  Differential wavefront sensing is an essential technique for optimising the
performance of many precision interferometric experiments. Perhaps the most
extensive application of this is for alignment sensing using radio-frequency
beats measured with quadrant photodiodes. Here we present a new technique that
uses optical demodulation to measure such optical beats at significantly higher
resolutions using commercial laboratory equipment. We experimentally
demonstrate that the images captured can be digitally processed to generate
wavefront error signals and use these in a closed loop control system for
correct wavefront errors for alignment and mode-matching a beam into an optical
cavity to 99.9\%. This experiment paves the way for the correction of even
higher order errors when paired with higher order wavefront actuators. Such a
sensing scheme could find use in optimizing complex interferometers consisting
of coupled cavities, such as those found in gravitational wave detectors, or
simply just for sensing higher order wavefront errors in heterodyne
interferometric table-top experiments.
