  Whispering gallery mode (WGM) lasers are well-known for their small sizes,
low thresholds and narrow linewidths, and have been the subject of research and
development for more than two decades. However most of the initial reports and
studies have focused on near-IR and visible WGM microlasers. Recent advances in
the synthesis, doping and processing of high quality mid-IR transparent glasses
have provided the means to extend the wavelengths of these microlasers to the
mid-IR range. In this paper, we present a detailed characterization of a
compact and low-cost mid-IR spherical WGM microlaser based on heavily doped
Er:ZBLAN glass operating near 2.7 um with an estimated linewidth less than 1.1
MHz. We describe experimental results on observed single-mode and multimode
spectra, thermal tuning, and polarization characteristics of such microlasers.
Using a simple analysis of spherical microresonator modes and including
considerations of pump and laser mode overlap we provide guidelines for
optimized microlaser design.
