Maximilian Lackner

Simultaneous Spectroscopy of NH

A fiber-based remote measurement setup for tunable diode laser absorption spectroscopy, introducing an electrically pumped, micromechanical vertical-cavity surface-emitting laser with single-mode emission spectrum, narrow linewidth of 40 MHz, and broadband, continuous wavelength coverage of 51 nm around 1.55 mum is presented. The tunable laser spectrometer is employed for analysis of heterogeneous gas compositions and simultaneous detection of two species, ammonia and carbon monoxide, in a single continuous wavelength sweep. Broadband absorbance spectra are captured at elevated temperatures up to 300 degC revealing opposed temperature dependencies for selected transitions.

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The spectroscopic application of a new broadband microelectromechanical-system-tunable vertical cavity surface-emitting laser with single-mode coverage of 60 nm (245 cm−1) in a single, continuous sweep is described. The operation of the device is illustrated with high-resolution spectra of CO and CO2 over 110 cm−1 (27 nm) and 67 cm−1 (17 nm), respectively, with the CO band shown for high-pressure scans between 1 and 3 bars (0.1-0.3 MPa). The achieved tuning range opens up new opportunities for tunable diode laser absorption spectroscopy. The spectra were compared with HITRAN-derived model calculations. The benefits of a sensor based on this laser are greater speed, laser power, and tuning range.

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A 1.8 µm wavelength single frequency InGaAlAs–InP vertical-cavity surface-emitting laser (VCSEL) was used successfully for the first time for high resolution absorption spectroscopic measurements. As a demonstration of the multi-species measurement capability, spectra of HCl, CH4 and H2O gases were measured at pressures ranging from 0.07 to 1.5 bar and compared with calculations based on the HITRAN2000 database. In general, good agreement was observed. The laser threshold was 0.9 mA, the temperature tuning rate was 0.125 nm K−1 (0.38 cm−1 K−1) and the current tuning rate was 0.9 nm mA−1 (2.75 cm−1 mA−1). A continuous mode-hop-free single frequency current tuning range of 8.4 cm−1 (2.8 nm) was achieved using a 0–4.1 mA driving current. Single frequency VCSELs show a variety of advantages compared to edge emitting semiconductor lasers, such as wide current tuning range, very high tuning speeds (MHz) and therefore very good time resolution, little susceptibility to optical feedback and low manufacturing costs. The availability of long wavelength single frequency VCSELs (1.3–2 µm) should make VCSEL the preferred choice for diode-laser gas sensing and process control applications and help to significantly expand the application fields for infrared diode-laser gas sensors.

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Methane-air mixtures at high fill pressures up to 30 bar and high temperatures up to 200°C were ignited in a high-pressure chamber with automated fill control by a 5 ns pulsed Nd:YAG laser at 1064 nm wavelength. Both, the minimum input laser pulse energy for ignition and the transmitted fraction of energy through the generated plasma were measured as a function of the air/fuel-equivalence ratio (λ). The lean-side ignition limit of methane-air mixtures was found to be λ=2.2. However only λ<2.1 seems to be practically usable. As a comparison, the limit for conventional spark plug ignition of commercial natural gas engines is λ=1.8. Only with excessive efforts λ=2.0 can be spark ignited. The transmitted pulse shape through the laser-generated plasma was determined temporally as well as its dependence on input laser energy and properties of the specific gases interacting. For a first demonstration of the practical applicability of laser ignition, one cylinder of a 1 MW natural gas engine was ignited by a similar 5 ns pulsed Nd: YAG laser at 1064 nm. The engine worked successfully at λ=1.8 for a first test period of 100 hr without any interruption due to window fouling and other disturbances. Lowest values for NO x emission were achieved at λ=2.05 (NO x =0.22 g/KWh). Three parameters obtained from accompanying spectroscopic measurements, namely, water absorbance, flame emission, and the gas inhomogeneity index have proven to be powerful tools to judge laser-induced ignition of methane-air mixtures. The following effects were determined by the absorption spectroscopic technique: formation of water in the vicinity of the laser spark (semi-quantitative); characterization of ignition (ignition delay, incomplete ignition, failed ignition); homogeneity of the gas phase in the vicinity of the ignition; and the progress of combustion.

Continuously Tunable MEMS-VCSEL

Optical access to combustion chambers via windows is desirable for combustion diagnostics as well as for laser ignition. By nature, combustion deposits can form on the inner surface of the light-transmitting window, leading to malfunction. We investigated whether a Nd:YAG ignition laser could cope with combustion-chamber deposits by means of ablation. In a 1.8-kW four-stroke internal combustion engine an optical window was installed to couple in the laser light. Ignition was carried out by a spark plug. Due to inherent high fuel and oil consumption, a deposit layer would form on the substrate within some tens of minutes. Elementary analysis showed carbonaceous as well as inorganic compounds gradually reducing light transmission. With cyclic 5-ns laser shots through the window, the pass-through stayed essentially free of deposits provided the energy fluence was around 10 mJ/mm2. Microanalysis showed evidence of the soundness of the principle. In addition, even single shots with a higher flux were enough to remove a relatively thick layer of deposits at once. Thus an optical window in an internal combustion engine can in principle be kept transmissive by the action of a compact solid-state laser.

CO and CO 2 spectroscopy using a 60 nm broadband tunable MEMS-VCSEL at ~1.55 μm

The laser-induced ignition of methane/air-mixtures at elevated pressures was investigated by an absorption spectroscopic technique. A room temperature continuous wave InGaAsSb/AlGaAsSb quantum well ridge diode laser was wavelength tuned around 2.55 mum by periodically modulating the injection current from 0 to 174 mA at a 5 kHz repetition rate. The laser heat sink temperature was fixed at 291 K. The infrared laser beam was sent through the pressurized combustion vessel perpendicularly to the igniting laser beam (Nd:YAG laser, 10 ns pulse duration, 20 mJ) at the position of the ignition spark. Fuel-rich to fuel-lean mixtures of methane/air (air equivalence ratio 0.89, 1.06, 1.42, 2.50) were investigated at initial pressures of up to 3 MPa. The initial temperature was 473 K, the volume of the combustion vessel 0.9x10(-3) m(3). The formation of water vapor in the vicinity of the laser spark was tracked by the diode laser. The time resolution of the measurements was 0.2 ms for a total continuous measurement time of up to 1 s. In this way, the laser-induced ignition and its accompanying effects could be investigated on a time scale spanning four orders of magnitude. Apart from the absorbance of water vapor which could be determined semi-quantitatively (due to the effects of severe pressure broadening at high pressures and the ignorance of the exact temperature distribution after ignition), the emissions from the flame (broadband, 1-10 mum) and a gas inhomogeneity index were recorded. The gas inhomogeneity index was obtained by extracting a frequency variable from the time-dependent fluctuations of the transmitted laser intensities and calculating its derivation. The absorbance of water vapor, the emissions from the flame and the gas inhomogeneity index were found to be a powerful tool to characterize laser-induced ignition. Major implications of in situ species concentration measurements at high pressures for the design and development of high-load combustors are presented.

1.8??m vertical-cavity surface-emitting laser absorption measurements of HCl, H2O and CH4

Abstract Oberflächenemittierende Diodenlaser (engl. VCSEL, Vertical-cavity surface-emitting laser) werden zur raschen direkten In-situ-Molekülspektroskopie eingesetzt. Nach dem Verfahren der Absorptionsspektroskopie mittels durchstimmbarer Diodenlaser wird Sauerstoff bei 760 nm, Ammoniak bei 1540 nm, Methan bei 1680 nm sowie Chlorwasserstoff und Wasser bei 1810 nm detektiert. Druckverbreiterte und hochaufgelöste Spektren werden gezeigt und das Prinzip eines langzeitstabilen Spektrometers vorgestellt. Die Wellenlängenmodulation der VCSEL mit der Temperatur und dem Strom wird untersucht. Während der Temperaturkoeffizient in etwa derselbe ist wie für herkömmliche Diodenlaser im nahen Infrarot (DFB-Laser), lassen sich VCSEL deutlich weiter mit dem Strom durchstimmen. Darüber hinaus können VCSEL thermisch wesentlich schneller moduliert werden als konventionelle Kantenemitter. Repetitionsraten bis 5 MHz werden demonstriert. Die neu eröffneten Anwendungsfelder im Hinblick auf den weiten, modensprungfreien Durchstimmbereich (Messung bei hohem Druck, mehrere Spezies, Temperaturverteilungen) und die rasche Modulierbarkeit (Messung extrem transienter Prozesse) werden diskutiert. Weiter werden spektroskopisch interessante Eigentümlichkeiten der VCSEL (geringer Schwellstrom und Strombedarf als Vorteil für batteriebetriebene mobile Geräte, Austestmöglichkeit auf der Waferebene) beleuchtet. Die langwelligen VCSEL mit λ > 1 μm auf InP-Basis existieren noch nicht lange. Es wird angenommen, dass diese demnächst verstärkten Einzug in die Molekülspektroskopie halten werden und das Einsatzgebiet von auf Diodenlasern basierenden Geräten beträchtlich nach höheren Drücken und schwierigen Bedingungen hin erweitern werden.

Laser Ignition of Methane-Air Mixtures at High Pressures and Diagnostics

Absorption spectroscopy of CO and CO2 at wavelengths around 1.55mum via an all fiber based remote measurement setup is presented as application where a singlemode MEMS-VCSEL can feature its broadband (51 nm) continuous tuning characteristics

Laser cleaning of optical windows in internal combustion engines

The design, technology and characteristics as well as sensing applications of micromachined long-wavelength (~1.55μm) tunable vertical-cavity surface-emitting lasers are reported. The laser combines an active optical component (so-called half-VCSEL) and an agile mechanical component (MEMS) in a hybrid assembly. Electrothermal actuation expands the enclosed air-gap and continuously shifts the cavity resonance towards longer wavelengths. A curved mirror membrane is deployed to solely excite the desired fundamental mode with high output power and high sidemode suppression. The comparatively high stiffness of the MEMS lifts its mechanical resonance frequency to values around 150 kHz as measured by laser Doppler vibrometry under electrostatic actuation and - at the same time - reduces its susceptibility to Brownian motion. Laser linewidths as narrow as 32MHz are demonstrated by using the self-heterodyning technique and the wavelength dependent linewidth variation is presented for the first time. After successful absorption spectroscopy experiments under steady laboratory conditions the tunable VCSEL is used for trace gas detection in a combustion process. Preliminary experimental results are shown and practically encountered problems are discussed.

In situ investigation of laser-induced ignition and the early stages of methane–air combustion at high pressures using a rapidly tuned diode laser at 2.55 μm

A laser-based system should be advantageous to a spark-plug based ignition system. Free choice of the ignition spot and precise timing constitute two major advantages. Multi point laser ignition could lead to higher efficiencies, and laser ignition as such is capable of igniting leaner mixtures than a spark plug, thereby decreasing thermal NOx and soot emissions. This paper is devoted to advances in optical diagnostics of laser ignition for future internal combustion engines. The focus of this paper is on diagnostics at high pressures, that is engine-like conditions. Laser ignition tests were performed with the fuels methane, hydrogen and biogas in static combustion cells with dimensions comparable to stationary engines. A Nd:YAG laser (5 ns pulse duration, wavelength 1064 nm, 1–20 mJ pulse energy) was used to ignite gaseous fuel/air mixtures at initial pressures of 1–3 MPa. Schlieren photography and laser-induced fluorescence (LIF) were used for optical diagnostics (flame kernel development, shock wave propagation). The lean burn characteristics were investigated. Schlieren photography was used to determine the velocity of the shock wave and to study the influence of the shock wave on temperature rise and energy loss. Using planar laser-induced fluorescence (PLIF), the spatial distribution of the combustion intermediates OH and formaldehyde were recorded. The temporally resolved imaging shows that the initial stages of the flame front evolution closely follows the turbulence and density fluctuations caused by the shock and pressure wave induced by the laser spark. In this paper, results from LIF spectroscopy and Schlieren photography are compared. Depending on the laser pulse energy and focus size, at later stages after the ignition the flame front propagation approaches the laminar burning regime and flame front speed decrease. Flame front break up at lean conditions indicates the limit of the ignitable mixture fraction when the speed due to spark-induced convection exceeds the flame propagation rate.© 2004 ASME