High Responsivity and Reliability of InP-Based Waveguide Avalanche Photodiodes with Butt-Joint Coupled Structure

This paper reports on the performance of InP-based waveguide avalanche photodiodes (APDs) with a butt-joint (BJ) coupled structure for intensity-modulation and direct detection (IMDD) transmissions. Though waveguide APDs are able to perform with both high responsivity and wide bandwidth operation, they have generally been considered to suffer from high polarization dependent loss (PDL) and low reliability. To overcome this assumption, we have fabricated BJ waveguide APDs integrated with polarization insensitive spot size converters (SSCs) by using InP-based regrowth techniques. As a result, responsivities as high as 0.69 A/W at 1.31 μm and 0.84 A/W at 1.55 μm and PDL less than 0.05 dB are obtained. Moreover, the high reliability of BJ waveguide APDs has been verified through a 5,000-hour accelerated aging test at 175 °C. Due to sufficient passivation of the active region, a drift of breakdown voltage of 18 APDs after the test has been only less than 0.5 V. We have assembled receiver optical sub-assemblies (ROSAs) with the BJ waveguide APD installed. Compared with our previously fabricated InP-based APDs which have a surface-illuminated structure, these ROSAs achieve an improvement of more than 2 dB in receiver sensitivity of 25-Gbit/s non-return-to-zero (NRZ) transmission.