A multigigahertz microwave signal was generated and up-converted in a GaAs substrate coplanar strip line. Two 630-nm-wavelength laser pulses-one with a normal wavefront, another with a titled wavefront- were respectively used to generate and frequency up-convert the signal. The relativistic plasma front, induced by the tilted optical wavefront, frequency up-shifts the counter-propagating electromagnetic wave via the Doppler effect. When the speed of the plasma front was about 0.4 times the speed of electromagnetic wave in the coplanar strip lines, the experiments showed that the fall time of a step signal decreased more than 30% after the reflection. Given the bandwidth limitations of the data acquisition system, it is possible that a factor of 2 increase was achieved. A transmission line model was employed to simulate this process. The simulation results were consistent with experimental observations. Using coplanar strip lines on a GaAs substrate for microwave signal compression device has the advantage of a high reflection coefficient, frequency tunability, small laser trigger energy, and all-solid-state construction, making this technique suitable for impulse radar applications.
A transmission-line model was used to study the asymmetric illumination of photoconductive switches. We studied the evolution of the electric field within the first 2 ns of optical illumination. The electric fields were significantly compressed in the regions of low photocarrier density. Also, the field collapse was slower and the switching efficiency reduced in the assymetric illumination case. Both the symmetric and the asymmetric cases were investigated experimentally with our electro-optic sampling system. The experimental results confirmed the predictions of the transmission-line model.
Using an electro-optic imaging system, we have measured the spatial and temporal structure of electric fields in photoconductive switches with four different contact configurations. Contacts with heavy doping under the metallization had the best performance. Modeling has verified these results.