Backward collinear guided-wave-acousto-optic interactions insingle-mode fibers

A theoretical analysis of backward collinear guided-wave-acoustooptic interactions (BCGWAOI) in single-mode fibers is presented. Single-mode optical and acoustic waves are guided in the fiber core simultaneously. At very high acoustic frequencies ( >10 GHz), the wavenumber of the guided acoustic modes can be twice that of the guided optical mode. The coupling coefficient and wavelength selectivity of BCGWAOI in a single-mode optical fiber (SMOF) are calculated for the lowest guided longitudinal-acoustic mode. Considerations related to all the other guided acoustic modes, applications, and acoustic losses are also discussed     

Guided-wave acousto-optic diffraction in Zn:LiNbO3

The acousto-optic control of light in Zn-diffused LiNbO₃ optical waveguides is demonstrated. The guided optical mode diffraction by the surface acoustic waves is consistent with the Klein and Cook model for the intermediate diffraction regime. The SAW induced refractive index change is 1.5times10^-4 at acoustic power 130 mW. The Zn-diffused lithium niobate waveguides appear to be good candidates for integrated light-control devices based on acousto-optic interaction     

Thin-film acousto-optic interaction in LiNbO3

100% Bragg diffraction of guided optical waves by surface acoustic waves in metal-diffused layers in LiNbO3 substrates has been observed. The predicted efficiency, which compares favourably with experimental measurement, is shown to arise mainly from the electro-optic contribution of the electric field associated with the yz acoustic surface waves instead of the clasto-optic contribution due to the strain or from the surface ripple components.     

共线声光模式转换效率的分析与计算

利用声表面波共线声光效应的衍射光栅可使波导中导模向泄漏模转换,此现象在光通信在线解复用系统具有潜在应用前景。利用耦合模理论求解这一过程,模式转换效率可以化作声波频率、声光作用长度、波导厚度和声波功率密度的函数,利用数值计算得出结果并给出其关系。由数值计算结果可知,x切质子交换铌酸锂波导模式转换效率最大,适当选择波导长度和声驱动功率密度,其TE0→TMυ转换效率可达90%以上。     

两通道声表面波全光纤声光调制器的研究

研制了一种新型的两通道声表面波(SAW)全光纤声光调制器,建立了多通道声波与光纤中光导波相互作用的耦合模方程并求出其解。SAW全光纤声光器件由两通道SAW器件和单模光纤构成。光纤嵌入SAW器件,光纤中的光导波被声表面波调制,由于两个通道中的SAW具有不同的中心频率,所以光纤中的光导波可以被具有不同频率的两个通道的SAW同时调制。测试了器件,实现了全光纤相干调制解调。     

On diffraction and focusing in anisotropic crystals

This paper discusses various aspects of diffraction and focusing for waves in anisotropic crystals, and the concepts are applicable to electromagnetic, ultrasonic bulk, or ultrasonic surface waves in homogeneous but arbitrarily anisotropic crystals. A numerical technique based on an angular spectrum of plane waves representation is shown to produce theoretical intensity profiles in good agreement with acoustooptic probe measurements of the diffraction field of a slit-type ultrasonic transducer launching quasi-longitudinal and quasi-shear waves in quartz along on axis of extreme anisotropy where the wave normal and the direction of energy flow are separated by angles greater than 20°. A theoretical study of two-dimensional focusing structures for use in anisotropic crystals reveals that a launching transducer fabricated in the shape of the relevant group-velocity surface can be far superior in producing a well-defined focus to a structure having the shape of a circular arc. The computation techniques can be applied to obtain the diffraction field of any arbitrary aperture function and the examples described, theoretical and experimental, concern acoustic waves in crystals demonstrating extreme elastic anisotropies, in fact much more extreme than would be encountered in analogous cases of optical diffraction. Because of current relevance to device research, emphasis in the latter sections of the paper is placed on ultrasonic surface waves.     

LiNbO3TE-TM mode converter using collinear acoustooptic interaction

A highly efficient TE-TM mode conversion device utilizing the acoustooptic effect has been demonstrated. Mode conversion has been made by an acoustic surface wave propagating collinearly with an optical guided wave. The maximum mode conversion, although the fraction was restricted to 70 percent, has been obtained with an acoustic power of only 55 mW (electrical input 0.55 W). Effective interaction length of the present device is limited to 5 mm due to optical and/or acoustic inhomogeneity of the waveguides. For selecting TE and TM waves, a polarizer is constructed monolithically and the extinction ratio is improved by up to 21 dB. Performance characteristics of a tunable optical filter of this type of the mode conversion are also discussed.     

Acousto-optic diffraction of optical guided waves in crystal quartz

A waveguide supporting a single propagating mode has been fabricated by ion beam modification of X-cut crystal quartz. The gas ion implantation reduced the index of a subsurface layer, and guiding occurred in the surface region. The acoustic properties of the quartz are little affected. Bragg diffraction of guided light by surface acoustic waves was observed with efficiencies of approximately 0.25% per milliwatt acoustic power.     

Acoustic diffraction of light in anisotropic media

The Bragg formalism which describes the diffraction of optical radiation by traveling acoustic waves is generalized to include diffraction in anisotropic media. When the diffracted and incident optical polarizations are different, the Bragg relations require important modification. The correctness of the suggested modifications is confirmed by experiments using optical diffraction from longitudinal and transverse acoustic waves in crystal quartz and in sapphire. A special case of diffraction from longitudinal waves in an anisotropic medium is discussed as a simple example of a collinear parametric interaction.     

Multifrequency Magneto-optic Bragg Diffraction and Radio Frequency Signal Parallel Processing

Magneto-optic （MO） coupling of guided optical waves with microwave magnetostatic waves （MSWs） simultaneously excited by multiple radio frequency（RF） signals can lead to multifrequency diffraction effects and then parallel processing of RF signals can be realized by using of the characteristics that diffraction efficiencies（DEs） are approximately in direct proportion to RF signals intensities and diffraction angles are related to frequencies of the corresponding RF signals within linear MO interaction region. In this paper, studied is the multifrequency MO Bragg diffraction in first-order MO interaction approximation, and obtained was the approximate analytical expression for principle diffraction efficiency（PDE）. Also, put forward was a parallel imaging method of relative intensity of RF signals based on single-frequency diffraction. By calculation and analysis, it is shown that the relative error is not more than 0.3 dB for the case of three RF signals within the frequency space of 60 MHz.     

Shear Mode Ultrasonic Transducers from Flexible Piezoelectric PLLA Fibers for Structural Health Monitoring

Shear mode guided waves are highly demanded for underwater structural health monitoring (SHM) applications due to their simplified non-dispersive feature and minimal acoustic energy loss in the presence of liquid. Excitation and detection of pure shear wave are challenging using conventional piezoelectric materials used in the current ultrasonic transducers because they have complex piezoelectric responses mixed with multiple longitudinal, transverse, and shear modes. They also suffer from aging issue due to depoling. Here, conformable shear mode ultrasonic transducers are designed and made of flexible piezoelectric poly (L-lactic acid) (PLLA) fibers on both flat and tubular structures. The electromechanical responses over a macroscopic area of the transducers are evaluated in a wide frequency range up to 500 kHz. The PLLA fiber-based shear mode ultrasonic transducers exhibit a consistent sensitivity of detecting defects in liquid and air. In addition, the only shear mode in PLLA fibers originates from crystal structure without requiring electrical poling to render piezoelectricity, thus does not depole due to aging. The theoretical analyses including ab initio calculations and experimental results on both flat and tubular structures show the great potential of PLLA material and significant advantage of PLLA fiber-based shear mode ultrasonic transducers for underwater SHM applications.     

Depth measurement of the periodic grooved reflectors of surface acoustic waves using the laser probing

An optical method for depth measurement of the periodic grooved structures that serve as reflectors of surface acoustic waves (SAWs) is presented and analyzed. The method is based on the diffraction of the laser beam on a system of the SAW reflection elements. The measurement method is theoretically validated, and the experimental results are presented.     

Modeling of noncollinear multifrequency acoustooptic interaction inguided-wave Bragg cells

We present an original detailed theoretical modeling of guided-wave noncollinear acoustooptic (AO) interaction produced by a multifrequency surface acoustic wave (SAW) in planar guided-wave structures. The modeling of the multifrequency AO interaction is based on a modified version of the coupled-mode theory. The first- and third-order diffraction efficiency and optimum dynamic range of the Bragg cell have been calculated as a function of acoustic power, acoustic radiofrequency, and optical waveguide parameters in Y-cut Ti:LiNbO₃ planar waveguides. Polynomial approximations, describing the first and third order diffraction efficiency versus normalized acoustic power in the cases of two- and three-frequency interactions, are derived and presented for the first time for arbitrarily large RF signals     

A wide-band acoustooptic TE-TM mode converter using a doubly confined structure

Bandwidth limitations of collinear acoustooptic TE-TM mode converters have been overcome by a method in which the optical and surface acoustic waves have been interacted in a doubly confined waveguide structure. A 5 percent bandwidth centered at 550 MHz and a conversion efficiency of 50 percent at a total acoustic input power of less than 1 mW have been obtained withY-cut LiNbO3. It is found that the power transfer efficiency does not degrade significantly when the phase mismatch increases.     

Phase mismatching slope optimization for magneto-optic pulse compression

The collinear diffraction effects of linearly chirped Gaussian optical pulses with continuous magneto-static surface waves (MSSWs) excited in magneto-optic (MO) film waveguides under a horizontal bias magnetic field are theoretically studied. MO pulse coupling equations and the analytic solution are presented. It is shown that optical pulse compression based on MO Bragg diffraction is dependent on phase mismatching slope, which can be greatly increased by using different mode-number of incident and diffracted guided optical waves (GOWs) and appropriately choosing normalized thickness of MO film (ratio of the film thickness to the light wavelength). In TE₀ → TM₈ diffraction full-width-at-half-maximum (FWHM) ratio of the diffracted and incident optical pulses comes up to 38%.     

静磁前向体波—光导波调制器研究

在理论和实验两个方面研究了YIG/GGG结构中静磁前向体波(MSFVW)色散、辐射阻抗和输入阻抗。按模式耦合理论对YIG/GGG结构中MSW-GOW互作用问题作了分析。成功地实现了MSFVW对光导波的调制,作用效率达21％,带宽0.5GHz。     

提高液体中脉冲激光致声转换效率的理论研究

从液体中脉冲激光致声的理论出发,讨论了热膨胀机制下提高光声转换效率的措施,并着重讨论了不同液体媒质的特性对转换效率的影响;基于Helmholtz共振器的共振频率和放大特性理论,讨论了通过共振光声池的设计提高光声转换效率的可行性;指出在对光致声波可重复性和可控制性没有特殊要求的情况下,可采用高功率光纤激光器,通过汽化机制或介电击穿机制激发声波,提高转换效率.     

声扭波在单模光纤中的应用

光纤中的基模LP01有两种偏振态,通过在油光纤方面加入声扭波可以引起两个偏振模发生耦合,本文主要介绍了声扭波和振幅和色散关系,同时也发现,采用高阶声扭波可以提高耦合系数的值。     

具有高模场限制的异质纳米棒结构波导(英文)

现代光学及纳米光学的一个主要发展趋势是从理论和实验两方面探究用于突破传统衍射极限的亚波长级别的电磁波波导结构。表面等离子激源提供了解决此问题的有效突破口。文中根据异质波导结构的耦合以及锲型波导的表面等离子激发理论,提出了异质纳米棒结构波导,该结构具有非常高的模场限制能力,对其传输的模式理论上可实现亚波长级别的限制能力。文中主要对此结构进行了数值仿真,分析了该波导结构的能量以及传输损耗,其结果表明该结构的模场限制能力可达到衍射极限的1/500,而且通过调整结构参数,将模场的传输距离延长到毫米级别。     

质子交换LiNbO_3波导中声光共线相互作用的实验研究

本文报导了对质子交换LiNbO_3波导中声光共线相互作用的实验研究,给出了在这样相互作用下光导模向基板辐射模转换的效率与声表面波功率的关系以及衍射光束的空间分辨率。