Rotatable joints for communication lines

摘要

A system of communication across a rotatable joint comprising discriminating between channels by means of a combination of wavelength division multiplexing and optical polarisation multiplexing using phase compensated quarter wave plates, or alternatively achromatic quarter wave plates to effect conversion of respective orthogonally polarised signals to circularly polarised signals, which circularly polarised signals are arranged to traverse a boundary between relatively rotatable parts and using similar plates to effect re-conversion of the respective circularly polarised signals back to orthogonally polarised signals.