Acoustooptic nonpolar light controlling devices and polarization modulators based on paratellurite crystals

A complex of investigations and projects aimed at solving problems of highly effective acoustooptic control of nonpolarized laser radiation is generalized on the basis of anisotropic Bragg’s diffraction in a TeO₂ crystal from a slow acoustic wave, which is characterized by extraordinarily high acoustooptic quality. Modulators of the zeroth diffraction order, modulators of the first Bragg’s order, and a polarization-insensitive deflector are considered. The developed systems ensure the control of the polarization state of optical radiation. These systems are employed in setups and devices based on high-power solid-state and fiber lasers with a wavelength of 1.06 μm.     

Highly effective acoustooptic diffraction of light by multifrequency sound using a nonaxial deflector

The subject of investigation is multiple-beam Bragg diffraction of light, i.e., the formation of a highly effective multiple-beam field representing a superposition of equidistant monochromatic acoustic waves with determinate phases and amplitudes using a nonaxial TeO₂ deflector. It is shown theoretically and experimentally that the anisotropic character of interaction considerably changes the basic parameters of multiple-beam diffraction (compared with the isotropic case): the net diffraction efficiency grows to 100%, and the frequency band expands noticeably. A modulator splitting the initial laser radiation into five beams of equal intensity with a net efficiency approaching 100% is designed.     

Diffraction of pulsed laser radiation from an acoustic wave with frequency- and phase-shift keying

Bragg acoustooptic diffraction of nanosecond pulsed laser radiation from a frequency- and/or phase-keyed acoustic wave is studied experimentally and theoretically for exact synchronization of laser pulses and signal keying. It is shown that the diffraction field for short pulses is practically stationary and is determined by the positions of acoustic signal keying over signal aperture. The application of this type of signals for the formation of a multibeam diffraction pulsed radiation field is considered. A method is proposed for transforming the angular spectrum of laser radiation intensity from the initial Gaussian to a nearly rectangular spectrum. This may considerably increase the efficiency of high-power technological lasers used in material processing (laser cutting, welding, engraving, etc.), in which the action of radiation is of the thresh-old type in light intensity. The possibility of correcting the angular intensity distribution for a pulsed fiber laser, which increases the thermal efficiency of radiation from such a laser, is established experimentally.     

Effect of the spatial structure of an acoustic field on Bragg’s acoustooptic diffraction under strong acoustic anisotropy conditions

Bragg’s acoustooptic diffraction in an acoustically anisotropic medium is considered taking into account the two-dimensional spatial diffraction structure of the acoustic beam. The conditions are determined under which reverse transfer of optical power from the diffracted to the transmitted beam in the regime of 100% efficiency of diffraction is considerably suppressed. It is shown that this effect is due to diffraction bending of wave fronts of the acoustic beam in the acoustooptic diffraction plane. The problem of optimization of the piezoelectric transducer size and the spatial position of the input light beam is solved using the criterion of the minimal required power of the acoustic field. The results of simulation in a wide range of the acoustooptic interaction parameters for a Gaussian light beam are reported. The correctness of the model is confirmed experimentally. Recommendations for designers of acoustooptic devices are formulated.     

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.     

An acoustooptic method of measuring the noise equivalent power of optical heterodyne systems

The noise equivalent power of optical heterodyne detection at 10.6 m has been measured with a method based on Raman-Nath diffraction of a CO₂ laser beam. One of the frequency shifted first order diffracted beams is used as the signal radiation. The local oscillator radiation is obtained by splitting off a part of the laser beam incident upon the device used for the acoustooptic diffraction. The signal power can be varied over a large dynamic range by changing the acoustic input power. A study of the probable errors shows that the total error in the NEP measurement is less than 30%.     

Investigation into the acoustooptic properties of tellurium crystals by anisotropic diffraction of light

The optical, acoustic, and acoustooptic properties of tellurium crystals viewed as a candidate material for mid-and far-IR acoustooptic devices are considered. The phase velocities, polarization, and drift angles of the acoustic energy in different crystal planes are calculated. The acoustooptic figure of merit for tellurium under the anisotropic diffraction conditions is estimated, and light-sound interaction geometries promising for acoustooptic applications are discussed. Measuring data for the optical and acoustooptic parameters of tellurium crystals are given.     

Inverse acoustooptic problem: Coherent summing of optical beams into a single optical channel

A highly efficient summing of mutually coherent beams (channels) into a single beam with the same divergence and aperture (an inverse acoustooptic problem) is realized via diffraction in a Bragg cell. The multibeam field to be converged is formed as a result of the diffraction (splitting) of a single laser beam. Theoretical and experimental evidence is obtained for the fact that the repeated diffraction can provide a highly efficient (up to 100%) reconstruction of beam with initial parameters. The experiments are performed with a single-mode laser radiation at 0.63 μm and multimode radiation at 0.96 μm. The virtually attained summing efficiency is on the order of 70%. The factors that act to diminish the experimental efficiency below the predicted value, the ways to raise the efficiency, and possible applications of the results of this study are discussed.     

Acoustooptic multiplication of the frequency shift of optical radiation on the basis of bragg polarization splitting

A technique for the acoustooptic multiplication of the frequency shift of an optical beam is proposed. The technique is based on the cascade diffraction of the beam by a single acoustic wave with the use of Bragg polarization splitting in a uniaxial crystal. The fundamental possibility of the practical realization of the technique is confirmed experimentally by using anisotropic acoustooptic diffraction in LiNbO₃.     

Isotropic diffraction of a light beam by acoustic waves of fundamental frequency and harmonics

A set of equations describing acoustooptic diffraction of a light beam by acoustic waves of a fundamental frequency and its harmonics in an isotropic medium is obtained. The possibility of suppressing higher diffraction orders by adding the second or third harmonic to the fundamental monochromatic acoustic signal is theoretically justified. It is demonstrated that the maximum degree of suppression decreases with an increase in the light beam divergence. Results of simulation are presented for some particular cases of diffraction.     

Acoustooptic Bragg diffraction without overemodulation with a phased-array transducer

Acoustooptic Bragg diffraction without overemodulation (i.e., with the extended high-efficiency region on the diffraction efficiency dependence vs. sound amplitude) is studied theoretically and experimentally. It is found that this effect appears in the case of a symmetrically nonuniform acoustic field and is due to the equality of additional opposite phase shifts of light beams passing through symmetric regions of the acoustic field. The situation is considered when an acoustic field is excited by a three-section phased-array transducer. The conditions are determined, in which reverse optical power transfer from the diffracted beam to transmitted beam (overemodulation) in the case of a high (close to 100%) diffraction efficiency is considerably suppressed. In the case of a phased array, the effect weakly depends on the frequency of sound and the size of transducer sections, which makes it possible to observe it in a wide range of acoustooptic interaction parameters.     

Light modulation by acoustic signals for high diffraction efficiency

A theoretical discussion is presented on strong Bragg acoustooptic interaction AOI of light beams in the dynamic field of an acoustic signal. A system of integrodifferential equations is formulated to describe the evolution of the angular and frequency spectra of the beams in the AOI region for a high level of acoustooptic coupling. The third-order approximation in the perturbation method is used to obtain an analytic solution. Calculations are presented on the modulation of monochromatic beams by acoustic pulses having rectangular envelopes and propagating in a lithium niobate crystal, and the same for a signal having linear frequency modulation LFM in a paratellurite crystal, which demonstrate the broadening of the beam spectrum as the depth of the acoustooptic coupling increases, together with the occurrence of an asymmetry specific to strong AOI in the response of the light field to the symmetrical acoustic signal. Tomsk State University for Control Systems and Electronics, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 42, No. 1, pp. 99–106, January, 1999.     

New acoustooptic effect: Constant high diffraction efficiency in wide range of acoustic power

A new effect, viz., acoustooptic Bragg diffraction without the overmodulation mode, in which the efficiency of the Bragg order attains its maximal value (close to 100%) upon an increase in the intensity of an acoustic wave and then remains practically unchanged, is predicted theoretically and observed experimentally. The effect takes place in the case of considerable bending of phase fronts of the acoustic field in the acoustooptic diffraction plane and attains its maximal value at a relatively low frequency of sound, a small width of a piezoelectric transducer, strong acoustic anisotropy of the medium, and a large distance between the light beam and the piezoelectric transducer.     

Diffraction characteristics of acoustooptic interaction in an acoustic field with a curved wave front

A theoretical model is presented for the strong two-dimensional acoustooptic interaction (AOI) of finite-size beams with arbitrary profiles in an acoustic field with a curved wave front. Transfer functions are derived in a universal normalized form for the AOI. The results of numerical simulation of the diffraction characteristics show that during AOI in an acoustic field with a curved wave front the product of the diffraction efficiency and bandwidth increases at least 1.5 times for a weak interaction and more than five times for a strong interaction. The optimal values of the normalized parameters (of the acoustooptic coupling and the curvature), for which the efficiency and bandwidth are maximum, are determined.Tomsk State Academy of Control Systems and Electronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 41–50, April, 1995.     

Orientation phenomena in nematic liquid crystals under a periodic compressional deformation

The orientation behavior of homogeneous planar layers of nematic liquid crystals with open and closed ends in the field of compressional deformations caused by an acoustic effect is studied. The mechanisms determining the connection of the optical response of a nematic liquid crystal (the variable component of an optical signal and its spectrum) with the acoustic parameters (the oscillation amplitude and frequency, and the amplitude of sound pressure) and the layer thickness are revealed. The factors responsible for the mechanism and modes of acoustooptic conversion are considered. It is demonstrated that, by varying the layer thickness, it is possible to implement different modes of signal conversion. The possibility of designing a new modification of a sound receiver based on a nematic liquid crystal and the specific features of this design are discussed. Its advantages over conventional sound pressure receivers based on nematic liquid crystals are indicated, in particular, the absence of limitation of the frequency of the received signal in the low-frequency range.     

Optical reflectometric detection of a three-dimensional acoustic field

In laser ultrasonics, when focusing strongly the pump beam, a three-dimensional (3D) diffracted acoustic field can be generated. Considering weakly absorbing materials, the 3D interaction of this elastic perturbation with the optical detection beam must be taken into account. A semi-analytical model is proposed to describe such phenomena in reflectometric measurements. Once the acoustooptic interaction has been partially linearized, the resulting inhomogeneous differential system is solved using the matricant method. Good agreement is found comparing the calculated results with the experimental data in an aluminum film.     

Optical-gyroscope application of efficient crossed-channel acoustooptic devices

An acoustooptic (AO) deflector/modulator using a single-mode crossed-channel waveguide of 2n type and an interdigital transducer is fabricated in aY-Z LiNbO₃ substrate. This module has shown a high diffraction efficiency. A 50% diffraction efficiency and a bandwidth of 13.4 MHz were obtained with 0.13 W of surface-acoustic wave power centered at 320MHz. Since the cross section of the channel waveguide and that of the optical fiber are comparable, the interfacting of the resultant acoustooptic devices with fiber optical systems would greatly simplified. In addition, the frequency-shifted optical beam can be conveniently used as a reference signal or local oscillator in heterodyne detection. Consequently, this cross-channel acoustooptic device should find a variety of applications in realizing an integrated-optics module with a 50–50% power slit, optical communication, and an optical fiber system. One of those applications, optical gyroscopes, is proposed by using this kind of crossed-channel AO device.     

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.     

TeO2 single-crystal acoustooptic 2×2 switch for light beams of different wavelengths

The principles of acoustooptic switching of two light beams of different wavelengths by means of an acoustooptic interaction with two acoustic waves are examined. A version in which the acoustic waves form parallelograms and a version with strictly orthogonal propagation are considered. The latter version is the more attractive one when a TeO₂ single crystal is used as the acoustooptic cell. Experiments on the switching of two-color Ar laser radiation with wavelengths λ₁=0.5145 and λ₂=0.488 μm are carried out and demonstrate a high switching efficiency. On the whole, the experiment confirms the basic conclusions of the theory. Zh. Tekh. Fiz. 67, 66–71 (February 1997)