Optical field analysis of a circular grating-coupled surface-emitting laser with integrated focusing outcoupler

The lasing characteristics of a circular grating-coupled surface-emitting laser (GCSEL) with an integrated focusing outcoupler are investigated. Measured near and far fields suggest lasing in two near orthogonal and mutually incoherent linear elements, rather than ideal circular modes of the planar circular resonator, resulting in a degradation of the focused spot quality. Modeling of the evolution of the emitted optical field by Fresnel diffraction calculations, using a new orthogonal projection scheme to account for polarization, supports this assumption. Using the theory, we have also estimated the degradation of the focused spot quality induced by random errors in the period of the focusing outcoupler caused by the limited resolution of the electron-beam lithography system used to fabricate the device. We found this effect to be small compared to the effect of the nonideal mode characteristics.