What is a slip ring for RF signals and how does it differ from an optical rotary joint?

A slip ring for RF signals is a mechanical device that transfers RF (and often DC power and digital data) across a continuously rotating interface using sliding electrical contacts. It differs from an optical rotary joint (FORJ), which transfers signals across the rotating interface using a light beam. The RF slip ring consists of: concentric rings (one for each channel) mounted on the rotor, and spring-loaded brushes (contacts) mounted on the stator that slide on the rings as the rotor turns. Each ring/brush pair provides one electrical channel. For RF signals: the slip ring must maintain a controlled-impedance transition across the rotating interface to minimize reflections and loss. The RF slip ring's performance: frequency range: DC to approximately 2-6 GHz for standard slip rings; specialized designs to 18-40 GHz; bandwidth is limited by the impedance discontinuity at the brush-to-ring interface and the parasitic capacitance/inductance of the structure. Insertion loss: 0.3-1.0 dB (higher than a dedicated rotary joint due to the multi-channel design compromises). VSWR: less than 1.5:1 to 2.0:1 (worse than a dedicated rotary joint). The FORJ: transfers signals as modulated light through a rotary optical coupling (lens-to-lens or fiber-to-fiber across the rotating gap). Performance: bandwidth: 100 MHz to 10+ Gbps (determined by the optical transceivers, not the rotary joint). Insertion loss: 1-3 dB (optical, including the gap loss). Advantages over RF slip ring: no EMI (optical signals do not create or receive electromagnetic interference), much wider bandwidth (single-mode fiber supports 10+ Gbps), no wear-related degradation of signal quality (no physical contact in single-mode FORJ), and no frequency limitation (the optical carrier is at 193 THz, far above any RF frequency).