Mode-Division Multiplexing For Ultrafast Optical Fibre Communications
The advent of global telecommunications has revolutionised our society. We now have the ability to communicate with anyone in the world instantly from anywhere. At the click of a button we have access to a vast repository of all the world’s knowledge. The backbone of this extensive global communication network has long been the humble optical fibre. An optical fibre is a very pure and elongated piece of glass capable of sending light over long distances. In fact, optical fibres are the only technology we have that can transmit high-bandwidth signals thousands of kilometres across the earth’s surface. They are critically important to telecommunications and the modern internet simply would not exist without them.
The usage of fibre optics continues to increase dramatically and in 2011 the global demand for optical fibre was 195 million km – enough fibre to reach from earth to the sun. However, the world’s increasing demand for high-speed data is proving insatiable. For instance there are currently almost 6 billion mobile internet subscribers and demand for data from these users is predicted to increase 6 fold in the next 3 years alone. The challenge facing industry now is that the speeds achievable in practice are beginning to reach theoretical limits – that is, they are constrained by the fundamental properties of light itself.
Figure 1 Principle of Mode Division Multiplexing
Light has a finite number of physical properties that can be used to encode data and with wavelength, polarisation and amplitude/phase all reaching their limits, connection speeds will soon fall far short of demand. There is but one remaining degree of freedom that is as yet largely unexplored: space. In this sense space is the final frontier of fibre communication. The focus of this research is Mode Division Multiplexing (MDM) as shown in Figure 1. In MDM, light beams for different channels are assigned different spatial profiles (i.e. different shapes) called modes. A simple example would be to send one channel on a laser beam shaped like a circle, one like a square and one like a triangle. In practice the shapes used are more complicated and have special mathematical and physical properties. This method represents arguably the most revolutionary change in how information is transmitted down fibres since at least the 1980s, perhaps even since fibres were invented.
Figure 2 Principle of holographic modal multiplexing
The COMIMO team in CMMPE, has developed a novel MDM system. The light for each channel is coupled into the fibre by way of a computerised hologram specially designed to generate the desired mode in the fibre as shown in Figure 2. At the receiver, there is a corresponding hologram that is designed to detect the presence of that mode and direct the light to the appropriate output. This can be done with such precision that the modes can be recovered after many kilometres. So far we have demonstrated the multiplexing of two channels over a distance of 2km with each operating at 56Gb/s giving a total speed of 112Gb/s unsing the system shown in Figure 3. This bandwidth is enough to download an entire DVD in just 0.36 seconds. This is an effective doubling of the capacity of these optical fibres – a significant achievement for our first result.
Figure 3 Overall layout of the COMIMO optical system
In the past 18 months we have published different results from this system in 5 major journals, as well has having presented at 5 international optics conferences, including giving invited workshops to expert audiences at 2 leading conferences in the field. This, however, only represents the beginning and our MDM system has the potential to support 3, 4 or even more data channel. This will enable creation of some of the fastest optical links ever made and is a true leap forward in telecommunications technology. Over the next 4-5 years we plan to develop this project as part of the COMIMO project, funded by EPSRC in collaboration with UCL, Oxford and Southampton. With analogy to periods such as the Stone, Bronze, Atomic Age etc. the past twenty to forty years is sometimes referred to as the Information Age. If mastery over stone, metallurgy and the atom defined those ages, then it is mastery over light that defines this one. MDM represents a new frontier in telecommunications and we are very well positioned to contribute to this field at the very earliest stages of its development.
Related journal publications:
Aberration correction for free space optical communications using rectangular zernike modal wavefront sensing
Free space communications with beam steering a two-electrode tapered laser diode using liquid-crystal SLM
Optical Mode-Multiplexing Using Holography and Multimode Fiber Couplers
of Multimode Fiber by Selective Mode Excitation
Offset Launch for Dynamic Optimization and Characterization of Multimode
space optical wireless communications using directly modulated two-electrode
high brightness tapered laser diode
of high-stability DC balancing scheme for ferroelectric liquid crystal
on silicon holograms using graphics processing units
Selected conference papers:
Holographic wavefront sensing and correction for free space optical communications
Degenerate mode-group division multiplexing using MIMO digital signal processing
Mode division multiplexing exploring hollow-core photonic bandgap fibers
Optical vortex based Mode Division Multiplexing over graded-index multimode fibre
Demonstration of Radio-over-Fibre Transmission of Broadband MIMO over Multimode Fibre using Mode Division Multiplexing
multiplexing at 2x 20Gbps over 19-cell hollow-core photonic band gap
mode generation for mode division multiplexing
control of a 50µm core diameter multimode fibre using a spatial
implementation of optical multiple-inputs, multple-outputs (mimo) over
a multimode fibre
Optical Switching: The ‘ROSES’ Demonstrator”
Some of this telecoms research was carried out as part of the COMIMO project, and was done in collaboration with the Centre for Photonic Systems.