Abstract: A method and apparatus is disclosed for modulating a laser by a current resulting from the combination fo a threshold current and a current due to a data signal and in which a fraction of the laser output signal is taken and converted into a monitoring current, such monitoring current is sampled, an actual value of the peak power is obtained as an average over a number of samples of the monitoring current, the actual value of the peak power thus found is compared with a desired value of the peak power and an updated value for the laser threshold current is obtained from the difference, if any, between the desired value and the actual value, the updated value being such as to compensate for the difference.