PROBLEM TO BE SOLVED: To provide a mechanically induced long period optical fiber grating. SOLUTION: A coupling with high wavelength selection properties is induced between a wave guide mode and a cladding mode of a fiber, by pressing a normal single mode optical fiber (normally, dispersion-mode fiber) onto an external long period mechanical grating. A series of such induced couplings being all disposed on the identical fiber surface are used so as to produce a loss bandwidth of several tens nm width with arbitrary shapes having a resolution equal to or smaller than 3 nm. The light intensity of every wavelength can be continuously adjusted from zero to 17 dB or more. The inducing apparatus is ideal to equalize dynamic gains of optical amplitude devices because of the simplicity, background loss of approximate zero, and in particular, capability of continuous readjustment.
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