Constantin P. Cristescu

He-Cd hollow-cathode oscillations in an axial magnetic field

The influence of the axial magnetic field on the output of a hollow-cathode Cd-He laser oscillating on the 533.7 and 537.8 nm lines is studied for various discharge conditions. The application of the axial magnetic field causes a decrease in the laser intensity. This behaviour can be explained by taking into account the action of the magnetic field both on the atomic transition and on the plasma inside the hollow cathode.

Error analysis for elastic-backscattering lidar

Elastic backscattering LIDAR represents one of the most promising tools for attaining remotely information about the distribution of aerosols and particulates in the atmosphere, which could lead to a better understanding of climate processes. In this paper we shall examine the uncertainties associated with such measurements and try to determine which of the LIDAR parameters need to be optimized in order to minimize the measurement errors. An optimization method of the operative characteristic associated with the LIDAR signal validation is also proposed.

The fluorescence of liquid micro spheres induced by laser irradiation

The fluorescence radiation of droplets rhodamine 6G solution in ethylene glycol is studied. The radiation of an argon ion laser was used for pumping. Our experiments show that amplification is taking place in whispering gallery modes (WGMs) of the micro spheres. The propagation of the pumping light in the micro spheres as well as the WGMs are discussed in the framework of geometrical optics. The dependence of the intensity of the fluorescence radiation on the position of the micro spheres as they fall through the pumping beam was studied. The temporal change of the fluorescence radiation during the falling of the micro spheres through a system of horizontal fringes obtained with a Michelson interferometer shows a saturation behavior. For droplet diameters of the order of tens of micrometers the fluorescence radiation is isotropical, whereas a diameter dependent anisotropy of fluorescence is observed for larger micro spheres.

Influence of laser irradiation on the morphology of fractal electrodeposition

Abstract This work presents the effect of cw argon ion laser irradiation on far from equilibrium fractal growths from CuSO 4 aqueous solutions in a quasi two-dimensional geometry. We find that the polarization orientation of the laser electric field influences the growth process. A laser electric field parallel to the field generated by the applied voltage causes an increase in the rate of the growth process. A slight inhibition of the process is observed when the two fields have crossed polarization. An interpretation based on the hypothesis of the polarization of the molecules and of the fractal growth caused by the laser electric field is given. A numerical simulation based on a perturbed diffusion model is shown to give results in agreement with the experimental data.

A numerical model to improve the derivation of aerosols optical parameters from elastic backscatter lidar data

LIDAR systems have demonstrated their ability to map aerosol variations throughout the atmospheric column and therefore they have has become a central technology in current strategies for tropospheric aerosol research. Its use is complicated, however, by the fact that the lidar signal contains a convolution of two basic optical properties of the aerosol particles: the backscatter coefficient and the extinction coefficient. A quantitative retrieval of either property requires knowledge of their relationship along the laser path which is referred as lidar ratio. If the lidar ratio can not be measured by high spectral resolution lidar, or Raman lidar, then either an assumed value of LRa must be used in the lidar retrieval, leading to very large uncertainties in light extinction, or models can be used for determination of LRa profile. Our research refers to the development of an iterative hybrid regularization technique for elastic backscatter lidar data processing and retrieval of the aerosols optical parameters using the atmospheric model, Mie model and Fernald-Klett, but also Ackermann algorithm for lidar ratio calculation based on relative humidity profile. This study focuses on a numerical investigation about the lidar ratio of tropospheric aerosols characterizing Romanian atmosphere. The model can be also used for other type of atmosphere in order to improve the derivation of aerosols optical parameters from elastic backscatter lidar data when no other information than meteorological data are available.

The influence of the rise time of the excitation pulse upon the output of a copper bromide laser

Considering the rise time (understanding the rise speed as well) of the electrical intensity of the pumping pulse as the key parameter characterizing the output power of a copper-type laser, we present here a simplified model of the main kinetic processes occuring during the electrical pumping pulse and afterglow in a Ne-CuBr laser. The a priori knowledge of the typical temporal behavior of the electron density and electron temperature reduces the number of coupled differential equation to be solved. The importance of the parasitic radiative filling of the lower laser levels 2D via channels that are not involving the upper laser levels 2P is enhanced. The comparative green-yellow lasing efficiencies and the temporal timing of the optical pulses with respect to the excitation pulse are consistent with the experiments.

Diffraction of a laser beam at the edge of a fractal electrodeposit

The diffraction of a laser beam while crossing a quasi-twodimensional cell in which a fractal electrodeposition takes place is presented. The features of the diffraction on the fractal edge are dependent on the current intensity through the cell. We observe that the diffractive properties of the fractal change in time after the stopping of the growth process, and totally disappear after a lapse of time of the order 300 s. In the anode zone, the behavior is entirely different. It is characterized by the appearance of high gradients of the refractive index generated by the metal ions created in the anode region. The observed beam divergence is explained using a model based on a curved surface phase optical wedge. The results are tested by analysis of moire´ fringe patterns.