OPTICAL DEMULTIPLEXER

PURPOSE:To simplify the constitution and reduce the cost by forming a filter film, which reflects one wavelength at least and couples this reflected light to a light reception-side optical fiber, on the end face of a rod lens in the side to which a multiplexed light is projected. CONSTITUTION:When a multiplexed light having wavelengths lambdaa-lambdad is projected from an optical fiber 10 to the position on the optical axis of a rod lens 15 on an end face 15a of the rod lens 15 of the optical branching filter, this multiplexed light is made incident on a filter film 16 through a filter film 20. Lights having wavelengths lambdaa and lambdab are reflected on the filter film 16 and are made incident on the rod lens 15 and are projected to a filter film 17. The light having the wavelength lambdaa is transmitted through the filter film 17 and is made incident on a fiber 11. The light having the wavelength lambdab is projected to a position deviated from the optical axis on the filter film 16 and is made incident on an optical fiber 12. The light having the wavelength lambdac out of lights having wavelengths lambdac and lambdad which are transmitted through the filter film 16 is made incident on an optical fiber 13, and the light of the wavelength lambdad is made incident on an optical fiber 14. Thus, the constitution is simplified, and the cost is reduced.