We evaluate the effects of the external optical feedback on the on-off keying (OOK) and four-level pulse amplitude modulation (PAM4) signals in the single-mode fiber (SMF) link implemented by using a commercial 10G-class 1.55-mu m vertical-cavity surface-emitting laser (VCSEL). The results show that the PAM4 signal is much more sensitive to the external optical feedback than the OOK signal. For example, in the back-to-back condition, an error floor is observed at the bit-error rate (BER) of similar to 8 x 10(-3) for the 10-Gbaud PAM4 signal when we set the optical feedback ratio to be -14 dB. In comparison, the 10-Gbaud OOK signal has a power penalty of only 0.4 dB (@BER = 5 x 10(-5)) under the same conditions. The results also show that the PAM4 signal becomes more sensitive to the optical feedback after the SMF transmission due to the shallowed BER curve by the fiber dispersion. In addition, we confirm that the effects of the optical feedback are independent of the baud rate. From these results, we conclude that the error-free transmission of the high-speed PAM4 signal cannot be achieved if there is even one bad fiber connector having >2.5% reflection in the VCSEL-based SMF link.