Karin M. Gruenewald

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.

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.

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.

Design and Investigation of a Hybrid Optical Resonator for Medium Energy COIL

High brightness laser radiation of chemical oxygen iodine lasers (COIL) requires resonator types especially adapted to low gain medium and rectangular cavity geometry. For medium energy class lasers, those resonators suffer from small magnification numbers and therefore often imply sophisticated optical concepts. Selected solutions should not inhibit the demands of a field deployment of the laser system. Outside the laboratory additional nonoptical properties like compactness, ruggedness and ease of operation become important. Resonators that enable reliable operation under field conditions should be composed of a reasonable number of optical components and a straightforward architecture. Based on the amplification characteristics of a 10 kW-class COIL, off-axis hybrid resonator configurations are designed and pre-evaluated theoretically. The most promising candidates are technically realized and applied to the DLR-COIL device. The resonators are tested for their alignment sensitivity and brightness parameters. Positive branch and negative branch hybrid resonators are discussed. Particularly, the negative branch hybrid resonator meets the above challenges. A further promising design approach, the multi-pass hybrid resonator, will be depicted within this paper.

Near-diffraction limited high-power COIL emission

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.

COIL fiber transmission for material cutting applications

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.

Recent results of iodine and water diagnostics in supersonic COIL

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.

Long-Term Storage of BHP for Tactical COIL Application

Tactical COIL deployment expects an any-time, any-place operational laser system. Such a system affords a permanent availability of the operating supplies at any filling station within reach. For COIL especially a supply with basic hydrogen peroxide (BHP) is a challenge. Sufficient quantities of ready-to-use BHP have to be prepared and stored without any loss of reactivity. Since such BHP is not necessarily prepared at COIL bases, the stored BHP should be transportable to appropriate filling stations. The BHP suitability for storage is tested over several months. The BHP is stored at low temperatures. The BHP decomposition is controlled by titration. Optimum storing conditions were derived and safety aspects are developed. Long-term storing of BHP could be verified at low storing temperatures without considerable loss of reactivity. I. Introduction OR multi-kilowatt COIL operation, up to now, only the thin film reaction between gaseous chlorine Cl2, and basic hydrogen peroxide (BHP) yields the necessary flow rates of singlet-delta oxygen. The classical BHP is an aqueous solution of potassium hydroxide KOH and hydrogen peroxide H2O2. The reaction between gas and liquid takes place in a layer of about 50 nm in the liquid at the gas-liquid interface. The reaction can be described in the following overall equation 1,2

Performance of a Modified Negative Branch Confocal Unstable Resonator for COIL

Far field deployment of high power chemical oxygen iodine laser (COIL) affords optimally adapted resonator architectures. Such resonators shall enable high brightness operation. The different resonator concepts suffer from the low gain medium and the rectangular geometry typical of COIL. Even full exploitation of the laser active medium yields small outcoupling fractions and in consequence small magnifications for unstable resonator design. Additionally, the realization of suitable aspect ratios for the output aperture is difficult. An off-axis modified confocal negative branch unstable resonator (MNBUR) is recommended for high power COIL. For first verification tests, such a resonator is especially designed and coupled to a medium-sized COIL. Near field and far field characteristics are measured and compared to the theoretical predictions. The results and experiences gathered at the 10 kW-class DLR-COIL are encouraging to recommend MNBUR as a well suitable resonator for tactical COIL systems.

Measurement of molecular electronic ground state oxygen O 2 (3Σ) in COIL device

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.