An optical device consists of integrated parts and is used for an optical amplifier. The optical device has first to third assemblies. The first assembly includes two optical fibers each for transmitting a beam and a lens for receiving the beams and providing collimated beams oriented in different directions at given angles. The second assembly includes an optical fiber and a lens to receive one of the collimated beams. The third assembly includes an optical fiber and a lens to receive the other collimated beam. At least one of the second and third assemblies has a second optical fiber. An optical film is arranged in front of the lens of the assembly having the second optical fiber. The optical film reflects an optical signal whose wavelength is different from that of the collimated beam. The two optical fibers of the assembly having the second optical fiber are coupled together through the lens and optical film, to pass the optical signal between the two optical fibers.