We report a 108 Gbit/s fully coherent optical-wireless transmission in the 125 GHz band for 6G mobile fronthaul. Using an injection-locked heterodyne detection scheme, we generated a 125 GHz-IF signal with a single-sideband phase noise of 3.2 degrees, which was sufficiently low for the demodulation of a 64 QAM data signal. By applying a digital pre-equalization scheme, we successfully transmitted an 18 Gbaud 64 QAM signal over a 10 km single-mode fiber and over 70 m wirelessly with a spectral efficiency of 5.71 bit/s/Hz.

We report a polarization-division-multiplexed (PDM) fully coherent optical-wireless transmission in the 125 GHz band for 6G radio access networks (RANs). A two-dimensional (2-D) I/Q equalizer based on a 2×2 complex transfer matrix was employed to compensate for waveform distortions caused by both imperfect frequency responses and IQ imbalance in the analog IQ mixer, which was used to down-convert the received intermediate frequency (IF) signal to baseband I/Q signals. Compared with a conventional one-dimensional (1-D) complex equalizer using a finite impulse response (FIR) filter, the 2-D I/Q equalizer enabled a lower crosstalk of 2.7 dB, which could increase the modulation multiplicity from 64 QAM to 128 QAM. By further employing PDM, we successfully demonstrated a 252 Gbit/s, PDM 18 Gbaud 128 QAM transmission over a 10 km single-mode fiber (SMF) and a 40 m wireless link.

We describe the operating mechanism of a photonic Ising machine (PIM) using ultrahigh-speed optical pulse transmission technology in an optical fiber loop from the perspective of the AM mode-locking of lasers. Although this PIM does not generate pulses like a conventional mode-locked laser, the theory of the AM mode-locking of a laser can be used to explain the operating principle of the present PIM when the feedback of coherent pulses is considered as equivalent gain. Specifically, we describe the existence of Gaussian-like steady-state pulses in the fiber loop of the PIM, the relationship between the recirculating pulse width and the spectrum, and the consistency with AM mode-locking theory as regards the optical filter width and pulse width. The time-bandwidth product of the pulses in the loop is approximately 0.44, and the waveform in the time domain closely matches that of a Gaussian pulse. Furthermore, the calculation of (Δf)¹/²Δτ, where Δf is the full width at half maximum (FWHM) of the optical filter and Δτ is the FWHM of the recirculating pulse width in intensity, revealed that it remains nearly constant, indicating that the pulses in the loop satisfy the relationship predicted by the AM mode-locking theory. In addition, an investigation of the bandwidth dependence of bifurcation switching showed that narrowing the bandwidth results in broader pulses, thereby reducing the peak value of the pulses and hence suppressing bifurcation switching caused by the Kerr effect. Finally, we investigated the auto stabilization effect of the pulse width and pulse amplitude caused by the gain saturation of the erbium doped fiber amplifier (EDFA) and showed that a very stable bifurcation can be achieved.

We present the design and numerical analysis of negative curvature antiresonant hollow-core fibers employing multiple hyperbola-arc cladding rims. Triangular, square, pentagonal, and hexagonal cores with three or four hyperbola-arc rims significantly reduce light leakage from the core, and the calculated confinement loss is less than 0.001 dB/km based on COMSOL simulations. The hyperbola-arc cladding rims extend linearly toward the outer glass jacket wall, and all rim edges converge to a single point at both ends of each cladding segment. This geometry behaves similarly to a nodeless tubular cladding and reduces confinement loss by eliminating nodal defects. Furthermore, since the hyperbola-arc curvature changes more gradually than that of conventional circular claddings, the incidence angle at the cladding boundary approaches grazing incidence, thereby increasing the Fresnel reflection back into the core compared with conventional tubular claddings. In addition, light leakage into the rim-edge regions between adjacent cladding segments is effectively suppressed thanks to an antiresonant wedge-shaped mirror effect. The proposed fiber structure is expected to be easier to fabricate than nested tubular hollow-core fibers because it does not require the precise positioning and fixation of many unsupported glass tubes in the preform.

We report a polarization-division-multiplexed (PDM) fully coherent optical-wireless transmission in the 125 GHz band for 6G radio access networks (RANs). A two-dimensional (2-D) I/Q equalizer based on a 2×2 complex transfer matrix was employed to compensate for waveform distortions caused by both imperfect frequency responses and IQ imbalance in the analog IQ mixer, which was used to down-convert the received intermediate frequency (IF) signal to baseband I/Q signals. Compared with a conventional one-dimensional (1-D) complex equalizer using a finite impulse response (FIR) filter, the 2-D I/Q equalizer enabled a lower crosstalk of 2.7 dB, which could increase the modulation multiplicity from 64 QAM to 128 QAM. By further employing PDM, we successfully demonstrated a 252 Gbit/s, PDM 18 Gbaud 128 QAM transmission over a 10 km single-mode fiber (SMF) and a 40 m wireless link.

We propose a digital-coherent Ising machine using complex I / Q signals that employs both amplitude and phase of optical pulses. Stable max-cut calculation without cut-value dips due to Kerr effect was performed with the proposed machine.

We propose a novel oscillator-less photonic Ising machine (PIM) in which continuous-wave (CW) light is injected into a synchronously AM-modulated passive fiber loop. Unlike conventional approaches relying on pulse laser sources, the proposed method generates steady-state pulses in the loop through the combined action of an optical filter and AM modulation. We show that the generated steady-state pulse width is uniquely determined by the filter bandwidth and modulation frequency, independent of the input waveform, thereby enabling stable PIM operation even with a CW input. Numerical analyses confirm that bifurcation and Ising computations are achievable with CW injection. Although a CW-PIM requires a longer buildup time with approximately twice the computation time of pulse-input PIMs, it ultimately achieves comparable max-cut performance. Furthermore, the circulating pulse characteristics under CW injection are analogous to the AM mode-locking of lasers, validating the applicability of established theoretical models. These results would establish CW-PIM as a cost-effective and practical alternative to pulse-input PIMs, eliminating the need for expensive ultrafast laser sources while maintaining high computational performance.

We show that bifurcation exists not only in the I channel but also in the Q channel, noting that the in-phase part I and the quadrature part Q of coherent quadrature amplitude modulation (QAM) communication are simply phase-shifted by 90 degrees from each other. To realize bifurcation in the quadrature, it is important to add an optical phase shift of π/2 and feed it back to the fiber loop as an imaginary amplitude. Then, we report that the bifurcation switching of I-bifurcation and Q-bifurcation occurs for different amounts of nonlinear phase rotation where cos φNL and sin φNL become zero, respectively. We also show that stable bifurcation exists where I and Q are coupled through the Kerr effect, where I changes Q and Q changes I. This simultaneous I/Q-bifurcation can be used to maintain a constant intensity and therefore, a constant phase when both are converging to stable bifurcation. This phenomenon can be used to realize a complex bifurcation machine, i.e., an I/Q-bifurcated photonic Ising machine, where a stable max-cut calculation without a cut-value dip can be performed even in the presence of the Kerr effect. We also report that a new phenomenon named “bifurcation splitting” exists when the Kerr effect has a strong influence on a 50 km-long single fiber-loop propagation.

We demonstrate a novel polarization-dependence-free injection-locking circuit with a lambda/4-phase-shift-free HR DFB LD. This circuit enabled us to achieve stable phase-locking for randomly polarization-modulated injection-light with a modulation-speed of 600 krad/s. Furthermore, a 240-Gbit/s-64 QAM transmission was demonstrated under a random polarization-rotation condition. (c) 2025 The Author(s)

We present the first demonstration of coherent Nyquist pulse TDM transmission in a fully digital scheme. 40-Gbaud, 16-QAM signals were transmitted over 4,000-km, which is 300-km longer than an NRZ transmission with a Nyquist filter.

A severe cold air outbreak hit the US and parts of Canada in January 2019, leaving behind many casualties where at least 21 people died as a consequence. According to Insurance Business America, the event cost the US about 1 billion dollars. In the Midwest, surface temperatures dipped to the lowest on record in decades, reaching −32 °C in Chicago, Illinois, and down to −48 °C wind chill temperature in Cotton and Dakota, Minnesota, giving rise to broad media attention. A zonal wavenumber 1–3 planetary wave forcing caused a sudden stratospheric warming, with a displacement followed by a split of the polar vortex at the beginning of 2019. The common downward progression of the stratospheric anomalies stalled at the tropopause and, thus, they did not reach tropospheric levels. Instead, the stratospheric trough, developing in a barotropic fashion around 70° W, turned the usually baroclinic structure of the Aleutian high quasi-barotropic. In response, upward propagating waves over the North Pacific were reflected at its lower stratospheric, eastward tilting edge toward North America. Channeled by a dipole structure of positive and negative eddy geopotential height anomalies, the waves converged at the center of the latter and thereby strengthened the circulation anomalies responsible for the severely cold surface temperatures in most of the Midwest and Northeast US.

We describe theoretical and experimental analyses of guided acoustic wave Brillouin scattering (GAWBS) noise in few-mode fiber (FMF). First, we theoretically show the mechanism of GAWBS noise generation through mode-coupling between different linearly polarized (LP) modes in FMF. We show that acoustic modes with wavenumbers along the fiber propagating axis contribute to the mode-coupling in the FMF. Next, we calculate the power spectra of GAWBS noise generated between identical LP modes, different LP modes, and degenerate LP modes in an FMF supporting six LP modes. We then measure the GAWBS noise spectra using a self-delayed heterodyne detection method and confirm good agreement with the calculated results. We find that GAWBS noise generated through mode-coupling between different LP modes exhibits reversibility with respect to the direction of the mode-coupling. Furthermore, we show that GAWBS phase noise generated between different LP modes is converted into intensity noise due to the different group velocities among LP modes in the FMF. We also clarify that the GAWBS noise coefficient is 3 to 4 dB higher than that in standard single-mode fiber (SSMF) since it experiences simultaneous mode-coupling from all LP modes. Finally, we evaluate the impact of GAWBS noise on digital coherent transmission with FMF using the Gaussian noise (GN) model and show that it can cause a substantial transmission distance penalty of as high as 27.7 ∼ 30.0%.

This paper reviews research on GAWBS noise in MCF and FMF. Theoretical and experimental analyses of GAWBS noise in these fibers are described, and the impact of GAWBS noise on SDM transmission is discussed.

We describe in detail the optical Kerr nonlinearity in our photonic Ising machine (PIM), which employs ultrahigh-speed optical pulse propagation in a lossless fiber loop. Although the peak power of the pulses in the fiber loop is set as low as ∼1 mW, the present PIM requires ultralong-distance pulse propagation of the order of 100,000 km (e.g., ∼2,000 circulations in a 50 km loop) to calculate large-scale optimization problems. As a result, a nonlinear phase rotation of greater than π/2 is accumulated due to the Kerr effect. This nonlinear phase rotation makes it possible to couple between the real (I) and imaginary (Q) parts of the recirculating optical pulse. Thus, as the amplitude of the I-channel changes due to the nonlinear phase rotation, the Q-channel also varies accordingly, and vice versa. We show that this mutual coupling gives rise to a new phenomenon, which we name Kerr-resonanced bifurcation switching, where the accumulated nonlinear phase rotation results in a periodic dip in the cut value of a max-cut problem. This dip phenomenon can be understood as a consequence of optical power peaking in a nonlinear optical fiber loop resonator with Kerr phase rotation. Finally, we propose a method for preventing dip generation by combining a large core fiber and a chirped fiber Bragg grating (CFBG) over a short length, which can reduce the Kerr-induced nonlinear phase rotation.

Recent advances in fully coherent mobile fronthaul transmission using an injection-locked heterodyne detection technique are described. A 108 Gbit/s-64 QAM transmission over a 10-km SMF and 10 m wirelessly is demonstrated at 30 GHz-IF.

We demonstrate a single-channel 108 Gbit/s fully coherent mobile-fronthaul transmission at 125 GHz-IF. By using a digital pre-equalization scheme, an 18 Gbaud-64 QAM signal was successfully transmitted over a 10 km-SMF and 70 m wirelessly.

We propose a large-scale photonic Ising machine (PIM) employing ultrafast pulse propagation in 50-km recirculating loss-less fiber loop and digital feedback nonlinearity without pulse oscillation. A 10(6)-node max-cut problem is solved with the PIM software.

We describe a new multiplexing technique and its application to demultiplexing in the time domain by using higher -order Hermite-Gaussian (HG) pulses, which are solutions of the Schrodinger equation. We call this technique eigen-function division multiplexing (EDM). This method enables us to further increase the total transmission capacity by superimposing many different HG pulses in the same time slot. This technique is different from a conventional optical time domain multiplexing (OTDM) technique using interleaving, where one pulse exists only in one time slot. The transmitted EDM HG pulses can be demultiplexed by adopting the time -domain orthogonality of the HG pulses (eigen-function orthogonality). The information carried by the mth-order HG pulse (HGm pulse) can be coherently detected by a photo detector, where photo -mixing with a phase -locked HGm pulse generated by a local oscillator can realize demultiplexing. The overlap integral with a different HG pulse becomes zero due to the time domain orthogonality. First, we show numerically that such a new EDM transmission scheme in the time domain is possible. We then show experimentally that we could successfully carry out an EDM HG coherent pulse transmission with four different HG pulses (HG0, HG1, HG2, and HG3), where we report a 400 similar to 480 Gbit/s (10 Gbaud x 4 eigen-functions x 2 pol-mux.) 32 similar to 64 QAM EDM transmission over 300 similar to 450 km.

We propose a new mode-division-multiplexing (MDM) technique in time-domain using higher-order Hermite-Gaussian pulses. 32-QAM, 450-km MDM transmission was successfully demonstrated with HG0, HG1, HG2, and HG3 pulses, where the time-domain orthogonality was used for demultiplexing.

We describe the GAWBS noise characteristics in few-mode fibers (FMFs). We found that the GAWBS noise is newly generated due to an interaction between different LP modes through longitudinally propagating acoustic waves.