A spectrum analyzer includes an input member, a predetermined output plane and a reflection type diffraction grating. The input member receives an optical signal. The reflection type diffraction grating includes a non-Rowland circle curved grating profile curved surface and a plurality of diffraction structures. The diffraction structures, each having a pitch and disposed on the grating profile curved surface, are configured to separate the optical signal into a plurality of spectral components. At least some pitches of the spectral components are different from each other. One of the spectral components indicating a central wavelength is emitted to the predetermined output plane in a direction substantially perpendicular to the predetermined output plane. The grating profile curved surface is used for focusing the spectral components on the predetermined output plane.