Abstract
Data centre optical links are migrating to four-level pulse amplitude modulation (PAM-4) as a method to extend the data rate while covering longer distances. Vertical-cavity surface-emitting lasers (VCSELs) enable the integration of low-power transmitters, but their non-linear and bias-dependent behaviour makes conventional feed-forward equalisation less effective. This Letter presents a 0.13 μ m SiGe BiCMOS PAM-4 driver that boosts the falling-edge to the bottom level through a selective pre-emphasis technique. Experiments at 25 GBd (50 Gb/s) reveal that adding selective pre-emphasis to a 4-tap equalised current driving a 20.6 GHz 1.5 μ m VCSEL, relaxes the critical PAM-4 link budget requirements by >1 dB at the KP4 forward error correction threshold of 2.2 × 10 − 4 . The potential of PAM-4 VCSEL transmitters can be significantly enhanced by including selective pre-emphasis to the equalisation topology while requiring minimal overhead.
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