全文获取类型
收费全文 | 6619篇 |
免费 | 1507篇 |
国内免费 | 470篇 |
专业分类
化学 | 1071篇 |
晶体学 | 60篇 |
力学 | 948篇 |
综合类 | 156篇 |
数学 | 1764篇 |
物理学 | 4597篇 |
出版年
2024年 | 28篇 |
2023年 | 64篇 |
2022年 | 139篇 |
2021年 | 164篇 |
2020年 | 183篇 |
2019年 | 167篇 |
2018年 | 174篇 |
2017年 | 248篇 |
2016年 | 277篇 |
2015年 | 248篇 |
2014年 | 502篇 |
2013年 | 522篇 |
2012年 | 389篇 |
2011年 | 443篇 |
2010年 | 339篇 |
2009年 | 388篇 |
2008年 | 440篇 |
2007年 | 448篇 |
2006年 | 432篇 |
2005年 | 376篇 |
2004年 | 313篇 |
2003年 | 281篇 |
2002年 | 270篇 |
2001年 | 279篇 |
2000年 | 240篇 |
1999年 | 205篇 |
1998年 | 168篇 |
1997年 | 155篇 |
1996年 | 139篇 |
1995年 | 106篇 |
1994年 | 77篇 |
1993年 | 52篇 |
1992年 | 53篇 |
1991年 | 51篇 |
1990年 | 36篇 |
1989年 | 27篇 |
1988年 | 24篇 |
1987年 | 30篇 |
1986年 | 18篇 |
1985年 | 16篇 |
1984年 | 15篇 |
1983年 | 9篇 |
1982年 | 10篇 |
1981年 | 14篇 |
1980年 | 10篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 6篇 |
1971年 | 2篇 |
1957年 | 2篇 |
排序方式: 共有8596条查询结果,搜索用时 15 毫秒
991.
Abstract A survivable wavelength division multiplexing passive optical network enabling both point-to-point service and broadcast service is presented and demonstrated. This architecture provides an automatic traffic recovery against feeder and distribution fiber link failure, respectively. In addition, it also simplifies the protection design for multiple services transmission in wavelength division multiplexing passive optical networks. 相似文献
992.
Abstract A recent study discussing different ring-resonator- and fiber-based gyroscopes concluded that the performance of a single-loop resonant fiber-optical gyro exceeds that of more complex ring-resonator-based gyros. It is demonstrated here that this conclusion applies as well to a gyro that consists of a closed loop of coupled-ring resonators with internal feedback. While this device is more sensitive to rotation than other gyro structures with the same footprint and losses, a simpler alternative consisting of a resonant single ring with an area equal to complex gyro structures still displays a higher degree of sensitivity. 相似文献
993.
Abstract This article proposes various combinations of optical amplifiers for a dense wavelength division multiplexed system and investigates the impact of reduced channel spacing at high bit rates in terms of quality factor, bit error rate, eye closure, and output power. It is reported that the hybrid optical amplifier (Raman–erbium-doped fiber amplifier [EDFA]) provides better results with a maximum covered single span distance (220 km) at channel spacing of 6.25 GHz. The maximum acceptable bit rate for the 12.5-GHz channel spacing dense wavelength division multiplexed system is also investigated, and the recommendation is provided that for the Raman–EDFA, Raman–EDFA–Raman, EDFA–Raman–EDFA, and EDFA–semiconductor optical amplifier–EDFA, the operating bit rate should not be greater than 20, 16, 19, and 20 Gbps, respectively. 相似文献
994.
Wei Ji 《Fiber and Integrated Optics》2013,32(4):268-279
Abstract Video on demand is a very attractive service used for entertainment, education, and other purposes. The design of passive optical networking+Ethernet over coaxial cable accessing and a home gateway system is proposed. The network integrates the passive optical networking and Ethernet over coaxial cable to provide high dedicated bandwidth for the metropolitan video-on-demand services. Using digital video broadcasting, IP television protocol, unicasting, and broadcasting mechanisms maximizes the system throughput. The home gateway finishes radio frequency signal receiving and provides three kinds of interfaces for high-definition video, voice, and data, which achieves triple-play and wire/wireless access synchronously. 相似文献
995.
Yi-Lin Yu Edvin Skaljo Hoa Le Minh Zabih Ghassemlooy 《Fiber and Integrated Optics》2013,32(5-6):383-394
AbstractThis article presents a novel bidirectional wavelength reconfigurable optical network utilizing a remotely pumped erbium-doped fiber amplifier and tunable fiber Bragg gratings. The system is experimentally demonstrated at a 10-Gb/s per channel over 20-km fiber span that verifies the metro-network range system performance. The achieved power penalty is less than 1 dB when compared to the back-to-back transmission link. An example of practical application where the proposed module is used as an add/drop multiplexer and a remote node in the bidirectional wavelength division multiplexing passive optical network system is described. 相似文献
996.
ADOLFO V. T. CARTAXO JOÃO L. REBOLA NATAŠA B. PAVLOVIĆ PEDRO M. A. CHARRUA DANIEL D. T. FONSECA JOSÉ A. P. MORGADO 《Fiber and Integrated Optics》2013,32(3-4):331-352
Abstract 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. 相似文献
997.
Abstract The performance assessment of orthogonal frequency division multiplexing signals in direct-detection transmission systems by using the error vector magnitude and several bit error ratio approaches is analyzed and compared through numerical simulation. It is shown that excellent accuracy of the bit error ratio estimates is obtained by a semi-analytical Gaussian approach for all the orthogonal frequency division multiplexing system configurations analyzed and that the error vector magnitude only provides reliable estimates of the system performance when the system is dominantly impaired by noise. Additionally, a novel Q-factor approach for orthogonal frequency division multiplexing optical signals showing improved bit error ratio estimates is also presented. 相似文献
998.
High frequency estimates for the Dirichlet-to-Neumann and Neumann-to-Dirichlet operators are obtained for the Helmholtz equation in the exterior of bounded obstacles. These a priori estimates are used to study the scattering of plane waves by an arbitrary bounded obstacle and to prove that the total cross section of the scattered wave does not exceed four geometrical cross sections of the obstacle in the limit as the wave number k → ∞. This bound of the total cross section is sharp. 相似文献
999.
The order of convergence for the low frequency asymptotics of the exterior Dirichlet problem for the Helmholtz equation with variable, possibly non-smooth coefficients is shown to be Ω2, the square of the frequency, except of some singular cases. In these cases the asymptotics are characterized completely by some lower order terms in the spherical harmonics expansion of the solution to the static problem. 相似文献
1000.
《偏微分方程通讯》2013,38(5-6):1085-1111
Abstract In the present article, we study the system of eikonal and transport equations arising in geometrical optics. The mathematical analysis is performed by using the suitable notion of solution, i.e., the viscosity solution for the Hamilton–Jacobi equation and the measure solution for the transport equation defined via the generalized Filippov characteristics. We study the stability as well as the geometry of the solution to the system. 相似文献