The present disclosure provides in a first aspect a mode-locked laser for generating laser pulses. The mode-locked laser comprises an optical coupler and a first optical path capable of carrying optical signals from and to the optical coupler. The first optical path includes an optical amplifier that is arranged so that saturation of optical amplification causes amplitude modulation of the light. The optical amplifier has a saturation time that is shorter than a pulse transition period of the mode-locked laser and is arranged for recovery of amplifying properties after the saturation within a period of time that is shorter than the pulse transition period of the mode-locked laser. The laser further comprises a second optical path capable of carrying optical signals from and to the optical coupler. The second optical path includes an optical isolator. The first optical path has a non-linear property and is arranged so that, when a light pulse that is received from the second optical path is split by the optical coupler into component light pulses that are directed in opposite directions in the first optical path, the component light pulses experience differing shifts in phase and wherein the mode-locked laser is arranged so that mode-locked lasing is initiated by a light pulse that is amplitude modulated by the optical amplifier.