Acousto-optic interaction in a non-homogeneous acoustic field excited by a wedge-shaped transducer

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.     

Intermediate and Bragg acousto-optic interaction in elastically anisotropic medium

The Raman-Nath, intermediate and Bragg regimes of acousto-optic interaction in an acoustically anisotropic medium are theoretically examined in the paper. The general model of the interaction is extended over the case of an elastically anisotropic medium. Basic results of computer modeling of diffraction processes taking place in crystals are presented in the paper. The interaction of light and ultrasound is investigated in media with arbitrary elastic anisotropy in a wide range of the Klein-Cook and Raman-Nath parameters. The influence of the acoustic anisotropy on the process of light diffraction, especially on the light intensity in diffraction maxima, is also discussed.     

Thermal behavior of acousto-optic devices: effects of ultrasound absorption and transducer losses

In the present paper we analyze the electric and acoustic losses in acousto-optic devices, especially in their ultrasonic transducers and the related thermal effects. We include electric and acoustic losses into the classical electric equivalent model of the transducer, to explain the characteristics of the measured electric and thermal behavior. Measured temperature distributions on the acousto-optic crystal faces serve visualization of the conversion efficiency of the radio-frequency input to bulk acoustic waves. We show that the pronounced acoustic frequency dependence of the temperature distribution is in correlation with the frequency dependent losses in the transducer and in the bulk. We also demonstrate experimentally the effectiveness of our active and passive heat removing and temperature stabilization methods.     

The dynamics of a mode locked and frequency-doubled laser

The theory for the transient build-up of mode locked pulses in a mode-locked and frequency-doubled laser is presented. The time required for the mode-locked pulsewidth to reach a steady-state value is computed. It is found that steady-state is reached faster in the presence of internal frequency doubling because of the broadening effect of the doubling crystal. The effects of different conversion efficiencies and modulation depths on the mode-locked pulsewidth are investigated and the results are graphically presented.     

复合腔CuBr激光器中的模式研究

本文报道了在两种结构的多镜复合腔中CuBr激光器的纵模淬灭现象,并得到了重复频率几倍于基频纵模间隔的多重锁模脉冲串,最高重复频率可达440MHz.实验结果表明,通过调整复合腔的结构参数,可以在不缩短增益介质长度的条件下,实现CuBr激光多重锁模脉冲高重复频率的大范围调节.     

Design, simulation and structural optimization of a longitudinal acoustic resonator for trace gas detection using laser photoacoustic spectroscopy (LPAS)

The design of the acoustic resonator is critical for the optimization of the sensitivity of laser photoacoustic spectroscopy (LPAS) in trace gas detection applications. In this paper, an LC circuit model is used for the simulation of a 1D acoustic resonator. This acoustic resonator is designed for CO photoacoustic spectroscopy. The effects of the structural parameters, quality factor and resonant frequency on the performance of the device are theoretically analyzed. The role of the buffer volume as an acoustic filter is investigated and optimized dimensions of the buffer volume, to achieve minimum noise transmission coefficient, are calculated. The effects of the ambient temperature, variety of pressure and gas flow velocity on the resonant frequency of photoacoustic resonator and PA signal are simulated. The temperature dependence of the microphone sensitivity is also introduced.     

薄板声谐振子扰动的声学特征检测研究

为进行快速ＮＤＴ，研究了薄板声谐振子扰动和声学特征变化的关系，用有限元方法分析在睡无缺陷薄板的振动频率和振型。利用数字信号处理技术，实验研究了样品中缺陷对声学特征的影响。实现和理论分析结果一致性很好。研究结果有助于正确利用样品声学特征进行无损检测。     

Feasibility study of complex wavefield retrieval in off-axis acoustic holography employing an acousto-optic sensor

We propose and experimentally demonstrate a new method for complex-valued wavefield retrieval in off-axis acoustic holography. The method involves use of an intensity-sensitive acousto-optic (AO) sensor, optimized for use at 3.3 MHz, to record the acoustic hologram and a computational method for reconstruction of the object wavefield. The proposed method may circumvent limitations of conventional implementations of acoustic holography and may facilitate the development of acoustic-holography-based biomedical imaging methods.     

复频聚焦超声换能器水中焦区商场特性的研究

本文对新研制的一种复频聚焦超声换能器的辐射声场进行了实验研究,测定了辐射压在轴向上的分布曲线,进而确定了焦区位置,这与理论所得结果符合较好,测定了辐射声压在焦平面上的分布曲线,并对焦区声场进行了定位,最后对换能器在水中焦区辐射声的频谱进行了研究,不仅观测到了两个源波,而且还观测到了和频波,差频波以及倍频波,证实了声散射声效应的存在。     

激光辐照下非稳腔镜变形对激光模式的影响

 分析了正支虚共焦非稳腔中腔镜变形对激光模式的影响,采用时间离散化、快速傅里叶变换（FFT）和有限元分析(FEA)方法解决腔镜变形和激光模式两者之间的耦合问题。计算结果表明,随着激光器工作时间的增加,腔镜温升和变形相应增大,谐振腔模式变差,输出光场相位均匀性变差,呈现散焦现象,输出光束质量下降。实验验证了激光器输出光束质量随出光时间的延长而逐渐变坏。     

Interplay of local resonances and Bragg band gaps in acoustic waveguides with periodic detuned resonators

This letter is concerned with acoustic wave propagation and transmission in acoustic waveguides with periodically grafted detuned Helmholtz resonators. The interplay of local resonances and Bragg band gaps in such periodic systems is examined. It is shown that, when the resonant frequencies of the resonators are tuned close to a Bragg band gap, the behavior of the Bragg band gap can be affected dramatically. Particularly, by introducing appropriately tuned resonators, the bandwidth of a Bragg band gap can be reduced to zero, leading to a very narrow pass band with great wave attenuation performance near both band edges. The band formation mechanisms of such periodic waveguides are further examined, providing explicit formulae to locate the band edge frequencies of all the band gaps, as well as the conditions to achieve very narrow pass bands in such periodic waveguides.     

MEMS环形谐振器椭圆模态中的锚损分析（英）

环形谐振器是微机电系统（MEMS）谐振器中典型结构之一。其中,椭圆模态是环形谐振器面内振动的一阶模态,也是使用最为普遍的模态。以降低谐振器振荡时的锚损为目标,以有限元分析为技术手段,采用能量法对环形谐振器锚点的几何尺寸进行定量分析。分析结果表明,与研究者普遍认为的观点相反,谐振器锚点振荡模态的激发并不有助于提高谐振器的Q值。     

Measurement of acoustic particle velocities in enclosed sound field: Assessment of two Laser Doppler Velocimetry measuring systems

The performances of two Laser Doppler Velocimetry (LDV) systems adapted for measuring the acoustic particle velocities are assessed in enclosed sound field. This assessment is performed by comparing the acoustic velocities measured by means of LDV to reference acoustic velocities estimated from sound pressure measurements. The two LDV systems are based on a single optical bench which delivers an optical signal called Doppler signal. The Doppler signal, which is frequency modulated, is analyzed by means of two signal processing systems, the BSA (Burst Spectrum Analyser from Dantec) on the one hand, and a system specifically developed for the estimation of the acoustic velocity on the other hand. Once the experimental setup has been optimized for minimizing the errors made on the reference velocities, the assessment is performed and shows that both systems can measure the acoustic velocity in enclosed field in two the frequency ranges [0-4 kHz] and [0-2 kHz] respectively for acoustic velocity amplitudes of 10 mm/s and 1 mm/s.     

Passive harmonic mode locking of soliton bunches in a fiber ring laser

We report on the experimental observation of passive harmonic mode locking of bunches of single-pulse solitons or twin-pulse solitons in an Erbium-doped fiber ring laser. Experimental investigations on the phenomenon revealed that, although the soliton interaction between the adjacent single-/twin-pulse solitons in a bunch is weaker than that of the pulse interaction in the twin-pulse solitons, a soliton bunch could also function as a unit and form the state of passively harmonic mode-locking. Harmonic mode-locking is one of the intrinsic characteristics of soliton emission in passively mode-locked fiber ring lasers. It can be formed based on the single-pulse soliton, twin-pulse soliton, or bunch of solitons.     

二极管激光阵列在Talbot外腔中同相模的选择

从理论和实验上对二极管激光阵列在Talbot外腔中的锁相进行了研究。在同相模和异相模近场分布的基础上,利用1维情况下的菲涅耳衍射公式计算了其远场分布。根据同相模和异相模在Talbot腔中的分布特性,采用1/2 Talbot腔并将外腔镜倾斜一个角度a的方法既能选择同相模,又能使模式的功率损耗最小。二极管激光阵列芯片采用CD金刚石材料,“三明治”结构对其进行封装,明显地减小了阵列的“smile”效应。在实验中实现了二极管激光阵列同相模的锁相输出,远场单瓣模的半高全宽为0.11 mm。     

How diffraction limits ultrasonic screening in phononic plate composed of a periodic array of resonant slits

We explore experimentally the role played by diffraction in the phenomenon of acoustic shielding provided by a plate that is periodically perforated with subwavelength slits and immersed in water. We carried out ultrasonic transmission measurements for all directions of propagation in order to check the omnidirectionality of acoustic shielding. While a single slit acts as a Fabry–Perot resonator in the frequency range of interest, the coupling between adjacent slits provides an attenuation frequency band centered around the resonant frequency that is mostly independent of the angle of incidence. Beyond the incident angle of 45 degrees, however, we observe the appearance of scattered radiation that limits the attenuation of ultrasound. This spurious scattering is shown to arise from diffraction by the grating of slits.     

二极管激光阵列在Talbot外腔中同相模的选择

 从理论和实验上对二极管激光阵列在Talbot外腔中的锁相进行了研究。在同相模和异相模近场分布的基础上,利用1维情况下的菲涅耳衍射公式计算了其远场分布。根据同相模和异相模在Talbot腔中的分布特性,采用1/2 Talbot腔并将外腔镜倾斜一个角度a的方法既能选择同相模,又能使模式的功率损耗最小。二极管激光阵列芯片采用CD金刚石材料,“三明治”结构对其进行封装,明显地减小了阵列的“smile”效应。在实验中实现了二极管激光阵列同相模的锁相输出,远场单瓣模的半高全宽为0.11 mm。     

Real time NDE of laser shock processing with time-of-flight of laser induced plasma shock wave in air by acoustic emission sensor

Acoustic emission sensor is used to research the time-of-flight of the shock wave induced by laser-plasma in air for real time nondestructive evaluation (NDE) of laser shock processing. The time-of-flight of the shock wave propagating from the source to the sensor declines nonlinearly and similarly at the different distances for different laser energies. The velocity of the shock wave at the distance of 30 mm increases faster than that of the distance of 35 mm. The relationship between the laser energy and the distance is almost linearly when the signal with distortion is measured by acoustic emission sensor. Finally, Taylor solution is used to analyze the experimental results, and the empirical formula between the energy of the shock wave and the laser energy is established, which will provide a theoretical basis for real time NDE of laser shock processing.     

Engineering of band gap and cavity mode in phononic crystal strip waveguides

The optimal parameters for the largest band gap were investigated in three typical phononic crystal strip waveguides. Single cavity mode was created inside the band gap region by proper design of a defect. The band structures and the displacement distributions were discussed with the variation of the defect. Results show possibilities to guide extremely slow phonon cavity mode in strip waveguide with chosen displacement components, frequencies and symmetries.     

Origin of the acoustic phonon frequency shifts in semiconducting nanoparticles

The surface and size effects on the acoustic phonon properties of semiconducting nanoparticles, such as ZnO, are studied using the s-d model and a Green's function technique. We have shown that the electron-phonon and anharmonic phonon-phonon interactions play an important role in ZnO nanoparticles and must be taken into account in order to explain the experimental data. Due to surface and size effects on the electron-phonon constants, the acoustic phonon frequency and their damping increase with decreasing of particle size.