Abstract

The introduction of electro-optic sampling techniques now allows the study of very high speed pulse propagation on planar transmission lines. In the frequency domain, recent work on microstrip and coplanar waveguides (CPW) on semiconductor substrates has shown the existence of slow-wave phenomena [1][2]. Since these phenomena are frequency dependent, strong dispersive effects on picosecond pulses would be expected. For a lossless substrate, a spectral domain technique has been used to calculate pulse dispersion for a coplanar waveguide (CPW) and coplanar strips [3], and for a particular geometry and lossy layer both mode matching and finite element techniques have been used to predict pulse behavior in microstrip and CPW [4]. In a typical structure exhibiting strong slow wave effects there are three distinct layers in the substrate: first, a thin, lossless spacer layer immediately below the CPW; second, a somewhat thicker lossy (i.e. doped) layer; and third, a very thick lossless layer. The slow-wave phenomenon (and corresponding variation in effective dielectric constant) occurs as a result of the different interactions of the electric and magnetic fields of the propagating wave with the lossy layer below the CPW. The complex effective dielectric constant of the guide is a function of the conductivity of the lossy layer, the separation of the CPW from this layer, the transmission line dimensions, and the frequency. For frequency domain applications, we have recently proposed a new method to control the slow wave factor in these structures by replacing the doped lossy layer with an (CW) optically generating electron-hole plasma layer in the semiconductor substrate [5]. The device could then serve as a phase-shifter, controlled by varying the optical excitation level, and thus the conductivity of the lossy layer. In this paper we discuss the the effects of the lossy layer on picosecond pulse dispersion.

© 1987 Optical Society of America