Wolfgang Paa

Two-dimensional temperature measurements in particle loaded technical flames by filtered Rayleigh scattering

Filtered Rayleigh scattering (FRS) is applied to determine two-dimensional temperature distributions in a hexamethyldisiloxane loaded propane/air flame intended for combustion chemical vapor deposition processes. An iodine cell as a molecular filter suppresses background scattering, e.g., by particles, while the wings of the spectrally broadened Rayleigh scattering can pass this filter. A frequency-doubled Nd:YAG laser is tuned to a strong absorption line of iodine. The gas temperature is deduced from the transmitted Rayleigh scattering signal. Since FRS also depends on molecule-specific scattering cross sections, the local gas composition of majority species is measured using the Raman scattering technique. Limits and restrictions are discussed.

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.

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.

Enhanced laser-induced deflection measurements for low absorbing highly reflecting mirrors

A new concept enhances the capability of photo-thermal absorption measurements with transversal probe beam guiding by overcoming drawbacks such as a lack of sensitivity for materials with low photo-thermal response and/or round substrate geometry. The sandwich concept using the laser-induced deflection technique is introduced and tested for the investigation of highly reflecting (HR) coatings. The idea behind the sandwich concept is based on the decoupling of the optical materials for the pump and probe beams. This is realized by either placing a HR coated rectangular substrate in between two optical (sandwich) plates or attaching a HR coated thin round substrate onto one optical plate. For both configurations, the sandwich concept results in a strong increase in sensitivity for the measurement of HR coatings deposited onto photo-thermally insensitive substrates. Experiments reveal that for a CaF2 substrate, up to two orders of magnitude enhancement in sensitivity can be achieved.

Laser induced deflection (LID) method for absolute absorption measurements of optical materials and thin films

We use optimized concepts to measure directly low absorption in optical materials and thin films at various laser wavelengths by the laser induced deflection (LID) technique. An independent absolute calibration, using electrical heaters, is applied to obtain absolute absorption data without the actual knowledge of the photo-thermal material properties. Verification of the absolute calibration is obtained by measuring different silicon samples at 633 nm where all laser light, apart from the measured reflection/scattering, is absorbed. Various experimental results for bulk materials and thin films are presented including measurements of fused silica and CaF2 at 193 nm, nonlinear crystals (LBO) for frequency conversion and AR coated fused silica for high power material processing at 1030 nm and Yb-doped silica raw materials for high power fiber lasers at 1550 nm. In particular for LBO the need of an independent calibration is demonstrated since thermal lens generation is dominated by stress-induced refractive index change which is in contrast to most of the common optical materials. The measured results are proven by numerical simulations and their influence on the measurement strategy and the obtained accuracy are shown.

Note: Broadband cavity ring-down spectroscopy of an intra-cavity bulk sample

A cavity ring-down (CRD) setup equipped with a pulsed broadband light source (480 nm ≤ λ ≤ 650 nm) and a multichannel detection system (temporal gate width Δτ = 20 ns) is used to simultaneously record the optical loss spectrum of an intra-cavity CaF2 sample and its changes upon transverse ArF laser irradiation at 193 nm. The CRD setup with mirrors of high reflectivity (R > 99.93%) allows to register loss changes of 5 × 10(-5) with a spectral resolution of 0.3 nm in less than 2 min.

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.

Laser diagnostics as a tool to improve combustion based atmospheric SiO 2 layer deposition

SiO-LIF fields, and temperatures based on OH-LIF and OH∗ where used to optimize the burner to substrate distance and the thread temperature during SiO x layer deposition on glass and threads in an atmospheric HMDSO/propane/air flame.