Transmission Performance of OOK and 4-PAM Signals Using Directly Modulated 1.5-μm VCSEL for Optical Access Network

We evaluate the possibility of realizing optical access networks using 1.5-μm vertical-cavity surface-emitting lasers (VCSELs) in the on-off keying (OOK) and four-level pulse amplitude modulation (4-PAM) formats. In order to maximize the power budget of the networks, we optimize the extinction ratio of the 10.7- and 21.4-Gb/s signals and exploit the electrical equalization at the receiver. The experimental comparison made at 10.7 Gb/s shows that the OOK format vastly outperforms the 4-PAM format in terms of tolerance to chromatic dispersion. For example, we achieve the 80-km transmission of the OOK signal over standard single-mode fiber (SSMF), but the maximum reach is halved when the 4-PAM signal is employed. Nevertheless, the 4-PAM format is still an effective way to double the data rate of the VCSEL links in optical access networks. We successfully transmit the 21.4-Gb/s 4-PAM signal generated by using the 1.5-μm VCSEL over 18-km long SSMF.