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The fabrication of whispering gallery lasers (WGL) is used to experimentallyevaluate the evaporation rate (mol/$\mu$m) and ratio (mol/mol) of erbium andsilica lost from a doped fiber during heating. Fixed lengths of doped silicafiber are spliced to different lengths of undoped fiber and then evaporated byfeeding into the focus of a CO$_{2}$ laser. During evaporation, erbium ions areprecipitated in the doped silica fiber to control the erbium concentration inthe remaining SiO$_2$, which is melted into a microsphere. By increasing thelength of the undoped section, a critical point is reached where effectively noions remain in the glass microsphere. The critical point is found using thelasing spectra of the whispering gallery modes in microspheres with equalsizes. From the critical point, it is estimated that, for a given CO$_{2}$laser power, $6.36 \times 10^{-21}$~mol of Er$^{3+}$ is lost during theevaporation process for every cubic micron of silica fiber. This is equivalentto $1.74 \times 10^{-7}$~mol of Er$^{3+}$ lost per mol of SiO$_{2}$ evaporated.This result facilitates the control of the doping concentration in WGLs andprovides insight into the kinetics of laser-induced evaporation of dopedsilica.

Evaporation characteristics of Er$^{3+}$ doped silica fiber and its application in the preparation of whispering gallery mode lasers