Optical fiber device, optical monitor and optical switch

In order to enable coupling two multi-mode optical fibers with relatively high efficiencies while increasing the distance between lenses to be interposed as compared with the prior arts, with a device according to the present invention, a first condenser lens and a second condenser lens are arranged such that they are spaced apart from each other by four times the focal length and their optical center axes are coincident with each other. A first (second) multi-mode optical fiber is arranged such that it is spaced apart from the first (second) condenser lens by a distance larger than the focal length and the center axis of light emitted from the first (second) multi-mode optical fiber is substantially coincident with the optical center axis of the first (second) condenser lens.