Vladimir Ya. Molchanov

Tunable acoustooptic filters and equalizers for WDM applications

We describe a high performance acoustooptic tunable filter for add-drop application and for signal equalization in WDM telecommunication crossconnects. It results from a thorough investigation in TeO/sub 2/ of bulk collinear interaction, the geometry of which, particularly the direction of propagation of the acoustic wave, has been chosen in order to obtain the best compromise between the spectral resolution of the device and the acoustooptic figure of merit. Less than 40 mW of electric power is needed either to deviate 100% of a selected light wavelength /spl lambda/ at resonance, or to induce a 30-dB attenuation of its intensity. The sidelobes practically vanish for this configuration and the resolution is equal to 0.75 nm (or 94 GHz) for /spl lambda/=1.55 /spl mu/m. Polarization splitters combined with half-wave plates allow to completely get rid of polarization sensitivity problems. The use of optical fibers to collect the signal at the filter outputs, actually contributes to the high performance of the device as a whole. Experiments have been performed by multiplexing three signals in the input fiber, separated by 4, 2, and 1 nm. The transmission of the filter has been examined through the bar and cross state.

Adaptive acousto-optic technique for femtosecond laser pulse shaping

We discuss the theoretical and experimental investigation of acousto-optic dispersive tunable filters, based on quasi-collinear geometry of light-sound interaction in a tellurium dioxide single crystal. The geometry uses the effect of strong acoustic anisotropy in the paratellurite as well as peculiarities of acoustic wave reflections at the free boundary of the crystal. A mathematical concept for determination of optical, electrical, and constructional parameters of the filters is developed. Different experimental acousto-optic filters intended for femtosecond pulse shaping are designed and tested. Preliminary experiments are performed in a subpetawatt optical parametric chirped pulse amplification laser system. The experimental data conform completely with the predicted data.

Acousto-optic collinear diffraction of a strongly divergent optical beam

Collinear diffraction of a strongly divergent optical beam by a monochromatic acoustic wave is investigated theoretically and experimentally. Three-dimensional patterns of acousto-optic (AO) phase matching are calculated and their transformation with varying acoustic frequency and ultrasound propagation direction is analysed. Two effects are discussed which appear at spectral collinear filtration of optical images, namely distortion of filtered image structure and spectral resolution deterioration. It is shown that the effects result in contradictory requirements on AO cell parameters. However, a compromise is possible which allows filtering of high-spatial-resolution images with an acceptable reduction in spectral resolution. Experimental results on diffraction of a strongly divergent He-Ne laser beam in a CaMoO4 AO cell are also presented in this paper.

Compensation of thermal effects in acousto-optic deflector

Results on investigation of thermal effects which appear during operation of an acousto-optic deflector are presented in the paper. Influence of the effects on characteristics of the scanning device is evaluated. For a case of the tellurium dioxide deflector, it is proved that the most principal thermal effect consists in an undesirable spatial shift of a scanning optical beam. The shift leads to the optical beam positioning error which in a typical deflector may be of dozens of resolvable spots. A method of the thermal shifts compensation based on acoustic frequency tuning is proposed. For various regimes of the deflector operation, laws of the frequency tuning are found thus providing the compensation of the shifts with the required precision.

Two-dimensional selection of optical spatial frequencies by acousto-optic methods

We theoretically and experimentally examine the selection of spatial frequencies of optical beams carried out by acousto-optic meth- ods. Control of the optical rays is provided in two orthogonal directions with the help of a single acousto-optic cell fabricated of a birefringent paratellurite single crystal. The investigation is performed using a cell of a tunable acousto-optic filter illuminated by coherent radiation of a He-Ne laser at optical wavelength l5633 nm. The filter is capable of a 2-D selection of optical beams propagating all over the range of spatial fre- quencies un xu5un yu5<300 cm 21 in the spectral passband Dn550 to 100 cm 21 . Optimal regimes of the acousto-optical interaction character- ized by high efficiency of the filtration are considered during the re- search. New results on processing of optical images formed by divergent optical rays are presented and regular trends of the processing are dis- cussed.

Influence of acoustic energy walk-off on acousto-optic diffraction characteristics

Influence of acoustic beam energy walk-off on characteristics of Bragg diffraction of light is studied theoretically and experimentally by the example of a paratellurite single crystal. Two cases of isotropic and anisotropic light scattering are examined. Angular and frequency characteristics of acousto-optic interaction are calculated in wide ranges of Bragg angles and ultrasound frequencies by means of modified Raman-Nath equations. It is shown that the walk-off can substantially change the width of angular and frequency ranges, resulting in their narrowing or broadening subject to position of the operating point in the Bragg angle frequency characteristic. Coefficients of broadening are introduced for characterization of this effect. It is established that frequency dependences of the broadening coefficients are similar to the Bragg angle frequency characteristics. Experimental verification of the calculations is carried out with a paratellurite cell of 10.5° crystal cut.

Advanced spectral processing of broadband light using acousto-optic devices with arbitrary transmission functions.

In the paper, we developed a dispersive method for transmission function synthesis of collinear and quasi-collinear acousto-optic tunable filters. General theoretical consideration was performed, and modelling was made for broadband and narrowband signals. Experimental results on spectral shaping of femtosecond laser emission were obtained. Binary spectral encoding of broadband emission was demonstrated.

Phenomenological method for broadband electrical matching of acousto-optical device piezotransducers

We use Masons equivalent circuit formalism for evaluation of the parameters of a piezoelectrical transducer. An influence of the inter- mediate bonding layer to the acoustical impedance matching of the transducer and acousto-optical medium is analyzed. The phenomeno- logical method of electrical circuit synthesis for broadband transducer matching using the Smith chart is discussed. The experimental results of the electrical matching of the shear wave LiNbO3 transducer bonded on an acousto-optical medium from TeO2 are presented.

Deflectors with a feedback: new possibilities for image processing

Some results on theoretical and experimental investigation of acousto-optic deflectors containing an optoelectronic feedback of a hybrid type are presented. Two modifications of these devices in which the feedback signal controls amplitude or frequency of acoustic waves are considered. A possibility of realization of high order multistable regimes is shown. Different applications of such deflectors for optical information processing are discussed.

Acousto-optical imaging spectropolarimetric devices: new opportunities and developments

We report design and prototype performance of an acousto-optical imaging spectropolarimeter aimed for the 2.5 m telescope at Caucasian Mountain Observatory of Lomonosov Moscow State University. Special geometry of the acousto-optical interaction provides two diffracted beams polarized along the slow and the fast axes of the crystal. The optical system of the spectrometer consisting of plane and elliptical mirrors delivers the images of the object with two orthogonal polarizations to a single CCD matrix, increasing the focal ratio of the telescope.