Some personal recollections on the acousto-optics as an interesting field of physics and its applications will be shared with the participants in a short welcome address. The consecutive number of the present Spring School on AO and A, 14-th, indicates a long history of events (39 years) initiated in 1980 by the University of Gdańsk as the triennial international meeting of specialists of science and technology over the world, being continued till now.The fundamentals of the acousto-optics domain covering light (fotons) and ultrasonics (fonons) interactions originally come from works of L. N. Brillouin whose anniversaries the 130 years of his birthday and the 50 years of his death are being celebrated during the current year. The next principles were widely developed by many others in 1930’s after essential two independent papers by P. Debye and F. W. Sears (USA) and R. Lucas and P. Biquard (France), then fundamentals by C.V. Raman and N. S. Nagendra Nath, followed by many papers and applications after the II World War. The wide and rich literature of the subject of the last 100 years built up a branch of knowledge to which a piece of contribution of the AO and A Schools has its own place. The further development of the branch is prospective, too. Recently, such problems as very short time (pico- and femtoseconds) acousto-optic interactions, nonlinear processes in bio- and nano-structures and their applications in acousto-optic and optoelectronic instrumentation as well as many others are subjects of the current interest and promising topics for the future.
Optically isotropic media which are susceptible to acoustically induced birefringence can be used as acousto- optic polarization converters. A comparative study between fused silica and dense flint shows that at normal light incidence 52% of the light can be converted from linear to circular in the case of fused silica and only 20% in the case of dense flint. In each case the conversion appears at moderate sound amplitudes and at frequencies which are typical for the intermediate regime of diffraction. Applying oblique light incidence, most interesting effects can be obtained with fused silica at high sound frequencies which are typical for the Bragg regime of diffraction and in the neighborhood of the Bragg angle. The possibility is shown to use an AO cell fabricated of fused silica as a laser-beam splitter, converting a linearly polarized beam of light partially into a circularly polarized beam and a linearly polarized beam, the light intensity of the two beams being equal. In addition, it is seen that the temporal light intensity modulations which can be observed in the near field of the light diffracted under these specific conditions, can be understood from the polarization changes taking place.
Photoacoustic spectroscopy method has been applied to study nonradiative processes in the excited states of Ce3+ and Cr3+ ions in YAG crystals. A comparative analysis between photoacoustic and absorption spectra has been performed.
A theoretical model is presented for the study of the near field of light diffracted by ultrasound in an isotropic medium which becomes birefringent in the presence of an acoustic wave. Due to the interference with the sound wave, the diffracted light wave in its near field is seen to act as a quasi-monochromatic light wave which is partially polarized. The degree of coherence ins sen to coincide with the degree of polarization and can be controlled acoustically. Hence coherent laser light can be transformed into incoherent light by sending it through an isotropic material with acoustically induced birefringence. In the experimental part of the work, the non-diagonal elements of the diffracted light beam coherency matrix were measured and a good agreement with the theoretical model was obtained.
The model layered system of thin olive oil layers spread on the water surface was studied using a photoacoustic method. Dramatic changes in the signal phase and rather slight variations in the signal amplitude were observed in reference to the oil-water interface probing. Applying a principal relation of the depth profiling approach between the thermal diffusion length and thermal diffusivity the latter for a particular layer thickness 25 micrometers , was evaluated to be 1.22 by 10-3/s being a characteristic value for kind of oils examined.
A number of problems of light diffraction in a nonstationary acoustic field are considered in the paper with using the spectral method. Transient processes arising in the acoustooptic cell during an abrupt change of acoustic amplitude, frequency or phase are analyzed in detail. The effects of diffracted mode concurrence and modulation nonreciprocity which are typical for multifrequency diffraction in the strong acoustooptic interaction regime are studied both theoretically and experimentally.
In the ultrasonic light diffraction phenomena in an isotropic medium in the intermediate range (between Raman-Nath and Bragg) variations in polarization state of diffracted light have been predicted and experimentally observed. In the paper some experimental and numerical results for 0 order of light diffracted by ultrasonic wave of 50 MHz frequency are presented. A possibility of continuous control of the polarization state from linear to elliptical or circular against the Raman-Nath parameter has been confirmed in good agreement between theoretical and experimental values.
Isotropic media which become anisotropic in the presence of an acoustic wave are demonstrated to act as optical polarizors. In contrast to traditional polarizing plates or rotators, the final state of polarization of light passing through a crystal with ultrasonically induced anisotropy, can smoothly be controlled by varying the acoustic parameters. The main diffraction and polarization effects are visualized in the case of fused quartz and dense flint.
Polarization effects that arise during diffraction of an arbitrarily polarized light by an acoustic wave in an anisotropic medium are studies both theoretically and experimentally. Peculiarities of these effects in the cases of isotropic and anisotropic scattering of the light are considered in detail. Results of polarization state measurements performed with a paratellurite acousto-optic cell in the zeroth order of diffraction are presented as well.
Some conclusions resulting from the solution of the Raman-Nath system of difference- differential equations relating to the amplitude and phase of diffracted light by an ultrasonic wave for the Raman-Nath, the intermediate, and the Bragg regions are presented and exemplified by some numerical and experimental results.
Double ultrasonic beam acousto-optical modulators require high precision driving electric signals for stable control of frequency ratio and phase shift. The high precision digital synthesis method has been developed for the generation of high quality periodic signals. Maximum frequency range of synthesized waveforms is determined by the speed of available random access memory (RAM) and an available D/A converter. A computer-based digital function generator can generate any arbitrary type of signal in the frequency range determined by the speed of RAM and the D/A converter. Frequency stability is determined by the quartz clock of the D/A converter and is in the order of 1/107. Periodic waveforms can be synthesized, stored, and then played continuously by looping through the RAM buffer.
The proposal of application of acousto-optical methods for nondestructive evaluation in the layered system described by O. Leroy and J. M. Claeys and the reconstructing of an ultrasonic field method used by Y. Nishida et al., have been reviewed. A suggestion is made to adapt the Nishida et al.'s, method of measuring the intensities in given light diffraction orders in pairs: I+1 + I-1, I+1 - I-1, I+2 + I-2, I+2 - I-2, ... to evaluate Raman-Nath parameters and phase shift between UW components for the layered system similar to that described by O. Leroy and Claeys.
Polish achievements in acousto-optics are reviewed, including optical holographic interferometric methods for ultrasonic fields and vibrating transducer examination, ultrasonic light diffraction phenomena and their modulation problems, material examination of anisotropic liquids and solids, and construction of acousto-optical devices. Topics reflect research being conducted in university centers in Gdansk, Gliwice, Poznan, and Warsaw. The studies are described and illustrated by means of arbitrarily chosen examples. Over one hundred references are cited.
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