An interferometer comprises a light source unit, a first splitter, a reference beam unit and a detection unit. The light source unit provides a laser beam. The first splitter receives the laser beam from the light source unit and splits the laser beam into a first beam and a second beam. The reference beam unit comprises a frequency shifter, a stopper and a spherical mirror. A center of the frequency shifter is located on a curvature center of the spherical mirror, the first beam traveling from the first splitter to the frequency shifter, the frequency shifter splitting the first beam into a diffraction beam and a zero-order beam, wherein the diffraction beam travels to the spherical mirror, reflected by the spherical mirror toward the frequency shifter, passing the frequency shifter to become a reference beam, and the zero-order beam is stopped by the stopper. The detection unit receives the reference beam from the reference beam unit.