In this paper, we experimentally demonstrate the transmission of 56 Gbaud four-level pulse amplitude modulation (PAM4) signal over 2-km single mode fiber (SMF) with intensity modulation and direct detection (IM/DD) scheme, while the bit-error-ratio (BER) of the PAM4 signal is under hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10-3. Linear pre-equalization is implemented in the transmitter side with a 3-tap finite-impulse-response (FIR) filter to compensate for the intersymbol interference (ISI) induced by system bandwidth limitation. Receiver side equalization is realized with training sequence (TS) based feed-forward equalizer (FFE) with decision-feedback equalizer (DFE). Furthermore, an Adaptive Notch Filter (ANF) is proposed to suppress the digital-to-analog converter (DAC) clock leakage induced narrowband interference for the first time, and the bandwidth of the ANF is optimized to achieve the best BER performance.