RAMAN-ENHANCED POLARIZATION BEATS WITH PHASE-CONJUGATION GEOMETRY

The Raman-enhanced polarization beats (REPB) with phase-conjugation geometry in carbon disulfide are investigated using chaotic and phase-diffusion models. The overall accuracy of using REPB to measure the resonant frequency of Raman-active vibrational mode is determined by the relaxation rates of the Raman mode and the molecular-reorientational grating. Different stochastic models of the laser field only affect the sixth- and fourth-order coherence function. The difference between the REPB and the fourth-order coherence on ultrafast modulation spectroscopy is discussed as well from a physical viewpoint.     

差频瑞利增强极化拍的零差探测研究

基于马尔可夫随机场模型的理论给出了差频瑞利增强极化拍（REPB）的解析解，并且研究了三种马尔可夫随机场模型下场关联效应和瑞利型共振增强效应对拍频信号的影响，指出了振幅涨落和位相涨落在时域和频域的不同作用.差频REPB信号展现出了时域对称特性，信号在三种马尔可夫随机场模型下的差别随着带宽的增大逐渐消失.     

Quantum coherence dynamics of a three-level atom in a two-mode field

The correlated dynamics of a three-level atom resonantly coupled to an electromagnetic cavity field is calculated (Λ, V, and L models). A diagrammatic representation of quantum dynamics is proposed for these models. As an example, Λ-atom dynamics is examined to demonstrate how the use of conventional von Neumann’s reduction leads to internal decoherence (disentanglement-induced decoherence) and to the absence of atomic coherence under multiphoton excitation. The predicted absence of atomic coherence is inconsistent with characteristics of an experimentally observed atom-photon entangled state. It is shown that the correlated reduction of a composite quantum system proposed in [18] qualitatively predicts the occurrence and evolution of atomic coherence under multiphoton excitation if a seed coherence is introduced into any subsystem (the atom or a cavity mode).     

Prevention of decoherence by two femtosecond chirped pulse trains

We study the vibrational energy relaxation (VER) and collisional dephasing as channels of coherence loss in a vibrational mode that is selectively excited using chirped pulse adiabatic passage in the Raman configuration. Based on the dressed state picture analysis we propose a method to reduce decoherence using femtosecond chirped pulse trains. When applied with a period close to the VER time, the pulse trains allow one to sustain high coherence in the selected vibrational mode.     

Temporal coherence of sound transmissions in deep water revisited

This paper examines the signal coherence loss due to internal waves in deep water in terms of the signal coherence time and compare to data reported in the literature over the past 35 years. The coherence time of the early raylike arrivals was previously modeled by Munk and Zachariasen ["Sound propagation through a fluctuating stratified ocean: Theory and observation," J. Acoust. Soc. Am. 59, 818-838 (1976)] using the supereikonal approximation and by Dashen et al. ["Path-integral treatment of acoustic mutual coherence functions for arrays in a sound channel," J. Acoust. Soc. Am. 77, 1716-1722 (1985)] using the path integral approach; a -1 [corrected] power frequency dependence and a -1/2 [corrected] power range dependence were predicted. Recent data in shallow water in downward refractive environments with internal waves suggested that the signal coherence time of the mode arrivals follows a -3/2 power frequency dependence and a -1/2 power range dependence. Since the temporal coherence of the acoustic signal is related to the temporal coherence of the internal waves, based on the observation that the (linear) internal waves in deep and shallow waters have a similar frequency spectrum, it is argued that the modelike arrivals in deep water should exhibit a similar frequency dependence in deep and shallow waters. This argument is supported by a brute-force application of the path integral to mode arrivals based on the WKB relation between the ray and mode. It is found that the data are consistent with the -3/2 power frequency dependence but more data are needed to further test the hypothesis.     

Coherence properties of individual femtosecond pulses of an x-ray free-electron laser

Measurements of the spatial and temporal coherence of single, femtosecond x-ray pulses generated by the first hard x-ray free-electron laser, the Linac Coherent Light Source, are presented. Single-shot measurements were performed at 780?eV x-ray photon energy using apertures containing double pinholes in "diffract-and-destroy" mode. We determined a coherence length of 17 μm in the vertical direction, which is approximately the size of the focused Linac Coherent Light Source beam in the same direction. The analysis of the diffraction patterns produced by the pinholes with the largest separation yields an estimate of the temporal coherence time of 0.55?fs. We find that the total degree of transverse coherence is 56% and that the x-ray pulses are adequately described by two transverse coherent modes in each direction. This leads us to the conclusion that 78% of the total power is contained in the dominant mode.     

多模Ar^＋激光时间相干性与输出功率的关系

本文就多纵模Ar~+激光时间相干性(即相干长度)与输出功率的关系进行了实验观测,得到了相干长度与输出功率关系近似为双曲线函数的实验结果,并根据简单的Smith模型,对多纵模激光的时间相干性与输出功率进行了理论分析与数值计算.研究结果表明,理论与实验结果基本吻合.     

Coherence vortices of partially coherent beams in the far field

Based on the propagation law of cross-spectral density function, studied in this paper are the coherence vortices of partially coherent, quasi-monochromatic singular beams with Gaussian envelope and Schell-model correlator in the far field, where our main attention is paid to the evolution of far-field coherence vortices into intensity vortices of fully coherent beams. The results show that, although there are usually no zeros of intensity in partially coherent beams with Gaussian envelope and Schell-model correlator, zeros of spectral degree of coherence exist. The coherence vortices of spectral degree of coherence depend on the relative coherence length, mode index and positions of pairs of points. If a point and mode index are kept fixed, the position of coherence vortices changes with the increase of the relative coherence length. For the low coherent case there is a circular phase dislocation. In the coherent limit coherence vortices become intensity vortices of fully coherent Laguerre--Gaussian beams.     

用于干涉测量的短相干固体激光器光谱纵模特性

对固体激光器的光谱纵模特性与复相干度的关系进行了理论分析,推导出了在任意纵模数下的光程差与复相干度的关系式。通过比较窄谱宽激光器与谱宽较宽的测量用普通固体激光器在不同纵模数下的复相干度特性,发现固体激光器谱宽较宽,复相干度衰减较快,不同纵模数下的衰减速度和周期不同,纵模数越多复相干度震荡衰减越快,不会出现周期性的高相干区域。根据理论分析搭建了一台用于干涉测量的短相干多纵模固体激光器,并对其光谱纵模特性和相干长度进行了测量,实验结果与计算结果一致。     

截断平顶光束的广义M2因子和相干模分解

针对Li提出的平顶光束的新描述模型，分别采用广义二阶矩方法和矩形函数的复高斯函数展 开方法，推导出平顶光束通过硬边光阑的广义光束传输M²因子和模相关系数的解析表 达式.以此为基础，对截断平顶光束的模相关、模结构以及相干模分解进行了分析，建立了 截断平顶光束的相干模分解理论. 关键词： 截断平顶光束 2因子')" href="#">广义M²因子 模相关系数 相干模分解     

Effect of coherence property of incident light on prism waveguide coupling

The accurate reflection coefficient of prism waveguide coupler that takes coherence property of incident light into account is obtained at first. Based on the reflection coefficient, the effect of coherence property of incident light on prism coupling is discussed in this work. It is shown that extra damping in waveguide caused by the coherence property is larger for high-order resonant mode than that for low-order mode. For 100 fs Gaussian pulses incidence, the reflected spectra at different resonant modes are obtained theoretically and experimentally, which show that coherence property has a strong effect on prism coupling at high-order modes.     

Scintillations of partially coherent Laguerre Gaussian beams

Scintillations of Laguerre–Gaussian (LG) beams for weak atmospheric turbulence conditions are derived for on-axis receiver positions by using Huygens–Fresnel (HF) method in semi-analytic fashion. Numerical evaluations indicate that at the fully coherent limit, higher values of radial mode numbers will give rise to more scintillations, at medium and low partial coherence levels, particularly at longer propagation distances, scintillations will fall against rises in radial mode numbers. At small source sizes, the scintillations of LG beams having full coherence will initially rise, reaching saturation at large source sizes. For LG beams with low partial coherence levels, a steady fall toward the larger source sizes is observed. Partially coherent beams of medium levels generally exhibit a rising trend toward the large source sizes, also changing the respective positions of the related curves. Beams of low coherence levels will be less affected by the variations in the refractive index structure constant.     

Spatial coherence in the high-frequency limit

Two expressions for the spatial coherence of acoustic signals scattered by a randomly rough pressure-release surface using composite-roughness theory are presented. One model is the conventional result, obtained by splitting the surface spectrum. The second model includes diffractive corrections which in principle remove the first model's dependence on choice of spectral cut-off wavenumber. We find that both models produce outcomes that do not differ significantly from one another, particularly for very high frequencies, for any reasonable choice of spectral wavenumber cut-off. Thus, for the purpose of coherence computations, the diffractive corrections are unnecessary in the high frequency/roughness regime. We show, as well, that in this regime the composite roughness models agree with Kirchhoff integral formulations for the coherence.     

Coherent fluorescence emission by using hybrid photonic–plasmonic crystals

The spatial and temporal coherence of the fluorescence emission controlled by a quasi‐two‐dimensional hybrid photonic–plasmonic crystal structure covered with a thin fluorescent‐molecular‐doped dielectric film is investigated experimentally. A simple theoretical model to describe how a confined quasi‐two‐dimensional optical mode may induce coherent fluorescence emission is also presented. Concerning the spatial coherence, it is experimentally observed that the coherence area in the plane of the light source is in excess of 49 μm², which results in enhanced directional fluorescence emission. Concerning temporal coherence, the obtained coherence time is 4 times longer than that of the normal fluorescence emission in vacuum. Moreover, a Young's double‐slit interference experiment is performed to directly confirm the spatially coherent emission. This smoking gun proof of spatial coherence is reported here for the first time for the optical‐mode‐modified emission.     

Experimental deterministic coherence resonance

We demonstrate coherence resonance in a dynamical system without external noise. The experimental evidence is reported in the low frequency fluctuations of a chaotic diode laser with optical feedback. The phenomenon is also verified numerically using the Lang-Kobayashi equations for a single solitary mode laser, without noise terms. Fast deterministic dynamics plays the role of an effective exciting noise, narrowing the resonance in the autonomous slow power drop cycles of the laser. This new result is the natural extension of deterministic stochastic resonance and noise induced coherence resonance predicted and observed in recent years.     

Coherence and intensity moments of laser light

Taking the modal behaviours of the laser field into account, the coherence properties of laser light can be characterized by mode coherence functions (MCFs) and mode coherence coefficients (MCCs). Changes in laser light properties due to optical operations can be described by transformation laws for the MCFs and the MCCs. The transformation laws as well as transformation matrices for propagation in free space, phase transformations, spatial amplitude modulations and aperturizing are given. Furthermore, intensity distributions and intensity moments in the spatial and in the spatial frequency domains are expressed in terms of the MCCs. General expressions for the beam quality factors,M ²s, obtained using the intensity moments and some useful expressions for intensity moments of higher order are discussed.     

Injection locking semiconductor lasers with phase conjugate feedback

Phase conjugate injection locking is shown to couple coherently two semiconductor lasers through a double phase conjugate mirror geometry. The mutual coherence and stability of the lock band are characterized from the visibility and relative intensity noise measurements for various levels of phase conjugate coupling and frequency detunings. Side mode phase conjugate injection locking is demonstrated by high mutual coherence with visibility 0.93 and detunings up to nine longitudinal mode spacings at injection levels of −22 dB. Central mode phase conjugate injection locking with high mutual coherence is also possible for feedback levels of −36 dB and exhibiting an asymmetric stable locking region of 350 MHz bandwidth within the locking band. The phase locking is unstable though, the reasons for which are discussed. Increased phase conjugate feedback extends the lock band to greater detunings and asymmetry as in direct injection locking with only a small region showing high mutual coherence. Intensity noise measurements are also discussed.     

Kerr 介质中三能级原子与双模场非共振相互作用的量子统计性质

研究了Kerr介质中双模场与V型三能级原子的非共振相互作用. 对Cauchy-Schwartz不等式的研究表明,在两模场失谐量相等(δ₁=δ₂)条件下,当初始场强ξ一定而两模场的光子数差q减小时,场的非经典相关程度加强;q一定而ξ增大时,场的非经典相关程度减弱. 当Δδ=|δ₂-δ₁|≠0且q和ξ的取值一定时,随介质常量x的增大,光场两模间的非经典相关程度减弱,且Δδ的大小对这种非经典相关程度的减弱有重要影响. 对场的 关键词： Kerr介质 非共振相互作用 Cauchy-Schwartz不等式 场的二阶关联函数     

计数率和分辨时间对光场统计性质测量的影响——单探测器直接测量的实验分析

利用工作在盖革(Geiger)模式下的单光子探测器(single-photon-counting module, SPCM)，采取单个探测器直接对光子计数的方法，对相干光场及热光场的光子数统计性质进行了测量和分析. 通过改变计数率，调整分辨时间，系统地研究了光场二阶相干度测量值受实验条件的影响. 结果表明，在综合考虑系统中的各种因素对测量影响的情况下，通过选择合适的测试条件，可以利用单个单光子探测器直接探测的方法快速确定一个待测光场的二阶相干度. 实验表明在实测计数率为109 kc/s，分辨时间范围为2⁸ns—2¹²ns的条件下，该系统能很好地揭示相干光场和热光场的光子统计性质. 关键词： 光子统计 单光子探测器 二阶相干度     

Eigenmode analysis of phased-coupled VCSEL arrays using spatial coherence measurements

We apply the modal coherence theory to evaluate the spatial mode structure of a 2×2 phase-coupled array of vertical cavity surface emitting lasers (VCSELs). The eigenmode structure is extracted for different pump currents by measuring the degree of spatial coherence of all VCSEL pairs in the array. The results reveal the impact of optical disorder and spatial hole burning on the modal discrimination. The approach is useful more generally for the evaluation of spatial mode content of other laser array.