  We demonstrate the integration of Nd3+ doped Barium-Titanium-Silicate
microsphere lasers with a Silicon Nitride photonic platform. Devices with two
different geometrical configurations for extracting the laser light to buried
waveguides have been fabricated and characterized. The first configuration
relies on a standard coupling scheme, where the microspheres are placed over
strip waveguides. The second is based on a buried elliptical geometry whose
working principle is that of an elliptical mirror. In the latter case, the
input of a strip waveguide is placed on one focus of the ellipse, while a
lasing microsphere is placed on top of the other focus. The fabricated
elliptical geometry (ellipticity=0.9) presents a light collecting capacity that
is 50% greater than that of the standard waveguide coupling configuration and
could be further improved by increasing the ellipticity. Moreover, since the
dimensions of the spheres are much smaller than those of the ellipses, surface
planarization is not required. On the contrary, we show that the absence of a
planarization step strongly damages the microsphere lasing performance in the
standard configuration.
