PURPOSE:To suppress frequency noise simultaneously with amplitude noise by providing an optical fiber with an optical isolater and a waveform selecting means. CONSTITUTION:Laser beams radiated from one end face 10a of a laser device 10 are passed through the optical fiber 11 and extracted from a photocoupler 12 to the outside. On the other hand, laser beam radiated from the other end face of the device 10 and passed through the fiber 11 in the reverse direction are interrupted at its passage by the isolator 13 and only a specific beam is selected by selecting any one of the wavelengths of the laser beams obtained from one end face 10a, by a wavelength selecting means 14. Therefore, the amplitude noise is suppressed because the competition, i.e. interference, of beams having different advance modes is prevented. Since the specific wavelength of the light beams passed through the isolator 13 is selected and fed back, the spectrum line width of the passed beams is narrowed. Consequently, the frequency noise can be suppressed simultaneously with the amplitude noise.
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