An optical communication system includes a first wavelength filter, a bidirectional amplifier, and a second wavelength filter. A connecting portion projects integrally from the ball tension portion, and leg portion extends from the connecting portion with a post engaging surface parallel with the ball tension portion post engaging surface for engagement on a leg of the right-angled post. When the adiabatic compression stroke is longer than or equal to CAth, IVC is advanced. The diffusion layer has a negative hardness profile with the hardness increasing from the diffusion surface in contact with the substrate to the surface away from the substrate. The refraction varying layer in the active layer of the first DFB section has a refractive index different from that of the refraction varying layer in the active layer of the second DFB section.