Focal plane filters are being used to implement wave front sensors in high-resolution adaptive optics systems. We present experimental results obtained with a closed-loop adaptive optics system that uses a differential phase- contrast focal plane filter. The differential filter in our system is implemented using an electrically-addressed liquid-crystal spatial light modulator (LCSLM) operated in a phase-mostly mode. Two additional phase-mostly LCSLM devices are used in our experiment. One is used as a phase screen to introduce simulated turbulence into the propagating input beam, and the other is used as the wave front corrector driven by closed-loop feedback signals from the phase- contrast wave front sensor. This paper describes the system implementation and presents some preliminary experimental results.