We experimentally demonstrate long-haul C+L-band transmission using an inline repeater that combines erbium-doped fiber amplifiers (EDFAs) and low-loss variable-spectrum equalizers (LLVSEs). Our LLVSEs offer a potentially low-cost solution for compensating for spectral distortions in a C+L-band wavelength division multiplexing (WDM) signal, primarily caused by EDFA gain tilts and inter-channel stimulated Raman scattering (SRS), even at repeater sites. To evaluate their equalization capability, C+L-band recirculating transmission experiments were conducted with spectral equalization performed solely by the LLVSEs in the recirculating loop. Net bitrates exceeded 1 Tbps per channel for transmission distances up to 1120 km, and total net bitrates of 74.9, 67.0, and 59.2 Tbps were achieved after 640-, 1120-, and 1600-km transmission, respectively. These results demonstrate the effectiveness of LLVSEs in long-haul transmission and their potential to enhance the performance of multiband WDM transmission systems.

We demonstrate ultra-wideband long-haul fiber-optic transmission with 27.0-THz signal bandwidth across the S, C, L, U, and extremely long wavelength (X) bands spanning a wavelength range of 238 nm (1464.01-1702.64 nm). The longer wavelengths beyond the U-band (1625-1675 nm) are lossy in silica-core fibers, but with support from power transition caused by inter-channel stimulated Raman scattering (ISRS) in multiband transmission, such wavelengths can be utilized as low-loss wavelength resources for data transmission. We denote this band beyond the conventional telecommunication bands as the X-band and explored its potential in ultra-high-capacity long-haul transmission systems. Inline amplification in bands other than the C/L-band, including the X-band, was performed using an optical hybrid repeater with waveband conversion using optical parametric amplifiers (OPAs) based on in-house periodically poled lithium niobate (PPLN) waveguides and erbium-doped fiber amplifiers (EDFAs). By designing the fiber launch condition to account for the ISRS, we achieved net capacities of 189.5 Tbps and 160.2 Tbps after 560-km and 1040-km transmission, respectively, over an 80-km-span standard single-mode fiber lumped-amplified link with Tbps-class channels in the X-band.

We demonstrated 92-channel 192-GBaud WDM transmission in 18.4-THz triple-band with our proposed low-noise forward Raman pumping technique, achieving the highest average channel rate of 1.117 Tb/s/λ over 2000 km with a total net-bitrate 102.8 Tb/s.

This paper presents high-capacity, long-haul, wavelength division multiplexing (WDM) transmission exceeding 100 Tb/s over 1000 km in the S+C+L band, which utilizes forward (FW)- and backward (BW)-pumped distributed Raman amplifications (DRAs). From transmission experiments, we confirmed the relationship between the FW Raman on-off gain and improved signal-to-noise ratio of the signal to evaluate the effect of relative-intensity-noise transfer from the FW-pumped DRA on the transmitted signal quality and determine the optimal FW Raman on-off gain. We successfully demonstrated 100-Tb/s-class high-capacity long-haul transmission using 122 channels of 144-GBaud signals in the 18.3-THz triple-band WDM configuration with FW- and BW-pumped DRAs for 80-km fiber spans. The total achievable (and net) bitrates were 117.6 (112.0) Tb/s over 1040 km, 113.8 (107.7) Tb/s over 1200 km, 109.9 (103.0) Tb/s over 1360 km, and 104.0 (96.3) Tb/s over 1600 km.

We experimentally demonstrate good agreement between numerical estimation and measured results for on-off Raman gain, inter-core crosstalk, and GSNR in C+L-band transmission with backward Raman amplification over hybrid SMF-MCF links.

This paper demonstrates 100-Tb/s-class long-haul single-mode fiber transmission over 2000 km in the S+C+L band using a hybrid optical amplification system comprising forward- (FW) and backward-pumped distributed Raman amplification (DRA) and rare-earth-doped fiber amplifiers. To mitigate signal quality degradation due to relative intensity noise transferred from FW Raman pumps, we applied our proposed low-noise FW-pumps featuring polarization-interleaved narrow longitudinal modes using Fabry-Perot laser diodes without fiber Bragg gratings. Experimental results showed that this low-noise FW-pumped DRA enabled a signal-to-noise ratio improvement of more than 1.3 dB at the center channel in the S band compared with the conventional FW-pumped DRA with coherent pumps after 2000-km transmission. Using the proposed technique, we successfully achieved a net bitrate of 105.6 Tb/s over 2000 km (80 km × 25 span) in an 18.3-THz wavelength division multiplexing bandwidth with 122 channels of 144-GBaud signals, significantly outperforming previous transmission records in the S+C+L band.

We present ultra-wideband long-haul transmission systems based on waveband conversion using PPLN-based optical parametric amplifiers and the WDM-bandwidth extension to extremely long wavelengths up to 1702 nm by effectively utilizing inter-channel stimulated Raman scattering.

We demonstrate 86.5-Tbps (801.5 Gbps × 108 channels) wavelength division multiplexing (WDM) transmission using the L- and U-bands over a 10.8-THz bandwidth across 70.4 km of field-deployed dispersion-shifted fiber (DSF). The key enabling technology is the wavelength-band conversion using periodically poled lithium niobate (PPLN) waveguides, which extends the usable bandwidth into the U-band where higher-chromatic dispersion reduces Kerr nonlinear impairments. Although U-band operation faces increased fiber loss, which is slightly exacerbated by bending, mechanical stress, and splicing in the field environment, these penalties are mitigated by the inter-channel stimulated Raman scattering (ISRS) gain derived from co-propagating L-band channels. The experimental results confirm the feasibility of bandwidth extension to the U-band and high-data-rate transmission over legacy DSF infrastructure.

We demonstrated 22.05-THz four-band long-haul transmission with a S-to-U-band lumped repeater consisting of PPLN-based optical parametric amplifiers and EDFAs over an 80-km-span SMF link. The achieved net bitrate was 133.06 Tbps at 1040 km with the 25.5-dBm fibre launch power designed by accounting for ISRS.

We demonstrated 27-THz lumped-amplified transmission within 1464.00–1702.64 nm with PPLN-based OPA/EDFA hybrid repeater extended to extreme longer-wavelength band (X-band), achieving the record capacity of 160.2 Tbps in 1040-km (13×80-km) transmission supported by designed ISRS.

We investigate 100-Tb/s-class C+L+U-band 14.85-THz bandwidth inline-amplified transmission over non-zero-dispersion-shifted fiber (NZ-DSF). The WDM signal bandwidth is extended to the U-band away from the zero-dispersion wavelength to boost the total throughput. The bandwidth extension to the U-band is enabled by a U-band repeater composing periodically poled lithium-niobate-based optical parametric amplifiers (PPLN-OPAs) and erbium-doped fiber amplifiers (EDFAs). The launch power of the C+L+U-band WDM signal is optimized by calculation accounting for stimulated Raman scattering and the wavelength-dependent accumulation of amplified spontaneous emission noise from PPLN-OPAs and EDFAs and nonlinear interference from NZ-DSF transmission to maximize the total throughput. To select an appropriate calculation method for launch power optimization, we analyze the impact of multi-channel interference, which is ignored in most closed-form expressions of the Gaussian noise (GN) model. We experimentally demonstrate net bitrates of 115.6 and 101.4 Tb/s after 240- and 400-km NZ-DSF transmission with bandwidth extension to the U-band and launch power optimization using a closed-form GN model.

We demonstrate ultra-wideband fiber-optic transmission over an ITU-T G.654.E-compliant cutoff-shifted single-mode fiber (CSF), which has low-loss and low-nonlinearity characteristics. By extending the transmission bandwidth to long wavelengths in the U-band and the extremely long wavelength band (X-band), a signal bandwidth reached 18.6 THz within 1530.53-1702.64 nm without using wavelengths below 1530 nm, the maximum cutoff wavelength defined in G.654.x. Using an optical inline lumped repeater combining erbium-doped fiber amplifiers and optical parametric waveband converters based on in-house periodically poled lithium niobate waveguides, we conducted an 80-km-span C + L + U + X-band lumped-amplified long-haul transmission up to 3040 km. We achieved net transmission capacity of 133.9 Tbps at 1040 km (13 × 80 km), 108.0 Tbps at 2000 km (25 × 80 km), and 87.6 Tbps at 3040 km (38 × 80 km). These results demonstrate that ultra-wideband transmission above 18 THz can be achieved using only the single-mode region of CSFs by extending bandwidth to long wavelengths with the support of inter-channel stimulated Raman scattering.

We demonstrate a net 107.7-Tb/s S+C+L-band long-haul transmission over 1200 km with the WDM-launch and the forward and backward Raman-pump powers optimized by a closed-form GN model and by experimental evaluation of the RIN transfer.

We have successfully demonstrated that our proposed forward-pumped distributed Raman amplification (DRA), which reduces noise transfer by using pump light with polarisation-interleaved narrow longitudinal modes, can dramatically extend transmission distance in conjunction with backward-pumped DRA, achieving net 105.6 Tb/s over 2000- km transmission in S+C+L band. (c) 2025 The Author(s)

This paper reviews ultra-wideband inline-amplified WDM transmission systems, including bandwidth extension to the U-band, using optical parametric amplification and waveband conversion based on our high-efficiency periodically poled lithium niobate (PPLN) chi((2))-nonlinear waveguides.

We propose coupled-power equations accounting for stimulated Raman scattering and Rayleigh backscattering to estimate inter-core crosstalk in multiband WDM transmissions over weakly-coupled multicore fibres. Experimental validation demonstrated the dynamics of forward- and backward-propagating inter-core crosstalk can be accurately estimated in C+L-band transmissions utilizing step-index-4-core fibre. (c) 2025 The Author(s)

We show the improvement of signal quality in S+C+L-band transmission using a low-beatnoise forward pumping. After 1200-km transmission, the measured signal-to-noise ratios were improved by more than 0.7 dB compared to using a conventional technique.

We propose an accurate power profile calculation method for GN-model-based SNR estimation and demonstrate an average SNR estimation error of 0.22 dB in C+L-band 101-ch WDM 96-Gbaud PCS-36QAM signal 1120-km hybrid Raman-EDFA amplified transmission.

We investigate the estimation of inter-channel stimulated Raman scattering (ISRS), signal-to-noise ratio (SNR), and generalized mutual information (GMI) throughput in 14.1-THz signal bandwidth (S+C+L-bands) inline-amplified transmission over up to 400-km G.652.D single-mode-fiber (SMF). The bandwidth across spectral edges (i.e., including the guard band) reached 15.2 THz. ISRS noticeably transfers signal power from shorter-wavelength channels to longer-wavelength channels in ultra-wideband wavelength division multiplexing (WDM) systems, changing the wavelength dependence of signal impairments. The accuracy of ISRS estimations, which are a key step in the performance estimation or design of ultra-wideband WDM systems, is analyzed for different Raman gain coefficients by comparing estimated and experimentally measured ISRS. By taking into account the wavelength dependence of Raman gain coefficients, an accurate ISRS estimation with less than 0.55-dB peak-to-peak errors is demonstrated. Then, we combine the accurate ISRS estimation with a closed-form Gaussian noise model and evaluate the estimation accuracy of SNR and GMI throughput in the 14.1-THz bandwidth system for different transmission distances. We experimentally demonstrate less than 0.92 dB mean-absolute SNR estimation errors and less than 2.19% absolute total GMI throughput estimation errors after up to 400-km transmission. Finally, using the validated estimation model, we investigate a better launched WDM signal spectrum to enhance the total GMI throughput beyond that achieved in the experiment. Our calculation shows that modifying the launched spectrum can increase the total GMI throughput by ∼3.4% for 160-km transmission and by ∼2.9% for 400-km transmission.

We demonstrate 14.85-THz-bandwidth WDM transmission over G.654.E fiber using hybrid PPLN-OPA/EDFA for 4.5-THz U-band. Capacities of 115.3-Tb/s over 800 km and 72.6-Tb/s over 2,400 km for 144-Gbaud PCS-QAM signals are achieved with lumped amplifiers only.