超短脉冲信噪比单次测量技术

【作者】 张东方；

【导师】 钱列加；

【作者基本信息】 复旦大学 ， 光学， 2009， 硕士

【摘要】 飞秒激光是一种以脉冲形式运转的激光,持续时间特别短,因而容易实现非常高的瞬时功率,极大地推动了超快光物理、飞秒化学、飞秒生物和非线性光学等前沿科学研究。飞秒激光的一个重要发展方向是建设高强度固体激光系统（聚焦光强达到相对论性或超相对论性）。近年来,随着宽带固体激光材料的广泛应用和啁啾脉冲放大（CPA）技术的日趋成熟,高强度激光在峰功率方面得到了长足的发展,已经实现了超过拍瓦PW(10¹⁵W)的功率指标。0目前,高强度固体激光尚未攻克的一个重大科学技术问题是聚焦光束的脉冲信噪比。当聚焦光强达到相对论性10¹⁹W/cm²(超相对论性～10²⁵W/cm²)时,脉冲信噪比必须高于10⁸(超相对论性光强时要求信噪比高于10¹⁴),从而控制噪声脉冲的光强不超过等离子体产生的阈值(～10¹¹W/cm²)。为解决脉冲信噪比这一重大技术难题,首先需要具备高动态范围信噪比单次测量技术,但现有技术尚未足以让我们能够单次测量10⁶。以上的脉冲信噪比。本文重点研究设计了新型脉冲信噪比单次测量装置,我们在很大程度上解决了现有技术的主要缺陷,将可能推动信噪比测量技术的发展,从而帮助解决脉冲信噪比这一重大技术难题。同时,信噪比测量需要应用二阶光学非线性作为测量的脉冲相关过程,如倍频（SHG）、和频（SFG）和光参量放大（OPA）过程。为此,本文也研究了准位相匹配（QPM）结构中的倍频过程,通过建立模型理论计算的方式,细致研究了QPM结构中非线性相移对SHG的影响,并对QPM结构设计作了讨论。本文主要进行了如下工作:一、研究了OPM结构中的倍频过程,包括级联非线性相移的作用以及OPM中畴结构的设计。QPM结构可支持应用最大的非线性极化矩阵元,可以人工设计频率转换过程的晶体相位匹配要求,已经被广泛地应用于倍频和级联非线性相移产生等过程中。我们在本文中将主要讨论了非线性相移在高强度激光倍频过程中的作用,同时对高强度激光倍频的QPM结构设计作了讨论。二、设计了一种新型的脉冲信噪比单次测量装置,解决了传统信噪比测量系统的一些缺陷,并实验得到更高的脉冲信噪比测量动态范围。随着高功率激光技术的发展,对测量技术的要求也越来越高。在本章节中主要是介绍了一种新型的脉冲信噪比单次测量方法。利用光纤阵列的方式解决了传统激光系统中的一些问题。光纤阵列组成的探测系统来代替传统单次测量方式中的CCD或光电二利管阵列,使用光纤衰减器来调节相关信号峰值的衰减量,避免了使用中性衰减片（NDF）,同时使系统可以使用低噪声高灵敏度的光电倍增管（PMT）进行单次测量。这种基于光纤阵列的串并转换的数据采集方式同时具有常规扫描测量方式的优势（除了信号的平均）。我们在实验室已经得到～2×10⁷:1的信噪比测量动态范围。三、提出一种基于粗细光束的脉冲信噪比测量方式。第二章主要针对信噪比的探测系统,作为第二章的延续,本章主要是介绍了另外一种信噪比测量方式,提出了一种新型的脉冲非线性相关信号产生的方法。本章节提出了一种基于粗细光束的信噪比测量方法,使对测量时间窗口宽度的影响由晶体的横向尺寸转移到纵向长度上。同时,由于细光束的应用,群速度失配（GVM）对最终测量结果的影响更小,可以得到更精确、分辨率更高的测量结果。并且在实验中很好地解决了波长简并非线性过程引发的光噪声和探测器响应波段等问题,实验上演示了～10⁸:1的动态测量范围。更多还原

【Abstract】 Femtosecond lasers are pulses that have very short duration and high instantaneous power.It has greatly enhanced the development of ultrafast-physics, femtosecond chemistry,biology,nonlinear optics and so on.One of the main aims of femtosecond lasers is to build high energy solid laser system with focused intensity reaching relativistic or super-relativistic.Recently,with the use of wide band solid laser materials and Chirp pulse amplification（CPA）,the technology of ultrafast lasers has experienced tremendous progress and power in excess of 10¹⁵W has been achieved.Currently,the contrast,defined by the ratio of the peak intensity of the main pulse to its background,especially in the leading part,is one of the most important parameters of a high-power laser pulse.When Focused intensity in excess of 10¹⁹W/cm²(super-relativistic intensity-10²⁵W/cm²),it is often critically important that these systems produce pulses of high temporal contrast(about 10⁸,＞10¹⁴ for super-relativistic intensity) so as to preclude preplasma and adversely affect(with intensity above 10¹⁰ W/cm²).In order to solve such important scientific problem,firstly we should have the high pulse contrast detecting technique.But current single-shot pulse contrast measurement only has a dynamic range of 10⁶:1.In this paper,we focus on designing of new pulse contrast measurement systems which may greatly resolve many of the defects in the current technologies and enhance its development.Meanwhile Quadratic(x^（2）) nonlinear in many pulse correlation process,such as second-harmonic generation（SHG）,sum-frequency generation（SFG） and optical-parametric amplification（OPA）,is also an important process in the pulse contrast measurement.So we also studied the SHG process in quasi-phase-matched（QPM） structures and the influence of relative-phase shift to SHG through our theoretical model analysis.This paper will include the following contents:1.Investigate the SHG process in QPM structures,which includes the influence of cascaded nonlinear phase shift and the design of domain structure. The scheme of quasi-phase-matching（QPM） may take the advantages of a larger nonlinear coefficient and noncritical phase-matching configuration by an engineerable design.QPM structures have been extensively studied for quadratic(x^（2）) nonlinear interactions.In this paper,we mainly talk about the influence of nonlinear phase shift and the design of QPM for better conversion efficiency in the high-intensity SHG process.2.We propose and demonstrate a fiber-array-based detection scheme for single-shot pulse contrast characterization which has resolved some defects in the traditional method and reached a higher dynamic range in experiment.With the development of high power laser system,the demand for pulse measurement technologies is much greater.In this chapter,a new single-shot pulse contrast measurement system is proposed.We use the fiber-array system to solve some problems in the traditional method.The fiber-array-based scheme eliminates the NDF necessary in the conventional method while allowing the use of a high-sensitivity PMT instead of the multielement photodiode array.The parallel to serial mapping by the fiber array forms a measurement configuration that is similar to the time-scanning mode except for the pulse averaging.The proof-of-principle experiments have shown a dynamic range of 2×10⁷:1 with sub-millijoule pulse energy in a SHG-DFG configuration.3.A new single-shot pulse measurement system based on thin-thick beam structure is proposed.Chapter 2 mainly discusses the detecting component of the measurement system. As a continuation of chapter 2,here we discuss another pulse contrast measurement system.This system uses the thin-thick beam structure to produce the pulse nonlinear correlation signal.The detection window duration depends on the crystal length instead of the width of the crystal.The influence of group-velocity-mismatch（GVM） can be lowered because of the thin beam and we can obtain much higher resolution. The problems of degeneration and limiting response bandwidth of detector have also been solved in experiment.A dynamic range of 10⁸:1 is shown in experiment.更多还原