OPTICAL FIBER TYPE WAVELENGTH FILTER AND WAVELENGTH PERIOD ADJUSTMENT METHOD THEREOF

PROBLEM TO BE SOLVED: To provide a low-cost optical fiber type wavelength filter capable of realizing a low insertion loss, and to provide a wavelength period adjustment method of the optical fiber type wavelength filter capable of easily adjusting the wavelength period. SOLUTION: A twin core optical fiber 4 is provided with a first core and second cores having refractive index distribution allowing propagation of single- mode light in a nearly central part and its peripheral part in a sectional view of a section perpendicular to the longitudinal direction. In the twin core optical fiber 4, two extension parts 14, 16, (i.e., optical coupling parts) generating optical coupling between the first core and the second cores apart from each other in the longitudinal direction are formed. The twin core fiber 4 is bent in the longitudinal direction between the extension parts 14, 16.