Acousto-optic interaction of diverging Gaussian beams in anisotropic media

A strong acousto-optic interaction of bounded light and sound beams of a Gaussian shape is considered for different geometries permitting long-term interaction. The anisotropic spreading of an acoustic beam in the course of its propagation is taken into account. The spectra of light beams are described by a Gaussian-polynomial expression, and a set of differential equations is derived which allows one to describe various geometries of acousto-optic diffraction. It is demonstrated that the transmission function of an acousto-optic cell essentially depends on the ratio between the dimensions of the light and sound beams and on the angle between the propagation direction of the diffracted beam and that of the incident beam. The degree of spreading of the acoustic beam noticeably affects the suppression of the transmission side lobes.     

Ultrasonic diffraction of Bessel light beams in uniaxial gyrotropic crystals

Specific features of the acousto-optic diffraction of Bessel light beams propagating in the vicinity of the optical axis of a uniaxial gyrotropic crystal have been investigated. The dependences of the diffraction efficiency on the acousto-optic interaction length, ultrasound power, and polarization state of the incident Bessel light beam have been analyzed using the coupled-wave equations and the overlap-integral method. It is shown that polarization-independent diffraction of Bessel light beams is observed in paratellurite crystals, when the Bragg diffraction efficiency is independent of the polarization state of the incident beam. The physical reason for this diffraction has been established (both theoretically and experimentally) to be simultaneous implementation of two processes of anisotropic scattering, at which the Bragg synchronism conditions are satisfied for orthogonal polarized Bessel beams with elliptical polarization.     

The Optical Characteristics of the Acousto-optic Tunable Filter with the Equivalent Point

In this paper, the interaction curves of acoustic and optical waves in a non-collinear AOTF (acousto-optic tunable filter) cell under the parallel-tangent momentum matching condition are analyzed systematically. A new theory of equivalent point for AOTF design is put forward. The wavelength characteristics and the wavelength resolution of two diffraction beams with equivalent point design in AOTF are emphatically analyzed. The optical characteristics at δ=0 (ideal case) and δ≠0 (real case) are also discussed. The advantages of the AOTF with equivalent point design include high wavelength resolution,more quantitative energy and larger incident angle aperture.     

The Optical Characteristics of the Acousto-optic Tunable Filter with the Equivalent Point

In this paper, the interaction curves of acoustic and optical waves in a non-collinear AOTF (acousto-optic tunable filter) cell under the parallel-tangent momentum matching condition are analyzed systematically. A new theory of equivalent point for AOTF design is put forward. The wavelength characteristics and the wavelength resolution of two diffraction beams with equivalent point design in AOTF are emphatically analyzed. The optical characteristics at δ = 0 (ideal case) and δ≠0 (real case) are also discussed. The advantages of the AOTF with equivalent point design include high wavelength resolution, more quantitative energy and larger incident angle aperture.     

Acousto-optic diffraction at liquid-solid interface with light incident at critical angle

A new acousto-optic diffraction configuration is described. At a liquid-solid interface, a light beam is incident from the solid side of higher refractive index. The maximum acousto-optic diffraction of the Raman-Nath type occurs in the liquid with light incident at the critical angle. The angular dependence of the diffraction efficiency indicates accurate optical alignment requirement. Application to acoustic imaging is briefly discussed.     

The spatial structure of acousto-optic phase matching in biaxial crystal of alpha-iodic acid

Anisotropic acousto-optic diffraction in biaxial crystal of alpha-iodic acid has been studied in the case in which the wave vector of the ultrasound is almost orthogonal 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, owing to the complex form of wave surfaces in the field of optical axes of a biaxial crystal, the considered variant of acousto-optic interaction is very sensitive to changes in the direction of the wave vector of the ultrasound that imposes special requirements to accuracy of manufacturing of acousto-optic devices.     

Acousto-optic study of the unusual cases of reflection of bulk elastic waves in a paratellurite crystal

The propagation and reflection of bulk ultrasonic waves in a paratellurite crystal have been investigated by the acousto-optic method. Simultaneous excitation of two acoustic waves by one piezoelectric transducer has been observed in a cell of complex, specially chosen configuration. Maximally efficient conversion of the energy of a quasi-longitudinal wave into a single reflected quasi-shear wave with a walkoff angle 72° is implemented for the reflection from a free crystal boundary. The wave beam compression by a factor of 7 is observed for this reflection. A reflection (close to backward) of waves with an angle of spatial separation of the incident and reflected beams equal to 8° is also implemented. The acousto-optic figure of merit values of the medium are calculated for arbitrary propagation and polarization directions of the interacting light and ultrasonic waves. Possible applications of the effects studied in acousto-optics and acoustoelectronics are indicated.     

Control of light polarization by acousto-optic diffraction from leaky acoustic wave in LiNbO₃

The acousto-optic control of light polarization due to diffraction by leaky acoustic waves in ZX-LiNbO? has been demonstrated. The randomly polarized light of 633 nm wavelength is converted by the anisotropic diffraction into two beams with mutually orthogonal polarizations, the relative intensities of which depend on the light incidence angle and acoustic frequency. Variation in acoustic frequency from 108 to 112 MHz rotates the polarization of the output optical beam by 90°. The acousto-optic control is accomplished entirely by electronic means and can be applied for implementation of fast polarization converters.     

线偏光闪耀反射光栅衍射特性研究

通过数值方法对线偏光在Littrow装置中的理想导体闪耀光栅的偏振特性进行研究.用时域有限差分程序计算出任意偏振方向线偏光经过闪耀光栅衍射后的电磁场分布,提取衍射波的S偏振和P偏振分量,讨论了与光栅闪耀角和入射光波偏振角的关系.分析了在1530～1570nm光谱范围内,入射光波为线偏振波和园偏振波时,衍射光波的偏振色散特性与光栅闪耀角的关系,并给出了存在偏振色散的光栅闪耀角范围.这些分析和结果对工程设计具有一定的指导意义.     

声光可调谐滤波器(AOTF)消色散设计

介绍了非共线声光可调谐滤波器的构成及基本原理,计算了非共线声光可调谐滤波器衍射出的±1级衍射偏振光的偏转角,采用在光路中添加光楔的方法补偿非共线声光可调谐滤波器晶体色散引起的图像漂移.分析结果表明使用TeO_2材料设计的光楔在楔角为7.6°的时候可以控制图像漂移在一个像元以内,并在400～900 nm的工作波段范围内满足稳定成像的要求.     

Optimal conditions of the acousto-optic interaction in crystals with gyrotropy

The conditions of the acousto-optic interaction in the crystals with gyrotropy are considered. The optimal length of the acousto-optic interaction and the diffraction efficiency in the gyrotropic direction are calculated. The diffraction efficiency is determined by the coupled wave method which allows calculation of the field interaction in the gyrotropic crystal [1]. It is theoretically shown and experimentally confirmed that the diffraction efficiency in the gyrotropic direction is independent of incident light polarization, when the polarization plane rotates by 45° along the interaction length.     

基于级联体光栅的光纤激光阵列谱组束

提出了一种基于级联体光栅(VBG)的谱组束方案,组柬阵元数可随级联体光栅的数目倍增.给出了一个双光栅级联谱组束系统实例,将组束阵元按波长分为两组,利用具有相应波长选择性的体光栅分别对它们进行组束,由于两个级联体光栅的角度选择性互不重叠,所有光束经第二个体光栅后都沿其布拉格(Bragg)角方向出射,实现近场和远场的功宰叠加.为保证各光束传播方向的一致性,采用严格耦合波理论推导出了准确计算光束入射角的解析式.数值计算结果表明各光束的平均衍射效率优于80%,在入射角偏离理论值不超过±5'(≈2.424×10-5rad)的条件下组束光中所有光束的传播方向偏差小于4×10-7rad.     

Acousto-optic filter of nonpolarized electromagnetic radiation

An acousto-optic tunable filter that is insensitive to the polarization of incident light is studied both theoretically and experimentally. The fundamental possibility of designing an acousto-optic filter of nonpolarized light is demonstrated. The filter operates in the visible range of electromagnetic waves with λ=480–750 nm and provides a high spatial and temporal resolution. The number of resolvable spots of the filtered image reaches a value of 400×400 with the spectral bandwidth of the device being Δλ=1.7 nm. The results of the processing of optical images formed by arbitrarily polarized light beams are discussed. An acousto-optic filter based on a TeO₂ crystal can be used in optics and spectroscopy in the processing of light beams with an arbitrary polarization, as well as in analyzing optical images formed by light beams whose polarization varies with time.     

Influence of acoustic anisotropy of paratellurite crystal on the double acousto-optic Bragg light scattering

Influence of acoustic anisotropy on acousto-optic interaction in optically and acoustically anisotropic media is theoretically and experimentally studied. A specific type of acousto-optic diffraction is analyzed with allowance for the phase-matching conditions for two diffraction maxima. Analytical expressions for the phase-mismatch parameters versus the angle between the phase and group velocities of acoustic wave are derived. Light intensity in the diffraction peaks is numerically calculated, and experimental data on the diffraction in the paratellurite crystal at an acoustic walk-off angle of 54° are presented.     

Polarization characteristics and magneto-optic diffraction efficiency at oblique light incidence

In a great number of papers that deal with the study of the magneto-optic diffraction (MOD), theoretical analysis and interpretation of experimental data are performed without regard for reflection effects of the incident and diffracted light beams on the entrance and exit surfaces of the transparent film. In consequence the MOD parameters are not connected with the angle of light incidence and the orientation of the incident light polarization plane. In the present work the reflection effects are taken into account. For the Raman-Nat diffraction region the general expressions involving the relationships of diffracted light polarization and intensity with the symmetry of the grating, angle of incidence and polarization of the incident light are obtained. In particular it is shown that even for a symmetrical grating the polarization of the zeroth-order diffracted beam differs from that of the incident light, and the polarization of higher-order beams differs from the orthogonal one providing that the incident light is not s- or p-polarized. A maximum in the angular dependence of diffraction efficiency is predicted being observed experimentally.     

Geometric Arrangement with Multiplicity of Laser Beams in Target Area

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关于非同向二氧化碲声光可调谐滤波器的实验研究

介绍了一种具有大光入射角度的非共线TeO2声光可调谐滤波器(AOTF)的结构和特点,并对实验样品的特性进行了测量。实验结果表明,该器件在100nm的可调谐范围内有效衍射效率可以达到96%,带宽小于2nm,试验结果与理论值一致。这种AOTF将用于动态光分插复用器(OADM),在未来光网络系统中起到重要作用。     

超衍射极限相干反斯托克斯拉曼散射显微成像技术中空心光束的形成

空心光束的质量是超衍射极限相干反斯托克斯拉曼散射显微成像技术中决定成像质量的一个至关重要的因素. 本文基于菲涅耳衍射理论,分析了螺旋相位片法产生空心光束的物理机理,并且模拟了不同的入射条件对产生的空心光束的影响. 模拟结果表明:波长与相位片中心波长匹配且光强呈圆对称分布的高斯光垂直入射到相位片上,当高斯光束中心与相位片中心完全对准时,可获得较理想的空心光束;入射光光强分布的圆对称性以及入射光中心与相位片中心的对准程度都会影响产生的空心光束的强度分布;同时,高斯光束小角度倾斜入射时,空心光的强度分布仍呈圆对称,却在观察面发生一定的位移;此外,入射光中心波长偏离相位片中心波长不大时,对产生的空心光束的强度分布几乎没有影响. 上述分析结果对用于超衍射相干反斯托克斯拉曼散射显微成像技术中理想空心光束的获取具有重要的指导意义. 关键词： 空心光束 超衍射极限 相干反斯托克斯拉曼散射 螺旋相位片     

Some effects of optical alignment errors in a differential laser Doppler velocimeter

A formulation has been developed to treat the effects of alignment errors of the optical system used in a differential laser Doppler velocimeter. It is then applied to analyse the properties of output beat signals obtained by the velocimeter. The effects of optical alignment errors on the spectral width and signal-to-noise ratio of the output beat signals and on the probing volume are investigated in some detail. The spectral width is not affected very much by the deviation of the angle between two beams incident on the probing volume, but depends mainly on the number of interference fringes produced in that volume. The signal-to-noise ratio is very sensitive to the displacement of two incident beams on the probing volume and is also affected by the intensity and beam width differences of these two beams. The probing volume is evaluated in the case of two incident beams having different beam widths.     

Continuous tuning of an electrically tunable external-cavity semiconductor laser

We propose and demonstrate a novel approach to achieving rapid and phase-coherent continuous broadband tuning of a single-mode external-cavity semiconductor laser. Two internal acousto-optic devices control the angle of incident light on a diffraction grating and the effective round-trip phase electronically. Although the present implementation derives the phase from a long delay line and is not yet mode-hop free, the new approach lends itself to fully electronic control.