Multipath Interference in a 4 × 4 Mirror-Array Intelligent Reflecting Surface for Free-Space Optical Communication

We present a flexible, centimeter-scale 4 × 4 macro mirror array serving as an intelligent reflecting surface (IRS) for free-space optical communication (FSOC) links in urban environments. The realized array integrates a compact, high-precision tip-tilt mechanism with a minimum scanning increment of 0.87 μrad and achieves a fill factor of 86%, enabling high-power transmission and supporting long-range connectivity. We investigate the multipath interference arising from different optical path lengths between the tiles, which introduce relative time delays at the receiver and can constrain the usable modulation bandwidth. A geometric model is developed to simulate the frequency response of the fabricated mirror array by explicitly accounting for tile-dependent optical path length differences. The simulated responses show excellent agreement with experimental data, validating the theoretical framework. Notably, the measured frequency responses exhibit pronounced minima at specific frequencies, evidencing multipath-induced destructive interference. The results demonstrate that the proposed model can accurately predict the frequency-dependent performance and establish practical bandwidth limits for IRS-assisted FSOC links based on specific array geometries.