Fibre optic rotary joints

What a fibre optic rotary joint is

A fibre optic rotary joint, or FORJ, is the optical equivalent of a slip ring: it passes light signals between a fixed and a rotating fibre without a physical join. In a single-channel unit the two fibre ends sit on the rotation axis with collimating lenses so the beam crosses a small gap. Multi-channel units use a derotating optical element, such as a Dove prism, so several fibres can share the axis. Joints are built for single-mode or multimode fibre and can be combined with electrical slip rings.

When to choose a fibre optic rotary joint

Choose a FORJ when the rotating part needs very high bandwidth, immunity to electromagnetic interference or long transmission distances, as in subsea vehicles, cable reels, radar and video systems. It is passive and wear is very low because nothing touches. The trade-offs are insertion loss, which must fit the optical budget, sensitivity to contamination and a higher cost for multi-channel units. If data rates are modest and EMC is manageable, an electrical Ethernet slip ring may be simpler.

How to specify fibre optic rotary joints

• Number of optical channels
• Fibre type: single-mode or multimode, core size (µm)
• Operating wavelengths (nm)
• Maximum insertion loss and loss variation during rotation (dB)
• Return loss requirement (dB)
• Connector type, such as FC, ST, LC or SC
• Speed (rpm), environment and any electrical circuits to combine

Common mistakes

• Not including the FORJ insertion loss in the full link optical budget
• Mixing single-mode and multimode fibres between the joint and the transceivers
• Leaving connectors uncapped during assembly, which contaminates the optical faces
• Specifying many channels when wavelength multiplexing would cut cost and size

Series compared

Maximum figures across each series, summarised from manufacturers' published data on the date shown on each series page. Always confirm against the current datasheet.

Where fibre optic rotary joints are used

Fibre optic rotary joints: frequently asked questions

What is the difference between single-mode and multimode fibre optic rotary joints?

Single-mode joints carry light in one guided mode through a very small core, which suits long distances and high data rates but needs precise alignment. Multimode joints use a larger core, which is more tolerant of alignment and suits shorter links. The joint must match the fibre and transceivers already used in the system, so state the fibre type and wavelength.

Can a fibre optic rotary joint be combined with an electrical slip ring?

Yes. A common arrangement places the FORJ on the axis, with the electrical slip ring built around it in a through-bore format, so power and optical data pass through one assembly. This is widely used on cable reels, ROVs and radar. Specify the electrical circuits and optical channels together so the supplier can design one housing and sealing system.