Pulse Stretcher and Method

An apparatus for increasing a pulse length of a pulsed radiation beam, the apparatus comprising:

• a beam splitter configured to split an input radiation beam into a first beam and a second beam;
• an optical arrangement, wherein the beam splitter and the optical arrangement are configured such that at least a portion of the first beam is recombined with the second beam into a modified beam after an optical delay of the first beam caused by the optical arrangement;
• and
• at least one optical element in an optical path of the first beam, the at least one optical element comprises phase regions across a surface of the at least one optical element configured such that the phase of different parts of a wavefront of the first beam is varied to reduce coherence between the first beam and the second beam,
• wherein the at least one optical element is configured such that the angle at which the radiation is diffracted from the at least one optical element is less than a fraction of the acceptance angle of the radiation in an optical column, the fraction being the reciprocal of the number of first beams that substantially contribute to the transmitted intensity of the modified beam.

A method for increasing a pulse length of a pulsed radiation beam using an apparatus, the method comprising:

• passing an input radiation beam through a beam splitter of the apparatus to split the input radiation beam into a first beam and a second beam;
• passing the first beam through an optical arrangement of the apparatus, wherein the beam splitter and the optical arrangement are configured such that at least a portion of the first beam is recombined with the second beam into a modified beam after an optical delay of the first beam caused by the optical arrangement;
• and
• varying the phase of different parts of a wavefront of the first beam using at least one optical element in an optical path of the first beam to reduce coherence between the first beam and the second beam.