The growing demand for high-capacity, long-distance terrestrial and ground-to-space communication applications highlights the limitations of conventional transceivers, which are primarily designed for controlled data-center environments. To address the unique challenges of free-space optical communication, particularly wavefront distortions induced by atmospheric turbulence, we present and experimentally demonstrate a 4 & times; 4 array of coherent receivers integrated on a CMOS-compatible silicon photonics (SiPh) platform. This receiver array performs spatial sampling of the incoming distorted wavefront, enabling enhanced signal recovery in turbulent and low-power conditions. Central to our signal processing approach is the maximum-ratio combining (MRC) algorithm, specifically designed to optimally combine spatially sampled signals and effectively mitigate turbulence-induced phase and amplitude distortions. The system is experimentally validated under moderate atmospheric turbulence conditions using binary phase-shift keying (BPSK) modulation, achieving nearly theoretical combining efficiency, restoring a clean eye diagram at -37.95 dBm average received power and a BER improvement from 1E-2 (single receiver) to 1E-15 (16 receivers with MRC).

We propose a novel GSNR-aware spectral slot and inter-band wavelength converter (IWC) assignment algorithm that improves SCL-band network utilization, achieving 38% higher network capacity with IWCs versus CL-band network, and only 8% improvement without IWCs.

We discuss various techniques to optimize the performance of probabilistic and geometric constellation shaping for linear shaping gain and improved tolerance to fiber Kerr nonlinearity. These techniques are practical with reduced implementation complexity.

We implemented an array of 4x4 coherent receivers on a CMOS-compatible SiPh chip which can spatially sample distorted wavefronts. The spatially sampled coherent fields are combined using maximum ratio combining to compensate for turbulence-induced wavefront distortion, achieving nearly theoretically-limited combining efficiency. (c) 2025 The Author(s)

We propose an efficient and accurate numerical solver for estimating power profiles in UWB transmission systems with arbitrary bidirectional signal and Raman pump configurations. A significant speedup and estimation error below 1 % is achieved.

Four-dimensional (4D) constellations are optimized using geometric shaping (GS) with orthant symmetry (OS) over realistic optical channel models targeting next-generation single-span systems. The optical fiber is modeled via the Manakov equation and is implemented using the split-step Fourier method (SSFM) in a Monte-Carlo based approach. Additional noise sources and losses are added making the link model relevant in practice. Constellations are optimized for different cardinalities, forward error correction code rates, and symbol rates. The SSFM-optimized constellations are reported to offer up to 4% in reach increase with respect to conventional quadrature amplitude modulation (QAM). Comparisons against existing additive white Gaussian noise (AWGN)-optimized constellations show that SSFM-optimized constellations with OS generally negligibly outperform AWGN-optimized counterparts. This somewhat unexpected result leads to the conclusion that AWGN-optimized constellations are a good choice for realistic single-span optical links.

We propose anomaly detection based on power profile estimation in bidirectional transmission system. We demonstrate anomaly localization of 0.5 dB loss at any location along 80 km span in band-wise bidirectional C+L band transmission.

We propose band-wise bidirectional SCLU transmission. By counterpropagating S- and U-band signals and optimizing launch powers, we achieve maximum throughput that is 1.5% lower compared to aggregate throughput of individual band transmission.

We propose machine learning based 2nd order backward Raman amplifier design using autoencoder and a two-step training process. We demonstrate 0.18 dB RMS error for target Raman gain profile of 10 dB in C+L band transmission. (C) 2024 The Author(s)

Digital-coherent-receiver-based fiber-longitudinal power profile estimation (PPE) over multiple spans is presented. We then review three specific examples of applications of photonics tomography based on PPE and discuss the comparison between hardware and software implementation.

We show that subcarrier symbol rate optimization and better laser frequency stability can maximize transmission reach of super-channels. Configurations with 1.6Tb/s subcarriers achieve longer reach with benefits of smaller size/power and easier management.

We review our correlation based technique for fiber longitudinal power profile estimation which is a key for photonics tomography. Then, we discuss several applications based on this technique, anomaly loss monitoring, polarization dependent loss (PDL) monitoring, fiber-type identification, and nonlinear SNR estimation for optimizing quality of transmission. We show that these applications can detect the position of 3dB anomaly loss or PDL, and differences between deployed and designed fiber types in a multi-span transmission testbed. In addition, we discuss implementation of this technique in hardware and in software.

We demonstrate the nonlinear SNR estimation based on longitudinal power profile obtained with coherent receiver. The estimation error is less than 0.6 dB in WDM transmission over hybrid Raman-EDFA link.

Four-dimensional (4D) constellations with up to 131 072 points (17 bit/4D-sym) are designed for the first time using geometric shaping. The constellations are optimized in terms of mutual information (MI) and generalized MI (GMI) for the additive white Gaussian noise (AWGN) channel, targeting a forward error correction (FEC) rate of 0.8 at finite signal-to-noise ratios. The presented 15–17 bit constellations are currently the highest-performing constellations in the literature, having a gap to the AWGN capacity as low as 0.17 dB (MI) and 0.45 dB (GMI) at 17 bit/4D-sym. For lower cardinalities, our constellations match or closely approach the performance of previously published optimized constellations. We also show that (GMI-)optimized constellations with a symmetry constraint, optimized for a FEC rate of 0.8, perform nearly identical to their unconstrained counterparts for cardinalities above 8 bit/4D-sym. A symmetry constraint for MI-optimized constellations is shown to have a negative impact in general. The proposed procedure relies on a Monte-Carlo-based approach for evaluating performance and is extendable to other (nonlinear) channels. Stochastic gradient descent is used for the optimization algorithm for which the gradients are computed using automatic differentiation. This article is part of the theme issue ‘Celebrating the 15th anniversary of the Royal Society Newton International Fellowship’.

We propose a power profile estimator using MMSE, that automatically adjusts the scaling and nonlinear rotation of constellation with complex scaling factor. We demonstrate robust performance in simulation and experiment, even at higher launch powers.

Four dimensional geometric shell shaping (4D-GSS) is introduced as an approach for closing the nonlinearity-caused shaping gap. This format is designed at the spectral efficiency of 8 b/4D-sym and is compared against polarization-multiplexed 16QAM (PM-16QAM) and probabilistically shaped PM-16QAM (PS-PM-16QAM) in a 400ZR-compatible transmission setup with high amount of nonlinearities. Reach increase and nonlinearity tolerance are evaluated in terms of achievable information rates and post-FEC bit-error rate. Numerical simulations for a single-span, single-channel show that 4D-GSS achieves increased nonlinear tolerance and reach increase against PM-16QAM and PS-PM-16QAM when optimized for bit-metric decoding (R-BMD). In terms of R-BMD, gains are small with a reach increase of 1.7% compared to PM-16QAM. When optimizing for mutual information, a larger reach increase of 3% is achieved compared to PM-16QAM. Moreover, the introduced GSS scheme provides a scalable framework for designing well-structured 4D modulation formats with low complexity.

We evaluate longitudinal power profile estimator based on coherent receiver in WDM configurations and identify the impact of nonlinear rotation of constellation caused by copropagating neighboring channels. We also review prototype of optical tomography.

We demonstrate optical network tomography software prototype for localization of anomaly loss and fiber-type identification based on the estimated optical power profile using experimentally measured waveform data in a multi-span transmission testbed.

We propose, simulate, and preliminarily demonstrate experimentally optically-pumped SOAs which achieve high gain, high saturation output power, and low noise figure spanning entire C+L bands.

We discuss performance and implementation aspects of geometric and probabilistic constellation shaping, which can be optimized for different applications. We show that symbol rate optimization with probabilistic shaping can further improve reach/capacity.