  We have tuned the whispering gallery modes of a fused silica micro- resonator
over nearly 1 nm at 800 nm, i.e.over half of a free spectral range or the
equivalent of 10^6 linewidths of the resonator.This has been achieved by a new
method based on the stretching of a two-stem microsphere.The devices described
below will permit new Cavity-QED experiments with this ultra high .nesse
optical resonator when it is desirable to optimise its coupling to emitters
with given transition frequencies. The tuning capability demonstrated is
compatible with both UHV and low temperature operation which should be useful
for future experiments with laser cooled atoms or single quantum dots.A general
overview of the current state of the art in microspheres is given as well as a
more general introduction.
