Optical amplifying device

An optical amplifying device, comprising:

• an optical system including a first end and a second end, the optical system configured to receive signal light through the first end, to lead the received signal light to an optical amplifying medium, and to output the signal light amplified by the optical amplifying medium through the second end, the optical system including a plurality of optical isolators, each of which is capable of allowing light propagating in a direction the same as a direction in which the signal light propagates to pass therethrough and blocking light propagating in the opposite direction,
• wherein in isolation-wavelength properties for light propagating in the opposite direction which the plurality of optical isolators have, a peak wavelength at which each of the plurality of optical isolators exhibits the minimum isolation falls within a gain bandwidth of the optical amplifying medium, the peak wavelengths of the plurality of optical isolators differ from one another, and an isolation bandwidth of each of the plurality of optical isolators is narrower than the gain bandwidth of the optical amplifying medium.