  We present a compact, fibre-coupled interferometer with high sensitivity and
a large working range. We propose to use this interferometer as a readout
mechanism for future inertial sensors, removing a major limiting noise source,
and in precision positioning systems. The interferometers peak sensitivity is
$2 \times 10^{-{14}}$ m/${\sqrt{\rm{Hz}}}$ at 70 Hz and $8 \times 10^{-{11}}$
m/$\sqrt{\rm{Hz}}$ at 10 mHz. If deployed on a GS-13 geophone, the resulting
inertial sensing output will be dominated by suspension thermal noise from 50
mHz to 2 Hz.
