We experimentally demonstrate 10- and 20-Gb/s pulse amplitude modulation-4 (PAM4) signal generation using two directly modulated lasers (DMLs) and one photodiode (PD) over 10km multi-core fiber (MCF). In this paper, the two DMLs produce OOK signals with different extinction ratios, and by an optical coupler (OC), an optical signal can be in the form of PAM4. Compared to the conventional scheme, the proposed scheme not only can provide similar performance, but also is insensitive to the offset of the DML's bias current.