Exploiting the bandwidth potential of multimode optical fibres

Lead Research Organisation: University College London
Department Name: Electronic and Electrical Engineering

Abstract

Historically the optical fibre was perceived to provide "unlimited" bandwidth, however, the capacity of current communications systems based on single mode optical fibre technology is very close to the limits (within a factor of 2) imposed by the physical transmission properties of single mode fibres. The major challenge facing optical communication systems is to increase the transmission capacity in order to meet the growing demand (40% increase year-on-year) whilst reducing the cost and energy consumption per bit transmitted. If new technologies are not developed to overcome the capacity limitations inherent in single mode fibres and unlock the fibre bandwidth then the growth in the digital services, applications and the economy that these drive is likely to be curtailed. The need for increased capacity in the core and metro areas of the network and within computing data centres is likely to become even more acute as optical access technologies, providing far greater bandwidths directly to the users, take hold and services such as ubiquitous cloud computing are adopted.
Multimode optical fibres (MMF) offer the potential to increase the capacity beyond that of current technologies by exploiting the spatial modes of the MMF as additional transmission paths. To fully exploit this available capacity it is necessary to use coherent optical (CO) reception and multiple-input multiple-output (MIMO) digital signal processing techniques analogous to those already used in wireless communication systems such as WiFi. This project aims to develop the technologies and sub-systems required to implement a CO-MIMO system over MMF that exceeds the capacity of current single mode fibre systems and reduces the cost and energy consumption per bit transmitted. To achieve this goal the project addresses the following key engineering challenges necessary to realise a complete system demonstrator.
Engineer the channel: The multimode optical fibre MIMO channel, unlike its wireless counterpart, presents the opportunity to engineer the optical channel to optimise its performance for MIMO operation by designing and fabricating new optical fibres, using proven solid core technology, to maximise the MIMO capacity of the fibre.
Dynamically control the channel: The transmission characteristic of the multimode optical fibre channel varies with time. We will exploit both the flexible and fast adaptive nature of digital signal processing, and the less energy intensive and slower adaptation of liquid crystal spatial light modulator based optical signal processing to compensate for the channel variation and recover the spatially multiplexed data channels.
Employ energy efficient optical amplification: In order to reduce both the energy consumption and cost per bit and to extend the propagation distance into the hundreds of kilometres region it is essential to develop optical fibre amplification technologies that provide amplification to multiple spatial and wavelength channels and thus share the cost.
Coherently detect the optical signal to exploit the wavelength and spatial domains: The developed system will combine spatial multiplexing with existing dense wavelength division multiplexing, polarisation multiplexing and multilevel modulation techniques to maximise the capacity. The key to achieving this is the use of coherent optical detection and digital signal processing, which is essential not only to fully exploit the spatial capacity of the MMF channel, but also facilitates the use of existing multiplexing techniques that are difficult to realise in conventional multimode transmission systems.
The technologies and systems developed within this project will find applications, ranging from capacity upgrades of existing MMF data networks in data and computer processing centres, through to the installation of new high capacity metro and long haul fibre transmission systems using the MIMO optimised fibres and technologies developed in this project.

Planned Impact

All members of society will benefit from improved access to the communications infrastructure that this research will enable. In particular the continuing growth in the digital economy with its requirement for universal access and increased bandwidth to support services such as cloud computing requires the development of new communications technologies, such as those proposed here, to provide increased capacity at lower cost and with reduced energy consumption. Cloud computing in particular has the potential to significantly decrease the energy footprint of computing, but its successful adoption is entirely dependent on a low cost, high capacity and reliable communications infrastructure between the users and the computing centre. It also requires energy efficient, high capacity links for interconnection of computing resources within the data centre. Of particular importance here is low energy dissipation and small form factor as the data centre environment is space constrained. The use of high capacity MMF transmission technology developed within this proposal will be particularly attractive in this application particularly as these centres already have installed MMF links and this represents an attractive capacity upgrade path. Moreover, simply adding more fibre to meet the capacity in metro and long haul communications does not reduce the cost or energy consumption.
To ensure that economic and societal benefits are realised from this work it is essential to facilitate the commercialisation of the technologies and subsystems that will be developed in this proposal. To achieve this we will contribute to the standardisation activities in this area to ensure that the systems developed are able to be widely deployed and work closely with our industrial project partners to commercialise the subsystem technologies. The industrial project partners and supporters have been carefully chosen to have interests and capabilities that are closely aligned across the full range of outcomes from this project. This includes: RSoft an optical systems simulation software provider with whom we will work to distribute the models developed; Oclaro an optical components manufacturer with a particular interest in integrated optical transmitters and optical amplifiers; SPI Lasers who in collaboration with Southampton develop and manufacture specialised optical amplifiers and have devices that may be well suited to multimode amplification; PMC Sierra a semiconductor chip designer and manufacturer for communications applications with interest in the implementation of advanced DSP for coherent optical receivers.

Publications

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Carpenter J (2012) Degenerate Mode-Group Division Multiplexing in Journal of Lightwave Technology

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Carpenter J (2012) Mode Division Multiplexing of Modes With the Same Azimuthal Index in IEEE Photonics Technology Letters

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Carpenter J (2013) Optical vortex based Mode Division Multiplexing over graded-index multimode fibre in 2013 Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference, OFC/NFOEC 2013

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Gomez A (2015) Beyond 100-Gb/s Indoor Wide Field-of-View Optical Wireless Communications in IEEE Photonics Technology Letters

 
Description We have developed and demonstrated a new ring core fibre (RCF) technology, that supports 10 spatial modes, increasing the data transmission capacity by a factor of 10 over that of conventional single mode fibre technology. The RCF developed in this project was designed to support 10 spatial channels in approximately the same size core as a conventional single mode fibre, thus dramatically increasing the capacity per unit area. In addition the transmission properties of the RCF were designed so that they reduce the required complexity of the receiver digital signal processing. During the course of this research we developed the RCF fibre, and the spatial multiplexing, optical amplification and digital signal processing components and technologies in order to demonstrate optical data transmission over ring core fibre.

Ring core fibre: A low loss 25km RCF has been designed and fabricated with a measured loss of 0.3dB/km, comparable to the 0.2dB/km loss of conventional single mode fibre. This fibre supports the transmission of 10 spatial channels simultaneouly. The ring structure of the fibre was designed to achieve a large effective index difference between mode groups, which minimises crosstalk between mode groups during propagation and simplifies the required receiver signal processing.

Spatial MUX and DEMUX: A spatial multiplexer (MUX) and demultiplexer (DEMUX) are required to efficiently couple signals from multiple transmitters into and out of the RCF. In this project we have developed and demonstrated both a flexible SLM based MUX/DEMUX and a compact and practical all fibre based Photonic Lantern MUX/DEMUX. The SLM based system is very flexible in that it lets you couple into arbitrary mode profiles and can support the launch of multiple modes, in this work we demonstrated the simultaneous launch of 6 modes. This makes it ideal for characterisation of the RCF and system performance, however the insertion loss and physical size of this system make it impractical for a deployed optical communications system. In collaboration with researchers from The University of Central Florida we developed and tested an all fibre photonic lantern that is cable of simultaneously multiplexing and demultiplexing 10 spatial modes with an insertion loss of less than 4dB and a mode selectivity better than 4.5dB.

Optical Amplification: Optical amplification is required to overcome the loss of the transmission fibre to support long distance transmission. An Erbium doped ring core fibre compatible with the passive RCF used for transmission has been designed and fabricated. A RCF optical amplifier that provides a gain of 10dB with a mode dependent gain variation of less than 1dB has been realised, however, further work is required to reduce the intrinsic loss of this fibre to increase the gain. One of the key advantages of the RCF multimode fibre over other competing multimode fibre designs for spatial multiplexing is the low mode gain variation that can be achieved with the ring core structure when using a simple pumping scheme.

Digital Signal Processing: Spatial multiplexing systems that use mode multiplexing rely on Multiple Input Multiple Output (MIMO) digital signal processing (DSP) to undo the crosstalk between the different spatial channels that occurs due the imperfections in the optical components. Typically the required signal processing scales with the square of the number of spatial channels and linearly with transmission distance. As such approaches to reduce this square law scaling and distance scaling in complexity are needed to make these systems practical. The RCF developed in this project minimises the crosstalk during transmission, however the spatial multiplexers still introduce crosstalk at discrete locations so some MIMO signal processing is still required. We have developed reduced complexity MIMO DSP that exploits the discrete nature of the crosstalk in a RCF optical transmission system to reduce the DSP complexity, by using separate MIMO processing blocks per mode group rather a single large MIMO processing block for all the modes, so it is independent of the transmission length and scales at a lower rate than the square of the number of modes.
Exploitation Route These findings show the potential of Ring Core Fibres to increase the capacity of optical fibre transmission whilst minimising the complexity of the digital signal processing. They also show that the fibre itself and the required spatial multiplexing, amplification and signal processing technologies are feasible and practical for deployment in a real system. We have also shown ring core fibre design will only scale to support at most 14 spatial modes whilst maintaining the low mode group crosstalk properties.

These findings will be used by fibre manufacturers and component manufacturers when deciding which variant of spatial multiplexing should be used to meet the demand for future optical communications capacity.
Sectors Digital/Communication/Information Technologies (including Software)

URL https://www.ee.ucl.ac.uk/ong/group-research/comimo
 
Description Big Bang Fair (London) 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact A few thousand students attend the Big Bang Fair London. We ran an interactive exhibit designed to reveal the technology and engineering behind the internet. This stand revealed the hidden the optical communications infrastructure and the contribution that our research on optical communications makes to this infrastructure.
Year(s) Of Engagement Activity 2014,2015
URL https://nearme.thebigbangfair.co.uk/Event/?e=2505
 
Description Progress and Challenges in MIMO Signal Processing and Channel Modelling for Space Division Multiplexed Transmission systems, ECOC 2016 Workshop 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact As part of this project I organised a Sunday Workshop at The European Conference on Optical communications with the aim of diseminating the progress that has been made in the development of spatial multiplexing technology, considering the most likely fibre technology from the technologies that have been demonstrated to date and identifying the challenges that still remain.

The workshop was attended by around 80 people and had speakers (see below) from both university and industrial research organisations giving their views on the current state of the technology in this area. With two of the speakers (Kai Shi and Yongmin Jung) coming from this project.

Optical MIMO Processing (in Modal Basis) for Direct-Detection MDM, Karthik Choutagunta, Stanford, USA.
Elliptical core fibres MDM without DSP, Ezra Ip, NEC, USA
OAM fibres for DSP free MDM, Siddharth Ramachandran, Boston University
Multimode fibre channel modelling in the linear and nonlinear regime, Georg Rademacher, TU Berlin, Germany.
Multimode optical amplification performance and implications for channel equalisation, Yongmin Jung, Optoelectronics Research Center, University of Southamption, UK.
The challenges of Mode division multiplexed transmission with 30+ modes, Nick Fontaine, Nokia Bell Labs, USA.
Optimising MIMO-DSP for different crosstalk and distance regimes, Kai Shi, UCL, UK.
Taking MIMO-DSP from offline to online implementation, Sebastian Randel, Karlsruhe Institute of Technology, Germany

This workshop identified that the coupled core fibre technology looked be be extremely promising in terms of offering a good spacial density and compatibility with existing fibre and component manufacturing techniques whilst minimising the DSP complexity however like the ring core fibre technology developed in this project it is limited in in terms of number of spatial channels that it can support to less than 10.
Year(s) Of Engagement Activity 2016
URL http://conference.vde.com/ecoc-2016/Programme/Pages/Workshops.aspx