Patent ID: 9645500
Date: 2017-05-09
CPC Classifications: G02B,G03F,H01S,H05G

Claim:
1. A radiation source comprising: a nozzle configured to direct a stream of fuel droplets along a droplet path towards a plasma formation location; the radiation source being configured to receive a gaussian radiation beam having gaussian intensity distribution, having a predetermined wavelength and propagating along a predetermined trajectory, the radiation source further being configured to focus the radiation beam on a fuel droplet at the plasma formation location; a phase plate structure comprising one or more phase plates, the phase plate structure having a first zone and a second zone, the zones being arranged such that radiation having the predetermined wavelength passing through the first zone and radiation having the predetermined wavelength passing through the second zone propagate along respective optical paths having different optical path lengths, wherein a difference between the optical path lengths between the radiation passing through the first zone and the radiation passing through the second zone is an odd number multiplied by the value of half the predetermined wavelength when the radiation passing through the first zone and the radiation passing through the second zone hit one of the fuel droplets at the plasma formation location.