The interaction between two neighboring n-decane droplets during the autoignition process in air was experimentally investigated under microgravity conditions in the drop tower Bremen. The initial droplet diameter was 0.8 mm in all experiments. Single droplets and droplet pairs with a center distance of 1, 2, 4 and 6 mm were investigated. The air temperature was varied between 650 and 850 K with an increment of 25 K. The examined pressure conditions were 0.3 and 0.5 MPa. This results in a total number of 90 drop experiments. The ignition process was observed applying the laser induced fluorescence (LIF) on formaldehyde with high temporal and spatial resolution. Both, the cool flame process and the hot flame ignition could be measured regarding induction times and the temporal and spatial evolution of the formaldehyde formation.

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.

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.

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.

Residual absorption in optical coatings and materials is directly measured by means of the laser-induced deflection (LID) technique. For transmissive coatings a measurement strategy is introduced that allows for the separation of different absorptions of the investigated sample (bulk, coating, surface) by use of only one sample. Laser irradiation yields absorption values between 2 x 10(-3) and 2.9 x 10(-2) for antireflecting and highly reflecting (HR) coatings at 193 nm and 30.6 x 10(-6) for a HR mirror at 527 nm. Use of laser-induced fluorescence at 193 nm excitation reveals trivalent cerium and prasodymium and hydrocarbons in different single layers and coatings. In addition to correlation with absorption data, the influence of a high fluorescence quantum yield on the absorption measurement is discussed.

We report on two approaches to strongly shorten life time testing of fused silica's absoption degradation upon 193 nm laser irradiation. Both approaches are based on enhancing the two photon absorption (TPA) induced generation of E' and NBOH defects centers in fused silica compared to common marathon test irradiation parameters. For the first approach the irradiation fluence is increased from typical values H<1 mJ/cm2 to H=10 mJ/cm2, therefore increasing the peak laser power for a more efficient TPA process. To avoid microchannel formation in the samples, being a common break-down criterion in marathon tests based on transmission measurements, a small sample of 10 mm length is irradiated and the absorption is measured directly by the laser induced deflection (LID) technique. For comparing the experimental results with a real marathon test at H=1.3 mJ/cm2, an experimental grade sample with very low hydrogen content, i.e. fast absorption changes due to reduced defect annealing, is choosen. During the fluence dependent absorption measurements after the prolonged irradiation at H=10 mJ/cm2 it is found, that both experiments reveal very comparable absorption data for H=1.3 mJ/cm2. For investigating standard material with high hydrogen content, i.e. slow absorption increase due to effective defect annealing, a sample is cooled down to -180 °C in a special designed experimental setup and irradiated at a laser fluence H=10 mJ/cm2. To control the increase of the defect density and to determine the end of the TPA induced defect generation, the fluorescence at 650 nm of the generated NBOH centers is monitored. Before and after the low temperature experiment, the absorption coefficient is measured directly by LID technique. By applying both, elevated laser fluence and low temperature, the ArF laser induced generation of E' and NBOH centers in the investigated sample is terminated after about 1.2*107 laser pulses. Therefore, a strong reduction of irradiation time is achieved in comparison to about 1010 pulses required in common marathon test applications.

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.

Calcium fluoride and fused silica are low absorbing key optical bulk materials for pulsed DUV laser application. Due to their large band gap fluoride thin films play a key role for applications in the DUV spectral region. For their main use in laser microlithography, these materials are commonly characterized by in situ transmission measurements. A differentiation is not possible between absorption and scattering. Therefore, experimental techniques are highly attractive which selectively characterize the absorption process using small samples.A direct absorption measurement method using laser induced deflection of a probe beam (LID) was introduced and applied to bulk and thin film materials. Laser induced fluorescence (LIF) in bulk materials and coatings is investigated to correlate the absorption with microscopic properties like intrinsic and extrinsic defects.

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.

Precise absorption measurements of bulk materials and coatings upon pulsed ArF laser irradiation are presented using a compact experimental setup based on the laser induced deflection technique (LID). For absorption measurements of bulk materials the influence of pure bulk and pure surface absorption on the temperature and refractive index profile and thus for the probe beam deflection is analyzed in detail. The separation of bulk and surface absorption via the commonly used variation of the sample thickness is carried out for fused silica and calcium fluoride. The experimental results show that for the given surface polishing quality the bulk absorption coefficient of fused silica can be obtained by investigating only one sample. To avoid the drawback of different bulk and surface properties amongst a thickness series, we propose a strategy based on the LID technique to generally obtain surface and bulk absorption separately by investigating only one sample. Apart from measuring bulk absorption coefficients the LID technique is applied to determine the absorption of highly reflecting (HR) coatings on CaF 2 substrates. Beside the measuring strategy the experimental results of a AlF 3 /LaF 3 based HR coating are presented. In order to investigate a larger variety of coatings, including high transmitting coatings, a general measuring strategy based on the LID technique is proposed.

An experimental setup was established for in situ transmission and laser induced fluorescence (LIF) measurements of CaF2 at 193 nm laser irradiation. The known rapid damage process in CaF2 upon ArF laser irradiation is shown to terminate for all tested samples within 3×104 laser pulses for the applied fluences. Furthermore, it is demonstrated that for typical application values the fluence dependent transmission (FDT) at the end of the rapid damage process is independent of the irradiation history and determined by the specific crystal quality. From the lifetimes and signal strengths of different present fluorescence bands the excitation and recording conditions for LIF investigations are derived. The results of laser induced fluorescence measurements at 193 nm excitation make evident that certain impurities or defects are responsible for the different transmission properties even of high purity CaF2 crystals. Comparing transmission and LIF data a quantitative correlation was found between selected emission bands and ArF laser stability of CaF2 material.

Temperature fields under cool flame conditions (T < 800 K) are determined using laser-induced fluorescence (LIF) of formaldehyde following excitation with a single broadband pulse from an XeF excimer laser. A detection system consisting of two intensified cameras (CCDs) with filters (395/5 and 405/15 nm) allows us to obtain two-dimensional temperature fields (light-sheet technique). A temperature calibration function based on two LIF signals, each spectrally integrated over selected wavelength regions, covers the 300-800 K temperature range. The LIF thermometry method has been used to record two-dimensional temperature fields in the neighborhood of a self-igniting 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.