We experimentally demonstrate the first mode-resolved fiber-longitudinal power monitoring (LPM) in a three-mode fiber link by extending the receiver-side-DSP-based LPM algorithm to multi-mode transmission, enabling location-resolved mode-dependent loss (MDL) monitoring down to 1 dB.

Signal gating combined with local-oscillator-frequency switching enables bandwidth scaling of offline coherent reception without costly receiver parallelization. We experimentally verify this concept at symbol rates of up to 288 GBaud.

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 propose a configuration for generating high-order quadrature amplitude modulation (QAM) signals using an optical IQ modulator with multi-electrodes of M different lengths. The most notable feature of this configuration is that the four arms in the IQ modulator are driven independently without the use of push-pull drives. Compared to the conventional configuration using classical IQ modulators, the Euclidean distance between symbols in constellation can be increased by 1.2 times, as our technique does not use the push-pull drives that are essential in the classical IQ modulator. Moreover, compared to another conventional configuration that uses a single Mach-Zehnder modulator and multi-electrodes of M different lengths, the proposed configuration requires half the value of M to achieve the same error vector magnitude.

We review recent progress in high-capacity long-distance SDM transmission and its enabling technologies. We also introduce a feedforward, non-data-aided frequency-offset estimator for attaining stable performance in strongly coupled SDM-MIMO channels.

RF fading induced by chromatic dispersion (CD) is a major limiting factor for transmission reach and symbol rate in direct-detection (DD) optical transmission systems. Conventional approaches for mitigating this limitation rely on increased electronic front-end complexity, such as using multiple digital-to-analog converters (DACs) and/or analog-to-digital converters (ADCs) per effective baseband channel or requiring non-conventional signaling beyond simple baseband pulse amplitude modulation (PAM) at the output of the DACs. Recently, we proposed cross-lane phase diversity (XLPD), a CD-tolerant DD scheme that significantly alleviates RF fading while preserving non-redundant DAC/ADC configurations (i.e. one DAC and one ADC per effective baseband channel) and compatibility with conventional baseband PAM signaling at the output of the DACs. In this paper, we present a detailed description of the operating principle of XLPD and develop analytical models for signal reconstruction and bandwidth limitation. In XLPD, two PAM signals are combined into a quadrature amplitude modulation (QAM) signal, which is conveyed over two transmission lanes using phase-diverse carriers aligned with the in-phase component in one lane and with the quadrature component in the other. After square-law detection, complementary spectral components of the QAM signal are distributed across the lanes despite CD-induced frequency-dependent phase rotation. Joint digital signal processing using the two detected waveforms enables reconstruction of the QAM signal without significant information loss due to RF fading. The analysis further reveals the effective RF bandwidth of wavelength XLPD and its dependence on transmission distance and carrier spacing. Experimental demonstrations using two wavelength lanes achieved net bit rates exceeding 200 Gbps/lane for 80-km C-band transmission with 4-level PAM signaling assuming hard-decision forward error correction (FEC) code and 300 Gbps/lane for 40-km transmission with probabilistically shaped PAM assuming soft-decision FEC code.

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 propose geometric mode-division multiplexing (GMDM) to overcome diffraction-induced limitations in mode-division-multiplexed FSO links. Using a multi-aperture single-mode transmitter, we demonstrate 140-Gbaud, three-mode, single-wavelength transmission, achieving up to net 3.6 Tb/s under diffraction.

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.

We propose a bidirectional transceiver architecture based on PPLN-based optical parametric amplifiers. A crosstalk suppression of 14 dB is experimentally achieved by exploiting the gain contrast between received signal and transmitted-signal-induced crosstalk using selective phase-sensitive de-amplification.

We propose numerically-stable and fast-converging QR-decomposition-based recursive-least-squares MIMO equalization for high-MDL SDM transmission, achieving up to 67% lower computational complexity. It simultaneously provides IQ-imbalance compensation and twofold-faster filter-coefficient learning in 12-coupled 140-GBd 3317-km signal transmission.

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 propose a method to evaluate tracking performances of MIMO equalisers in perturbated coupled SDM transmission, fully scrambling SDM channels at quantified speed by a SDM fibre stretcher. This enables evaluation of channel dynamics at larger SDM numbers, which are underestimated in conventional partial-channel scrambling. (c) 2025 The Author(s)