Dual Evanescently Coupled Waveguide Photodiodes with High Reliability for over 40-Gbps Optical Communication Systems

For over 40-Gbps optical communication systems, phase coded modulation formats, like differential phase shift keying (DPSK) and quadrature phase shift keying (QPSK), are very important for signal frequency efficiency and long-reach transmission. In such systems, differential receivers which regenerate phase signals are key components. Dual Photo Diodes (dual PDs) are key semiconductor devices which determine the receiver performance. Each PD of the dual PDs should realize high speed performance, high responsibility and high input power operation capability. Highly symmetrical characteristics between the two PDs should be also realized, thus the dual PDs are desired to be monolithically integrated to one chip. In this paper, we describe the design, fabrication, characteristics and reliability of monolithically integrated dual evanescently coupled waveguide photodiodes (EC-WG-PDs) for the purpose described above. The structure of the EC-WG-PDs offers the attractive advantages of high speed performance, high responsivity and high input power operation. Furthermore, their fabrication process is suitable for the integration of two PDs on one ship. First, the optimization was done for high products of 3-dB bandwidth and responsivity for 43-Gbps DPSK receivers. Excellent characteristics (50 GHz bandwidth with a responsivity of 0.95 A/W), and high reliability were demonstrated. The other type of optimization was done for ultra high speed operation up to 100-Gbps. The fabricated PDs exhibited the 3 dB-bandwidth of 80 GHz with a responsivity of 0.25 A/W. Furthermore, 43-Gbps RZ-DPSK receivers including the dual EC-WG-PDs based on the former optimization and differential transimpedance amplifiers (TIAs) newly developed for the purpose were also presented. Clear and symmetrical eye openings were observed for both ports. The OSNR characteristics exhibited 14.3 dB at a bit error rate of 10−3 that is able to be recovery with FEC. These performances are enough for practical use in 43-Gbps RZ-DPSK systems. key words: 100-Gbps, 40-Gbps, differential phase shift-keying (DPSK), reliability, waveguide photodiode