Karin M. Grünewald

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.

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.

[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%.

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

The DLR laser test range at Lampoldshausen allows for optical measurements under daylight conditions at distances up to 130 m. This infrastructure is very suitable for the development of standoff detection systems for biological, chemical and explosive hazardous substances. In a first step, laser-induced breakdown spectroscopy (LIBS) has been introduced to this test site. A basic LIBS setup and first LIBS spectra of selected samples are presented. A Nd:YAG laser beam was focused by a Cassegrain type telescope onto different samples at distances exceeding 50 m. The light of the generated plasma plume was collected by a Newtonian telescope and analyzed by a gated 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. LIBS spectra from 10 nm layers of gold on a silicon wafer were recorded. In addition, LIBS spectra from black powder were measured and compared to the spectrum of potassium nitrate, which is a main component of black powder and shows very characteristic emission lines. LIBS spectra of the above samples have also been acquired with an excitation laser wavelength in the eye-safe region. Recorded spectra are measured as a function of the laser wavelength, pulse energy and distance to the target substance.

Standoff detection: distinction of bacteria by hyperspectral laser induced fluorescence

Sensitive detection and rapid identification of hazardous bioorganic material with high sensitivity and specificity are essential topics for defense and security. A single method can hardly cover these requirements. While point sensors allow a highly specific identification, they only provide localized information and are comparatively slow. Laser based standoff systems allow almost real-time detection and classification of potentially hazardous material in a wide area and can provide information on how the aerosol may spread. The coupling of both methods may be a promising solution to optimize the acquisition and identification of hazardous substances. The capability of the outdoor LIF system at DLR Lampoldshausen test facility as an online classification tool has already been demonstrated. Here, we present promising data for further differentiation among bacteria. Bacteria species can express unique fluorescence spectra after excitation at 280 nm and 355 nm. Upon deactivation, the spectral features change depending on the deactivation method.

Experimental and numerical analysis of atmospheric propagation of high energy laser

The transmission of high power laser radiation through the air is influenced by atmospheric turbulence. As a result the beam experiences variations regarding its position and its distribution, which increase with increasing propagation length. In order to analyze the atmospheric influence on the laser beam propagation a disk laser with a maximum output power of 6 kW and a wavelength of 1.03 μm is operated on the 130 m long free transmission laser test range at Lampoldshausen. The test range is equipped with a variety of sensors, which continuously monitor the current status of the weather conditions. Power sensors and camera systems at the beginning and the end of the test range measure the laser beam parameters before and after propagation. First measurements of atmospheric power transmission, diameter change of the laser beam and deviation of its center of gravity are performed on a sunny and on a rainy day and are compared with turbulence strength, visibility and rainfall. The results show good correlation between the optical parameters and the weather conditions. Following measurements will be performed at different weather conditions and seasons. Experimental results will be compared to a numerical analysis.