全文获取类型
收费全文 | 4870篇 |
免费 | 982篇 |
国内免费 | 465篇 |
专业分类
化学 | 81篇 |
晶体学 | 3篇 |
力学 | 50篇 |
综合类 | 15篇 |
数学 | 21篇 |
物理学 | 1922篇 |
无线电 | 4225篇 |
出版年
2024年 | 12篇 |
2023年 | 41篇 |
2022年 | 60篇 |
2021年 | 91篇 |
2020年 | 92篇 |
2019年 | 52篇 |
2018年 | 76篇 |
2017年 | 133篇 |
2016年 | 135篇 |
2015年 | 185篇 |
2014年 | 316篇 |
2013年 | 342篇 |
2012年 | 383篇 |
2011年 | 478篇 |
2010年 | 329篇 |
2009年 | 372篇 |
2008年 | 383篇 |
2007年 | 432篇 |
2006年 | 381篇 |
2005年 | 302篇 |
2004年 | 284篇 |
2003年 | 223篇 |
2002年 | 226篇 |
2001年 | 189篇 |
2000年 | 165篇 |
1999年 | 134篇 |
1998年 | 81篇 |
1997年 | 80篇 |
1996年 | 77篇 |
1995年 | 71篇 |
1994年 | 41篇 |
1993年 | 45篇 |
1992年 | 27篇 |
1991年 | 26篇 |
1990年 | 12篇 |
1989年 | 4篇 |
1988年 | 10篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 9篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1980年 | 1篇 |
排序方式: 共有6317条查询结果,搜索用时 0 毫秒
51.
Seed laser pulses with average power of 146 μW and pulse duration of 480 fs were amplified to 14.5 mW. The pulse duration was compressed to 260 fs using 6 m high concentration E3+r-doped fiber under forward pumping. The amplified signal pulse energy was 0.691 nJ (corresponding to a peak power of 2 657.7 W) and the repetition rate was 20.84 MHz. Spectrum breakup was observed simultaneously. The spectrum of pulses amplified by 3 m E3+r-doped fiber remains a single peak under different pump power. The amplified pulse duration was compressed abnormally with the increasing pump power using the backward pumping; that is, the amplified pulses were compressed with the increasing pump power under low pump power. When the pump power reached 38 mW, the shortest amplified pulse duration was 309 fs. With further increase in pump power, the amplified pulses began broadening, accompanied by a single peak spectrum under different pump power. 相似文献
52.
Jacek Jasielski Stanisław Kuta Witold Machowski Wojciech Kołodziejski 《Microelectronics Journal》2014
In the paper we propose a novel architecture and implementation of 11-bit Digital Pulse Width Modulator (DPWM) circuit based on previously known building blocks. Linearized Class-AD Double-sided (LADD) algorithm has been used to calculate the DPWM signals of the 11-bit resolution hybrid DPWM for a Class-AD digital audio amplifier. Noise-shaping process is used to support high fidelity with practical values of time resolution. The proposed DPWM circuit is composed of 8-bit counter and Analog Delay Locked Loop (ADLL) using 4-bit tapped delay line. A dual ADLL employing coarse and fine programmable delay element is used to adjust the delay time of delay line and lock it to required time. The coarse- as well as fine-delay lines are implemented as a cascade of variable-delay elements based on shunt capacitor delay element or single-ended Schmitt trigger. The proposed 11-bit DPWM circuit, at a switching frequency of 352.8 kHz and clock generator frequency of 90.3 MHz allows us to attain SNR of 120 dB and THD of the output signal less than 0.1% within the audio baseband and modulation index M=0.95. Basic verification of circuit manufacturability and simulation results (Monte Carlo analysis) for real CMOS process are presented. 相似文献
53.
54.
We demonstrate a fiber ring laser with a dispersion compensation fiber (DCF) and a delayed interferometer (DI) with temperature control, which is able to switch eleven wavelengths one by one. In ring cavity, DCF supplies different effective cavity lengths for different wavelengths, DI generates a wavelength comb corresponding to the ITU grid, a flat-gain erbium-doped fiber amplifier (EDFA) provides uniform gain for each lasting wavelength, and a semiconductor optical amplifier (SOA) not only acts as active modulator, but also alleviates homogeneous broadening effect of EDFA. Stable pulse trains with a pulsewidth about 40 ps at 10 GHz have been obtained by injecting external optical control signals into the laser. Wavelength switching process among eleven wavelengths is achieved by merely tuning an intracavity optical delay line. 相似文献
55.
Yuxin Leng Lihuang Lin Wenyao Wang Yunhua Jiang Bin Tang Zhizhan Xu 《Optics & Laser Technology》2003,35(6):425-429
A birefringent crystal quartz plate of known thickness has been used as a spectral filter for spectral shaping in a Ti:sapphire regenerative amplifier. The spectral profile of the amplified pulse ejected from the regenerative amplifier was observed while adjusting the birefringent crystal plate in the cavity. By altering the gain spectrum, the bandwidth of the regeneratively amplified pulse was increased from 18 to 35 nm by using a 0.34-mm thick birefringent plate. The output pulse spectrum from the regenerative amplifier neared the bandwidth of the seed pulse. As a comparison, we used a coated filter outside the regenerative amplifier cavity, and the bandwidth of the regeneratively amplified pulse was stretched to 28 nm. When the bandwidth was stretched to 35 nm, the pulse was compressed to 35 fs. 相似文献
56.
57.
58.
基于非线性光学耦合波方程组建立了聚焦泵浦条件下熔石英玻璃材料中SBS过程的物理模型并编制了相应的数值求解程序,研究了SBS介质内激光场的时空发展特征,从而试图揭示SBS介质的内在破坏机制并寻求相应的加固措施.通过数值模拟程序研究了焦点处激光功率密度以及SBS反射率与激光能量的依赖关系,发现SBS对介质内的激光场具有"限幅"作用,即在注入能量大幅度提升的情况下,内部的激光场强度只有较小的增加,这种机制使大能量的固体介质PCM成为可能.分析激光脉宽以及耦合透镜焦距等参数对SBS过程的影响发现,精细选择系统参数,在熔石英玻璃材料中有可能无损伤地实现高效的SBS过程. 相似文献
59.
60.