Theoretical Investigating On The Polarization In VCSEL Subjected To Optical Feedback

As a new kind of semiconductor laser,vertical-cavity surface-emitting lasers(VCSEL) have such merits as low threshold,working in single longitudinal mode,small angle of divergence,high dynamic modulating frequency,easily implemented two-dimension integration than conventional edge emitting lasers.VCSEL have a significant implication prospect in optical communication, optical interconnection,image signal processing,etc.With its increasing application,the research of VCSEL's optical output characteristic becomes very important.The structure of conventional edge emitting lasers is not symmetrical,so the crystals in cavity show that there is strong anisotropy.Transverse electric modes of output are commonly linearly polarized along a fixed axis.However,the structure of VCSEL is cylindrical and symmetrical and the crystals in cavity are weak anisotropy.The polarization performances are usually described with well-known spin-flip model.The SFM model shows the emission of VCSELs is commonly linearly polarized along one of two preferred orthogonal directions.When external environment,such as bias current,optical feedback,optical injection,frequency detuning and so on,changes,the polarization may switches abruptly.In this paper,polarization switching performances and polarization bistability performances in VCSEL subjected to optical feedback had been investigated.The results show that,for polarization switching performance,polarization-preserved optical feedback made PS processes even slower,and the regional that two polarization modes coexistence appeared in P-I curve.Polarization-selected optical feedback can make one polarization mode lasing and the other polarization mode be suppressed.Which direction the feedback light is,VCSEL will oscillate in this direction.For polarization bistability performance,we respectively investigated it subjected to optical single trip feedback and multiple trip feedback.For single trip feedback approximation,along with feedback factors increase,regardless of polarization-preserved optical feedback or polarization-selected optical feedback,the VCSEL polarization bistable characteristic like such as the width of the bistablity loop,the switch-on and switch-off current and so on have the obvious change.For multiple trip feedback approximation,these changes are more obvious,and with the increase of the strength of external optical feedback,the difference is further increased.These results show that,for relatively higher facet reflectivity of VCSEL,though single trip optical feedback approximation can explain some physical processes to a certain degree,the error is inevitable due to the fact that the multiple trip feedback usually exists.Therefore,in order to correctly model the polarization bistability performance in VCSEL subject to optical feedback,the multiple optical feedbacks should be taken into account.The above results can provide an instructive insight to researchers who investigate polarization switching and polarization bistability performance.