Impact of polarization of supermodes on the high-frequency response of coupled aperture VCSELs

The shape of couped oxide-confined apertures governs the selective losses and dynamics of main optical supermodes in VCSELs and their polarization. (i) For the case of coupled apertures with broader (~1.5μm) and shorter (~2μm) bridge connecting the non-oxidized regions the VCSELs can demonstrate both co-polarized and cross-polarized lasing despite of the significant shape anisotropy of the coupled aperture. The loss discrimination among the polarized modes is weak. Strong antiphase intensity oscillations exist in the coupled apertures for symmetric (S) and antisymmetric (AS) supermodes are observed in streak camera studies in this case. Polarization switching and polarization hysteresises confirm low scattering losses in the coupled aperture system resulting in quasi-equal threshold currents and gains for the supermodes at different polarizations. The dephasing time is long, exceeding tens of nanoseconds. Upon current increase the splitting of the modes and the intensity oscillation frequency increase linearly with current reaching 50-70GHz. Once the device switches to self-injection locked (SIL) regime characterized by a single AS mode, the oscillation frequency decreases by ~25GHz and both apertures oscillate in phase. Upon further current increase the same linear slope of the frequency vs current is reestablished up to 60-70GHz. Cross-polarized modes revealed below the onset of SIL regime may be responsible for the resonance frequency feature in the modulation response observed in circular polarized studies, even when the intensity of the cross-polarized mode is weak. (ii) For the coupled apertures connected by a longer (~4μm) and narrower (~1μm) bridge only co-polarized along the coupling axis S and AS modes are observed, indicating that scattering loss mechanisms become important and significantly and selectively affect threshold gains for differently polarized modes. No antiphase oscillations are observed for such chips below SIL threshold, and the mode splitting is reducing with current. In the SIL regime defined by the AS mode, in-phase oscillations evolve in both cavities at a frequency matching the reduced mode splitting ~20GHz at currents in the very vicinity of the onset of the SIL regime. The frequency of the first post-excitation oscillation rapidly increases above SIL threshold linearly with current reaching effective bandwidths ~100GHz. The following two oscillations proceed at lower frequencies. Further oscillations demonstrate further reduced frequencies and decay rapidly with time. Our data indicates that shape engineering can effectively control the mode scattering mechanisms. Consequently, one can design the intrinsic modulation response either by making it suitable for ultrahigh data transmission rates in digital format, or, as opposite, for generation of stable frequencies controlled by drive current. Cavity engineering becomes, however, challenging once ultimate control over the shape is required. Avoidance of oxide-confined apertures by applying purely metal apertures for optical confinement enables drastic extension of shape and loss engineering options while keeping low threshold, high differential efficiency and controlled S and AS mode splitting. We show that coupled apertures can be also generated by introducing etch pattern in the top dielectric cap layer, which effectively confines optical emission within ~1μm-scale areas and generates specific multispot near filed patterns for high order modes.