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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.
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.
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.
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.
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.
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.
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.
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.
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