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
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 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
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.
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.
A modified unstable resonator, suitable for a laser with large gain medium cross section and a small or median
output coupling, is presented. The spherical mirrors of this resonator form an off-axis negative-branch confocal
unstable resonator. The special shaping of the scraper together with the negative-branch confocal unstable
resonator configuration yields a laser output beam in the shape of a half-ring. In contrast to the conventional
resonator with a ring-shaped output beam, the cross section of the half-ring is more compact and generates a
lower structured far-field distribution, but the rotational symmetry disappears. For a rectangular medium cross
section, the modified resonator is applicable, too.
Some properties of such a rectangular modified negative-branch confocal unstable resonator (MNBUR) are
investigated numerically. The theory is based on the integral equation of the Fresnel-Kirchhoff formulation of
The theoretical study pays attention to inaccuracies that may occur in mirror manufacturing. With respect to
deviations from the specified mirror radius of curvature, the MNBUR does not show important differences to the
performance of a conventional unstable resonator. The tilt of a resonator mirror affects the total coupling loss.
The sensitivity of the MNBUR to output mirror misalignment is smaller than that of the common negativebranch
confocal unstable resonator. A further important improvement achieved by resonator modification is an
increased beam quality.
For a 10kW-class Chemical Oxygen Iodine Laser (COIL), negative and positive branch hybrid resonators are investigated in theory and experiment. Calculations from Fresnel-Kirchhoff theory predict a nearly diffraction limited beam quality in the unstable direction. Theory is verified by measurements with a 2D-camera system in the far field.
High brightness performance of chemical oxygen iodine lasers (COIL) requires resonator concepts that enable efficient
power extraction from low gain medium while the beam quality is close to the diffraction limit. The resonator shall provide
straightforward alignment procedures and stable output performance for efficient laser operation. Such resonator
configurations are theoretically designed and implemented in a 10 kW-class COIL.
Different resonator concepts are pre-evaluated by numerical methods with regard to their design and alignment sensitivities.
Promising candidates are tested for their alignment performance and brightness parameters. Among others, hybrid
resonators are found to be of straightforward optical architecture meeting the above challenges. Theoretical predictions
and experimental results are in excellent agreement. Therefore, further promising design approaches, like multi-pass hybrid
resonators or the modified negative-branch unstable resonator, will be depicted and discussed within this paper.
Various schemes of geometrical coupling between optical resonator and gain medium were investigated for a 10-kW class Chemical Oxygen-Iodine Laser (COIL). Starting from theory, different types of resonator layouts were designed and optimized for COIL with a rectangular gain medium and an output coupling of about 10%. Hybrid resonators match these coupling conditions more easily than concentric unstable resonators. Compared to the negative branch type, the positive branch hybrid resonator shows very high sensitivity to the optical alignment in the unstable direction but avoids a focal line within the resonator. The obtained output power of both hybrid resonators is compared to the output power of the COIL device in a conventional stable resonator configuration. Measured margins for the sensitivity of resonator setup and alignment were found in close agreement with numerical calculations. Power density distributions were measured in the near field and in the far field. The divergence of the emitted laser beam in the unstable direction was nearly diffraction limited.
A rectangular negative branch off-axis hybrid resonator was coupled to a 10-kW class Chemical Oxygen-Iodine Laser (COIL). Various schemes of geometrical coupling between resonator and gain medium were investigated. The extracted power was 6.6 kW and reached about 70% of the output power for the COIL device in an optimized conventional stable resonator configuration. Experimentally measured margins for the sensitivity of resonator setup and alignment were found in close agreement with numerical calculations. Power density distributions were measured in the near field and in the far field. The divergence of the emitted laser beam in the unstable direction was lower than 2 times diffraction limited.
The improvement of the efficiency of singlet oxygen generators (SOG) in chemical oxygen iodine lasers (COIL) is still a key component for optimizing the performance of these lasers. Important parameters for the SOG operation are the utilization of chlorine and the singlet oxygen yield. In this work, the singlet oxygen yield is examined by two different methods: the absorption spectroscopy of ground state molecular oxygen O2(3Σ) based on a commercial diode laser system in conjunction with a multi-pass Herriott cell, and the measurement of spontaneous emission of the O2(3Σ)-O2(1Δ) transition in the near infrared (1.27 μm). A separate calibration cell has been build with geometry identical to the diagnostic duct of the DLR COIL for exact calibration of the absorption measurement. Results of simultaneously applied emission and absorption measurements are compared. This procedure allows the determination of the radiative lifetime of O2(1Δ) to a value of 3730 s.
A negative branch and a positive branch hybrid resonator, suitable for a laser with large optical cross section and small output coupling are explored numerically. The basis of the theory is the Fresnel-Kirchhoff integral equation, the calculations describe a passive resonator. The fabrication of cylindrical mirrors is difficult and deviations in mirror radius of curvature are possible. While a large concave radius of curvature of the mirror is allowed in the stable direction, a convex curvature is not tolerable. Near the ideal mirror parameters, the resonator in unstable direction is insensitive to mirror curvature variations, if the resonator length is appropriately adapted to the actual mirror curvatures. With respect to mirror tilt, the calculations show that in unstable direction the off-axis negative branch confocal unstable resonator is less sensitive than the off-axis positive branch confocal unstable resonator. In stable direction, the sensitivity to mirror misalignment is larger and dependent on the mirror curvature but independent of the unstable resonator part.
A negative branch hybrid resonator was coupled to the 10kW-class Chemical Oxygen-Iodine Laser (COIL) device of DLR. Resonator set-up and alignment turned out to be straight forward. Experimentally measured margins for mirror misalignment were found in close agreement to numerical calculations. The extracted power came up to 70% of the power coupled out of a stable resonator device, while the divergence of the emitted light obtained in unstable direction was lower than 2 times diffraction limited.
11 kW of COIL power are successfully transmitted through a 20 meter long modified commercial fiber system with optical losses below 10%. The transmission ratio is derived from simultaneous measurement of the laser output power behind the total reflector and the transmitted laser power behind the complete optical fiber system. The fiber delivery system is used to demonstrate cutting of various materials pertinent to dismantling & decommissioning of nuclear installations. Nitrogen and oxygen gas assisted cuts are performed for metal and nonmetal samples, which are made of steel, aluminum, concrete and graphite. The cutting results are validated with theoretical models from literature. The COIL cutting performance is compared to referenced data of laser materials processing.
The 10kW-class Chemical Oxygen-Iodine Laser (COIL) device of DLR1 was used for demonstration of efficient power coupling into a commercial fiber optic system (HIGHYAG, Berlin) modified for 1.315 μm radiation. Transmission investigations were performed with different intra-cavity apertures of the stable COIL resonator to match the out-coupled laser beam to the fiber optic system. The transmitted power through the 20 m long fiber with core diameter of 1000 μm exceeded 6 kW during a typical 8 s test run of the laser. The highest transmitted power amounts to 11 kW, the highest value reported in literature. The transmission ratio was above 90%. Samples of metal and non-metal materials were cut by using nitrogen as processing gas. The materials were selected with regard to their application in the contaminated area of nuclear power plants. The results of the cutting experiments were used in theoretical models2,3 for extrapolation of the cutting data. Scaling studies were performed to estimate achievable cutting depth with COIL systems of higher laser power.
A test facility was designed to operate a disk type singlet delta oxygen generator in a closed loop for the flow of basic hydrogen peroxide (BHP). The facility allows the investigations of the influence of operating conditions like BHP flow, gas flow, disk rotation speed, and system components like disk package parameters and type of liquid separator on the generator performance and the ascertainment of the facilities capability for long term COIL operation. The dimension of the generator in the closed flow loop is chosen in accordance with the 10 kW-DLR-COIL.
The large-spot out-of-band irradiation of Ge wafers and subsequent evaluation is discussed in this paper. The wafers were irradiated with a high-power, cw COIL system, operating at a wavelength of 1.315 μm. Damaging fluence values on the order of 1 kJ/cm2 were found for irradiation periods of several seconds. Thermal simulations were consistent with experimental findings. The damage morphology showed melt and microcrystallites. For sample evaluation, a compact modulation transfer function (MTF) test bench has been developed.
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.
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.
Water vapor measurements at DLRs supersonic chemical oxygen iodine laser (COIL) have been carried out applying a tunable diode laser system. During a typical hot flow of 8 s, the water molar flow rate increases from 35 mmol/s to 85 mmol/s. Simultaneous measurements in the subsonic and supersonic region showed that about 10 percent of water in front of the expansion nozzle is lost. Standard mathematical expressions for water vapor pressure have been used to estimate the water partial pressure over basic hydrogen peroxide from the gas temperature measured by thermocouples at the exit of the singlet oxygen generator. A second tunable laser diode system was used to measure the iodine small signal gain for different iodine flow rates. Gain values were found at 1.2 percent/cm. From the recorded iodine atom absorption profiles the temperatures were calculated to be 190-220 K. The high starting temperatures are in accordance with the water measurements and a temperature increase due to water condensation.
In the supersonic COIL the power output and the laser performance strongly depend on mixing conditions of the gas flows. For the 10 kW COIL of DLR, the effects of the gas mixing on the laser power have been experimentally investigated to work out the relations between maximum laser power and the corresponding optimum mixing conditions. For this purpose, optimally diluted iodine jets have been matched to different molar flow rates of singlet delta oxygen. These energetic considerations have been complemented by the investigation of the power dependence on fluid mechanic aspects.
The motion and distribution of a fluid in a rotating disk generator as used in chemical oxygen-iodine lasers for the generation of singlet delta oxygen has been investigated experimentally. A transparent mock-up with 3 disks has been built and glycerine solutions of different concentrations have been used as the liquid. This should simulate the various viscosities that the basic hydrogen peroxide, the actual working fluid in a singlet oxygen generator, can assume. Other important parameters in the investigation with a significant influence on the results were the rotation rate of the disks and the filling level of the liquid bath. Depending on these parameters substantial amounts of liquid are trapped between the disks and reduce the accessible disk area for the actual gas-liquid reaction in the production of oxygen. Simultaneously, the gas flow velocity through the disks can locally increase by a factor of 6.5. A strong vortex has been found in the liquid bath of a batch type generator which limits the exchange of reacted liquid with the bulk. Finally, an estimate of the thickness of the liquid film on the disk surface could be derived.
In a stable resonator configuration, the output power and the power density distribution of a chemical oxygen-iodine laser (COIL) were measured for various outcoupling geometries: Diaphragms with slit apertures of various size were introduced intracavity in front of the outcoupling mirror. The slit was placed at different positions along the flow axis. Furthermore, outcoupling mirrors of various reflectivity were used. For all experimental conditions the measured power density distribution at the outcoupling mirror reveals strong symmetry effects: The beam patterns are always symmetric to the resonator axis. The beam shape and the beam size are defined by the hardware aperture that is nearest to the resonator axis. As a function of the slit width, the laser output power saturates well before the aperture of the resonator is fully opened. Fifty percent of the maximum output power were achieved at a width of 6 mm only. Good agreement is found between the measured data and theoretical calculations, when taking into account the specific flow conditions. The data highlight the significance of deactivation processes and the strong iodine repumping mechanism within the cavity. For standard operating conditions a Rigrod type analysis reveals a small signal gain that is nearly constant throughout the cavity exceeding 1.3% cm-1. The outcoupling reflectivity for the maximum power output was found at a value of about 94%. These experimental data agree well with a simplified analytic model for gain saturation and power extraction as derived by G. D. Hager et al.
The design of an optimized resonator for a supersonic chemical oxygen iodine laser (COIL) requires detailed information of the saturated gain distribution. By using diaphragms with slit apertures of various sizes at different positions in a stable resonator configuration it was possible to work out detailed information on the coupling characteristics and the influence of iodine repumping and deactivation processes. The measurements were performed at the DLR Lampoldshausen 10 kW supersonic COIL. A high Fresnel number stable resonator is coupled to the active medium and extends about 40 mm downstream from the exit plane of the expansion nozzle array. Apertures with slit widths ranging from 14% to 100% of the full resonator width were introduced into the cavity directly in front of the outcoupling mirror at a fixed z-position of 17 mm downstream of the nozzle exit. The center of another aperture with a fixed slit width of 10 mm could be positioned in flow direction from near the nozzle exit plane to about 29 mm downstream. The optical alignment was controlled with respect to the laser beam size and power density distribution at the outcoupling mirror. As a function of slit aperture the laser output power saturates well before the aperture of the resonator is fully opened. The experimental results are compared with calculations based on the theoretical model of Hager et al. Measured pressure values along the flow direction from the nozzle exit plane to well beyond the laser cavity show a strong dependence on position and width of the diaphragm, resulting from deactivation of unexploited singlet delta oxygen. After taking these pressure effects into account good agreement is found between theory and experiment. The data highlight the significance of deactivation processes for the achievable output power.
The DLR successfully operates a supersonic chemical oxygen iodine laser (COIL) at power levels of up to 8 kW. We address substantial discrepancies between the measured oxygen yield and its theoretically deduced value. This major issue cannot be resolved by using the well-known heuristic model for COIL performance evaluation. Therefore, a new approach derived from a comprehensive enthalpy flow model is presented and validated. We conclude that the experimental yield value is consistent with the new model whereas the theoretical yield is largely overestimated.
The German Aerospace Research Establishment (DLR) is routinely operating its supersonic chemical oxygen-iodine laser (COIL). Meanwhile over 200 single tests have been performed at run times extending to 1 minute. Power levels of 5 KW have been exceeded. Parametric studies were performed resulting in chemical generator efficiencies of about 43% for the baseline operation with high power output. The BHP molarity was found as one of the most important parameters for a stable and reproducible operation. Yield measurements revealed lower numbers than expected from theoretical calculations. The paper gives an overview of the COIL device and discusses the experimental and calculated results of the investigations.
For a rotating disk type generator the dependencies of utilization and yield on the generator operating mode are experimentally investigated. The fundamental effects of rotational speed, reduced gas volume, gas flow and liquid phase composition on the efficiency of singlet delta oxygen generation are discussed together with theoretically predicted generator performace from literature.
The chemical oxygen iodine laser (COIL) is an attractive candidate for efficient power scaling at short wavelengths. High specific power output from supersonic operation leads to compact devices. The German Aerospace Research Establishment (DLR) started experimental investigations of a multikilowatt supersonic COIL at its Lampoldshausen rocket test site in 1994. The excited oxygen is produced by a rotating disk generator. Currently the laser is operated without a cold trap. After the injection of the iodine, the laser gas is expanded to an isentropical Mach number of 1.8 by a multi-element grid nozzle. At present, laser power at about 5 kW and radiation times up to one minute are realized. The paper discusses the general setup of the device and reports on some selected laser output characteristics.