光纤-光栅对激光脉冲压缩

本文报道用2m单模保偏光纤实现脉冲光谱加宽,经光栅对补偿后,将40ps脉冲压缩到小于10ps的实验结果,分析了影响光脉冲压缩质量的原因。实验结果与理论较好地吻合。     

梯形截面硅脊形波导的模式特性及其等离子体色散效应

本文运用有效折射率法和WKB法对具有梯形状截面的硅脊形波导的模式特性作了分析,导出了TE模和TM模的模方程以及截止方程,并运用一阶微扰法对因波导顶部注入载流子而引起的模式调制的大小作了数值估计。其结果可为全硅光波导器件的研制提供指导。     

Investigation of axial density modulations of core fibrils by interface distribution functions

The axial density modulation of core-fibrils in polypropylene and poly (1butene) crystallized from the oriented melt has been investigated by calculating interface distribution functions of the small angle x-ray scattering. The density modulation along the fiber axis originates from the migration of chain defects in the early stages of crystallization in the shear-field. The lengths of the needle crystals are approximately three times higher than those of the distorted zones. The values of the interface lengths between crystals and zones of lower electron density, are matching well with the lengths of the distorted zones. This result confirms the concept of defect clustering.     

On determining the profile of steady wind-induced currents

A technique is developed to solve the steady-state hydrodynamic equations which describe wind-induced currents at a point. By using the Galerkin approach, with an expansion of complex coefficients and real functions, a continuous current profile can be computed from sea surface to sea bed for arbitrary vertical variations of viscosity. The method can incorporate slip or no-slip bottom boundary conditions, and gradients of sea surface elevation. The technique is used to investigate the influence of viscosity profile, bed stress and elevation gradient on surface current magnitude and direction. Calculations suggest that viscosity profile, bed stress, and elevation gradient all affect surface current, offering some explanation for the range of surface current magnitudes and directions reported in the literature.     

Overview of DWDM/ODC Project

This article introduces the main achievements resulting from the DWDM/ODC project. The five areas of research activity within the DWDM/ODC project cover some of the main issues of design and development of dense wavelength division multiplexing systems for transparent optical networks. These issues are: performance assessment with arbitrary optical filtering; performance of signaling formats; dispersion compensation strategies for directly and externally modulated systems in presence of nonlinear transmission-induced degradation; and the impact of noise and crosstalk in the extent of transparent optical networks. All five areas of research activity have contributed significantly to a better understanding of the limitations present in dense wavelength division multiplexing systems.     

The study of third harmonic demodulation in phase modulation optical heterodyne technique using optical resonator

With the technique of phase modulation optical heterodyne(PMOH), we detected the beat-frequency signal of the reflected wave from a F-P cavity. The frequency of the beat signal was three times of that of the modulation electric field, when the 3rd harmonic of the modulation electric field was used as the phase reference signal. The line shapes of the signal match those of the theoretical calculations, and the dispersion signal has good frequency-discrimination characteristic.     

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).     

重复脉冲注入下半导体光放大器皮秒增益和折射率非线性的数值模拟

对重复周期小于载流子寿命的重复脉冲注入的情况，从速率方程出发，用分段模型对半导体光放大器的皮秒增益和折射率非线性进行了数值模拟，给出了重复脉冲注入下输出光脉冲时域波形的畸变、注入光脉冲引起的增益和相位变化，输出光脉冲上的频率啁啾随注入脉冲数增加的变化，以及归零光信号码流引起的增益和相位变化。     

Two-dimensional microwave band-gap structures of different dielectric materials

We report the use of low dielectric constant materials to form two-dimensional microwave band-gap structures for achieving high gap-to-midgap ratio. The variable parameters chosen are the lattice spacing and the geometric structure. The selected geometries are square and triangular and the materials chosen are PTFE (ε = 2.1), PVC (ε = 2.38) and glass (ε = 5.5). Using the plane-wave expansion method, proper lattice spacing is selected for each structure and material. The observed experimental results are analyzed with the help of the theoretical prediction     

Moment-volume instabilities in alloys with compositions around the C14 laves phase

We measured the thermal expansion and the specific heat of Ti_xFe_100-x alloys with x = 30.5, 32.5 and 35, all with hexagonal C14 laves phase structure (MgZn₂) like TiFe₂, and determine the temperature dependence of the magnetic contributions to the thermal expansion and the specific heat c_mag. For fixed composition and c mag ( T ) show the same type of behavior, demonstrating that both anomalies are of the same microscopic nature. They originate from moment-volume fluctuations (antiferromagnetic Invar-effect) as a comparison with total energy calculations as a function of atomic volume and moment for TiFe₂ reveals. Received: 26 January 1998 / Accepted: 17 April 1998