Improvement of performance of optically controlled microstrip phase shifters

Two optically controlled phase shifters are proposed. The first structure is based on the microstrip ring resonator inserted between two gaps. Phase-shifting is performed by optically controlling the gaps capacitance. The structure has shown excellent characteristics, however the rejection rate is high. To minimise the reflected power, the second phase shifter, which is based on the rat race coupler, is proposed. In this case, the phase-shifting is performed by adding extra lengths to two gaps, judiciously placed along the microwave ports. The gaps act as short-circuits under sufficient illumination conditions. At 10 GHz, the structure provides a continuous variation of Δφ(S21) between 0 and 90°, with Δ{norm of matrix}S21{norm of matrix} less than 5dB. On a 500 MHz band around 10 GHz, the phase variation is less than 5%. {norm of matrix}S11{norm of matrix} keeps <-20 dB within the experimental bandwidth 8.9-10.7 GHz. Both structures were fabricated on high-resistivity silicon substrate and the experimental results confirm the three-dimensional electromagnetic simulations. © 2007 The Institution of Engineering and Technology.