Optical links in datacentres operate mainly in burst mode. The utilisation might be as low as 10%, i.e. most of the time the link is idle, but still consumes power. It would therefore be desirable to put these links into a low-power sleep mode during idle signal by reducing the bias, and to operate the electronics and laser only when data is actually being transmitted. The laser is typically implemented with vertical cavity surface-emitting lasers (VCSELs). VCSELs should not be biased below their threshold current as otherwise an extensive turnon delay will be incurred. A significant wake-up time is still observed in VCSELs even if the bias is not reduced during idle and only logic zeros are transmitted. With reduced bias, an unacceptable wake-up time is observed. The authors present measured data and propose a high-amplitude wake-up pulse at the beginning of the data burst, to significantly speed up the wake-up time. Measurements show the effectiveness of this pulse.