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1、激光原理,第一章 激光的基本原理,光学发展简史（省略） 激光发展 激光的物理基础 激光特性 典型激光器简介,光学的范畴,狭义来说，光学是关于光和视见的科学，optics（光学）这个词，早期只用于跟眼睛和视见相联系的事物。 广义来讲，是研究从微波、红外线、可见光、紫外线直到 X射线的宽广波段范围内的、关于电磁辐射的发生、传播、接收和显示、以及跟物质相互作用的科学。,分支：几何光学、波动光学、大气光学、 海洋光学、 量子光学、光谱学、生理光学、 电子光学、集成光学、 空间光学,激光发展简史,1917年Einstein的理论预言：光子和原子相互作用包含三种过程:自发辐射、受激吸收和受激辐射（按此模型
2、推导出和实验完全符合的黑体辐射Planck公式），提出在物质与辐射场的相互作用中，构成物质的原子或分子可以在光子的激励下产生光子的受激发射或吸收，预示了有可能利用受激发射实现光放大（Light Amplification by Stimulated Emission of RadiationLASER ） 理论工作指出：受激辐射光波与激励光波同相位、同方向、同频率、同偏振,1954年汤斯(Townes)、巴索夫(Basov)、普洛霍诺夫(Prokhorov) 利用原子、分子的受激辐射来放大电磁波的新概念，实现氨分子微波量子振荡器(Maser)-量子电子学 1958年汤斯、肖洛(Schawlow
3、)尺度远大于波长的开放式光谐振腔（Fabry-Perot） 布隆伯根（Bloembergen）利用光泵浦三能级原子系统实现粒子数反转分布 1960年梅曼（Maiman）世界上第一台红宝石激光器,1961年：He-Ne激光器、第一台调Q激光器、钕玻璃激光器 1962年：砷化镓（GaAs）半导体激光器 1963年：激光器的半经典理论 1964年：氩离子（ Ar+）激光器、二氧化碳激光器、掺钕钇铝石榴石（Nd:YAG）激光器 1965年：铌酸锂光参量振荡器 1966年：固体锁模激光器、染料激光器 1970年：准分子激光器 1977年：红外波段的自由电子激光器,发明了近百种不同类型的激光器件，其发明者
4、都有完全不同 的专业背景 各种学科和技术应用激光并形成了许多重要的交叉学科：如光电子学、激光光谱学、 激光化学、非线性光学、激光全息术、激光生物医学、 激光计量、超快光电子学、激光加工处理 形成了一系列重要技术应用领域，光子产业方兴未艾,它的历史是一部典型的交叉学科的发展史：,激光的特性 (Laser output-beam properties),单色性： 指光强按频率的分布状况，激光的频谱宽度非常窄 相干性：时间相干性和空间相干性都很好 方向性：普通光向四面八方辐射，而激光基本沿某一直线传播，激光束的发散角很小 高亮度：在单位面积、单位立体角内的输出功率特别大,空间相干性和方向性是紧密联系
5、的，与激光的横模结构相联系。（横模代表光腔模式的横向光场分布） 如果激光是单横模结构，同一模式内的光波场是空间相干的；而另一方面，单横模结构又具有最好的方向性。反之，如果激光是多横模结构，由于不同模式的光波场是非相干的，所以激光的空间相干性程度减小；另一方面，多横模意味着方向性变差（高次模发散角加大）,为了提高激光器的空间相干性，应限制激光器工作在单横模；合理选择光腔的类型以及增加腔长以利于提高光束的方向性 激光所能达到的最小光束发散角还要受到衍射效应的限制，它不能小于激光通过输出孔径时的衍射角m 设输出孔径为2a，则衍射极限为,激光束的空间相干性和方向性对它的聚焦性能有重要影响。 当一束发散
6、角为的单色光被焦距为F的透镜聚焦时，焦平面光斑直径D为,时间相干性和单色性是紧密联系的c=1/ ， -频带宽度 光源的单色亮度定义为单位截面、单位频带宽度和单位立体角内发射的光功率,四性本质上归结为：激光具有很高的光子简并度。激光可以在很大的相干体积内有很高的相干光强。这一特性正是由于受激辐射的本性和激光腔的选模作用才得以实现的。,There are several important and fundamental differences in detail, however, between the “incoherent” light emitted by any thermal lig
7、ht source, such as the flashlight and the “coherent” light emitted by a laser oscillator.,Ideal laser monochromaticity and frequency stability,The flashlight, like any other thermal light source, emits a generally broadband continuum of light at many different wavelengths. There are light sources, s
8、uch as discharge lamps, that emit only comparatively few spectral lines or narrow bands of wavelengths, but the spectral widths of the light emitted by even the best such sources are still limited by the linewidths of the atomic transitions in the discharge atoms.,The output beams from most lasers c
9、an be, by contrast, highly monochramatic, and in ideal lasers can consist almost entirely of a single frequency. That is, the output signal for a near-ideal laser will be a nearly pure, constant-amplitude, highly stable, single-frequency sine wave, exactly like the signal generated by a highly stabl
10、e electronic oscillator in any other frequency range.,The term coherence necessarily refers not to one property of a signal at a single point in space and time, but to a relationship, or a family of relationships, between one signal at one point in space and time, and the same or another signal at o
11、ther points in space and time. More colloquially, a signal is called “temporally coherent” if there is strong correlation in some sense between the amplitude and/or phase of the signal at any time and at earlier or later time.,Both the amplitude and the phase of a good-quality laser oscillator will
12、in fact change only slowly with time, so that the amplitude and the phase of the output sine wave from the laser at any one time will be strongly correlated with the amplitudes and phases at considerably earlier or later times. A good laser beam might thus be said to be temporally coherent because o
13、f this strong correlation between the amplitudes and phases of the signal at not very different points in time.,A good-quality laser oscillator can also oscillate in a single transverse-mode pattern, which has a definite and specific amplitude and phase pattern across any transverse plane inside the
14、 laser, and particularly across the output mirror. In this situation there is a very high degree of correlation between the instantaneous amplitudes, and especially between the instantaneous phase angles, of the wavefront at any two points across the output beam. We can then also say that the output
15、 beam possesses a very high degree of “spatial coherence” (in the transverse direction).,A single-transverse-mode laser oscillator can produce (usually in practice, and always in principle) an output beam that is more or less uniform in amplitude and constant in phase (“uniphase”) across its full ou
16、tput aperture of width of diameter d. Such a beam can propagate for a sizable distance with very little diffraction spread; will have a small far-field angle at still larger distances; and can be focused into a spot only a few wavelengths in diameter.,Elementary diffraction theory says, for example, that a uniphase plane wave coming from an aperture of diameter d will have a minimum angular diffraction spread in the far field given by,
