A fiber optic rotary joint (FORJ) enables continuous rotation while maintaining optical connection between two or more fiber cables. To ensure reliable performance, key parameters include low insertion loss, minimal insertion loss variation (wow), and high return loss.

• Insertion Loss: This refers to the signal loss as it passes through the FORJ. A 3 dB loss equals 50% transmission. Since multiple components can contribute to overall system loss, it’s important that the FORJ maintains low insertion loss—typically around 0.5 dB for single-channel and up to 3 dB for multi-channel designs.
• Insertion Loss Variation (Wow): As the FORJ rotates, slight variations in signal loss may occur due to changing coupling conditions. Excessive variation can degrade signal quality. Wow also serves as a diagnostic indicator—sudden increases may suggest a fault in the FORJ or elsewhere in the system.
• Return Loss: This measures how much signal is reflected back, which can affect laser stability and cause power fluctuations. A return loss of 40 dB is standard for single-mode passive components, with values up to 60 dB achievable in high-performance designs.

Single-channel FORJs have a simple mechanical design, making them compact, fast, reliable, and durable. A single-mode fiber can carry dozens or even hundreds of data channels, and combining it with an external multiplexer is often more cost-effective than using a multi-channel FORJ.

Dual-channel FORJs can be designed with a coaxial structure to maintain compactness and consistent performance across all rotation angles—without any “dead spots.”

Multi-channel FORJs require more complex mechanical and optical design to maintain low insertion loss, minimal variation (wow), and high return loss. Poor alignment can lead to issues such as:

• Wavelength Dependent Loss: Variation in insertion loss across wavelengths (e.g., 850, 1310, 1550 nm). A difference of 0.5–1.0 dB between 1310 nm and 1550 nm is generally acceptable.
• Polarization Dependent Loss (PDL): Caused by changes in input polarization; less than 0.1 dB is the industry standard.
• Polarization Mode Dispersion (PMD): Pulse stretching due to birefringence; typically kept below 0.1 ps for high-speed systems.
• Crosstalk: Signal leakage between channels; 60 dB crosstalk means only one part per million crosses over, which is a common benchmark for high-performance systems.