Hydrogenation of buried passive sections in photonic integrated circuits: a tool to improve propagation losses at ∼1.56 μm

We demonstrate here that hydrogenation offers a simple and attractive solution to fabricate low-loss InGaAsP/InP photonic integrated circuits operating at 1.56 μm. Using InGaAsP/InP buried waveguide test structures closely imitating the passive sections of photonic circuits, we first demonstrate that hydrogenation helps to bring down significantly the propagation losses. Secondly, with the aid of modal calculations, as well as the optical and electrical measurements on test and control structures, this drop in losses subsequent to hydrogenation is principally attributed to a decrease in free hole concentration (and consequently to a reduction of absorption by free holes) in the upper p+ InP cladding layer. Lastly, we demonstrated that the propagation losses improved by hydrogenation are thermally stable up to temperatures as high as 350°C.