ABSTRACT
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.
Subject(s)
Acoustics , Optics and Photonics , Temperature , Ultrasonography , LightABSTRACT
An acousto-optic (AO) device can be configured to operate in a standing wave (SW) mode. The effectiveness of an SW AO device surpasses the effectiveness of a conventional, running-wave one, when the driving signal frequency meets the SW condition. This SW frequency depends primarily on the acoustic wave velocity and the crystal length. While promising significant benefits, this situation imposes serious restrictions on the AO device performance. In this study, we examine the SW frequency dependency of such a device on its electrical impedance matching circuit (EIMC) parameters and configuration both theoretically and experimentally. Our analysis is aimed at this effect utilization for the purpose of expanding of the standing-wave AO devices applicability domain.
Subject(s)
Optics and Photonics , Sound , Acoustics , Electric Impedance , VibrationABSTRACT
The development of the optical frequency comb (OFC) generation and practical application methods is one of the most important and rapidly developing areas of the modern optical electronics. One of the comb types is acousto-optical (AO) OFCs. This type of OFCs is obtained by the multiple passage of an optical signal through a closed loop containing an acousto-optic frequency shifter (AOFS). Despite the fact that AO OFCs have been studied quite intensively lately, the published papers did not focus on the influence of the main element, the AO cell used as AOFS, parameters on the characteristics of the obtained optical comb, primarily on the comb spectral width, number of spectral components and its envelope shape. In this paper, we perform a theoretical analysis of all possiblities in paratellurite crystal wide-angle AO diffraction geometries in order to determine the most suitable for the application as AOFS in a frequency shifting loop.
ABSTRACT
An acousto-optic mode locker (AOML) operating in a standing wave mode has a fixed working frequency, defined by the length of the crystal, which makes the device prone to temperature variations. In this study, we examine the effect of the electrical matching circuit parameters on the acoustic resonances and AO diffraction in the AOML both theoretically and experimentally. Acoustical and electrical models of the AOML are introduced. We outline the ways of utilizing this effect for compensation of ill thermal influence. Our analysis allows us to broaden the temperature and driving power domains of an AOML's reliable operation without a heat sink or a complicated temperature stabilization system.
ABSTRACT
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.
ABSTRACT
The paper presents a short survey on theoretical and experimental investigations of an acousto-optic (AO) method of optical wavefront visualization proposed and developed at Moscow State University. The method is based on angular selectivity of Bragg diffraction. It is shown that distribution of light intensity in the visualized image is proportional to the phase gradient in the AO interaction plane. Spatial resolution and contrast of the visualized image are determined primarily by the divergence angle of the acoustic beam. Most attention in the paper is concentrated on the problem of AO visualization of two-dimensional phase objects. The important advantage of the AO method consists of the possibility of optical field phase-structure registration in the presence of amplitude modulation of the initial optical field. Examples of computer simulations as well as some experimental results are presented for illustration of the potentialities of this method.
ABSTRACT
Influence of acoustic beam energy walk-off on characteristics of Bragg diffraction of light is studied theoretically and experimentally by the example of a paratellurite single crystal. Two cases of isotropic and anisotropic light scattering are examined. Angular and frequency characteristics of acousto-optic interaction are calculated in wide ranges of Bragg angles and ultrasound frequencies by means of modified Raman-Nath equations. It is shown that the walk-off can substantially change the width of angular and frequency ranges, resulting in their narrowing or broadening subject to position of the operating point in the Bragg angle frequency characteristic. Coefficients of broadening are introduced for characterization of this effect. It is established that frequency dependences of the broadening coefficients are similar to the Bragg angle frequency characteristics. Experimental verification of the calculations is carried out with a paratellurite cell of 10.5° crystal cut.
ABSTRACT
Peculiarities of collinear acousto-optic interaction of a strongly divergent optical beam are examined theoretically by examples of two crystals widely used in acousto-optics: calcium molybdate (CaMoO(4)) and paratellurite (TeO(2)). These materials demonstrate essentially different diffraction characteristics because of peculiar features of optical and acousto-optic anisotropy in these crystals. The dependence of the integral diffraction efficiency and the transmission band of collinear acousto-optic filters on the optical beam divergence and acoustic power is studied. It is shown that with increasing light divergence these characteristics of the filter are worsened according to the same law, and the product of the relative bandwidth and the diffraction efficiency remains constant and independent of the optical wavelength and the acousto-optic interaction length.
ABSTRACT
Acousto-optic processing of images is based on the angular selectivity of acousto-optic interaction resulting in spatial filtration of the image spectrum. We present recent theoretical and experimental investigations carried out in this field. Much attention is given to the analysis of the acousto-optic cell transfer function form depending on the crystal cut, the geometry of acousto-optic interaction, and the ultrasound frequency. Computer simulation results of the two-dimensional acousto-optic spatial filtration of some elementary images are presented. A new method of phase object visualization is suggested and examined that makes it possible to separate amplitude and phase information contained in an optical image. The potentialities of the acousto-optic image processing are experimentally demonstrated by examples of edge enhancement and optical wavefront visualization effects.
ABSTRACT
The article is devoted to theoretical analysis of light diffraction in a non-homogeneous acoustic field created by a wedge-shaped piezoelectric transducer. Relationships describing electrical, acoustic and acousto-optic properties of the acousto-optic cell are derived in the approximation of a small thickness of the piezoelectric plate. Principal characteristics of acousto-optic interaction are examined such as dependences of the light diffraction efficiency on the phase mismatch and the acoustic wave amplitude. It is shown that the acoustic field has a complicated amplitude-phase structure changing with the acoustic frequency. It is demonstrated that the diffraction efficiency in the Bragg regime can approach 100% in spite of a noticeable phase mismatch. The appropriate optimal values of ultrasound power and incidence angles of light are found.
