Lidar Systems for the measurement of three-dimensional wind or cloud and aerosol formations in the earth atmosphere
require highly stable pulsed single frequency laser systems with a narrow line width. The lasers for ESAs ADM-Aeolus
and EarthCARE missions require frequency stabilities of 4 and 10 MHz rms at a wavelength of 355 nm and a line width
below 50 MHz at 30 ns pulse duration. Transferred to the fundamental wavelength of the laser systems the stability
requirement is 1.3 and 3.3 MHz, respectively. In comparison to ground based lidar systems the vibrational load on the
laser system is much higher in airborne and spaceborne systems, especially at high frequencies of some hundred Hertz or
even some kHz. Suitable frequency stabilisation methods have therefore to be able to suppress these vibrations
sufficiently. The often used Pulse-Build-up method is not suitable, due to its very limited capability to suppress vibration
frequencies of the order of the pulse repetition frequency.
In this study the performance of three frequency stabilisation methods in principle capable to meet the requirements, the
cavity dither method, the modified Pound-Drever-Hall method and a modified Ramp-Fire method - named Ramp-Delay-
Fire - is theoretically and experimentally investigated and compared.
The investigation is performed on highly efficient, passively cooled, diode end-pumped q-switched Nd:YAG oscillators,
which are breadboard versions of the A2D (ADM-Aeolus) and possible ATLAS (EarthCARE) oscillators. They deliver
diffraction limited output pulses with up to 12 mJ pulse energy at a pulse duration of 30 ns and 100 Hz pulse repetition
Design and experimental characterization of a nonlinear optical converter module for the generation of widely tunable
UV radiation is presented. The module combines units for second, third and fourth harmonic generation of tunable
Ti:Sapphire lasers. A modified conversion scheme based on the combination of BIBO and BBO crystals reduces the
complexity of our former published UV setup - resulting in a significant increase of performance and long-term stability
of the system. Experimental characterization of the former and the improved UV setup are compared. The investigations
of the converter module are carried out with a widely tunable Ti:Sapphire laser with nanosecond pulses and a repetition
rate of 1 kHz. This laser provides a continuous tuning range of 690 nm to 1010 nm with pulse energies up to 2.0 mJ and
a spectral line width of less than 10 GHz resulting in an output power of the converter module of 1000 mW, 400 mW
and 200 mW respectively for the second, third and fourth harmonic generation. The new converter module is a decisive
step in the development of a hands-off solid-state laser system with a continuous tuning range from the UV to the NIR -
200 nm to 1000 nm.
Generating the difference frequency of a frequency-doubled, widely tunable Ti:Al<sub>2</sub>O<sub>3</sub> laser and a Nd:YAG laser provides tunable laser radiation in the visible spectrum range. The generated wavelength region closes the spectral gap between the fundamental and the second harmonic of the Ti:Sapphire laser. A prototype has being developed with a fully automated wavelength tuning, i.e. the wavelength tuning of the Ti:Sapphire laser, the angel tuning of the nonlinear crystals and the tuning of the temporal delay between the Ti:Sapphire and the Nd:YAG laser operate self-controlled. Design, theoretical modeling and experimental characterization of the
system are closely discussed. At a repetition rate of one kilohertz, the frequency-doubled Ti:Sapphire laser provides pulses of approximately 20 ns, a spectral line width of 20 GHz, a nearly diffraction limited beam quality and pulse energies of up to 850 μJ. The tuning range reaches from 340 nm to 510 nm. For the three wave
interaction process in a 8 mm long BBO crystal the Ti:Sapphire pulses (pump wave) are mixed with 3.5 mJ pulses of a Nd:YAG laser (signal wave). The generated idler wave has pulse energies of up to 280 μJ and pulse durations of approximately 10 ns in the spectral range between 510 nm and 680 nm. This yields to a conversion
efficiency of about 33% and a quantum conversion efficiency of more than 50%. To our knowledge, this clearly exceeds the values that has been obtained with comparable setups so far. Further increase of the efficiency is currently under investigation.