Peter Mahnke

Lidar research network water vapor and wind

This paper reports on a workshop which took place on April 08/09, 2002 at the University of Hohenheim, Stuttgart, Germany. During this workshop, laser and lidar experts as well as experts in atmospheric sciences came together. The goals of this workshop were to establish an interdisciplinary network on lidar research, to develop operational reference systems for water vapor and wind lidar measurements and to demonstrate the significant impact of water vapor and wind lidar systems on climate and weather research. The main result was the setup of an interdisciplinary expert network Lidar research water vapor and wind. This network is responding to large gaps in our knowledge of the global water vapor and wind distribution and shall improve weather and climate research. The network has mainly the following objectives: 1) The development of high-performance, high-power laser transmitters for wind and water vapor measurements based on novel pump laser designs, 2) research on efficient frequency conversion techniques of the high-power pump light into wavelengths required for water vapor and wind lidar systems, 3) the development of an operational reference water vapor differential absorption lidar system, also as a demonstrator for space borne missions like the Water Vapor Lidar Experiment in Space (WALES), 4) the performance of observation system simulation experiments (OSSEs) in order to demonstrate the impact of lidar data on nowcasting and short-range weather forecast, 5) the extension of the application of current and future lidar data in combination with other instruments for process studies, 6) the design of future field studies including a new generation of remote sensing systems. The network shall investigate key questions in atmospheric sciences, particularly, the role of water vapor in the Earths weather and climate system by the application of lidar systems with unprecedented accuracy, resolution and range.

Non-linear optical frequency conversion crystals for space applications

Reliable, long term operation of high-power laser systems in the Earth orbit is not a straightforward task as the space environment entails various risks for optical surfaces and bulk materials. The increased operational risk is, among others, due to the presence of high energy radiation penetrating the metallic shielding of satellites and inducing absorption centers in the bulk of optical components, and vacuum exposure which can deteriorate coating performance. Comprehensive testing for analyzing high-energy radiation effects and mitigation procedures were performed on a set of frequency conversion crystals and are discussed in this paper. In addition to a general resistance to space environmental effects, the frequency conversion crystals were subjected to a comparative analysis on optimum third harmonic efficiency, starting from pulsed 1064 nm laser radiation, with the goal of exceeding a value of 30%. Concomitant modeling supported the selection of crystal parameters and the definition of crystal dimensions.

Fast tuning of external-cavity diode lasers

We report on a method for characterizing the tuning capabilities of small-bandwidth external-cavity diode lasers. The step response of a piezo-driven Littman-Metcalf external-cavity diode laser and the compensation of its optoelectromechanical frequency response is investigated. When an approximately 50-V compensated Gaussian pulse is applied to the piezo element, a detuning of 13.6 GHz is observed. This modification of the laser is useful for several spectroscopical applications and as a tunable seed laser for lidar applications.

Wavelength switching in the acceptance bandwidth of a dual injection seeded optical parametric oscillator

We describe the investigation of a dual laser injection seeding and switching of an optical parametric oscillator. This option is part of a differential absorption lidar (DIAL) transmitter system for measuring environmental species at two wavelengths. For instance the realization was accomplished by using an injection seeded diode pumped Nd:YAG laser as pump for an optical parametric oscillator (OPO). The purpose of these studies was an evaluation of different injection seeding techniques and their practical applicability. The suitable wavelengths were generated by injection seeding the OPO at the signal wavelength. Experiments with one and two external cavity diode lasers were performed to generate the required two idler wavelengths for the measurement. By this method line-narrowing occurs. A spectral width of the pulsed OPO of 200 MHz was achieved. Within the seeding and switching range at these two wavelengths inter-mode mixing was observed.

1 kHz 3.3 μm Nd:YAG KTiOAsO 4 optical parametric oscillator system for laser ultrasound excitation of carbon-fiber-reinforced plastics

We present a new laser prototype for laser ultrasonics excitation. The fundamental wavelength of a Q-switched Nd:YAG laser with a repetition rate of 1 kHz is converted to 3.3 μm with a KTiOAsO4 optical parametric oscillator. The achieved pulse energy at 3.3 μm is 1.7 mJ, and the pulse duration at the fundamental wavelength of 1.06 μm has been measured to be 21 ns. The ultrasonic excitation efficiency is about 3.5 times better compared to the application of state-of-the-art CO2 lasers.

Injection seeded ns-pulsed Nd:YAG laser at 1116 nm for Fe-LIDAR

We present a side-respectively end-pumped Nd:YAG laser system at a laser wavelength of 1116 nm. The third harmonic of this wavelength at 372 nm can be used for LIDAR detection of molecular iron in the middle atmosphere at the height of about 70 to 120 km. Such measurements give information about temperature profiles and wind speeds in this atmospheric region and thus help in validating and calibrating global circulation models [1, 2].

Frequency stabilized injection seeded Q-switched Ho:YAG laser for 2 μm Doppler wind LIDARs

For obtaining a single frequency Q-switched laser output injecting a continuous wave single frequency master laser into a Q-switched laser is a commonly used technique [1]. For a stabilized injection seeding a Fast Fourier Transform (FFT) based single heterodyne beat signal technique will be presented.

2 µm Ho:YAG Thin Disk Laser

A Thulium fiber laser pumped Ho:YAG thin disk laser with 15W (cw) or several mJ (pulsed) operation will be presented. Additionally, a narrow (<0.5nm), tunable (30nm) cw operation near 2.09 µm, will be shown.

Characterization of a coherently coupled 2x2 W fiber amplifier array

Experimental investigations concerning the operation characteristics of a coherent fiber laser MOPA array are presented. The experimental set up consists of a single frequency fiber coupled 35 mW DBR diode laser at 1063 nm as master oscillator and two polarization maintaining two stage 2 W Yb doped commercial fiber amplifiers. Phase control is accomplished by fiber coupled acousto-optic frequency shifters prior to the amplifiers and an opto-electric phase locked loop operating at 100 MHz. Phase measurement at the amplifier output is achieved by a combination of a free space and fiberoptic interferometer in combination with RF photodiodes. A heterodyne signal of the amplifier output signal is generated with respect to a reference signal derived from the master oscillator and works as input signal for the phase control. Phase coupling of the array is demonstrated and the degree of coherence is determined from the contrast of the far field diffraction pattern of the output beam as well as from analysis of the RF photodiode signals. The characteristics of the phase control and phase stability are investigated and residual phase disturbances resulting from thermal and acoustic effects as well as depolarization are identified. Achievable beam quality as a function of fill factor is compared to theoretical computations. Finally, perspectives concerning a coherent 4x15 W MOPA array with three stage amplifying systems are outlined.

OPO resonator length stabilisation for injection seeding using fibre coupled heterodyne detection

By employing optical heterodyne measurements of injection seeded optical parametric oscillator pulses with the frequency shifted seed laser we present a fibre coupled solution for resonator length stabilisation.