  Precise mode matching is needed to maximize performance in coupled cavity
interferometers such as Advanced LIGO. In this paper we present a new mode
matching sensing scheme that uses a single radio frequency higher order mode
sideband and single element photodiodes. It is first order insensitive to
misalignment and can serve as an error signal in a closed loop control system
for a set of mode matching actuators. We also discuss how it may be implemented
in Advanced LIGO. The proposed mode matching error signal has been successfully
demonstrated on a tabletop experiment, where the error signal increased the
mode matching of a beam to a cavity to 99.9%.
