基于金属/介质混合反射镜的高调制深度半导体可饱和吸收镜锁模的飞秒光纤激光器

介绍了一种新型半导体可饱和吸收镜(SESAM)的原理与特性,并用它来实现掺铒光纤激光器的锁模飞秒脉冲的生成.这种新型宽带SESAM调制深度可以达到20%,用在1.5μm波段的环形腔掺铒光纤激光器里,实现自启动锁模.锁模光谱以1559 nm为中心,半峰全宽为9 nm,锁模激光脉冲串重复频率为25.6 MHz,输出功率14 mW,自相关仪测得脉宽为170 fs.     

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用时域有限差分法研究了电磁波在等离子体光子晶体中的传播特性。数值模拟中使用完全匹配层吸收边界条件,计算了电磁波通过等离子体光子晶体的反射和透射系数。讨论了等离子体密度、等离子体温度、介电常数比和引入缺陷层对等离子体光子晶体光子带隙的影响。     

Coupling coefficients evaluation of a directional coupler using gain guided and index antiguided fibers

In this paper, we have investigated the mutual and self coupling characteristics of directional couplers using two individual gain guided and index antiguided fibers, which can deliver robust single mode operation with large mode area, respectively owing to the combined adequately large gain coefficients in the core; Some interesting and distinguishing properties from conventional index guided fiber couplers are obtained that: The variations of coupling coefficients are complex-valued and spatially damping oscillated, but not exponential-like decreasing as conventional index guided couplers.     

Differential variation of laser longitudinal mode spacing induced by small intra-cavity phase anisotropies

Intra-cavity phase anisotropy causes each laser longitudinal mode to split into two orthogonally polarized frequencies. When the frequency difference generated by small anisotropy is about 40 MHz or less, only one frequency of the splitting mode oscillates while the other is always in lock-in state due to strong mode competition. In that case the longitudinal mode spacing deviates from its original value. And the spacing of two adjacent longitudinal modes shows differential variations depending on direction of cavity length tuning and mode polarization. In this paper, the relationship between spacing variation and small intra-cavity anisotropy is demonstrated experimentally and theoretically. The equivalent fast axis of anisotropy is determined additionally by its correlation with laser intensity tuning of the two orthogonally polarized components. The novel measurement method is successfully tested in several applications.     

Complete chaotic synchronization mechanism of polarization mode of VCSEL with anisotropic optical feedback

We numerically investigate the complete chaotic synchronization mechanism of polarization mode of VCSEL with anisotropic optical feedback. Firstly, the synchronization quality of the x-linear polarization (LP) mode and the y-LP mode are both periodically changed with the angle θ between the polarizer and the light axes. Secondly, in a enough large scale region of the feedback coefficient and the injection current, where each LP mode can obtained good synchronization quality when the angle θ exists in the former half period region and the injection current is fixed at a certain value, which is over the threshold current. Under this condition, their synchronization quality is independent of the polarization states. By contrast, in the latter half period region of the angle θ where each LP mode can be obtained very instable and inferior synchronized quality. At last, the robustness of the synchronization scheme is sensitive to the internal and external parameter mismatch between the transmitter-VCSEL and the receiver-VCSEL.     

Sound propagation in soundproofing casement windows

Casement windows consisted of two wooden frames that can be opened and closed at various angles are widely used in developing countries with tropical climates. However, an annual increase in a number of motorcycles and automobiles and traffic noise level these countries have rendered these windows to be useless. In this paper, we present a model for manufacturing windows which are suitable for the developing tropical countries. These windows are capable of ventilating, regulating sunlight, protecting against coldness, and reducing traffic noise and fumes from motor vehicles. The ventilation and soundproofing unit hold an importance place in the design of the windows and are calculated using the wave equation and observations of higher-order mode waves. In order to maximize the soundproofing ability, the selection of size and placement of input and output openings in such a way that would minimize the effects of higher-order mode waves are considered in details.     

Efficient equalization of multi-exciter distributed mode loudspeakers

An efficient digital equalization method is applied successfully to the problem of spectral equalization of multi-exciter distributed mode loudspeakers (DML). It is based on a chain of second-order sections of infinite impulse response parametric filters with very low computational cost. The method compensates for the measured multi-exciter DML response in order to achieve a desired frequency response. The sound radiation of these flat loudspeakers is a complex superposition of excited modes that vary strongly with frequency. Therefore, the characteristic multi-exciter DML spectrum is very irregular and is equalized with the method presented here for a natural, uncolored response. In multichannel systems, such as wave field synthesis (WFS), the use of efficient filters to equalize a large amount of drivers is an advantageous approach. The equalization process has been applied to two multi-exciter DML prototypes, comprising three and five exciters per panel. Both panel and exciter equalization have been addressed, which consequences on the filtered responses are discussed. Finally, some subjective assessments are carried out to optimize the order of the filter while maintaining the perceived quality of the equalization.     

The molecular symmetry group of the CO dimer and the assignments of the intermolecular vibrations

Vibrational assignments for some of the lowest energy levels of the CO dimer have been derived purely spectroscopically on the basis of the molecular symmetry, nuclear spin statistics, observed selection rules, and sequence of the transition doublets. Most of the previous assignments of Surin et al. [L.A. Surin, D.N. Fourzikov, T.F. Giesen, S. Schlemmer, G. Winnewisser, V.A. Panfilov, B.S. Dumesh, G.W.M. Vissers, A. van der Avoird, J. Phys. Chem. A 111 (2007) 12238-12247] are confirmed in the present work, and are supported by much clear evidences. One significant revision of that analysis in the present work is identification of the torsional (out-of-plane) excited state, which had been assigned previously to an excited state of the geared (in plane) internal rotation.     

A MIXED MODE FRACTURE CRITERION BASED ON THE MAXIMUM TANGENTIAL STRESS IN BRITTLE INCLUSION

A closed-form solution for predicting the tangential stress of an inclusion located in mixed mode Ⅰ and Ⅱ crack tip field was developed based on the Eshelby equivalent inclusion theory. Then a mixed mode fracture criterion, including the fracture direction and the critical load, was established based on the maximum tangential stress in the inclusion for brittle inclusioninduced fracture materials. The proposed fracture criterion is a function of the inclusion fracture stress, its size and volume fraction, as well as the elastic constants of the inclusion and the matrix material. The present criterion will reduce to the conventional one as the inclusion having the same elastic behavior as the matrix material. The proposed solutions are in good agreement with detailed finite element analysis and measurement.     

OPERATIONAL MODAL ANALYSIS SCHEMES USING CORRELATION TECHNIQUE

For some large-scale engineering structures in operating conditions, modal parameters estimation must base itself on response-only data. This problem has received a considerable amount of attention in the past few years. It is well known that the cross-correlation function between the measured responses is a sum of complex exponential functions of the same form as the impulse response function of the original system. So this paper presents a time-domain operating modal identification global scheme and a frequency-domain scheme from output-only by coupling the cross-correlation function with conventional modal parameter estimation. The outlined techniques are applied to an airplane model to estimate modal parameters from response-only data.