Why Multimode was Superseded by Singlemode
The tiny light guiding core required by the vision of a Single Mode fibre guide, was a major impediment in the early days: Difficult to align sufficiently accurately, and it made accurate measurements of the fibre attenuation nearly impossible. So the focus switched almost immediately to using Multi-mode fibre with a much larger core. This continued until around 1977 when a field trial was being installed between the towns of Hitchin and Stevenage. At that point a major problem was discovered:
Modal Noise
The story of the discovery of Modal Noise:
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Modal Noise forced a return to Charles Kao’s original vision of a Single Mode guide.
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