PURPOSE: To provide a wide dynamic range with low insertion loss and to modulate or attenuate the light wave propagating in an optical fiber with a reduced size, weight and electric power consumption by pressing the optical fiber by a standing wave of a plain-sliding and oscillating member, thereby changing the refractive index of the optical fiber itself.

CONSTITUTION: When voltages V1, V2 from power sources 10, 11 are attenuated through attenuators 12, 13 and excitation voltages V1', V2' are applied to plain-sliding and oscillating members 3, 6, the members 3, 6 are excited by resonance frequencies to generate standing waves. A prepressurizing voltage is preliminarily impressed as a reference to the members 3, 6 so as to exert preliminarily the pressure to an optical fiber 1 and the voltage superposed thereon is impressed to said members. When stress is applied to the fiber 1 by the oscillation of the members, 3, 6 and a core part 1c through which a light wave passes is displaced, the refractive index changes and therefore the incident light Pi from the inlet part of the member 3 is attenuated according to the change in the refractive index in the outlet part of the member 6. More specifically the amplitude-modulated output light Po is obtd. Since the amplitude modulation is accomplished by using the standing wave, the insertion loss is decreased and the electric power consumption is reduced by using the voltage signal of the resonance frequency of the oscillating members.