In this letter, the design and measurement results of a high-swing 42-Gb/s vertical-cavity surface-emitting laser (VCSEL) driver with an optical modulation amplitude of 1.4 dB is presented. To keep the power dissipation low, no equalizer was employed at the highest rate. At the output stage circuit, a series-shunt peaking technique is used, and it is optimized to lower the group-delay variation at the laser input. The transmitter circuitry can be disabled when there is no data transmission to save half of the power required for the full-rate functionality. Then, it can be enabled in less than 10 ns to full rate operation. The chip is fabricated in a 14-nm bulk CMOS technology and bonded to a common-cathode 20-GHz VCSEL. Optical measurements show that the data transmission up to a data rate of 42 Gb/s with bit error rate better than 10-12 was possible. The total power dissipation, including the one of the VCSEL, is 81.5 mW, which provides a power efficiency of 1.94 pJ/b. To the best of our knowledge, this is the fastest VCSEL driver in any CMOS process suitable for burst mode operation.