【作者】 朱时军；

【导师】 蔡阳健；

【作者基本信息】 苏州大学 ， 光学， 2014， 博士

【摘要】 激光束的传输和变换是激光物理和激光应用研究的重要基础课题。近几十年的研究发现，在某些领域，如：自由空间光通讯、惯性约束核聚变以及材料处理等领域，高相干性激光会容易带来散斑和更强的衍射。适当降低激光束的相干性，不仅能保持激光束的一般特性，还能减低光学元件以及湍流介质对激光束的不利影响。以往人们都是把光当做完全相干光或完全非相干光来研究。严格的理论表明，所有的光场都是部分相干场，利用部分相干理论处理激光束的传输是最理想的。相干和偏振是光场的两个基本的特性，而通常用的标量理论则忽略了光束的偏振特性。近些年的研究表明，标量部分相干光理论在表征一些实际光束的时候，会丢失偏振信息。2003年，Wolf总结了以往的研究成果，并提出了部分相干部分偏振统一理论，该理论明确指出，光场的相干和偏振这两个特性是紧密联系的，部分相干矢量光束可以用交叉谱密度矩阵来表征。随后的理论和实验研究也证明了光束的相干特性和偏振特性是相互影响，相互联系的。越来越多的研究发现，具有特殊形状的部分相干光束，如空心光束、平顶光束等，在很多领域具有重要的实用价值。因此，部分相干光束的合理整形显得非常有意义。我们在实验上利用相位元件、多模光纤以及液晶空间光调制器产生了部分相干空心光束、平顶光束、涡旋光束并测量了这些光束的光参数。携带涡旋相位的光子具有轨道角动量，其轨道角动量的大小由拓扑荷数所决定，在光镊、光捕获、冷原子以及自由光通讯方面有重要应用价值。除了涡旋相位，带有扭曲相位的部分相干光束同样也可以具有轨道角动量。利用张量方法，得到了部分相干扭曲光束经过象散光学元件调制后的轨道角动量流表达式，计算结果显示，携带扭曲相位的部分相干光束轨道角动量会受象散光学元件调制。实验产生了部分相干各项同性高斯-谢尔模(GSM)光束，并通过测量光束在两个不同传输距离处的束腰宽度和相干长度，确定了产生光束的波前曲率半径。另外，还实验产生了部分相干电磁高斯-谢尔模(EGSM)光束，并利用光场的四阶关联测量了光束的参数。1986年，Wolf发现了部分相干光在不满足“定标率”时，即使在自由空间传输，光谱也会发生频移，这一现象也成为“关联诱导光谱频移”。光谱频移是部分相干光的一个很重要的特性，在很多领域，如光谱测量、医疗诊断等具有重要应用。我们利用多模光纤以及径向偏振转换器产生了部分相干空心光束、平顶光束、径向偏振光束并测量了这些光束的聚焦光谱频移特性。基于部分相干偏振统一理论，研究了部分相干电磁光束经过复杂象散光学系统、高斯谐振腔、非线性介质、人体组织后的光参数特性。特别是课题组提出的张量方法，为研究了部分相干光束在谐振腔内的传输提供了更为简便的方法。这些理论研究可以帮助人们深入了解各种部分相干光束的传输整形特点，为寻找实际需要的部分相干光束提供了重要依据。部分相干光束在自由空间光通讯中的研究受到了广泛关注。利用张量方法、Wigner分布函数法以及二阶矩研究了部分相干扭曲GSM光束和EGSM光束在大气湍流中传输演化特性，包括光束的光场分布、光谱频移以及M2因子等特性。研究发现，初始光参数对光束的传输有较大的影响，通过选取合适的初始光束，可以有效的降低大气湍流对光信号的影响。径向偏振光束在信息加密、高密度存储、超分辨测量、超分辨成像、材料加工以及粒子捕获等领域有重要的应用。基于象散光学系统中的广义Collins公式，研究了部分相干径向偏振光束经过象散系统后的光场分布，偏振度以及相干度。研究发现可以利用象散元件来调制部分相干径向光束的统计特性。此外，还从实验上研究了部分相干径向偏振光的杨氏双缝干涉实验，研究发现初始光束的相干性对远场干涉图像、相干度以及偏振度会产生显著地影响。更多还原

【Abstract】 The propagation and transformation of laser beams play an important role in laserphysics and laser applications. In the past decades, more and more studies have found thatlaser beams with lower coherence are more useful than general laser beams in free-spaceoptical communications, inertial confinement fusion and laser processing. Low coherentlaser beams not only reserve the general characteristics of laser beams, but also can reducedisadvantages caused by random turbulent medium and optical elements.Usually, people treat the light as completely coherent light and completely incoherentlight to investigate them. Strictly theory indicates that all light beams are partially coherentlight beams and the best way to deal with the laser beams is the partially coherent theory.Coherence and polarization are two basic properties of light field. The scalar theory weoften used ignores the polarization characteristics of light beams. In the past decades,scalar theory of partially coherent beams can’t completely characterize any practical beams,because of the polarization properties is lost. In2003, Wolf summarized previous studiesand gave out the partially coherent and partially polarized unified theory. This theoryclearly pointed out that coherence and polarization were closely related to each other, andpartially coherent electromagnetic beams can be characterized with the cross-spectraldensity(CSD). After then, lots of theoretical and experimental studies have shown that thecoherence and polarization properties of light beams affect each other.More and more studies have found that partially coherent beam of special shapes,such as hollow beams, flat-topped beams have important practical value in many areas. So,reasonable shaping of partially coherent beams are becoming very meaningful. With thehelp of phase elements, multi-mode fiber(MMF) and liquid crystal spatial light modulator, we experimentally generated partially coherent hollow beams, flat-topped beams, vortexbeams and measured the beam parameters.It’s well known that a light beam carrying vortex phase has orbital angularmomentum(OAM) determined by its topological charge, and vortex beam has importantapplications in optical tweezers, optical trapping, atom optics and free-space opticalcommunications. Besides the vortex phase, a general partially coherent beam carrying atwist phase proposed by Simon et al., may have OAM, as well. We derive the generalexpression for the OAM flux of an astigmatic partially coherent beam carrying twist phase(i.e., twisted anisotropic Gaussian Schell-model (TAGSM) beam) propagating through anastigmatic ABCD optical system. The numerical results show that we can modulate theOAM of a partially coherent beam by varying the parameter of the cylindrical thin lens. Weexperimentally produced partially coherent isotropic Gaussian-Schell model beam(GSM)and determined the radius of curvature by measuring the transverse beam widths and thetransverse coherence widths at two different planes. Moreover, we report the experimentalgeneration of an electromagnetic Gaussian-Schell-model (EGSM) beam and measure thebeam parameters based on the technique for measuring the fourth-order correlationfunction.In1986, it is revealed by Wolf that the spectrum of a partially coherent beamgenerally changes on propagation, even in free space, unless the degree of coherencesatisfies a certain scaling law. This phenomenon is also called “correlation-induced spectralshift”. Spectral shift is an important property of partially coherent beams and can be foundimportant applications in spectroscopy and medical diagnostic. We experimentallygenerated partially coherent hollow beams, flat-topped beams and radially polarized beamsby using MMF and radial polarization converter(RPC). Furthermore, we experimentallymeasured the spectral changes of such beam focused by a thin lens.Based on the unified theory of coherence and polarization, we investigated the beamparameters of partially coherent electromagnetic beams propagation through complexastigmatic optical system, Gaussian cavities, nonlinear media and human tissue. Especially, with the help of the tensor method proposed by our group before, the investigation ofpartially coherent beams passing through cavities became very convenient. All of thesetheoretical studies will help us in-depth understanding the propagation properties ofvarious partially coherent beams and finding the right partially coherent beams we need.Partially coherent beams have attracted much more attention in application offree-space optical communications. By using a tensor method, Wigner distribution functionand second-order moments, we studied the properties of spectral intensity, spectral shiftand the M2-factor of partially coherent TGSM beam and EGSM beam propagation throughturbulent atmosphere. Studies have shown that initial beam parameters have great impacton the properties of output beam. By choosing appropriate initial beam can effectivelyreduce the disadvantages of atmospheric turbulence bring to the optical signal.Radially polarized beams have been found useful for information encryption,high-density storage, super-resolution measurement, super-resolution imaging, materialthermal processing and particle trapping. Based on the generalized Collins formula of theastigmatic optical system, we analyzed the focusing properties of spectral intensity, degreeof polarization and degree of coherence of partially coherent radially polarized beam. It isdemonstrated that the astigmatism of the optical system can be used for modulating thestatistical properties of a focused partially coherent radially polarized beam. In addition,we also carry out theoretical and experimental study of Young’s double-slit interference ofpartially coherent radially polarized beam. It is shown that the initial coherencesignificantly affect the far-field interference pattern, degree of coherence and degree ofpolarization.更多还原