. In recent years, cells with other shapes have come into focus. In these cases, the sound spectrum and other quantities of interest cannot be obtained by analytical means. Finite element tools, however, are well suited to deal with unconventional geometries. In this article we present results concerning the eigenmodes of T-shaped photoacoustic cells.

Acoustic resonances in HID lamps: model and measurement

A finite element model including plasma simulation is used to calculate the amplitude of acoustic resonances in HID lamps in a 2D axisymmetric geometry. Simulation results are presented for different operation parameters and are compared with experimental data.

Finite element estimation of acoustical response functions in HID lamps

High intensity discharge lamps can experience flickering and even destruction when operated at high frequency alternating current. The cause of these problems has been identified as acoustic resonances inside the lamps arc tube. Here, a finite element approach for the calculation of the acoustic response function is described. The developed model does not include the plasma dynamics.

Quantum cascade laser linewidth investigations for high resolution photoacoustic spectroscopy

High detection selectivity is extremely important for gas analyzers in order to correctly identify the measured compound. Therefore, laser-based systems require a high optical resolution, which primarily depends on the spectral linewidth of the radiation source. This study examines the effective linewidth (chirp) of a pulsed distributed feedback (DFB) quantum cascade laser (QCL) in a photoacoustic (PA) gas detection system. The influence of the QCL operating parameters pulse duration and pulse current as well as the impact of the modulation technique are investigated. Effective QCL linewidths for pulse gate modulation, pulse frequency modulation, and chopper modulation are compared. The investigations are performed by measuring the PA spectra of nitrogen monoxide absorption lines. The results prove the strong influence of pulse duration and pulse current. They also demonstrate that the modulation technique has a considerable influence and, consequently, affects the detection selectivity of the PA analyzer. The aim of this research is to determine optimum operational parameters for high resolution PA spectroscopy.

Solving a coupled field problem by eigenmode expansion and finite element method

The propagation of sound in fluids is governed by a set of coupled partial differential equations supplemented by an appropriate equation of state. In many cases of practical importance one restricts attention to the case of ideal fluids with vanishing transport coefficients. Then, the differential equations decouple and sound propagation can be described by the wave equation. However, when loss mechanisms are important, this is in general not possible and the full set of equations has to be considered. For photoacoustic cells, an alternative procedure has been used for the calculation of the photoacoustic signal of cylinder shaped cells. The method is based on an expansion of the sound pressure in terms of eigenmodes and the incorporation of loss through quality factors of various physical origins. In this paper, we demonstrate that the method can successfully be applied to photoacoustic cells of unconventional geometry using finite element analysis.

Numerical investigation of symmetry breaking and critical behavior of the acoustic streaming field in high-intensity discharge lamps

For energy efficiency and material cost reduction it is preferred to drive high-intensity discharge lamps at frequencies of approximately 300 kHz. However, operating lamps at these high frequencies bears the risk of stimulating acoustic resonances inside the arc tube, which can result in low frequency light flicker and even lamp destruction. The acoustic streaming effect has been identified as the link between high frequency resonances and low frequency flicker. A highly coupled 3D multiphysics model has been set up to calculate the acoustic streaming velocity field inside the arc tube of high-intensity discharge lamps. It has been found that the velocity field suffers a phase transition to an asymmetrical state at a critical acoustic streaming force. The system behaves similar to a ferromagnet near the Curie point. Furthermore, it is discussed how the model allows to investigate the light flicker phenomenon. Concerning computer resources the procedure is considerably less demanding than a direct approach with a transient model.

Modeling and Numerical Investigation of Photoacoustic Resonators

Photoacoustic spectroscopy is based on the photoacoustic effect, that was discovered in 1880 by A. G. Bell (Bell, 1880). One year later, W. C. Rontgen published a paper on the application of photoacoustic spectroscopy on gas (Rontgen, 1881). Sensors based on the photoacoustic effect are devices which allow the detection of molecules of very low concentration. It is even possible to discriminate different isotopes of one molecule. In a photoacoustic sensor (PAS) a gas sample contained in the measuring cell is subjected to a laser beam. The wavelength of the laser is tuned to a vibrational or rotational line of the searched molecules. The technique takes advantage of the fact, that absorbed electromagnetic radiation is due to non-radiant transitions partially transferred into thermal energy of the surrounding molecules. This leads to an increase of the pressure in the sample. A modulated emission generates a sound wave. The resulting acoustic wave is detected by a microphone and phase-sensitively measured. A typical set-up for photoacoustic investigation is shown in Figure 1. To detect low molecule concentrations one enhances the microphone signal by utilizing the acoustic resonances of the measuring chamber. The achievable amplification depends on the shape of the resonator and on the precise coupling of the laser profile and the acoustic modes. Experimental investigations of different PAS set-ups are very time consuming and expensive. Addressing the related questions numerically is much more efficient. The theoretical treatment of PAS has a long history. Analytical calculations have been performed for cylinder shaped resonators, which play an important role among the variety of measuring chambers. Resonator shapes of higher complexity, however, are not amenable to these methods. Numerical techniques like the finite element method (FEM) represent a suitable tool to investigate such systems. Generally, the investigation of the excited gas requires the solution of a system of coupled partial differential equations. The FEM allows the treatment of such coupled problems. However, this is rather computer time consuming and, considering the numerous design variants, should be avoided. In literature, methods are discussed, that allow to circumvent the coupled problem. We combine these methods with the FEM and are now able to calculate the photoacoustic signal for arbitrary resonator shapes. This offers the possibility