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We demonstrate a record-long 10-mode-multiplexed transmission over 1300 km using a 6-LP few-mode fiber (FMF). Long-haul mode-division-multiplexed (MDM) transmission systems face challenges posed by the pulse broadening effect caused by differential mode delay (DMD), which hampers performance and increases computational complexity for receiver-side multiple-input multiple-output digital signal processing (MIMO-DSP) to mitigate mode mixing effects. In this study, we extend the application of mode permutation techniques, specifically the cyclic mode-group permutation (CMGP), as a solution to address these performance limitations in 10-spatial-mode-multiplexed long-haul transmission. We present experimental results conducted over a 6-LP graded-index (GI) FMF, demonstrating the effectiveness of CMGP in improving spatial channel coupling and reducing pulse spreading with comparisons with several CMGP-adopted configurations. The introduction of CMGP realized a modal-dispersion-unmanaged link, even when employing a 6-LP FMF with a modal dispersion coefficient of 157 ps/km, achieving the world's longest 1300-km transmission utilizing 10 spatial modes while reducing the impact of the modal dispersion accumulation by 82%.

In this paper, we investigate the deployment properties of step-index multi-core fibers (SI-MCFs) and trench-assisted multi-core fibers (TA-MCFs) in terrestrial high-density cables and links. We reveal that SI-MCFs and TA-MCFs can be designed with the same core pitch while maintaining their compatibility with conventional single-mode fiber (SMF) compliant with ITU-T Recommendation G.652. When SI-MCFs and TA-MCFs were implemented in high-density cables, we observed no significant increase in loss during the cabling and installation processes. Furthermore, the crosstalk deviation between processes is consistent with power coupling theory, indicating that effective bending is a key factor in determining the crosstalk under deployed conditions. We also confirm that good fusion splicing for MCF link connection is achievable by ensuring sufficient angular alignment accuracy. Additionally, our field experiments verify that accounting for the randomness of MCFs in the longitudinal direction of the link can reduce the characteristic deviation between MCF cores, leading to improved transmission characteristics.

We demonstrate a 38.4-Tbps (60 ch. × 640 Gbps) wavelength-division multiplexing (WDM) transmission across 5 × 80-km G.654.E single-mode fibers over 6 THz within L- and U-bands. The U-band (1625–1675 nm) is promising as a next transmission band for multiband networks because of its low transmission loss compared with E- and O-bands, but there are few demonstrations of coherent WDM transmission with the U-band. One reason is that promising optical components, especially for optical transceivers, are underdeveloped for the U-band. An optical parametric amplifier (OPA) using a periodically poled LiNbO3 (PPLN) waveguide has wide-gain bandwidth and wavelength conversion function, which allows the use of commercially available optical components for new bands. We achieve L- and U-band digital coherent WDM transmission within 1597.2–1649.9 nm (181.7–187.7 THz) using PPLN-based optical parametric inline amplification and wavelength-band conversion.

To economically manage the rapid traffic growth in data center networks, transmission technologies need to be studied for next-generation high-speed Ethernet, such as 1.6 TbE and beyond. This article describes a demonstration of 1.6 Tb/s (4 × 400-Gb/s/lane) O-band transmission over 10 km of installed four-core fiber. Since chromatic dispersion limits the transmission distance of the high-speed intensity-modulated direct detection (IM-DD) signals, a space division multiplexed technology using the 10-km four-core fiber is suitable to parallelly transmit 400-Gb/s/lane signals at near-zero dispersion wavelengths. A net-rate of 400 Gb/s/lane IM-DD signals with 155-GBd pulse amplitude modulation-8 (PAM-8) is generated by using an in-house broadband amplifier based on an InP double hetero-junction bipolar transistor (InP-DHBT). Our nonlinear maximum likelihood sequence estimation (NL-MLSE) enhances the performance of the 400-Gb/s/lane signals. We also introduce a technique called trellis path-limitation MLSE (TL-MLSE) for reducing computational complexity with temporarily decided results and a truncated trellis diagram. The trellis path-limitation MLSE with nonlinear calculation function (TL-NL-MLSE) achieves 1.6-Tb/s 10-km transmission in the O-band with the same performance as and lower computational complexity than the NL-MLSE.

We demonstrate a technique for generating high-quality optical signals at a symbol rate twice the sampling rate of digital-to-analog converters (DACs). Optical time-division multiplexing (OTDM) with a carrier-suppressed return-to-zero (CSRZ) wave doubles the symbol rate in a spectrally efficient manner. An 8×4 digital calibrator effectively compensates for the spurious spectral components in the CSRZ waveform, relaxing the required accuracy of the pulse generator and enhancing the signal integrity. We implemented a 224-GBaud CSRZ-OTDM transmitter with a single Mach-Zehnder modulator as the CSRZ carver and an integrated delayed dual in-phase-and-quadrature modulator as the data modulator, both fabricated using an InP n-i-p-n platform, and achieved single-carrier net bit rates of 2.59 Tbps at back to back and 2.52 Tbps after 80-km transmission.

We propose a novel low-complexity, fast-converging frequency-domain multiple-input multiple-output (MIMO) equalization technique suitable for long-haul space division multiplexed (SDM) transmission. Based on the recursive least squares (RLS) method, the proposed scheme, referred to as out-of-band exclusive frequency domain equalization (OBE-FDE), provides a robust learning of MIMO filter coefficients even in the presense of accumulated mode dependent loss (MDL), while achieving reduced complexity by selectively performing computations only within the signal's bandwidth. Numerical evaluation results show that complexity reduction effect is enhanced with increased number of spatial modes in a specific roll-off factor range. Also demonstrated is an improved convergence tolerance against MDL introduced by the RLS adaptation, contrasting with the widely-used least mean squares (LMS) adaptation in scenarios involving long-haul MDL accumulation and/or high-mode-count mode division multiplexing (MDM) transmissions. We also show experimental demonstrations, proving that the proposed low-complexity RLS-adapted MIMO-FDE achieved a ten-fold faster learning of filter coefficients in 4-coupled core multi-core fiber transmission at 5528 km. Additionally, by newly applying the scheme into 10-spatial-mode transmission, a record-long signal transmission distance over 1560 km was achieved while reducing computational complexity requirements by 40%.