Comparative study of the high-speed digital modulation performance of single- and multimode oxide confined VCSELs for free space optical interconnects

We have performed an extensive experimental study of the high-speed digital modulation characteristics of BCB-planarized oxide confined 850 nm VCSELs. In particular, we have compared the performance of single- and multimode VCSELs intended for high capacity free space optical interconnects. The digital modulation characteristics were evaluated by recording eye diagrams from 2 to 12 Gbit/s and measuring bit-error-rates (BER) at 10 Gbit/s. The single-mode VCSELs produce open and symmetric eyes at all bit rates and this behavior is maintained under large variations in bias current and modulation depth. For the multimode VCSELs, symmetric eyes can only be achieved under certain bias and modulation conditions. Both VCSELs allow for error free transmission at 10 Gbit/s. The receiver sensitivity at a BER of 1-9 under optimum bias and modulation conditions was -12.9 and -13.4 dBm for the single-mode and multimode VCSELs, respectively. The single-mode VCSELs produce near Gaussian beams ideal for free space optical interconnects where well defined and stable beams are needed for high efficiency and low cross-talk. The multimode VCSELs, on the other hand, have beam characteristics that are unpredictable and dependent on bias current, and are therefore less favorable for free space optical interconnects.