Optical Communications

About

Our experienced research team draws on a variety of disciplines and academic backgrounds to perform research that addresses urgent questions around networking and communications.

As pressure on hardware and bandwidth capacities continues to increase, with near-instantaneous global communications a necessity for many businesses, we are researching new methods of increasing network capacity and robustness while decreasing installation costs.

We perform in-depth research that pushes forward the frontiers of knowledge around optical communications.

Our research includes:

Novel approaches to overcoming hardware and infrastructure limitations
New ways of increasing total information throughput in optical fibres
Reducing the cost of access networks to connect with end users
Software solutions to increasing the maximum potential bit rate of wireless links
Questions surrounding Internet Protocol stability and network robustness
Net-Neutrality and questions surrounding bandwidth management during a crisis.

Our Projects

Record reduction in impact of nonlinearity

We experimentally demonstrated, for the first time, a significant 34dB reduction in the net nonlinear power produced from a transmission link by adding a mid-link optical phase conjugator and an optimised dual-order distributed Raman amplification technique. The dual-order backward-pumping scheme demonstrated a record signal power symmetry of 97% using 50.4km single mode fibre spans. Power precision was key, with the span-to-span launch power required to be within ±1dB in order to maintain 99% reduction in nonlinearly generated components. For 256Gb/s dual-polarisation-16QAM transmission over 100.8km (2x50.4km) with mid-link OPC, the proposed amplification scheme enabled the OPC to improve the nonlinear threshold by ~7dB and the optimum signal launch power by ~5dB. We later further improved the symmetry by adding short lengths of erbium doped fibre in order to overcome component insertion losses.

M.A.Z. Al-Khateeb, M. Tan, T. Zhang, A.D. Ellis, “Combating Fibre Nonlinearity Using Dual-Order Raman Amplification and OPC”, Photonics Technology Letters, doi: 10.1109/LPT.2019.2911131 (2019).

Mingming Tan, Mohammad Al-Khateeb, Tingting Zhang, and Andrew Ellis, “Fiber Nonlinearity Compensation Using Erbium-Doped-Fiber-Assisted Dual-Order Raman Amplification”, at CLEO 2019, paper SW3.O.1, (2019).

Record amplification bandwidth

In recent years, ultra-wideband transmission utilising the unused spectral bands of deployed fibre has gained considerable attention from the research community and system operators because of its potential to be a short to medium term solution to the inevitable “capacity crunch”, and featured as special workshop at ECOC 2019, co-organised by Aston University.

One of the key challenges for realising future ultra-wideband transmission is the proper design and optimisation of continuous optical amplifiers, giving clear advantages over parallel systems. Such systems have been demonstrated using multi-band rare-earth doped fibre amplifiers and hybrid Raman/semiconductor optical amplifiers.

However, the overall transmission performance was limited by energy transfer from short to long wavelength bands. Research at Aston University has shown that distributed Raman amplification may be used to reduce such energy transfer.

In a recent study, we demonstrated a dual-stage design 150nm bandwidth transmission systems with 15dB average gain and only 3dB ripple. The key innovation was improving the noise performance of the shortest wavelength signals without requiring high-power pumps to propagate through the remaining transmission fibre. This enabled errorless transmission through 70km SSMF with 152×100GHz-spaced channels using 30GBaud PM-QPSK signals, capable of simultaneously transmitting 228,000 broadband internet connections, representing every household in Birmingham, with each operating continuously at 80Mbit/s.

M. A. Iqbal, L. Krzczanowicz, I. Phillips, P. Harper and W. Forysiak, "150nm SCL-Band Transmission through 70km SMF using Ultra-Wideband Dual-Stage Discrete Raman Amplifier," 2020 Optical Fiber Communications Conference and Exhibition (OFC), San Diego, CA, USA, 2020, pp. 1-3.

Burst mode amplifiers to increase the reach of optical networks

When economics permits their deployment, state-of the-art 10G-PON passive optical networks are limited in reach to 20 km due to the power budget allowance in low cost direct detection systems. Cost trade-offs benefit dramatically from the use of so-called reach extension using optical amplifiers to reduce the number of active nodes. However, since all customers are different, amplifiers should accommodate compatibility with rapidly switching between users, that is, they should support burst transmission. The most common optical amplifier, the erbium doped fibre amplifier suffers problems when presented bursty nature of the traffic due to metastable electronic lifetime of the erbium ions (~1ms) used to store the energy used for amplification.

At Aston University, we have developed two alternative optical amplification techniques, with radically different energy transfer mechanisms. Raman fibre amplifiers and Fibre Optical Parametric Amplifiers (FOPA), both of which promise almost instantaneous response time. Furthermore, as these devices rely on vibrational, rather than electronic states for energy transfer, their operating wavelength is less constrained than amplifiers relying on electronic energy levels. Recently we conducted a series of experiments comparing the performance of the three amplifier types for varying traffic bursts. The FOPA was the first truly polarisation independent high gain amplifier (see Fig below) giving gain up to 20dB which is a drop-in replacement for conventional amplifiers. Our results show a significant improvement in the performance of the system when using the Parametric Amplifier due to the near instantaneous response time of the gain. These results indicate a new area of application for FOPAs in reach extended access network as an in-line amplifier and in any environment where traffic patterns are rapidly varying.

C. B. Gaur, F. Ferreira, V. Gordeinko, A. Iqbal, W. Forysiak and N. Doran, "Comparison of Erbium, Raman and Parametric Optical Fiber Amplifiers for Burst Traffic in Extended PON," 2020 Optical Fiber Communications Conference and Exhibition (OFC), San Diego, CA, USA, 2020, pp. 1-3.

Enhanced transponders

Higher-order modulation formats and higher symbol rates are being deployed to support the rapid traffic growth in optical communication systems. Moving towards dual-polarization 64GBaud 64-QAM transmission and beyond, coherent optical systems become increasingly sensitive to practical impairments in electrical and optical components, particularly in a transmitter and as a result of nonlinear effects in optical transmission. Significant performance penalty can be caused by transmitter skew, amplifier distortion, and the nonlinear Kerr effect. To maintain associated impairments within reasonable limits, transmitters and receivers are typically calibrated during manufacture (or power-up). As the modulation order and symbol rate of the system increases, requirements for the precision of calibration and compensation circuits increase accordingly.

Recent work at Aston University has applied an advanced post-equalization approach in the Rx allowing highly tolerant and robust transceiver operation under severe impact of Tx impairments. Similar work has deployed Machine Learning algorithms to compensate for nonlinear transmission impairments.

P. Skvortcov, C. Sanchez-Costa, I. Phillips and W. Forysiak, "Receiver DSP Highly Tolerant to Transmitter IQ Impairments," 2019 Optical Fiber Communications Conference and Exhibition (OFC), San Diego, CA, USA, 2019, pp. 1-3.

S. Sygletos, A. Redyuk, and O. Sidelnikov, "Nonlinearity Compensation Techniques Using Machine Learning," in OSA Advanced Photonics Congress (AP) 2019 (IPR, Networks, NOMA, SPPCom, PVLED), OSA Technical Digest (Optical Society of America, 2019), paper SpT2E.2.

Riemann-Hilbert problem for optical communications

In this work, for the first time, a full-spectrum periodic nonlinear Fourier transform (NFT) based communication system with the inverse transformation at the transmitter performed by using the solution of Riemann-Hilbert problem (RHP), is proposed and studied.

The entire control over the nonlinear spectrum rendered by our technique, where we operate with two qualitatively different components of this spectrum represented, correspondingly, in terms of the main spectrum and the phases, allows us to design a time-domain signal tailored to the characteristics of the transmission channel.

In the heart of our system is the RHP-based signal processing utilised to generate the time-domain signal from the modulated nonlinear spectrum. This type of NFT processing leads to a

computational complexity that scales linearly with the number of time-domain samples, and we can process signal samples in parallel. For the first time, we explain how to modulate the phases of individual periodic nonlinear modes.

M. Kamalian Kopae, A. Vasylchenkova, D. Shepelsky, J. E. Prilepsky and S. K. Turitsyn, "Full-spectrum periodic nonlinear Fourier transform optical communication through solving the Riemann-Hilbert problem," Journal of Lightwave Technology, doi: 10.1109/JLT.2020.2979322 (2020).