Stimulated Raman scattering (SRS)-induced inter-channel power transfer introduces global cross-channel coupling in the amplitude domain, causing power fluctuations across the spectrum that further interact with accumulated Kerr nonlinearities. The C + L-band system modeling and generalization become challenging under various nonlinear effects, especially with power pre-tilt. In this Letter, we propose an enhanced self-attention-assisted multi-channel waveform modeling to achieve efficient and accurate modeling with SRS and non-flat launch power. To capture these cross-channel and long-range dependencies, we apply rotary positional encoding to the query (Q) and key (K) matrices in the attention mechanism. Benefitting from enhanced self-attention, we realize waveform modeling with strong generalization ability across different non-flat launch power profiles and transmission distances in ultra-wideband (UWB) wavelength-division multiplexing (WDM) systems. We compare the split-step Fourier method (SSFM) with the proposed method over a 10-span link at the optimal launch power, and the Q-factor differs from SSFM by only 0.31 dB. In a 5-span scenario with pronounced nonlinear effects, our method reduces runtime by 99.4% while maintaining a Q-factor deviation of just 0.24 dB from SSFM.

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).

In this work, we explore the performance of a polarization-insensitive fiber optical parametric amplifier (PI-FOPA) used as an in-line amplifier in a multi-span transmission experiment. We transmit an equivalent of 24 × 35 GBaud PDM-16QAM signals with a capacity of 4.8 Tbps over 770 km, or 38 × 35 GBaud PDM-16QAM signals with a capacity of 7.6 Tbps over 231 km. The former is the record reach, and the latter is the record capacity for PI-FOPA-amplified multi-span links. These achievements have been facilitated by the employment of a Mach-Zehnder-like polarization diversity configuration for PI-FOPA, which reduces interchannel nonlinear crosstalk and optimization of a pump phase modulation waveform applied for mitigation of the stimulated Brillouin scattering. We demonstrate that the impact of pump phase modulation on amplified signals in state-of-the-art PI-FOPAs is the primary factor limiting the reach of multi-span links employing PI-FOPAs, while nonlinear crosstalk involving the pump limits the signal bandwidth employable for long reach transmission.

We compare the long-haul coherent transmission performance of 30 GBaud DP-16-QAM WDM signals using five different S-band optical amplifiers: a thulium doped fiber amplifier (TDFA), a distributed Raman amplifier (DRA) and three different lumped Raman amplifiers (LRAs) using 10 km inverse dispersion fiber (IDF), 10 km, and 5 km Raman optical fibers (ROFs). Over 1050 km standard single-mode fiber (SSMF), the DRA performed the best with an SNR of 14.2 dB at the optimum launch power of -2.3 dBm, followed by the TDFA with an SNR of 13.0 dB and the LRA using 10 km IDF with 11.6 dB of SNR.

We investigate the potential to expand the trans mission 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.

The proposed characterization method for electro-optic modulators was utilized to compare two distinct lithium nio-bate modulator implementations over an optical bandwidth of 400 nm. The results show insertion loss of the designed multi-band transmitter below 8.2 dB and confirm its broadband capabilities over 59 THz.

We investigate the power consumption of ultra-wideband SCL-band transmission systems using hybrid amplification. We measure the electrical-to-optical power conversion efficiencies of commercial EDFAs and TDFAs and distributed Raman amplification (DRA) pumps. We use the closed-form Gaussian Noise (GN) model including ISRS alongside these measurements to investigate the energy-per-bit of hybrid amplification under backward-DRA pump and launch power optimisation. Our results show that the S-band plays a critical role in the energy-efficiency of UWB systems, due to a combination of low TDFA efficiency and power depletion from inter-channel stimulated Raman scattering. In short links, DRA provides only nominal throughput gain and the energy-per-bit increases for all DRA pump configurations. However, in the long-haul regime (>1000 km) the throughput gain provided by DRA offsets the increased power consumption, resulting in up to 20% reduction in the total energy-per-bit. Taking into account the power consumption of a typical pluggable transceiver, we show that if the total transceiver power consumption exceeds that of the amplifiers, or if the power consumption overheads are large, DRA becomes an energy-per-bit efficient amplification technique for high-capacity ultra-wideband systems.

We studied the power consumption of hybrid-amplified SCL-band links using commercial benchtop amplifiers and Raman pumps. We show a reduction in energy per bit for multi-span hybrid Raman amplified links of up to 26% versus lumped amplification. (c) 2025 The Author(s)

We compare S-band long-haul transmission of 30GBaud DP-16-QAM WDM signals using TDFA, distributed Raman and three types lumped Raman amplifiers over 1050km SSMF. Distributed Raman amplifier performed the best, followed by TDFA and lumped Raman.

In bidirectional mode-division multiplexing (Bi-MDM) systems, which have the potential for significantly higher transmission spectral efficiency, the complex interplay between mode-coupling and Rayleigh scattering (RS) exacerbates channel instability, posing significant challenges for dynamic network management. To address this, we propose DeepRS, an innovative deep-learning-based scheme for high-precision, in-service RS noise monitoring. By utilizing deep neural networks (DNNs) to extract features from the filtered frequency amplitude histogram (FFAH) of received signals, it enables the efficient signal-to-RS ratio (SRR) monitoring without disrupting signal traffic. In a 3-mode coherent Bi-MDM experiment, DeepRS achieves an impressive SRR prediction accuracy, with a coefficient of determination ( R 2 ) exceeding 0.9927 when incorporating crosstalk (XT) pre-prediction. It demonstrates strong adaptability across various operating wavelengths, modulation formats, and Baud rates. The scheme’s outstanding performance has also been experimentally validated in a 10 km-long 6-mode fiber. Furthermore, DeepRS exhibits high robustness to XT prediction errors, maintaining an SRR prediction R 2 above 0.9884 when the absolute XT error is within 2 dB. Finally, simulation results confirm its insensitivity to optical distortions such as chromatic dispersion (CD), differential mode group delay (DMGD), and laser linewidth (LW), further validating its robustness for practical Bi-MDM systems.

A 223 km unrepeated transmission link is experimentally demonstrated using 121 nm optical bandwidth. Optimised bidirectional Raman amplification as well as Thulium- and Erbium-doped fibre amplifiers enable a record throughput of 122.62 Tb/s.

We combine 6 doped-fiber amplifiers (O-(x2), E-, S-, C-, L-bands) with discrete Raman U-band amplifiers and distributed Raman-amplification to transmit in each of the low-loss transmission bands of a standard optical fiber. For transmission distances up to 100 km, we explore ultra-wideband transmission of up to 1505 x 25 GHz spaced channels covering the O-, E-, S-, C-, L- and U-bands from 1281.2 nm to 1649.9 nm. After describing the amplifier and spectrum flattening technology, we first characterize the transceiver in back-to-back configuration. Then, for 50 km transmission, we report a record aggregate transmission bandwidth of 37.6 THz with a record standard single-mode fiber transmission data-rate of 402.2 Tb/s, estimated from GMI, and 378.9 Tb/s after FEC decoding. 100 km transmission allows a 36.6 THz bandwidth with 339 Tb/s GMI estimated data-rate and 322.8 Tb/s decoded data-rate. These results show the potential of ultra-wideband transmission covering the low-loss window of silica fibers as well as the challenges of building such systems on longer fiber spans.

Pump phase modulation, employed to mitigate stimulated Brillouin scattering in fiber optical parametric amplifiers (FOPAs), is a key source of signal degradation. We demonstrate significant potential to overcome this limitation and enable high-performance FOPA operation as an inline amplifier.

Optical phase conjugation (OPC) is a pivotal all-optical technique aimed at enhancing the received signal quality by compensating for nonlinear distortions. Integrating the OPC into a CMOS-compatible, highly nonlinear silicon photonic chip holds promise for developing fully integrated transceivers with a compact footprint, low loss, and minimal power consumption. Despite its potential, silicon-based OPC demonstrations have been limited, primarily due to challenges, such as inefficient conjugation and significant losses. In this work, we demonstrate an effective OPC technique utilizing a single passive silicon photonic waveguide spiral. This silicon photonic waveguide is meticulously designed with an optimal cross-section to achieve an ultralow loss and high conversion efficiency. The silicon photonic waveguide spiral was fabricated via standard multiproject-wafer processes, and the measured result shows an ultralow loss of 0.25 dB/cm and a high conversion efficiency of -5 dB, marking the highest conversion efficiency reported for passive silicon photonic waveguides to date. The experimentally demonstrated OPC significantly enhances idler generation, resulting in a 3-dB improvement in launched signal power within a 160 Gbit/s 16-QAM transmission system without the need for dispersion compensation for over an 80-km transmission distance.

Transmission with few-mode fiber (FMF) is one of the most promising methods for increasing single fiber capacity, with urgent demands for low-noise and wideband amplification. In this work, we experimentally demonstrate a few-mode hybrid Raman amplifier (FM-HRA) based on a second-order few-mode distributed Raman amplifier (FM-DRA) cascaded with a lumped Raman amplifier (LRA). The few-mode hybrid amplification with a 100-nm bandwidth (1520 to 1620 nm), covering the S + C + L bands, is realized using a weakly-coupled FMF with a transmission length of 100 km and a dispersion compensating fiber (DCF). The maximum on-off gains for three modes ( LP 01 , LP11, and LP21) are 22.6, 25.1, and 25.5 dB, and the average on-off gains over 100-nm bandwidth are 19.6, 21.4, and 21.3 dB, respectively. The differential modal gain (DMG) is less than 2.9 dB. The effective noise figure (NF) of the three modes is lower than - 1.1 dB over 100-nm bandwidth, and the lowest effective NF is - 6.8 dB, which is close to the effective NF of FM-DRA. This means HRA can fully utilize the advantages of ultralow noise of DRA and high gain of LRA. The MIMO-less transmission experiment demonstrates the highest data rate of 56 Gbps for LP01 mode. This work features pure Raman amplification with high gain, ultralow noise, and wideband. The configuration of high-order DRA in FMF and first-order LRA in DCF also offers the benefits of better pump power utilization and dispersion compensation.

We demonstrate the first SCL-band long-haul transmission using G.654.E-compliant fibre, achieving 100.8 Tb/s (GMI) over 1552 km, despite its 1520 nm cutoff wavelength. Due to the fibre's ultra-low loss and low nonlinearity, the achievable-information-rate with lumped amplification is comparable to that of G.652.D-compliant fibre links with distributed-Raman-amplification.

Efficient and precise monitoring on mode-coupling-induced crosstalk (XT) is crucial for the operation of mode-division multiplexing (MDM) systems, particularly for optimizing digital signal processing (DSP) parameter configurations and maintaining stable transmission performance. In this paper, we propose a mode pilot-tone (MPT) monitoring scheme and investigate its performance for XT (MPT-XT) monitoring in a three-mode MDM transmission system. By deploying a photodiode (PD) with a 1 GHz bandwidth for each mode to extract the MPT responses, the complete XT information from 21 GBaud 16-QAM signals is obtained by the proposed scheme. In the experiment, the root-mean-square-error (RMSE) for XT-prediction achieves 0.11 dB, with a coefficient of determination R2 of 0.9992 across the OSNR range from 15dB to 25dB. By applying the transfer learning-based crosstalk neural network (TL-XT-NN) scheme, which transfers the NN model trained exclusively with simulation data to experimental XT monitoring scenarios, the training overhead required by the NN is reduced more than 48.3%. Moreover, we further evaluate the monitoring performance of the proposed scheme on the multiplexing density, the chromatic dispersion (CD), the differential mode group delay (DMGD), and the mode differential loss (MDL). Simulation results confirm its exceptional and consistent performance across all conditions. Finally, we investigate the impact of MPT modulation on the compensation behavior of multi-input multi-output (MIMO) equalizer and propose an inverse mapping label erasure (IMLE) scheme to mitigate the resulting signal degradation. Through the MPT erasure procedure, the error vector magnitude (EVM) penalty of transmitted signals caused by MPT operation is reduced from 2.92% to 0.19%.

We review recent Raman amplifiers technologies for ultra-wideband coherent transmission and compare Raman amplifiers and doped-fibre amplifiers for unconventional bands.

We experimentally investigate the transmission performance of the constituent PAM-4 signals of a 400GBASE-LR4 O-band transceiver over extended reach of 25 km SSMF. Using a single wavelength (1195nm), low power Raman amplifier, improved BERs of up to a decade were measured for the 1273 nm, 1290 nm and 1315 nm lanes.