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Exploiting the bandwidth potential of multimode optical fibres (COMIMO)

EPSRC funded project using coherent optical (CO) reception and multiple-input multiple-output (MIMO) digital signal processing techniques


The project is funded by EPSRC.
Start date: 25 June 2012
End date: 24 December 2016

Project Partners

  • UCL (lead organisation) - optical and wireless MIMO and coherent optical transmission
  • University of Cambridge - multimode fibre transmission and optical signal processing using spatial light modulators
  • University of Oxford - modelling and characterisation of multimode fibre propagation
  • University of Southampton - fabrication of novel optical fibre and amplifier technologies

Industry Partners

  • Gennum UK Ltd
  • Oclaro Technology UK
  • Rsoft Design Group 


Dr Benn Thomsen, UCL - overall project lead. Leads on transmitter and receiver development, receiver based DSP and systems demonstration
Dr Tim Wilkinson, University of Cambridge - leads on efficient spatial multiplexing using Spatial light modulators
Dr Frank Payne, University of Oxford - leads on multimode fibre modelling, design and characterisation
Prof David Richardson, University of Southampton - leads on multi fibre fabrication and development of multimode optical amplification

Project Overview

This project aims to develop the technologies and systems required to exploit the spatial dimension of multimode optical fibre using coherent optical reception. It aims to increase the capacity of a single fibre beyond that of existing fibre communication systems in a cost effective and energy efficient manner.

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 increase 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.

Project Outcomes

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.

A full system demonstration of 10×10 MDM Transmission over 1 km Ring-Core Fiber using Mode Selective Photonic Lanterns and Sparse Equalization has been completed at a raw data rate of 1.12 Tb/s using a Nyquist DP-16QAM signal. Compact photonic lanterns are used as the mode MUX/DEMUX with a mode selectivity of approximately 4.5dB and the MDL of the system is found to be less than 3 dB. A sparse MIMO equalizer with 2048 taps is used to achieve an OSNR penalty of less than 1 dB and the performance of a reduced complexity delayed FDE with fixed number of taps for increased transmission distances is also investigated with the number of taps reduced to 384.




  1. Z. S. Eznaveh, J. C. A. Zacarias, J. Antonio-Lopez, Y-M. Jung, K. Shi, B. C. Thomsen, D. Richardson, S. G. Leon-Saval, R. A. Correa, "Annular Core Photonic Lantern Spatial Mode Multiplexer," Optical Fiber Communication Conference (OFC), paper Tu3J.3., 2017.
  2. Y. Jung, J. Hayes, Y. Sasaki, K. Aikawa, S. Alam and D. J. Richardson, “All-fiber optical interconnection for dissimilar multicore fibers with low insertion loss,” OFC’17, paper W3H.2, Los Angeles, CA, USA, 19-23 March 2017.
  3. Y. Jung, S. Alam, and D. J. Richardson, “Compact few-mode fiber collimator and associated optical components for mode division multiplexed transmission,” OFC’16, Paper W2A. 40, Anaheim, USA, 20-24 Mar 2016.

  4. Y. Jung, Q. Kang, S. Yoo, S. Raghuraman, D. Ho, P. Gregg, S. Ramachandran, S.U. Alam, D.J. Richardson, “Optical Orbital Angular Momentum Amplifier based on an Air-Core Erbium Doped Fiber,” OFC’16, Paper Th5A.5, Anaheim, USA, 20-24 Mar 2016 (Post-deadline paper).

  5. Yongmin Jung, Qiongye Kang, Shaiful Alam, and David J. Richardson, “Integrated SDM amplifiers for practical deployment,” OFC’16 Monday workshop, Will SDM systems make financial sense to justify commercial deployment in terrestrial networks, Anaheim, USA, 20-24 Mar 2016 (Invited speaker).

  6. Y. Jung, “Multimode optical amplification performance and implications for channel equalization,” ECOC’16 Sunday workshop, Dusseldorf, Germany, 18-22 Sep 2016 (Invited speaker).

  7. Y. Jung, Q. Kang, H. Zhou, R. Zhang, S. Chen, H. Wang, Y. Yang, X. Kin, F. Payne, S. Alam, and D. J. Richardson, “Low-loss 25.3km few-mode ring-core fibre for mode-division multiplexed transmission,” ECOC’16, paper W3.B2, Dusseldorf, Germany, 18-22 Sep 2016.

  8. F. Feng, X. Guo, G. S. Gordon, X. Jin, F. Payne, Y. Jung, Q. Kang, S. Alam, P. Barua, J. Sahu, D. J. Richardson, I. H. White, and T. D. Wilkinson, "All-optical Mode-Group Division Multiplexing Over a Graded-Index Ring-Core Fiber with Single Radial Mode," in Optical Fiber Communication Conference, W3D.5., 2016.
  9. Yongmin Jung, “Few mode doped fiber amplifiers,” ECOC’15 Sunday workshop, WS- Multimode photonics: optical waveguides, components and systems, Valencia, 27 Sep- 1 Oct 2015 (Invited speaker).

  10. Yongmin Jung, Shaif-ul Alam, David Richardson, “Compact higher-order mode converter based on all-fiber phase plate segment,” ECOC’15, Paper Mo.4.1.4, Valencia, 27 Sep-1 Oct 2015.

  11. X.Q. Jin and F.P. Payne, “Modelling of microbending loss in optical fibres,” Optical wave and waveguide theory and numerical modelling workshop, London, UK, Apr. 2015.
  12. Yongmin Jung, “Few mode doped fiber amplifiers,” ECOC’15 Sunday workshop, WS- Multimode photonics: optical waveguides, components and systems, Valencia, 27 Sep- 1 Oct 2015 (Invited speaker).

  13. Yongmin Jung, Shaif-ul Alam, David Richardson, “Compact higher-order mode converter based on all-fiber phase plate segment,” ECOC’15, Paper Mo.4.1.4, Valencia, 27 Sep-1 Oct 2015.

  14. X.Q. Jin, A. Gomez, D.C. O'Brien, and F.P. Payne, “Influence of Refractive Index Profile of Ring-Core Fibres for Space Division Multiplexing Systems,” IEEE Photonics Summer Topicals 2014, Montreal, Canada, Jul. 2014.
  15. Shi, K., Gomez, A., Jin, X., Jung, Y., Quintana, C., O'Brien, D., Kang, Q. (2016). Simplified Impulse Response Characterization for Mode Division Multiplexed Systems.
  16. Shi, K., Gorden, G., Thomsen, B.C. (2014). Degenerate Mode-Group Division Multiplexing using Delayed Adaptive Frequency-Domain Equalization.
  17. Sato, M., Maher, R., Lavery, D., Shi, K., Thomsen, B.C., Bayvel, P. (2014). Frequency Diversity MIMO Detection for Dual-Carrier DP-16QAM Transmission.
  18. X.Q. Jin, R. Li, D.C. O'Brien, and F.P. Payne, “Linearly Polarized Mode Division Multiplexed Transmission over Ring-Index Multimode Fibres,” IEEE Photonics Summer Topicals 2013, Waikoloa, HI, Jul. 2013
  19. E. L. Lim, S.Dasgupta, Q. Kang, J. M. O. Daniel, F. Poletti, S.-U. Alam, and D. J. Richardson, “The impact of fiber core ellipticity and modal coherency on few moded Erbium doped fiber amplifiers,” ECOC’13, Paper P.1.15, London, UK, 22-26 Sep, 2013.
  20. Shi, K., Gordon, G., Paskov, M., Carpenter, J., Wilkinson, T.D., Thomsen, B.C. (2013). Degenerate Mode-Group Division Multiplexing using MIMO Digital Signal Processing.
  21. Carpenter, J.A., Thomsen, B.C., Wilkinson, T.D. (2013). Optical vortex based Mode Division Multiplexing over graded-index multimode fibre.
  22. Carpenter, J., Thomsen, B., Wilkinson, T.D. (2012). 2x56-Gb/s Mode-Division Multiplexed Transmission Over 2km of OM2 Multimode Fibre Without MIMO Equalization.
  23. Thomsen, B.C. (2010). MIMO enabled 40 Gb/s transmission using mode division multiplexing in multimode fiber.




  1. Yongmin Jung, Qiongyue Kang, Raghuraman Sidharthan, Daryl Ho, Seongwoo Yoo, Patrick Gregg, Siddharth Ramachandran, Shaif-ul Alam, David J. Richardson, "Optical orbital angular momentum amplifier based on an air-hole erbium doped fiber," J. Lightwave Technol., 2017 (accepted on 10 Jan. 2017)
    Yongmin Jung, Qiongyue Kang, Hongyan Zhou, Rui Zhang, Su Chen, Honghai Wang, Yucheng Yang, Xianqing Jin, Frank P. Payne, Shaf-ul Alam, David J. Richardson, "Low-loss 25.3km few-mode ring-core fiber for mode division multiplexed transmission," J. Lightwave Technol., 2017 (accepted on 25 Jan. 2017)

  2. Xianqing Jin, Ariel Gomez, Kai Shi, Benn C. Thomsen, Feng Feng, George S. D. Gordon, Timothy D. Wilkinson, Yongmin Jung, Qiongyue Kang, Pranabesh Barua, Jayanta Sahu, Shaif-ul Alam, David J. Richardson, Dominic C. O'Brien, and Frank P. Payne, "Mode Coupling Effects in Ring-Core Fibers for Space-Division Multiplexing Systems," J. Lightwave Technol. 34, 3365-3372 (2016)
  3. X.Q. Jin and F.P. Payne, “Numerical Investigation of Microbending Loss in Optical Fibres”, J. Lightw. Technol., vol. 34, no. 4, pp.1247-1253, Feb. 2016.
  4. Gomez, A., Shi, K., Quintana, C., Faulkner, G., Thomsen, B.C., O'Brien, D. (2016). A 50 Gb/s Transparent Indoor Optical Wireless Communications Link With an Integrated Localization and Tracking System. Journal of Lightwave Technology, 34 (10), 2510-2517. doi:10.1109/JLT.2016.2542158
  5. Gomez, A., Shi, K., Quintana, C., Maher, R., Faulkner, G., Bayvel, P., ...OBrien, D. (2016). Design and Demonstration of a 400 Gb/s Indoor Optical Wireless Communications Link. Journal of Lightwave Technology, 34 (22), 5332-5339. doi:10.1109/JLT.2016.2616844
  6. Jin, X., Gomez, A., Shi, K., Thomsen, B.C., Feng, F., Gordon, G.S.D., ...Barua, P. (2016). Mode Coupling Effects in Ring-Core Fibers for Space-Division Multiplexing Systems. Journal of Lightwave Technology, 34 (14), 3365-3372. doi:10.1109/JLT.2016.2564991
  7. MARTINS, H.F., Shi, K., THOMSEN, B.C., MARTIN-LOPEZ, S., GONZALEZ-HERRAEZ, M., SAVORY, S.J. (2016). Real time dynamic strain monitoring of optical links using the backreflection of live PSK data. Optics Express, doi:10.1364/OE.24.022303
  8. Qiongyue Kang, Patrick Gregg, Yongmin Jung, Ee Leong Lim, Shaif-ul Alam, Siddharth Ramachandran, and David J. Richardson, "Amplification of 12 OAM Modes in an air-core erbium doped fiber," Opt. Express 23, 28341-28348 (2015)
  9. Gomez, A., Shi, K., Quintana, C., Sato, M., Faulkner, G., Thomsen, B.C., O Brien, D. (2015). Beyond 100-Gb/s Indoor Wide Field-of-View Optical Wireless Communications. Photonics Technology Letters, IEEE, 27 367-370. doi:10.1109/LPT.2014.2374995
  10. Sato, M., Maher, R., Lavery, D., Shi, K., Thomsen, B.C., Bayvel, P. (2015). Frequency Diversity MIMO Detection for DP- QAM Transmission. Journal of Lightwave Technology, 33 1388-1394. doi:10.1109/JLT.2015.2388860
  11. Shi, K., Feng, F., Gordon, G.S.D., Wilkinson, T.D., Thomsen, B.C. (2015). SLM-Based Mode Division Multiplexing System With 6 \times 6 Sparse Equalization. Photonics Technology Letters, IEEE, 27 1687-1690. doi:10.1109/LPT.2015.2436067
  12. Shi, K., Thomsen, B.C. (2015). Sparse Adaptive Frequency Domain Equalizers for Mode-Group Division Multiplexing. Journal of Lightwave Technology, 33 311-317. doi:10.1109/JLT.2014.2374837
  13. Y. Jung, E. L. Lim, Q. Kang, T. C. May-Smith, N. H. L. Wong, R. Standish, F. Poletti, J. K. Sahu, S. U. Alam, and D. J. Richardson, "Cladding pumped few-mode EDFA for mode division multiplexed transmission," Opt. Express 22, 29008-29013 (2014)
  14. Carpenter, J., Thomsen, B.C., Wilkinson, T.D. (2012). Degenerate Mode-Group Division Multiplexing. Journal of Lightwave Technology, 1. doi:10.1109/JLT.2012.2206562
  15. Carpenter, J., Thomsen, B.C., Wilkinson, T.D. (2012). Mode Division Multiplexing of Modes With the Same Azimuthal Index. IEEE Photonics Technology Letters, 24 (21), 1969-1972. doi:10.1109/LPT.2012.2220538