  Short (40-200 microseconds) single focused CO2 laser pulses of energy of
about 100 microJ were used to fabricate high quality concave micromirror
templates on silica and fluoride glass. The ablated features have diameters of
20-100 microns and average root-mean-square (RMS) surface microroughness near
their center of less than 0.2 nm. Temporally monitoring the fabrication process
revealed that it proceeds on a time scale shorter than the laser pulse
duration. We implement a fast feedback control loop (20 kHz bandwidth) based on
the light emitted by the sample that ensures an RMS size dispersion of less
than 5 percent in arrays on chips or in individually fabricated features on an
optical fiber tip, a significant improvement over previous approaches using
longer pulses and open loop operation.
