Browsing by Subject "Spektroskopie"

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10 W-Average-Power Single-Frequency Ti:sapphire Laser with Tuning Agility – A Breakthrough in High-Resolution 3D Water-Vapor Measurement
(2018) Metzendorf, Simon; Wulfmeyer, Volker
The differential absorption lidar (DIAL) technique is well suited for measuring the humidity field of the atmosphere with high spatial and temporal resolution as well as accuracy. The water-vapor DIAL of the University of Hohenheim is a mobile, ground-based, scanning system. The DIAL methodology and the application in the Hohenheim-DIAL impose stringent requirements on the laser transmitter. In this thesis, a new laser transmitter was realized and employed. It is a pulsed, actively frequency-stabilized titanium-sapphire laser system, pumped with a Nd:YAG master-oscillator power-amplifier (MOPA) and alternately seeded by two diode lasers. As pump source, two commercially custom-made, diode-pumped, Q-switched, and frequency-doubled Nd:YAG lasers in MOPA architecture were employed. The relevant properties for pumping the Ti:sapphire laser were studied. The second Nd:YAG MOPA provides a considerably higher average output power (up to P = 63 W at 532 nm, or a pulse energy of up to E = 210 mJ at a repetition rate of f = 300 Hz) and an almost ideal top-hat beam profile. Thus, efficient end-pumping of the Ti:sapphire crystal was enabled without any optical damage. The components for injection seeding of the titanium-sapphire laser, making narrowband operation at two alternating frequencies (online and offline) possible, were substantially improved. Now, advanced commercial external-cavity diode lasers (ECDL) are applied. With an analog regulation signal of a wavelength meter, the frequency of an ECDL can be stabilized precisely to a defined value (standard deviation < 1 MHz). Optionally, the frequency can be tuned according to various mathematical functions. The online-offline-switching is accomplished with a fiber switch. The crosstalk is extraordinarily low (< -61 dB), the switching time sufficiently short (~ 1.5 ms), and the spatial overlap of the signals, due to the waveguide, almost perfect. The power of the seeders in front of the resonator is more than sufficient, 17-20 mW. The Ti:sapphire laser consists of a ring resonator with four mirrors in a bow-tie layout. With adequate components, the operation wavelength at 818 nm is pre-selected and unidirectional propagation is ensured. The laser crystal is installed in an in-house-manufactured cooling mount, of which two designs were utilized and compared. The gain-switched Ti:sapphire laser was developed to operate in a dynamically stable state of the thermal lens, which arises in the crystal at high powers. To this end, the resonator was theoretically analyzed beforehand and the focal length of the thermal lens measured. The implementation of a cylindrical lens compensates the stronger contraction of the eigenmode in the tangential plane. By these means, a stable operation with an average output power of P = 10 W (corresponding to E = 33.3 mJ at f = 300 Hz; pulse duration ~ 30 ns) was realized. With a modified configuration of the cylindrical lens a maximum output power of P_max = 11.8 W (E_max = 39.3 mJ) was achieved. These values are the highest which were obtained so far for a laser of this kind, i.e., a laser transmitter whose power originates from a single radiation source (without further amplification or conversion). The laser cavity is actively stabilized to the frequency of the seeder, following a Pound-Drever-Hall technique. This yields permanent single-frequency operation with very high frequency stability (standard deviation < 2 MHz) and a narrow linewidth (< 63 MHz). These results correspond to the resolution limit of the characterizing wavelength meter. Laser emission occurs in the fundamental transverse mode, TEM_00 (M² <= 1.06). The laser system of the Hohenheim-DIAL has been successfully operated on several field campaigns. Its robustness has been demonstrated, for instance, during an uninterrupted operation for over 30 hours and an overseas transport to the USA which the system endured without damage. This work presents a vertical pointing and two scanning water-vapor DIAL measurements, confirming a high resolution and accuracy. The vertical measurement was executed for the first time at 10 W laser operation. Furthermore, two special DIAL measurements are discussed: The measurements on a strongly backscattering target demonstrate a high spectral purity >= 99.97% of the laser transmitter. Finally, an atmospheric measurement with a tuning online wavelength shows the frequency-agility of the laser and allows to determine the water-vapor absorption line experimentally. The comparison with the spectrum of a database shows a very good agreement (~ 5-10 % deviation in the absorption cross sections absolute value).
Development of coupled mid-infrared spectroscopic and thermal analytical approaches for the characterization and modeling of soil organic matter dynamics of arable soils
(2013) Demyan, Michael Scott; Cadisch, Georg
Soil organic matter (SOM) is a large part of the global carbon cycle both as a stock, as a source of fluxes (gaseous, dissolved, or sediments) to other stocks, and is also an important component of soil fertility and likewise plant productivity. Due to the growing need for additional data for both global studies related to climate change and soil fertility, additional information is needed not only on the total quantity of SOM, but its distribution within time and space and also its quality. In this study the use of mid-infrared spectroscopy in different applications was explored as an indicator of soil quality or composition, to measure the distribution of quality in different soils and fractions, and how these new methods could be used for SOM model parameterizations compared to other methods for both short and medium term model simulations. Firstly, certain mid-infrared active functional groups as measured with diffuse reflectance spectroscopy (DRIFTS) were studied in a long term fertilization experiment (Bad Lauchstädt) to ascertain the suitability of these different functional groups as indicators of the long term impacts of different fertilizer applications and also in various SOM fractions as separated by size-density approaches. Secondly, a coupled mid-infrared thermally evolved gas analysis was combined with in-situ monitoring of changes in vibrational functional groups to assign different qualities to different temperature ranges during a thermal oxidation experiment to 700 °C. Lastly, these two approaches were compared to traditional SOM fractionation as more rapid alternatives to parameterizing SOM pool sizes in the Century multi-compartment SOM model applied to arable soils at sites in the Kraichgau and Swabian Alb areas in Southwest Germany. In the long-term experiment (Bad Lauchstädt) it was found that certain vibrational functional groups (i.e. aliphatic (2930 1/cm) and aromatic (1620 1/cm)) in bulk soil varied (P < 0.05) according to long-term farmyard manure (FYM) and/or mineral fertilizer application. The application of 30 Mg/ha every second year of FYM increased the proportion of aliphatics as compared to aromatics, while the opposite was true for the control treatment (without any mineral and FYM fertilizer). The ratio of the aromatic to aliphatic relative peak areas were found to be positively related to the ratio of stabilized (SOC in heavy density fractions and clay size fraction) to labile SOC (light density fraction). This indicated that this peak area ratio (aromatic to aliphatic) is an indicator for the relative contribution of stabile to labile SOM as a stability index. In the next phase of the methodological development, evolved gas analysis (EGA) was used during a programmed heating of soil samples to 700°C to link EGA characteristics with SOM. An additional methodological step was the utilization of in-situ diffuse reflectance (in situT DRIFTS) measurements during heating as an indicator of the nature of SOM being decomposed at different temperatures. Thermal stability was found to be affected by experimental conditions and also sample type. The heating rate, amount of C in the sample, and volume of the sample in the heating chamber changed the rate and overall shape of the CO2 evolution curve and needed to be optimized when comparing different SOM fractions. In the long term experiment of Bad Lauchstädt, a decreasing thermal stability as measured by temperature of maximum CO2 evolution was found in the order from control > mineral fertilizer > manure > manure and mineral fertilizer. Furthermore, after a 490 day soil incubation at 20°C the thermal stability of SOC increased, but only slightly. In the in-situT DRIFTS method, the intensity of previously identified vibrational functional groups decreased (degraded) at different temperatures. The functional groups decreased in the order of aliphatic, alcoholic, and carboxylates, and at higher temperatures, also aromatic groups decreased. These findings were used as rules for fitting multiple peaks to the total evolved CO2 curve to derive SOM pools of different reactivity. Pools derived from the measured fractions of mid-infrared functional groups (aliphatic, carboxylate/aromatic, aromatic), evolved gas analysis (CO2) fitted peaks (centered at 320, 380, 540°C), and size-density fractionation (particulate organic matter, heavy density fraction, silt and clay fraction) in addition to a long-term equilibrium model run, were used to parameterize the SOM pools of the Century model as implemented in the Land Use Change Assessment tool (LUCIA) and compared to measured soil surface CO2 fluxes and soil organic carbon (SOC) contents after 2 years. The best fits for the short term study were found to be the SOM fractionation DRIFTS and EGA pool initialization methods, but the differences over two years were very small for the three different parameterization methods and generally CO2 fluxes were underestimated. A 20 year simulation, keeping all rate constants the same, on the other hand, showed large changes in both the SOC (14 Mg/ha, 0 to 30 cm) and the distribution in the pools. As compared to the 2010 baseline SOC, the DRIFTS, EGA-1, and SOM fractionation methods were closest in the Kraichgau site, while the equilibrium method was closest in the Swabian Alb. Overall, DRIFT mid-infrared spectroscopy showed its utility as a rapid assessment of the general distribution of stable to labile SOM in bulk soil. Additionally, when coupled with EGA and in-situ DRIFTS measurements, the integrated method can provide additional information during the thermal degradation of SOM during heating. All methods investigated found changes as a result of soil fertilization management, and between SOM fractions. Lastly, it was shown that such information can be used for direct SOM model inputs, although the methods should be tested on further land uses and soil types. These mid-infrared thermally coupled spectroscopic techniques represent an advance in the use of mid-infrared spectroscopy in the field of detailed SOM characterization for modeling SOM dynamics.
Prediction of soil properties for agricultural and environmental applications from infrared and X-ray soil spectral properties
(2013) Towett, Erick Kibet; Cadisch, Georg
Many of today?s most pressing problems facing developing countries, such as food security, climate change, and environmental protection, require large area data on soil functional capacity. Conventional assessments (methods and measurements) of soil capacity to perform specific agricultural and environmental functions are time consuming and expensive. In addition, repeatability, reproducibility and accuracy of conventional soil analytical data are major challenges. New, rapid methods to quantify soil properties are needed, especially in developing countries where reliable data on soil properties is sparse, and to take advantage of new opportunities for digital soil mapping. Mid infrared diffuse reflectance spectroscopy (MIR) has already shown promise as a rapid analytical tool and there are new opportunities to include other high-throughput techniques, such as total X-ray fluorescence (TXRF), and X-ray diffraction (XRD) spectroscopy. In this study TXRF and XRD were tested in conjunction with IR to provide powerful diagnostic capabilities for the direct prediction of key soil properties for agricultural and environmental applications especially for Sub-Saharan Africa (SSA) soils. Optimal combinations of spectral methods for use in pedotransfer functions for low cost, rapid prediction of chemical and physical properties of African soils as well as prediction models for soil organic carbon and soil fertility properties (soil extractable nutrients, pH and exchangeable acidity) were tested in this study. These state-of-the-art methods for large-area soil health measurement and monitoring will aid in accelerating economic development in developing sub-Saharan Africa countries with regards to climate change, increasing water scarcity and impacts on local and global food security as well as sustainable agricultural production and ecosystem resilience in the tropics. This study has developed and tested a method for the use of TXRF for direct quantification of total element concentrations in soils using a TXRF (S2 PICOFOXTM) spectrometer and demonstrated that TXRF could be used as a rapid screening tool for total element concentrations in soils assuming sufficient calibration measures are followed. The results of the current study have shown that TXRF can provide efficient chemical fingerprinting which could be further tested for inferring soil chemical and physical functional properties which is of interest in the African soil context for agricultural and environmental management at large scale. Further, this thesis has helped to improve understanding of the variation and patterns of element concentration data for 1034 soil samples from 34 stratified randomly-located 100-km2 ?sentinel? sites across SSA and explored the link between variability of soil properties and climate, parent material, vegetation types and land use patterns with the help of Random Forests statistics. Our results of total element concentration were within the range reported globally for soil Cr, Mn, Zn, Ni, V, Sr, and Y and in the high range for Al, Cu, Ta, Pb, and Ga. There were significant variations (P < 0.05) in total element composition within and between the sites for all the elements analysed. In addition, the greatest proportion of total variance and number of significant variance components occurred at the site (55-88%) followed by the cluster nested within site levels (10-40%). Our results also indicated that the strong observed within site as well as between site variations in many elements can serve to diagnose their soil fertility potential. Explorations of the relationships between element composition data and other site factors using ?randomForest? statistics have demonstrated that all site and soil-forming factors have important influence on total elemental concentrations in the soil with the most important variables explaining the main patterns of variation in total element concentrations being cluster, topography, landuse, precipitation and temperature. However, the importance of cluster can be explained by spatial correlation at distances of <1 km. This study has also analysed the potential of combining analyses undertaken using MIR spectroscopy and TXRF on 700 soil samples from 44 ?sentinel? sites distributed across SSA. MIR prediction models for soil organic carbon, and other soil fertility properties (such as soil extractable nutrients, pH, exchangeable acidity and soil texture) were developed using Random Forests (RF) regression and the current study has added total element concentration data to the residuals of the MIRS predictions to test how they can improve the MIR prediction accuracies. The RF approach out-perfomed the conventional partial least squares regression (PLSR) on simultaneous determination of soil properties; and in addition, RF results were also easily interpretable, computationally much faster and did not rely on data transformations or any other assumptions about data distributions compared to PLSR. With respect to the potential of combining TXRF and MIR spectra, including total element concentration data from TXRF analysis in the RF models significantly reduced root mean square error of prediction by 63% for Ecd, 54% for Mehlich-3 S, and 53% for Mehlich-3 Na. Thus, TXRF spectra were a useful supplement to improve prediction of soil properties not well predicted by MIRS. The prediction improvement from including TXRF was due to detection of a few outliers that did not appear as MIR spectral outliers. MIR showed remarkable ability to capture total elemental composition effects on physico-chemical soil properties but TXRF may have potential for outlier detection in large studies. This study has also helped to develop high-throughput spectral analytical methods and provided recommendations on optimal spectral analytical methods for the Globally Integrated Africa Soil Information Service (AfSIS) Project. Successfully developed methods in this study will become part of the standard AfSIS procedures.