Acousto-optic Devices Research Articles

Дифракция света на ультразвуке в пространственно периодическом акустическом поле

AbstractCharacteristic features of anisotropic light diffraction in a spatially periodic acoustic field created by a sectioned piezoelectric transducer with antiphase excitation of adjacent sections are studied theoretically. The numerical calculation is carried out for a cell of a paratellurite crystal with a shear acoustic mode propagating at an angle of 3° to the (001) plane of the crystal. It is shown that this type of acoustooptic interaction has some interesting features that can be used when developing acoustooptic devices, in particular, modulators of nonpolarized light.

Laser-Doppler vibrometer microscope with variable heterodyne carrier

The generation of a heterodyne carrier frequency for coherent detection via offset-locking in an optical phase-locked loop was demonstrated in communication, spectroscopy and other fields. In state-of-the-art laser-Doppler vibrometers (LDV), common acoustooptic devices (Bragg cells) allow efficient heterodyning with a static frequency. Moving mechanical parts vibrate in many technical applications, e.g. moving cars, trains, robots, ect.. In such applications, a large laser-Doppler-shift generated by the movement impairs the measurement of the relevant vibrations. A variation of the frequency offset of the interfering laser beams can suppress the Doppler effect of high-speed movements with slow speed variations and adjusts the signal spectrum to the optimal value. Thus, the electronic bandwidth can be set to an optimal value within the modulation bandwidth of the relevant vibrations. However, Bragg cells are not well applicable to shift laser frequencies without affecting the interferometer adjustment. In this paper, we propose offset locking of two independent lasers with an optical phase-locked loop to exploit the possibility of flexible heterodyne-carrier generation for laser-Doppler vibrometry. We demonstrate a LDV setup with offset locked visible DBR semiconductor lasers for a flexible heterodyning up to 200MHz and present first measurements.

Acousto-Optic Double Side-Band Amplitude Modulation of a Fiber Bragg Grating in a Four-Holes Suspended-Core Fiber

A dynamic multiwavelength reflection filter is acoustically induced by the reflectivity modulation of a fiber Bragg grating in a suspended-core fiber. The fiber composed by four air holes is combined with a specially designed acousto-optic modulator to increase the modulation efficiency and to reduce significantly the device size. The effect of the two strongest modulator modes on the grating is evaluated at 374 and 474 kHz. The high-frequency acoustic waves are amplitude modulated by a low-frequency signal from 100 to 500 Hz, generating resonant wavelengths peaks in the grating spectrum. The modulated grating reflectivity is experimentally characterized in the spectral domain. The modulation depth, bandwidth, and number of the induced resonances are tuned by the voltage and frequency of an electrical signal. Up to 15 wavelength peaks with a spectral bandwidth of ∼5 pm and a modulation depth up to 74% are generated, indicating new possibilities for compact and fast all-fiber acousto-optic devices.

Acoustically tunable photonic band gap generation in MgO doped Lithium Niobate micro-dimension plate

Refractive index contrast generation in MgO doped LiNbO3 plate via Lamb wave perturbation for induction of acoustically tunable photonic band gap is theoretically studied. Fundamental symmetric Lamb mode is excited in micro dimension thin plate structure to produce symmetric disturbance for periodic contraction and rarefaction along the length of plate. Effect of contraction and rarefaction on production of refractive index contrast is studied using photo–elastic effect. The generated quasi static low–high optical index region in plate modifies electromagnetic wave propagation and provides photonic band gaps, which are investigated using Bloch's theorem and transfer matrix method. The induced photonic band gap shows great dependency on optical contrast and unit cell dimension created by acoustic frequencies. These acoustically tunable photonic band gaps have rewarding applications in acousto optical devices, laser and sophisticated sensing.

Examination of the temperature influence on phase matching frequency in tunable acousto-optic filters

The temperature effect on the acousto-optic (AO) phase matching condition was examined both theoretically and experimentally on an example of wide-angle acousto-optic filter fabricated from tellurium dioxide crystal. It was shown that the AO crystal temperature variation changes the acoustic wave velocity that is involved into the AO interaction and shifts the phase matching frequency of AO diffraction. The AO phase matching frequency shift temperature coefficient was introduced, characterizing the magnitude of the frequency shift. The examination of frequency shift magnitude was carried for the optical wavelength band from 440 nm to 1.52 µm. It was shown that the temperature coefficient decreases with increasing the optical wavelength. A method was introduced that makes it possible to calculate the temperature shift coefficients for the real AO devices in a wide range of optical wavelengths. The coefficients obtained with the proposed method are in good agreement with the experimental data. Ultrasound attenuation was also examined in the given AO cell. Attenuation caused by the acoustic power absorption is the main mechanism leading to the appearance of the inhomogeneous temperature distribution inside the AO cell during the operation.

Peculiarities of acousto-optic interaction in biaxial crystal of alpha-iodic acid.

With the example of an alpha-iodic acid crystal, we demonstrate the unusual peculiarities of acousto-optic interaction in gyrotropic biaxial crystals. Basic attention is given to the most interesting cases of anisotropic diffraction in geometry, when the wave vector of ultrasound is directed almost orthogonally to one of the optical axes, and the directions of incident and diffracted light beams are close to the optical axis. It is shown that in this case a peculiar character of optical anisotropy originates unique variants of acousto-optic interaction that are fundamentally impossible in uniaxial crystals. A wide variety of frequency dependences of the Bragg angles allows choosing optimal configurations of crystal cuts for each specific acousto-optic device. The influence of the effect of optical activity on diffraction characteristics is examined as well.

Laser-ultrasonic temperature mapping of an acousto-optic dispersive delay line

Many acousto-optic paratellurite-based devices use a slow shear bulk wave for interaction with light beams. Strong bulk absorption of this wave leads to a significant inhomogeneous crystal heating, a drift of material constants, and to the consequent change in light diffraction patterns. Current paper is focused on a study of thermal regimes in acousto-optic devices. A method of laser ultrasonic time-of-flight thermography of solid-state devices is proposed. The method is used to experimentally evaluate temperature maps of an acousto-optic dispersive delay line with different acoustic pump frequencies. The comparison of obtained results with thermal imaging is carried out, showing good agreement within measurement error.

Randomizing phase to remove acousto-optic device wiggle errors for high-resolution optical tweezers

Acousto-optic (AO) devices have been used extensively in optical tweezers because of their flexibility and speed; however, these devices have trap positioning inaccuracies that limit their usefulness, especially for high-resolution applications. We show that these inaccuracies are due to interference patterns within the AO device sound fields. We have devised a method that removes these inaccuracies by reducing the coherence of the sound fields by directly controlling and randomizing the phase of the radio frequency voltage input signal. We demonstrate that the trapping inaccuracies are eliminated, for both constant trap position and force-ramp measurements, and that no additional noise is added. We show that this random phase method is applicable to both acousto-optic modulator and deflector type devices and can be easily integrated via software upgrade into existing instruments.

Enhanced photoelastic modulation in silica phononic crystal cavities

The enhanced photoelastic modulation in quasi-one-dimensional (1D) phononic crystal (PnC) cavities made of fused silica is experimentally demonstrated. A confined acoustic wave in the cavity can induce a large birefringence through the photoelastic effect and enable larger optical modulation amplitude at the same acoustic power. We observe a phase retardation of ∼26 mrad of light passing through the cavity when the exciting acoustic frequency is tuned to the cavity mode resonance of ∼500 kHz at 2.5 V. In the present experiment, a 16-fold enhancement of retardation in the PnC cavity is demonstrated compared with that in a bar-shaped silica structure. Spatially resolved optical retardation measurement reveals that the large retardation is realized only around the cavity reflecting the localized nature of the acoustic cavity mode. The enhanced interactions between acoustic waves and light can be utilized to improve the performance of acousto-optic devices such as photoelastic modulators.

No More “Wiggles”: Decoherent Acousto Optic-Based High-Resolution Tweezers Combined with Multi-Color Fluorescence

Recent advances in hybrid single molecule methods that couple angstrom-resolution optical trapping with single molecule fluorescence (high-resolution fleezers) have demonstrated the utility of measuring protein activity and conformation simultaneously e.g., revealing detailed helicase conformation coupled to DNA unwinding. Here we present advancements in multi-color high-resolution fleezers methods. First, we have developed a high-resolution fleezers instrument with three-color confocal fluorescence microscopy. To reduce photobleaching caused by the optical traps, timeshared dual optical traps were interlaced and synchronized with three fiber coupled fluorescence excitation lasers (473 nm, 532 nm, and 633 nm) and three single-photon counting fluorescence detectors. We report performance with a set of fluorophores varying both optical trap and fluorescence excitation intensity. In the fleezers, the Cy5 fluorophore dramatically outperforms other commonly used fluorophores. Second, we have devised a method that completely removes acousto-optic device trap positioning errors. Acousto-optic devices have been applied extensively in optical tweezers because of their flexibility and speed, however these devices have well-known trap positioning inaccuracies (‘wiggles’) that limit their usefulness, particularly for high-resolution applications. We show that these inaccuracies are due to interference patterns within the device sound fields. We have devised a method that removes these inaccuracies by reducing the coherence of the sound fields by directly controlling and randomizing the phase of the radio frequency control. We demonstrate that the trapping inaccuracies are completely eliminated, and that no additional measurement noise is added, for both constant trap position and force-ramp measurements. We show that this random phase method is applicable both to acousto-optic modulator and deflector type devices and can be easily integrated via a software upgrade into existing instruments.

Plasmonic enhancement of mid- and far-infrared acousto-optic interaction [Invited

We present a concept of an acousto-optical device with plasmonic efficiency enhancement at a 10.6 μm wavelength. Interaction of light, Rayleigh surface acoustic wave, and surface plasmon polariton excited via prism coupling using Otto geometry enhances the diffraction efficiency. In the case of Otto geometry, the surface acoustic wave affects both the refraction index of the optical medium and the air gap thickness between the prism and the metal. Dependencies of the reflective coefficient of the prism-air-metal system on gap thickness and dielectric permittivity modulation were analyzed. The analytical results were confirmed by experimental measurements of the angular spectrum of the reflected beam.

Acousto-optic control of internal acoustic reflection in tellurium dioxide crystal in case of strong elastic energy walkoff [Invited

Peculiar cases of acoustic wave propagation and reflection may be observed in strongly anisotropic acousto-optical crystals. A tellurium dioxide crystal serves as a prime example of such media, since it possesses record indexes of acoustic anisotropy. We studied one of the unusual scenarios of acoustic incidence and reflection from a free crystal-vacuum boundary in paratellurite. The directions of the acoustic waves in the (001) plane of the crystal were determined, and their basic characteristics were calculated. The carried-out acousto-optic experiment at the wavelength of light 532 nm and the acoustic frequency 73 MHz confirmed the theoretical predictions. The effects examined in the paper include the acoustic wave propagation with the record walkoff angle 74°. We also observed the incidence of the wave on the boundary at the angle exceeding 90°. Finally, we registered the close-to-back reflection of acoustic energy following the incidence. One of the stunning aspects is the distribution of energy between the incident and the back-reflected wave. The unusual features of the acoustic wave reflections pointed out in the paper are valuable for their possible applications in acousto-optic devices.

Wavelength characterization of an acousto-optic notch filter for unpolarized near-infrared light.

A cascaded system of two acousto-optical cells is proposed and experimentally demonstrated for optical notch rejection filtering in the spectral range from 1400 nm to 1600 nm. Two similar paratellurite acousto-optical devices for unpolarized light are used in a free-space gap of a fiber line. Compensation for birefringence in anisotropic paratellurite devices provides a diffraction regime that is insensitive to polarization of light. High extinction ratios up to 40 dB are measured.

Acousto-Optic Control of the Energy Profile of Laser Radiation

An acousto-optic shaper is developed for transformation of the Gaussian beam profile into almost rectangular profile. As distinct from lens devices, an acousto-optic device allows modification of the shape of the output beam in a fixed optical configuration using changes in radio control with relatively short response times of about 10 μs. The method is based on multibeam high-efficiency Bragg diffraction when the diffraction orders are overlapped with respect to angle. It is shown that the beam profile can be transformed into rectangular and more complicated profiles with an efficiency of no less than 80%. The experiments are performed using the TeO2 crystal.

DYNAMIC SPECTRAL FILTERING OF SIGNALS IN OPTOELECTRONIC SYSTEMS OF TARGET DETECTION

In the paper, principles of the dynamic spectral filtering, which is a further development of the imaging spectroscopy, are generalized and developed. In contrast to the imagining spectrometers, the optoelectronic systems with dynamic spectral filtering implement a controllable spectral filtering which provides maximum suppression of spectral components of background radiation with a minimum attenuation of the optical signal from a target in the pre-detector region. The study is aimed at generalization and development of the principles of the dynamic spectral filtering in behalf of creation of optoelectronic systems of target detection by spectral features. It proves possible to reduce the problem of optimum extraction of optical signals against a noise background to solving the detection problem for signals with a priori known parameters used in the radar theory. The paper presents the principles of dynamic spectral filtering of optical emission. Also, algorithms of optimal detection of targets using spectral features and synthesis of optimal detector of optical signals are described. In the paper, quantitative characteristics of the detector are determined that make it possible to estimate the conditional probability of true detection of objects of interest in dependence on the signal-to-noise ratio with a constant false-alarm rate. The quantitative characteristics of the detector have been obtained on the assumption that target echoes and noise background follow the normal distribution law, and their correlation characteristics (correlation matrices of the noise background ant target echo) are identical. The possibility has been established experimentally of creation of a controllable acousto-optical selective device that provides spectral selection of two optical signals with different spectral compositions by means of changing the frequency component amplitudes of an ultrasonic wave. Three lasers operating in the red, green and blue spectral ranges are served as radiation sources. In the course of the experiment, the spectral selection was performed due to diffraction of a polychromatic laser emission by a multi-frequency ultrasound. The filtering process efficiency was determined from the value of picture contrast before and after applying the filtering procedure. It has been shown experimentally that using three spectral channels of selection it proves possible to enhance the contrast at the output by several times.

Simultaneous Guidance of Surface Acoustic and Surface Optical Waves in Phoxonic Crystal Slabs

Phoxonic crystals, which exhibit simultaneous phononic and photonic bandgaps, are promising artificial materials for optomechanical and acousto-optical devices. In this paper, simultaneous guidance of surface acoustic and surface optical waves in truncated phoxonic crystal slabs with veins is investigated using the finite element method. The phoxonic crystal slabs with veins can show dual large bandgaps of phononic and photonic even/odd modes. Based on the phononic and photonic bandgaps, simultaneous surface acoustic and optical modes can be realized by changing the surface geometrical configurations. Both acoustic and optical energies can be highly confined in the surface region. The effect of the surface structures on the dispersion relations of surface modes is discussed; by adjusting the surface geometrical parameters, dual single guided modes and/or slow acoustic and optical waves with small group velocity dispersions can be achieved. The group velocities are about 40 and 10 times smaller than the transverse velocity of the elastic waves in silicon and the speed of light in vacuum, respectively.

Electronic frequency tuning of the acousto-optic mode-locking device of a laser

The effect of the electronic tuning of the acoustic resonances in an acousto-optic mode-locking device of a laser is investigated theoretically and experimentally. The problem of the excitation of a Fabry–Perot acoustic resonator by a plate-like piezoelectric transducer (PET) is solved in the approximation of plane acoustic waves taking into consideration the actual parameters of an RF generator and the elements for matching the PET to the generator. Resonances are tuned by changing the matching inductance that was connected in parallel to the transducer of the acousto-optic cell. The cell used in the experiment was manufactured from fused silica and included a lithium niobate PET. Changes in the matching inductance in the range of 0.025 to 0.2 μH provided the acoustic-resonance frequency tuning by 0.19 MHz, which exceeds the acoustic- resonance half-width.

Influence of target-substrate distance during pulsed laser deposition on properties of LiNbO3 thin films

This paper shows the results of studying the influence of target-substrate distance during pulsed laser deposition on electrical properties of LiNbO3 thin films. It has been shown that changing the target-substrate distance we can obtain thin films with a different composition. EDX spectra indicate that the LiNbO3 thin films fabricated under 120 mm target-substrate distance depleted Nb (0.7 atm. %) compare to LiNbO3 films fabricated under 20 mm target-substrate distance (11.0 atm. %). Varying target-substrate distance in the range from 20 mm to 120 mm charge carrier mobility increasing from 24 cm2/V·s to 395 cm2/V·s and concentration of charge carrier decreasing from 3·1013 cm-3 to 1·1012 cm-3. Obtained results can be used under development and fabrication of integral acousto-optic and surface acoustic wave (SAW) devices.

Coupled polaritonic band gaps in the anisotropic piezoelectric superlattices

Anisotropic piezoelectric superlattices (APSs) with the periodic arrangement of polarized anisotropic piezoelectric domains in a certain direction are presented, in which the coupled polaritonic band gaps (CPBGs) can be obtained in the whole Brillouin Zone and the maximum relative bandwidth (band-gap sizes divided by their midgap frequencies) of 5.1% can be achieved. The general characteristics of the APSs are similar to those of the phononic crystals composed of two types of materials, with the main difference being the formation mechanism of the CPBGs, which originate from the couplings between lattice vibrations along two different directions and electromagnetic waves rather than from the periodical modulation of density and elastic constants. In addition, there are no lattice mismatches because the APSs are made of the same material. Thus, the APSs can also be extended to the construction of novel acousto-optic devices.

音響光学素子を用いたホログラフィック3次元動画像表示技術の現状と将来展望 : 立体画像の入力・処理・表示関連

音響光学素子を用いたホログラフィック3次元動画像表示技術の現状と将来展望 : 立体画像の入力・処理・表示関連