We demonstrate a simple and ultra-sensitive all-optical sampling system suitable for the characterization of high capacity (> 100 Gbit/s) single channel systems operating around 1.5 µm. The system is based on the nonlinear effect of two-photon-absorption in a commercial 1.3 µm semiconductor laser and, using only direct detection of the unamplified two photon absorption photocurrent, we have achieved a temporal resolution of around 2 ps, and a sensitivity of less than 2 mW².