1. A device for controlling an excitation laser in an extreme ultraviolet (EUV) radiation source, the EUV radiation source comprising a droplet generator configured to generate target droplets and the excitation laser configured to heat the target droplets using excitation pulses, the device comprising:a first radiation source configured to irradiate each of the target droplets at a first position;
a second radiation source configured to irradiate each of the target droplets at a second position a fixed distance away from the first position;
a droplet detector configured to detect a first signal of radiation scattered by a given target droplet at the first position and a second signal of radiation scattered by the given target droplet at the second position; and
a timing module configured to receive the first signal and the second signal, measure a speed of the given target droplet based on the received signals, estimate a trigger time for providing an excitation pulse to heat the given target droplet based on the measured speed and provide the excitation laser the trigger time.