Vertical coupling of laser glass microspheres to buried silicon nitride ellipses and waveguides

TL;DR

This paper demonstrates the integration of laser glass microspheres with silicon nitride photonic platforms, introducing an elliptical geometry that enhances light collection efficiency without surface planarization.

Contribution

The study presents a novel elliptical coupling scheme for microsphere lasers on silicon nitride platforms, improving light collection by 50% over standard methods.

Findings

Elliptical geometry increases light collection capacity by 50%.

Surface planarization is unnecessary with smaller spheres in elliptical configuration.

Standard coupling performance is negatively affected by lack of planarization.

Abstract

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.