Effects of Size and Surrounding Medium on Whispering-Gallery-Mode Lasers in Er3+-Doped Silica Microspheres

Abstract

This work reports the experimental fabrication and characterization of the Er3+-doped silica microspheres (mS) and numerical investigation of the effects of size and surrounding medium on the optical properties of whispering-gallery-mode (WGM) lasers. The heat melting method of two discharge electrodes was used to produce the Er3+-doped silica mSs of diameters up to several tens of micrometers. The 125-mm diameter single-mode optical fiber was tapered with a cone angle formed by chemical etching in hydrofluoric acid (HF) solutions. It was used to produce the mSs and couple the pumped laser into mS surface as well was coupled out the lasing emission. The WGM lasers at telecom regime of ~1520-1570 nm were characterized in both clockwise (CW) and counterclockwise (CCW) propagation directions. By adjusting the coupling gap between the tapered optical fiber and the mS surface, the selectivity of the multi- or single-emitted modes of the mS laser was achieved. We performed finite-difference time-domain (FDTD) simulations to examine the size dependence and analyze the effect of the surrounding medium’s refractive index on the optical characteristics, such as emission wavelength, intensity, as well as the shape of WGM lasing emission. The facile approach and quantitative investigation of this work has attracted much attention from researchers in the micro-photonic field and may be useful in many applications from tunable single-mode lasing sensing to optical micro-devices.