  We demonstrate the suppression of parametric instability using through
optical actuation in an electro-optical feedback loop, stabilising the high
order optical mode content in an 80 metre long Fabry-Perot cavity. The loop
suppression of the high order mode is achieved by injecting a high order mode
with the same frequency and opposite phase. Frequency matching is achieved by
measuring the beat note signal between the fundamental and high order mode in
transmission of the cavity and applying that signal to an electro-optical
modulator to create the required frequency sideband. Spatial mode matching of
the sideband to the high order mode is accomplished through the inherent mode
overlap between the input injected beam and the high order mode of the cavity.
The paper presents the theoretical analysis and experimental demonstration of
parametric instability suppression, for an instability which would normally
ring up with a parametric gain of approximately 2.5.
