A high-responsivity high-bandwidth asymmetric twin-waveguide coupled InGaAs-InP-InAlAs avalanche photodiode

The authors demonstrate an integratable high-responsivity high-bandwidth long wavelength InGaAs-InP-InAlAs avalanche photodiode based on an asymmetric twin-waveguide structure. The device has an external quantum efficiency of 48 /spl plusmn/ 1.5% at /spl lambda/ = 1.55 μm, a 3-dB bandwidth of 28.5 /spl plusmn/ 0.5 GHz with gain of up to 4, and a fiber-to-waveguide misalignment tolerance of /spl plusmn/ 1.0 μm in the vertical and /spl plusmn/ 1.3 μm in the horizontal directions resulting in a 1-dB sensitivity penalty.

FIBER-OPTIC communication systems operating at 40 Gb/s (OC-768) require detectors with high speed and high responsivity. Avalanche photodiodes (APDs) are attractive for these applications due to the significant improvement in sensitivities afforded by their internal gains. While planar InGaAs–InP APDs for 1.3 and 1.55- m-long wavelength communication systems employing floating guard ring (FGR) structures that avoid edge breakdown have been demonstrated to have a gain-bandwidth product of 60 GHz [1], they are not suitable for ultrahigh bandwidth (40 Gb/s) applications due to the tradeoff between high speed and high responsivity. Edge-coupled multimode waveguide structures have been employed in APDs to overcome this shortcoming. Since the total optical absorption is determined by the device length, the absorption layer thickness can be small (0.2 m in [2]) to reduce carrier transit times across the device active region without sacrificing internal quantum efficiency [2]–[3] [5]. In these edge-coupled configurations, however, fiber-to-waveguide alignment is difficult, resulting in a low fiber coupling efficiency. Also, monolithic integration with other optical components is difficult, and the fabrication process can be complex. Fig. 1. (a) Three-dimensional schematic view of an asymmetric twin waveguide APD. (b) Index profile of the twin waveguide APD epitaxial structure.