啁啾脉冲高斯光束在大气湍流中的传输特性北大核心CSCD

基于广义惠更斯-菲涅耳原理,推导出啁啾脉冲高斯光束在湍流大气中传输的光谱解析表达式,并对解析表达式进行了数值仿真。结果表明:啁啾参数越大,光源谱宽越宽;当光源相对谱宽大于0.336时,轴上点光谱产生蓝移;湍流使得轴上点光谱的相对频移量减小,相对频移量随源光谱宽的增大而非线性增大;增大光束束腰半径可减小湍流对光谱频移、光束展宽的影响。     

高斯-谢尔模型光束通过光阑像散透镜传输轴上光谱变化

从部分相干光的传输理论出发,研究了高斯-谢尔模型(GSM)光束通过光阑像散透镜传输轴上的光谱变化规律.结果表明,与光源光谱相比,高斯-谢尔模型(GSM)光束通过光阑像散透镜传输轴上光谱发生了中心频率的移动,这种现象被称为光谱位移.详细分析了透镜的像散、光束的空间相关性和光阑的截断参量对光谱变化的影响.数值计算表明,这些参量会对光谱移动产生比较大的作用.     

On-axis spectral shifts and spectral switches of Gaussian Schell-model beams focused by an astigmatic aperture lens

Starting from the propagation law of partially coherent light, the on-axis spectral shifts and spectral switches of Gaussian Schell-model (GSM) beams focused by an astigmatic aperture lens are studied. It is shown that, as compared with an aberration-free case whose spectral shifts and spectral switches are induced by spatial correlation and aperture diffraction, the spectral shifts and spectral switches of GSM beams also depend upon the astigmatism of the lens for an astigmatism case. Detailed numerical calculations are made to illustrate the behavior of spectral shifts and spectral switches of GSM beams focused by an astigmatic aperture lens.     

Oscillatory mode coupling and electrically strobed gratings in photorefractive quantum-well diodes

Oscillatory mode coupling between two coherent laser beams is produced when an interference pattern moves against a quasi-static electrically strobed grating in a photorefractive AlGaAs/GaAs multiple-quantum-well diode operated in the quantum-confined Stark geometry. The oscillation frequency is equal to the frequency difference between the two laser beams and provides a method to measure high-frequency Doppler shifts or large surface displacements for laser-based ultrasound. Combined photorefractive gains (normally forbidden by symmetry in the Stark geometry) and absorptive gains approach 1200cm(-1)during two-wave mixing using moving gratings.     

Phase control of Goos-Hänchen shifts in a four-level quantum dot nanostructure

In this letter, phase control of the Goos-Hänchen shifts of the reflected and transmitted probe light beams through a cavity containing four-level InGaN/GaN quantum dot nanostructure is theoretically discussed. In order to achieve the wave functions and their corresponding energy levels of the mentioned quantum dot nanostructure, Schrödinger and Poisson equations must be solved in a self consistent manner for carriers (here electron) in quantum dot. It is found that the coupling field, the pumping field as well as the cycling field can enhance the GH shifts of the reflected and transmitted probe beams. The effect of relative phase and the detuning of the probe light on the GH shifts of the reflected and transmitted probe beams are also investigated. We find that the GH shifts can be switched between the large positive and negative values by adjusting the controllable parameters.     

拉盖尔-高斯光束在界面反射和折射的质心偏移特性研究

在圆柱坐标系中研究了傍轴线偏振拉盖尔-高斯光束在两种各向同性介质界面反射和折射后光强质心的偏移. 基于菲涅耳近似和泰勒级数展开,分别得到了部分反射和全反射两种情形下,质心的横向偏移和纵向偏移与光束拓扑荷的解析关系式. 研究表明,部分反射时,反射和折射光束的横向偏移的大小与光束的拓扑荷成正比,方向由拓扑荷的符号决定;而纵向偏移仅仅大小与光束的拓扑荷有关. 全反射时,反射光束质心偏移不受拓扑荷影响. 通过数值模拟验证了解析结果的正确性,并得到了解析公式的适用条件. 拉盖尔-高斯光束的质心偏移特性可应用于测量光 关键词： 拉盖尔-高斯光束 横向偏移 纵向偏移 拓扑荷     

Experimental study of the formation of nonlinear acoustic waves in focused beams

The shock wave formation in focused beams produced by spherical hydroacoustic transducers with different apertures and an operating frequency of 3 MHz, as well as in weakly divergent high-intensity beams of the same frequency, is studied experimentally. The profiles of the received signals are analyzed for different receiving points in the acoustic beam and for different combinations of nonlinear and diffraction effects. It is found that the distortion of the initial waveform (i.e., of the compression and rarefaction phases) is asymmetric. The asymmetry of the wave profile in a focused beam is more pronounced than that in a quasi-plane wave while the asymmetric distortion of the high-frequency carrier causes an asymmetric distortion of the pulse envelope. The angular characteristics of the difference-frequency waves produced by parametric sound radiators are compared using both focused and weakly divergent beams of pump waves. The experiments also show that the appearance of a bubbly phase screen in the region before the point of the shock formation either shifts this point to greater distances or makes the discontinuity formation impossible. Results illustrating the changes that occur in the shock wave characteristics when the bubbly phase screen is placed in the region of the fully developed shock are presented.     

Sub-Doppler laser spectroscopy

Ideal primary data in atomic and molecular physics would include the energy levels of single isolated atoms and molecules, and an approximation to these is obtained from spectra of gases at low pressures. In classical spectroscopy a severe barrier to spectral resolution and measurement precision is often encountered because of the Doppler effect. The absorption frequency of an atom or molecule is modified by its velocity, and so a spectral line will be broadened because of the thermal distribution of velocities in a gas. The barrier can, however, be overcome by using lasers in a variety of different ways. This article introduces the two primary techniques now employed, namely Lamb dip and two-photon spectroscopy, and a variety of examples are discussed. Alternative strategies employing molecular beams, laser cooling and time resolution are also outlined. Limitations on extreme resolution are considered, and the article concludes with a section on laser frequency stabilization and frequency standards which often rely on sub-Doppler spectra.     

Reducing the first-order Doppler shift in a Sagnac interferometer

We demonstrate a technique to reduce first-order Doppler shifts in crossed atomic/molecular and laser beam setups by aligning two counterpropagating laser beams as part of a Sagnac interferometer. Interference fringes on the exit port of the interferometer reveal minute deviations from perfect antiparallelism. Residual Doppler shifts of this method scale with the ratio v/(4d) of the typical atomic/molecular velocity v and the laser beam diameter d. The method is implemented for precision frequency calibration studies at deep-UV wavelengths, both in one- and two-photon excitation schemes: the 6s(2) --> 6s30p(3/2)J=1 line in Yb at 199 nm and the 4p(6) --> 4p(5)p[1/2](0) transition in Kr at lambda=212 nm. The achieved precision of 6 x 10(-10) is limited by the characteristics of the laser system.     

Image formation by inertialess dynamic holograms recorded in a medium with quadratic nonlinearity at different frequencies of the object and reference waves

It is shown theoretically and experimentally that the hologram recorded in a quadratically nonlinear medium is capable of forming the image in the case when the wavelengths of the object and reference beams are essentially different. The relations are derived that determine the wavelength of the reconstructed image, the direction of propagation of the restored beam, and the longitudinal shift of the reconstructed image depending on the wavelength of the beam scattered by the object. The experiment is carried out in a noncollinear arrange-ment of interaction of the object and reference waves in the KTP crystal. As a light source, a YAG:Nd laser (λ=1.064 μm) with a pulsewidth of 300 ps was used. The wavelength of the object wave was tuned by the Ba(NO₃)₂ crystal, which generated from one to three Stokes SRS components with the frequency shifts Δν, 2Δν, and 3Δν (Δν=1047 cm^?1). These components, upon interaction with the reference wave of the fundamental frequency formed the image at the wavelengths 0.532, 0.563, and 0.6 μm with an almost diffraction-limited quality. The experimental values of the angles of propagation of the beams that formed reconstructed images at different frequency shifts of the object wave corresponded to those predicted theoretically. We have confirmed the presence and direction of longitudinal shifts of the reconstructed images upon variation of the wavelength of the light scattered by the object. Thus, we have demonstrated the feasibility of the multicolor (multiwave) holographic generation of high-quality images separated in space. The inertialess nature of the beam coupling makes it possible to use the holograms of this type for ultrafast commutation of the information fluxes in optical computers and communication lines.     

Enhancement of the Goos–Hänchen shift by electromagnetically induced transparency with amplification

The manipulation of the Goos–Hänchen (GH) shifts of the reflected and transmitted probe beams through a cavity containing ?-configuration artificial or realistic atomic medium is investigated. Adjusting the coherent control fields of atomic medium, the electromagnetically induced transparency with amplification (EITA) can be yielded. When the frequency of probe beam is around EITA, the negative as well as positive GH shifts of the reflected and transmitted probe beams can be greatly enhanced by EITA. Meantime, the GH shift can be switched between the considerably large positive and negative values by adjusting the collective phase of the external fields.     

Temporal interference effects by a pair of two-frequency laser beams — application to extremely low velocity measurements

A new proposal is described for the detection of low Doppler-frequency shifts or small displacements. The temporal interference term is expressed in terms of a beat-frequency wave with no phase term. Its amplitude is the same as the intensity formula produced by two equal-frequency laser beams. The significant low Doppler-frequency shifts included in this intensity formula, usually imbeded in the unwanted low frequency noise-spectra, becomes detectable from the amplitude of the beat-frequency wave. An application to a cross-beam Doppler velocimetry is successful for measurements down to the Doppler shifts as low as 0.03 Hz, corresponding to the velocity of 0.1 μm/s.     

Theoretical Analysis and Experimental Measurement of Digital Multi-Beam Phased Antenna Array in the C-Range

The choice of element for constructing a phased antenna array (PAA) providing a relative frequency bandwidth up to 9% for the transmission or reception of wireless communication system information was carried out. There are three methods to suppress of side lobes and diffraction of PAA in the radiation far field. Superposition of excitation signals of a flat PAA for simultaneous emission of several independent beams in a radiation far field was used. Сalculation and optimization of progressive distribution of phase shifts in the excitation signal group in the horizontal and vertical directions in the plane of PAA for radiation (reception) of independent beams in spherical coordinates (azimuth and elevation) in space was performed. An experimental sample of a phased antenna array was developed using microstrip technology, which forms several beams in a radiation far field. An experimental measurement of the voltage standing wave ratio and relative frequency bandwidth of the sample developed by phased array microstrip and printing technology was carried out.     

Precise measurement of the line width of the photoassociation spectra of ultracold molecules by a frequency shifter

We propose a technique to precisely measure the line width of the photoassociation spectra of the excited cesium molecule by using a frequency shifter to generate two laser beams with a precise frequency difference. A series of photoassociation spectra are recorded with two laser beams induced molecular lines, whose peak separation serves as an accurate frequency ruler to measure the line width of the photoassociation (PA) spectra. The full width half maximum line width was studied as a function of PA laser intensity. The extrapolated value at zero laser intensity is (34.84 ± 0.22) MHz. By analyzing other broadening mechanisms, a value of (32.02 ± 0.70) MHz was deduced. It is shown that this scheme is inexpensive, simple, robust, and is promising for applications in a variety of other atomic species.     

Broadly tunable dual-wavelength light source for coherent anti-Stokes Raman scattering microscopy

The signal and idler beams from a picosecond, synchronously pumped optical parametric oscillator (OPO) provide the two colors necessary for coherent anti-Stokes Raman scattering (CARS) microscopy. The OPO provides a continuously tunable frequency difference between the two beams over a broad range of Raman shifts (100-3700 cm(-1)) by varying the temperature of a single nonlinear crystal. The near-infrared output (900-1300 nm) allows for deep penetration into thick samples and reduced nonlinear photodamage. Applications of this light source to in vivo cell and ex vivo tissue imaging are demonstrated.     

Focal shift of focused truncated random electromagnetic beams

The focal shifts of focused truncated random electromagnetic beams are investigated. Based on the complex Gaussian expansion method for a hard-edged aperture function, the analytical propagation formula of cross-spectral density matrix for a random electromagnetic beam focused by an optical system with a thin lens and a circular aperture is derived. The Fresnel numbers related with the beam and system parameters are defined and used to examine focal shifts. The dependence of the focal shifts on the different Fresnel numbers and polarization distribution are discussed in detail with numerical examples.     

衍射扭曲高斯-谢尔模型光束在远场的光谱移动和光谱开关

从部分相干光的传输理论出发,研究了被光阑衍射扭曲高斯-谢尔模型光束远场的光谱变化规律。结果表明,扭曲高斯-谢尔模型光束在远场也会出现光谱移动和光谱开关效应。与衍射高斯-谢尔模型光束情况相比,光谱移动和光谱开关效应不仅与光束空间相关性、截断参量和源光谱谱宽有关,光束的扭曲因子也会对衍射扭曲高斯-谢尔模型光束远场的光谱移动和光谱开关效应产生影响。通过数值计算结果详细讨论了光束扭曲因子影响衍射扭曲高斯-谢尔模型远场光谱的规律。     

Giant lateral shift of a light beam at the defect mode in one-dimensional photonic crystals

It is found that when a light beam is incident on a one-dimensional photonic crystal (1DPC) containing a defect layer, the lateral shifts of both the reflected and the transmitted beams are greatly enhanced near the defect mode of the 1DPC, whose location depends on the angles at a fixed frequency. The effect was studied by use of a Gaussian beam. The giant lateral displacement is due to the localization of the electromagnetic wave.     

Acoustic distortion in the ear canal. I. Cubic difference tones: effects of acute noise injury

Acoustic emissions in the form of cubic difference tones (CDT's), 2f1-f2, were measured in the ear canals of gerbils and cats. The state of the cochlea was manipulated by means of acute exposure to noise and was monitored with the aid of the whole-nerve response to tone pips. The resulting shifts in the levels of emissions generated by pairs of primary tones of equal intensity were then compared to the corresponding threshold shifts of the whole-nerve response across frequency. Data obtained from normal ears before injury indicate that the absolute thresholds of the whole-nerve responses across frequency are not necessarily good predictors of the absolute levels of CDT emissions generated by 70- and 80-dB SPL primaries. While high emission levels were often linked to low whole-nerve thresholds in pre-exposed ears, instances of animals with sensitive whole-nerve thresholds coupled with very weak emissions were also found. Conversely, animals with poor whole-nerve thresholds (shifted by up to 30 dB) could occasionally have high levels of emissions. After acute noise injury, however, the shifts of emission levels as a function of the center frequency of the primary-tone pair largely corresponded to the threshold shifts seen in the whole-nerve response. In other words, the temporary level shift of an acoustic emission largely reflected the acute change to a specific cochlear region associated with the primary frequencies.     

电介质膜对受抑全内反射结构中古斯-汉欣位移的增强

受抑全反射结构中,反射光束和透射光束的古斯汉欣(Goos-Hanchen)位移同时存在,对称双棱镜结构的受抑全反射古斯汉欣位移通常只有波长量级,在实验中很难测量。计算了在入射角大于棱镜与空气界面的临界角小于棱镜与薄膜界面临界角时,镀有电介质膜的对称双棱镜的受抑全反射过程中入射光束的反射系数和透射系数的复数表达式。利用稳态相位法计算得出透射光束和全反射光束的古斯汉欣位移。结果表明,反射光束和透射光束古斯汉欣位移量相同,与入射角大小、薄膜厚度以及空气层厚度有关。在入射角小于但接近棱镜与薄膜界面的临界角,薄膜厚度和空气层厚度一定时,古斯汉欣位移量共振增强达到波长的数百倍。