Optical matching layer structures in evanescent coupling photodiodes at a wavelength of 1.55 μm: physics, design and simulation

We have studied the optical matching layers (OMLs) and external quantum efficiency in the evanescent coupling photodiodes (ECPDs) integrating a diluted waveguide as a fibre-to-waveguide coupler, by using the semi-vectorial beam propagation method (BPM). The physical basis of OML has been identified, thereby a general designing rule of OML is developed in such a kind of photodiode. In addition, the external quantum efficiency and the polarization sensitivity versus the absorption and coupling length are analysed. With an optical matching layer, the absorption medium with a length of 30 μm could absorb 90% of the incident light at 1.55 μm wavelength, thus the total absorption increases more than 7 times over that of the photodiode without any optical matching layer.