Trace gas measurements using optically resonant cavities and quantum cascade lasers operating at room temperature

Achieving the high sensitivity necessary for trace gas detection in the midinfrared mol. fingerprint region generally requires long absorption path lengths. In addn., for wider application, esp. for field measurements, compact and cryogen free spectrometers are definitely preferable. An alternative approach to conventional linear absorption spectroscopy employing multiple pass cells for achieving high sensitivity is to combine a high finesse cavity with thermoelec. (TE) cooled quantum cascade lasers (QCLs) and detectors. We have investigated the sensitivity limits of an entirely TE cooled system equipped with an .apprx.0.5 m long cavity having a small sample vol. of 0.3 l. With this spectrometer cavity enhanced absorption spectroscopy employing a continuous wave QCL emitting at 7.66 micro m yielded path lengths of 1080 m and a noise equiv. absorption of 2 * 10-7 cm-1 Hz-1/2. The mol. concn. detection limit with a 20 s integration time was found to be 6*108 mols./cm3 for N2O and 2 * 109 mols./cm3 for CH4, which is good enough for the selective measurement of trace atm. constituents at 2.2 mbar. The main limiting factor for achieving even higher sensitivity, such as that found for larger vol. multi pass cell spectrometers, is the residual mode noise of the cavity. On the other hand the application of TE cooled pulsed QCLs for integrated cavity output spectroscopy and cavity ring-down spectroscopy (CRDS) was found to be limited by the intrinsic frequency chirp of the laser. Consequently the accuracy and advantage of an abs. internal absorption calibration, in theory inherent for CRDS expts., are not achievable

Non-symmetrical line broadening effects using short-pulse QCL spectrometer as determined with sub-nanosecond time-resolution

Quantum cascade lasers (QCLs) have attracted considerable interest as an alternative tuneable narrow bandwidth light source in the mid-infrared spectral range for chemical sensing. Pulsed QCL spectrometers are often used with short laser pulses and a bias current ramp similar to diode laser spectroscopy. Artefacts in the recorded spectra such as disturbed line shapes or underestimated absorption coefficients have been reported. A detailed time-resolved high-bandwidth analysis of individual pulses during a laser sweep has been performed. Quantitative results for CH4 absorption features around 1347 cm-1 (7.42 µm) fell short of the expected values for reasonable operating conditions of the QCL. The origin of the artefacts using short pulses was identified to be partly of the same nature as in the case of long laser pulses. A complex combination with the tuning principle was found, leading to an apparently increased instrumental broadening (effective line width) and underestimated concentrations at low-pressure conditions

Spectroscopical diagnostics of molecular microwave plasmas

The paper presents a review of recent results of emission (UV/visible) and absorption (visible and IR) spectroscopical investigations in molecular tube and planar microwave discharges. As a new approach by emission spectroscopy seven band systems of the hydrogen molecule have been analysed to investigate H2 radiative characteristics and to determine the gas temperature completed by Doppler broadening measurements of Hα, Dα and H2 spectral lines. H2 dissociation continuum was used for an estimation of the radiative dissociation rate/degree. By absorption spectroscopy the populations of 4s ArI levels and concentrations of hydrocarbons (e.g. CH3, C2H2, CH4) were determined. In particular, investigations were focused on spatial distributions of plasma parameters.La publication présente un aperçu de nouveaux résultats de mesures de spectroscopie démission (UV/visible) et d'absorption (visible et Infrarouge) dans des décharges microondes moléculaires dans des tubes et en structure plane. La nouveauté consiste en l'analyse de l'émission de système à sept bandes de la molécule d'hydrogène. On étudie ainsi les caractéristiques radiatives de H2 et on détermine la température du gaz par des mesures d'élargissement Doppler des lignes spectrales de Hα, Dα et H2. Le continuum de dissociation de H2 est utilisé pour estimer le degré de dissociation radiative. Les populations des niveaux 4s de l'Ar I et les concentrations des hydrocarbures (par exemple CH3, C2H2, CH4) sont déterminés par spectroscopie d'absorption. En particulier on s'est intéressé à la distribution spatiale des paramètres du plasma

In situ diagnostic of etch plasmas for process control using quantum cascade laser absorption spectroscopy

The combination of quantum cascade lasers and infra red absorption spectroscopy (QCLAS) opens up new possibilities for plasma process monitoring and control. First measurements are reported with an especially designed quantum cascade laser arrangement for application in semiconductor industrial environments to track the approach for in situ process control in silicon etch plasmas. In gas mixtures with N2 and in microwave (MW) plasmas at pressures below 30 Pa concentrations of C4F6 and of SiF4 were measured simultaneously online for the first time. It could be demonstrated, that using quantum cascade lasers (QCL) it is possible to control ex situ mass flow controllers (MFC) based on in situ measured species concentrations in the gas phase and in the MW plasma bulk

Quantum cascade laser based chemical sensing using optically resonant cavities

Progress in the development of compact semiconductor-based mid-infrared light sources, more specifically of quantum cascade lasers (QCLs), has been astonishingly rapid in the 2 decades since their first realisation. Their performance makes them superior to conventional sources and has led to significant improvements and new developments in chemical sensing techniques encompassing cavity enhanced methods. The aim of this compilation is to provide an overview about useful combinations of QCLs with optical cavities and to highlight recent achievements thereby focussing on potential sensing applications

Time-resolved study of a pulsed dc discharge using quantum cascade laser absorption spectroscopy : NO and gas temperature kinetics

In a pulsed dc discharge of an Ar–N2 mixture containing 0.91% of NO the kinetics of the destruction of NO has been studied under static and flowing conditions, i.e. in a closed and open discharge tube (p = 266 Pa). For this purpose quantum cascade laser absorption spectroscopy (QCLAS) in the infrared spectral range has been applied as a new approach for fast in situ plasma diagnostics which is capable of achieving a time resolution below 100 ns. The time decay of the NO concentration was measured in single discharge pulses of 1 ms duration. Additionally, the temporal behaviour of the electric field and the applied power was followed during the pulse. The comparison of the time evolution of the NO concentration under static and flowing conditions and simplified model calculations enabled an analysis of the dynamics of the plasma heating to be made. The temperature increase during the pulse is below 40 K, but has a strong influence on the line strength of the NO absorption line. The apparent decrease in the NO concentration in a single pulse of about 20% is due to the heating of the gas which in turn makes the line strength vary while the concentration remains constant for several successive pulses. Therefore the QCLAS measurements combined with model calculations are a powerful non-invasive temperature probe with a remarkable time resolution approaching the sub-microsecond time scale