Optical semiconductor resonator, optical semiconductor device, and optical module

In order to prevent non-uniformity in emission wavelength among different sites along an optical axis direction, provided is a resonator portion including: a waveguide which includes at least two areas where an effective refraction index varies in the optical axis direction; and diffraction gratings formed along the optical axis direction of the waveguide. The diffraction grating that is formed in one of the at least two areas of the waveguide where the effective refraction index is large has a pitch narrower than a pitch of the diffraction grating that is formed in another of the at least two areas of the waveguide where the effective refraction index is small.