Band Gaps of an Amorphous Photonic Materials

A new kind of amorphous photonic materials is presented.Both the simulated and experimental results show that although the disorder of the whole dielectric structure is strong,the amorphous photonic materials have two photonic gaps.This confirms that the short-range order is an essential factor for the formation of the photonic gaps.     

非对称进气双面离心压气机不同后弯角离心叶轮拓稳特性研究

以非对称进气双面离心压气机为研究对象,采用数值方法研究了两侧叶轮后弯角不同的双面离心压气机在稳定工作范围上存在的差异。通过改变两侧叶轮出口后弯角的大小,平衡两侧叶轮的流量分配,进而延迟工作模式的转变,抑制后方叶轮的提前失速,达到减小双面离心压气机失速流量的效果.首先采用理论计算的方式总结出了一种求取最佳叶轮组合的方法,其次选择了两侧叶轮后弯角相差20°,10°以及原始对称叶轮三个模型进行对比分析。结果表明,两种改型方案都能够减小双面离心压气机的失速流量。两侧叶轮后弯角相差10°的叶轮组合方案能够延缓工作模式转变,并使失速流量减小.两侧叶轮的后弯角相差20°则会使得拓稳范围更宽,最终会进入前方叶轮失速状态。     

获得1K级低温及其热分析

本文报道由液氦(~4Hc)获得1K级低温的减压降温装置技术及其热分析.实验结果已达到1.17 K.测量了实验杜瓦的轴向温度分布.已用于超流氦温度下复合材料导热系数的实验研究和氦λ点温度以下温度计的标定.     

硅光子晶体TE模式光下路分束器设计

设计了基于自准直效应的硅光子晶体TE模式13和12光下路分束器。13光下路分束器由4个分束镜组成,而12下路分束器由3个分束镜和1个反射镜组成。利用多光束干涉原理推导出光下路分束器各个出口的透射谱理论公式。通过选择合适的分束镜,可以得到不同分束比例的光下路分束器。对于13光下路分束器,设计了1∶1∶1和1∶2∶3两种分束比例;对于12光下路分束器,设计了1∶1和1∶2两种分束比例。再利用时域有限差分软件数值模拟了透射谱,其结果与理论设计一致。当下路波长为1550 nm时,13和12两种光下路分束器的大小均约为10 m 10 m,自由光谱区为36 nm,覆盖了整个光通信C波段。     

Investigation of transient two-photon excited fluorescence in biological tissue

The fluorescence power from biological tissue excited by a femtosecond laser pulse compared with excitation power does not appear to obey a simple quadratic relationship given by the steady non-linear theory.A more reliable analysis is developed based on transient two-photon absorption because the response time of two-photon absorption is longer than the width of a femtosecond pulse.Good agreement is obtained between the theoretical analysis and the experimental results of fluorescence power versus excitation power.This letter offers potential value to non-linear optics in biological tissues.     

高下路空气孔型光子晶体环形谐振腔的设计

提出一种基于二维正方晶格光子晶体空气孔型高下路效率的环形谐振腔,通过压缩线缺陷波导的宽度实现单模有效控制,同时讨论内围光子晶体列数对传输场的影响,然后运用二维时域有限差分方法数值分析了耦合强度及环区局部折射率调制对下路效率、品质因子以及下路波长等参量的影响.结果表明:当波导宽度为0.7个晶格常量,耦合强度为0个晶格常量,在信道波长为1 528.1nm时,下路效率为99%,品质因子Q为379;当耦合强度提高到1个晶格常量,下路波长稍微漂移为1 524.3nm,品质因子显著提高到1 397,而下路效率下降为89%.同时,下路波长会随着环区折射率的增加呈线性红移.     

HIRFL装置主动式点扫描束流配送系统的Monte Carlo模拟优化

为获得适用于HIRFL 装置主动式点扫描束流配送系统配送的碳离子束,利用Monte Carlo (MC) 工具SHIELD-HIT12A 研究了配送距离和微型脊形过滤器结构周期对治疗室等中心处束斑半高宽(FWHM) 和剂量平坦度的影响。模拟研究发现:束流配送距离越短,束斑FWHM越小,但剂量平坦度越差;微型脊形滤器结构周期是影响剂量平坦度的关键因素,周期越小,剂量平坦度越好。通过模拟研究得出:在HIRFL装置重离子治疗终端将真空窗设置在距等中心距离小于125 cm 时,采用结构周期为2 mm微型脊形过滤器可以满足主动式点扫描照射对束斑FWHM和剂量平坦度的要求。To obtain carbon ion beams suitable for the active spot scanning beam delivery system at the Heavy Ion Research Facility in Lanzhou (HIRFL), the Monte Carlo program SHIELD-HIT12A was used to study the influences of beam delivery distance and structure period of mini ridge filter on full width at the half maximum (FWHM) of beam spot and dose flatness at the isocenter of the treatment room. The present simulation study shows that the shorter was the beam delivery distance, the smaller was the FWHM of beam spot, but the worse was the dose flatness. The structure period of mini ridge filter was a key factor to account for the dose flatness at the isocenter. The smaller was the structure period of mini ridge filter, the better was the dose flatness. Based on the simulation results, we conclude that 2 mm structure period for a mini ridge filter statisfies the requirements on the FWHM of beam spot and dose flatness at the isocenter for the active spot scanning beam delivery system at HIRFL when the distance between the vacuum window and the isocenter is set shorter than 125 cm in the nozzle.     

Interleaving Gradient Magnetic Field Method for Diffusion Weighted Spectroscopy

Diffusion-weighted magnetic resonance spectroscopy （DWS） has considerable potential in clinical and research applications. However, it is seldom implemented in conventional magnetic resonance imaging （MRI） scanners due to the strict hardware requirements. We propose an interleaving gradient magnetic field （IGMF） method based on point resolved spectroscopy （PRESS）. Four interlaced powerful diffusion sensitive gradient magnetic fields are positioned around the two π refocusing rf pulses in the PRESS sequence. This method utilizes the interval time in the PRESS pulse sequence, doubles the duration time of the diffusion sensitive gradient magnetic field and decreases the detrimental effect of the induced eddy current. The results of theoretical analysis and experimental observation demonstrate that the IGMF method is suitable for conventional MRI scanners.     

Phase-sensitive fat suppression steady-state free procession sequence with phase correction

Robust and fast fat suppression is a challenge in balanced steady-state free precession （SSFP） magnetic resonance imaging. Although single-acquisition phase-sensitive SSFP can provide fat-suppressed images in short scan time, phase errors, especially spatially-dependent phase shift, caused by a variety of factors may result in misplacement of fat and water voxels. In this paper, a novel phase correction algorithm was used to calibrate those phase errors during image reconstruction. This algorithm corrects phase by region growing, employing both the magnitude and the phase information of image pixels. Phantom and in vivo imagings were performed to validate the technique. As a result, excellent fat-suppressed images were acquired by using single-acquisition phase-sensitive SSFP with phase correction.     

Self-Starting Passively Mode-Locked Er^3＋/Yb^3＋ Codoped Phosphate Glass All-Fibre Ring Laser

A self-starting, passively mode-locked all-fibre ring laser based on the homemade Er^3＋/Yb^3＋ codoped phosphate glass fibre is reported. Dual-pump sources were used to achieve the mode-locked operation in the experiment. The fundamental mode-locked pulse repetition rate is 10.67MHz. The pulse duration, assumed as the fit of hyperbolic secant pulse shape, is 5.82ps. We also observe the multi-pulse output of the fibre laser.