Frank Duschek

Modified negative-branch confocal unstable resonator

A new type of unstable resonator, suitable for a laser with a large medium cross section and a small or median output coupling, is presented. The resonator configuration, a modification of a negative-branch confocal unstable resonator, is numerically investigated. The basis of the theory is the Fresnel-Kirchhoff integral equation, and the calculations describe a passive resonator. With respect to output mirror tilting, the calculations confirm that the modified negative-branch confocal unstable resonator is less sensitive to mirror misalignments than the conventional negative-branch confocal unstable resonator. Furthermore, the modified resonator improves the beam quality in comparison with the conventional unstable resonator.

Chemical oxygen-iodine laser power generation with an off-axis hybrid resonator

A rectangular negative branch off-axis hybrid resonator was coupled to a 10 kW class chemical oxygen-iodine laser. Resonator setup and alignment turned out to be straightforward. The extracted power was 6.6 kW and reached approximately 70% of the power for an optimized stable resonator. The divergence of the emitted laser beam in the unstable direction was lower than two times the diffraction limit. Experimentally measured margins for mirror misalignment were found in close agreement with numerical calculations.

Small signal gain and temperature profiles in supersonic COIL

In the cavity of the supersonic COIL of DLR, the time dependence and the spatial dependence of small signal gain (ssg) and intra-cavity temperature (ict) are investigated for a broad range ofoperating conditions. The ssg is measured by a commercial diagnostic system of PSI with a software package upgraded by the Air Force Research Lab ofKirtland, U.S.A.1,2,3,4,5 The line-shape ofthe COIL gain profile is scanned in frequency by a diode laser of narrow linewidth operating in the region of the COIL transition frequency. The ict is derived from the frill bandwidth at halfmaximum ofthe inscribed Gaussian profile. The experiments are performed for different combinations of secondary gas flow at unchanged primary baseline conditions. The results are interpreted with regard to the ssg and the temperature distribution for optimized COIL operation.

COIL emission of a modified negative branch confocal unstable resonator

A modified negative branch confocal unstable resonator (MNBUR) was coupled to the chemical oxygen-iodine laser (COIL) device of the German Aerospace Center. It consists of two spherical mirrors and a rectangular scraper for power extraction. Experimentally measured distributions of the near- and far-field intensities and the near-field phase were found in close agreement to numerical calculations. The extracted power came up to approximately 90% of the power as expected for a stable resonator coupled to the same volume of the active medium. The output power revealed a considerable insensitivity towards tilts of the resonator mirrors and the ideal arrangement of the scraper was found to be straightforward by monitoring the near-field distributions of intensity and phase. The beam quality achieved with the MNBUR of an extremely low magnification of only 1.04 was rather poor but nevertheless in accordance with theory. The demonstrated consistency between theory and experiment makes the MNBUR an attractive candidate for lasers that allow for higher magnification. In particular, it promises high brilliance in application to 100 kW class COIL devices, superior to the conventional negative branch confocal unstable resonator.

Comparative studies on small signal gain and output power for COIL systems

In the 10-kW chemical oxygen-iodine laser (COIL) of DLR, the small signal gain and the laser output power were measured for identical gas flow conditions. The comparison of both results is used to elaborate the expressiveness of small signal gain for COIL laser design. For these investigations the temporal and spatial dependencies of small signal gain and laser power are measured along the flow axis of the cavity. The measurement of small signal gain is perlonned by a commercial diagnostic system of PSI with a software package upgraded by the Air Force Research Lab of Kirtland, U.S.A. 1,2,3,4 The laser power is extracted in a stable resonator configuration. In case of spatially resolved investigations, slit apertures of 6 mm width in flow direction are integrated in the cavity.

[Hybrid Resonator in a Double-pass Configuration for a Chemical Oxygen Iodine Laser]

A double-pass negative-branch hybrid resonator is applied to a 10 kW chemical oxygen iodine laser. The resonator is folded in such a way that the dimension of the stable direction is reduced. The intensity distributions of the near and far fields of the laser beam and the sensitivity against tilts of the output mirror are investigated. A comparison between theory and experiment is performed. It is shown that the folded hybrid resonator provides a better beam quality and therefore a higher power density in the far field than a single-pass hybrid resonator. The sensitivity against tilts of the resonator mirrors in the stable direction is reduced.

Unstable resonator with reduced output coupling

The properties of a laser beam coupled out of a standard unstable laser resonator are heavily dependent on the chosen resonator magnification. A higher magnification results in a higher output coupling and a better beam quality. But in some configurations, an unstable resonator with a low output coupling in combination with a good beam quality is desirable. In order to reduce the output coupling for a particular resonator, magnification fractions of the outcoupled radiation are reflected back into the cavity. In the confocal case, the output mirror consists of a spherical inner section with a high reflectivity and a flat outer section with a partial reflectivity coating. With the application of the unstable resonator with reduced output coupling (URROC), magnification and output coupling can be adjusted independently from each other and it is possible to get a good beam quality and a high power extraction for lasers with a large low gain medium. The feasibility of this resonator design is examined numerically and experimentally with the help of a chemical oxygen iodine laser.

Variations in Fluorescence Spectra of a Bacterial Population During Different Growth Phases

Biological agents like pathogenic bacteria represent a major threat to the public if spread. Bacteria may replicate in their host and can spread in an unpredictable way. Standoff detection based on laser induced fluorescence may help to mitigate the associated risks. Nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) and tryptophan are among others the most important fluorophores in bacteria. Bacteria adapt to their environment and thus the same bacterial species may be composed of different components and relative concentrations of them depending on environmental conditions. Fluorescence spectra of a Bacillus thuringiensis population were compared during different growth phases. Laser pulses with two different excitation wavelengths, 280 nm and 355 nm, were used. For 280 nm excitation the measured spectra show a difference in spectral features between the bacterial population before sporulation and after it was partially sporulated. In comparison a smaller variation in 355 nm excited LIF spectra of a bacterial population during exponential growth and the ageing population is observed.

Standoff detection and classification of bacteria by multispectral laser-induced fluorescence

Abstract Biological hazardous substances such as certain fungi and bacteria represent a high risk for the broad public if fallen into wrong hands. Incidents based on bio-agents are commonly considered to have unpredictable and complex consequences for first responders and people. The impact of such an event can be minimized by an early and fast detection of hazards. The presented approach is based on optical standoff detection applying laser-induced fluorescence (LIF) on bacteria. The LIF bio-detector has been designed for outdoor operation at standoff distances from 20 m up to more than 100 m. The detector acquires LIF spectral data for two different excitation wavelengths (280 and 355 nm) which can be used to classify suspicious samples. A correlation analysis and spectral classification by a decision tree is used to discriminate between the measured samples. In order to demonstrate the capabilities of the system, suspensions of the low-risk and non-pathogenic bacteria Bacillus thuringiensis, Bacillus atrophaeus, Bacillus subtilis, Brevibacillus brevis, Micrococcus luteus, Oligella urethralis, Paenibacillus polymyxa and Escherichia coli (K12) have been investigated with the system, resulting in a discrimination accuracy of about 90%.

Standoff detection applying laser-induced breakdown spectroscopy at the DLR laser test range

The DLR laser test range at Lampoldshausen is designed for a wide field of laser application studies under central European atmospheric conditions. Micrometeorological measurements are performed simultaneously and nearby to the laser propagation. The infrastructure is very suitable for the development of laser based standoff detection systems of biological, chemical, and explosive hazardous substances. In a first approach, laser-induced breakdown spectroscopy (LIBS) has been introduced for investigation of surface contaminants at distances up to 135 m. A basic LIBS set-up and LIBS spectra of selected samples using different excitation wavelengths from IR to UV are presented for detection at different distances. A Nd:YAG laser beam was focussed by a Cassegrain type telescope onto different samples. The light of the generated plasma plume was collected by a Newtonian telescope, analysed and detected by a broadband CCD-spectrometer system. The Nd:YAG laser yields pulse energies up to 800 mJ at a wavelength of 1064 nm and a pulse width of 8 ns. Optionally the second and third harmonics can be extracted at reduced energy. LIBS spectra produced on gold layers as thin as 5 nm deposited on silicon wafers were recorded for test of detection sensitivity and comparison of wavelengths effects. In addition, black powder as ordinary substance representing explosives was detected by LIBS technology. Spectra were recorded in single and repetitive pulsed scheme of the Nd:YAG laser at various daylight and atmospheric conditions.