Wolfgang Triebel

Single-shot imaging of formaldehyde in hydrocarbon flames by XeF excimer laser-induced fluorescence

A novel way to excite laser-induced fluorescence (LIF) of formaldehyde was applied to measure LIF of hydrocarbon flames (diethyl ether, ethene, n -heptane, and kerosene). LIF excitation spectra of formal-dehyde in a reference cell and of flames were obtained by tuning a spectrally narrowed XeF excimer laser (Δλ=4pm, 30 mJ pulse energy) between 353.1 and 353.6 nm and integrating the emission between 360 and 550 nm. The LIF excitation spectra of the flames showed a contribution from formaldehyde and a broadband background from, for example, polycyclic aromatic hydrocarbons (PAHs). Exploiting the high excitation efficiency and pulse energy of the narrowband XeF excimer laser, single laser shots were sufficient to measure two-dimensional distributions of formaldehyde LIF (light sheet technique [3×3 cm 2 ], intensified charge-coupled device [CCD] camera). Fast wavelength switching between on-and off-resonant excitation of formaldehyde was successfully applied to subtract background emission in the case of diethyl ether, n -heptane, and ethene flames. Furthermore, it was shown that the images of formaldehyde LIF complement the measurements of OH LIF. OH is only found in higher-temperature flame regions.

Spectrally resolved cavity ring down measurement of high reflectivity mirrors using a supercontinuum laser source

We investigate a cavity ring down setup that offers the possibility to measure the spectrally resolved reflectivities of high reflectivity mirrors. The setup consists of a resonator (ring down cavity) and an intensified CCD camera system combined with a spectrograph for spectral resolution. A commercial supercontinuum laser (350-1750 nm) is applied as a compact excitation source. It is based on a photonic crystal fiber that is pumped by a q-switched microchip laser (1.6 ns pulse duration, 25 kHz repetition rate). This combination allows simultaneously recording the transmittance of the cavity over a wide wavelength range determined by the excitation source and the spectral sensitivity of the detector. The photon lifetimes inside the cavity (ring down times) are measured with high spectral resolution by means of an intensified camera system. Subsequently shifting the gate of the image intensifier from short to long delay times after the excitation pulse allows calculation of the reflectivity spectrum of the mirrors. Comparison of these results with measurements using a conventional setup (laser diode 675 nm and photomultiplier tube) clearly shows the high potential of the method due to the multichannel excitation and the detection scheme.

Single-shot imaging of gas temperatures in low-temperature combustion based on laser-induced fluorescence of formaldehyde

Temperature fields under cool flame conditions ( T n -decane droplet in 1 bar air at 500–730 K. The derived LIF temperatures are in good agreement with a thermocouple measurement (standard deviation =14 K). The experimental findings of thermometry are consistent with a Boltzmann distribution in the 4 0 and 4 1 levels of S 1 state formaldehyde.

The "advanced disk laser" — An onboard laser diagnostics system for drop tower experiments

We report about recent developments concerning the Advanced Disk Laser system, which is intended for use in combustion experiments under microgravity conditions at the drop tower Bremen. Since the system will be integrated into a modified drop capsule it has to be a compact and efficient solid state laser which can withstand decelerations of up to 40 g. The existing laboratory set up was redesigned to fit into the capsule. Therefore, the system layout was split in three platforms for optical components (seed laser, regenerative amplifier and frequency conversion) and one additional platform for media supplies. Afterwards, a special developed ultra stable structure was equipped with newly designed mechanical and electro-optical components, which in part have already been tested in drop experiments. Access for alignments is possible via remote control of a number of motorized mechanical mounts. The three optical platforms build a vibration-insulated, dust-free laser capsule of their own inside the drop capsule and can be used both in the capsule and in a laboratory environment. The new laser system was already optically adjusted and tested in the laboratory and reached a pulse energy of 20 mJ, as was expected. An extensive evaluation period is going to follow and will be finished with a series of combustion experiments.

Combined multispecies PLIF diagnostics with kHz rate in a technical fuel mixing system relevant for combustion processes

Concentration distributions of formaldehyde were measured in a technical fuel mixing system by Planar Laser Induced Fluorescence (PLIF) using a novel all solid state disk laser system. This compact and efficient laser generates tunable, narrow bandwidth pulses with kHz repetition rate and energies of up to 25 mJ around 1030 nm. After frequency conversion to the UV spectral region, laser pulses with energies of up to 4 mJ excite different combustion relevant species inside of a semi-technical reactor. This reactor generates a homogeneous fuel vapor/air-mixture using the so-called cool flame. Since the mixture of fuel and air is a key factor concerning efficiency of combustion, the fast fuel injection and mixing processes were investigated with this laser system. Directing a light sheet into the reactor and collecting the fluorescence with an intensified CCD camera, we recorded PLIF images of formaldehyde concentration distributions using an excitation wavelength of 343 nm. In this way we characterized the turbulence of the injection process close to the fuel injection nozzle with 1 kHz repetition rate, and proved the excellent homogeneity of the fuel-air mixture close to the end of the reactor, where fuel-air mixture was burned in a hot flame. By means of scattered light from fuel droplets the mean flow velocity could be estimated. In the hot flame above the reactor spectrally resolved LIF of OH radicals could be recorded.

FAST WAVELENGTH SWITCHING OF NARROW-BAND EXCIMER LASERS

A novel system was developed, which allows one to switch the wavelength of a narrow-band excimer laser between two successive light pulses at a repetition rate of at least 250 Hz. This is realized by a periodically driven piezo actuator, which is attached to the diffraction grating of the narrow-band KrF excimer laser. The achieved position accuracy of the grating leads to a wavelength reproducibility of ±0.2 pm, which allows one to apply this system to laser spectroscopic investigations like LIF or LIPF of OH in flames. Using the fast wavelength switching system background reduced concentration and temperature fields in flames can be measured within one sequence. Some possible realized and planned applications like the measurement of gas temperature, the diagnostic of turbulent combustion processes, and the investigation of combustion processes under microgravity are discussed.

Diode pumped solid state kilohertz disk laser system for time-resolved combustion diagnostics under microgravity at the drop tower Bremen.

We describe a specially designed diode pumped solid state laser system based on the disk laser architecture for combustion diagnostics under microgravity (μg) conditions at the drop tower in Bremen. The two-stage oscillator-amplifier-system provides an excellent beam profile (TEM00) at narrowband operation (Δλ < 1 pm) and is tunable from 1018 nm to 1052 nm. The laser repetition rate of up to 4 kHz at pulse durations of 10 ns enables the tracking of processes on a millisecond time scale. Depending on the specific issue it is possible to convert the output radiation up to the fourth harmonic around 257 nm. The very compact laser system is integrated in a slightly modified drop capsule and withstands decelerations of up to 50 g (>11 ms). At first the concept of the two-stage disk laser is briefly explained, followed by a detailed description of the disk laser adaption to the drop tower requirements with special focus on the intended use under μg conditions. In order to demonstrate the capabilities of the capsule laser as a tool for μg combustion diagnostics, we finally present an investigation of the precursor-reactions before the droplet ignition using 2D imaging of the Laser Induced Fluorescence of formaldehyde.

Single-frequency tunable pulsed thin disk laser for drop tower applications

The combination of laser diagnostics and short term microgravity (/spl mu/g) conditions has become an important tool for the investigation of combustion processes. In this paper, a pulsed Yb:YAG laser tunable around 1030 nm is designed for autonomous operation under microgravity conditions. A pulse energy of 21 mJ at 1 kHz and a pulse duration of 20 ns is demonstrated.

Extending the narrow-bandwidth tunability of a thin disk Yb:YAG laser regenerative amplifier

We investigate the characteristics of a thin disk laser system based on the master oscillator-power amplifier design. The amplifier emits parasitic laser oscillation (PL) when the wavelength of the tunable narrow-bandwidth seed pulse is detuned more than 4 nm from the gain maximum at 1030 nm. For suppression of this unwanted PL, a birefringent filter (Lyot filter) was inserted into the amplifier cavity in order to generate wavelength selective losses, especially at the gain maximum of the Yb:YAG crystal. It is shown that the tunability range of the laser system can be extended by a factor of 4, when a properly chosen Lyot filter is applied and its angle of rotation is adapted to the seed wavelength of the system.

Use of a novel tunable solid state disk laser as a diagnostic system for laser-induced fluorescence

An all solid state disk laser system-named Advanced Disk Laser (ADL) -particularly tailored for laser induced fluorescence (LIF) in combustion processes is presented. The system currently under development comprises an Yb:YAG-seedlaser and a regenerative amplifier. Both are based on the disk laser concept as a new laser architecture. This allows a tunable, compact, efficient diode pumped solid state laser (DPSSL) system with repetition rates in the kHz region. After frequency conversion to the UV-spectral region via third and fourth harmonics generation, this laser-due to its unique properties such as single-frequency operation, wavelength tuneability and excellent beam profile-is well suited for excitation of small molecules such as formaldehyde, OH, NO or O2, which are characteristic for combustion processes. Using the method of planar laser induced fluorescence (PLIF) we observed concentration distributions of formaldehyde in cool and hot flames of a specially designed diethyl-ether burner. The images recorded with 1 kHz repetition rate allow visualizing the distribution of formaldehyde on a 1 ms time scale. This demonstrates for the first time the usability of this novel laser for LIF measurements and is the first step towards integration of the ADL into capsules for drop towers and the international space station.