Acousto-optic Crystal Research Articles

Investigation of acousto-optic interaction with momentum mismatching considering acoustic anisotropy in birefringent crystal.

In this work, the momentum mismatching based on which the acousto-optic (AO) transfer function and diffraction efficiency was acquired, was calculated considering the properties of AO crystals in AO interactions in acousto-optic tunable filter (AOTF). Transfer functions were obtained using a 4f optical system combined with AOTF and compared with theoretical calculations. It demonstrated the influence of acoustic energy shift on the AO interaction which should be considered in the design of AOTF.

Annealing modification and defect analysis of Hg2Br2 crystal

With the urgent demand for wavelength extension of acousto-optic devices in military and civil fields, long wavelength infrared acousto-optic crystals with high performance have become indispensable. In this context, Hg2Br2 crystals with dimensions of Ф 50 × 40 mm3 were successfully obtained by a self-designed physical vapor transport furnace. The transmittance spectra reflect that the Hg2Br2 crystal has a wide spectral transmission range, and the maximum transmittance in the UV–vis IR range exceeds 80%. Annealing is proven to be an effective measure to optimize crystal properties, and the optimal annealing temperature of the Hg2Br2 crystal is defined. The crystal quality and transmittance of the Hg2Br2 crystal are improved, and the stress is reduced after annealing at the optimal temperature. In addition, the dislocation defects of the Hg2Br2 (110) and (001) wafers were characterized via the thermal etching method. The average defect density was assessed on the order of magnitude of ∼105 cm−2, which indicates the high quality of the as-grown Hg2Br2 crystal.

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.

Examination of an acoustic field longitudinal power distribution in quasicollinear acousto-optic cells

The quasicollinear geometry of acousto-optic (AO) diffraction is notable as makes it possible to achieve an extremely high AO interaction length and, consequently, an anomalously high spectral resolution for AO devices. This geometry is especially convenient for the implementation of multifrequency AO diffraction, which has found wide application for solving the problems related to the laser pulse shaping. Since acoustic beams propagate over long distances in quasicollinear AO devices, and optical radiation spectral components diffract in the acoustic field in different parts of the AO crystal, accurate calculation of the characteristics of such devices requires knowing the distribution of the acoustic field amplitude inside the AO cell. The acoustic beam structure is affected by several factors in quasicollinear AO cells: the dimensions of the piezoelectric transducer, the geometry of acoustic wave propagation in the AO cell, acoustic anisotropy and the acoustic energy absorption along the chosen direction in the crystalline material used. In this paper, we propose a generic method to measure the acoustic beam power distribution along the direction of its propagation in the quasicollinear AO cell in the presence of ultrasound power absorption and media acoustic absorption. The measurements were carried out for the ultrasound frequency range from 72 to 176 MHz, for the case when the wave vector of the acoustic beam is directed at an angle of 1.58∘ to the [110] axis in the (11¯0) plane of the paratellurite crystal. The ultrasound attenuation coefficients were obtained for frequency interval between 87 and 176 MHz and their linear dependence on ultrasound frequency was confirmed.

Modes of wide-aperture acousto-optic diffraction in a uniaxial birefringent crystal

Acousto-optic (AO) anisotropic interaction in uniaxial birefringent crystals is a widespread and versatile approach to tunable spectral filtration of light. Its principal constraints are limited values of spectral and angular bandwidth restricting the light transmission and imaging performance of AO filters. A way to overcome these drawbacks is simultaneous Bragg diffraction of two or more light beams. In this paper, we discuss and compare the main features of four available modes of anisotropic wide-aperture AO interactions in uniaxial birefringent crystals. We calculate the angular aperture, spectral resolution and other parameters and estimate the spectral image quality features related to each one. Theoretical consideration and computational modeling are carried out for tellurium dioxide (TeO2)—the main AO crystal. Derived results are important for the development of AO imaging devices, particularly multi-channel AO tunable filters.

Double-Frequency-Shift Acousto-Optic Modulator with Controllable Pulse Pair Frequency Difference

A scheme for controlling the frequency difference of output pulse pair with double frequency shift loops is proposed. The frequency shift system includes two loop elements of 20 and 200 MHz. The first one carries out a single selective positive frequency shift of 1–20 MHz, and the second one can satisfy a single fixed positive frequency shift of 200 MHz. The reverse cascade technology of two acousto-optic crystals is introduced to solve the limitation of the small frequency shift of crystal size. A multichannel synchronization signal completes the time domain control of each acousto-optic modulator. Finally, the frequency shift difference of the output pulse pair ranges of 0–2 GHz, and the frequency shift accuracy is 5 MHz.

Acousto-optic Q-switched Er:CaF2-SrF2 laser at 2.73 μm

A diode-end-pumped Er:CaF2-SrF2 laser actively Q-switched by a TeO2 acousto-optic crystal was demonstrated for the first time, to our best knowledge. By setting the repetition rate of radio frequency signal, 2.73 μm Q-switched lasers with repetition rates of 100–1000 Hz were obtained. With 100 Hz repetition rate, a 390 μJ pulse energy and a 176 ns pulse duration were employed at an absorbed pump power of 2.17 W, and the corresponding peak power was 2.2 kW. These results prove that Er3+-doped CaF2–SrF2 crystal is promising for actively Q-switched laser with high peak power in the mid-infrared region.

A Fading Tolerant Phase-Sensitive Optical Time Domain Reflectometry Based on Phasing-Locking Structure

The demand for phase-sensitive optical time domain reflectometry (φ-OTDR), which is capable of reconstructing external disturbance accurately, is increasing. However, φ-OTDR suffers from fading where Rayleigh backscattering traces (RBS) have low amplitude and may be lower than the noise floor. Therefore, signal-to-noise ratio (SNR) is reduced. In conventional coherent φ-OTDR, an acoustic optical modulator (AOM), which consists of an RF driving source and an acousto-optic crystal, is commonly used to generate optical pulses and frequency shifts. Since RF driving and external modulation signals come from an independent oscillation source, every intermediate frequency (IF) trace has a different phase bias. Therefore, it is difficult to average the IF signals directly for noise reduction. In this paper, a coherent φ-OTDR system based on phase-locking structure was proposed. This structure provided a clock homologous carrier signal, a modulation signal and a data acquisition (DAQ) trigger signal. Then, moving average methods were taken on IF signals before phase demodulating to reduce the overall noise floor of the system. This new φ-OTDR is more tolerant to fading, which can provide higher accuracy for vibration reconstruction. The frequency response range of vibration was as low as 1Hz, and a 25dB improvement of SNR was achieved.

Measuring the radiation coefficient distribution and surface temperature distribution of a tungsten body heated by a powerful laser

In this article, we present what we believe to be the first successful demonstration of the emissivity distribution together with the temperature distribution of a tungsten plate at the laser heating spot. The measurements were made in the 740–800 nm wavelength range by using the multispectral imaging method on a laser-heating apparatus with a tandem acousto-optic filter (TAOTF). The TAOTF consists of two conjugated acousto-optic crystals connected to a high-definition video camera. The emissivity measurement error was less 7% for a maximum point temperature (2540 K) at the laser heating spot.

Characteristics of Acoustic Waves upon Refraction at the Lithium Niobate–Pratellurite Interface in Acousto-Optic Devices

Results are prevented from studying the propagation and refraction patterns of bulk acoustic waves in acousto-optic crystals. The strong anisotropy of the elastic properties of the considered materials is responsible for new effects that cannot be achieved in isotropic media. Ways of using the acoustic anisotropy of crystals in acousto-optic devices are discussed, along with the advantages of such devices over similar ones in which this anisotropy is not crucial.

Investigation of the possibility of using a system of two acousto-optical crystals to determine the parameters of pulsed signals

The work explores the possibility of using the acousto-optical effect to demodulate a sequence of radio pulses. Measuring setup circuits for determining sequence parameters are proposed. The methods for analyzing the diffraction pattern that relate the intensity distribution to the signal characteristics are described. Experimental and theoretical curves are constructed. Based on the data obtained, a conclusion is made regarding the requirements for the materials of crystals which is the main part of acousto-optic cells. The conditions for the appearance of uncertainty regions are described.

Reflection of Plane Acoustic Waves for an Oblique Incidence on a Tellurium Dioxide Crystal Face

This paper describes the phenomenology of the reflection of acoustic body waves in an acoustooptical paratellurite crystal in the case of arbitrary elastic wave incidence on a free interface between the crystal and vacuum. The analysis is performed at arbitrary angles of incidence of acoustic waves in the XOY plane of the material. Anomalous variants of acoustic wave reflection are considered, which fundamentally differ from the known ones and for which the transformation of incident elastic wave energy into reflected wave energy is studied. In the case of oblique incidence, cases of strict reflection of a wave towards the incident wave with an energy conversion efficiency close to 100% are noted.

Tunable terahertz reflection spectrum based on zinc-blende structure crystals excited by ultrasonic

The tunable terahertz reflection spectrum of zinc-blende structure crystals (ZnSe, CuCl and InP), formed in the first bandgap, excited by ultrasonic is investigated in the present paper. Numerical simulation results show that, under the pressure of 0 GPa–100 GPa, the volume change of the crystal indicates that the zinc-blende structure crystals are suitable acousto-optic crystal. According to the optical phonon resonance dispersion, the frequency selective bandwidth of ZnSe, CuCl and InP crystal are 22.5 meV, 21.4 meV and 32.4 meV, respectively, indicating that the frequency selection range of InP crystal is wide. The transverse optical polariton is dis-propagating in the first bandgap and reflects. With the increase of ultrasonic frequency, the reflection peaks moves towards the restrahlen band, and the amplitude of the reflection spectrum decreases until it disappears. More transverse optical phonon combinations make the width of the reflection peak wider. The tunable terahertz reflection spectrum can be achieved by adjusting the ultrasonic frequency, a satisfactory condition for tunable selection and filtering in terahertz range.

Study on spectral transfer function based on dual AOTF acousto-optic crystal

Study on spectral transfer function based on dual AOTF acousto-optic crystal

Polarizer-Free Acousto-Optic Monochromators

Monochromators based on a noncollinear acousto-optic filter without external polarizers have been studied. As an element to select an operating diffracted light beam, (1) a spatial filter-telescope and (2) an output plane of a filter crystal rotated in a diffraction plane have been used. In the latter case, dispersion of the acousto-optic filter crystal is compensated using a correcting prism made from the same crystal and placed behind an output filter plane along a path of a filtered light beam. As a result, the transmission coefficient of the monochromator is increased upon an effective compensation of the angular drift of the filtered light beam, the monochromator design is simplified, and its sizes are decreased.

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.

Study on diffraction efficiency of acousto-optical crystal in AOTF imaging spectrometer

Study on diffraction efficiency of acousto-optical crystal in AOTF imaging spectrometer

Acousto-optic modulators based on a KYW crystal

Two types of acousto-optic modulators based on new acousto-optic crystals KY(WO4)2 are developed and experimentally investigated. Their characteristics are compared with those of existing acousto-optic quartz modulators. It is shown that the new modulators provide an almost 10-fold efficiency gain, which eliminates the need for water cooling of such devices.

Acousto-optic deflector based on a paratellurite crystal using broadband acoustic adhesive contact

A broadband acoustic matching of a piezoelectric transducer made of a lithium niobate crystal with an acousto-optic TeO2 crystal has been studied. The transducer is acoustically attached to the crystal by the method of adhesive contact. The experimental method of creating a piezoelectric transducer with an acoustic matching layer from a tin film is developed. The conditions of the optimal technological regime when applying the matching layer are determined. The results of the research are used in creating a broadband high-performance deflector with a central ultrasound frequency of 37 MHz, with the frequency band scanning more than 30 MHz, and a diffraction efficiency of about 90% at a wavelength of 1.06 μm.

纳秒至微秒可调脉宽输出CO<sub>2</sub>激光器

研制了一台激光脉宽可变、输出波长可调谐的重复频率脉冲CO2激光器，并重点从理论和试验两个方面研究了Ge晶体声光调制时腔内光子数的时间变化过程。实验结果显示，在激光谐振腔内应用时，声光调制器理论并不适合解释激光器输出脉冲的时间行为，而必须由腔内Q开关速率方程所描述，分析其原因在于谐振腔内激光能级粒子数反转跃迁产生的开关增益效应远远大于声光晶体渡越时间的影响。对于Ge声光调制器施加射频控制信号时，激光器可以实现从ns至s范围的脉冲激光输出：平均功率3~8 W左右，重复频率为1 Hz~100 kHz范围，脉冲功率最高达到10 kW量级。使用金属衍射光栅替代输出耦合镜时，可以实现9.2~10.8 m范围三十余条谱线的可调谐输出。 A repetitive pulsed CO2 laser with variable pulse widths and tunable wavelengths was developed and the photon number change behavior inside the resonant cavity which has an inserted Ge crys- tal acousto-optical modulation was researched on both theory and experiment. The experimental results show that acousto-optical modulation theory is not suit to explain output characteristic of the laser pulses, the felicitous description should make use of the Q switched rate equation instead of, due to the reason that the action of switched gain effect produced from laser level population inversion transition is much larger than that of the transit-time along the acousto-optical crystal. When the Ge crystal is added with controlled radio frequency modulation signals, the laser can realize laser pulses with a range from nanosecond to microsecond under the conditions of 3 - 8 W average power and 1 Hz - 100 kHz repetition rate, corresponding the maximumpulse power 10 kW. Over 30 laser spectral lines between 9.2 - 10.8 m can be achieved if the metal grating is used for substituting the cavity output coupler.