Browsing by Subject "Wasserdampf"

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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).
3-D observations of absolute humidity from the land surface to the lower troposphere with scanning differential absorption lidar
(2016) Späth, Florian Heiko; Wulfmeyer, Volker
The water vapor (WV) distribution in the atmospheric boundary layer (ABL) is spatially and temporally highly variable. To investigate this behavior, the Institute of Physics and Meteorology at the University of Hohenheim (UHOH) developed a unique scanning differential absorption lidar (DIAL). This instrument allows for water vapor measurements with high temporal and spatial resolutions of the orders of seconds and tens of meters in the range of several kilometers from the surface up to the lower troposphere. Additionally, the UHOH DIAL system can perform scanning measurements which allows for observations down to the surface as well as for observations of the horizontal moisture variability. Within this thesis, three aspects regarding high-resolution observations of moisture in the ABL with scanning DIAL are demonstrated: 1) the development of a new seeder system for the laser transmitter, 2) the presentation of three scan modes, and 3) applications of 2-D to 3-D WV DIAL data. The newly developed seeder system is based on distributed feedback (DFB) laser diodes as seed lasers and an electro-optical deflector as optical switch. The setup and its specifications are presented. Scanning measurements were performed to capture the spatial WV structures. For this purpose, three scan modes with measurement examples are presented: 1) Range-height indicator (RHI) scans provide vertical cross-section images of the atmospheric humidity distribution. The presented series of four measurements show several humidity layers with different WV content and their evolution. Clouds appear in the last scan. 2) A volume scan captures the whole three-dimensional WV structure made out of several conical scans of different elevation angles. The horizontal variation of the layer heights can be related to the terrain profile with a small hill near the DIAL site. 3) Low elevation scans observe the WV distribution directly above the surface. Thus, relationships of the ground characteristics and vegetation with the humidity content above can be investigated. It is shown that there was more moisture above a maize field and above a forest than above grassland. For the analysis of scanning measurements, new analysis and visualization routines as well as new methods for the error estimation were developed. More scientific applications of high-resolution WV data from DIAL measurements are presented in three publications. A evaluation study compared humidity profiles from model simulations with different land-surface schemes with horizontal mean profiles of scanning DIAL measurements. High-resolution humidity fluctuations from vertical measurements were used to determine higher-order moments up to the fourth-order as well as skewness and kurtosis. Furthermore, such WV profiles were combined with profiles of temperature and vertical wind velocities and used for the development of new turbulence parameterizations and for model validation.
Convective-scale data assimilation of thermodynamic lidar data into the weather research and forecasting model
(2022) Thundathil, Rohith Muraleedharan; Wulfmeyer, Volker
This thesis studies the impact of assimilating temperature and humidity profiles from ground-based lidar systems and demonstrates its value for future short-range forecast. Thermodynamic profile obtained from the temperature Raman lidar and the water-vapour differential absorption lidar of the University of Hohenheim during the High Definition of Clouds and Precipitation for advancing Climate Prediction (HD(CP)2) project Observation Prototype Experiment (HOPE) are assimilated into the Weather Research and Forecasting model Data Assimilation (WRFDA) system by means of a new forward operator. The impact study assimilating the high-resolution thermodynamic lidar data was conducted using variational and ensemble-based data assimilation methods. The first part of the thesis describes the development of the thermodynamic lidar operator and its implementation through a deterministic DA impact study. The operator facilitates the direct assimilation of water vapour mixing ratio (WVMR), a prognostic variable in the WRF model, without conversion to relative humidity. Undesirable cross sensitivities to temperature are avoided here so that the complete information content of the observation with respect to the water vapour is provided. The assimilation experiments were performed with the three-dimensional variational (3DVAR) DA system with a rapid update cycle (RUC) with hourly frequency over ten hours. The DA experiments with the new operator outperformed the previously used relative humidity operator, and the overall humidity and temperature analyses improved. The simultaneous assimilation of temperature and WVMR resulted in a degradation of the temperature analysis compared to the improvement observed in the sole temperature assimilation experiment. The static background error covariance matrix (B) in the 3DVAR was identified as the reason behind this behaviour. The correlation between the temperature and WVMR variables in the background error covariance matrix of the 3DVAR, which is static and not flow-dependent, limited the improvement in temperature. The second part of the thesis provides a solution for overcoming the static B matrix issue. A hybrid, ensemble-based approach was applied using the Ensemble Transform Kalman Filter (ETKF) and the 3DVAR to add flow dependency to the B matrix. The hybrid experiment resulted in a 50% lower temperature and water vapour root mean square error (RMSE) than the 3DVAR experiment. Comparisons against independent radiosonde observations showed a reduction of RMSE by 26% for water vapour and 38% for temperature. The planetary boundary layer (PBL) height of the analyses also showed an improvement compared to the available ceilometer. The impact of assimilating a single lidar vertical profile spreads over a 100 km radius, which is promising for future assimilation of water vapour and temperature data from operational lidar networks for short-range weather forecasting. A forecast improvement was observed for 7 hours lead time compared with the ceilometer derived planetary boundary layer height observations and 4 hours with Global Navigation Satellite System (GNSS) derived integrated water vapour observations. With the help of sophisticated DA systems and a robust network of lidar systems, the thesis throws light on the future of short-range operational forecasting.
Development of an eye-safe solid-state tunable laser transmitter around 1.45 my m based on Cr 4+:YAG crystal for lidar applications
(2008) Petrova-Mayor, Anna; Wulfmeyer, Volker
A gain switched tunable Cr4+:YAG laser was developed using a Q-switched flashlamp?pumped Nd:YAG pump laser at 10 Hz. A vacuum spatial filter (VSF) was designed in order to filter the ?hot spots? of the pump beam profile. As a result of applying the VSF, a nearly Gaussian-shaped beam profile was achieved which enabled safe pumping of the Cr4+:YAG crystal with pulse energies in excess of 100 mJ. An extensive experimental optimization of the efficiency of the wavelength converter was performed. A maximum output energy of ~7 mJ at 1430?1450 nm, corresponding to ~7% conversion efficiency (with regard to absorbed pump energy), and a pulse duration of 30?35 ns were obtained with a 25-cm-long stable resonator. Tunability in the range 1350?1500 nm and spectral linewidth of ~200G Hz were demonstrated using a 3-plate birefringent filter. The laser was multimode with a flat-top profile and sufficiently good M2~4. The performance and size of the laser are acceptable for use in a laboratory based non-scanning lidar system if a narrow-band birefringent filter is installed. In order to employ a scanning mobile lidar, high pulse frequency (>100 Hz) of the pump laser for the Cr4+:YAG laser is required. The tunability permits the improvement of the laser transmitter for water-vapor DIAL measurements at on-line wavelengths of approximately 1459 nm or 1484 nm if injection-seeding is applied.
High-resolution measurements of temperature and humidity fields in the atmospheric boundary layer with scanning rotational Raman lidar
(2016) Hammann, Eva; Wulfmeyer, Volker
The Institute of Physics and Meteorology of the University of Hohenheim (UHOH) operates a scanning rotational Raman lidar (RRL) for high-resolution temperature and water vapor measurements. The measurement performance of the RRL was improved in several aspects. The statistical error of temperature measurements was reduced by up to 70% through optimization of the filter passbands for various solar background conditions. The optimization method, based on detailed simulations, was written for one specific wavelength and was not applicable to other Raman lidar systems. Therefore the simulation results were parametrized in respect to temperature and background level and expressed in units of wavenumbers. A new interference filter transmitting rotational Raman lines near the excitation wavelength was installed, resulting in a higher transmission and eliminating possible leakage signal. A detection channel for the vibrational Raman line of water vapor was added for the retrieval of water vapor mixing ratios during day-and nighttime. More than 300 hours of temperature and more than 200 hours of water vapor measurements were performed and the acquired profiles used in several publications. Atmospheric variance and higher order moment profiles of the daytime atmospheric boundary layer were derived.
Theoretical analysis and design of high-performance frequency converters for LIDAR systems
(2009) Wagner, Gerd; Wulfmeyer, Volker
Frequency converters based on parametric and nonparametric frequency conversion are analyzed with respect to the specifications for high-average power water-vapor DIAL transmitters (DIAL: Differential Absorption LIDAR; LIDAR: Light Detection and Ranging). A Ti:Sapphire laser was selected as a suitable frequency converter to fulfill simultaneously all the requirements in the wavelength range of 935 nm and 820 nm. As thermal effects have a decisive influence on the overall performance and laser resonator design, they were simulated on Ti:Sapphire laser crystals in detail for different crystals, pump, and cooling configurations using finite element analysis (FEA). The performance and spectral properties of the Ti:Sapphire laser transmitter were modeled with a rate-equation approach for stable and unstable resonators. First theoretical results of an end-pumped Ti:Sapphire laser based on an optimized, asymmetric confocal unstable ring resonator design are presented. The obtained results can especially be used for the further development of a Ti:Sapphire laser to serve as a demonstrator for a future space-borne DIAL system transmitter according to the WALES (Water Vapor Lidar Experiment in Space) specifications. Furthermore, the adaptation of the developed theory modules to other lasing materials and configurations is straightforward.