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为光源的亮度定义为单位截面、单位立体角内发射的光功率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。Thereareseveralimportantandfundamentaldifferencesindetail,however,betweenthe“incoherent”lightemittedbyanythermallightsource,suchastheflashlightandthe“coherent”lightemittedbyalaseroscillator.Theflashlight,likeanyotherthermallightsource,emitsagenerallybroadbandcontinuumoflightatmanydifferentwavelengths.Therearelightsources,suchasdischargelamps,thatemitonlycomparativelyfewspectrallinesornarrowbandsofwavelengths,butthespectralwidthsofthelightemittedbyeventhebestsuchsourcesarestilllimitedbythelinewidthsoftheatomictransitionsinthedischargeatoms.Theoutputbeamsfrommostlaserscanbe,bycontrast,highlymonochramatic,andinideallaserscanconsistalmostentirelyofasinglefrequency.Thatis,theoutputsignalforanear-ideallaserwillbeanearlypure,constant-amplitude,highlystable,single-frequencysinewave,exactlylikethesignalgeneratedbyahighlystableelectronicoscillatorinanyotherfrequencyrange.Thetermcoherencenecessarilyrefersnottoonepropertyofasignalatasinglepointinspaceandtime,buttoarelationship,orafamilyofrelationships,betweenonesignalatonepointinspaceandtime,andthesameoranothersignalatotherpointsinspaceandtime.Morecolloquially,asignaliscalled“temporallycoherent”ifthereisstrongcorrelationinsomesensebetweentheamplitudeand/orphaseofthesignalatanytimeandatearlierorlatertime.Boththeamplitudeandthephaseofagood-qualitylaseroscillatorwillinfactchangeonlyslowlywithtime,sothattheamplitudeandthephaseoftheoutputsinewavefromthelaseratanyonetim