An Optically Controlled Voltage Variable Waveguide Attenuator From 35–50 GHz

Abstract

This work presents a fibre-free, fully integrated tuneable absorptive variable waveguide attenuator at U-band (35–50 GHz). The device utilizes photo-generated solid-state plasma (SSP) within a 250 µm high-resistivity silicon (Si) vane to control microwave signal amplitude. To minimize parasitic reflection and insertion loss, a 5 mm tapered geometry is implemented, providing superior impedance matching compared to rectangular vanes. Measurement results demonstrate that the tapered design achieves a minimum insertion loss of 0.12 dB at 37 GHz and maintains a return loss consistently higher than 17.3 dB in ON-state (light-off). By employing a symmetric dual-side illumination scheme with packaged laser diodes, a peak isolation of 13.2 dB is achieved at 50 GHz, representing a 32.3% improvement over single-side excitation. The attenuator remains absorptive across the entire band with a return loss exceeding 9 dB in OFF-state (light-on). This approach eliminates the need for complex photolithography and fibre-optic coupling, offering a cost-effective and fast-switching solution for integrated millimetre-wave communication and radar systems.