UC San Diego

As analog optical links continue to mature and fulfill communication needs, the requirements for output power and linearity continue to be a main focus. The receiver end of a link is a limiting factor for such applications, and therefore photodiode research continues to be at the forefront of these issues. In order to compete, photodiodes need to be able to maintain high bandwidth, high power and high linearity simultaneously. The study of photodiodes for analog links has focused on linearity, in particular the third order intermodulation distortions (IMD3), which occur near the fundamental signal. Although the output third order intercept point (OIP3) is an important figure of merit, there are still many questions about how OIP3 is measured. The goal of this thesis is to assess the systems used to measure OIP3, in order to develop a better understanding of nonlinearity allowing us to perform accurate modeling and design for waveguide style photodiodes that require high power and high linearity. First, the different measurement systems are discussed. A three laser two-tone setup is demonstrated as an alternative to the two laser two-tone setup, which suffers from link component nonlinearities. The setup is experimentally and analytically characterized. Next the one- and two-tone heterodyne setups and a four laser three -tone setup are compared using mathematical relationships to equate the results. Second, two PIN waveguide photodiodes are presented with similar layer structures. The diodes are characterized for bandwidth, DC responsivity, and OIP3. The devices are also modeled electrically with Silvaco and thermally with Comsol. The results are used to discuss the benefits for certain design tradeoffs, such as bandwidth and responsivity, as it pertains to power and linearity. Finally, a uni- traveling carrier style waveguide photodiode with a directional coupler is presented. The directional coupled waveguide controls the optical absorption profile along the length of the device, so that the front facet does not have high current density. The device is characterized for responsivity, bandwidth, and OIP3, as well as modeled electrically and thermally. Additionally, variations of the device, including the coupler width, photodiode width, and photodiode length, are characterized and modeled