Analysis of AM-to-PM conversion in MUTC photodiodes based on an equivalent circuit model

High-speed, high power-handling photodiodes with sufficiently low amplitude-to-phase (AM-to-PM) conversion coefficients are critical components in the systems that generate ultra-stable microwave signals. This paper reports the AM-to-PM conversion in modified unitraveling carrier photodiodes (MUTC-PDs) with 20 gm and 40 gm diameters. The contributions of AM-to-PM conversions from the carrier transit-time and impedance were quantified systematically based on a photocurrent-dependent nonlinear equivalent circuit model. It is found that the AM-to-PM conversion in 40 mu m PD is dominated by the nonlinear impedance, while for 20 mu m PD, the transit-time impacts the AM-to-PM conversion more significantly. These results imply that, for large PDs, the nonlinearity of the PDs' photocurrent-dependent impedance is the critical reason causing AM-to-PM conversion. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement