  The fabrication of whispering gallery lasers (WGL) is used to experimentally
evaluate the evaporation rate (mol/$\mu$m) and ratio (mol/mol) of erbium and
silica lost from a doped fiber during heating. Fixed lengths of doped silica
fiber are spliced to different lengths of undoped fiber and then evaporated by
feeding into the focus of a CO$_{2}$ laser. During evaporation, erbium ions are
precipitated in the doped silica fiber to control the erbium concentration in
the remaining SiO$_2$, which is melted into a microsphere. By increasing the
length of the undoped section, a critical point is reached where effectively no
ions remain in the glass microsphere. The critical point is found using the
lasing spectra of the whispering gallery modes in microspheres with equal
sizes. 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 the
evaporation process for every cubic micron of silica fiber. This is equivalent
to $1.74 \times 10^{-7}$~mol of Er$^{3+}$ lost per mol of SiO$_{2}$ evaporated.
This result facilitates the control of the doping concentration in WGLs and
provides insight into the kinetics of laser-induced evaporation of doped
silica.
