Improving the time-averaged thermal efficiency of a laser beam by acoustooptic correction of the directional pattern

Applicability of the acoustooptic method for raising the time-averaged thermal efficiency of laser radiation is substantiated theoretically and confirmed experimentally. The effect produced by laser radiation on materials being processed (laser cutting, welding, engraving, etc.) has a threshold in light intensity. Importantly, a beam with the most frequently used normal (Gaussian) angular distribution of intensity is not optimal from the technological viewpoint. A method proposed for its optimization is based on acoustooptic refraction, i.e., fast nonlinear scanning of the initial beam around its central position, which improves (at certain values of the parameters) the time-averaged angular distribution of the beam intensity. In the experiment, the thermal efficiency of laser radiation is raised by several times.     

Dynamic optical transfer function of an acoustooptic modulator with a damped sound wave

Based on a spatially dependent dynamic optical transfer function of an acoustooptic modulator, a dynamic model of light modulation by a sound signal under conditions of acoustic damping is developed. A system of equations describing the dynamics of the acoustooptic interaction is given. Solutions of this equation for arbitrary power level and spatial-time structure of a sound signal are found. It is shown that acoustooptic damping has the strongest effect in a nonlinear modulation regime. Here, the dissipation of an acoustic signal suppresses the higher harmonic of the dynamic optical transfer function. An analytical model of the dynamic optical transfer function for a low level of acoustooptic coupling is given. It is shown that when the pump beam aperture is much greater than the spatial size of an acoustic signal, the time response of the acoustooptic modulator response is identical within a phase factor to the amplitude profile of a pump beam apodized by the exponential dependence of the amplitude of a damped sound wave. Otherwise, the sound damping produces almost no distortions of a plane top of the acoustooptic modulator response to a pulsed signal and shows itself only under phase mismatch conditions. Here, the asymmetry of overshoots at the edges of the acoustooptic modulator response is observed. Calculated plots are presented, which illustrate the aforementioned specific features of the transient process under conditions of acoustic damping in the acoustooptic interaction in paratellurite.     

Two-coordinate diffusion of optical beams by an acoustooptic Raman-Nath modulator based on a paratellurite crystal

Acoustooptic Raman-Nath diffraction by a standing acoustic wave in a paratellurite crystal is investigated. An acoustic line is made in the form of a polished cube and serves as a high-Q acoustic resonator. A slow shear wave is excited by a single piezoelectric transducer. Multiple lossless sound reflections lead to two-coordinate light diffusion. When the acoustic intensity introduced into the crystal is about 2 W/cm² at a sound frequency of 7 MHz, there appears a diffraction pattern in the form of a homogeneous light spot with a solid angle of about 0.5 sr. An explanation for the features of the acoustooptic interaction is given. It is shown that this type of diffraction is helpful in designing acoustooptic two-coordinate diffusers of light beams.     

Conditions for the suppression of spatial harmonics at a two-frequency acoustooptic interaction in the Raman-Nath mode

It is demonstrated that the diffraction of a plane light wave by an acoustic wave of fundamental frequency in the Raman-Nath mode with admixture of the second acoustic harmonic may be accompanied by a considerable change in the higher diffraction orders with numbers ±2, ±3, etc. Conditions for the selective suppression of the orders are obtained in the weak and strong interaction modes. Results of a numerical simulation are presented.     

Calculation of the minimum scattering field of an arbitrary antenna by its directivity pattern

A rigorous statistically closed expression has been obtained to calculate the elements of the polarization scattering matrix S _p which describes the quadratically minimum scattering field of an arbitrary passive matched antenna. The elements S _p are analytically expressed only through the complex vector pattern of the antenna.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 8, pp. 1026–1035, August, 1996.     

Anisotropic acoustooptic modulator of nonpolarized light based on diffraction by a slow acoustic wave in a paratellurite crystal

The problem of anisotropic Bragg diffraction of nonpolarized light by a slow acoustic wave in a TeO₂ crystal is solved. Two independent acoustic waves are excited in the crystal. Nonpolarized light splits in the crystal into two orthogonally polarized eigenmodes, either diffracting by its associated acoustic beam. Conditions under which the angles of incidence and diffraction are the same for both diffraction processes are found. Depending on the acoustic frequency, the diffracted light at the exit from the crystal may be represented either by a single nonpolarized beam or by two orthogonally polarized beams with different directions and orthogonal polarizations. This may provide a high diffraction efficiency (up to 100%) for nonpolarized light in a TeO₂ crystal. Theoretical calculations are supported by experiments. Modulators capable of controlling a high-power laser operating at a wavelength of 1.06 μm are fabricated.     

Radiation directivity pattern of a semiinfinite planar dielectric waveguide with a multilayer transformer

Mowsow State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 33, No. 2, pp. 222–225, February, 1990.     

The directivity pattern of an interferometer for measuring thermal acoustic radiation

The directivity patterns of a pair of piezoelectric transducers for measuring the spatial correlation function of sound pressures produced by sources of thermal acoustic radiation in the megahertz frequency range are calculated. Sources in the form of a heated plane or strip are considered. The signal detection by two circular or rectangular piezoelectric transducers and by focusing transducers is studied. It is demonstrated that, for measuring the correlation function, the piezoelectric transducers must partially overlap. To determine the directivity pattern with a strong dependence on the distance between the heated object and the pair of piezoelectric transducers, focusing piezoelectric transducers should be used. The results obtained offer possibilities for a noninvasive measurement of the absorption coefficient of a medium and also for the realization of the previously proposed [20] passive acoustic thermotomograph, which does not use a priori information on the absorption coefficient of the medium.     

Reduction of the duration of pulses emitted by a thin-walled piezoceramic cylinder with the use of a correcting electric circuit

On the basis of an electric circuit’s analog of a thin-walled cylindrical piezoceramic transducer connected with an electric circuit, the shapes of the pulses emitted into water are calculated. The optimal circuit’s parameters that provide the minimal pulse’s duration are calculated for different values of the cylinder height and relative wall’s thickness. The possibility to considerably reduce the emitted pulse’s duration is demonstrated, and specific results of numerical calculations are presented.     

Nonstationary radiation mode of a thin-walled piezoceramic sphere with a correcting electric circuit

A transducer in the form of a water-loaded thin-walled piezoceramic sphere is considered. On the electric side, the transducer is connected to an RL circuit. The pulsed radiation mode of the transducer is studied using the theory of equivalent schemes of piezoelectric transducers and the spectral Fourier transform. For the system under study, the parameters that provide the minimal emitted signal duration are determined for different values of the relative wall thickness of the sphere. The durations and amplitudes of acoustic pulses emitted by the transducer in the presence and absence of the electric circuit are compared.     

The application of an electro-optical frequency modulator with a rotating electric field in optical Doppler velocimeters

The possibility of using an electro-optical frequency modulator with a rotating electric field (MREF) in LDV is discussed. Prototype systems measuring one component of the velocity vector are described. Photographs of the experimental signals obtained are given.     

Improving the security of multiparty quantum secret sharing against an attack with a fake signal

We come up with a special attack strategy to the multiparty quantum secret sharing protocol [Z.J. Zhang, Y. Li, Z.X. Man, Phys. Rev. A 71 (2005) 044301]. Using fake signal and Bell measurement, the agent Bob who generates the initial signals can elicit Alice's secret message. With little modification, our strategy also works for an improved version of this protocol. At last, a further improvement of the multiparty quantum secret sharing protocol is proposed.     

Optical-to-acoustic energy conversion efficiency upon interaction of pulsed laser radiation with a liquid: II. Conversion efficiency measurement by holographic interferometry upon acoustooptic interaction

The efficiency of conversion of pulsed CO₂ laser energy to acoustic energy is measured with holographic interferometry. The method suggested is simple and does not require that fringe shifts in the interferogram be converted to local pressure values in the acoustic wave. The efficiency measured for the thermal mechanism of energy conversion is in good agreement with analytical calculations [1] where the temperature dependence of the volume thermal expansion coefficient of water is taken into account.     

Optimization of the transfer function of an acoustooptic cell with an apodized piezoelectric transducer

It is shown that an apodized piezoelectric transducer can significantly reduce the side lobe level of the acoustooptic cell transfer function. Series and symmetric connections of the transducer sections and measures to suppress the effect of spurious elements arising in the electric circuit are proposed. In particular, the effect of spurious capacitances and inductances on the frequency response of the transducer is studied. It is shown that they violate the optimal condition for the suppression of the transfer function side lobes, especially at high frequencies. It is shown by calculations that the effect of spurious elements can be eliminated by the insertion of additional capacitors at 80 and 150 MHz.     

Amplitude-frequency response of an acoustooptic deflector with an apodized surface piezoelectric transducer

For a deflector with a surface multielement transducer having a variable length of acoustooptic interaction, amplitude-frequency responses taken at different angles of incidence of light on an ultrasonic column excited by the transducer are analyzed theoretically and experimentally.