Acousto-optic Efficiency Research Articles

Tuning stability enhancement of all-fiber acousto-optic tunable filter based on multi-piezoelectric transducer

All-fiber acousto-optic tunable filters (AOTFs) have advantages of wide wavelength tuning range, fast tuning speed and low insertion loss. The resonance wavelength of the all-fiber AOTF can be tuned by adjusting the driving frequency of the piezoelectric transducer (PZT). In general, the driving voltage needs to be tuned in wide range in the wavelength tuning process, due to the existence of resonance frequency of the PZT. In this work, we propose and demonstrate a stability-enhanced all-fiber AOTF based on multi-PZT, which can reduce the dependence on the resonant frequency of a single PZT. In the experiment, the acousto-optic coupling efficiency can maintain larger than 80%, when the resonance wavelength of the AOTF is tuned from 1400 nm to 1600 nm, under the condition of a fixed driving voltage. The high tuning stability of the proposed multi-piezoelectric driving configuration can facilitate the practical applications of the all-fiber AOTF, such as the laser wavelength tuning, wavelength-division-multiplexing optical communication, fast optical spectroscopy, and so on.

Анизотропия упругих и фотоупругих свойств кристаллов CaМоО4

The anisotropy of the elastic and photoelastic properties of calcium molybdate crystals has been studied for Bragg diffraction of light by longitudinal acoustic waves propagating in the (001) plane. It is shown that the efficiency of the acousto-optic interaction can be characterizedby the introduced acousto-optic efficiency coefficient, which is determined from the elasticand photoelastic constants of the crystal. A strong dependence of the coefficients of the acousto-optic quality and the acousto-optic efficiency on the direction of propagation of acousticwaves in the investigated plane is revealed.

High-sensitivity acoustic impedance sensing based on forward Brillouin scattering in a highly nonlinear fiber.

By using radial acoustic modes induced forward Brillouin scattering (FBS) in a highly nonlinear fiber (HNLF), to the best of our knowledge we have demonstrated acoustic impedance sensing with the sensitivity reaching beyond 3MHz for the first time. Benefiting from the high acousto-optical coupling efficiency, both radial acoustic modes (R0,m) and torsional-radial acoustic modes (TR2,m) induced FBS in HNLF have larger gain coefficient and scattering efficiency than those in standard single-mode fiber (SSMF). This provides better signal-to-noise ratio (SNR) and hence larger measurement sensitivity. By using R0,20 mode in HNLF, we have achieved a higher sensitivity of 3.83 MHz/[kg/(s · mm2)], in contrast to that of 2.70 MHz/[kg/(s · mm2)] when measured using R0,9 mode (with almost the largest gain coefficient) in SSMF. Meanwhile, with the use of the TR2,5 mode in HNLF, the sensitivity is measured to be 0.24 MHz/[kg/(s · mm2)], which is still 1.5 times larger than that reported when using the same mode in SSMF. The improved sensitivity would make the detection of the external environment by FBS based sensors more accurate.

Effect of ellipticity of optical eigenwaves on the enhancement of efficiency of acousto-optic Bragg diffraction. A case of optically active Pb5Ge3O11 crystals

We show that the existence of optical activity in an optical material can lead to essential enhancement of acousto-optic (AO) figure of merit. The reason is that the ellipticity of interacting optical eigenwaves approaches unity near the optical axis and so additional elasto-optic (EO) tensor components become involved in the effective EO coefficient. We demonstrate on lead germanate crystals that the increase in the efficiency of AO diffraction manifests itself for all types of AO interactions whenever the incident optical wave propagates close to the optical axis. In the particular case of diffraction in the interaction plane XZ of Pb5Ge3O11 crystals, the maximal enhancement of the AO figure of merit takes place under conditions of the isotropic diffraction of ordinary and extraordinary optical waves with a pure transverse acoustic wave (AW) polarized parallel to the Y axis (a so-called AW PT2), with the AO figure of merit increasing from zero to 7.5 × 10−17 s3/kg, and the types of anisotropic diffraction on the AWs QT1 and PT2 when the AO figure of merit increases twice (e.g., from 7.6 × 10−17 to 1.5 × 10−16 s3/kg). The maximal AO efficiency in the interaction plane XZ is reached at types I and II of the isotropic AO interactions of ordinary and extraordinary optical waves with a quasi-longitudinal AW; in these cases, the AO figure of merit increases from 6.8 × 10−15 to 12.4 × 10−15 s3/kg and from 5.6 × 10−15 to 12.4 × 10−15 s3/kg, respectively.

Acousto-optic modulation of a wavelength-scale waveguide

Since the advent of the laser, acousto-optic modulators have been an important tool for controlling light. Recent advances in on-chip lithium niobate waveguide technology present new opportunities for these devices. We demonstrate a collinear acousto-optic modulator in a suspended film of lithium niobate employing a high-confinement, wavelength-scale waveguide. By strongly confining the optical and mechanical waves, this modulator improves a figure-of-merit that accounts for both acousto-optic and electro-mechanical efficiency by orders of magnitude. Our device demonstration marks a significant technological advance in acousto-optics that promises a novel class of compact and low-power frequency shifters, tunable filters, non-magnetic isolators, and beam deflectors.

Full set of piezo-optic and elasto-optic coefficients of Ca3TaGa3Si2O14 crystals at room temperature.

The photoelasticity of the Ca3TaGa3Si2O14 (CTGS) crystal was studied by an interferometric method based on a single-pass Mach-Zehnder interferometer. The maximum number of sample orientations for the piezo-optic experiments was applied to prove accuracy in the determination of the piezo-optic coefficients. Based on the matrices of the piezo-optic coefficients and the elastic stiffness coefficients, all the coefficients pik of the elastic-optic matrix are calculated. For the highest pik coefficient, the acousto-optic efficiency is evaluated. The results obtained for CTGS are compared with the corresponding results for La3Ga5SiO14 (langasite) crystals. The highest acousto-optic figure of merit of CTGS M2=1.66⋅10-15s3/kg is two and three times higher, compared with langasite and strontium borate, respectively, which are often used for acousto-optic modulation of light in the ultraviolet spectral range.

Direct Acoustic Imaging Using a Piezoelectric Organic Light-Emitting Diode

Conventional ultrasonic imaging requires acoustic scanning over a target object using a piezoelectric transducer array, followed by signal processing to reconstruct the image. Here, we report a novel ultrasonic imaging device that can optically display an acoustic signal on the surface of a piezoelectric transducer. By fabricating an organic light-emitting diode (OLED) on top of a piezoelectric crystal (lead zirconate titanate, PZT), an acousto-optical piezoelectric OLED (p-OLED) transducer is realized, converting an acoustic wave profile directly to an optical image. Due to the integrated device architecture, the resulting p-OLED features a high acousto-optic conversion efficiency at the resonant ultrasound frequency, providing a piezoelectric field to drive the OLED. By incorporating an electrode array in the p-OLED, we demonstrate a novel tomographic ultrasound imaging device that is operated without a need for conventional signal processing.

Characterization of photoelastic materials by combined Mach-Zehnder and conoscopic interferometry: Application to tetragonal lithium tetraborate crystals

Mach-Zehnder and conoscopic interferometry are used to explore photoelastic properties of anisotropic crystal materials. In a number of cases an application of both techniques significantly improves an accuracy of piezooptic and photoelastic measurements. The performance of such combined approach is demonstrated on tetragonal lithium tetraborate (LTB) single crystals, as an example. Special attention is paid to methodological and metrological aspects, such as measurement accuracy and the quantitative error analysis of the resulting measurements. Performing the interferometric measurements for different geometries of piezooptic coupling the full sets of piezooptic and photoelastic tensor constants of LTB crystals have been determined. The acoustooptic efficiency, on the other hand, has been evaluated using the magnitudes of photoelastic constants derived from the piezooptical measurements. For the geometries with strong photoelastic coupling LTB demonstrates quite large acoustooptic performance with figure of merit value, М2, achieving 2.12 × 10–15 s3/kg. It is several times larger than that of strontium borate crystals, nowadays the best acoustooptic material in deep-ultraviolet spectral region.

Photoelastic Properties of Potassium Sulfate Crystals

The spectral dependences of the absolute piezooptic coefficients of potassium sulfate crystals are studied. It is established that their dispersion dependence is insignificant. The matrix of elastooptic coefficients pin of these crystals is filled using the elastic stiffness coefficients and the absolute piezooptic coefficients. The coefficient of acoustooptical efficiency M2 is calculated. It is shown that, considering the maximum values, it is more than an order of magnitude higher than that of quartz and strontium borate crystals. Potassium sulfate crystals are proposed for use for acoustooptic modulation of ultraviolet radiation, taking into account the short-wavelength limit of their transparent region (~170 nm).

Фотоупругие свойства кристаллов сульфата калия

The spectral dependences of the absolute piezo-optic coefficients of potassium sulfate crystals are studied and their insignificant dispersion dependence is established. The matrix of elasto-optic coefficients рin of these crystals is filled using the values of elastic stiffness coefficients and absolute piezo-optic coefficients. The coefficient of the acousto-optic efficiency М2 is estimated and it is shown that, considering the highest values, it is more than an order of magnitude higher than that of quartz and strontium borate crystals. Taking into account the short-wavelength boundary of the transparency region of potassium sulfate crystals (~ 170 nm), it is proposed to use them for acousto-optic modulation of ultraviolet radiation.

High-efficiency acousto-optic coupling in phoxonic resonator based on silicon fishbone nanobeam cavity

We investigate the acousto-optic coupling rates between different acoustic resonance modes and a specified optical resonance mode in a one-dimensional phoxonic crystal fishbone nanobeam formed by periodically arranging semi-cylinders of air on both sides of a suspended silicon waveguide. The gradually tapered unit cells form optical and acoustic resonators. In acousto-optic coupling rate calculation, the acoustic fields and optical fields are obtained by steady state monochromatic analysis and eigen-mode computation, respectively. Results showed that the acoustic polarizations and symmetries of the acoustic resonance modes are dominant factors in the acousto-optic coupling efficiency, and appropriate selection of these parameters can prevent cancellation of acousto-optic interactions, thereby enhancing acousto-optic coupling rates. This study provides important insights that can be applied to acousto-optic device designs.

Piezooptic coefficients and acoustooptic efficiency of TGS crystals

We have determined ten piezooptic tensor components for monoclinic triglycine sulfate crystals, using an interferometric method. Acoustooptic figure of merit has been...

UV-induced acoustooptics of matrices containing BaHf(BO3)2 microcrystallites embedded into olygoetheracrylate photopolymer

UV-induced acoustooptics was explored for BaHf(BO3)2 microcrystallites with sizes varying within 5–30 μm range. The titled microcrystallites were embedded into the polymer matrices. The results were analyzed using both experimental optical as well as theoretical DFT approach. The measurements were done for principal acoustical frequencies: 0.5 MHz, 1 MHz, 2 MHz, and 3 MHz. The acoustical waves were excited by an electromechanical LiNbO3 piezoceramics transducers from acoustical generator. We explored dependence of the acoustooptical efficiency versus the photoinducing laser beam power, angle between the beams, acoustical power light polarization and temperature. We explored variation of the acoustooptical efficiency at 1150 nm probing cw He-Ne laser wavelength under influence of pulsed nitrogen laser at wavelength 371 nm working in the 7 ns regime with frequency repetition 6 Hz at power densities up to 900 MW/cm2. Existence of some optimal conditions at about 1 MHz and UV photoinduced power equal to about 0.8 MW/cm2 was found. The spatial acousooptical gratings formation was observed. The DFT simulations of the charge density in main structural fragments under photoinducing UV beams is presented and principal role of the BO3 structural fragments is established. This is confirmed by crystallochemistry analysis. The phenomenological description is also presented.

Anisotropy of an acousto-optic figure of merit for NaBi(MoO4)2 crystals.

We develop a technique for analyzing the anisotropy of an acousto-optic figure of merit for crystals belonging to tetragonal symmetry groups 4/m, 4, and 4¯. The technique is based on phenomenological relations that describe anisotropies of the effective elasto-optic coefficients and the acoustic wave velocities. Our approach is verified on the example of NaBi(MoO4)2 crystals for all possible types and geometries of acousto-optic interactions. We list and characterize the acousto-optic interaction geometries at which the maximal acousto-optic figures of merit are reached for both isotropic and anisotropic interaction types. Finally, we show that the acousto-optic efficiency of NaBi(MoO4)2 originates mainly from large elasto-optic coefficients.

An improved Mach-Zehnder acousto-optic modulator on a silicon-on-insulator platform

The interdigital transducer (IDT) of the traditional Mach-Zehnder (MZ) acousto-optic modulator on a silicon-on-insulator (SOI) platform is located outside its two arms. The crest and trough of the surface acoustic wave (SAW) are used to modulate the two arms of the MZ interferometer so as to achieve a high modulation efficiency. Therefore, the distance between the two arms must be odd multiples of half acoustic wavelength. However, since the substrate is usually not uniform, the wavelength of the SAW changes as it transmits through the surface of the device. As a result, the exact distance between the two arms is difficult to choose. On the other hand, the SAW losses a portion of energy after passing through the first arm of the MZ interferometer, so the modulation of the second arm becomes much weaker. To solve these problems, we propose a new structure where its IDT is situated in the middle of the two arms of the MZ interferometer. With this scheme, the two arms of the MZ interferometer are located exactly at the crest and the trough of the SAW, while they are modulated with equal strength. In this paper, we first use the finite element method to simulate the acoustic frequency and the surface displacement of the excited SAW. Then we deduce the refractive index variations of all layers according to their acousto-optic effects. After that, we analyze the influences of different factors on the acousto-optic modulation efficiency, including the type and size of waveguide, the thickness of zinc oxide (ZnO) layer, and the area it covers, the number of electrodes, etc. These parameters are accordingly optimized to enhance the modulation efficiency. Modeling result based on COMSOL Multiphysics indicates that when the width of the strip waveguide is 6 m, the ZnO layer covers only the area under the IDT and has a thickness of 2.2 m, and the number of the electrodes is 50, the effective refractive index variation of the waveguide reaches 4.0810-4 provided that the amplitude of the driving voltage is 1 V. This value is 12% higher than that of the traditional structure.

Acousto-optic modulation of a photonic crystal nanocavity with Lamb waves in microwave K band

Integrating nanoscale electromechanical transducers and nanophotonic devices potentially can enable acousto-optic devices to reach unprecedented high frequencies and modulation efficiency. Here, we demonstrate acousto-optic modulation of a photonic crystal nanocavity using Lamb waves with frequency up to 19 GHz, reaching the microwave K band. The devices are fabricated in suspended aluminum nitride membrane. Excitation of acoustic waves is achieved with interdigital transducers with period as small as 300 nm. Confining both acoustic wave and optical wave within the thickness of the membrane leads to improved acousto-optic modulation efficiency in these devices than that obtained in previous surface acoustic wave devices. Our system demonstrates a scalable optomechanical platform where strong acousto-optic coupling between cavity-confined photons and high frequency traveling phonons can be explored.

Elastic and photoelastic properties of KY(WO4)2 single crystals

The complete elastic modulus matrix of a KY(WO4)2 single crystal has been determined for the first time using sound velocities measured in different directions of the crystal by a pulse-echo method. We have studied the entire isotropic part of the elasto-optic modulus matrix of KY(WO4)2 and identified high acousto-optic coupling efficiency directions. Calculated sound velocities in the three main crystallographic planes demonstrate that there are directions with considerable group velocity dispersion.

Compact dip-style viscometer based on the acousto-optic effect in a long period fiber grating

An acousto-optic modulator (AOM) is used to tune the spectral properties and add a dynamic component to a long period fiber grating (LPG). Most of the physical parameters that an LPG is sensitive to, can be obtained through passive analysis of the spectral changes. Here, we show that by adding a temporal response component to the grating sensitivity, dynamic parameters such as fluid viscosity, can be extracted. By measuring the modulation signal rise time and/or the acousto-optic efficiency, we demonstrate a compact, dip-style, multi-parameter sensor, which is able to measure, among various parameters, the viscosity of a fluid in frequencies up to 700 Hz. Further, we show that taking the temporal response into consideration, the sensor has a linear response against viscosity, in contrast to average measurements based on spectral properties. The non-linear component of the average measurements arises from a relaxation of the initial acoustic perturbation on a timescale longer than the initial rise time.

Acousto-optic efficiency of two-dimensional photonic crystals

The Bragg regime of the acousto-optic (AO) interaction in two-dimensional (2D) photonic crystals (PhCs) is considered. Approximate formulas for the AO figures of merit of PhCs are obtained and their frequency dependences for 2D PhC of the Si-SiO2 system are calculated. It is shown that the figures of merit of a composite PhC can exceed the values of these parameters for the components.

Bragg light diffraction in nontransparent crystals

As is well known, in absolutely transparent crystals an acoustooptical interaction is lacking. In this connection's acoustooptical investigations are presented greatest interest in optical range, in which the crystal is not transparent. In order to carry out similar experiments, it is necessary to use the crystal with a small light absorption coefficient at applied wavelength. The main problem is selection of an appropriate buffer crystal that the light beam can penetrate into sample by Bragg angle at required frequency. At present work the acoustooptical properties of Si crystal were investigated at light wavelength 632.8 nm. Dy2S3 crystals were used as a buffer sample. It was detected the diffracted light intensity is much more powerful in comparison with that for LiNbO3 crystals. Acoustooptical efficiency M2 (it is defined the acoustic-optical quality of material) has been calculated from the values of optic coefficients and elastic constants for appropriate directions of sound and light. The results of executed investigations have shown the possibility of determination of acoustooptical properties of nontransparent crystals by Bragg diffraction method. At that rate, the very high intensity of diffracted light can be obtained in comparison with that in transparent crystals.