Filter device and laser apparatus

A filter device comprising:

• an optical fiber that propagates multimode light having a predetermined wavelength;
• a first higher-order mode filter that removes at least part of the propagating multimode light that has a mode order higher than a predetermined mode order;
• and
• a fiber Bragg grating that:
• transmits the multimode light transmitted through the higher-order mode filter, and
• reflects light having a particular wavelength longer than the predetermined wavelength, wherein
• the optical fiber includes a cladding and a core with no active element,
• the first higher-order mode filter is a bent section of the optical fiber,
• the bent section has a predetermined diameter, and
• the fiber Bragg grating is disposed downstream of the first higher-order mode filter in a propagation direction of the light.