MITSUBISHI

MITSUBISHI OPTICAL SEMICONDUCTORSPRINCIPLES OF LASER OSCILLATION2,STRUCTURE OF AIGaAs LASERLaser Beam-------~-----r.~~~--~n-GaAsSubstrate(-)Energyn-AlxGal_xAs(Clad layer)nr ,---------,H00oo00oI..Q.IIIp-AlxGal~xAs(Clad layer)"I I.. d1 1I 1••• ~ •• h---.......,(E~ctron)!:•••••••• • • ••• IIT'TTTT'I"rrT''''''''''11I 1~ !-,----I :1 11 1p(Y)~n..l--.L:~1AlyGal ~yAs (y< x)(Active layer)(a)(+)Structure ofCross sectionLayer(b) Energy band(C)Injection carrierdensity (Gain)(d) Refractive index(e)Distribution oflight intensityFig. 2 Operational principle of double hetero junction laserA laser diode can't oscillate if amplication hasoccured, Laser oscillation never happens unless ithas feedback. In order to achieve this a "Fabry­Perot" oscillator is often used. (See Fig. 2-(a)).The operation principles are: use the two cleavedfacets as reflection mirrors. At first the beams goin every direction within the active layer. However.only selected beams that correspond to the Fabry- Perot oscillator's direction would be confinedbetween these mirrors and the result that opticaloscillation occurs.AIGaAs laser diode for example, has a doublehetero junction structure, where an AlyGal - Asactive layer is sandwiched between nand p-AlxGal-xAs cladding layers (y< x). (See Fig. 2-(a)). Theamplifying media is a double hetero junction narrowband gap layer (AlyGal- yAs active layer). By makingthe p - clad layer E& and the n - clad layer e, andadding forward directed bias, electrons and holeswill be injected from the nand p-cladding layers.Carriers will be confined to the active layer andcreate 'population inversion'. The result is that ahigh gain will be obtained, because' the band - gapof the cladding layer is larger than that of activelayer and a barrier will be produced in the junction.(Fig. 2- (b), (c)). Light has a characteristic that itgathers to' the area of high refractive index. Therefractive index of the activation layer is higherthan that of cladding layer. in an AIGaAs LASER.(Fig. 2 - ( d) ) .The light will therefore remain in the active layer.This is called 'optical confinement'. In this way.stimulated emission will be performed effectively,because carriers and light are confined in the activelayer.• ,MITSUBISHI..... ELECTRIC 1 - 13