Institut für Physik und Meteorologie

Permanent URI for this collectionhttps://hohpublica.uni-hohenheim.de/handle/123456789/3

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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).
A backscatter lidar forward operator for aerosol-representing atmospheric chemistry models
(2020) Geisinger, Armin; Wulfmeyer, Volker
State-of-the-art atmospheric chemistry models are capable of simulating the transport and evolution of aerosols and trace gases but there is a lack of reliable methods for model validation and data assimilation. Networks of automated ceilometer lidars (ACLs) could be used to fill this gap. These networks are already used for the detection of clouds and aerosols, providing a 3D dataset of atmospheric backscatter profiles. But as the aerosol number concentration cannot be obtained from the ACL data alone; one needs a backscatter-lidar forward model to simulate lidar profiles from the model variables. Such an operator allows then for a qualitative and quantitative model validation based on ACL data. In this work, a newly developed backscatter-lidar forward operator and the related sensitivity studies are presented and results of the forward operator applied on model output data are compared to measured ACL profiles in the frame of a case study. As case study, the eruption of the Icelandic volcano Eyjafjallajökull in 2010 was chosen and extensively analyzed. The Consortium for Small-scale Modeling - Aerosols and Reactive Trace gases (COSMO-ART) model of DWD (Deutscher Wetterdienst) was operated during this event for ash-transport simulations over Europe. For the forward model, the attenuated backscatter coefficient is used as lidar-independent variable, which only relies on the laser wavelength. To calculate the attenuated backscatter coefficient, the size-dependent aerosol number concentration and the scattering properties of each aerosol type and size have to be simulated. While the aerosol number concentration is a model output variable, the scattering properties were determined by extensive scattering calculations. As these scattering calculations require assumptions about the aerosol refractive indices and shapes, sensitivity studies were performed to estimate the uncertainties related to the particle properties as represented by the model system. An analysis of the particle shape effect for the extinction and backscatter coefficients resulted in huge differences of the scattering properties between spherical, ellipsoidal and cylindrical particle shapes. Due to a particle shape mixture in typical volcanic ash plumes, the application of non-spherical scattering calculation methods for estimating the effective optical properties requires more information related to the particle shape distribution (specifically: a particle size and shape distribution). As such information was not available for the present case study, it was necessary to assume spherical shaped volcanic ash particles but estimate the uncertainty related to this assumption within the frame of additional sensitivity studies. Finally, the forward modeled lidar profiles were compared to ACL measurements from stations of the German ACL network. The comparison required an extraction of common time and height intervals of the ACL and forward modeled COMSO-ART data as well as reshaping the datasets to the same vertical and temporal resolution. Significant differences between ACL profiles and the output of the forward operator applied to the COSMO-ART data were found. Some ash layer structures were at similar coordinates which is remarkable due to the uncertainties related to the model dynamics and the limited amount of measurement data that could be used for model validation. In detail, however, the major fraction of the compared time and height interval differed both in the relative signal intensity and the layer structures of the volcanic ash plume. Based on such quantitative comparison, a future data assimilation system could correct the model prediction of the forward modeled attenuated backscatter coefficient, the time of arrival, as well as the vertical structure of the volcanic ash plume. In summary, the continuous and distributed data stream provided by ACL stations was found to deliver valuable verification information for dispersion simulations of aerosol events. But major issues have been determined which limit current realizations of backscatter-lidar forward operators for aerosol transport simulations: First, it is suggested that the ACL systems improve their dynamic range and perform automatic calibration to increase the precision of ACL data and for calculating the measured attenuated backscatter coefficient with a minimum leftover of uncertainties. This will allow for the calculation of the attenuated backscatter coefficient in the presence of clouds as well as of faint aerosol signals. Second, the aerosols scattering properties have to be analyzed even more extensively which includes both the variety of aerosol sizes or types as well as the size distribution information. From the findings within this study, the particle size distribution was indentified to be a critical component when using monodisperse size classes.
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.

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