Method for high-power illumination of silicon for optically configurable microwave circuits

Photoexcited intrinsic silicon "pixels" are applied as coupling elements between microstrip lines. Under proper illumination, the free-carrier concentration of the silicon increases sufficiently to pass a microwave signal across the pixels. Illumination is accomplished through the use of multimode fiber coupling between high-power laser diodes and multimode prismatic-waveguide output couplers. The prismatic couplers are fabricated on the endfaces of thick glass slab waveguides to direct the light onto the silicon pixel. This configuration delivers up to similar to75% of the total laser diode light to the prism-waveguide couplers and these couplers, in turn, deliver up to similar to70% of the input light to the silicon pixels. The prismatic waveguide couplers provide illumination uniformity within approximately +/-15% over the length of a 1mm x 5mm pixel. As a simple in-line coupler between microstrip lines, the fully illuminated pixel allowed an increase in tranmitted signal of >5dB over most of the range from 0.5GHz to 15GHz with >10dB obtained over intermediate ranges. A silicon pixel tunable transmission line termination exhibits impedance matching at increasing wavelengths by successive illumination of multiple pixels, effectively increasing the length of the termination. This is illustrated by shifts in the resonant frequencies of the device reflection characteristics.