Direct visualization of the in-plane leakage of high-order transverse modes in vertical-cavity surface-emitting lasers mediated by oxide-aperture engineering

Oxide–confined apertures in vertical cavity surface emitting laser (VCSEL) can be engineered such that they promote leakage of the transverse optical modes from the non– oxidized core region to the selectively oxidized periphery of the device. The reason of the leakage is that the VCSEL modes in the core can be coupled to tilted modes in the periphery if the orthogonality between the core mode and the modes at the periphery is broken by the oxidation–induced optical field redistribution. Three–dimensional modeling of a practical VCSEL design reveals i) significantly stronger leakage losses for high–order transverse modes than that of the fundamental one as high–order modes have a higher field intensity close to the oxide layers and ii) narrow peaks in the far–field profile generated by the leaky component of the optical modes. Experimental 850–nm GaAlAs leaky VCSELs produced in the modeled design demonstrate i) single–mode lasing with the aperture diameters up to 5μm with side mode suppression ratio >20dB at the current density of 10kA/cm2; and ii) narrow peaks tilted at 37 degrees with respect to the vertical axis in excellent agreement with the modeling data and confirming the leaky nature of the modes and the proposed mechanism of mode selection. The results indicate that in– plane coupling of VCSELs, VCSELs and p–i–n photodiodes, VCSEL and delay lines is possible allowing novel photonic integrated circuits. We show that the approach enables design of oxide apertures, air–gap apertures, devices created by impurity–induced intermixing or any combinations of such designs through quantitative evaluation of the leaky emission.