全文获取类型
收费全文 | 374341篇 |
免费 | 3464篇 |
国内免费 | 1035篇 |
专业分类
化学 | 175560篇 |
晶体学 | 5534篇 |
力学 | 13248篇 |
综合类 | 14篇 |
数学 | 31624篇 |
物理学 | 98309篇 |
无线电 | 54551篇 |
出版年
2021年 | 2610篇 |
2020年 | 3047篇 |
2019年 | 3254篇 |
2018年 | 3968篇 |
2017年 | 4059篇 |
2016年 | 6133篇 |
2015年 | 3701篇 |
2014年 | 5995篇 |
2013年 | 15071篇 |
2012年 | 11147篇 |
2011年 | 13564篇 |
2010年 | 9973篇 |
2009年 | 10570篇 |
2008年 | 13620篇 |
2007年 | 13810篇 |
2006年 | 13328篇 |
2005年 | 12356篇 |
2004年 | 11337篇 |
2003年 | 10605篇 |
2002年 | 10175篇 |
2001年 | 12471篇 |
2000年 | 9669篇 |
1999年 | 7741篇 |
1998年 | 6251篇 |
1997年 | 6268篇 |
1996年 | 6030篇 |
1995年 | 5678篇 |
1994年 | 5521篇 |
1993年 | 5222篇 |
1992年 | 6205篇 |
1991年 | 6191篇 |
1990年 | 5881篇 |
1989年 | 5695篇 |
1988年 | 5508篇 |
1987年 | 4973篇 |
1986年 | 4649篇 |
1985年 | 5788篇 |
1984年 | 5857篇 |
1983年 | 4680篇 |
1982年 | 4779篇 |
1981年 | 4646篇 |
1980年 | 4419篇 |
1979年 | 4810篇 |
1978年 | 4900篇 |
1977年 | 4883篇 |
1976年 | 4685篇 |
1975年 | 4286篇 |
1974年 | 4226篇 |
1973年 | 4234篇 |
1972年 | 2936篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
K.P. Pipe R.J. Ram 《Photonics Technology Letters, IEEE》2003,15(4):504-506
By measuring the total energy flow from an optical device, we can develop new design strategies for thermal stabilization. Here we present a comprehensive model for heat exchange between a semiconductor laser diode and its environment that includes the mechanisms of conduction, convection, and radiation. We perform quantitative measurements of these processes for several devices, deriving parameters such as a laser's heat transfer coefficient, and then demonstrate the feasibility of thermal probing for the nondestructive wafer-scale characterization of optical devices. 相似文献
3.
We propose a standardization procedure that provides a convenient, quantitative and reproducible laboratory-based method for measuring the state of polarization (SOP) fluctuations produced by polarization varying devices. This method is based on the SOP distributions generated by commercial polarization scramblers. We show that these devices generate distributions of the maximum change of the SOP (in a given sample time) that follow Rayleigh statistics, which scale linearly with scrambling frequency and the sample time. We use this procedure to measure the SOP fluctuations in a short length of coiled fiber subject to mechanical perturbations. 相似文献
4.
A common computing-core representation of the discrete cosine transform and discrete sine transform is derived and a reduced-complexity algorithm is developed for computation of the proposed computing-core. A parallel architecture based on the principle of distributed arithmetic is designed further for the computation of these transforms using the common-core algorithm. The proposed scheme not only leads to a systolic-like regular and modular hardware for computing these transforms, but also offers significant improvement in area-time efficiency over the existing structures. The structure proposed here is devoid of complicated input/output mapping and does not involve any complex control. Unlike the convolution-based structures, it does not restrict the transform length to be a prime or multiple of prime and can be utilized as a reusable core for cost-effective, memory-efficient, high-throughput implementation of either of these transforms 相似文献
5.
6.
7.
This article presents a graph-theoretic method for constructing low-density parity-check (LDPC) codes from connected graphs without the requirement of large girth. This method is based on finding a set of paths in a connected graph, which satisfies the constraint that any two paths in the set are either disjoint or cross each other at one and only one vertex. Two trellis-based algorithms for finding these paths are devised. Good LDPC codes of practical lengths are constructed and they perform well with iterative decoding. 相似文献
8.
Hogari K. Tetsutani S. Jian Zhou Yamamoto F. Sato K. 《Lightwave Technology, Journal of》2003,21(2):540-545
Many cables containing 1.3-/spl mu/m zero-dispersion single-mode (SM) optical fibers are installed in trunk and access networks. Recently, there have been a number of studies on wavelength-division-multiplexing (WDM) systems designed to increase transmission capacity and flexibility. If we can construct WDM systems using SM optical-fiber cable networks designed to transmit using wavelengths in the 1.3-/spl mu/m window (O-band), this will prove very effective in reducing construction costs. It is therefore important to examine the wavelength dependence of the transmission characteristics of SM optical-fiber cables and networks that have already been installed and in which several optical fibers are joined. In this paper, we describe the measured optical characteristics of SM optical-fiber cables and installed optical-fiber cable networks at various wavelengths. The optical characteristics were stable in the 1.46 to 1.625-/spl mu/m wavelength range and we confirmed that the installed SM optical-fiber cable networks could be used for WDM system applications. 相似文献
9.
For realizing a naturalistic collaboration between the human and the robot, we have to establish the intention sharing from the series of motion data that are observed and exchanged between the human and the machine. In a word, this is a problem to detect "meanings" out of the digitized data stream. In this paper, we propose a novel approach based on semiosis, and present a method of interpreting bodily motions using recurrent neural networks called Elman networks. We made some experiments using the raw data acquired while a human performs a simple task of fetching objects by stretching and folding his/her arm, and demonstrate that the network can learn invariant features of the generalized motion concepts, classify the motion by referring to self-organized memory structure, and understand a task structure of the observed human bodily motion. These capabilities are essential for machine intelligence to establishing the human-robot shared autonomy, a new style of human-machine collaboration proposed in the area of robotics. 相似文献
10.
A VQ-based blind image restoration algorithm 总被引:5,自引:0,他引:5
Learning-based algorithms for image restoration and blind image restoration are proposed. Such algorithms deviate from the traditional approaches in this area, by utilizing priors that are learned from similar images. Original images and their degraded versions by the known degradation operator (restoration problem) are utilized for designing the VQ codebooks. The codevectors are designed using the blurred images. For each such vector, the high frequency information obtained from the original images is also available. During restoration, the high frequency information of a given degraded image is estimated from its low frequency information based on the codebooks. For the blind restoration problem, a number of codebooks are designed corresponding to various versions of the blurring function. Given a noisy and blurred image, one of the codebooks is chosen based on a similarity measure, therefore providing the identification of the blur. To make the restoration process computationally efficient, the principal component analysis (PCA) and VQ-nearest neighbor approaches are utilized. Simulation results are presented to demonstrate the effectiveness of the proposed algorithms. 相似文献