Andreea-Rodica P. Sterian

Dynamics of a two-level medium under the action of short optical pulses

Optical Bloch equations are used to describe the dynamics of a two-level atomic medium subjected to ultrafast optical pulses. Starting with an uninverted system, the final atomic state is numerically studied. The generalization of the Bloch equations for a dense medium such that induced near dipole-dipole interactions are significant can result in a switching behavior of the atomic population inversion. Based on numerical investigations for typical pulse shapes, the mechanism of switching is explained in terms of dynamical systems theory.

Rate equations for an Erbium laser system, a numerical approach

In our work, we develop a numerical model for studding the variation of phenomenological and optical parameters in an Er+3 system of 6 excited levels that is subject to a lasing condition. The model is using a four order Runge- Kutta method to solve a system of eight time-dependent variables. The simulations, based on the change of pumping wavelength, prove that the highest efficiency is reached for a 970 nm pump wavelength, concordant with the existing literature results. Different processes, including: three ion-ion up-conversion processes, two pumping processes and the inherent spontaneous transition, are considered.

Filtering possibilities for processing optoelectronic current for acceleration measurements

This paper presents robust methods for filtering the optoelectronic current generated by laser signals in case of vibration measurements. First is presented a general procedure for obtaining signals corresponding to certain spatial vibrations by using diffracting grating. The diffracting grating is attached to the body supposed to present vibrations, and a set of four photodetectors is placed in the measuring area in symmetrical positions as related to a point of maximum intensity for the received light. The photoelectric current generated by this set is further filtered and processed, so as three electronic signals corresponding to displacements of the body along all three rectangular spatial coordinates axes to be obtained. Then is presented a method for determining the derivative of the spatial coordinates of the body as related to tune, in conditions of low-amplitude vibrations overlapped to a general translation ofthe body along a certain direction, under the influence of an external force. It is shown that such a structure is suitable for controlling the position of material bodies, due to the fact that the derivative of the position as related to time represents the acceleration of the body (proportional to the external force acting upon it).

Switching effect in dense media

Maxwell-Bloch equations generalized for a dense medium of two- level atoms are used to describe the dynamics of the system under the action of an ultrafast optical pulse. Starting with an uninverted system, the final population is one of the two extreme cases, depending on the ratio of the Rabi frequency and the parameter describing the near dipole-dipole interactions. Based on numerical investigations for typical pulse shapes, the mechanism of switching is explained.

The dynamics of erbium-doped fiber laser

The nonlinear dynamics of an erbium-doped fiber laser is explained based on a simple model of the ion pairs present in heavily doped fibers. The single-mode laser dynamics is reductible to four nonlinear differential equations. Depending on the ion pair concentration, the pumping level and the photon lifetime in the laser cavity, numerical calculations predicts cw, self-pulsing and sinusoidal dynamics. The regions of these dynamics in the space of the laser parameters are determined.

Infrared radiometer with digital signal processing for medical applications

In this paper we present our results concerning the study, design and construction of an infrared radiometer with digital signal processing for medical applications. The design and the practical realization of the system included an a priori study for performing the parameters analysis, both for the radiation source and the detector range, and it can be used for measuring with the desired resolution of the signal power in relation with the diagnosed situations. The pyroelectric type sensor represents the reference element of the system, so that in function of its type and parameters we have designed the source pointing optical ensemble as well as the electronics required for the detector output signal processing. The interface with the operator gives us the possibility of controlling the accuracy of the measurements by using a computer terminal, and also to monitorize the measured values and to modify the parameters by means of an external signal.

Laser signals’ nonlinear change in fatty acids

Abstract Previous works showed that thin layers of fatty acids and fatty acid–cholesterol mixtures behaved as optical liquid crystals, even at low incident laser power. The paper presents an experimental and computer study of laser signals, emergent from such samples, in presence of fluctuations. The optical emergent laser beams’ features at different incident parameters were experimentally determined for different type (c.w. and pulsed) lasers, as for example helium–neon and Nd 3+ glass lasers. The results were correlated with the amount of cholesterol in mixtures and with their response in external electric field. These measurements are in all cases affected by fluctuations. We developed some computer-based procedures, by using the TableCurve3D from Jandel Scientific software and equations Runge–Kutta in MATLAB for taking into account these fluctuations.

Spectral behavior and nonlinear optical properties of aluminophosphate semiconductor-doped glasses

Until now, all studies were done over the semiconductor glasses doped with CdSxSe1-x, in which the microcrystallite phase was grown into a vitreous silica matrix. Our study was made as for growing microcrystallites of CdSxSe1-x into a vitreous aluminophosphate matrix. In the first step, the samples of semiconductor glass were obtained by melting the raw materials batch and then by adding sulphur selenium and cadmium into the molten glass. In the second step, the resulted glass was annealed in order to bring about the microcrystallite nucleation and growing process. Both linear and non-linear optical properties are deeply influenced by the microcrystallite size, which, in its turn is determined by the duration and temperature of the annealing treatment. By means of a spectrophotometer (SPECORD M-42), were drawn absorption, transmission and fluorescence spectra, in visible domain. From the shape and the absorption edge of these spectra, we are able to characterize the microcrystallite growing process. From these spectra it can be observed a shift of the absorption edge to red which depends on the microcrystallite size.

Calibration method of a thermographic system for medical applications

The process of medical unconventional diagnosis is started by the user through a search process of appearance of some possible areas of abnormal temperatures which can be seen on a suitable monitor and the comparison of these areas with the unaffected ones. To guarantee an increased successful of this search process is necessary that the system design to take into account those displaying modalities of thermal images in order to highlight critical areas. These areas are all the more so full of information as: calibration of the system is more strictly achieved; image is rather well-defined, both geometrical and thermal (through depth and colour conversions). The present paper is made up as a contribution to the existing methodologies in calibration process of such a system. The authors leave from the reason that the existence of a set of calibration filters can contribute with sufficient data which can compensate the inherent errors cam e out by the use of a blackbody source at different temperatures necessry to calibration. Starting from the reasons mentioned above can be demonstrated that is sufficient to have a source constant temperature for the assurance of a strictly calibration.

Laser study of thermal neutron irradiation influence on simple biological membrane models

In this paper, thin films of some fatty acids and fatty acid-cholesterol mixtures were investigated as biological membrane simple models, in order to establish the thermal neutron irradiation influence on these systems, while in the smectic C liquid crystal state. Samples are nonlinear dielectrics with a weak conduction, and nonlinear optical media. They generally present some memore effects. Cholesterol modifies these properties depending on its percentage in mixture and saturated or unsaturated character of the acid. After exposed to a flow of thermal neutrons, texture and behavioral changes, in terms of content of cholesterol in mixtures, and depending on the number of carbon atoms, were emphasized.