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美国量子物理研究的新发现可能为光通讯领域带来变革
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【科研中国SciEi.com整理】美国加里佛尼亚大学得到的实验结果也许会给光通讯领域带来深刻的变革。这一研究报告刊登在10月28日一期的《自然》杂志上。 据physorg网2005年10月28日报道：美国加里佛尼亚大学得到的实验结果也许会给光通讯领域带来深刻的变革。这一研究报告刊登在10月28日一期的《自然》杂志上。 加里佛尼亚大学的物理学家马克·舍温说：“随着信息技术的进步，科学家们正努力提高信息传输的速率。我们的目标是以当前100倍的速度发送信息。”他的研究小组已在这一项目上花费了5年时间。实验用到了大学里和房间差不多大小的自由电子激光器。 他解释说：“我们使用了一种电控半导体快门，同时我们尝试以每秒3万亿分之一秒的速度开关快门，这时，我们发现快门自身产生了振荡现象。” 舍温说：“这些振荡现象也许可以仅用微弱的激光束就能打开快门而无需采用高电压。光通讯过程中存在着各种通讯频道，所以我们能够使用不同的激光束对应不同的频道。这才是真正快速切换通讯频道的一个途径。现在光通讯里切换频道是个非常缓慢的过程。” 舍温解释说，电子要比光子慢得多，光纤传输信息的速率可以比电脑等电子设备快一千倍。 “我们在加里佛尼亚大学所使用的是一种特殊的发射源——自由电子激光器。它可以产生每秒振荡几万亿次的电磁场，”舍温说，“我们发现当以这种高速驱动调制器，或快门时，它会以一种独特的方式快速运转。与一般只能以单一频率吸收光不同，它可以同时以第二个频率吸收光线。这就可能制造出新型的交叉调制器，一束有特定吸收频率的激光能打开或关闭其它的光。” 舍温说，人类使用光进行远距离传递信息已有3000多年的历史了。例如，按荷马在《伊利亚特》中的描述，古希腊人使用火从一个山头向另一个山头传递讯号。为了发送信息，必须对光进行调制——这意味着，要能打开或关闭光线。在第二次世界大战期间，船队之间的通讯使用人工调制的探照灯来发送编码信号，而现代的光调制器是由电压控制。 在电吸收调制器中，光接近特定频率——也就是载波频率，就能通过与载波信号调谐或异谐的方法来关闭或发送。一个普通的电吸收调制器是由半导体量子井构成的，即薄薄一层带有较小的“能隙” （一种较大的正电子与负空穴之间的吸附力）的半导体夹在两层带有一个更大“能隙”的半导体层中间。 舍温解释说，当频率合适的光线附到一个量子井上时，它会创造出一对叫做“电子空穴对”的电子束缚洞，而光线会被其所吸收。垂直作用于量子井的表面的电场会改变吸收的频率。这样与零场谐振产生共振现象的光线就不再被吸收。目前的量子井电子吸收调制器可以以超过每秒100亿比特的速率调制光。 在这篇论文中科学家们指出，量子井电子吸收调制器在超过1太赫兹（1万亿个循环）时动力强劲，比通常的量子井调制器要快100倍。在这种超高频的情况下，电子空穴的内部量子震荡会被激发出来。当强烈的太赫兹级的震荡与电子空穴的震荡谐振时，对量子井电子空穴吸收频谱附近的弱光吸收光谱能使单波峰变为双波峰，或成对波峰。这一成对波峰意味着接近电子空穴吸收频谱的光线频率将不再是简单地最低能量状态上产生出电子空穴，而是必须在其基本态和激发态下产生量子机制叠加。 一个在光通讯上的潜在运用是由太赫兹级频率所分隔开的任意两个弱光束可以互相进行调制。“通常，这样的交叉调制只发生在光束的强度高于某一极限时才会发生，”舍温说。 在一个分开的注释中，舍温说，“在原子气中，观察到的成对波峰是在发明能够明显降低光速甚至阻止光线传播的系统的研究方面迈出了第一步。在半导体中减慢或停滞光线传播将提高光通讯设备和计算机系统的能力。然而，为实现减慢或停滞光线传播，我们必须大幅度降低在量子井调制器中的能量耗散机制。” 科学论文《光调制器中的量子相关性》的作者包括：参与加里佛尼亚大学实验并随后转到科罗拉多州立大学的S. G. Carter；加里佛尼亚大学的V. Birkedal；加里佛尼亚大学的C. S. Wang,；加里佛尼亚大学的L. A. Coldren；来自美国NASA阿莫斯研究中心纳米技术中心的A. V. Maslov，和位于法国梅斯的洛林地区的乔治亚技术研究中心分部的D. S. Citrin。 该研究得到了美国国家科学基金会的资助。 英文原文链接见：http://www.physorg.com/news7676.html |
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英文原文，方便大家
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Results from experiments conducted at the University of California, Santa Barbara may lead to profound changes in optical communications. The discovery is reported in the October 28th edition of the journal Science. Physicist Mark Sherwin at UCSB explained that as information technology advances, scientists are intent on transmitting information much more quickly. "We are working toward sending information 100 times faster than it can be sent now," he said. His research group has spent five years on this project. The experiments were performed using the university's room-sized, free-electron laser. "We took an existing semiconductor device that is essentially an electrically controlled shutter and we have tried to open and close the shutter at the rate of three trillion times a second," he explained. "We found that in addition to opening and closing the shutter we are making the shutter itself vibrate." Those vibrations of the shutter may enable the shutter to be opened and closed with weak light beams rather than strong voltages, said Sherwin. In optical communications there are different channels of communications, so these light beams could correspond to different channels. "It would be a way of changing channels really fast," he added. "Right now it is a very slow process to change channels in optical communications. Sherwin explained that electronics are much slower than optics and that one optical fiber could transmit information more than 1,000 times as fast as the information could be put on it by an electronic device like a computer. "What we have here at UCSB is a special source of radiation, the free-electron laser, that can generate electromagnetic oscillations at the rate of a few trillion per second," said Sherwin. "We found that when you drive the modulator, or shutter, that fast it acts in a peculiar way. Rather than absorbing light near a single frequency, it can absorb light near a second frequency as well. This opens the possibility of a new type of cross modulation, where a beam of light at one of the absorption frequencies can turn on or off the light of the other." Sherwin said that light has been used to send information rapidly over long distances for more than 3000 years. The ancient Greeks, for example, used large fires to flash signals from mountain top to mountain top, as described by Homer in the Iliad. In order to send information, light must be modulated—that is, one must be able to turn the light beam on and off. In World War II, ships communicated with one another in code using searchlights that sailors modulated manually with shutters. Modern modulators for light are controlled by electrical voltages, explained Sherwin. "In an electro-absorption modulator, light near a particular frequency, the carrier frequency, can be blocked or transmitted by tuning a material oscillation in or out of resonance with the carrier frequency," said Sherwin. "A common electro-absorption modulator is made of a semiconductor quantum well, a thin layer of a semiconductor with a relatively small "band gap" (or a relatively large affinity for negatively charged electrons and positively charged holes) sandwiched between two layers with a larger band gap." Sherwin explained that when light of the correct frequency is incident on a quantum well, it creates bound electron-hole pairs called excitons and is absorbed. An electric field applied perpendicular to the plane of the quantum well shifts the frequency of the excitonic absorption so that light resonant with the zero-field excitonic resonance is no longer absorbed. Quantum well electro-absorption modulators are currently used to modulate light at rates exceeding 10 billion bits per second. In this article, the scientists report that a quantum well electro-absorption modulator has been strongly driven at frequencies exceeding one Terahertz (1 trillion cycles). This is more than 100 times faster than quantum well modulators are usually operated. At these extremely high frequencies, internal quantum-mechanical oscillations of the excitons themselves were excited. When the strong Terahertz drive was resonant with the excitonic oscillations, the absorption spectrum of weak light near the excitonic absorption of the quantum well was transformed from a single peak to a double peak, or doublet. This doublet is a signature that light with frequency near the excitonic absorption can no longer simply create an exciton in its lowest-energy state, but must create a quantum mechanical superposition of an exciton in its ground and excited states. A potential application for optical communication is that two arbitrarily weak light beams separated by the frequency of the Terahertz drive could modulate one another. "Usually, such cross-modulation occurs only when light beams have power exceeding a certain threshold," said Sherwin. On a separate note, Sherwin said, "In atomic gases, the doublet observed here has been the first step toward creating a system that could greatly slow or even stop the propagation of light. The ability to slow or stop light in a semiconductor would also enhance the toolbox for optical communications and computation. However, in order to achieve slowing or stopping of light, the mechanisms for energy dissipation in the quantum well modulator would have to be significantly reduced." The Science article, "Quantum Coherence in an Optical Modulator," was co-authored by S. G. Carter, who worked on the experiments at UCSB and then moved to the University of Colorado; V. Birkedal, from UCSB; C. S. Wang, from UCSB; L. A. Coldren, from UCSB; A. V. Maslov, from the Center for Nanotechnology at the NASA Ames Research Center; and, D. S. Citrin from the Georgia Institute of Technology and Georgia Tech Lorraine in Metz, France. The research was funded by the National Science Foundation. Source: University of California, Santa Barbara |
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