A novel application of range-gated underwater laser imaging system (ULIS) in near-target turbid medium

Range-gated imaging system improves signal-to-backscattering noise ratio (SBR) by rejecting backscattered light from the target irradiance. This is achieved by synchronizing the arrival of pulsed target irradiance with the gating of an intensified camera. Witherspoon and Holloway (Ocean Optics X 1302 (1990) 414–420) indicated that the image quality of range-gated imaging system might be further improved by a delay in the camera gating towards the tail of reflected image temporal profile (RITP). This phenomena has, however, not been further elaborated. This paper extends on Witherspoon's observation. The MCRITP (Monte Carlo for RITP) algorithm is validated with theoretical and experimental results for medium attenuation coefficients of 0.26–5.90/m. By temporal convolution of the simulated results with outgoing Gaussian light pulse, Tail RITP region shows more reduction in the unwanted backscatter effects than the target intensity. This can be observed as an improvement in image contrast and a modified fidelity index (MF).     

热应力双折射介质中的光传输研究

计算了简化的平行平板中的温度和热应力分布,根据光弹性理论得到了介质内部折射率的改变,采用矢量微分算法模拟光线在介质中传输的情况,预测了接收面上的光强分布和条纹形状,程序输出图形和作为对照的光弹性实验的结果符合良好,证明了上述分析方法和光传输计算的正确性,为进一步完善光学系统成像质量提供参考. 关键词： 光弹性 应力双折射 光线追迹     

距离选通技术对后向散射抑制作用的理论分析

 从辐射度学的基本理论出发，建立了单CCD像元的距离选通激光主动成像模型。而后考虑成像质量，借助单CCD像元信噪比和图像均方差误差两个指标，阐明了距离选通技术是抑制后向散射、增大成像距离、提高图像质量的有效手段。最后从模型出发，导出了距离选通技术在激光主动成像中应用时一些关键参数的选取原则:(1) 尽可能减小激光脉冲宽度，以克服后向散射的影响；（2） 要能够达到尽可能远的观测距离，在激光功率一定的条件下，激光占空比越大越好；（3）在激光满足一定条件的情况下，脉冲频率越高越好。     

Propagation of light polarization in a birefringent medium: Exact analytic models

Driving the analogy between the coherent excitation of a two-state quantum system and the torque equation of motion, we present exact analytic solutions to different models for manipulation of polarization in birefringent medium. These models include the one-dimensional model, the Landau-Zener model, and the Demkov-Kunike model. We also give an example for robust, broadband manipulation of polarization by suitably tailoring the birefringence vector.     

Impact of echo broadening effect on active range-gated imaging

Analytical formulas and experimental proof of the echo broadening effect in active range-gated imaging, including atmospheric interference, currently exist. We investigate the impact of this effect on target detection. Our research demonstrates that the echo broadening effect affects the energy profile of the depth of view and collects only part of the signals of targets in head and tail zones. Under bad weather conditions, the effect weakens the signal-to-noise ratio (SNR) of images, especially in cases with large laser pulse width. Fortunately, by modifying the laser pulse width, the effect can be controlled. These results are valuable to the applications of active range-gated imaging.     

激光照明水下目标距离选通成像系统的研制

基于水下目标探测的应用需求,研制开发了一套激光水下距离选通成像系统。系统采用波长532 nm、最大单脉冲能量400 mJ的Nd:YAG脉冲激光器作为照明光源,采用最小选通门宽3 ns、像元10241024的ICCD相机作为门选通器件和图像记录器件,利用DG535型数字脉冲发生器作为精确延时和同步控制器件实现激光脉冲和ICCD相机选通门的同步以实现距离选通功能。利用该系统在某水库进行了水下目标探测实验,实验结果表明,该系统可在6.5倍的衰减长度上识别目标,在8倍衰减长度上发现目标。     

激光照明水下目标距离选通成像系统的研制

基于水下目标探测的应用需求,研制开发了一套激光水下距离选通成像系统。系统采用波长532 nm、最大单脉冲能量400 mJ的Nd:YAG脉冲激光器作为照明光源,采用最小选通门宽3 ns、像元10241024的ICCD相机作为门选通器件和图像记录器件,利用DG535型数字脉冲发生器作为精确延时和同步控制器件实现激光脉冲和ICCD相机选通门的同步以实现距离选通功能。利用该系统在某水库进行了水下目标探测实验,实验结果表明,该系统可在6.5倍的衰减长度上识别目标,在8倍衰减长度上发现目标。     

Effective dichroism in forward scattering by inhomogeneous birefringent medium

The Mueller matrix model for inhomogeneous medium characterized by simultaneous linear and circular birefringence for single scattering in the forward direction is derived. We find that the presence of inhomogeneity results in the occurrence of new polarization effects. In particular, forward scattering by inhomogeneous birefringent medium exhibits linear and circular dichroism. We show that in the case of weak depolarization (when Cloude's entropy is less than 0.5), the initial parameters for linear and circular birefringence can be obtained from deterministic Mueller matrix associated with the largest eigenvalue of Cloude's coherency matrix. Sample calculations are given for quartz.     

激光距离选通技术在微光成像系统中的应用

激光距离选通技术提高了微光条件下的远距离观测的视距,有效的克服了大气的后向散射对微光成像系统成像质量的影响。采用改进的同步控制技术解决了距离选通技术中激光器与摄像机的同步问题。实验结果表明,采用同步控制技术的距离选通技术对提高成像的质量和探测距离起到了关键的作用。     

双折射吸收非线性介质薄膜中倍频的产生

从二次谐波电场满足的波动方程出发,考虑到介质对二次谐波的吸收以及把完全边界条件应用于基频波和二次谐波在介质的入射面和出射面之间的反射效应,推导出了单光轴非线性薄膜介质中的二次谐波输出功率的计算式.结果表明：当采用复折射率的概念时,可以把非线性无吸收介质的二次谐波输出功率计算式应用到非线性吸收介质二次谐波输出功率计算式中去. 关键词： 非线性 倍频 二次谐波功率     

双折射吸收非线性介质薄膜中倍频的产生

从二次谐波电场满足的波动方程出发，考虑到介质对二次谐波的吸收以及把完全边界条件应用于基频波和二次谐波在介质的入射面和出射面之间的反射效应，推导出了单光轴非线性薄膜介质中的二次谐波输出功率的计算式.结果表明：当采用复折射率的概念时，可以把非线性无吸收介质的二次谐波输出功率计算式应用到非线性吸收介质二次谐波输出功率计算式中去.     

距离选通成像系统分辨力模拟测试装置的设计

距离选通成像系统采用的是脉冲激光主动照明与ICCD选通技术相结合的方法。为了在室内模拟测试在一定大气条件下距离选通成像系统的分辨力,设计了一套模拟测试装置。以辐射传输理论为基础,建立了大气距离选通成像系统后向散射光和信号光的模型。该模型给出了后向散射光和信号光在光电阴极面上照度随时间的变化规律。采用两个模拟光源,分别模拟了信号光和后向散射光。采用阴极面等效辐射照度法,给出了模拟光源功率的方法。以S 20光电阴极的像增强器与CCD的耦合为例,计算了模拟光源的功率。     

Interaction of waves with a birefringent medium moving with acceleration

Recent experiments demonstrated that the energy of a neutron traversing an accelerated sample of a refractive medium changes. Later, it was realized that such an accelerated-medium effect (AME) is quite a general phenomenon characteristic of waves and particles of different nature. This paper discusses some special features of the effect for a birefringent medium. In this case, AME shows quite new features. In neutron optics, where birefringence is due to the spin dependence of the refractive index, AME results in a nonstationary state with a precessing spin. In the case of the propagation of a two-flavor neutrino through an accelerated layer of matter, AME affects substantially the ensuing evolution of a neutrino flavor state as it propagates through a free space.     

3D imaging: wave front sensing utilizing a birefringent crystal

Utilizing a rotatable thin birefringent crystal and two polarizers, the three-dimensional (3D) wave front of any light wave can be measured conveniently. In first experiments phase-front distortions as small as 15 &mgr;m are detected with a dynamic range of 3 mm and a spatial resolution of 50 &mgr;m. Such a dynamic range and spatial resolution exceed the performance of conventional wave front sensors of, e.g., the Shack-Hartmann type, significantly. Furthermore, the new system is rather insensitive against mechanical instabilities in opposition to interferometric and holographic techniques.     

Optical bistability in plasmonic nanoparticles: Effect of size,shape and embedding medium

We theoretically investigate the optical bistability, which one input signal allows two possible outputs, from single spherical/cylindrical nanoparticles and also nanoshells in the frame work of quasi-static formalism. It is shown that the bistability behavior greatly depends on several parameters such as the nanoparticle size, material and the surrounding dielectric environment. We demonstrated the width of the bistability region and also the bistable threshold depends on the geometrical parameters, and can be tuned by adjusting the size of nanoparticle, the shell thickness and the dielectric constant of the embedding medium. It is also shown that the optical bistable behavior depends strongly on the shape of plasmonic nanoparticles and nanoshells. However, these dependences of optical bistability of spherical/cylindrical nanoparticles and nanoshells on changing of their geometrical parameters can be used for realize optical switching and sensing purposes.     

Light transmission in inhomogeneous birefringent medium as a function of propagation and observation angles

We use the Mueller matrix for inhomogeneous linear birefringent media derived in Savenkov et al. [Mueller-matrix model of an inhomogeneous linear birefringent medium: single scattering case. JQSRT 2007;106:475–86] to generate new results on the forward scattering of light using Cloude's coherence matrix method. We show that the intensity of scattered light (m₁₁) as a function of observation angle depends on the difference between refractive indices along the eigen polarizations resulting in intensity lost when unpolarized light propagates parallel to the optical axes compared to propagation orthogonal to the optical axis. For a given inhomogeneity (roughness), depolarization strongly depends on the direction of light propagation in the medium. The depolarization at 90° propagation angle is minimal for any value of inhomogeneity. Sample calculations are based on calcite.