High-Performance Dual-Step Evanescently Coupled Uni-Traveling-Carrier Photodiodes

In this paper, we discuss a dual-step evanescently coupled photodiode (PD), which is composed of an improved evanescently coupled optical waveguide and a uni-traveling-carrier PD (UTC-PD). The hitherto serious dependence of the responsivity on the coupling length, which is determined by the cleaving process, or the necessity of a long (~ 700 mum) passive waveguide with a complex tapered structure, can be eliminated. The optimization of the waveguide structure, and the doping profile of the p-type absorption layer, allow the integrated UTC-PD to achieve a high responsivity (0.9 A/W, 1.04 A/W), with a large cleaving tolerance (~ 50 mum), a wide invariable electrical 3-dB bandwidth (60 GHz, 40 GHz) from a low (~ 0.5 mA) to a high output current (>13 mA), and a high saturation current-bandwidth product (around 780 mAGHz) simultaneously under a load of 50 Omega .