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Green Initiative for Future Optical Networks

Final Report Summary - GRIFFON (Green Initiative for Future Optical Networks)

Summary overview of result:

The GRIFFON overall scientific objective is to develop an a green optical network, viz. fibre optic coherent communication system with increased capacity, distance (over 300 km) and reduced power consumption based on an ultra-long 1st or/and 2nd cascade fibre Raman amplification with supressed polarisation impairments, viz. polarisation dependent gain (PDG) and polarisation mode dispersion (PMD), and digital signal processing to suppress linear (chromatic dispersion, phase noise, polarisation mode dispersion) and intra-channel nonlinear (Kerr nonlinearity) transmission impairment. The Griffon consortium have addressed this overall objective by developing and testing unrepeatered link with distributed 1st/2nd cascade Raman amplification. This design allowed signals transmission over 240 km by using high-capacity modulation formats DP-16QAM and DP-64QAM. It has been found that results can be scaled for transmission over 320 km in the case of application of more advanced erbium-doped preamplifier at the receiver end.
In addition to the addressing main objective, the project the following innovative results have been obtained.
New phenomena – Stochastic Anti-Resonance (SAR), i.e. random birefringence enhanced gain fluctuations, - has been discovered.
New mechanism of the pump-to-signal intensity noise transfer caused by SAR has been revealed. It has been found that SAR leads to the pulse broadening and emergence of the rare events taking the form of low output power pulses with the probability heavily deviated from the Gaussian distribution. New algorithms for stochastic calculations based on the separation of the deterministic and stochastic evolution has been discovered. This results in a library of stochastic trajectories that allowed us to decrease substantially the computational time and required memory. This improvement in computational efficiency can find potential applications in telecommunications for machine learning based modulation format recognition, linear and nonlinear transmission impairments mitigation, stochastic digital backpropagation as well as in fibre lasers engineering for machine learning based optimal laser parameters self-adjustment. Comparison of the results of stochastic simulations with the results for two analytical models has helped to determine the margins of the approaches to averaging over the random birefringence. This allowed us to justify applicability of the generic Mankov equation for a long (>25) transmission links. However, for short length (<10 km) approach to averaging developed in Aston is better applicable and so was used to develop an advanced theory of a fibre Raman Polariser, i.e. devise using polarisation pulling effect of a Raman amplification to repolarise completely unpolarised signal. Results on the random birefringence-based RIN transfer leading to emergence of the rare events (rogue waves) stimulated experimental study of rogue waves in fibre Raman lasers. The practical implementation of the obtained results can be in the design of stabilised multi-wavelength fibre Raman lasers (FRLs) for telecommunication and sensing applications. This idea has resulted in a new approach to the stable mode-locked operation of erbium doped fibre lasers.
In addition, for erbium doped fibre lasers, the conditions for mapping complex self-pulsing and rogue waves in terms of ellipticity of the pump wave and in-cavity birefringence have been found experimentally and theoretically. These results will have a potential application in fibre optic telecommunication and metrology in the context of development of ultra-stable mode-locked lasers (local oscillators). The new generation of pulsed laser sources with tunable states of polarisation and repetition rate (12 MHz – 900 MHz) along with narrow radiofrequency linewidth of 1 Hz has been developed.
It has been proved that a classical fibre model describing signal and noise power distributions in Raman-amplified links can be successfully applied for simulations of ultra-long links 300-500 km using the 1st or/and 2nd cascaded pumping schemes without numerical instabilities and with reasonable simulation times, which is important for design and optimization of such links. Using this type of model in VPI software simulation-based design studies of unrepeated links over 300km utilizing ultra-long fibre Raman laser (URFL)-based distributed amplification have been carried out to characterize and optimize the power and noise characteristics. The on/off Raman gain and its bandwidth, the signal excursion over the fibre length, OSNR spectra, and the accumulated nonlinearities have been evaluated. To achieve best performance the laser design has been optimized with respect to the forward/backward pump powers and wavelengths, input/output signal powers, reflectivity profile of the FBGs and other parameters.

The analytical vector fibre model developed within this project has been verified versus an established stochastic vector full-field model based on Manakov equations that were numerically integrated using the split-step Fourier method. The simulation results with VPI software have shown a good agreement with the predictions of the analytical model concerning the statistical properties of Raman gain distribution and Polarization Dependent Raman Gain (PDG) for different PMD values and in the asymptotic limits for large and low PMD. An advanced RIN reduction technique based on Raman interactions in short fibres (1.5-2.5 km) has been numerically investigated and characterized. A significant RIN reduction (which may prove to be sufficient for the ultra-long unrepeated Raman-amplified transmission systems) could be achieved in DCF fibres. The obtained results can be used to mitigate the RIN impairment being one of the major limiting effects.
Using simulations, transmission impairments have been characterized in unrepeated SSMF links of 200km length with a bidirectional Raman pumping (whose 5% forward to 95% backward power splitting ratio was chosen to suppress the RIN transfer while keeping an acceptable OSNR level) using the DP-16QAM modulation format at a symbol rate of 28Gbaud. Performance of different DSP algorithms for signal equalization have been investigated. It has been found that the training sequences-assisted FDE-algorithm can cope well with Raman amplification-induced PDG. A comparison of several nonlinearity compensation schemes and modulation format optimization scenarios targeting >200 Gb/s unrepeated Raman-amplified system over 420 km of ultra-low loss (ULL) fibre have been carried out using simulations. In order to find the ultimate transmission limits, the RIN transfer and PDG were deliberately neglected in further numerical studies, assuming these effects can be effectively diminished by using advanced RIN reduction techniques and depolarized pump sources. It has been shown that for 200Gb/s transmission, single carrier 28 Gbaud PM-16QAM combined with DBP and PCTWs is outperformed by single carrier 56Gbaud PM-QPSK, not requiring any nonlinearity mitigation techniques.

A successful transmission of net 1 Tb/s PM-QPSK Nyquist-spaced superchannel with intra-superchannel net spectral efficiency of ~ 3.6 b/s/Hz over 420 km of Raman amplified ULL fiber without using any nonlinearity compensation has been reported. The superchannel performance has been further improved by mitigating nonlinearities using symbol-rate-optimization. The best performance has been obtained for 20 x 14 Gbaud superchannel. With DBP requiring high computational complexity and PCTWs halving the overall link spectral efficiency, a superchannel approach using modulation and baud-rate optimization has been identified as the most promising technique to unlock the capacity of very long (>400km) unrepeated Raman amplified links.

Experimentally characterized the pump to signal relative intensity noise (RIN) and phase noise (PN) transfer and their impact on the fibre Raman based transmission performance has been done experimentally and reported. Experimentally demonstrated unrepeatered transmission with distributed 1st/2nd cascade Raman amplification to allow signals transmission over 240 km by using high-capacity modulation formats 16QAM and 64QAM

Theoretical analysis and simulations have been carried out in Acreo on compensating the fibre chromatic dispersion (CD) and correcting the equalization enhanced phase noise (EEPN) by comparing different equalization methods, including novel carrier phase recovery algorithms. We have now incorporated a full transmission model including effects of fibre dispersion and fibre non-linearity. The model considers digital back propagation techniques to mitigate the non-linear effects.

The impact of frequency noise in the local oscillator (LO) laser in the coherent receiver on system performance has been thoroughly investigated in collaboration with the ICONE project. The theory, which has been validated by simulations and experiments, is general and hence valid for both white frequency noise (Lorentzian linewidth) and slow 1/f frequency noise (Gaussian linewidth), explains what is known as Equalization Enhanced Phase Noise (EEPN). Slow noise frequency causes jittering on the receiver side, whereas faster noise causes inter- and intrasymbol interference. The analytical model gives guidelines and simple rule-of-thumb formulas for system and laser designers and has been accepted for publication in Nature Scientific Report. System experiments using multilevel modulation (PAM-4, PAM-8, DMT, duobinary) and direct detection using a 100GHz integrated external modulator laser developed at KTH have been carried out, to demonstrate the use in Datacom applications. This work has generated large interest in the scientific community and lead to an invited paper at ECOC 2016. An analytical, small signal, multi-mode model for the noise and modulation response of multi-section lasers has been implemented, including graphical user interface, in the LaserMatrix laser design tool. The model is longitudinally resolved and takes into account the influence of the important effects in a semiconductor laser like spatial hole-burning, gain compression, detuned loading, 1/f current noise and the influence of weak side modes on laser dynamics. The multi-mode model has been used in collaboration with Finisar to develop a new type of directly modulated lasers that utilize weak side modes for bandwidth enhancement, leading to the fastest directly modulated lasers in the world at 1.3µm wavelength with up to 55GHz bandwidth for 112 Gbit/s datacom applications (56 Gbaud, 4-PAM modulation) published as a post-deadline paper at OFC 2016.A Matlab model has been developed that makes it possible to calculate the corresponding optical spectrum and test how different linewidth measures depend on the shape of the frequency noise spectrum. The model can also be used to calculate the optical spectrum from a measured frequency noise spectrum and has been used for noise emulation in the measurements. A new improved semi-analytical study of the trade-off between and power consumption, spectral efficiency, transparent optical reach, taking into account power consumption and performance of transmitters, receivers, optical amplifiers and optical regenerators for different modulation formats have been published. An investigation how thermal drift of DWDM optical receiver filters affect the system performance in radio fronthaul systems have been carried out in collaboration with Infinera.

Conclusion: Obtained results addressed the disruptive change in information and communication solutions towards drastic energy saving (green) technologies by delivering a bunch of technologies related to fibre Raman-based unrepeatered transmission. GRIFFON demonstrated increased communication capacity through coherent technique with advanced modulation formats and polarisation multiplexing of signals, increased distance and reduced power consumption by applying an ultra-long first or/and second cascade fibre Raman amplification with supressed polarisation impairments, viz. polarisation dependent gain (PDG) and polarisation mode dispersion (PMD). The project merges four complimentary fields of expertise brought by well-recognized participants: VPIPhotonics – advanced modelling and design of fibre optic communications links; Aston – high-speed communication systems and Raman technologies; KTH – the development of high-speed transmitters, receivers and modulators for high-capacity fibre optic communications; and ACREO – digital signal processing techniques and algorithms for compensation of linear/nonlinear transmission impairments in coherent communication systems, to create synergy in development of innovative devices and products for new generation of optic fibre communication and secure communication systems.
Economic impact: Project outcomes resulted in innovations in cost-efficient and flexible access technologies that are fostering global economic performance, and specifically the economic competitiveness of EU and will create jobs. Project outputs can be potentially commercialised to EU companies such as VPIPhotonics (Germany), Xtera Communications Ltd. (UK) and EU multinationals (Ericsson AB, Finisar, etc.). The project contributes to the EU fibre optic based information-carrying digital communications infrastructure through developing low-cost and high-capacity coherent optical communication system and to photonics industry through increased demand in advanced fibre optic components and systems. The GRIFFON research outcomes will lead to the technical excellence, and so contribute to the transition of Europe to a knowledge-based society, driving the transformation of industry towards higher added value and sustainable development as outlined in ICT strategic objectives.
Impact on standards: The project will have impact on standardisation through bringing to market new laser sources with ultra-narrow linewidth of 1 Hz for advanced transmission schemes and high-speed optical communications. The project results demonstrated the opportunity to increase the span for unrepetered transmission beyond the 400 km and, thus, – to decrease of number of amplifiers and reduction of a consumed power per transmitted bit over fixed distance.
Societal impact: The demonstrated reduction of the fast optical communications cost together with simultaneous increase of bandwidth and speed will enable far more citizens across the EU to take advantages of fibre communication links in the context of improved quality of service in e-banking and e-business, HD and 3D TV, high-quality telephony, and medical diagnostics. Thus, project outputs will improve quality of life, increase the number of employees in the sector of ICT service, and will remove the separation between largely connected districts and those who left behind.
Environmental impact: Demonstrated system has reduced power consumption in view of absence of electrical cable embedded into the optical cable to drive on-line amplifiers.
Impact of the Project on Researchers’ Industrial Career Prospects: The secondment programme of GRIFFON has been designed to have maximum positive impact on improving the technical skills, expertise and career prospects of the researchers involved in the secondments to industrial partner. Both seconded and recruited researchers demonstrated a research output that includes both scientific papers, internal company reports and memos, and software.
Impact on Industrial Partner and Sustainable Knowledge Transfer Linkages: The project will have strong impact on the industrial partners (Acreo and VPI). The project will allow the industrial partners to develop new products and services that results, thus leading to new markets and to create novel high-technology products leading to commercial impact.
The GRIFFON publication list demonstrates how successful this project has been.Thanks to recruitment and secondments of skilled researchers, effective communication and good collaboration within the Consortium the scientific results are beyond expectations.

GRIFFON website: http://www.astonishgriffon.net/

Contact: Dr. Sergey Sergeyev, GRIFFON Coordinator s.sergeyev@aston.ac.uk