A light source has a structure in which a 3-dB beam splitter is integrated with a Febry-Perot laser diode having a cleaved plane. A first waveguide grating and a first refractive index modifier changing a Bragg wavelength of the first waveguide grating are provided at one branch of the 3-dB beam splitter. A second waveguide grating and a second refractive index modifier changing a Bragg wavelength of the second waveguide grating are provided at another branch of the 3-dB beam splitter.