Abstract: An optical device includes a first fiber including a first reflector, a second fiber including a second reflector, and an amplifying fiber that has a normalized frequency that is greater than or equal to 5.13 and is wound so as not to intersect on one plane. The amplifying fiber includes an outer section wound in a circular manner. The amplifying fiber includes an inner section having first arc portions and wound such that at least two of the first arc portions have a radius of curvature in which a higher-order mode that is higher than an LP11 mode has loss. The amplifying fiber includes a first end connected to the first fiber and disposed on an inside of the amplifying fiber. The amplifying fiber includes a second end connected to the second fiber.