Experimental Performance Of An Optically Controlled Coplanar Waveguide Phase Shifter With A Thick GaAs Buried Layer.

Abstract

The experimental performance of an a optically-controlled coplanar waveguide phase-shifter fabricated on a heterojunction substrate containing a thick GaAs layer buried between two A1GaAs layers is presented. The measured performance shows that relative phase-shifts of 1000 can be obtained even for optical input powers in the tW range. Introduction Studies of coplanar waveguides (CPW) on semiconductor multi-layered substrates have shown, that within that the range of resistivities covered by most semiconductor substrates and the frequency spectrum of interest for microwave and millimeter wave applications, a slow wave (SW) mode of propagation can exist [1]. A typical structure consist of a thin lossless layer immediately below the CPW, followed by a thicker lossy layer and a third lossless layer. The slow wave factor (SWF), which is the ratio of the effective guide wavelength to the free space wavelength, is a measure of the change of the effective refractive index of the substrate. The magnitude of the SWF is a function of both the resistivity of the buried lossy layer and the of the first layer. Because of the dependence of the SWF on the properties of the underlying substrate, a CPW on a multi-layered substrate can be used as a variable phase-shifter if either the conductivity or layer thicknesses can be actively controlled. At the 10th IR & MMW conference [2], a method of optically controlling a coplanar-waveguide (CPW) phase-shifter was proposed. By the appropriate selection of semiconductor heterojunction substrate materials (A10.4Ga0.6As / GaAs / Al0AGa0.6As) and the corresponding selection of an illuminating source of the correct wavelength (wavelengths longer than 600 nm, but shorter than 860 nm), the conductivity of the buried GaAs layer can be controlled by the intensity of the optical source. The phase shift is then controlled directly by varying the intensity of the illuminator. At last year's IR & MMW conference [3] preliminary measurements of a prototype device were reported. The results indicated that relative phase shift can be obtained even at relatively low levels of intensity. Recently Kwon, et al [4] showed that the slow-wave mode propagating on the CPW can be modeled by a quasi-TEM circuit model. Using this approach we have developed an circuit model for an ideal optically controlled CPW phase-shifter [5]. The analysis showed that when a large percent of the total equivalent thickness of the substrate is lossy, large relative phase-shifts can be obtained. By conformal mapping this translates into a thick second lossy layer for a typical structure. To test this prediction, a device was fabricated on a substrate with a thick GaAs (711m) layer.