Control of spontaneous emission coupling factor beta in fiber-coupled microsphere resonators

The spontaneous emission coupling factor is studied for fiber-coupled microspheres with a thin gain layer of phosphorus codoped sol-gel erbium silica-aluminum glass. From the input-output characteristics of the lasing, =0.039 is estimated for a sample with a gain layer on the sphere surface. is estimated to increase to 0.19 when an additional silica glass thin layer is overcoated to improve the overlap between the gain layer and the optical mode of the lasing inside the sphere