Space Division Multiplexing

The technical simplest approach is to use multiple optical fibers for obtaining higher overall bit rates. One essentially multiplies most of the hardware for transmitting information via multiple parallel information channels. The used components are essentially the same as for single fiber-optic links. Only to a moderately extent, one tries to reduce the cost by using a single component for multiple channels:

• One may use a single laser diode in conjunction with fiber splitters for pumping multiple fiber amplifiers.
• Multiple fibers can be integrated into a single fiber cable. For small distances, fiber ribbons may be used.
• Multiple laser transmitters maybe he realized as a laser array on a single chip, e.g. a VCSEL array.
• Multiple receivers may also be combined on a single chip (→ photonic integrated circuits), and some of the electronics can be shared between the channels.

The disadvantage of this approach is that the cost still rises substantially for an increasing number of transmission channels. However, the development is relatively simple, and no essential new optical components are required.

A potentially more economical solution – particularly for large transmission distances and a substantial number of transmission channels – is to use a single fiber containing multiple fiber cores. Such fibers are called multi-core fibers [21]. Each individual core can provide one communication channel.

In the simplest situation, the fiber cores are sufficiently separated from each other such that there is negligible light coupling between the fibers, thus negligible cross-talk between the channels. The signal processing for such uncoupled SDM fibers is relatively simple. However, that requirement severely limits the number of fiber cores, as their distance needs to be substantial. One may try to “insulate” relatively closely spaced cores with refractive index trenches. Alternatively, one may use fibers with larger cladding diameter, but that approach has serious disadvantages, particularly the increased tendency for breaking fibers even for moderate bend radii, but also difficulties for fiber interfaces, e.g. with fusion splicing.

Due to the limitations of uncoupled fibers, there is a tendency to use fibers with more cores, put so closely together that a significant amount of coupling between the fibers occurs (coupled SDM fibers), and supermodes are formed. In order to deal with the resulting cross-talk issue, one uses refined MIMO techniques (multiple inputs / multiple outputs), based on electronic signal processors, which are in principle similar to those used in wireless data transmission (e.g. for WLAN with multiple antennas). As the group velocities of the different fiber cores are quite similar, and a moderate amount of coupling does not lead to a substantial spread of group velocities, MIMO receivers with limited complexity are sufficient to obtain high bit rates with sufficiently small bit error rates.

Various technical challenges are encountered. The perhaps smallest one is the fabrication of multi-core fibers. Although traditional techniques of fabricating fiber preforms can not be used, there are alternative solutions e.g. based on photonic crystal fibers which are very flexible in that respect. Once a multi-core fiber preform has been made, it can be drawn into fiber in traditional fiber drawing towers.

Connecting Multi-core Fibers

Spatial Multiplexers

Fiber Amplifiers for Multi-core Fibers

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 or 19. The specific term for that technique is mode division multiplexing.

When comparing that approach with using multi-core fibers, there are various important differences:

• The used fibers are simpler to make. In some cases, one may even use fibers of existing multimode fiber-optic links, where previously one has used only a single spatial channel.
• Connecting multimode fibers e.g. with fiber connectors or splices is much easier than coupling multi-core fibers, where the cores of the two fibers need to be aligned. However, the quality requirements for fusion splicing are higher than for ordinary multimode fiber communications, because mode-dependent loss must be minimized.
• Interfacing to single fibers is technically different; special spatial mode multiplexers are required. They can be realized with certain phase plates or as photonic lanterns, for example.
• The cross-talk between different fiber modes is more substantial than that between separate cores. Besides, there are significant differences in group velocity between different modes. Therefore, a MIMO receiver (digital signal processor) with higher complexity is required. One may also optimize few-mode fibers such that their spread of group velocities (modal differential group delay, MDGD) is minimized – typically by using a graded-index profile. At the same time, one should try to minimize mode coupling.
• Again, special fiber amplifiers are required, which provide sufficiently similar gainfully different fiber modes [8].

When comparing with the use of multi-core fibers, the ease a trade-off: the optical set up is simpler with multimode fibers, but on the other hand one requires more sophisticated electronics.

It is also possible to combine the two approaches described above: using a firewall which contains multiple cores, each one having several spatial modes.

In recent years, a lot of new hardware has been developed for space division multiplexing. Partially, such items are commercially available:

• There are various kinds of SDM fibers, particularly multi-core fibers and few-mode fibers, partially with minimized modal differential group delay. Fiber cables containing such fibers are also available.
• There are various kinds of spatial multiplexers, mode connectors and the like.
• Special erbium-doped fiber amplifiers for few-mode fibers and for multi-core fibers have been developed.
• Further, there are fusion splicers which are suitable for splicing multi-core fibers, sometimes also with increased cleaning diameters.

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