A method at a transmitter of a tunable laser of controlling a mode change of the laser during a transmission of data to a receiver of a terminating node is based on the determination of a time instance for initiation of the mode change on the basis of the state of the tunable laser. In addition, buffer occupancy and/ or the state of the data transmission may be considered when determining such a time instance. In addition, data of the data transmission is buffered during the mode change, such that data loss is avoided during the change of mode. A transmitter is also provided which can execute the suggested mode changing method.