UC San Diego

Large optical power at the photodiode is especially beneficial to the high speed fiber optic link that is demanded by the exponential internet traffic growth. The power handling capability of waveguide photodiode (WGPD) has been limited by one of its intrinsic feature, which is the lack of uniform absorption distribution in the absorber in the longitudinal direction. As the required bandwidth kept increasing, an increasing photocurrent is needed in a reduced absorber length, which challenges the traditional WGPD designs for high power fiber optic link.

In order to study absorption uniformity in a high speed high power WGPD, supermode analysis is developed to replace the traditional WGPD analysis, which optimizes the absorption profile via adjusting the waveguide confinement factor. It is point out that the waveguide confinement factor has opposite effects on the bandwidth and power handling of a WGPD, and makes the traditional WGPD analysis ineffective for the high speed high power WGPD design. the supermode analysis focuses on finding out the features of supermodes that improve the absorption uniformity. The designs of the waveguide structure and the layer structure are guided by the supermode analysis to improve the absorption uniformity in a WGPD.

The supermode analysis is applied to improve the absorption uniformity in a traditional WGPD. Traditional WGPD with the optimized structure experimentally demonstrates power handling capability higher than most of the reported WGPD with similar bandwidth. A layer structure with diluting layer is proposed to increases the power handling capability by 30% in the demonstrated WGPD. The supermode analysis also points out that the traditional WGPD has a trade-off of absorption uniformity and bandwidth-efficiency product.

The supermode analysis is applied to re-design the directional coupler photodiode (DCPD), which is pre-studied in our group. With the optimization of absorption uniformity in a DCPD that has RC time limited bandwidth being close to 50 GHz, the re-designed DCPD shows better power handling capability than that of any other traditional WGPDs with similar bandwidth.

Finally, we have proposed and demonstrated the incorporation of cascaded optical bandpass filters in a fiber optic link can significantly improve the link performance.