(SDM) spatial division multiplexing
Ø As the services supplied to users become more and more faster , The urge of higher capacity becomes a continuous matter and a motivating for the ambitious telecom vendor to develop and invent.
-The Multiplexing of several services in the same Transmission media is one of the key factors to increase the Transmission speed . Through out the history of telecom development there are many multiplexing methods Most common are wavelength division multiplexing (WDM, using multiple wavelength channels) and time division multiplexing (TDM) , Another option is space division multiplexing or also called spatial division multiplexing (SDM), where one fiber used for multiple spatial channels.
SDM fibers
There are 2 types of SDM fibers :-
1- Multi-Core Fibers (MCFs), where capacity is multiplied by the number of cores present in the fiber.
2- Few-Mode Fibers (FMFs), where capacity is multiplied by the number of modes supported by the fiber.
MCF & FMF
Multi-Core Fibers (MCFs)
The fiber is fabricated to have multi-core in a single fiber , this kind of fiber could be classified into 2 types :-
Uncoupled SDM fibers :- the fiber cores are sufficiently separated from each other such that there is negligible light coupling between the fibers.
That made the processes of the channels much easier, but In the other hand, that severely limits the number of cores in the fiber , also limits the distance .
coupled SDM fibers :- the fiber has a lot of cores that are closely located in the fiber, that caused a significant amount of coupling between the fibers .
In order to deal with the resulting cross-talk issue , we need a refined MIMO (multiple inputs / multiple outputs) which are in principle similar to those used in wireless data transmission (e.g. for WLAN with multiple antennas) .
The MIMO receivers with limited complexity are sufficient to obtain high bit rates with sufficiently small bit error rates.
Some of the disadvantage of using MCF Multi Core Fibers are:-
Ø Connecting Multi-core Fibers
Connecting multi-core fibers is obviously more difficult than for standard fibers , the fibers need to be rotationally aligned such that the core positions match. For example, special provisions are needed to ensure the correct rotational orientation when fusion splicing multi-core fibers.
Ø Spatial Multiplexers
It is also necessary to couple multi-core fibers to separate single-mode fibers, either for combining input signals from such fibers or for distributing output signals to separate photodetectors.
Ø Fiber Amplifiers for Multi-core Fibers
When using multi-core fibers, one requires special multi-core fiber amplifiers . They need to be optimized such that the obtained amplifier gain is sufficiently uniform over the spatial modes, in other words, the differential gain must be small.
Few-mode Fibers (FMF)
It is also possible to realize space division multiplexing with multimode fibers, where each spatial mode is used for one transmission channel. Typically, one uses few-mode fibers, having a quite limited number of modes, for example 3, 6, 7, 12 ,15 or 19. The specific term for that technique is mode division multiplexing.
The FMF can be used for 10s of kilometers , so its found so suitable for connecting DC in near distance .
Conclusion
SDM is a promising technology that start to be used for expanding the fiber capacity .
The SDM is classified to MCF and FMF , the MCF is intended to be used in long distance like marine cables , while FMF is intended to be used in the short distance.
There is a tendency pointing out that SDM can be used on top of wavelength division multiplexing (WDM). This allows a single fiber to transport multiple signals or services that are transmitted at different wavelengths without interfering with each other.
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Resources :- https://www.rp-photonics.com/space_division_multiplexing.html
https://www.prysmiangroup.com/en/insight/telecoms/nexst/space-division-multiplexing-sdm-enables-extremely-high-capacity-networks
