Why few-mode fibres?

Optical fibre is used for the vast majority of communications carrying nearly all internet communications and phone calls. Optical fibres are flexible and transparent, and are typically made of glass or plastic with a diameter slightly greater than that of a human hair. Optical fibres allow for much greater bandwidth transmission and cover longer distances than standard electrical cables.

Conventional optical fibres usually consist of a glass core surrounded by a cladding of a different glass with lower refractive index which allows light to be guided in the core by total internal reflection. Depending on the size of the core, optical fibres can support one or more propagation paths known as modes.

The most commonly used fibres in long-distance transmission are single-mode fibres which as their name suggests only support a single propagation path along the fibre core. This ensures that pulses of light do not spread out along the length of the fibre which would otherwise lower the achievable data rates or bandwidth. Since single-mode fibres have smaller cores it is more challenging to connect two fibres together due to the smaller tolerances.

Multi-mode fibre is commonly used in short-haul local networks because its larger core size means joining and connecting the fibre is easier. The larger core however means that more propagation paths are possible along the length of the fibre, which means that a pulse of light spreads out along the length of the fibre, limiting the data rate possible. This has motivated the replacement of old multi-mode fibre links in local area networks with single-mode fibre.

Single mode fibre has an upper limit to the amount of data it can transmit. The theoretical data capacity limit of single-mode fibre is 100 Tb/s. In practise the maximum data capacity of single-mode fibres, such as that of Facebook’s MAREA undersea cable is  approximately 20 Tb/s.¹ The need to enable higher data capacity has motivated the development of new types of fibre for future long-haul fibre networks.

Few-mode fibre has a core diameter (see Fig 1b) slightly larger than that of single-mode fibre but much smaller than multi-mode fibre. Few-mode fibres typically come in 2 mode group and 3 mode group types. The slightly large core enables them to handle more optical power than a single-mode fibre, and hence carry more data channels. This fibre type has been used in world record spectral efficiency demonstrations over 3500 kms with capacities beyond the limits of single-mode fibre.²

Another new class of fibre are multi-core fibres (Fig 1c and 1d). Multicore fibres come in both single-mode and few-mode variants and can be used to dramatically scale the capacity of fibre links. Several demonstrations using such fibres have already allowed for data transmission beyond the petabyte/s rate (that is the equivalent of 1000 terabytes per second) which is an order of magnitude larger than that possible with single-mode fibre.³

References:

1. https://www.theverge.com/2019/2/28/18244357/microsoft-facebook-marea-cable-16qam-20-percent-speed-boost
2. Rademacher, G.; Ryf, R.; Fontaine, N. K.; Chen, H.; Essiambre, R.-J.; Puttnam, B. J.; Luis, R. S.; Awaji, Y.; Wada, N.; Gross, S. In 3500-km mode-multiplexed transmission through a three-mode graded-index few-mode fiber link, 2017 European Conference on Optical Communication (ECOC), IEEE: 2017; pp 1-3.
3. Luís, R. S.; Rademacher, G.; Puttnam, B. J.; Eriksson, T. A.; Furukawa, H.; Ross-Adams, A.; Gross, S.; Withford, M.; Riesen, N.; Sasaki, Y., 1.2 Pb/s Throughput Transmission Using a 160 micron Cladding, 4-Core, 3-Mode Fiber. Journal of Lightwave Technology 2019, 37 (8), 1798-1804.