Ruben Soares Luis

The increasing capacity demand of optical communication systems has driven research on space-division multiplexing (SDM) using few-mode fibers (FMFs). While FMFs provide high spatial efficiency, their practical deployment is limited by the rapidly growing complexity of multiple-input multiple-output (MIMO) digital signal processing (DSP). Partitioned-MIMO architectures reduce DSP complexity by exploiting the existence of mode groups (MGs) in FMFs, where intra-group coupling is strong and inter-group coupling is weak. In this work, we experimentally investigate a partitioned MIMO architecture with mode-group specific interface quantization for a 15-mode FMF transmission system, which would otherwise require a full 30 × 30 MIMO equalizer. The equalizer is decomposed into five MG-specific sub-equalizers with low-resolution, MG-dependent inter-partition signal exchange to compensate residual inter-group coupling. Using 24.5-GBd dual polarization QPSK transmission over a 58.9-km graded-index FMF, we evaluate the impact of interface quantization on the achievable signal-to-noise ratio (SNR). The results show that weak inter-group coupling can be omitted entirely, while stronger neighboring MG interactions require only three to four bits to limit SNR penalties to below 1.7 dB relative to a uniform five-bit reference. This approach significantly reduces inter-partition interface throughput while preserving equalization performance, demonstrating the scalability of partitioned-MIMO architectures for high-capacity SDM systems.

We demonstrate a cost-effective, space-division multiplexed (SDM) reconfigurable optical add & drop multiplexer (ROADM) capable of petabit-per-second-class switching that can be integrated into the switching fabric of next-generation SDM-wavelength-division multiplexed high-capacity optical network elements. The SDM network node is a two-line-side ROADM supporting petabit-per-second line signals, consisting of 22 compact, low-cost, low-loss, 2×2 MEMS switches supporting 22-core multicore fibers. The functionality of the low-cost SDM ROADM is demonstrated through the network scenarios of full add & drop, full express, and partial add & drop, handling a total traffic capacity of 1.08 Pb/s.

We experimentally demonstrate a set of U-band discrete Raman amplifiers using backward incoherent pumping in 1 km HNLF, achieving up to 22.3 dB net gain, and 4.2–5.8 dB NF. Three different types of Raman gain fiber have been investigated, including 1 km HNLF, 0.51 km HNLDSF, 8 km and 7.6 km IDF. Using HNLF achieved the highest gain and the lowest NF, while using 8 km IDF yielded up to 16.2 dB net gain and a minimum of 6.3 dB NF due to its low Raman gain coefficient and higher fiber loss. Using 0.51 km HNLDSF gave up to 21 dB net gain, but at a cost of over 8.1 dB noise figure. These amplifiers were incorporated in a C + L + U-band coherent transmission system using 516 × 24.5GBd DP-64/256QAM channels over 50 km SSMF. We achieved a maximum decoded data rate of 123.5Tb/s across C + L + U bands, with 25.6Tb/s specifically in the U-band (1625–1650 nm).

We demonstrate a phase-coherent transmission system supported by the transmission of a seed lightwave in a dedicated multi-core fiber (MCF) core used as a reference for parametric optical frequency comb (OFC) generation at both the transmitter and receiver. We exploit the phase coherence of the wavelength/space-division multiplexed channels to implement a shared digital signal processing (DSP) scheme that reduces the complexity of carrier phase recovery (CPR). In the scheme, the carrier phase noise estimated from a small set of reference channels is used to recover other channels across both wavelengths and cores. The system performance is evaluated over transmission distances of up to 176 km in a 4-core MCF, showing that just 6 reference channels enable CPR of 3 (cores)$\times 150\times$24.5-GBaud polarization-multiplexed 64-ary quadrature amplitude modulation C-band channels, with data-rates >30 Tb/s/core. This is achieved with <0.5 dB Q-factor compared to independent CPR. The results demonstrate the potential of OFC-based MCF systems for high-capacity, short- to medium-reach optical interconnects with reduced hardware requirements and simplified DSP.

We investigate the potential to expand the transmission bandwidth beyond the C-band to include the L- and U-bands over an unrepeatered link. We characterize three fibers - a G.654 compliant Sumitomo Z+ fiber, a G.657 compliant Corning SMF28ULTRA fiber, and a G.652 compliant OFS AllWave Zero Water Peak (ZWP) fiber - for their attenuation profiles and Raman gain efficiencies in the L- and U-bands. We demonstrate the transmission of C+L+U bands over an unrepeatered link of 257.1 km. The transmission system includes band-specific erbium-doped fiber amplifiers for the C- and L- bands, lumped Raman amplifiers for the U-band, and bidirectional distributed Raman amplification for the three bands. We report a decoded data-rate of 84.5 Tb/s over a spectral bandwidth of 109.2 nm using 520 polarization-multiplexed 16-ary quadrature amplitude modulated channels across C+L+U bands. We investigate ways to manage the effects of stimulated Raman scattering while expanding the transmission window beyond 100 nm over an unrepeatered link.

We report the first experimental characterization of stimulated Raman scattering (SRS) in coupled-core multi-core fibers (MCFs). The experiment was performed using field-deployed fibers, with two complementary methodologies. First, the effective Raman gain coefficient was directly measured over a bandwidth of 120 nm – covering the upper S-, C- and lower L-bands – with the Raman pump being injected into different combinations of fiber cores and using either co- or counter-propagating signal. Second, we analyzed the spectral power tilt induced by SRS when transmitting a wavelength-division multiplexed signal spanning the entire C-band from 1529.4 nm to 1564.2 nm, which was found to be in agreement with the Raman gain coefficient measurements. Measurements performed on a co-deployed standard single-mode fiber (SMF) produced similar results. This observation, together with the notion that the Raman gain coefficient should be similar in MCFs and SMFs, validates our earlier derived model of SRS in fibers with strong mode mixing.