Fiber laser device

A fiber laser device, comprising:

• a pumping light source that outputs pumping light;
• an amplifying fiber comprising a core doped with a rare earth element excitable by the pumping light;
• an input side fiber that is fusion-spliced on an input side of the amplifying fiber and that comprises a high reflectivity-fiber Bragg grating (HR-FBG);
• an output side fiber that is fusion-spliced on an output side of the amplifying fiber and that comprises an output coupler-fiber Bragg grating (OC-FBG),
• having a reflectivity smaller than a reflectivity of the HR-FBG;
• an output end that outputs a laser beam;
• and
• a mode filter that removes at least a portion of a predetermined higher-order mode from the core of the amplifying fiber,
• wherein
• the input side fiber or an intermediate fiber disposed between the amplifying fiber and the input side fiber is fusion-spliced with the amplifying fiber via a fusion splice portion, and
• at least a portion of the mode filter is disposed between the fusion splice portion and a position distant from the fusion splice portion by a coherence length of beating caused by mode interference of signal light propagating in the amplifying fiber.