Radiation source

According to a first aspect of the present invention, there is provided a radiation source comprising: a nozzle configured to direct a stream of fuel droplets (70) along a trajectory towards a plasma formation location; a laser configured to direct laser radiation at a fuel droplet at the plasma formation location to generate, in use, a radiation generating plasma; wherein the laser comprises: a seed laser (50) for providing a seed laser beam (52); a beam splitter (54) for receiving the seed laser beam from the seed laser; an optical amplifier (58) for receiving the seed laser beam from the beam splitter and performing optical amplification; a first reflector (60) located downstream of the optical amplifier, configured to direct the seed laser beam back through the optical amplifier and on to the beam splitter; and a second reflector (70) located further downstream of the beam splitter, configured to receive the seed laser beam from the beam splitter and to direct at least a portion of the seed laser beam back toward the beam splitter.