How the fibre communications revolution began

Why Multimode was Superseded by Singlemode

Multimode fibre dominated early fibre transmission systems, but in the 1980’s there was a return to the ideal Single-mode waveguide envisaged by Charles Kao & George Hockham.

The reasons

1/. The risk and unpredictability of Modal Noise

  • Discovered during the Hitchin Stevenage system development
    Coherent Lasers +lossy Multimode = Multipath interference

2/. Unpredictability of the bandwidth concatenated fibres

  • Multimode fibres from different suppliers had different refractive index profiles.
    Joined sections have unpredictable modal dispersion

3/. Mechanical precision of fibre and connectors improved

4/. The move to longer (lower loss) wavelengths made the light spot size larger

5/. Transoceanic systems demanded very low dispersion