  With the use of twin, co-located, 3D interferometers, Cardiff University's
Gravity Exploration Institute aims to observe quantum fluctuations of
space-time as predicted by some theories of quantum gravity. Our design
displacement sensitivity exceeds that of previous similar experiments, which
have constrained the magnitudes of the fluctuations in the 1-25 MHz band. The
increased sensitivity comes in large part from the comparably higher
circulating power we aim to achieve, which reduces the overall shot noise. One
complication of higher circulating power is an increase in contrast defect
light, which includes higher-order modes. We will use the DC-readout scheme,
whose dark-fringe offset must sufficiently dominate the contrast defect in
order to detect faint signals. However, too much total output power risks
saturating the high-bandwidth photodetectors. Suppressing the higher-order mode
content of the contrast defect is a key strategy to realising the high
circulating power and eliminating non-signal-carrying power that contributes to
shot noise. For this, the inclusion of an output mode cleaner, whose design is
described, is required.

Instrumentation and Methods for Astrophysics;General Relativity and Quantum Cosmology