Contents I. Introduction
The performance of microwave and millimeter-wave photonic systems would benefit from the use of photodiodes (PDs) with high saturation power, high-speed and high responsivity performance [1]. There are two major ways to satisfy these three requirements of PDs. One is to distribute and uniform the photocurrents along the edge-coupled PDs by improving the structure of optical and electrical waveguides, such as evanescently-coupled photodiode (ECPD) [2], [3]; the other is to minimize the space-charge effect by changing the structure of epitaxial layers, such as uni-traveling carrier PD [1] (UTC-PDs) and partially depleted absorber PD (PDA-PD) [2], [3]. Recently, several research groups have demonstrated state-of-the-art performance of evanescently-coupled PD with a short coupling length and partially depleted absorber [2], [3]. However, the tolerance of cleaving process of such devices is very small (less than ) and different length of coupling region will affect the performance of responsivity seriously [2], [3]. For the case of PD with a leaky optical waveguide and Distributed Bragg-Reflectors (DBRs) [4], the problem of cleaving tolerance can be released greatly [4] and high speed with high saturation current-bandwidth product has been demonstrated. However, the e-beam lithography processes for DBR mirrors may be necessary. In this paper, we demonstrate a dual-step evanescently-coupled UTC-PD. By separating the fiber-guide region and coupling-guide region into different parts of optical waveguide [5], the dependence of responsivity on cleaved-length or the necessity of a long passive waveguide [6] with complex taper stages can be eliminated. A high responsivity (0.9A/W), wide electrical 3-dB bandwidth , and a high saturation current-bandwidth product (over 780mA-GHz,) have been achieved simultaneously under a load.
