Investigation On The Polarization Switching Characteristics Of 1550 Nm Vertical-cavity Surface-emitting Lasers Under Double Polarization Optical Injection

Vertical-cavity surface-emitting lasers(VCSELs),as a new type of optoelectronic devices,have attracted much attention due to their unique advantages such as low threshold current,single-longitudinal mode operation,large modulation bandwidth,low cost,and easy integration into two-dimensional arrays,etc.In particular,due to the special structure of VCSELs,and the dichroism and birefringence effects of active region gain material,a free running VCSEL can output two orthogonal polarization components.When bias current or device temperature are changed,or these external disturbances such as optical feedback and optical injection are introduced into the free-running laser,a power alternation between two polarization components can be observed,which denotes that polarization switching(PS)happens.In general,the optical injection is extensively adopted to study the PS characteristics in VCSELs.So far,most of attentions are mainly focused on continuous optical injection method,while the study for PS characteristics of VCSELs under pulsed injection,especially under double polarization pulsed injection is relatively scarce.Additionally,the switching time of PS is an important issue in information processing rate and system stability.Therefore,studying the PS characteristics of VCSELs has great significance for the development of future all-optical information processing systems based on VCSELs.In this paper,based on the spin-flip model(SFM),the PS characteristics of two polarization outputs in a 1550 nm VCSEL subject to double polarization pulsed injection(DPPI)are investigated theoretically.The simulated results show that,compared with the case,in which the 1550 nm VCSEL is simultaneously injected by the pulses of orthogonal polarized light and a continuous wave(CW)parallel polarized light(double optical injection with pulsed orthogonal polarization(DIPOP)),the shorter switching time and larger extinction ratio can be achieved for DPPI method under suitable parameters.For the pulse injection signal with 50 MHz modulation frequency,the PS characteristic of the 1550 nm VCSEL under DPPI is affected by the time difference(?t)in the switching of two injection pulses,and an optimized ?t value can be determined for each bias current,and the value tend to decrease with the increase of bias current.For the fixed bias current,optimized ?t value and frequency detuning,the switching time of X(Y)polarization output mainly dependent on the polarized pulsed injection strength Einj,x(Einj,y)and the switching time becomes shorter with increasing Einj,x(Einj,y).For a fixed injection strength,the negative frequency detuning is helpful to shortening the switching time of X and Y polarization outputs.As a result,the switching times for two polarization outputs in 1550 nm VCSEL can be controlled by adjusting the injection strength and frequency detuning.Furthermore,the corresponding system response of the 1550 nm VCSEL under the two 2.5 Gb/s NRZ signals injection are also given.