Carsten Pargmann

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.

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.

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.

Modelling studies of transmission and scattering of high energy laser radiation under remote continental conditions

A numerical model is developed to simulate the angle dependent light scattering. The model is based on Mie theory and uses the complex refractive indices of aerosol particles and rain droplets together with their corresponding experimental number size distributions as input parameters. The laser beam parameters of the high energy laser at the DLR laser transmission test range in Lampoldshausen and the geometry of the detection system are taken into consideration. It is demonstrated that the numerical model accurately predicts the absolute scattered powers obtained by a calibrated multiangle light scattering probe measuring under five different scattering angles. The model is applicable for dry and rainy weather conditions. In addition, Mie calculations are performed to determine the extinction coefficients at 1030 nm. The calculated extinction coefficients are correlated with meteorological parameters (i.e. rainfall intensity and visibility) obtained from different types of instruments. The calculated extinction coefficients are compared with the extinction coefficients derived from laser transmission experiments at 1030 nm. A good agreement between numerical results and measurements is observed under rainy weather conditions.

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.

Atmospheric propagation of high power laser radiation at different weather conditions

Applications based on the propagation of high power laser radiation through the atmosphere are limited in range and effect, due to weather dependent beam wandering, beam deterioration, and scattering processes. Security and defense related application examples are countermeasures against hostile projectiles and the powering of satellites and aircrafts. For an examination of the correlations between weather condition and laser beam characteristics DLR operates at Lampoldshausen a 130 m long free transmission laser test range. Sensors around this test range continuously monitor turbulence strength, visibility, precipitation, temperature, and wind speed. High power laser radiation is obtained by a TruDisk 6001 disk laser (Trumpf company) yielding a maximum output power of 6 kW at a wavelength of 1030 nm. The laser beam is expanded to 180 mm and focused along the beam path. Power and intensity distribution are measured before and after propagation, providing information about the atmospheric transmission and alterations of diameter and position of the laser beam. Backscattered laser light is acquired by a photo receiver. As a result, measurements performed at different weather conditions show a couple of correlations to the characteristics of the laser beam. The experimental results are compared to a numerical analysis. The calculations are based on the Maxwell wave equation in Fresnel approximation. The turbulence is considered by the introduction of phase screens and the “von Karman” spectrum.

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.