Waveguide AlInAs/GaInAs APD for 40Gb/s optical receivers

Avalanche photodiodes (APDs) are key components for future optical transmission systems because their internal gain provides better sensitivity. However, their fabrication have been mastered so far up to limited bit rates of 10Gb/s using back-side illumination of the photodiode junction. For >40Gb/s applications, a waveguide structure is required to reduce the trade-off between the diode responsivity and its bandwidth [1]. We demonstrate in this paper a waveguide AlInAs/GaInAs APD with a high bandwidth (>26GHz up to M=7-8) and a primary responsivity of 0.5-0.6 A/W as required for 40Gb/s applications. The waveguide AlInAs/InGaAs APD structure (Fig. 1) is grown by Metal Organic Vapor Phase Epitaxy on a semi-insulating InP substrate. The diluted multimode waveguide is composed of a succession of GaInAsP/InP and an optical matching layer that allows a gradual increase of the optical refractive index from the diluted waveguide to the absorption layer offering better quantum efficiency. Beam Propagation Method was used to calculate the light power propagation into the device structure. Moreover, a genetic algorithm was used to optimize the number of periods and layers thicknesses in order to achieve high quantum efficiency and low TE/TM polarization dependence loss (PDL). Compared to PIN waveguide diodes, the design of such waveguide is critical on APDs because of the low refractive index of AlInAs multiplication layer that reduces coupling efficiency of the light input with the GaInAs absorption layer located on the top of the avalanche region.