We carried out a parameter optimization of the multimode fiber (MMF) Raman amplifier via numerical simulations of signal amplification based on inter-modal stimulated Raman scattering (IM-SRS), showing that the signal on–off gain induced by IM-SRS, defined as the ratio of the output signal power with and without the pumps, overcomes the fiber losses when the total injected pump power is 1 W.
The next goal has been the delivery of a suitable Mode-MUX/DEMUX device, based on the multi-plane light conversion technology [1]. Three MUX/DEMUX pairs have been designed and fabricated. After fabrication, we characterized the modal selectivity, insertion loss, and tolerance to high optical power of the MUX at signal and pump wavelengths.
The first pair of MUX was optimized at 1550 nm; it is functional all over the C-band and designed for a standard commercial OM4 (50/125) graded-index (GRIN) MMF. The corresponding GRIN fiber modes are the first 15 Laguerre-Gaussian (LG) modes. However, the experimental characterization of the response of the MUX/DEMUX pair for LG modes of an OM4 fiber using 5 km of MMF has revealed that over such fiber lengths there is an unacceptable high level of modal X-talk, owing to linear RMC. This means that the OM4 MMF platform cannot be used, because of the strong RMC-induced level of modal X-talk.
As a result, we used a specialty GRIN fiber, the low-differential-mode-group-delay fiber (9LP) [2]. This MMF supports 15 different spatial modes. We designed and fabricated a MUX pair for the LP9 MMF, optimized for the C band of the signal only. Although the modal IL and X-talk for the MUX pair optimized for the C band were satisfactory also in the presence of the 8 km of 9LP MMF, the IL at the pump wavelength of 1455 nm was excessive. Hence, we made a redesign of the MUX pair, optimized for the wavelength range of 1450-1550 nm.
An optical signal transmission setup was implemented: pumps were injected at the transmitter or at the receiver sides and optimized according to numerical simulations. Signals and pump were directly applied to the modal multiplexers and coupled to distinct modes.
In a first test, co-propagating signals and pumps were used. The 1 W pump power was equally divided among the pump modes. With a co-propagating pump, the obtained modal gains have an average of 2.6 dB, which is in relatively good agreement with our numerical predictions. The Raman gain compensates for the transmission fiber losses. In a second test, counter-propagating signals and pumps were used. In this configuration, the measured average gain is 1.89 dB.
In conclusion, the capability of obtaining Raman gain for mode-multiplexed signals was numerically and experimentally demonstrated. Signal amplification in the transmission fiber, at the expense of the pumps, was obtained via IM-SRS, which was distributed along the fiber. At the receiver, power in the signal modes was retrieved via a corresponding modal demultiplexer. The measured gain entirely compensated for the fiber losses experienced by the mode-multiplexed signals.
Numerical simulations confirmed the experimental data, and suggested that, by increasing the pump power to 10 W distributed over a larger number of modes, the signal gain could increase up to 19.3 dB over 8 km of GRIN FMF. The injection of such pump power appears to be feasible, as far as the MMF is concerned, but may pose challenges to the MUX/DEMUX design.
The successful demonstration of the IM-SRS architecture was published [3]. This demonstration opens the way to the use of high-capacity, optical unrepeatered systems over long-haul systems, which are typical of international transport networks. In turn, this may permit to install long-haul cables including hundreds of MMFs, carrying no electrical current for unwanted in-line repeaters.
We carried out a Freedom to Operate analysis by performing a patent search on the literature and public databases, to identify existing patents and published results that may hinder the IP protection of MULTIBRIDGE technology. On July 27, 2022, M. Zitelli and S. Wabnitz of Sapienza filed a patent application for the MULTIBRIDGE concept [4].
We carried out a market analysis of the existing unrepeatered systems, focusing on the performances and pricing, and identified the value proposition of MULTIBRIDGE with respect to the state-of-the-art. We analysed competitors, customer segmentation, market size and trends, regulatory issues, and related risk analysis. Details are provided in the core report.
1. G. Labroille, et al., Characterization and applications of spatial mode multiplexers based on multi-plane light conversion, Opt. Fiber Technol., Mater. Devices Syst. 35 (2017) 93–99
2. P. Sillard, et al., Low-differential-mode-group-delay 9-LP-mode fiber, J. Lightwave Technol. 34 (2) (2016) 425–430
3. M. Zitelli, et al., “Inter-modal Raman amplification in space-division multiplexed systems,” Optical Fiber Technology, 86, 103854 (2024)
4. M. Zitelli and S. Wabnitz, Mode-division multiplexed fiber Raman amplifier system and method, 2023, USPTO US20230170993A1
