Metrology for improving DUV laser alignment

An imaging apparatus comprising:

• an illuminator configured to direct imaging light into a gas discharge stage outputting a light beam, the illuminator distinct from a gas discharge stage;
• a sensing apparatus;
• and
• an optical arrangement configured to selectively image the light beam from the gas discharge stage and the imaging light from the illuminator from a plurality of distinct object planes within the gas discharge stage onto the sensing apparatus, wherein the optical arrangement includes one or more focusing optical elements that are adjustable along an optical path to thereby select which object plane within the gas discharge stage is imaged onto the sensing apparatus.

A method of aligning a gas discharge stage outputting a light beam and comprising a chamber having a first optical port and a second optical port, an input/output optical element in optical communication with the first optical port, and a spectral feature adjuster in optical communication with the second optical port, the method comprising:

• aligning both imaging light from an imaging apparatus and amplified spontaneous emission (ASE) from the chamber with:
• a first aperture of the input/output optical element;
• and
• a second aperture of the spectral feature adjuster;
• aligning the input/output optical element at the first aperture with imaging light;
• and
• aligning the spectral feature adjuster at the second aperture and the chamber with the light beam.