复杂传播路径条件下激光光强起伏特性研究

对复杂地形情况下实际湍流大气中的激光强度起伏进行了系统的实验观测,分析了光强起伏统计特征的周日及全年变化规律。结果发现：复杂地形情况下实际湍流大气中的激光光强起伏,不论是概率密度分布还是其功率谱等统计特征均与理论上的偏离很大。     

城市路径下激光光强起伏特性测量及链路余量估算

 在城市环境下进行了3.5 km的激光大气传输实验。实验中对光强起伏和到达角起伏进行了同步测量，分析了接收光强起伏的统计特性以及传输路径上大气折射率结构常数的特性。基于实验结果，对自由空间光通信中不同闪烁指数下的衰落冗余以及不同探测阈值下的衰落概率进行了估算，从而为空间光通信系统的设计提供可靠的实验基础。     

SPGD算法光束净化中光强起伏对校正效果的影响研究

尽管无波前传感自适应光学技术可不受闪烁效应等波前畸变条件的限制,能够有效地补偿高能激光的波前畸变以提高光束质量,但是激光器光强起伏和噪声干扰对校正效果却存在严重的影响。分析了光强起伏和噪声干扰对随机并行梯度下降(SPGD)算法光束净化效果的影响,并进行了相关实验研究。理论分析表明,在光强起伏频率大于等于算法扰动频率时,系统性能随着光强起伏增加而降低;在低频光强起伏时,SPGD算法的抗干扰能力较强。为消除光强起伏对校正效果的影响,提出采用桶中功率作为系统性能评价函数的光束净化方案,理论分析和实验结果都表明,该方案能够有效地消除光强起伏和噪声干扰对校正效果的影响,实现接近校正极限的理想校正效果。     

振幅起伏对自适应光学系统的限制

本文讨论了补偿前以及经柔形镜对位相畸变完全校正后的光强分布和斯特列尔比,分析了振幅起伏对自适应光学技术的基本限制。结果表明:振幅起伏的影响最严重时补偿后的中心光强仅为衍射极限的一半,斯特列尔比的极限值在最低时为0.5。     

大气湍流对激光通信系统的影响

从分析激光在大气湍流场中的传输方程出发,忽略系统中的其它噪声,仅考虑由大气喘流引起的系统误码率,讨论了激光信号在传输过程中的振幅起伏以及强度起伏;推导出由大气湍流引起的光强起伏和系统误码率的关系,结果表明:在弱起伏条件下,对于系统误码率为10－9以下的要求,光强起伏应小于0.67;随着湍流强度C2n的增大,误码率增加很快;采用长波长的激光进行传输可以有效地降低系统误码率.     

激光大气闪烁的分形分析

 采用分形理论分析了激光大气闪烁的统计特征。研究结果表明：在弱起伏条件下，激光大气闪烁的分形维和奇异性随光强起伏的增强而增大，而其长期相关性则减小；不同Fresnel尺度下具有相同闪烁指数的激光大气闪烁的分形特征存在着明显的差别；在强起伏条件下，有限的数据中尚未发现分形维有饱和现象，因此可以用来描述激光大气闪烁的强度。     

多光束发射对湍流效应下激光传输的影响

大气湍流引起的光强起伏会导致大气闪烁、光束漂移、到达角起伏和波前畸变等效应.多光束发射技术是克服激光大气传输中光强起伏的有效途径之一.以单光束、双光束、四光束以及八光束发射为例,通过计算峰值光强、环围功率半径以及像散参数等光束质量参数,得到单光束和多光束大气传输的光强起伏特性.结果表明:随着光束数目的增加,峰值光强显著...     

兰州地区光强分布及其起伏特性研究

在兰州地区夜间进行了链路长为610m的激光传输实验,研究了高斯激光在不同天气条件下的光强分布及其起伏特性.采用三维伪彩色变换方法,分析了光斑光强的空间分布规律,得到了光斑主瓣光强服从类高斯分布,并按照晴天、多云天、阴天的顺序,光斑几何中心位置附近光强分布的陡峭度依次减小,而其衰减则依次增大的结论.同时,利用实测光强值分析了大气闪烁指数,结果表明:晴天、雨后晴天和阴天下的闪烁指数分别为0.225 4、0.189 2、0.188 8,这说明晴天下的光强起伏大于雨后晴天和阴天的,且均为弱起伏.通过对归一化光强的频数分布进行非线性拟合得到了光强的概率密度分布,它们均服从对数正态分布,其中晴天下的概率密度曲线的拟合优度更是达到了0.997 50.     

泵浦光强起伏对SBS反射率的影响

 在忽略介质吸收和其它非线性效应的假设条件下，从理论上分析了泵浦光强起伏对受激布里渊散射反射率的影响。当泵浦光强为阈值光强的1.9倍时，反射率的稳定性接近泵浦光的稳定性，当泵浦光强大于阈值光强1.9倍时，反射率的稳定性优于泵浦光的稳定性。当泵浦光强大于阈值光强5倍时，反射率的统计起伏小于泵浦光强起伏的2.4%。     

兰州地区夜间光强起伏特性实验

在兰州地区夜间进行了链路长为610m的激光传输实验,研究了晴、阴、沙尘、雨夹雪天气下光强起伏的统计特性.采用闪烁法测量了不同天气下的大气折射率结构常数C_n~2,分析其最大值、最小值、均值和标准差,得到实验期间C_n~2值的范围为1.06×10~(-15) m~(-2/3)~1.05×10~(-13) m~(-2/3),均属于中等湍流;而且晴天和雨夹雪天气下C_n~2值以及起伏程度均大于阴天和沙尘天的值.同时,利用实测光强值分析了Rytov方差,结果表明:晴、雨夹雪、沙尘天气下光强起伏大部分时间属于弱起伏,有小部分时间跨入中等起伏区,而阴天下光强起伏均属于弱起伏,且晴天和雨夹雪天气下光强起伏大于阴天和沙尘天.通过对光强频数分布进行对数正态、Gamma-Gamma和指数威布尔非线性拟合,不同天气下光强起伏概率分布最为接近指数威布尔分布,其拟合优度均大于0.989 95,而在弱到中等起伏区,Gamma-Gamma分布拟合效果优于对数正态分布.     

A laser feedback interferometer with an oscillating feedback mirror

A method is proposed to solve the problem of direction discrimination for laser feedback interferometers.By vibrating the feedback mirror with a small-amplitude and high-frequency sine wave,laser intensity is modulated accordingly.The modulation amplitude can be extracted using a phase sensitive detector(PSD).When the feedback mirror moves,the PSD output shows a quasi-sine waveform similar to a laser intensity interference fringe but with a phase difference of approximately ±π/2.If the movement direction of the feedback mirror changes,the phase difference sign reverses.Therefore,the laser feedback interferometer offers a potential application in displacement measurement with a resolution of 1/8 wavelength and in-time direction discrimination.Without using optical components such as polarization beam splitters and wave plates,the interferometer is very simple,easy to align,and less costly.     

Optical resonators with inhomogeneous mirrors

It is shown that the possibility of changing a laser light beam intensity distribution exists when the output mirror transmittance depends on the transverse plane coordinates. Methods of manufacture of inhomogeneously transmitting laser mirrors with constant phase of the reflected beam over the whole surface are described. Two examples of application of such systems are considered: one for obtaining a laser beam with constant intensity distribution in the transverse plane, the other for single-segment energy extraction from high-order transverse modes. The total laser output power is shown to be almost the same as for use of a homogeneous output mirror.     

激光回馈波片位相延迟测量的误差源及消除方法

激光回馈法是一种新型的波片位相延迟测量方法。将波片放置在激光器与回馈镜之间,可使激光回馈波形产生偏振90°旋转（即跳变）现象,而两偏振态所占的周期比（占空比）与波片的位相延迟相关。通过测量占空比即可由计算机自动给出波片位相延迟。采用双向扫描回馈镜,2次获得并测出占空比,这样可以很好地消除两偏振态损耗的波动造成的测量误差,提高了该方法长期测量的稳定性。测量的重复性达到0.5°。该方法结构简单,在线测量精度高,满足工业化生产的需要。     

腔调谐过程中激光器中两垂直偏振光的回馈特性

腔调谐过程中同时对HeNe激光器进行光回馈，设定回馈镜驱动信号的周期远小于腔调谐信号的周期，此时便获得腔调谐过程中谐振腔中两垂直偏振光的回馈信号. 回馈的形式分为仅垂直光回馈、仅平行光回馈以及两光同时回馈三种情况. 当两光同时回馈时，两模式间的模竞争较弱，两光回馈曲线有一定的相位差，该相位差小于180°，两光总体波动幅度变化趋势相反. 当仅单偏振光回馈时，两模式间的模竞争较强，两光回馈曲线始终反相，而且两光波动幅度变化趋势相同，此时可以将两光强度信号相减以实现成倍提高回馈信号幅度，从而提高系统灵敏度. 尤其仅垂直光回馈时，两光各自光强波动幅值都较大，并且在较大的增益曲线范围内保持光强波动幅值无明显变化，该特性可用于提高回馈系统的抗干扰能力，减少因激光功率漂移而引起的错误计数. 关键词： 自混合干涉 光回馈 模竞争 位移测量     

Mode profiles in resonators with a near 90° roof-top mirror

The transverse modes of a laser resonator comprising a near 90° roof-top mirror and a feedback mirror are studied theoretically. We relate this configuration to an almost plane roof-top resonator and apply the Fox and Li technique to compute the lowest-order symmetric and antisymmetric modes. The effects of perturbations to the vertex angle of the roof-top mirror, misalignments of the feedback mirror and curved feedback mirrors are examined.     

Coherent beam combination of broad-area laser diode array using off-axis external cavity with double feedback

In this letter, we demonstrate coherent beam combination of laser diode array using the external Talbot cavity with double feedback. The double feedback elements consist of grating and high-reflection plane mirror. Compared with single high-reflection plane mirror feedback, the external Talbot cavity with double feedback reduce the number of interference strips in the far-field pattern and narrow spectral line-width of the laser diode array. The results indicate that the application of the external Talbot cavity with double feedback produces a clear far-field interference pattern. In addition, line-width is reduced to 0.15 nm full-width at half-maximum (FWHM).     

Modes competition in a dual modes HeNe laser with optical feedback

The optical feedback characteristics in a short cavity dual modes HeNe laser are studied under different modes strength condition. Modes competition can be obviously observed during optical feedback. The whole laser intensity curve has intensity branches and its leaning direction is related to the moving direction of the external feedback mirror. There are two cross points at different voltage levels between the two orthogonally polarized lights in a period of the optical feedback fringe and the order that the cross points come out is different when the feedback mirror moves at different direction which can be used for direction discrimination. Sometimes there are two peaks of the same size in a period of the optical feedback curve which can be used to improve the resolution of an optical feedback system.     

Backscattering from different objects in turbulent media

Abstract

The connection between diffraction characteristics of the scatterer and distribution of average backscattered intensity of a spherical wave is considered. In experiments with an ‘infinite’ plane mirror it is shown that the distribution of average backscattered intensity coincides with the correlation function of the intensity fluctuation of a virtual point source located at the mirror and observed from the real source plane. Non-monotonic dependence (with a minimum at the Fresnel number of scattered mirror?1) between the enhancement factor and the size of reflected mirror is observed in experiments.     

Fringe Locking in a Laser Diode Interferometer by Optical Feedback During Modulation of Injection Current

A fringe locking phenomenon in a two beam interferometer using a semiconductor laser subject to optical feedback was, whose injection current is modulated. When a path difference of the interferometer is sufficient, fringes taken by a charge coupled device camera are seen to be stationary and the rms fluctuations of fringe phase is reduced to as low as 0.2ϖ radians from more than 8ϖ radians without the optical feedback. The rms phase fluctuation is independent of frequency and amplitude of the current modulation. The fringe locking has also been observed in the presence of both injection current modulation and piezoelectric transducer mirror vibration.     

非即变相位共轭反馈对半导体激光器动态特性的影响

从四波混频产生相位共轭的物理原因出发,定义了相位共轭镜（PCM）的响应时间．建立起非即变相位共轭反馈条件下半导体激光器的外腔模型。以响应时间及频率失调为参变量,对其分岔及噪声等动态行为进行数值分析。结果表明,不考虑噪声影响时,增加相位共轭镜响应时间会使混沌带出现的次数和范围得到较大的抑制,当响应时间增大到1．5ns时,混沌带消失,半导体激光器保持稳定的单周期状态;考虑噪声影响后,随若响应时间的相对强度噪声（RIN）可减小几dB甚至十几dB,产生突变需要的反馈量也增大一个数量级以上,且其频谱的峰值向高频方向移动;另外,由于共轭反馈引起的频率失调低于半导体激光器激射频率3个数量级以上．它只对分岔特性有影响．对相对强度噪声的影响几乎为零。