A laser device includes a seed laser, an amplifier, a detector, and an optical element arranged to direct radiation emitted by the seed laser towards a plasma generation site. The optical element is arranged to direct towards the detector amplified spontaneous emission radiation which has been emitted by the seed laser and has been reflected from a droplet of fuel material. The detector is arranged to trigger generation of a laser radiation pulse by the seed laser when the reflected amplified spontaneous emission radiation is detected.