Efficient operation of a diode-bar-pumped Nd:YAG laser on the low-gain 1123-nm line.

Efficient operation of a Nd:YAG laser on the low-gain 1123-nm transition by end pumping with the reshaped output from a 7-W diode bar is reported. Using a simple standing-wave laser configuration and pumping with 5.6 W of incident power yielded a laser output power of 1.7 W in a near-diffraction-limited TEM00 mode with a beam quality factor of M2 < or approximately equal to 1.1. A unidirectional single-frequency ring laser was also constructed, yielding 180 mW of single-frequency output. The prospects for further increase in power by optimization of the resonator design are discussed.

Ground-based differential absorption lidar for water-vapor profiling: assessment of accuracy, resolution, and meteorological applications.

The accuracy and the resolution of water-vapor measurements by use of the ground-based differential absorption lidar (DIAL) system of the Max-Planck-Institute (MPI) are determined. A theoretical analysis, intercomparisons with radiosondes, and measurements in high-altitude clouds allow the conclusion that, with the MPI DIAL system, water-vapor measurements with a systematic error of <5% in the whole troposphere can be performed. Special emphasis is laid on the outstanding daytime and nighttime performance of the DIAL system in the lower troposphere. With a time resolution of 1 min the statistical error varies between 0.05 g/m(3) in the near range using 75 m and-depending on the meteorological conditions-approximately 0.25 g/m(3) at 2 km using 150-m vertical resolution. When the eddy correlation method is applied, this accuracy and resolution are sufficient to determine water-vapor flux profiles in the convective boundary layer with a statistical error of <10% in each data point to approximately 1700 m. The results have contributed to the fact that the DIAL method has finally won recognition as an excellent tool for tropospheric research, in particular for boundary layer research and as a calibration standard for radiosondes and satellites.

Ground-based differential absorption lidar for water-vapor and temperature profiling: methodology.

A comprehensive formulation of the differential absorption lidar (DIAL) methodology is presented that explicitly includes details of the spectral distributions of both the transmitted and the backscattered light. The method is important for high-accuracy water-vapor retrievals and in particular for temperature measurements. Probability estimates of the error that is due to Doppler-broadened Rayleigh scattering based on an extended experimental data set are presented, as is an analytical treatment of errors that are due to averaging in the nonlinear retrieval scheme. System performance requirements are derived that show that water-vapor retrievals with an accuracy of better than 5% and temperature retrievals with an accuracy of better than 1 K in the entire troposphere are feasible if the error that results from Rayleigh-Doppler correction can be avoided. A modification of the DIAL technique, high-spectral-resolution DIAL avoids errors that are due to Doppler-broadened Rayleigh backscatter and permits simultaneous water-vapor and wind measurements with the same system.

Single-mode operation of an injection-seeded alexandrite ring laser for application in water-vapor and temperature differential absorption lidar.

A major improvement of a differential absorption lidar (DIAL) system for measurements of tropospheric water vapor and temperature is introduced. A Q-switched unidirectional alexandrite ring laser is injection seeded by a cw Ti:sapphire ring laser. Using an especially developed single-mode electronic, one starts the Q switch when the slave resonator is in resonance with the frequency of the Ti:sapphire laser. Long-term single-mode operation of the alexandrite laser is achieved. A single-shot spectral linewidth of <40 MHz and a frequency stability of 15 MHz rms can be specified. Thus what is to our knowledge the first single-mode DIAL system in the near infrared is presented.

Injection-seeded alexandrite ring laser: performance and application in a water-vapor differential absorption lidar.

A new laser system for use of differential absorption lidar (DIAL) in measurements of tropospheric water vapor and temperature is introduced. This system operates in the 720-780-nm region and is configured as an alexandrite ring laser injection seeded by a cw Ti:sapphire ring laser. This combination provides for the necessary narrow-bandwidth, high-frequency stability and excellent spectral purity. A bandwidth of <5.0 x 10(-3) cm(-1), a frequency stability of 2.1 x 10(-3) cm(-1) rms, and a spectral purity of 99.995% at 726 nm have been achieved during extended periods of operation. A comparison of a DIAL water-vapor measurement with a radiosonde in the boundary layer between 500 and 2000 m was performed. The maximum deviation between the humidity profiles is 15%, the standard deviation 1.6%, and the difference between the mean values 1%.

Depolarization of polarized light caused by high altitude clouds. 1: Depolarization of lidar induced by cirrus.

A scattering model is described for the investigation of depolarization of polarized light caused by ice clouds. The scattering by a single particle is described by refraction, reflection, and diffraction. The ice cloud is assumed to be a random mixture of hexagonal columns and plates of random orientation and size. Multiple scattering effects are included by means of a Monte Carlo method, where single photon histories are constructed from random samples of the distributions governing the basic scattering parameters. The dependence of depolarization on cloud extinction coefficient, receiver field of view, and mixing ratio of columns to plates are studied. Lidar measurements of depolarization by a high altitude cirrus cloud are presented and discussed within the frame of the present model. Good agreement can be obtained assuming a variation of crystal shape distribution with height.

Correction scheme for spectral broadening by Rayleigh scattering in differential absorption lidar measurements of water vapor in the troposphere.

An evaluation scheme is given to calculate the water vapor content from data obtained by differential absorption lidar (DIAL), taking into account that the Rayleigh scattered part of the return signal shows considerable spectral broadening in contrast to the Mie scattered part. To correct for errors caused by this effect, information on the aerosol backscattering properties is necessary. Sensitivity analysis performed by model calculations show that it can be retrieved with sufficient accuracy from the off-line signal in the same way as for backscatter lidar. It can be expected that water vapor retrieval will be possible with good accuracy even in the most critical cases, where steep gradients in aerosol backscattering exist in the upper troposphere.