Low polarization dependence for evanescently coupled avalanche photodiodes: theoretical analysis and design

In this report, we study the difference of propagation conditions for different polarized modes in evanescently coupled waveguides. We propose a factor, the sum of squared deviations of indexes ratios, to describe the difference of propagation conditions between transverse-electric mode and transverse-magnetic mode. A simple and intuitive method based on above theoretical analysis is presented that can be used to design evanescently coupled avalanche photodiodes (ECAPDs) with low polarization dependence. We also propose a structure of ECAPDs that air slots are introduced into the matching layer. This improved structure can reduce the polarization dependence and achieve efficient coupling with a short coupling length. Our approach is compared to the other methods, such as mode analysis and beam propagation method. Agreement between predicted values and calculated results with other methods confirms the feasibility of our approach in the accurate design of polarization insensitive ECAPDs. The polarization dependence is reduced to 0.2 dB and coupling efficiency above 80 % is achieved for both polarized modes with an absorption length as short as 15 mu m.