Optical Processing for Future Dynamic Optical Networks
This page presents the results from the research in dynamic optical networks that was carried out by Dr Benn Thomsen as part of his EPSRC Advanced research fellowship Grant EP/D074088/1, with key contributions from Dr Jose Manuel Delgado Mendinueta, Dr Ben Puttnam and Dr Robert Maher.
September 2006 - September 2011.
The demand for access from anywhere, increasing usage of limited processing power mobile devices to access computing resources, and the need to reduce energy consumption in IT infrastructure has given rise to ‘cloud computing’, where the processing and data storage is moved away from the users’ desktop. Such systems require a high bandwidth and efficient data transport network. Dynamic optical networks that are able to offer flexible bandwidth on demand and efficiently use the limited network resources are potentially attractive for supporting the future high bandwidth intra- and inter- data centre networks, and the access and core networks that allow customers to connect to these centres. In a dynamic optical network data is sent as short bursts that remain in the optical layer from source to destination with no electronic switching or routing, reducing latency and energy consumption.
This fellowship focused on the development of the physical layer subsystems that are required to implement such a network and assessed the interoperability of these subsystems in a network context. In particular it developed wavelength agile transmitters, burst tolerant optical amplification and adaptive burst mode receiver subsystems, for use in both low cost access networks and in future high capacity core networks.
The rapid increase in the speed of CMOS integrated circuits with respect to optical linerates has made the use of DSP in optical processing extremely attractive. In this work DSP has been exploited in the development of a 10Gb/s digital burst mode receiver for use in future optical access networks. The use of adaptive DSP greatly simplifies the burst detection, clock and data and also allows for the compensation of physical layer impairments. We have demonstrated a digital burst mode receiver that provides a 14.5dB dynamic range with respect to burst power variations. This subsystem has been tested a dynamic network test-bed that was developed to emulate the types of distortions encountered in both access and core network type applications, in particular those that arise from the variable path history that results in burst-to-burst power fluctuations, and interference from neighbouring burst channels resulting in power fluctuations across the burst. The digital burst mode receiver greatly relaxes the requirements on the performance of the optical amplifiers in the system and extends the number of links through which the signal can pass before regeneration as well as allowing for multi-rate operation to support both legacy 1.25Gb/s and future 10Gb/s customers on the same PON.
Coherent optical transmission, which is just beginning to be deployed for 100Gb/s links, provides another degree of flexibility in the design of dynamic optical networks. The inherent wavelength selectivity of coherent reception makes it possible to implement not only a tuneable transmitter but also a fast tuneable receiver by simply tuning the frequency of the local oscillator laser. In a network, the choice of transmission and reception wavelengths can be used to efficiently address different routes through the optical network. However, coherent reception also places new requirements on the laser performance in particular the linewidth of the transmitter and receiver lasers under dynamic operation. We have developed techniques to characterise the switching time and linewidth of fast wavelength tuneable lasers under dynamic operation. We have also developed a burst mode coherent receiver that uses physically realisable parallelised DSP. With this system, we have shown that the dynamic linewidth of the tuneable lasers is commensurate with coherent reception when differential decoding is employed. We have demonstrated fast receiver based channel switching speeds of less than 200ns in a 112Gb/s DP-QPSK 24-channel WDM system test-bed with transmission over 240km.
During the early stages of the fellowship the work was focused on SOA based regeneration of 10Gb/s systems and in particular the requirements for successfully cascading regenerators in a network or long haul applications. The aim of this work was to identify the optimum regenerator spacing and limits on the number of optical regenerators in a link. We showed that the signal quality can be maintained for spacing of up to 600km over distances of up to 20,000km and up to 100 cascaded regenerators in the link . The key design requirement derived from this work is that to enable cascading of optical regenerators it is necessary not only for the regeneration to suppress noise but also to maintain the extinction ratio. An investigation into the of the amplitude and phase response of nonlinear SOAs characteristics, using the frequency resolved optical gating technique, was carried out to evaluate their potential for regeneration at 160Gb/s . The results here whilst interesting are less favourable for regeneration or switching due to the reduced phase shift that is observed at 160Gb/s.
The next phase of work was focused on the development of a digital burst mode receiver (DBMRx) for 10Gb/s intensity modulated systems. The aim of this work was to develop a receiver that was not only able to carry out the required clock and data recovery on a burst mode basis but also compensate for the impairments that are introduced by dynamic network operation. The developed DBMRx was based on a standard AC coupled PIN+TIA photoreceiver whose output is then digitized. The impairment compensation, and burst mode clock and data recovery is then efficiently carried out in DSP. The DBMRx was performance was characterised in both dynamic core network and optical access network scenarios. In both scenarios the main receiver requirement is to provide sufficient dynamic range to accommodate the burst-to-burst power fluctuations that arise from different transmitters and path histories. The initial DBMRx design provided a dynamic range of 7dB  which was found to be adequate for most dynamic network scenarios, however, the access network standards specify 15dB. It was shown that the dynamic range could be significantly extended by using a current controlled SOA based equaliser before the DBMRx which extended the dynamic range to 16.5dB . Further work has also shown that the dynamic range of the DBMRx can also be extended without the need for an SOA by employing clipping in the digitizer providing a dynamic range of 14.5dB .
We have developed a multi-rate DBMRx for access network applications that is able to support upstream burst mode transmission of both legacy 1.25Gb/s and future 10Gb/s customers on the same network [5, 6]. The DBMRx has also been utilised as the upstream receiver in a consolidated PON where multiple single wavelength legacy PONs are mapped using an all optical SOA based equaliser and wavelength converter onto a DWAM grid for backhaul. In this work the adaptive equalisation in the 10Gb/s DBMRx relaxes the requirements on the performance of the optical wavelength, providing a sensitivity of -27dBm and a dynamic range of 22dB over a backhaul distance of 60km .
In the core network impairments also come from the interference of other wavelength channels, in particular cross talk arising from cross gain modulation in the optical amplifiers. We investigated both the performance of optical gain clamping to reduce the gain modulation and the use of adaptive equalisation in the DBMRx to relax the requirements on the optical amplifiers in a dynamic network. The use of receiver based equalisation was shown to provide a six-fold reduction in the amount of additional optical feedback required at each amplifier . This improvement allows for both an increase in the span length, by 39% for a 25-node network, and the maximum number of hops.
From the perspective of the upper network layers the performance of a dynamic network depends on the traffic loading and packet error rate. In order to investigate the impact that physical layer components, transmitter, optical amplifiers, and DBMRx have on this a test bed that allows for the variation of traffic loading and burst power was developed along with the processing required to estimate the packet error rate (PER) that results from physical impairments . Characterising the PER in dynamic networks is essential as errors arise not only from the detection of individual bits but also from the packet detection process. We show that a burst/packet header of 16 which results in a 0.1% overhead on a 1.63µs packet is sufficient to ensure a PER less than 1e-3 over the full dynamic range of the DBMRx . We also find that the performance of the DBMRx is not impacted by the variation in traffic load with the fully loaded scenario providing a lower bound .
Recent work has focused on developing both burst mode transmitters and receivers for 100Gb/s coherent transmission. In this work the key issues are understanding performance of the tunable lasers that are used at both the transmitter and receiver to provide the dynamic configurability. In particular we have developed techniques to characterise the switching time and linewidth under dynamic operation . We have also developed a burst mode coherent receiver that uses physically realisable DSP algorithms based on parallel processing and shown the receiver is able to recover the transmitted bursts with a locking time of less than 200ns from the start of the wavelength switch .
 G. Gavioli, B. C. Thomsen, V. Mikhailov, and P. Bayvel, “Cascadability Properties of Optical 3R Regenerators Based on SOAs,” Journal of Lightwave Technology, vol. 25, no. 9, pp. 2766-2775, Sep. 2007.
 G. Gavioli, B. Thomsen, and P. Bayvel, “160Gb/s characterisation of gain and phase dynamics of a semiconductor optical amplifier,” Optical Communication (ECOC), 2007 33rd European Conference and Ehxibition of. pp. 1-2, 2007.
 B. C. Thomsen, B. J. Puttnam, and P. Bayvel, “10 Gb/s AC-Coupled Digital Burst-Mode Optical Receiver,” in OFC/NFOEC 2007 - 2007 Conference on Optical Fiber Communication and the National Fiber Optic Engineers Conference, 2007, pp. 1-3.
 B. C. Thomsen, B. J. Puttnam, and P. Bayvel, “Optically equalized 10 Gb/s NRZ digital burstmode receiver for dynamic optical networks,” Optics Express, vol. 15, no. 15, p. 9520, Jul. 2007.
 J. M. D. Mendinueta, J. E. Mitchell, P. Bayvel, and B. C. Thomsen, “Digital dual-rate burst-mode receiver for 10G and 1G coexistence in optical access networks,” Optics Express, vol. 19, no. 15, p. 14060, Jul. 2011.
 J. M. D. Mendinueta, J. E. Mitchell, P. Bayvel, and B. C. Thomsen, “Digital multi-rate receiver for 10GE-PON and GE-PON coexistence,” in Optical Fiber Communication Conference and Exposition (OFC/NFOEC), 2011 and the National Fiber Optic Engineers Conference, 2011, pp. 1-3.
 J. M. D. Mendinueta, B. Cao, B. C. Thomsen, and J. E. Mitchell, “Performance of an optical equalizer in a 10 G wavelength converting optical access network,” Optics Express, vol. 19, no. 26, p. B229, Nov. 2011.
 B. J. Puttnam, B. C. Thomsen, and P. Bayvel, “Performance of an Adaptive Threshold Receiver in a Dynamic Optical Burst-Switched Network,” IEEE Photonics Technology Letters, vol. 20, no. 3, pp. 223-225, Feb. 2008.
 J. M. Delgado Mendinueta, P. Bayvel, and B. C. Thomsen, “Cluster processing for the study of optical burst-mode digital signal processing receivers and subsystems for dynamic optical burst switching networks,” in 2008 5th International Conference on Broadband Communications, Networks and Systems, 2008, pp. 77-81.
 J. M. D. Mendinueta, P. Bayvel, and B. C. Thomsen, “Impact of network load and traffic sparsity on the performance of a digital burst-mode receiver,” in 36th European Conference and Exhibition on Optical Communication, 2010, pp. 1-3.
 R. Maher and B. Thomsen, “Dynamic linewidth measurement technique using digital intradyne coherent receivers,” Optics Express, vol. 19, no. 26, p. B313, Nov. 2011.
 B. C. Thomsen, R. Maher, D. S. Millar, and S. J. Savory, “Burst Mode Receiver for 112 Gb/s DP-QPSK with parallel DSP,” Optics Express, vol. 19, no. 26, p. B770, Dec. 2011.