Mode Division Multiplexing
Mode division multiplexing (MDM) is a technique which is applied in some modern systems for optical fiber communications for increasing the data capacity. Essentially, the idea is using different guided modes of a multimode fiber for different transmission channels.
Components for MDM
The following types of components are typically used for mode division multiplexing:
- Typically, the used multimode fibers have a relatively small number of guided modes and are thus called few-mode fibers. They may in principle be step-index fibers, but often one uses graded-index fibers optimized for minimum differential modal group delay (DMGD), because that allows the use of simpler receiver technology.
- Certain kinds of spatial multiplexers (SMUX) are required for (a) injecting signals from multiple single-mode fibers into the core of a few-mode fiber and (b) for separating the signals after transmission, sending them to different photodetectors. Different kinds of spatial multiplexers have been developed. Some of them are based on certain phase plates, others on photonic laterns.
- For maintaining sufficiently high signal powers over large distances, there are special fiber amplifiers (mostly erbium-doped fiber amplifiers = EDFAs) for few-mode fibers, which are optimized for minimum variations of modal gain.
Because there is always a substantial cross-talk between the different channels, receivers need to be equipped with special electronics based on MIMO techniques (multiple inputs / multiple outputs), similar to those which are already widely used for wireless data transmission, for example for WLAN.
The requirements on fusion splicers are more demanding than for splicing ordinary multimode fibers, since one needs to minimize mode -dependent losses.
Combination with Other Multiplexing Techniques
Mode division multiplexing can be combined with other techniques like wavelength division multiplexing. It is expected to become important for satisfying the first one rising demand of data transmission capacities.
For a more detailed discussion, including the comparison with other techniques, see the article on space division multiplexing.
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