KEYWORDS: Particles, Visibility, Mass attenuation coefficient, High power lasers, Atmospheric particles, Atmospheric propagation, Scattering, Signal attenuation, Laser scattering
Absorption and scattering by gaseous species, aerosol particles and precipitation lead to atmospheric attenuation of laser radiation. High power laser propagation over long horizontal and/or vertical distances is significantly affected by atmospheric attenuation in outdoor locations. Laser transmission experiments can be used to derive experimental extinction coefficients, which can be included in the assessment of the laser safety. However, a full representation of the effect of precipitation types on high power laser propagation is unknown to the authors.
Laser transmission experiments at 1030 nm under rainy and snowy weather conditions were performed on a laser test range to investigate the effect of atmospheric attenuation on high power laser radiation. Based on these laser transmission measurements, experimental extinction coefficients were derived. The experimental extinction coefficients are compared with theoretical values. In addition, the experimental extinction coefficients are correlated with visibility measurements.
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.
Measurements of parameters of the natural atmospheric environment, like aerosol and precipitation distributions as well as the visibility, are simultaneously performed together with the angle dependent scattering of high power laser radiation. These measurements were obtained on the free transmission test range operated by the German Aerospace Center (DLR) in Lampoldshausen. The applied high power disk laser system operates at a wavelength of 1030 nm and all presented measurements are performed at an output power of 3000 W in continuous wave. The laser beam is propagating along the 130 m test range and the focus is ten meters away from the detection setup for the scattered light. Each of the five detection systems consist of a fiber-coupled photoreceiver and all fiber-couplers are positioned on a stage at angles of 30°, 60°, 90°, 120° and 150°, respectively to the direction of the laser beam. The distance between the fiber-couplers and the detection volume is 500 mm and the aperture is about 22 mm. The laser beam is modulated at 10 Hz and the signals of the photoreceivers are transmitted to lock-in amplifiers. The transportable setup is protected by a weatherproofed box and is operated under various conditions and precipitations, like rain and snow. In order to obtain absolute values the detection systems are calibrated using an integrating sphere. The experimental setup of the calibration system is introduced and measurements of angle dependent absolute scattering powers in dependence on the power transmission loss, the geometric scattering coefficient and the visibility are shown.
Biological hazardous substances like 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 incalculable and complex consequences for first responders and people. The impact of such an event can be minimized by a combination of different sensor technologies that have been developed to detect bio-threats and to gather information after an incident. Sensors for bio-agents can be grouped into two categories. Sampling devices collect material from locations supposed to be contaminated, and they are able to identify biological material with high sensitivity and selectivity. However, these point sensors need to be positioned correctly in advance of an attack, and moving sources of biological material cannot be tracked. A different approach is based on optical standoff detection. For biological samples laser induced florescence (LIF) has been proven to get real time data on location and type of hazards without being in contact with the suspicious substance. This work is based on a bio-detector developed at the DLR Lampoldshausen. The LIF detection 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) as well as time resolved information for the fluorescence decay which can be used to classify suspicious samples. While the classification device had been trained on uncritical samples (like amino acids, NADH, yeast, chemicals, oils), this work presents the progress to more relevant, living bacteria of different strains. The low risk and non-pathogenic bacteria Bacillus thuringensis, Bacillus atrophaeus, Bacillus subtilis, Brevibacillus brevis, Micrococcus luteus, Oligella urethralis, Paenibacillus polymyxa and Escherichia coli (K12) have been investigated with the above set-up at both excitation wavelengths
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.
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.
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.
The high and still increasing number of attacks by hazardous bioorganic materials makes enormous demands on their detection. A very high detection sensitivity and differentiability are essential, as well as a rapid identification with low false alarm rates. One single technology can hardly achieve this. Point sensors can collect and identify materials, but finding an appropriate position is time consuming and involves several risks. Laser based standoff detection, however, can immediately provide information on propagation and compound type of a released hazardous material. The coupling of both methods may illustrate a solution to optimize the acquisition and detection of hazardous substances.
At DLR Lampoldshausen, bioorganic substances are measured, based on laser induced fluorescence (LIF), and subsequently classified. In this work, a procedure is presented, which utilizes lots of information (time-dependent spectral data, local information) and predicts the presence of hazardous substances by statistical data analysis. For that purpose, studies are carried out on a free transmission range at a distance of 22m at two different excitation wavelengths alternating between 280nm and 355 nm. Time-dependent fluorescence spectra are recorded by a gated intensified CCD camera (iCCD). An automated signal processing allows fast and deterministic data collection and a direct subsequent classification of the detected substances. The variation of the substance parameters (physical state, concentration) is included within this method.
The challenges of detecting hazardous biological materials are manifold: Such material has to be discriminated from other substances in various natural surroundings. The detection sensitivity should be extremely high. As living material may reproduce itself, already one single bacterium may represent a high risk. Of course, identification should be quite fast with a low false alarm rate. Up to now, there is no single technique to solve this problem. Point sensors may collect material and identify it, but the problems of fast identification and especially of appropriate positioning of local collectors are sophisticated. On the other hand, laser based standoff detection may instantaneously provide the information of some accidental spillage of material by detecting the generated thin cloud. LIF technique may classify but hardly identify the substance. A solution can be the use of LIF technique in a first step to collect primary data and – if necessary- followed by utilizing these data for an optimized positioning of point sensors. We perform studies on an open air laser test range at distances between 20 and 135 m applying LIF technique to detect and classify aerosols. In order to employ LIF capability, we use a laser source emitting two wavelengths alternatively, 280 and 355 nm, respectively. Moreover, the time dependence of fluorescence spectra is recorded by a gated intensified CCD camera. Signal processing is performed by dedicated software for spectral pattern recognition. The direct comparison of all results leads to a basic classification of the various compounds.
The application of standard unstable resonators does not allow for an independent adjustment of the resonator
magnification and the output coupling. Either you get a high magnification together with a high output coupling, or
vice versa. Certain laser types, like e.g. thin-disc lasers or chemical oxygen iodine lasers, permit only quite low
optimum output couplings. The corresponding low resonator magnification is equal to a poor beam quality. In order
to apply unstable resonators with a high magnification also to low gain media an additional mirror surface
retroreflects a part of the out coupled radiation back into the cavity. The output coupling is reduced efficiently,
whereas the resonator magnification stays high. Accordingly low gain media can be operated with high power
extraction in combination with a good beam quality. Numerical and experimental investigations are shown. The
experiments are performed with a chemical oxygen iodine laser operating at a wavelength of 1.315 μm and
demonstrate the feasibility of this resonator design.
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.
An off-axis configuration of the negative-branch confocal unstable resonator is examined numerically and
experimentally for a gain medium with rectangular cross-section. Due to less diffraction effects such a configuration
yields lower beam divergences than the standard on-axis resonator. The output coupling and the adaptation to the
geometry of the gain medium are attained by a scraper. Two different scraper profiles are examined. One profile
resembles to a rectangular bracket "[" and the other profile resembles to the letter "L". The experiments are performed
with a 10 kW class chemical oxygen iodine laser (COIL), which has a medium of low gain. Both scraper profiles are
applied to a resonator of the same magnification. Measurements of the intensity distributions in the near field and in the
far field are presented. The setup using the [-shaped scraper yields a higher output coupling and therefore a lower output
power and a lower beam divergence, whereas the setup using the L-shaped scraper makes use of the complete gain
medium. Furthermore, the L-shaped scraper is reusable for different resonator magnifications.
For laser media with large cross-section and low small signal gain neither the stable nor the unstable conventional
resonator is an applicable choice. Either the large Fresnel number leads to a multimode operation or the low output
coupling ring results in large diffraction effects and therefore in a heavily structured far field. In order to reduce these
diffraction effects a modified negative-branch unstable resonator (MNBUR) was introduced. In rotational symmetry the
off-axis setup provides an output coupling in the shape of a half ring and is accomplished with the help of a scraper. The
shape of this scraper can also be modified to adapt the MNBUR to a rectangular symmetry, whereas the spherical
resonator mirrors are kept unchanged. The scraper either takes the shape of a rectangular bracket "[" or of the letter "L".
The performance of both scrapers is tested using a chemical oxygen iodine laser (COIL) of the 10 kW class. The results
are compared to numerical calculations.
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.
KEYWORDS: Resonators, Mirrors, Near field, Diffraction, Chemical oxygen iodine lasers, Reflectors, Near field optics, Laser resonators, Aerospace engineering, Physics
Based on the experiences made with a negative-branch hybrid resonator (NBHR) a double-pass configuration of the
NBHR for a 10 kW class Chemical Oxygen Iodine Laser (COIL) is investigated. Measurements of the intensity
distribution of the far field are performed, as well as measurements of the sensitivity against tilts of one of the resonator
mirrors. The results are compared to calculations done with the help of the Fresnel-Kirchhoff theory. Enhanced results
for divergence, power density in the far field and sensitivity are achieved.
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