System for and method of calibrating measurement of duv laser beam spectral properties

A metrology method comprising:

• receiving a beam of input radiation;
• dividing the beam of input radiation into a pump beam, a first probe beam, and a second probe beam;
• directing the pump beam through a material having an energy transition at a specified wavelength along a first path in a first direction to saturate the material along the first path;
• directing the first probe beam through the material along the first path in a second direction opposite to the first direction;
• directing the second probe beam through the material along a second path in the second direction;
• receiving the first probe beam after the first probe beam has traversed the first path;
• receiving the second probe beam after the second probe beam has traversed the second path;
• measuring a spectral property of the first probe beam and a spectral property of the second probe beam;
• measuring a difference between the property of the first probe beam and the property of the second probe beam;
• generating a transition profile based on a difference between the spectral property of the first probe beam and the spectral property of the second probe beam;
• generating an absolute reference based at least in part on the transition profile;
• and
• calibrating a measurement of a spectral property of a laser beam based at least in part on the absolute reference signal.

A metrology system for measuring a spectral property of a beam of laser radiation, the metrology system comprising:

• a beam separator for separating a beam of metrology radiation from the beam of laser radiation;
• a source of a beam of calibration radiation;
• a measuring module arranged to receive the beam of metrology radiation and configured to measure a spectral property of the beam of metrology radiation and generate a measurement signal indicative of the spectral property;
• a calibration module arranged to receive either the beam of metrology radiation or the beam of calibration radiation as an input beam of radiation, the calibration module including
• a vapor of an elemental material having a concentrated proportion of an isotope of the elemental material, the isotope having an energy transition at a specified wavelength,
• an optical arrangement configured to
• divide the beam of input radiation into a pump beam and first and second probe beams,
• direct the pump beam through the material along a first path in a first direction, and
• direct the first probe beam through the material along the first path in a second direction opposite to the first direction and the second probe beam through the material along a second path in the second direction, and
• a detector configured to
• receive the first probe beam after the first probe beam has traversed the first path,
• receive the second probe beam after the second probe beam has traversed the second path,
• measure a property of the first probe beam and a property of the second probe beam as a wavelength of the beam of input radiation is caused to vary through a range of wavelengths including the specified wavelength,
• obtain a difference between the property of the first probe beam and the property of the second probe beam, and
• generate a first differential signal based on the difference between the property of the first probe beam and the property of the second probe beam when the input beam comprises the beam of metrology radiation and a second differential signal based on the difference between the property of the first probe beam and the property of the second probe beam when the input beam comprises the beam of calibration radiation;
• and
• a control system arranged to receive the first differential signal and the second differential signal and adapted to generate an absolute reference signal based on the first differential signal and the second differential signal.