【作者】 刘伯晗；

【导师】 张健；

【作者基本信息】 哈尔滨工业大学 ， 仪器科学与技术， 2007， 博士

【摘要】 目前,传统的基于几何光学的波前变换已经不能满足日益复杂的波前变换需要。如何灵活地改变入射光波的波前,使其满足不同领域的特殊需要,成为现代光学领域重要的研究内容之一。例如,如何根据需要改变激光束的能量分布和形态的问题就属于光学波前变换需要研究的课题。其应用背景就是形成一种全新的无惯量激光扫描技术,以克服传统机械扫描方法因自身固有机械惯量造成的缺陷,适应高精度,高响应,高集成度激光扫描的应用要求。液晶器件可通过外加电场使液晶分子变形,控制其等效折射率,从而改变光波通过液晶器件的波前。由于完全是弱电控制过程,不引入机械惯量,且可实现可编程控制。因此采用液晶空间光调制器,通过在线相位优化设计实现光束波形的快速可编程控制,是解决上述无惯量激光扫描问题的有效途径之一。液晶空间光调制器是一种可编程衍射元件,若希望一束平面波通过液晶空间光调制器产生的衍射场是我们所期望的光场分布,则在液晶空间光调制器上的相位分布必须满足一定的要求。然而,目前根据远场的光场分布反向求解衍射元件相位分布的相位恢复算法,因受自身缺陷的制约,其收敛速度很难做到相位实时恢复,所以目前利用衍射元件的波前变换一直停留在离线设计状态,国内也很少有研究者针对纯相位液晶空间光调制器进行波前实时变换的研究。本文主要研究基于液晶空间光调制器的纯相位调制器技术实现光波实时变换,讨论了在线相位恢复算法的收敛机理及改进方法,建立了二维激光束动态偏转系统,并在偏转系统基础上进行了空间目标阵列式激光束的动态跟瞄试验。研究目的是实现二维空间激光束动态可编程敏捷控制,验证纯相位液晶空间光调制器实现光波实时变换的可行性。研究的主要内容和取得的创新性成果如下:1.提出一种利用纯相位液晶空间光调制器实现波前可编程实时变换的新方法。该方法基于液晶可编程电控双折射特性和实时快速相位恢复算法,可将平面相干光实时变换成任意期望的衍射图形。突破了传统的离线相位恢复方法对波前变换的限制,使液晶空间光调制器成为能在线应用的可编程衍射元件,开辟了其新应用领域。2.提出一种伪随机相位编码初始化的改进相位恢复算法。该改进算法能按照统计概率、最大限度地将期望的幅值信息带入GS算法的初始相位值,从而避免因初始相位选择不当而陷入局部最小,以更快的速度逼近期望目标,增强了算法的鲁棒性。3.提出一种新颖的激光束搜索空间目标的方法,该方法由光束动态偏转系统产生阵列式激光束对空间动目标进行主动扫描,根据扫描摄像系统回馈的目标位置信息,自动集中并导引激光束锁定目标。跟踪试验结果显示,在200ms响应时间内,系统能完成扫描角度误差小于0.032°的追踪锁定任务。比传统的单光束扫描方式有更高的目标搜索捕获能力。更多还原

【Abstract】 Nowadays, with the increasing requirement of high precision, high response and high integration, the traditional optical scanning system is becoming to less competent in the applications of complicated wavefront transformation due to its mechanical inertia reason. How to change a laser beam into arbitrary beams is becoming to a critical problem in the wavefront transformation field.The effective refractive indices of liquid crystal devices can be changed by applying electrical volts on the Liquid Crystal (LC) cell. Because there is no mechanical inertia involved in the process of controlling LC refractive indices, the LC device acts as an agile and variable Diffractive Optical Elements (DOE). Therefore, it is a practical way to adopt Liquid Crystal-Spatial Light Modulator (LC-SLM) as the beam steering elements to generate programmable beam shapes and programmable patterns.When a plane wave is modulated by an LC-SLM and forms a certain expected diffractive pattern in the far field image plane, the phases distribution of the SLM pixels should satisfy some constraints. The phase constraints can be obtained by an optimal algorithm. However, for some inherent reasons, the phases retrieval algorithm is still in the state of off-line computing. It is difficult to realize the real-time phases retrieval by current algorithms, so there are few researchers who are studying the real-time wavefront transformation by phase-only LC-SLMs. It is necessary to develop a fast phase retrieval algorithm to realize the real-time wavefront transformation.In this thesis, a new type of dynamical beam steering system is designed with a phase-only LC-SLM, which has a high response time. The far-field arbitrary diffractive pattern approaching by non-mechanical laser beams is resolved by an in-line phases retrieval algorithm. Besides, on the base of our dynamical beam steering system, we design a moving objects tracing system with agile multiple beams pointing to the objects dynamically. The main contributions of this dissertation are as follows:Firstly, a new method is proposed which can perform the real-time wavefront transformation by a phase-only LC-SLM. This new method can transform a plane wave into arbitrary beams based on the liquid crystal theory of electrically controlled birefringence and the algorithm of real-time phase retrieval. The traditional viewpoint of off-line phase retrieval is broke through and a new field of using the LC-SLM as an in-line programming diffractive optical element is developed.Secondly, a modified fast phase retrieval algorithm is proposed, which adopts the pseudorandom phases encoding technology to initialize GS (Gerchberg–Saxton) algorithm. The modified algorithm is a robust and fast constringency one which has the merits of decreasing the sensitivity to initial phases and of decreasing the twinborn images effects.Thirdly, a new system of tracing moving objects by phased array beams is proposed and presented. A high speed camera is used to capture the moving objects information from the reflected image. According to the feedback information of the camer, the SLM generates another frame of diffractive pattern to focus the laser energy onto the objects dynamically. Results show that the tracing system has the position error of less than 0.032°and the response time is less than 200ms for one tacking frame. It is more capable of dynamically scanning and pointing the moving objects than the traditional scanning methods.更多还原