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.

Abstract: The light emitted by the laser diode 1 is put into an interference optical filter 5. The light passing through the interference optical filter 5 and the reflected light are respectively received in photo diodes PD1, PD2. Their output ratio is calculated by an adder 13, a subtracter 14, and a divider 15, and a wavelength signal is obtained. The difference of the output ratio and reference value is detected as error signal by an error detector 16, and the emission wavelength of the laser diode 1 is controlled so that the error signal may be zero.