Group-delay Dispersion in Double-prism Pair and Limitation in Broadband Laser Pulses

A general expression of group-delay dispersion is obtained without any approximation for a pair of double prisms with an arbitrary apex angle. This expression also includes the change of dispersion resulting from change in insertion of the prism material into the beam by translating prism-pairs. The high-order dispersion can be calculated by means of this expression. The limitation for generation of negative group-delay dispersion by use of prism-pairs is presented for ultrashort laser pulses with broadband spectrum.     

Group-delay Dispersion in Double-prism Pair and Limitation in Broadband Laser Pulses

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色散阶变光纤正常色散区啁啾演变特性之讨论

利用傅里叶变换法得到了色散阶变光纤正常色散区群速度色散导致啁啾的解析表达式;而且,采用数值法模拟了色散阶变光纤正常色散区啁啾演变过程。结果表明:色散阶变光纤中啁啾以色散缓变光纤中啁啾为中心跳跃式变化,但远小于常规光纤中啁啾。选择合适入射脉冲初始啁啾会使净啁啾等于零,从而形成暗孤子。     

The coefficient of material dispersion of a photonic crystal as a Fourier series

Material dispersion in a generic photonic crystal is investigated by deriving a mathematical expression in terms of a Fourier series for the coefficient of this type of dispersion. From the above formulation, the variation upon wavelength of material dispersion is discussed.     

Beam spot size evolution of Gaussian femtosecond pulses after angular dispersion

Beam spot size evolution of spatially Gaussian femtosecond laser pulses after angular dispersion is analyzed and verified with experiments. A general analytical expression for beam spot size at arbitrary propagation distances is obtained, which indicates that beam spot size evolution after angular dispersion is determined by the direct interaction of spectral lateral walk-off by angular dispersion and the intrinsic divergence of the Gaussian beam. This work reveals insights into the propagation of Gaussian femtosecond laser pulses and beams after angular dispersion and may be important for the generation and application of femtosecond laser pulses.     

受激拉曼散射对非线性介质波导的时空不稳定性影响

基于非线性包络方程我们研究了平面非线性介质波导中的时空不稳定性,得到了有拉曼散射效应情形下不稳定性调制的增益谱的表达式.结果表明在正常和反常两种群速度色散情形下,拉曼散射效应都会导致出现新的不稳定性区域,并且使原有的每一个谱分量的增益上升从而使谱的范围扩大.并通过分析反常色散情况下四种不同函数分布的非线性色散介质系数对不稳定性增益谱的影响,得到呈双曲函数分布的非线性色散介质最为理想.     

Improvement of dispersion tolerance using wavelength-interleaving and forward error correction

In this paper, we first derived the BER expression of an optical channel with chromatic dispersion impairment. The derived BER expression takes into account the influence of pulse spreading, transmitter rise/fall times, receiver Q-factor and noises. It is simpler and hence easier to use than many existing channel BER models. Then we proposed a wavelength-interleaved FEC scheme to mitigate the chromatic dispersion impairment in optical transmission system and analyzed its decoding performance. The minimum wavelength separation required for such a wavelength-interleaving FEC system to obtain the highest coding gain is also determined. Based on this design, the chromatic dispersion tolerance of the proposed system with 2-wavelength and 4-wavelength-interleaving are shown to improve by about 13% and 22%, respectively, over a non-interleaved system. In essence, wavelength-interleaving averages out the performances of a set of channels experiencing higher and lower dispersion, hence the interleaved channels have better performance than the non-interleaved channels experiencing higher dispersion, and worse performance than the non-interleaved channels experiencing lower dispersion. However, since WDM systems are typically designed based on the worst-case channel, the overall system performance is improved by wavelength-interleaving.     

Analysis of dispersion compensation for position-dependence in externally modulated CATV lightwave systems by using chirped fiber grating

The dispersion compensation characteristics of the chirped fiber grating (CFG) for different dispersion compensation positions are analyzed in externally modulated cable television (CATV) lightwave system and the analytic expression of the composite second order (CSO) distortion is derived. The analyses give a reasonable explanation for the position-dependent effect of CFG dispersion compensator, which was found in practical systems. Moreover, the theoretical result is also verified by an experiment. It is believed that the theory will be helpful in designing optical CATV fiber links with nodes at proper positions both for intensity amplification and dispersion compensation.     

极端相对论快电子分布等离子体中横振荡色散关系

本文对各向同性极端相对论快电子呈幂律分布等离子体中横振荡色散关系进行了解析和数值研究.得到长波支和短波支色散关系的解析表达式.由于解析近似对波数的限制,无法得到全波数空间色散曲线,因此对色散方程进行了数值求解.研究表明,在满足长波支和短波支条件时,解析色散曲线与数值色散曲线完全符合.此外,利用多项式回归的方法对数值结果进行拟合,得到不同参数下快电子分布等离子体横振荡色散关系近似表达式,该结果为进一步研究快电子分布等离子体的性质提供了重要的理论依据. 关键词： 相对论性等离子体 快电子分布 色散关系 数值计算     

Group-velocity dispersion in photonic crystal fibers

The dispersion properties of photonic crystal fibers are calculated by expression of the modal field as a sum of localized orthogonal functions. Even simple designs of these fibers can yield zero dispersion at wavelengths shorter than 1.27 mum when the fibers are single mode, or a large normal dispersion that is suitable for dispersion compensation at 1.55 mum.     

色散缓变光纤中飞秒光脉冲的调制不稳定性研究

研究了色散缓变光纤中飞秒光脉冲的调制不稳定性 ,发现当色散缓变光纤的色散参量满足一定关系式时 ,增益谱的谱宽最宽 ,获得了增益谱的表达式 ;三阶色散对调制不稳定性不起作用 ;自变陡效应使增益谱的谱宽变窄 ,振幅的增长速度减漫 ;拉曼效应改变了调制不稳定性的产生区域。     

单模光纤正常色散区零色散波长附近的啁啾演变特性

利用傅里叶变换法,首次推出了单模光纤正常色散区中二阶、三阶色散共同决定群速度色散效应时,所致啁啾的解析表达式.利用数位解法,模拟了零色散波长附近群速度色散效应和自相位调制效应共同导致的啁啾演变过程.计算结果表明:啁啾为非线性的,且有极值,因而导致脉冲形状的畸变.     

色散管理孤子中 Raman效应引起的时移及定时抖动

给出了色散管理孤子中Raman效应所引起的脉冲中心位置时移的解析表达式并讨论了孤子脉宽的波动所引起的Raman定时抖动.结果表明:时移的演化和不同的色散配置有关,并且色散管理孤子Raman时移和常规孤子一样与传输距离及平均色散的平方成正比,但是比常规孤子的比例大.     

各向异性内包层对双包层光纤特性影响的分析

提出了单轴各向异性材料为内包层，且其主轴沿光纤轴线（z轴）方向的双包层光纤模型，推出了矢量模特征方程，重点研究了主轴折射率比kcl对波导色散的影响，针对从矢量模特征方程求解波导色散因表达式极为复杂而无法直接求解的困难，提出了一种求解波导色散的有效方法，研究结果发现可以在不改变光波导结构参量的条件下，通过改变kcl可有效地改变光纤的波导色散，也分析了kcl、几何参量S、光学参量R对低次模的传输和截止特性的影响。研究结果为获得更为理想的色散补偿、色散平坦光纤及设计新型无源光器件提供了重要的依据。     

On the energy flux of stationary electromagnetic waves in anisotropic dissipative media with spatial dispersion

The special features of the propagation of electromagnetic wave in gyrotropic medium with dispersion and resonant dissipation (specifically, in a magnetoactive plasma) are studied. Even though the anti-Hermitian components of the permittivity tensor are substantial in magnitude, weakly damped waves can exist in such media. However, the well-known phenomenological expression for the energy flux of waves in a medium with spatial dispersion is inapplicable for them. A theory extending this expression to the case studied is developed. The modified expression for the energy flux is used to construct the Hamiltonian for the ray optics of such media.     

同轴电介质-金属-电介质结构表面等离激元色散研究

从平面电介质-金属薄膜-电介质对称结构中表面等离激元的色散方程出发,建立模型推导得到了同轴电介质-金属薄膜-电介质结构中表面等离激元的色散方程,在这种结构中各阶模式都发生了分裂,形成了高折射率高损耗和低折射率低损耗的两种模式,该模型为理解同轴电介质-金属薄膜-电介质结构表面等离激元模式的传播提供了直观的图像。用色散方程计算了0.5～1.5μm波长范围内各阶模式的等效折射率以及传输损耗,并将该结果与有限元方法数值模拟得到的结果进行对比分析,分析表明,两者吻合得很好。并分析了计算结果以及色散方程的适用范围。     

强色散管理孤子在密集周期光纤中的演化

利用变分方法,推导出了在强色散管理的密集周期光纤中脉冲稳态传输的解析条件,并给出了初始脉冲峰值功率的表达式.运用这些公式,对各种色散管理系统中孤子的演化进行了半解析的考察.三阶色散也被考虑在内.结果表明:不完善的色散斜率补偿将导致脉冲中心位置的偏移甚至严重的孤子形状劣化.     

Low-energy double plasmon satellites in the X-ray spectra of metals

Bohm and Pines' Hamiltonian and their dispersion relation have been extended up to second order to account for double plasmon oscillations in the X-ray emission spectra of solids. The effect of interaction between electrons and plasmons has also been taken into account in the dispersion relation. An expression for the relative intensity of low-energy double plasmon satellites has been derived. Fairly good agreement is obtained between values of the relative intensity calculated using this expression and values observed experimentally by Jenkins and Chung.     

Dispersion relation and relative intensity for double-plasmon satellites

An expression for the dispersion relation and the relative intensity of double-plasmon oscillations and satellites has been derived by extending the dispersion relation and the extended Bohm and Pines Hamiltonian to second order. The calculated value of the relative intensity of the double-plasmon satellite for Be agrees fairly well with the value observed experimentally by other workers.     

Optical surface modes of lattice vibrations

Two simple methods, based on simple crystal models, for estimating the longitudinal optical surface vibration mode frequencies which lie in a LALO gap are examined. One method consists of deriving and solving a single-variable equation for the surface mode frequency, which depends only on the experimental bulk dispersion curves. The other method consists of fitting the known dispersion curves of a simple one-dimensional model, for which the optical surface mode frequency is given by a simple expression, to experimental bulk dispersion curves of more complex crystals. The results are compared with detailed theoretical surface mode calculations. The latter method is found to give better estimates.