We experimentally demonstrate the use of a high-coherence hybrid silicon (Si)/III-V semiconductor laser as the light source for a transmitter generating 20 Gbaud 16- and 64- quadrature amplitude modulated (QAM) data signals over an 80 km single-mode fiber (SMF) link. The hybrid Si/III-V laser has a measured Schawlow-Townes linewidth of ${\sim}{10}\;{\rm kHz}$∼10kHz, which is achieved by storing modal optical energy in low-loss Si, rather than the relatively lossy III-V materials. We measure a received bit error rate (BER) of ${4.1} \times {{10}^{ - 3}}$4.1×10-3 when transmitting the 64-QAM data over an 80 km SMF using the hybrid Si/III-V laser. Furthermore, we measure a BER of $ {\lt} {1} \times {{10}^{ - 4}}$<1×10-4 with the Viterbi-Viterbi digital carrier phase recovery method when transmitting the 16-QAM data over an 80 km SMF using the hybrid Si/III-V laser. This performance is achieved at power penalties lower than those obtained with an exemplary distributed feedback laser and slightly higher than those with an exemplary narrow-linewidth external cavity laser.