激光的特性(laser.ppt

激光的特性(Laser output-beam properties) J单色性： 指光强按频率的分布状况，激光的 频谱宽度非常窄 J相干性：时间相干性和空间相干性都很好 J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 J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对基横模、单纵模激光束立体角的单位为sr，称为球面度。

1sr是这样的 立体角：其顶点位于球心，它在球面上所截取 的面积等于以球半径为边长的正方形面积Area• There are several important and fundamental differences in detail, however, between the “incoherent” light emitted by any thermal light 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, such 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 can 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 stable 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 other 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 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 of 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 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 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 output 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 (in radians)• Suppose this same uniphase laser beam with initial diameter d is focused down to a spot of diameter d0 by means of a simple lens of focal length f. The diameter d0 of the focused spot can then be calculated by applying the same angular spread condition in reverse, to obtain • 一般通称激光的四性为：单色性、相 干性、方向性和高亮度。

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。