Improved velocity mismatch in traveling-wave coplanar waveguide photodetector with thick ground-metal-line

Abstract

Bandwidth of a traveling-wave photodetector (TWPD) is limited by the optical absorption coefficient, velocity and impedance mismatches, and the drift time of photo-generated carriers in the intrinsic region. In these parameters, velocity and impedance mismatches have much influence on the bandwidth of TWPD. In this paper, we focus on mismatches, and propose a novel design to enhance the bandwidth. In the new structure, the thickness of a ground electrode increases as much as the ridge thickness. It forces the structure to have characteristics similar to a coplanar waveguide. We simulate this structure by finite different time domain Method in three dimensions and look-over frequency dependent parameters by Fourier transform for the detailed analysis of microwave characteristics such as characteristic impedance, microwave effective index, and microwave attenuation of TWPD. As a result, we obtain 50 (Omega) impedance matching and 89.7 % velocity matching using our novel structure.