OAM Fibers: Main Achievements

One of the main objectives of ROAM project is to develop new OAM fibers and to demostrate an increased transmission capacity for short-reach high data density applications as data centers.

Special fibers are developed within the ROAM Consortium to support a high number of OAM modes. Several kind of fibers are available within ROAM Consortium. Universitè Laval designed and developed three kind of OAM fibers: Ring Core Fibers (RCFs), Air Core Fibers (ACFs), and the Inverse-Parabolic Graded Index Fibre (IPGIF). A forth kind of fiber was made available from a private company (Fiberhome - China): Graded-Index Ring-Core Fibre (GIRCF). This fibre is designed to support multiple groups of OAM modes (e.g., |l| > 4) to provide over 10 OAM channels for transmission. Fibers produced by UL are tested at their extensive fiber characterization and systems experimental facility.

Fibers are also tested in the laboratories of the University of Bristol with their innovative OAM devices.
 

Design and testing

Three kind of fibers were designed and developed by UL and made available to the ROAM project: Ring Core Fiber (RCF), Air Core Fiber (ACF) and Inverse-Parabolic Graded Index Fibre (IPGIF). In RCF the wave-guiding region is a high-index annulus instead of a conventional circular core, which for reasons related to polarization-dependent differential phase shifts for light at waveguide boundaries, leads to enhanced stability for OAM modes. In ACF the air core enables conservation of OAM acting as a repulsive barrier, forcing the mode field to encounter the large index step between ring and cladding. IPGIF is capable of large effective index separations between the vector modes.

The OAM fibres have been tested. UL and UNIVBRIS have calculated and measured mode separations, coupling loss, propagation loss in all three types of OAM fibres, and measured the OAM-PMD in IPGIF and RCF. UNIVBRIS has studied the modal interactions in mode division multiplexing (MDM) links supporting OAM modes. The MIMO memory depth requirements have been investigated. Moreover UNIVBRIS conducted the testing of GIRCF, finding its main characteristics. UNIVBRIS implemented a receive-diversity architecture for different mode groups in OAM mode multiplexed transmissions to suppress the mode partition noise.

The coupling among OAM modes and mode groups has been investigated by UNIVBRIS. Inter-group and intra-group mode coupling has been measured over IPGIF and RCF. GIRCF has been tested over different distances up to 10km. The measurements have given feedback on the requirements to implement OAM-mode-division multiplexing transmission with GIRCF over distances >10 km.

UNIIVBRIS has investigated the optical equalisation techniques for mode coupling in MIMO-free systems using the IPGIF and RCF. Three types of mechanical gratings for the equalisation of coupling between different OAM mode groups have been fabricated. The joint effect of in-line polarization controller and mechanical grating for the complete equalization of inter- and intra-group crosstalk has also been investigated

A fibre-chip coupling interface using bulk optics has been developed by UNIVBRIS, and efficient coupling between integrated OAM emitters and few-mode fibres has been achieved. A high speed transmission link enabled by this fibre-chip coupling interface over a 2-km few-mode fibre has been demonstrated. UNIVBRIS has also presented a preliminary design of fibre-chip coupling interface employing integrated micro-lenses. UoG has developed the optimised integrated OAM emitters for the testing of the fibre-chip coupling interface.  

Coupling of the integrated OAM emitter with the OAM fibre

CNIT has studied nonlinear performance assessment in the case of OAM propagating fibers, with a focus on ACF and RCF fibers developed by UL. From simulations it came out that non-linear interference originated by WDM channels propagating on the same mode as the reference channel largely dominates the non-linear interference coming from WDM channels on different modes. The numerical results has shown that that nonlinear effects are not expected to be of any harm for the low distance transmissions ('<'2km) for data-center applications over OAM fibers.

 

OAM multiplexed transmission: crosstalk and power consumption investigation

UNIVBRIS and UL carried out an experimental activity to characterize the behaviour of the OAM multiplexed signal into the OAM fibre. An OAM multiplexed transmission experiment has been performed with a 100m IPGIF, in which different scenarios are experimented, including single-wavelength channel, with OAM crosstalk and with both OAM and WDM crosstalk. A rough estimation of the power consumption needed in the OAM-mode division multiplexing (MDM)-WDM (32Gbaud QPSK/channel) transmission experiments has been computed and compared with the one of a standard single mode fibre (SMF) WDM link.

 

OAM multiplexed transmission: crosstalk and power consumption investigation

MIMO algorithms have been developed by HWDU for OAM multiplexed transmission necessary to counteract the OAM mode coupling during propagation. The MIMO transfer matrices have been characterized by UNIVBRIS and UL on IPGIF and RCF. Different algorithms for cross-talk cancellation, i.e. channel equalization, have been studied and implemented for offline digital signal processing of coherent detection signals paying attention to universality, complexity and convergence properties.

Several OAM multiplexed transmission experiments have been implemented exploiting MIMO processing using at the receiver real-time oscilloscopes provided by the PSNC. A rigorous model has been built for comparison of space division multiplexing (SDM) and MDM techniques performed in various types of novel optical fibres, to investigate the system capacity allowed, system implementation, and complexity required for DSP equalisation. Efficient DSP algorithms to direct channel estimation methods have been extended and improved with respect to the state-of-the-art. This activity has been carried on mainly by HWDU, with the support of UNIVBRIS and PSNC regarding the experimental implementation.  

Experimental setup for MIMO algorithms verification

Optical equalization techniques

The optical equalization techniques have been implemented by UNIVBRIS to transmit 4 modes multiplexed over 15 wavelength channels without any MIMO processing at the receiver. The transmission has been done over 100m IPGIF exploiting 4 modes. The equalization has been implemented with polarization controller put on the IPGIF. The crosstalk among the OAM modes has been reduced below -10dB.

 

OAM-WDM multiplexed transmission

The final demonstration of the overall transmission capacity (16Tb/s) and distance (1-2km) as specified in Objective 1 of the proposal has been achieved. The final experiment with 10 OAM x 16 WDM channels has been setup at CNIT. A team composed by CNIT, UNIVBRIS, HWDU, UoG and PSNC staff met in Pisa to contribute to the OAM-WDM transmission demonstration. The transmission of 10 OAM modes multiplexed over 16 wavelengths has been demonstrated (28 Gbaud/s QPSK, with MIMO) over 1km GIRCF. Then the experiment has been repeated changing the modulation format from QPSK to 16QAM, keeping the baud rate at 28Gbaud. This last achievement demonstrates the project objective 1.  

Experimental setup for OAM-WDM multiplexed transmission

OAM-WDM multiplexed transmission: demonstrator with real data-traffic

A demonstrator of OAM multiplexing transmission in fibre has been performed at CNIT. A researcher from PSNC joined the experiment to provide support in the generation, detection and processing of the real-data traffic. The transmission testbed included the OAM fibre provided by UNIVBRIS, real-time oscilloscopes provided by PSNC and MIMO algorithms developed by HWDU. The field trial has been implemented multiplexing a comb of 16 WDM channels over 10 OAM modes. The tests included both laboratory and commercial, operational networking and measurement equipment. Error rate and its distribution has been used to estimate the impact on operational traffic. The obtained results show that OAM multiplexing transmission system can be used in high throughput short burst systems that are typically observed in data center scenarios.