Dispersion compensation using high-positive dispersive optical fibers

The common and traditional method for optical dispersion compensation is concatenating the transmitting optical fiber by a compensating optical fiber having a high-negative dispersion coefficient.In this Letter,we take the opposite direction and show how an optical fiber with a high-positive dispersion coefficient is used for dispersion compensation.Our optical dispersion compensating structure is the optical implementation of an iterative algorithm in signal processing.The proposed dispersion compensating system is constructed by cascading a number of compensating sub-systems,and its compensation capability is improved by increasing the number of embedded sub-systems.We also show that the compensation capability is a trade-off between the transmission length and bandwidth.We use the simulation results to validate the performance of the introduced dispersion compensating module.Photonic crystal fibers with high-positive dispersion coefficients can be used for constructing the proposed optical dispersion compensating module.     

Modeling dispersion in optical fibers: applications to dispersion tailoring and dispersion compensation

The phenomenon of temporal pulse dispersion, which is a key characteristic of an optical fiber communication system is described from the first principles. Beginning with the basics of dispersion in a bulk medium, these concepts are then applied to propagation of a pulse in an optical fiber. Details of modeling dispersion are then described in the context of dispersion tailoring and dispersion compensation with a view to form the foundation for subsequent chapters on dispersion compensation that follow in this report. Basic physics behind the design target for dispersion compensating fibers is discussed, which should be useful to fiber designers.     

汞光谱色散曲线实验研究

在现有理论基础上,利用汞光谱色散实验测出汞光谱8条谱线对棱镜的折射率后,通过多项式拟合得到了波长和折射率的经验关系,即色散特性函数,并由此得到了汞光谱的角色散率.本文有助于人们更深入认识色散现象.     

A comparative study of the full dispersion relation, Compton dispersion relation, and Raman dispersion relation of a free-electron laser in helical wiggler and guiding magnetic fields

The full dispersion relation obtained for a free-electron laser in the presence of circularly polarized, periodic, static wiggler and guiding magnetic fields by the kinetic approach incorporating the particle trajectories is reduced to Compton and Raman regime approximations in the case of a tenuous electron beam. The temporal growth rate has been compared between the full dispersion relation and the Compton dispersion relation, as well as between the full dispersion relation and the Raman dispersion relation in the microwave region. The results show the maxima of growths in the full dispersion relation and the Compton dispersion relation, as well as the full dispersion relation and the Raman dispersion relation, are at the same locations, but the growth rates in both regimes are enhanced with respect to the full dispersion relation for the same plasma frequency and cavity parameters.     

光栅压缩器中色散的矩阵计算

在ABCD矩阵基础上,通过引入角度色散项获得了可以方便描述色散系统的3×3阶矩阵.采用这种矩阵方程方便、准确地计算了标准光栅对压缩器中的二阶、三阶以及四阶色散,得到的结果与由常规积分法获得的结果相同.     

宽带色散补偿光子晶体光纤的设计与优化

提出了一种新型的宽带色散补偿光子晶体光纤。通过增大光子晶体光纤(PCF)包层第一环空气孔半径r1,同时优化孔间距和包层其它环空气孔,在1550nm波长处获得了低至-1906.4ps/nm/km的负色散值。针对常规单模光纤的色散特性,设计出了宽带色散补偿光子晶体光纤,可补偿23倍长度的常规光纤,补偿的带宽达330nm,这在WDM系统中对多个信道同时进行色散补偿具有非常重要的意义。     

Tailoring dispersion properties of photonic crystal waveguides by topology optimization

The paper describes a systematic method for the tailoring of dispersion properties of slab-based photonic crystal waveguides. The method is based on the topology optimization method which consists in repeated finite element frequency domain analyzes, analytical sensitivity analyzes and gradient based design updates. The goal of the optimization process is to come up with slow light, zero group velocity dispersion photonic waveguides or photonic waveguides with tailored dispersion properties for dispersion compensation purposes. Two examples concerning reproduction of a specific dispersion curve and design of a wide bandwidth, constant low group velocity waveguide demonstrate the efficiency of the method.     

Ultrasonic plate waves in paper

A theoretical and experimental study of ultrasonic plate waves in machine-made paper is described. The paper is assumed to behave as a homogeneous orthotropic plate. The dispersion equation for orthotropic plate waves in principal directions is then developed analytically. It is shown that at low frequencies the orthotropic dispersion equation depends only on in-plane parameters. Using appropriate elastic constants, a computer is used to construct the dispersion curves and describe the internal motion for plate waves in paper. Finally, a portion of the dispersion curve is checked experimentally.     

Dispersion theory of multiple-prism beam expanders for pulsed dye lasers

The single pass dispersion of multiple-prism beam expanders of interest for practical pulsed dye laser cavities is calculated. The theory is extended to enable evaluation of the overall (outward and return pass) dispersion of multiple-prism-grating combinations. It is found that although the contribution to overall dispersion from the prisms can be minimized (though not eliminated for practical expanders) by arrangement of the prisms in compensating pairs, it is in any case small (≈2%) compared to the contribution to dispersion from the grating in practical cavities. Formulae for dispersion of multiple-prism-grating combinations including up to four prisms arranged in additive or compensating configurations are given; the dispersion of an arbitrary number of prisms arranged in conjunction with a grating can also be calculated. The implications for output linewidth of pulsed dye lasers incorporating prism beam expanders are discussed.     

Second- and third-order dispersion in PW prism-sequence with arbitrary apex-angle

Numerical results for the second- and third-order dispersion in PW prism-sequence (double prism-pair) with arbitrary apex-angle by ray-tracing method are presented. The influence of the prism separation distance and apex-angle on second- and third-order dispersion is discussed. In certain conditions, the PW prism-sequence with small apex-angle (20°) is a better choice to reduce third-order dispersion than single prism-pair without requiring excessive length. As a less dispersion material, quartz is suitable to be used with the smallest possible value of third-order dispersion. This result could be the basis of dispersion compensation and pulse compression.     

横向各向同性地层快速各向异性反演方法

斯通利波的频散特性常用于反演横向各向同性地层的各向异性。该文重点对斯通利波频散曲线随井孔和地层弹性模量相关参数的变化规律进行了探讨,提出了频散曲线的快速插值计算方法,经过误差分析验证了插值频散计算的可行性。据此提出了通过对参数大间隔取值建立理论频散数值表,插值计算不同各向异性参数的频散曲线,对比波形频散快速反演横向各向同性地层各向异性的方法。基于实轴积分法计算的横向各向同性地层井孔声场分别进行了插值频散快速反演和传统方法反演,对比结果证实了该文提出的快速反演方法的准确高效性,并且对数据加噪后验证了快速反演方法的抗噪性。     

基于光纤自相位调制多波长全光再生的色散管理优化

针对基于光纤自相位调制效应的多波长全光再生系统,分析了光纤平均色散对再生器品质因子改进量的影响,提出一种色散管理优化方法.由于波长间色散差异会导致再生系统性能的稳定性下降,通过选择色散补偿光纤和单模光纤的长度,使其合成的色散曲线与各波长分别达到最优再生性能时对应的色散曲线近似拟合,同时保证该色散曲线落在性能满意度高的色散区域,然后调整信号输入功率和滤波器的偏移量,可有效改善多波长再生性能的稳定性,实现再生器的色散管理优化.     

Investigations on order and width of RZ super Gaussian pulse in different WDM systems at 40 Gb/s using dispersion compensating fibers

In optical wavelength division multiplexed (WDM) systems the dispersion management is a key issue. In optical systems a lot of research is going on to reduce dispersion by selecting proper dispersion compensating techniques and proper modulation format for input data. One way to reduce dispersion is by using dispersion compensating fibers in the WDM systems. This paper analyzes the use of RZ super Gaussian pulse inputs for different WDM systems i.e. for conventional, dense and ultra dense WDM systems employing dispersion compensating fibers. The pulse width and the order of the RZ super Gaussian pulse was varied to evaluate the performance at 40 Gb/s. The experiment showed that to get minimum BER, pulse width of 7.5 ps and 10 ps along with third-order RZ super Gaussian pulse were found suitable and recommended to be used.     

Transmission of light through a random small-angle scattering medium

A theory of random small-angle scattering is presented. The photon dispersion partial differential equation is derived and the dispersion coefficient characterizing the medium is introduced. The equation is solved for a spatial impulse. The modulation transfer function and the contrast loss are derived as a funcion of spatial frequency, dispersion coefficient, and object-to-image distance. The limitation on resolution is shown by an indeterminancy relation based on the dispersion coefficient. The dispersion angle (rms value of scattering angle) is calculated as an example.     

用于色散补偿的反射型棱栅的色散分析

利用角色散和材料色散得到了反射型棱栅的二阶、三阶色散完整的解析表达式,与光线追迹法的结果能很好地符合,利于工程中棱栅的设计.系统分析了温度、棱栅结构参数对其色散的影响.结果表明,温度、棱镜材料折射率、插入量、棱栅间距对棱栅色散影响很小.而入射角、光栅常数、棱镜项角等对色散的影响很大,可以调节上述参数来改变棱栅的色散,从而补偿放大介质等材料色散.     

一种40Gb/s单信道光纤通信系统中的动态色度色散补偿

提出了一种用于40 Gb/s单信道光纤通信系统中的动态色度色散(CD)补偿技术。采用2×2光开关,色散补偿光纤(DCF)等器件构成可调节色度色散补偿器;提取中心频率为12GHz的窄带电功率信号作为反馈信号控制可调节色度色散补偿器,提取的窄带电功率值随系统中的累积色度色散值的增大而减小。实验证明,整个补偿系统的最长响应时间为0.7 s;补偿范围和补偿精度分别为81.55 ps/nm和5.28 ps/nm,通过增加光开关的数量和缩短每段色散补偿光纤的长度可以进一步提高补偿范围和精度。通过对比补偿前后系统的眼图可以看出:该系统能有效地补偿40 Gb/s光纤通信系统中动态变化的色度色散。     

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。     

双光栅快速扫描光学延迟线的色散补偿

光学相干层析成像(OCT)系统的纵向分辨力不仅与光源的带宽有关,而且与系统中两干涉臂间的色散匹配有关。如果色散没有得到精确匹配,将使光学相干层析成像系统的纵向分辨力达不到所预期的理论值。色散问题在超高分辨光学相干层析成像系统中尤为突出。提出了一种基于双光栅快速扫描光学延迟线(RSOD),用于光学相干层析成像系统的色散补偿。该方法中新增的光栅引入了光栅间距这一独立变量,其与常规单光栅快速扫描光学延迟线机构中的光栅离焦量一起,可使光学相干层析成像系统中的群速度色散(GVD)和三阶色散(TOD)同时得到补偿。分析了双光栅快速扫描光学延迟线的色散特性和色散调节原则,并提供了一个典型光学相干层析成像系统中的色散补偿实例。     

Dispersion engineering of tellurite holey fiber with holes formed by two glasses for highly nonlinear applications

Tellurite nonlinear holey fiber is characterized by a high nonlinearity and a broad transparent window. However, these advantages are canceled by its unflattened dispersion in most practical applications. The unflattened dispersion is due to the fact that tellurite holey fiber with complex structure is difficult to fabricate. In this work we develop a dispersion flattened tellurite composite holey fiber (TCHF). The TCHF has only one ring of holes. The holes are formed by two tellurite glasses. The heavy deformation, which probably occurs for tellurite complex microstructured fiber during the fabrication process, is avoided by this simple structure. The fiber is made of two glasses with different refractive-indices, which improves the flexibility in dispersion engineering. By using this structure the dispersion is engineered to be the most flattened for the highly nonlinear soft glass fiber within 1.5–1.6 μm. Owing to the flattened dispersion and high nonlinearity, more than one octave supercontinuum generation is demonstrated by a femtosecond fiber laser.