One approach is to use multicore fiber (MCF). MCF can be described as multiple optical fibers in one. Zhu and colleagues from OFS Laboratories now suggest a novel MCF design consisting of seven 8-µm-diameter cores arranged in a hexagonal array with a spacing of 38 µm. Each core can carry signals to and from 64 end-users, so 1 fiber can serve 448 end-users simultaneously. The researchers find that the transmission losses of the 6 outer cores can be slightly higher than in standard optical fibers but also show that the losses can be reduced simply by increasing the fiber cladding diameter from 130 to 140 µm.
Nothing would be gained from this advanced design if the coupling of separate signals into the individual cores of the MCF required difficult and costly alignment. However, the researchers also demonstrate a new tapered multicore fiber connector (TMC). The TMC consists of seven single core fibers tapered together to match the MCF, so that one can couple light into each core of the MCF from separate single-core fibers. The TMC is fusion spliced to a MCF by use of a commercially available splicer with automatic alignment, thereby showing that the complex design of the MCF does not hinder its practical implementation. The splice losses are even found to be similar to splice losses of conventional fibers.
The researchers further test their MCF by examining crosstalk between adjacent cores—since it is important that the signals carried in one core do not interfere with the signals carried in other cores. The power transfer between cores is measured to less than -38 and -24 dB for signals propagating at 1310 and 1490 nm, respectively.
As a powerful demonstration of the applicability of the new fiber design, the researchers built a setup with an 11.3-km-long MCF connected to TMCs at both ends. In one end, each of the 7 single core fibers were connected to optical fiber splitters, dividing the signals from each single-core into 64 optical fibers. In this way, the network can supply 448 end-users. It was demonstrated that signals can be transmitted back and forth along the MCF at 2.5 Gbit/s in each core, while still achieving practically error-free transmission.
The network architecture and components used by the OFS Laboratories group can thus allow a single optical fiber to supply each of 448 end-users with 39 Mbit/s. This is more than enough for each user to watch DVD-quality video in real time. Since the data transmission is bidirectional, one other obvious application is online conferences with high-quality video. Looking into the future, further improvements to the individual network components can lead to an additional increase in both the number of end-users and the data rate. Increasing the number of end-users per fiber lowers the implementation cost per user, thereby making it more economically viable to start the deployment of new fibers.
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