Dr. Chris Emslie, Managing Director and Dr. A. Gillooly at Fibercore explore how next generation optical fibre is being revolutionised with multiple cores to considerably expand current communication bandwidth capabilities
The ever increasing demand placed on the Internet and other telecommunications systems has required optical fibre manufacturers to consider new ways of transmitting larger amounts of data in increasingly more efficient ways. Over the last few decades, optoelectronic systems manufacturers have repeatedly managed to find ways of cramming more data down the existing optical fibre network.
These techniques involve ever-increasing levels of complexity and cost in order to squeeze as many channels of light down the fibre as possible. But there comes a time when the physics of optical transmission catches up and radical new ideas need to be considered.
In response, there are techniques currently being developed to improve optical conversion efficiency of fibres used in optical amplifiers by creating fibres with multiple cores. This enables a multiplication factor in the communication bandwidth that can be transmitted down a single fibre.
Over the last few years, multicore and multimode telecommunication fibres have become a large research topic for both academic and commercial researchers around the world.
For multicore fibres the concept is simple, by multiplying the number of cores in a fibre, it is possible to multiply the communication bandwidth that can be transmitted down the fibre. This technique is known as space division multiplexing (SDM). There is a competing technology known as mode division multiplexing (MDM) which uses multimode optical fibres to enable a single core of a fibre to carry different channels on different models within the core.
By taking techniques such as those developed by Fibercore, in using a modified chemical vapour deposition (MCVD) performance manufacturing process and combining it with their ultrasonic drill, it is possible to fabricate multicore or multimode fibres and even to combine both methods and create multicore-multimode fibres; potentially creating a quantum leap in the bandwidth capacity of the optical fibre.
Even before the first use of multicore fibres, the company was using their ultrasonic drill to manufacture an all silica, high power optical amplifier fibre. The drilling process enables the inner cladding of the fibre to be formed into an efficient mode-mixing waveguide which, due to the all silica design, is environmentally stable. The company is also utilising its erbium doped fibre manufacturing process to create multicore fibres with erbium doped cores for multicore optical amplifiers.
Process improvement efficiencies
The company’s latest EDF IsoGain is the fifth generation of core composition. In EDF, control of core composition at the microscopic level is the most important factor. It is true, it must contain erbium to generate the fundamental gain, aluminium to both enhance efficiency by dispersing the erbium ions evenly within the core matrix and flatten the gain-profile – but to guarantee performance in the latest generation of commercial EDFAs, quantities and ratios of key constituents must be maintained, contaminants eliminated and the fundamental process of forming the core within the preform controlled to an exceptionally high degree.
Performance improvements tend to be evolutionary rather than revolutionary but a development programme begun by the company in March 2011 has succeeded in refining the core composition of this product in a way that has yielded increases in EDFA conversion efficiency by between 6 percent and 10 percent (a little more or less can be expected, dependent upon the precise amplifier design and inversion levels).
In other words, it is now possible to achieve the same output power with c. 330mW of pump that was used to achieve 350 – 360mW – potentially sufficient to substitute a lower-cost diode, or to under-run an existing pump to increase MTBF.
The cause, effect & advantages
The impact of refining the core composition has distinct advantages. For instance, not only is there a marked improvement in conversion efficiency of more than 6 percent, but also a significant reduction in variability from lot-to-lot (see Fig. 2) – enabling EDFA manufacturers to confidentially specify pump conditions.
These improvements come with the added benefit that they have been achieved without changes to the fibre specification or fundamental material composition. The combination of flexibility found with the company’s drilling and preform manufacturing process coupled with the new highly efficient erbium doped cores, enables telecommunications equipment manufacturers to design and manufacture products with much improved communication capacity.
The availability of multicore transmission fibres that are splice compatible with the new highly efficient amplifier fibres, represents a huge advance in both current communication bandwidth and for the future of the industry.
Fibercore
