Long-distance free-space optical clock synchronization system for the CEPC accelerator

The Circular Electron Positron Collider (CEPC) requires a high-precision clock synchronization system covering 192 control nodes distributed along a 100 km underground tunnel. While fiber-based White Rabbit (WR) synchronization achieves excellent performance, the harsh radiation environment in the tunnel necessitates either expensive radiation-hardened fibers or extensive shielding along the entire ring. This paper presents an alternative approach using Free-Space Optical Communication (FSO) links to replace fiber segments in the clock synchronization network. The FSO transceiver operates at 1550 nm. Its Tx chain uses an Erbium-Doped Fiber Amplifier (EDFA) booster; the Rx chain combines a 50 mm aperture with an Avalanche Photodiode (APD). Alignment is assisted by an on-board camera. The paper examines how launch power, divergence, and aperture size shape timing jitter inside the WR DDMTD loop, using a simplified noise-propagation analysis from APD shot noise through the PLL response, and shows that Tx-side EDFA boost helps operate the receiver closer to a shot-noise-dominated regime. The CEPC tunnel geometry (100 km circumference, 6.5 m diameter) gives adjacent nodes—520 m apart along the arc—a direct line of sight. Three experiments characterize the system: a 6-hour BER test ( < 3.5 × 1 0 − 14 ), an outdoor 500 m 3-level cascade (6.27 ps), and an indoor 25 m 7-level cascade (6.83 ps). Cascade noise analysis reveals an empirically observed scaling law σ 2 ∝ N 3 consistent with a Random-Walk Frequency Modulation-like accumulation whose physical origin remains to be verified. A complete noise-type identification would require additional Allan-deviation characterization; the present data establish the scaling law phenomenologically. The enclosed, temperature-controlled CEPC tunnel strongly suppresses the outdoor atmospheric contributions, so performance should more closely approach the hardware-limited fiber regime. The radiation-tolerance problem is localized to compact transceiver nodes, whose component-level radiation assessment is discussed. FSO clock distribution localizes the radiation-hardening problem to individual transceiver nodes—avoiding 100 km of shielded fiber.