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七郎 / Seven Liu

科学与艺术,恰如人生中的红颜知己。 本博内容保留所有权利.

 
 
 

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4.10 Coherence 相干性  

2012-06-26 17:19:55|  分类: 读点英文 |  标签: |举报 |字号 订阅

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Coherence is a possible aspect between light waves. The coherence phenomenon is used in optical disc technology. Some types of light waves show coherence. This is indicating how well these light waves can interfere with each other. There are several different forms of coherence. In this chapter an attempt to explain the temporal coherence and the spatial coherence will be done. Temporal coherence is the measure of hove well a wave can interfere with itself at a different time. If the amplitude and wavelength of a light source are very stable, the temporal coherence is high.

光波之间可能会发生相干,光盘技术中就用到了相干现象的原理。有些类型的光波会呈现相干性。相干性是指光波会在多大程度上发生相互干涉的现象。相干现象分为几种类型。本章试图解释时间相干性和空间相干性。时间相干性是测量一个光波在不同时间与自己发生干涉的程度。如果光源的振幅和波长非常稳定,那么时间相干性就高。

The delay over which the phase or amplitude wanders by a significant amount is defined as the coherence time Tc. The coherence length Lc is defined as the distance the wave travels in time Tc. A wave containing only a single frequency with constant amplitude has infinite coherence time.

经过一段延迟时间后,假若一个波的相位或波幅开始发生足够显著的变化,就可以定义这个延迟时间为"相干时间Tc"。相干长度Lc则定义成在相干时间内光波传播的距离。如果一个波具有单一频率,并且振幅恒定,那么其相干时间为无限。

(维基百科图片)

When the phase, wavelength or amplitude is changing over time the coherence time and coherence length are short.

当相位、波长或振幅在时间上是变化的,那么相干时间和相干长度都会是短的。

Laser light can have a very long coherence length, up to hundreds of meters. Holography is one field where light with a long coherence time is required. Spatial coherence describes the ability for two points P1 and P2 in the extent of a wave to interfere. The range of separation between the two points over which there is the significant interference is called the coherence area, Ac.

激光可拥有很长的相干长度,甚至长达几百米。全息摄影术就是使用长相干时间光波的例子。空间相干性描述了从P1、P2两点发出的光波相互干涉的能力。分离两点之间产生明显干涉的范围被称之为干涉面积Ac。

This aspect is important for optical disc technology as interference is one of the key phenomena for generation of signals from a disc. It is important that the waves from all parts of the focused laser beam are being in phase when they reach the laser spot. This is achieved by ensuring that the wave profile or wave front is plane, in the parallel beam before the focus optics.

干涉是从光盘再生信号的关键物理现象之一,对于光盘技术来说是非常重要的。聚焦光束的光波同相到达光斑处是非常重要的,这要通过使光波截面或"波前面"呈平面来实现的,即在聚焦之前为平行光束。

 

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Key Words:

·holography 全息术,[激光]全息照相术

·temporal coherence 时间相干性

·spatial coherence 空间相干性

·in the extent of 在…的范围内;到…的程度

 

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