Lasers for ICF with a Controllable Function of Mutual Coherence of Radiation

The current status of laser thermonuclear fusion research in the leading world scientific centers is characterized by the development of superhigh-power multi-channel laser facilities of megajoule pulse-energy level. The development of such laser installations operating in the pulse-repetition mode with a large number of laser beams, which are necessary for high-symmetry irradiation of a spherical thermonuclear target, is an extremely difficult physical and engineering problem. The concept of a special laser with a controllable function of mutual coherence of radiation is proposed. The studies performed demonstrate that a laser based on such a principle has a number of advantages as compared to the conventional schemes of lasers. In particular, the optical scheme of the laser is significantly simplified, and the cost of the output-energy unit is reduced by several times. The influence of radiation coherence on the homogeneity of the thermonuclear target irradiation is analyzed. The feasibility of suppressing the small-scale self-focusing without application of spatial filtration is shown. A module of the laser facility has been triggered to check the validity of the principles proposed for constructing a laser driver for power stations, and the first experimental results are reported. The possibility of controlling the coherence of laser beams used in ICF experiments without violation of the laser--target system matching is demonstrated, as well as controlling the distribution of the laser radiation intensity in the lens focus.     

The effect of the spontaneously generated coherence on the dynamical behaviors of the dispersion and the absorption

This paper investigates the effects of the spontaneously generated coherence (SGC) and incoherent pumping field on the dynamical behaviors of the dispersion and the absorption. Using a three-level Λ-type atomic medium, the influence of the relative phase between the applied fields on the transient dispersion and absorption is discussed. It is shown that the phase dependence of the dispersion and the absorption not only depend on the existence of SGC, but they depend on the existence of the incoherent pumping field as well. We show that the medium can be used as an optical switch in which the propagation of the laser pulse can be controlled by another laser field.     

Effects of energy spread on brightness and coherence of undulator sources

The (spectral) brightness for partially transverse coherent sources such as synchrotron radiation and free‐electron laser sources can be defined as the maximum of the Wigner distribution. Then, the brightness includes information on both coherence and wavefront characteristics of the radiation field. For undulator sources, it is customary to approximate the single‐electron electric field at resonance with a Gaussian beam, leading to great simplifications. Attempts to account for the modified spatial and angular profile of the undulator radiation in the presence of detuning due to energy spread, currently build on the simplified brightness expression derived under the assumption of Gaussian beams. The influence of energy spread on undulator radiation properties is becoming important in view of diffraction‐limited rings with ultralow emittance coming on‐line. Here the effects of energy spread on the brightness of undulator radiation at resonance are discussed, as well as relevant relations with coherence properties.     

Spectral linewidth of inversionless and Raman lasers in V-type three-level systems

A theoretical analysis of the spectral linewidth of V-type inversionless and Raman lasers is presented. First, we examine the effects of the atomic coherence between dressed states and the Autler-Townes splitting on the linewidth. It is demonstrated that near above threshold, the V inversionless laser has a narrower linewidth than that of the two-level laser. Instead of the dressed coherence, it is the Autler-Townes splitting that is responsible for the linewidth reduction though the dressed coherence determines the laser gain. Next, we explore the effects of the generated laser intensity on the linewidth. It is shown that the linewidths of the V inversionless and Raman lasers follow the usual 1/I decrease for smaller laser intensity I, but a slower decrease than 1/I for larger laser intensity. For the V Raman laser, even more surprisingly, with the laser intensity increasing, the linewidth appreciably increases as well. As a result, well above threshold, the V inversionless and Raman lasers may have a larger linewidth than that of the two-level laser. Finally, a comparison is made between the V lasers and the Λ lasers. It is found that the linewidth of the Λ inversionless laser shows a fast 1/I ² decay under optimum conditions. Received 25 October 1999 and Received in final form 10 March 2000     

Atomic coherence effects in the ion-trap laser

The influence of atomic coherence effects on the statistical and spectral properties of the ion-trap laser is investigated. Various pump configurations are considered for a realistic level scheme using a Ca ion. Compared to previous suggestions, 50% more output is obtained.     

Research for Signal Power Spectrum Distribution in Laser Doppler Rotational Speed Measurement

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Phase-noise-compensated optical frequency domain reflectometry with measurement range beyond laser coherence length realized using concatenative reference method

A novel type of optical frequency domain reflectometry with a measurement range much longer than the laser coherence length is proposed and experimentally demonstrated. To reduce the influence of laser phase noise, the measurement signal is compensated by using reference signals generated from a single auxiliary interferometer supported by a newly proposed compensation process. The compensation is accomplished numerically with a computer for each section of the delay fiber length in an auxiliary interferometer after only one data acquisition. By using the proposed technique, it is confirmed experimentally that the laser phase noise is well compensated even beyond the coherence length.     

Echo-wave scattering characteristics of the diffuse target in the slant atmospheric turbulence

The extended Huygens–Fresnel principle is utilized to make analysis of the received intensity for laser beam propagation through the atmospheric turbulence in the slant path. For a diffuse target, the effects of the turbulence on the statistical parameters such as the mutual coherence function and the mean intensity are studied in detail. The influence of the incident wave radius and the distance of the incident beam on these parameters are discussed.     

Coherence effects in surface roughness induced by vacuum ultraviolet F2 laser ablation

We analyze statistical fluctuations in the pulse-to-pulse spatial fluence distribution of a highly multimode F2 laser beam and the influence that coherence effects have on the attainable smoothness of an ablated surface. The magnitude of fluctuations is predicted to be a few percent, with associated roughness increasing as m1/2, where m is the number of ablation pulses. By use of a white-light optical interferometer, measurements have been made of roughness induced on N-BK7 glass surfaces ablated with a 157-nm laser, and reasonable agreement with predictions based on this roughening mechanism has been found.     

Molecular dynamics study of the stability of a carbon nanotube atop a catalytic nanoparticle

The effect of spontaneously generated coherence (SGC) on the quantum heat engine (QHE) consisting of a laser system is studied in terms of its dynamical evolution and the generation of coherences. The QHE is coupled to the two thermal photon reservoirs, a squeezed thermal bath as well as to a cavity mode. The coherence associated with the transition interacting with squeezed reservoir and the average thermal photon number of the hot (as well as cold) reservoir shows a non monotonous behavior between them. The dynamics along with generated coherences of the system and the laser power emitted depend sensitively on the hot, cold and squeezed reservoir parameters.     

部分相干光通过强湍流对通信系统误码率的影响

为研究部分相干光通过强湍流对系统误码率的影响,借助对激光在大气湍流场中的传输方程进行解析求解(忽略系统中其他噪声,仅考虑由大气湍流引起的系统误码率),得到不同湍流内尺度、传输激光波长和光源相干参数条件下,系统误码率和传输距离的关系.结果表明:在强湍流条件下,当发射天线数目达到一定时,随着传输距离的增加,系统误码率逐渐增大,但增大到一定程度后趋于饱和;光源相干参数越大,系统误码率越低;湍流内尺度越大,系统误码率越高;传输激光波长的变化对系统误码率无明显影响.     

激光空间相干性对照明均匀性的影响

 研究了激光相干性对照明均匀性的影响，为照明激光器的选择提供了理论参考。其中，部分相干高斯光束分解为模式间相互独立的厄米-高斯光束的迭加。采用相位屏的近似处理方法对激光通过大气湍流的传输进行计算模拟。数值模拟的结果表明：当照明光束的空间相干性降低时，其照明均匀度逐步提高。因此对于照明激光器而言，选择空间相干性较差的激光器对其照明均匀度更加有益。     

Mutual coherence of supercontinuum pulses collinearly generated in bulk media

We report the collinear generation of white light pulses in bulk media by phase-locked and time-delayed intense laser pulses. The mutual phase coherence of the pump pulses is fully transferred to the two supercontinua, which thus show a highly-modulated two-pulse spectrum extending from all of the visible down to the near-infrared. We have investigated the effects of changing the relative delay between the pulses as well as the role of imbalances in the pump energy, and we have found that, at least in a limited energy interval, the mutual phase coherence of the two white-light pulses is surprisingly robust.     

混沌光场光子统计分布及二阶相干度的分析与测量

利用通信波段双通道单光子探测器,采用Hanbury Brown-Twiss关联测量方案,理论分析并实验测量了光反馈半导体激光器产生的混沌光场的光子统计分布及不同混沌状态光场的二阶相干度.通过对混沌光场二阶相干度g~((2))(τ)的理论分析,得出随着延迟时间和相干时间的变化,其与相干光、热光及单光子态的二阶相干度可明显区分并呈现出不同分布.同时实验上产生了频谱宽度6.7 GHz的混沌光场,测量了不同光子数分布的结果,并用高斯随机分布、泊松分布、玻色-爱因斯坦分布对光子数分布进行理论拟合,发现随着入射平均光子数的增加,光子数分布从玻色-爱因斯坦分布过渡到泊松分布,但整个过程都与高斯随机分布符合较好,且光场的二阶相干度g~((2))(0)由2降至1.通过改变偏置电流(I=1.0Ith-2.0Ith)和反馈强度(0—10%),实验上研究了混沌光场由低频起伏到相干塌陷的过程中不同状态宏观动力学特性与二阶相干度的对应关系.结果表明:混沌光场在此过程中始终呈现出明显的聚束效应,并在频谱宽度最大时达到最强;同时给出了光子计数测量中聚束效应减弱的物理原因.实验表明该系统及方法能很好地揭示不同状态混沌光场的光子统计特性.     

激光聚变驱动器的光束相干性及其控制:回顾与展望

高功率强激光与物质相互作用蕴藏着丰富的非线性效应,激光聚变驱动器的光束具有高度相干性,它在光束传输过程中极大地凸显了这种效应,并不可避免地制约着激光功率的提升和激光能量的有效利用。回顾激光聚变驱动器的发展史,在提升激光输出能力的主线外,还存在一条与光束相干性做斗争的暗线贯穿其中。以激光与物质相互作用为牵引,从高功率强激光传输中非线性效应抑制和激光等离子体相互作用的抑制两方面回顾了激光聚变驱动器光束相干性的控制现状,并针对潜在需求,展望了未来高功率激光发展的创新技术。     

特殊关联结构光束的大气传输研究进展

近年来,光场调控逐渐成为光学领域的热点研究课题,光场相干性调控可以引发许多新颖物理效应。其中,通过相干性调控得到的特殊关联结构光束不仅展现出奇特的传输特性,而且可以有效地降低大气湍流引起的光强退化、光束漂移、光强闪烁和退偏振等负面效应。因此,特殊关联结构光束在自由空间光通信领域具有重要的应用前景。本文回顾了特殊关联结构光束的构建基础理论和大气传输研究方法及其发展历程,举例展示近些年典型特殊关联结构光束的大气传输研究成果。     

Strong reduction of the degree of spatial coherence of a laser beam propagating through a preformed plasma

A strong reduction of the spatial coherence of a laser beam after its propagation through a plasma has been measured using a Fresnel biprism interferometer. The laser beam was diffraction limited; the coherence width was reduced from 40 mm in vacuum down to a few mm with the plasma. Numerical results based on a paraxial model exhibit a coherence degree close to the experimental one; they also prove the importance of taking into account the nonlocal transport effects in numerical simulations for such plasma conditions.     

Transverse spatial coherence of a transient nickellike silver soft-x-ray laser pumped by a single picosecond laser pulse

The degree of spatial coherence in the direction perpendicular to the target surface is reported for a transient nickellike silver x-ray laser at 13.9 nm. An x-ray laser plasma column was produced by irradiating a slab silver target with a single shaped picosecond laser pulse with energy less than 3 J. Young's double-slit method was applied to measure the fringe visibility as a function of the slit separation for different target lengths. The diameter of the equivalent incoherent source and the coherence radius of the output radiation were determined as well.     

激光光源线宽对外差探测性能的影响

本文根据统计理论分析了激光线宽对外差探测结果的影响, 并讨论了激光线宽对杨氏干涉条纹对比度的影响. 本文基于解析结果做了数值仿真, 所得结果表明激光线宽增加时, 外差探测方式仍可以检测到中频信号, 但在线宽较大时, 受噪声影响无法准确地提取到中频频率. 为验证理论分析结果, 使用线宽为1 MHz的激光光源进行了8.1 km外场实验, 实验结果与数值分析结论一致, 即不会因为线宽增加而无法检测到中频信号. 文中所得结论对于外差探测光源的选择有重要指导意义, 因此根据测量目标的特性和测量要求, 可按照文中结论评估光源的线宽指标.     

Coherent transition radiation from thin targets irradiated by high intensity laser pulses

A theoretical examination on coherent transition radiations (CTR) from the surface of thin solid density target irradiated by high intensity laser is presented. The theory is extended to consider the expansion dynamics of thin foils. The motion of target surfaces leads to the modulation on the temporal structure of micro bunches in the electron beam as well as the spectrum of CTR. The spectral shifts of radiation are owing to the enhancement of electron bunch separation and the relativistic Doppler effects. The radiation power distribution is strongly affected by the temporal coherence of electron beam structure, so thus the electron temperature and velocity dispersions. With these effects accounted for, the spectral properties of coherent transition radiation can provide insights into the expansion of thin foil targets irradiated by intense laser pulse as well as the fast electron transport through it.