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Revolutionising optical fibre transmission and networking using the Orbital Angular Momentum of light

Periodic Reporting for period 2 - ROAM (Revolutionising optical fibre transmission and networking using the Orbital Angular Momentum of light)

Reporting period: 2016-08-01 to 2018-04-30

The growth of the Internet traffic, driven by the spreading of services such as video-on-demand, social networks, cloud computing, is pushing to the limit the performance of data centers, which play a fundamental role in maintaining high performance for those services. The main limitations are currently represented by the scalability, bandwidth and power consumption of the data center links and switches. The research community is addressing these issues with a number of different technical solutions and approaches.
The central idea in the Horizon2020 Project ROAM (Revolutioning optical fibre transmission and networking using the Optical Angular Momentum of Light) is to investigate the use of optical modes carrying orbital angular momentum (OAM) to improve the performance of communications and networking systems. Specifically, point-to-point optical links and optical switches were identified as two scenarios that could substantially benefit from the exploitation of OAM multiplexing techniques, as described by the two core objectives of the project:

Objective 1: Exploiting the use of OAM modes in optical fibres as a means for increasing optical fibre transmission capacity for short-reach high data density applications.

Objective 2: Exploiting the use of the OAM domain as a switching resource in conjunction with the wavelength domain to improve the scalability and the power consumption of the switches in data-centre applications.

To improve the power consumption, scalability and form factor of the systems, the ROAM project developed integrated silicon photonic circuits for OAM generation and multiplexing.
Moreover, in order to increase the throughput and performance of the point-to-point link special fibres supporting multiplexed OAM modes have been developed.
Work towards Objective 1:
Several new designs of optical fibres supporting OAM modes were developed and evaluated by University of Laval (UL), including air core fibre (ACF), ring core fibre (RCF), inverse parabolic graded index fibre (IPGIF).. Besides the assessment of their transmission characteristics, equalization methods, nonlinear mode coupling and MIMO algorithms were evaluated. . An alternative design based on a graded index ring core fibre (GIRCF) was developed by UNIVBRIS with the support of the company FiberHome. A 10 OAM x 16 WDM MIMO-aided transmission with both QPSK and 16 QAM modulation formats at 28Gbaud/s over 1Km were demonstrated. These experiments were complemented by demonstrating the transmission of real data-center traffic Ethernet frames through a 1km-long GIRCF fibre carried by 16 WDM channels multiplexed over 10 OAM modes. A number of photonic integrated devices were also implemented to facilitate the optical coupling between the OAM multiplexer and the OAM fibres..

Work towards Objective 2:
The core device architecture for the switch consisted of the cascade of photonic integrated OAM multiplexers and refractive element OAM demultiplexers developed by UoG. The OAM-wavelength switch architectures were tunable over 10 OAM modes and were demonstrated in systems with 16 WDM channels modulated up to 30 Gbaud. The experiments were implemented at CNIT with the contribution of University of Glasgow (UoG), Poznan Supercomputing and Networking Centre (PSNC), and UNIVBRIS. The packaging activity supported by IBM and CNIT-PNTL enabled the demonstration of a practical and compact device with OAM mode switching time down to the hundreds of nanoseconds timescale and power consumption in the mw/Gb/s range. A field trial implementing the two-layer OAM-WDM switch working under different real-data traffic conditions, including data-center and video traffic was also implemented.

Three possible products emerged from the ROAM activities. IBM, supported by HWDU, identified their market potential, application roadmaps and effective strategies to exploitation. The target cost for the ROAM products was identified carrying out a market analysis.
The project results have been disseminated through scientific journals, presentations at international conferences, project website, project Facebook page, and an OAM dedicated Workshop at the ACP 2017 conference.
Objective 1: current optical systems are utilising the quasi-totality of the optical bandwidth available through WDM or orthogonal frequency division multiplexing (OFDM) schemes. Further increase in bandwidth can only come from mode division multiplexing schemes. In this scenario, OAM modes represent an interesting subset of the available spatial modes that can be easily generated and manipulated by photonic integrated devices thanks to their azymuthal simmetry. Also, the low intermodal crosstalk amongst the OAM mode groups simplifies the digital signal processing at the receiver. By combining novel ring-core optical fibres specifically designed to guide OAM modes and compact multiplexers and demultiplexers, the ROAM project demonstrated a 10x bandwidth increase with respect to the state of the art of standard fibres with reduced MIMO processing. This scheme can be easily scaled to fibres containing multiple ring-cores to further enhance the total capacity of the system.

Objective 2: ROAM exploited the use of the OAM domain for switching. When combined with wavelength an OAM/WDM routing system offers a scalable option to support a large number of input and output optical ports. The switching scheme proposed by ROAM can support hundreds of ports per switch, with a total throughtput in the Tb/s range, and it is fully compatible with standard systems. The switch exploits an innovative integrated OAM multiplexer developped on standard integrated silicon technology, which offers compactness, low power and an established route to exploitation. The few mW/(Gb/s) power consumption per switch makes the ROAM architecture extremely competitive and scalable compared to state-of-the-art high port count integrated optical switches. Moreover the ROAM switching approach allows for an improvement with respect to the power consumption of commercial electronic switch fabrics. The sub-microsecond switching time makes the ROAM architecture suitable for long-lived data traffic routing in data-centre applications.

The innovative OAM technology developed in ROAM can have impact beyond the telecom market. In fact, alternative applications in quantum optics, bio-medical tissue diagnosis, related to the human health monitoring, and light detection and ranging, related to the sensing applications are rapidly emerging. In these scenarios, the technologies and systems demonstrated by the ROAM promise a step-change in terms of performance and future exploitation.