PURPOSE: To provide an optical fiber system with a fiber device with a taper carrying out filtering, wavelength shift detection or switching.

CONSTITUTION: This system is based on the discovery of finding that a multi- clad single mode optical fiber with the disconnection of some basic mode is suitable for forming compact elements useful within the optical fiber system such as a filter within a telephone communication system, a wave length shift detector within a sensing system or a switch within an optional optical system. To put it in the concrete, the characteristics of the system is that the system is composed of an optical fiber 14 of some length composed of a core and a plurality of clad layers surrounding the core and that the fiber 14 is provided with the disconnection of a basic mode within ±500nm of λ and with an area 19 with a single taper attenuating a signal at least 3dB-selectively at λ.