Quantum non-demolition (QND) measurements are of great interest in recent years as a method to increase the information capacity of communication systems. Common implementations of QND measurements in optical fiber are based on the Kerr effect: a probe beam acquires phase modulation which mimics the amplitude modulation of a signal beam. This phase is typically small and needs to be measured at high frequencies for practical fiber-optic communication systems. Here we present a self reflexive interferometer scheme adequate for this kind of measurements and describe its performance. Using this system we achieved fairly open eye-diagrams at 140 Mbit/sec.