Ultrahigh-speed 1.55 µm λ/4-shifted DFB PIQ-BH lasers with best-ever 17 GHz bandwidth as a DFB laser have been developed. This large bandwidth is attained by reducing the parasitic capacitance with a low doped p-InP burying layer and a PIQ (Hitachi Chemicals polyimide) layer, and by enhancing the relaxation oscillation frequency by optimisation of the waveguide structures.