Investigation On The Characteristics Of Polarization Switching And Bistability In VCSELs Under External Disturbances

As a new type of semiconductor laser, vertical cavity surface emitting lasers (VCSELs) possess many advantages such as small divergence angle, low threshold current, single longitudinal mode outputs, easy to integrate into two-dimensional arrays, and high modulating frequency, and then they have a broad application prospect in optical communications, optical interconnects, optical storage and optical switch etc. The researches have demonstrated that the output of a VCSEL generally includes two perpendicular linear polarized modes due to the weak cavity and material anisotropy, which leads to unstable linear polarization mode in VCSELs. For a VCSEL with determined polarization direction, its polarization direction can be the polarization mode of a single polarization mode VCSEL which could be abruptly hop when some parameters of lasers are changed, then the suppressed mode turns to be dominant model, and the original dominant mode is suppressed at the same time, which is named as polarization switching(PS). Studies also found that the positions of polarization switching could be changed with the change route of external parameter, which is named as polarization bistable (PB). The polarization switching and polarization bistability characteristics of VCSELs could be affected by adjusting the exterior perturbation of VCSELs such as optical injection, optical feedback, etc.In this paper, based on the spin-flip model(SFM) of vertical cavity surface emitting lasers(VCSELs), the characteristics of polarization switching and bistability in VCSELs under a variety of external disturbances especially under weak optical feedback and orthogonal time-varying optical injection were studied. The results show that:when the slave VCSEL(S-VCSEL) injected by the master laser(M-VCSEL) with orthogonal time-varying optical injection, the introduction of weak optical feedback will not chang the character that the bistable width enlarge with the increases of orthogonal time-varying injection scan rate of VCSEL, however, the introduction of weak optical feedback will cause VCSEL outputs chaotic state near positive scanning polarization switching(PS), which can lead the bistable region edges of positive scans appear fluctuate. For a fixed sweeping rate of the injection coefficient, with the increase of the feedback strength, the hysteresis width will narrow. Under the case that there exists frequency detuning Av (=vm-vs, vm and vs are the free running frequencies of M-VCSEL and S-VCSEL, respectively) between the M-VCSEL and S-VCSEL, with the increase of Δv from negative value to positive value, the hysteresis width narrows firstly, after reaches a minimum, and widens finally. Bistable loop width appear larger fluctuation when frequency detuning lies between-2GHz～-12GHz with no feedback, and the fluctuation can be suppressed to some extent after introducing optical feedback. Therefore, by controlling the scanning rate of time-varying injection coefficient, optical feedback coefficient and the frequency differences between M-VCSEL and S-VCSEL, the polarization bistable size and the polarization switch position will adjustable.