A phase pre-emphasis technique used in an all-optical sampling orthogonal frequency division multiplexing (AOS-OFDM) system is proposed and demonstrated. With the application of this technique, 50-Gb/s AOS-OFDM data are successfully transmitted over 20-km uncompensated single-mode fiber (SMF) with real-time direct-detection. The constructive interference effect between symbols is decreased with this technique.