A system includes an illumination system, an optical element, and a detector. The optical system is implemented on a substrate. The illumination system includes first and second sources and first and second generators. The illumination system generates a beam of radiation. The first and second sources generate respective first and second different wavelength bands. The first and second resonators are optically coupled to respective ones of the first and second sources and narrow respective ones of the first and second wavelength bands. The optical element directs the beam toward a target structure. The detector receives radiation from the target structure and to generate a measurement signal based on the received radiation.