Single-Mode Single-Polarization Multijunction VCSELs

Increasing the number of active regions is an effective approach for scaling optical output power in vertical cavity surface emitting lasers (VCSELs). However, due to a more complex cavity structure, these devices can exhibit simultaneous stimulation of multiple higher order transverse modes. This issue is further exacerbated especially in oxide confined VCSELs, in which the possible differences in length, shape, thickness, and composition of each oxide layer can give rise to stimulation of a higher order mode. Moreover, due to cylindrical symmetry of the cavity structure, in addition to complete isotropy of the semiconductor gain material, VCSELs do not exhibit a stable and dominant polarization direction throughout their operation. In this paper, we report on design, fabrication, and characterization of single-mode single-polarization 8-junction 940nm VCSELs. Single-mode operation has been achieved by using a surface relief mode filter to preferentially reduce the effective mirror reflectivity for higher order transverse modes, and consequently, increase their threshold gain. Optimizing the shape and the size of the relief feature has shown to be effective in inducing a dominant polarization direction for the VCSELs, in which a non-circular relief feature can perturbate the cylindrical symmetry of the cavity structure. The effect of elliptical relief feature on stabilizing the polarization behavior of VCSELs has been investigated. Room-temperature continuous-wave (CW) L-I-V characteristics of a 5um VCSEL with such relief feature shows a single-mode output power of similar to 13.5mW at similar to 3mA bias current (sidemode suppression ratio >30dB) and polarization extinction ratio greater than 20dB. Moreover, this VCSEL remains reliably single-mode with stable polarization characteristics at different temperatures up to 95 degrees C.