An optical link based on directly modulated vertical-cavity surface-emitting laser (VCSEL) at 850-nm wavelength using commercial VCSEL and photodiode operating error-free BER < 10^-12) at 40 Gbit/s is reported. The transmitter is implemented in a 14-nm FinFET CMOS while the receiver (RX) in 28-nm digital bulk-CMOS technologies. This is the fastest 850-nm VCSEL-based single-lane non-return-to-zero optical link with all the analog front-end components fabricated in CMOS. An energy-per-bit of 3.4 pJ/bit is measured at 40 Gbit/s. At lower data rates, the bandwidths of the circuits can be decreased adaptively to save power. This results in 2.7 pJ/bit at 20 Gbit/s. A sensitivity of -1.9 dBm optical modulation amplitude could be measured at the RX for 40 Gbit/s.