Formation of Hot Images in Laser Beams through a Self-defocusing Kerr Medium Slab

A nonlinear hot image is usually thought as of a special case of self-focusing, and thus occurs when a laser beam propagates through a slab of self-focusing medium. Here we show theoretically that a hot image may also be formed by a thin slab of self-defocusing medium. The physical origin for this hot image formation is akin to the in-line volume-phase holographic imaging due to the intensity-dependent refractive-index modulation of the self- defocusing medium. NumericM simulations confirm the theoretical prediction and further identify the dependence of the hot image on the beam power, the modulation depth of obscuration and the thickness of self-defocusing medium. The analysis presented here brings new insight into the physics of hot image formation in the high power laser system.     

GaN发光二极管表观电容极值分析

利用正向交流(ac)小信号方法对GaN发光二极管的电容-电压特性进行测量,可以观察到GaN发光二极管中的负电容现象。正向偏压越大,测试频率越低,负电容现象越明显。测量到的负电容现象是表象,不存在负电容;提出GaN发光二极管p-n结的结电容在特定的正向电压范围内等效于可变电容。分析可变电容对正向交流小信号响应得到:特定参数的可变电容使结电容电流相位落后于交流小信号电压相位π/2,使得在测量中表现为负电容。发现表观电容-正向电压曲线的极值点与理论模型相吻合,证明了该理论模型的正确性。     

发光二极管放电实验现象分析

利用直流电源对发光二极管(LED)的结电容充电,切断直流电源后对LED的电压-时间特性进行测量。当充电电压低于LED复合发光的门槛电压,LED的电压-时间特性与普通二极管的相似。当充电电压高于LED复合发光的门槛电压,首次观察到:开始放电的瞬间会出现一个快速下降过程,快速下降到门槛电压以下;LED上的电压越高,快速下降到的电压越低。对该现象进行分析,得到一些新的结论。当LED的正偏电压高于复合发光的门槛电压后,出现了注入到扩散区的非平衡载流子随正偏电压的提高而减小的现象,即dQ/du<0。     

Approximate analytical expressions of apertured broadband beams in the far field

The approximate analytical expressions of the apertured broadband beams in the far field with Gaussian and Laguerre-Gaussian spatial modes are presented.For the radially polarized Laguerre-Gaussian beam,the result reveals that the electromagnetic field in the far field is transverse magnetic.The influences of bandwidth(Γ) and truncation parameter(C 0) on the transverse intensity distribution of the Gaussian beam and on the energy flux distribution of radially polarized Laguerre-Gaussian beam are analysed.     

GaN发光二极管表观电阻极值分析

利用正向交流(ac)小信号方法对GaN发光二极管的电容-电压特性进行测量,可以观察到GaN发光二极管中的负电容现象。利用LED串联等效电路对表观电容和表观电阻进行了测量,表观电阻-正向电压曲线出现了一个极值点。通过对相关文献分析,提出负电容现象的根本原因是在较高的正偏下微分电容dQ/dU<0;推论出pn结的微分电容先随正向偏压的增大而急剧增大,当出现复合发光后随正向偏压的增大而减小,直到随正向偏压的增大而出现负值;正向偏置电压较大时,结电导电流的变化率根据I-V特性曲线取极大值,此时微分电容由于强复合效应已快速变小,表观电阻有极大值;得到了表观电阻极大值表达式。表观电阻与正向电压曲线的极值点与理论模型相吻合,证明了该理论模型的正确性。     

啁啾脉冲堆积宽带激光的时间与频谱特性分析

理论研究了啁啾堆积脉冲宽带激光时间与频谱特性与子脉冲参量的关系.结果表明,变换极限子脉冲堆积所得的脉冲平滑,频率啁啾型与时间啁啾型子脉冲堆积所得脉冲出现时间调制,平滑性变差,从频谱上看,前者出现单一谱线,而后者可看到多条谱线|随着子脉冲间延迟的增加,堆积脉冲由脉冲间干涉引起的时间调制增多,但幅度减弱,而频谱的谱线数量增加.所得结果可为适当地选择堆积脉冲参量以获得所需的时间与频谱形状的宽带激光提供参考.     

差分放大电路共模输出“异常”现象分析

从理论推导和计算机模拟仿真两方面论证了不完全对称的晶体管差分放大电路的共模 输出存在的各种可能性,解释了实验过程中存在的“异常”实验现象,并给出了改善实验 效果的方法与措施。     

被圆相位片衍射的空心高斯光束的远场矢量结构特征

基于麦克斯韦方程组解的矢量角谱表述与稳相法,在仅有远场近似的条件下,得到了被圆相位片衍射的空心高斯光束远场能流密度分布的解析表达式.结果表明,光束的总能流可以表示为自由项与附加项之和,其中附加项是相位片对能流分布的影响;总能流、自由项能流、附加项能流又可以分别表示为各自的TE项和TM项之和.利用所导出的公式,描绘了空心高斯光束的总能流、自由项能流、附加项能流及其各自的TE项与TM项能流的横截面分布图.讨论了截断参数与附加相移对纵向总能流分布的影响.     

A quasi-discrete Hankel transform for nonlinear beam propagation

We propose and implement a quasi-discrete Hankel transform algorithm based on Dini series expansion (DQDHT) in this paper. By making use of the property that the zero-order Bessel function derivative J' ₀(0)=0, the DQDHT can be used to calculate the values on the symmetry axis directly. In addition, except for the truncated treatment of the input function, no other approximation is made, thus the DQDHT satisfies the discrete Parseval theorem for energy conservation, implying that it has a high numerical accuracy. Further, we have performed several numerical tests. The test results show that the DQDHT has a very high numerical accuracy and keeps energy conservation even after thousands of times of repeating the transform either in a spatial domain or in a frequency domain. Finally, as an example, we have applied the DQDHT to the nonlinear propagation of a Gaussian beam through a Kerr medium system with cylindrical symmetry. The calculated results are found to be in excellent agreement with those based on the conventional 2D-FFT algorithm, while the simulation based on the proposed DQDHT takes much less computing time.     

啁啾脉冲堆积宽带激光非线性传输时域调制特性

为避免时域调制增长形成的高强度时间尖峰对光学元件的损坏,数值研究了啁啾堆积脉冲宽带激光非线性传输的时域调制增长规律。结果表明:对于正常色散介质,子脉冲间干涉引入的时间调制与外加调制随着传输距离的增加,不断振荡减弱;相反,对于反常色散介质,无外加时间噪声时,堆积子脉冲间的相互干涉引入的时间调制有微弱增长,但当传输距离进一步增加时,子脉冲间干涉引入的调制可以获得快速增长;而当外加非线性最快增长时间调制时,时间调制发生迅速增长,具有破坏威胁的时间尖峰易于形成。