Phase-tuned acousto-optic spatial filter.

An acousto-optic (AO) tunable filter with a phase-controlled dual-section piezoelectric transducer is designed and created for laser beam shaping (LBS). Owing to the acoustic beam steering effect, we experimentally observe splitting of the two-dimensional transfer function. As a result, we demonstrate generation of tunable bottle laser beams and dual-ring intensity distributions for the diffracted beam.

Independent multicolored bottle-beam generation using acousto-optic spatial shaping of a femtosecond laser beam.

We report on the development of a tunable spectral and spatial frequency shaping system for ultrashort laser pulses using acousto-optic filters. The system enables the creation of arbitrary axially symmetric multi-wavelength field configurations in the Ti:sapphire laser emission range near 800 nm and controlling them at a multi-kilohertz rate. We experimentally demonstrate independent generation of two-colored annular intensity distributions from a single femtosecond laser beam and a bottle beam having the hollow cylindrical volume with the aspect ratio of 9:1. This laser beam shaping system can be useful in creating advanced setups for an optical control of cold atoms.

Analysis of Acousto-Optic Figure of Merit in KGW and KYW Crystals.

Monoclinic potassium rare-earth crystals are known as efficient materials for solid-state lasers and acousto-optic modulators. A number of specific configurations for acousto-optic devices based on those crystals have recently been proposed, but the acousto-optic effect of those crystals has only been analyzed fragmentarily for some interaction directions. In this work, we numerically searched for the global maxima of an acousto-optic figure of merit for isotropic diffraction in KGd(WO4)2 and KY(WO4)2 crystals. It was demonstrated that the global maxima of the acousto-optic figure of merit in those crystals occur in the slow optical mode propagating along the crystal's twofold symmetry axis and in the acoustic wave propagating orthogonally, both for quasi-longitudinal and quasi-shear acoustic modes. The proposed calculation method can be readily used for the optimization of the acousto-optic interaction geometry in crystals with arbitrary symmetry.

Quasi-Collinear AOTF Spectral Transmission Under Temperature Gradients Aroused by Ultrasound Power Absotption.

Quasi-collinear geometry is a special configuration of acousto-optic (AO) diffraction that provides an extremely large AO interaction length for achieving extremely high spectral resolution of AO tunable filters (AOTFs). Large AO interaction length also makes it possible to implement multifrequency diffraction in quasi-collinear AOTFs, which has found multiple applications in modern optoelectronics. The most widespread of them being ultrashort laser pulse shaping when the pulse shape and spectral composition is controlled by the spectral composition of the ultrasound pulse aroused in the AO crystal. The operation of quasi-collinear AOTFs is accompanied by the appearance of the temperature gradients in the AO device mainly due to the acoustic power absorption by the material. We experimentally assessed the influence of these gradients on the AOTF spectral characteristics. A theoretical model based on the Raman-Nath equations was proposed, which allows to consider the influence of ultrasound attenuation and temperature gradients on the AOTF transmission. This model is valid for transmission simulations both in single- and modulated-frequency AOTF operation modes. The study includes the effects of acoustic wave attenuation and AO phase matching shift caused by inhomogeneous crystal temperature along the optical beam path. The compensation strategy based on ultrasound frequency and magnitude adjustment is proposed for minimizing the effect of temperature gradients and acoustic field attenuation on AOTF spectral transmission for broadband operation in ultrashort laser pulse shaping.

Characterization and alignment of acousto-optic devices using digital tailored RF waveforms.

Acousto-optic (AO) devices are used in a variety of light control and processing applications in different environments, including laboratory conditions and the field environment, as well as on air- and space-borne platforms. Accurate alignment and calibration of AO deflectors and tunable filters is crucial for correct operation and to achieve specified performances of those devices. We describe a simple routine to test and align AO devices with a programmable RF waveform generator. Using specially tailored RF waveforms as test signals enables single-shot measurement of an AO device's diffraction efficiency as a function of the applied RF signal frequency and amplitude. MATLAB code is available to implement the testing procedure and application demonstrations for an AO deflector and two types of AO tunable filters.

High-efficiency KYW acousto-optic Q-switch for a Ho:YAG laser.

A new, to the best of our knowledge, type of acousto-optic Q-switch was developed using slow shear acoustic mode in potassium yttrium tungstate (KYW) crystal. Two Q-switch configurations were created: one for vertical and one for horizontal light polarization, both providing over 50% diffraction efficiency at a wavelength of 2.1 µm and an RF driving power below 8 W. The laser-induced damage threshold of the KYW crystal was found to equal 650 MW/cm2. Operation of a nanosecond periodically pulsed Ho:YAG laser emitting 15 mJ pulses at 2.1 µm with the KYW Q-switch is reported.

Noncritical acousto-optic Bragg phase matching: analysis of orthorhombic and monoclinic crystal systems.

Anisotropic acousto-optic diffraction in crystals is the fundamental phenomenon that is used to design acousto-optic tunable filters. Noncritical and quasicollinear phase-matching geometries of Bragg acousto-optic diffraction are compared for acoustic symmetry planes in four crystal systems (tetragonal, trigonal, orthorhombic, and monoclinic). The results for uniaxial crystals are reviewed and generalized for biaxial crystals. It is shown that cubic frequency dependence on the Bragg angle exists in two symmetry planes of orthorhombic crystals and conditionally exists in the symmetry plane of monoclinic crystals. It is also shown that there are two points in the symmetry plane of monoclinic crystals where noncritical phase matching takes place in quasicollinear diffraction geometry that can be used to design high-resolution tunable filters. Phase-matching configurations in α-iodic acid and potassium gadolinium tungstate crystals are analyzed.

Ring-shaped optical trap based on an acousto-optic tunable spatial filter.

We report on a novel, to the best of our knowledge, optical scheme of an annular optical trap based on an acousto-optic tunable spatial filter. Design of the optical trap is proposed and validated. Experimental demonstration with polystyrene microspheres includes controllable arrangement of freely floating particles into a circular pattern, aggregation, and disaggregation of the particles. Dynamical adjustment of the trapping field potential diameter is achieved by programmable frequency-swept controlling of the acousto-optic filter.

Optimization of piezotransducer dimensions for quasicollinear paratellurite AOTF.

The method of quasicollinear acousto-optic tunable filters (AOTFs) piezoelectric transducer dimensions optimization is presented. The AOTFs with large interaction length apply the bulk acoustic wave (BAW) reflection from the input optical facet. Optimization is based on spectral approach to simulation of the BAW field in anisotropic media and Raman-Nath equations numerically solved for the inhomogeneous acoustic field. It was found that variation of the transducer dimensions can minimize RF power consumption of the AOTF. Comparison of the optimized transducer dimensions with those commonly used in quasicollinear paratellurite AOTFs showed that it is possible to improve the AOTF energy efficiency in about 2 times. It was shown that acoustic field simulation results obtained for one AOTF geometry can be applied to the other geometries but for equivalent frequency providing the same ultrasound beam ray spectrum width. It was also shown that when choosing the quasicollinear AOTF with reflection geometry, one should not rely only on the AO figure of merit value, since the energy efficiency of such AOTF type will be determined by the product of the AO figure of merit and the AOTF efficiency, which takes into account the change in the acoustic beam width in the reflection process. The results aim at improving the design of AOTFs for ultrashort laser pulse shaping.

Optimization of noncollinear AOTF design for laser beam shaping.

Optimization of a wide-angle paratellurite acousto-optic tunable filter (AOTF) is performed for applications in laser beam shaping systems. The AOTF configuration with annular transfer function is analyzed. It is demonstrated that the optimal AOTF design for single-frequency operation as a narrow-band spatial frequency filter is obtained at acoustic propagation angle of 5.6° relative to the [110] axis. The optimal design for maximization of AOTF resolution in multifrequency laser beam shaping operation mode is obtained at acoustic propagation angle of 13.8°.

Linear Phase Design of Piezoelectric Transducers for Acousto-Optic Dispersion Delay Lines Using Differential Evolution for Matching Circuit Optimization.

A numerical method for providing linear phase response of bulk acoustic wave (BAW) piezoelectric transducers (PTs) of acousto-optic devices is developed. Our approach is based on the analytic approximation of the phase response of the PT with the matching electrical circuit and optimization with a differential evolution genetic algorithm. Simulations and experiments were performed for two typical BAW excitation schemes: direct and reflective. Variance of the group delay in the specified frequency range was reduced by an order of magnitude for reflective excitation of BAWs.

AOTF-based hyperspectral imaging phase microscopy.

Phase imaging microscopy with incoherent object illumination is convenient and affordable for biomedical research and clinics since it provides easy integration with a variety of bright-field optical microscopes. We report the design of a new hyperspectral imaging system based on a combination of a spatial light modulator (SLM) and an acousto-optic tunable filter (AOTF) for phase imaging microscopy. Contrast of phase-only objects originates from matched spectral and spatial filtering performed by the SLM and the AOTF located in Fourier-conjugate optical planes in the back-end of the optical system. The system is designed as an add-on to a standard optical microscope with incoherent diascopic sample illumination.

Novel protocol of hyperspectral data acquisition by means of an acousto-optical tunable filter with synthesized transmission function.

A new protocol of hyperspectral data acquisition with an acousto-optical tunable filter is proposed and tested experimentally. Correction for the illumination source spectrum and regular spectral sensitivity factors is embedded in the data acquisition routine. The protocol is based on the adaptive electronic setting of the filter transmission passband inversely proportional to the power spectrum of the light source. Spectral equalization of the light source spectrum in a hyperspectral imaging spectrometer is demonstrated with 3% standard deviation over the bandwidth 465-820 nm. The broadening of the point-spread function does not exceed 20% at fourfold bandwidth extension owing to a confocal configuration of the spectrometer.

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.

Compact acousto-optic imaging spectro-polarimeter for mineralogical investigations in the near infrared.

Spectral imaging in the near infrared is a promising method for mineralogy analysis, in particular well-suited for airless celestial objects or those with faint atmospheres. Additional information about structure and composition of minerals can be obtained using spectral polarimetry with high spatial resolution. We report design and performance of laboratory prototype for a compact near infrared acousto-optic imaging spectro-polarimeter, which may be implemented for remote or close-up analysis of planetary surfaces. The prototype features telecentric optics, apochromatic design over the bandwidth of 0.8-1.75 µm, and simultaneous imaging of two orthogonal linear polarizations of the same scene with a single FPA detector. When validating the scheme, reflectance spectra of several minerals were measured with the spectral resolution of 100 cm-1 (10 nm passband at 1 µm). When imaging samples, the spatial resolution of 0.6 mm at the target distance of one meter was reached. It corresponds to 100 by 100 diffraction-limited elements resolved at the focal plane array (FPA) for each of the two light polarizations. A similar prototype is also being designed for the spectral range from 1.7 to 3.5 µm. This type of the spectro-polarimeter is considered as a potential reconnaissance and analysis tool for future planetary or moon landers and rovers.

Hyperspectral imaging acousto-optic system with spatial filtering for optical phase visualization.

We report a method for phase visualization in the images of transparent specimens using analog image processing in incoherent light. The experimental technique is based on adaptive bandpass spatial filtering with an amplitude mask matched with an acousto-optic tunable filter in a telecentric optical system. We demonstrate the processing of microscopic images of unstained and stained histological sections of human thyroid tumor with improved contrast.

70 GHz arbitrary modulation of chirped laser pulses by means of acousto-optics.

We report the arbitrary modulation of chirped ultrashort laser pulses with a 70 GHz bandwidth. In the proof-of-principle experiment, modulation of Ti:sapphire laser emission was performed by the acousto-optic dispersive delay line, and the temporal pulse shape was measured with a picosecond streak camera.

Anisotropic diffraction of bulk acoustic wave beams in lithium niobate.

The formalism of planar diffraction tensor was applied to the analysis of anisotropy of bulk acoustic wave diffraction and to build a full map of anisotropic diffractional coefficients for three bulk acoustic wave modes propagating in lithium niobate. For arbitrary propagation direction the diffractional coefficients derived allow estimation of ultrasonic beam divergence in far-field. Analysis of obtained data revealed that the maxima of acousto-optic figure of merit for anisotropic diffraction in the YZ plane correspond to moderate diffractional spreading of the beams exceeding isotropic diffraction 2-3 times.

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.