Active spectral control during spectrum synthesis

A method of estimating a spectral feature of a pulsed light beam produced by an optical source, the method comprising:

• modifying the wavelength of the pulsed light beam based on a predefined repeating pattern having a pattern period comprising a plurality of steps, the modification including shifting the wavelength of the pulsed light beam by a wavelength offset from a baseline wavelength for each step in the pattern period;
• measuring a baseline spectral shape of the light beam output from the optical source when the light beam is at the baseline wavelength;
• measuring a set of wavelengths of the light beam that are shifted relative to the baseline wavelength according to a predefined repeating pattern having a pattern period;
• determining a wavelength difference value between the measured wavelength of the light beam at each step in the pattern period and the baseline wavelength;
• and
• estimating an apparent bandwidth of the pulsed light beam based on the measured set of wavelengths, the determined wavelength difference value, and the measured baseline spectral shape across an evaluation window that includes all of the steps within the pattern period of the entire predefined repeating pattern.