Optical fibre is the backbone of all global internet communication. Every day, more than 100 exabytes of data is transferred across optical fibres; this is the same as transferring the gigabyte equivalent of all movies ever made, every five minutes. In simple terms, it’s a lot of data. Due to the basic human desire to communicate, the internet data demand is exponentially increasing. The total internet data demand has more than tripled over the last 5 years. However, there is a fundamental limit to how much data can be carried across currently deployed single-mode optical fibres. This is known as the nonlinear Shannon limit, and it is caused by optical nonlinearities intrinsic to the fibre design and material. This capacity limit has been reached by using different wavelengths, polarizations and encoding techniques. As a result, new technologies are required to further expand the transmission capacity and avoid the capacity crunch. The most promising technology is space-division multiplexing.
Figure 1: The increase in fibre capacity over the decades, with space division multiplexing representing the next frontier. Courtesy: Royal Society Publishing.
The basic idea of space-division multiplexing is to either have multiple single-mode optical cores within a common optical cladding, or to use a larger few-moded optical fibre. In both cases, the transmission capacity increases proportionally with the number of single-mode cores or the number of modes supported by the few-moded optical fibre, respectively. In recent demonstrations few-moded multicore fibers have also seen use to further increase the data rates.
Space-division multiplexing brings significant improvements in cost per bit as well as energy efficiency due to a reduction in the number of discrete components in optical communication systems when compared to individual single-mode fibres. When using few-moded optical fibres, one of the challenges is the ability to selectively excite and detect the individual modes of the fibre.
Several companies have recently developed compact mode-multiplexers and demultiplexers suitable for future networks using few-moded optical fibres. Such multiplexers take individual single-mode inputs and selectively convert them into the higher order modes of a few-moded fibre and vice versa.
The mode multiplexers manufactured by Modular Photonics for instance are inscribed into a monolithic block of glass using ultrafast laser inscription, whereby a femtosecond laser is tightly focused into the bulk glass, inducing a localised and permanent modification of the substrate and creating optical waveguides. The design of the multiplexers and demultiplexers takes advantage of the 3D capability of ultrafast laser inscription in order to achieve low losses, broad operational bandwidth and excellent mode-selectivity. This results in a highly scalable, compact and robust integrated photonic circuit. With technologies such as this, spatial division multiplexing is set to become more common in high-bandwidth optical networks.