小宽带光谱色散匀滑光束传输特性研究

利用激光聚变研究中心自主开发的小宽带传输放大软件对小宽带光谱色散匀滑(SSD)光束进行具有"时间、空间、光谱分辨"的传输模拟,研究了SSD光束通过空间滤波器和自由空间的传输特性.给出了时间、空间、光谱的对应分布图,对比分析了SSD光束与非SSD光束的传输效应.研究结果对于合理设计SSD光束的发散角与空间滤波器小孔之间的匹配关系、选择适宜的调制频率,以及选择束匀滑色循环数等都有重要意义,同时对于光路设计特别是细光束自由传输距离的设计具有指导意义.     

高功率激光光束经颗粒污染后的近场衍射效应

采用高功率激光驱动器设计软件Laser designer数值研究了激光光束经颗粒污染后的近场衍射特性. 对颗粒污染所产生的调制进行了分类, 并详细分析了真空和石英玻璃中, 振幅调制型和位相调制型颗粒污染对光束近场的影响. 研究结果表明, 无论在真空或石英玻璃中, 位相型颗粒污染对高功率激光光束的近场调制影响较大. 且在石英玻璃中, 位相型硬边颗粒污染所引起的高功率激光光束小尺度自聚焦热像调制现象相当严重. 而由颗粒污染引起光束强区的横向偏移, 却在一定程度对小尺度自聚焦有抑制作用.     

The statistical fluctuation of a single-mode laser system driven by coloured pump noise with signal modulation and the quantum noise with cross-correlation between its real and imaginary parts

Using the linear approximation method, this paper studies the statistical property of a single-mode laser driven by both coloured pump noise with signal modulation and the quantum noise with cross-correlation between its real and imaginary parts, and calculates the steady-state mean normalized intensity fluctuation and intensity correlation time. It analyses the influences of the modulation signal, the net gain coefficient, the noise and its correlation form on the statistical fluctuation of the laser system respectively. It is found that the coloured pump noise modulated by the signal has a great suppressing action on the statistical fluctuation of the laser system; the pump noise self-correlation time and the specific frequency of modulation signal have the result that the statistical fluctuation tends to zero. Furthermore, the `colour' correlation of pump noise has much influences on the statistical fluctuation of the laser system. Increasing the intensity of pump noise will augment the statistical fluctuation of the laser system, but the intensity of quantum noise and the coefficient of cross-correlation between its real and imaginary parts have less influence on the statistical fluctuation of the laser system. Therefore, from the conclusions of this paper the statistical property can be known and a theoretical basis for steady operation and output of the laser system can be provided.     

Evolution of low-frequency noise passing through a spatial filter in a high power laser system

The theoretical model of spatial noise passing through a spatial filter is established in high power laser system under the small signal approximation. The transmission characteristic for a noise signal passing through spatial filters with different magnifications is analyzed by numerical simulation, according to the actual structure of the high power laser system. The results show that the spatial modulation period of low-frequency noise getting through the pinhole will be proportional to the magnification...     

Understanding speech in modulated interference: cochlear implant users and normal-hearing listeners

Many competing noises in real environments are modulated or fluctuating in level. Listeners with normal hearing are able to take advantage of temporal gaps in fluctuating maskers. Listeners with sensorineural hearing loss show less benefit from modulated maskers. Cochlear implant users may be more adversely affected by modulated maskers because of their limited spectral resolution and by their reliance on envelope-based signal-processing strategies of implant processors. The current study evaluated cochlear implant users' ability to understand sentences in the presence of modulated speech-shaped noise. Normal-hearing listeners served as a comparison group. Listeners repeated IEEE sentences in quiet, steady noise, and modulated noise maskers. Maskers were presented at varying signal-to-noise ratios (SNRs) at six modulation rates varying from 1 to 32 Hz. Results suggested that normal-hearing listeners obtain significant release from masking from modulated maskers, especially at 8-Hz masker modulation frequency. In contrast, cochlear implant users experience very little release from masking from modulated maskers. The data suggest, in fact, that they may show negative effects of modulated maskers at syllabic modulation rates (2-4 Hz). Similar patterns of results were obtained from implant listeners using three different devices with different speech-processor strategies. The lack of release from masking occurs in implant listeners independent of their device characteristics, and may be attributable to the nature of implant processing strategies and/or the lack of spectral detail in processed stimuli.     

采用光谱色散平滑和连续相位板实现靶面均匀辐照的实验研究

为了改善高功率激光装置的靶面辐照均匀性, 在神光-Ⅲ原型装置的一路光上开展了结合光谱色散平滑(SSD) 和连续相位板(CPP)的高通量实验研究．实验基于调制频率9.2 GHz的体相位调制器开展, 输出的相位调制脉冲光谱展宽稳定, 脉冲波形顶部剩余调制很小. 结果表明SSD色循环数为1时预放和主放各级空间滤波器过孔顺利, 包含焦斑95%能量的通量对比度由窄带时的1.71下降到加SSD和CPP时的0.47．三倍频光脉宽1 ns, 能量1115 J时, CPP和终端光学组件元件未见损伤．通过实验解决了在高功率激光装置上采用SSD和CPP进行靶面均匀辐照的若干关键技术, 为将其应用于物理实验奠定了坚实基础.     

采用透镜列阵与光谱色散改善三倍频小焦斑辐照均匀性

通过数值模拟研究了高功率激光驱动器的光束均匀辐照情况, 分析了三倍频激光辐照下小焦斑的光强分布及空间功率谱. 采用透镜列阵可降低光束近场畸变的影响并控制靶面焦斑的包络形状; 采用二维光谱色散匀滑技术可明显地消除焦斑内部大反衬度的强度调制, 且该方法在焦斑非常小的情况下也仍然有效. 大量的数值模拟结果表明, 光谱色散匀滑技术应用于尺寸不同的焦斑时, 其效果是有差异的. 对大焦斑而言, 光谱色散匀滑主要抹平其内部细密的条纹, 即消除中高空间频率范围的强度调制; 对小焦斑而言, 光谱色散匀滑主要消除中低空间频率范围的强度调制, 故将影响焦斑的整体包络形状. 本文所得的数值结果对间接驱动实验中激光束匀滑方案的选择和优化具有参考价值.     

干涉测量用半导体激光器驱动电源

设计了一种可调制的高稳定度半导体激光器驱动电路。该电路的直流稳定度高达1.5×10-5，输出电流在0～200mA内连续可调，长时间工作（12h以上）电流变化小于1μA；在直流基础上注入100kHz～300kHz的调制电流，其调制深度为0～100mA连续可调，可实现在激光干涉测量中对光波频率和光波强度的调制。将该电路驱动的光栅外腔半导体激光器和辅助温度控制电路应用于光干涉测量技术中，得到了功率稳定、波长单一的激光输出，解决了激光器的跳模现象，完成了对远距离微小振动（纳米量级）的测量。     

光谱色散匀滑对焦斑光强频谱的影响

基于焦斑光强的空间频谱分析方法,建立了光谱色散匀滑技术（SSD）对焦斑匀滑的影响模型,提出了控制焦斑空间频率调制的响应函数.揭示了SSD参量对空间频率响应函数的影响规律.理论推导和数值模拟结果都表明：SSD对焦斑光强的频率响应具有周期调制性质,调制特性和色散角、光谱形状等相关.一般地,光谱形状控制空间响应函数形状,色散角决定所能匀滑的最低频率,色循环数可改变响应函数特点,调整对空间低频和高频的匀滑效果.空间频率响应函数的特点为多维SSD应用提出了应用准则. 关键词： 光谱色散匀滑 空间频谱 焦斑 响应函数     

Spatial modulation of Gaussian laser beams

A technique is described for obtaining deep spatial modulation of laser beams. A phase modulator at one plane in the beam path can yield amplitude modulation at a later plane with little loss in power. Explicit formulae are derived for the field distributions of the modulated beams, and applications are considered.     

强度调制532 nm激光水下测距

激光水下探测在水下目标搜寻、资源勘探等领域具有重要的应用,而散射是激光水下探测面临的主要挑战.载波调制激光雷达具有抗散射、抗干扰的优点,本文利用自行研制的532 nm强度调制激光源,在3 m长的水箱中搭建激光水下探测系统. 532 nm激光源最大输出功率为2.56 W,强度调制范围为10.0 MHz—2.1 GHz,光束发散角约0.5 mrad.通过在水箱中添加氢氧化镁(Mg(OH)_2)粉末,测量了不同浑浊度下水的衰减系数.采用相位测距的方法,目标反射光的调制信号为探测信号,对激光源进行调制的电信号作为参考信号,利用相关运算获得激光的延时时间,进而可以获得水下目标的距离.最大调制频率为500 MHz时,实现了距离为4.3个衰减长度目标的探测,测距误差约12 cm.探测距离越远,测距误差越大,调制频率越高,测距精度越高.     

The influence of smoothing by spectral dispersion on the beam characteristics in the near field

The influences of SSD on the beam characteristics in the near held are investigated. Results snow that it the SSD parameters are increased, the laser intensity modulation increases while fluence modulation decreases, which is attributed to the temporal and spatial variation of the SSD pulse phase. The variations of intensity and fluence modulations with the SSD parameters are given. The simulation results are presented along with a method for choosing appropriate SSD parameters according to the variations and the requirements of applications.     

Experimental research on spatial filtering of deformed laser beam by transmitting volume Bragg grating

Using transmitting volume Bragg gratings (TVBG) as a basis,an experiment on one-dimensional spatial filtering of a deformed laser beam is designed.The deformed laser beam results from a He-Ne laser beam modulated by an amplitude modulation plate with a spatial frequency of 7.2 mm·1.Results show that when the central wave vector of the deformed beam satisfies the Bragg law of TVBG,the spatial profile of the -1st forward-diffracted order is similar to that of the undeformed He-Ne laser beam due to the TVBG with a spatial frequency selective bandwidth of less than 5.0 mm·1.The higher frequency components of the deformed beam are filtered out in the optical near field.Thus,the TVBG cleanup of the spatiallydeformed laser beam is realized experimentally.     

Femtosecond pulse laser scanning using Acousto-Optic Deflector

Acousto-optic modulation of the femtosecond laser beam has wide potential in various fields such as microscopic imaging, micromachining and optical storage because of its rapidity, high accuracy and good repeatability. This paper reviews the latest research on acousto-optic modulation of femtosecond laser, describes the effects of spatial and temporal dispersion of such lasers after acousto-optic modulation, introduces the theoretical outcomes on pulse width evolution of the Gaussian laser pulses after acousto-optic modulation, discusses the characteristics of contemporary dispersion-compensating techniques for the acousto-optic modulation of femtosecond laser, and finally refers to the challenges of this technique. Supported by the National Natural Science Foundation of China (Grant Nos. 60278017, 30328014, and 30370463) and the National Basic Research Program of China (Grant No. 2004CB520804)     

高斯白噪声相位调制的激光光谱展宽

激光器是现代光学中一种常用的光源,分析其谱宽特性对于激光的研究具有重要意义.由于高性能窄线宽激光的半高全宽通常在几十M Hz以内,难以直接满足宽光谱领域的应用需求,因而无法发挥其在成本和性能方面的优势,这在一定程度上限制了激光的发展.如果在激光内部直接进行调制实现线宽展宽,又会导致频率的严重漂移,破坏稳频特性.为了在激...     

Investigation of stimulated rotational Raman scattering of the high-power broadband laser with applied angular dispersion

Smoothing by spectral dispersion （SSD） leads to considerable improvement on laser-irradiation uniformity in far field for fusion lasers. Phase modulation in time and spectral angular dispersion （SAD） across the beam introduced by SSD will affect the stimulated rotational Raman scattering （SRRS） gain in the near field. This paper focuses on the influence of SAD on SRRS gain under different laser conditions. Results show that the SAD will aggravate the generation of SRRS when the laser initial additional phase is constant. On the contrary, the SAD can reduce the SRRS gain if appropriate SSD parameters are adopted when the laser initial additional phase is variable. SSD has a certain application prospect in SRRS suppression.     

On the Mechanism of Absorption and Restoration of Radiation Transmission in the Channel of Small-Scale Self-Focusing of a Short Laser Pulse in Neodymium Glass

Physics of the Solid State - When diagnosing a small-scale self-focusing (SSSF) channel of a 0.5-ns laser pulse with an intensity of 3–5 GW/cm2 in neodymium glasses, an absorption jump was...     

Hyperbolic heat conduction effects caused by temporally modulated laser pulses

In non-destructive testing (NDT), the intensity of laser pulses may be temporally modulated to improve the signal-to-noise ratio of ultrasonic signals. The purpose of this work is to determine the time scale on which hyperbolic heat conduction effects are significant in ultrasonic displacements and stresses for temporally modulated laser pulses, under the assumption that the heat wave speed equals the longitudinal wave speed. A one-dimensional model with a finite train of heat flux pulses is investigated. Using the Green and Lindsay model, it is found that the critical modulation frequency in steel, above which hyperbolic heat conduction effects will become signficant, is 2.26 GHz. A dimensionless factor is given to calculate critical modulation frequencies in other isotropic materials. The signal enhancement potential of temporal intensity modulation is apparent in the numerical results.     

基于光纤激光相控阵的空间激光通信系统

为了满足超远程和高速空间激光通信的需求,设计了一种以1.06 m光纤激光4支路相控阵作为发射源的通信系统结构。采用53 Mb/s、120 Mb/s、155 Mb/s速率调制的伪随机序列,实验了相控阵种子源和一条支路处调制的2种方案。结果表明:相同速率下,种子源调制的接收眼图质量较好,通过相位控制,可以进一步提高接收质量。实验中,研究了2条支路光分别调制的相控阵多载波特性,调制信号采用5/5.08 MHz及20/40 MHz两组正弦波,经光束合成和相位控制,在接收端观测到相应的频域信号。实验初步验证了光纤激光相控阵空间通信系统的可行性。