Ultrashort Pulse Signal To Noise Ratio Of A Single Measurement Technique

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.