| In long-haul optical communication systems, there is an ever-increasing demand for high-speed and high-sensitivity photodetectors. An inherent tradeoff exists between these two characteristics, however, as the thin layers required are capable of only limited absorption of the optical signal. A waveguide configuration allows device speed and sensitivity to be decoupled. In a waveguide photodetector, the optical signal is incident on the edge of the device mesa and travels parallel to the plane of the absorption layer. Sensitivity thus becomes a function of device length rather than thickness.; A complimentary approach to boosting sensitivity is by electronic amplification of the incident signal. An avalanche photodetector (APD) makes use of internal gain to lift weak signals out of the noise. Photogenerated carriers are drawn into a high-field multiplication region where they undergo impact ionization, generating more carriers and thus amplifying the signal current.; Combining these optical and electronic methods of improving sensitivity into waveguide avalanche photodetectors (WG-APDs) yields fast, sensitive photodetectors. It has been the direction of this work to develop WG-APDs for use in optical communications systems. Avalanche photodiodes using the novel absorption material GaNAs were demonstrated for operation at 1.064 μm. Waveguide avalanche photodetectors were demonstrated in both the InGaAs/InP and AlGaAs/GaAs material systems. A record gain-bandwidth product of 320 GHz was achieved in an APD operating at 1.55 μm, the optimum wavelength for long-distance transmission. Finally, a novel wedge-facet design was demonstrated that improved the optical coupling in waveguide photodetectors. |
