Abstract: A semiconductor amplifying device 1 comprises, a substrate 2, a first optical waveguide 7 on the substrate 2 for transmitting an optical signal, a second optical waveguide 8 on the substrate 2, a third optical waveguide 11 on the substrate 2 forming an optical coupling with the first optical waveguide 7 along an interaction length and constructed as a semiconductor optical amplifier 11 that receives and amplifies at least a portion of the optical signal to the first waveguide 7, and antireflective means 16 covering light emitting edges 7a, 8a, 11a, 3a at corresponding ends of the respective waveguides 7, 8, 11 and an optical transmitting medium 3, the second optical coupling having a phase mismatch selected for a wavelength of the optical signal to provide for a power gain in the first optical waveguide 7 attributed to an amplified portion of the same wavelength of the optical signal coupled back from the amplifier 11.