Electromagnetic modeling of integrated optical wave controlled microstrip discontinuities

Abstract

The technological maturity in the area of microwaves, and the rapid advances of optics, coupled with the added advantage of the larger bandwidths of microwave systems compared to those of electronic circuits, has lead to the emergence of new fast-growing areas of research. The rea of optical control of microwave devices and signals is one of these areas. This class of devices is potentially capable of performing the high-speed/high-frequency functions demanded by the new communication systems applications.

This thesis envisions a future generation of optically controlled microwave structures that uses guided optical wave excitation in an integrated optical-microwave environment. The building block of these future devices is an optically-controlled microwave switch which is essentially a microstrip transmission line with a series gap. The optical signal is brought into the gap region using an optical waveguiding system buried in the semiconductor substrate of the microwave transmission line. Photo-generated carriers in the gap region produce a change in the transmission characteristics of the microstrip line.

One of the contributions of this Ph.D. work is the study and development of a systematic knowledge base for the technology of guided wave optical control of microwave devices. The introduction of guided optical waves as a replacement of bulk optics creates an integrated version of these devices and enhances the fabrication yield. This also leads to the improvement in the overall performance of the device or system that is being optically controlled. More complex devices and systems and complicated control mechanism could be implemented using guided wave optics. Examples of the proposed complex devices are presented.

The bulk of this thesis is dedicated to developing an accurate model of the microwave component of the switch. To model the propagation characteristics and the microwave discontinuity, an analysis that utilizes spatial Green's functions formulations is developed. This analysis is capable of modeling continuous and discontinuous shielded microstrip multi-conductor transmission lines with lossy multi-layer substrates. The formulation utilizes the concepts of the Green's function in the space domain and the principle of scattering superposition to obtain the propagation characteristics. The analysis is kept broad enough to allow for multi-layered substrates in which one or more of the layers are lossy.

The Green's functions of the enclosing shielding structure along with the boundary condition imposed by the transmission line mentalization yield a complex function representing the dispersion equation for the transmission line. This equation is solved using Muller Method to yield the unknown complex propagation constant, the characteristic impedance and the EM field patterns for the microwave signals propagated on the continuous microstrip transmission line. Samples of the results obtained using this method are presented.

Using the Method of Moments (MOM) coupled with the Electric Field Integral equation (EFIE), another formulation is developed to analyze the discontinuities in the transmission line. The MOM along with the EFIE, yield the current distribution on a transmission line with a certain discontinuity for a given excitation of the system. Using a curve fitting and optimization routine, the essential design and performance parameters are extracted from the current distribution and are used to compute the scattering ([S]) matrix and Impedance ([Z]) matrix of the structure. This analysis is kept general enough to allow for studying a variety of discontinuities and microstrip topologies.

The above analyses are developed into computer codes, and are used to conduct accurate modeling of a series gap discontinuity in the microstrip line. This series gap is a model of the microwave switch. The gap transmission behavior is studied for varied substrate conductivity, gap separations, and frequency ranges. The substrate conductivity represents the effect of photo-excitation generated by the optical control element.

Using the developed accurate model, the current approach of modeling optically controlled microwave switches using equivalent circuit models based on quasi-TEM assumptions is assessed and compared to the more accurate Green's function formulation. The domain of validity of these simplified models is defined.

The optical component of the proposed devices is also studied. The optical power distribution within the microstrip gap region is accurately modeled using ga Vector Beam Propagation Method (VBPM) and a Vector Finite Difference Time Domain (VFDTD) numerical simulators. Means to most effectively use the available optical power are studied. The optical guiding system proposed is an optical directional coupler. An iterative exact solution for the fields and effective index of the propagating optical power within the structure is presented and utilized in computing the input optical excitation field profile for the noted numerical simulators. The algorithm presented is particularly suited for modeling highly lossy optical waveguides, such as those utilized in the proposed structure.