Alexandru M. Preda

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.

Fractal-like diffractive arrangement with multiple focal points

In this paper we introduce a diffractive structure with a geometry which contains multiple Fresnel zone plates (MFZP) disposed in an arrangement based on a fractal-like rule. The corresponding diffracted intensity in planes perpendicular to the propagation axe presents two or more focal points. Their position and magnitude depend on geometrical parameters of the MFZP and on dimensions in the fractal-like arrangement. In our simulations we also analyze the influence in diffraction pattern of different non-binary phase levels in the MFZP plane. The MFZP structures with different values of geometrical and phase parameters are addressed to an optoelectronic device liquid crystal spatial light modulator (LCSLM), the experimental and simulation results are in a good agreement. The MFZP geometry with better optical parameters in diffraction pattern is then made on glass using electron beam lithography technique.

Quasi-periodic structure analysis by digital holography

We analyze an optical system consisting of a number of basically elements (BE) arranged in a quasi-periodic structure (QPS) of phyllotaxis type, which offers the best packing in many natural bodies. In our simulations, we change the parameters which generate this QPS to find its diffractive optical properties. This structure is then made on glass using e-beam lithography. To characterize this QPS, we use the digital holography (DH) method to reconstruct the amplitude and also the phase in object plane (QPS plane). Using different parameters of the spherical reference wave for holograms recording we change the contrast in holograms to improve the reconstruction of the object.

Iterative algorithm analysis for phase-only diffractive control access system

A new architecture with two phases-only diffractive elements and one decryption mask for optical control access system is presented. Only three different persons which keep this element have the permission to access together. The Iterative Fourier Transform Algorithm (IFTA) is analyzed for phase-only diffractive optical elements (PODE) design with different constraints in the input and output plane and the optimal variant is chosen for better image quality in the output plane (big value for diffraction efficiency and small value for merit function and signal to noise ratio). For higher security we propose different incident waves. That are compared with the case when the first phase-only diffractive element and decryption masks are designed together in an extended iteration and the output images of them (first desired image) is taken over the second phase-only diffractive element. In order to increase security level, this finally PODE are designed to increase some parts from the first desired image. Only with this condition the key image on the detector is formed.

Magneto-mechanical oscillations revisited

Here we study the temporal evolution of a magnetemechanical, inverted spherical pendulum. The oscillator is a rigid, stainless steel, hollow needle. The needle has one end fixed in a spherical-like articulation, while the other extremity has no mechanical contact. The oscillation is driven by a longitudinal, periodically magnetic field added to a constant value of a static field. A video camera takes simultaneously the optical images of the projections of the oscillating needle along two mutually normal directions. The pairs of temporal sequences are analyzed in the real space, phase space and Fourier space. Among all the external parameter that can be usually varied, like the strength of the static magnetic field, the amplitude and the frequency of the driving magnetic field, or the momentum of inertia of the oscillating body, of a crucial importance seems to be the magnetc-mechanical feed-back of the oscillating system.

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.

Structured light interferometer for space-time refraction index profile

In this paper we present a glycerin-filled interferometer which is suited to determine the thermally induced space-time refractive index profile of an optical, uranyl-ion-doped glass. The refractive index of the sample changes when irradiated with an Ar+ laser or a pulsed Nd-YAG laser and modifies the interference pattern of a probe beam. According to the task performed, the interferometer is used in longitudinal and transversal configurations. Using the model of the thermal absorption and diffusion, one can find the relation between the refractive index variation, temperature gradient and phase shift of the interference pattern. We calculate the refractive index changes of the on and off axis as well as its temporal variation during the relaxation process. The modifications of a thermally induced phase grating is interpreted in terms of a one-dimensional, structured gaussian beam obtained from a double refractive crystal of KDP (KH2PO3).

Interferometric study of the laser-induced melting stage of an absorbing glass

We study the processes of glass heating and melting by absorption of laser radiation using profilometry in reflected light and interferometric real time analysis. The sample irradiation is obtained by an argon ion laser and the interference pattern is generated by a He-Ne laser or by another argon ion laser. In the reflected beam we observe a change of the ring pattern caused by the increasing of pumping power. Above a certain level an additional ring pattern appears. We study the dependence of the structure of the ring patterns as function of the power of the pumping laser beam. From the diffraction pattern and the interference pattern given by a wedge shearing interferometer we estimate the shape and dimension of the mound and respectively the dimple generated for different pumping powers.

Determination of the refractive index of human serum by using a removable compact disk grating

Here we use a blank recordable compact disk as a diffractive device. A simple model for the disk grating is proposed. After the disk is properly cut, small pieces of it are completely immersed in the liquid of interest. Such a small piece is in fact a single-use reflective grating and we call it removable compact disk grating. Because its very cheap and, for this reason, easy to remove it, the compact disk grating is a suitable device for contaminating fluids and particularly for human serum. The principle of the method allows to calculate the refractive index of human serum by measuring the angular displacement of the diffracted maxima, or by fringe counting in a grating interferometer.

Interferometric detection of photothermal effects in uranyl optical glass

In this paper we present a sensitive photothermal interferometric detection technique using a Wollaston prism. The interference fringe pattern is formed by overlapping of the ordinary and extraordinary beams with adjustable intensities by a linear polarizer. The interferometric montage is made both for the pump beam (Ar-ion laser) and the probe beam (He-Ne laser). From the analysis of the interference patterns obtained separately for the two beams, we calculate the absolute change of the refractive index in the middle of the heated zone of an uranyl-doped optical glass, as well as the average transverse refractive index gradient induced by the pump laser. The experimental results are interpreted in terms of a Gaussian beam propagating through a weak absorptive medium whose refractive index is thermally modified by the pump beam.