US Patent 7577319 Semiconductor optical device and manufacturing method thereof

A low reflective window structure in an existent electro-absorption optical modulator involves a trading off problem between the increase in the parasitic capacitance and the pile-up. This is because the capacitance density of the pn junction in the window structure is higher compared with the pin junction as the optical absorption region, and the application of electric field to the optical absorption region becomes insufficient in a case of receding the electrode structure from the junction between the optical absorption region and the window structure making it difficult to discharge photo-carriers generated in the optical absorption region. An undope waveguide structure comprising a structure having such compositional wavelength and a film thickness that the compositional wavelength for each of multi-layers constituting the waveguide structure is sufficiently shorter than that of the signal light and the average refractive index is about identical with that in the optical absorption region may be disposed. In a case of forming the electrode structure so as to overlap the junction boundary between the optical absorption region and the undope waveguide, and do not extend on the joined boundary between the undope waveguide and the window structure, increase in the parasitic capacitance due to the pn junction of the window structure and pile up can be suppressed simultaneously.