Patent ID: 7570678

Claim:
A wavelength determining apparatus, comprising: reflection means for receiving at least part of a laser light beam emitted from an external cavity type semiconductor laser and emitting a reflected light beam that has a distribution of light intensities of fringes; light detection means for detecting an intensity of the light beam reflected from said reflection means in two or more different light reception positions; and determination means for obtaining a difference value of detection signals in the two or more light reception positions and determining the wavelength of the laser light beam on the basis of the difference value, wherein the two or more light reception positions are arranged in the direction of which the fringes take place, wherein the two or more light reception positions are set so that difference values are obtained for light beams having a first wavelength group that take place in a mode hop region of an external cavity, and wherein said determination means determines the wavelength of the laser light beam by identifying wavelengths corresponding to the difference values.