Abstract: An optical system includes: a difference-frequency-light generator that multiplexes output lights from a signal-light light source and an excitation-light light source and inputs the output lights to a nonlinear optical material to generate and output a difference-frequency light having a wavenumber that corresponds to a wavenumber difference between light from the signal-light and from the excitation-light light source, where one of the signal-light source or the excitation-light source is a wavelength-sweeping light source; a splitter that splits a portion of light output from the wavelength-sweeping light source into a first and a second reference light; and an equal-wavenumber-signal generator that receives the first reference light, via an interferometer, to generate and output an equal-wavenumber signal that fluctuates in equal wavenumber intervals according to wavelength sweeping of the wavelength-sweeping light source.