High-speed yet cost-effective intensity-modulation direct-detection (IM-DD) technology with a multilane architecture serves as an important role for optical interconnects in future edge and datacenter networks, while space division multiplexing (SDM) is emerging as a promising technological candidate for multilane. However, fiber chromatic dispersion (CD) remains a critical performance barrier for IM-DD. Current single-lane CD mitigation approaches are facing bottlenecks in complexity. Joint coding/transmission across multiple lanes, namely pairwise transmission, is an efficient solution to overcome CD. This work is extended from our upgraded invited paper on ECOC 2025 and investigates pairwise transmission over SDM lanes in more detail. We first theoretically quantify the impact of inter-lane CD mismatch on pairwise SDM transmission based on phase-diversity coding, revealing that weakly-coupled multicore fiber (WC-MCF) with minimal inter-lane CD mismatch is beneficial to reduce the required SNR or equalizer complexity in pairwise SDM transmission. Based on the theoretical insights, extensive experiments of C-band IM-DD interconnects via pairwise SDM transmission are performed, including up-to-210Gbps/lane over 35.5km WC-MCF and HD-FEC-compliant 120Gbps/lane over 85.4km WC-MCF. A record per-lane bitrate-distance product among SDM IM-DD systems is achieved with simple pairwise transceivers and Rx-side linear-only equalizers.

We demonstrate the ultralow nonlinearity of HCF for high-speed THz data transport and stable LO distribution, successfully delivering 100-Gb/s data and stable LO signal in the 300-GHz band at launch powers up to 29 dBm over a 3.9-km PBG-HCF link.OCIS codes: (060.5625) Radio frequency photonics; (350.4010) Microwave

We investigated the performance of a fiber–terahertz system for different sideband-to-carrier ratios and UTC-PD operating regimes, and successfully generated and transmitted a 125-Gb/s THz signal in the 300-GHz band, even when the UTC-PD operated in the high-power region.

A novel pilot-aided quaternion retrieval technique for full-field optical system identification without coherent receivers is proposed and demonstrated experimentally in a 63.25-Gbaud dual-polarization 16-QAM system.

Chromatic dispersion (CD) remains a critical performance barrier in high-speed intensity-modulation direct-detection (IM-DD) fiber-optic transmission systems, including the de-facto standard multilane ones. Current single-lane CD mitigation approaches are facing bottlenecks in DSP complexity. Here we unveil a CD-barrier-breaking paradigm in which transceiving processes are jointly designed across multiple spatial or wavelength lanes, termed as "pairwise/groupwise transmission". It aims to be compliant with the multilane architecture, meanwhile preserving the simplicity of IM-DD. We first theoretically show one of the viable pairwise coding conditions (PCC) that can remove fading, a dominant part of the CD barrier. Next, an efficient optoelectronic implementation of pairwise transmitter (Tx) satisfying this PCC is presented. Furthermore, theoretical bit error rate of pairwise transmission is derived and discussed considering ideal or mismatched lanes and digital equalizers. In addition, we experimentally validate 224Gb/s (112Gb/s/lane) transmissions with various link configurations in the challenging 1.55-mu m band. In optical amplification (OA)-free cases, pairwise transmission enables 4-fold reach extension compared to conventional IM-DD only using low-complexity linear equalizers. OA-assisted 80 km transmission is also verified.

Mode-division multiplexing (MDM) may enhance the total capacity of free-space optical (FSO) communication links by transmitting independent data channels on orthogonal spatial modes. A photodetector (PD) array can be a promising MDM receiver architecture due to its compactness and reconfigurability. Previous PD array-based MDM receivers have primarily focused on demultiplexing one-dimensional (1-D) modal sets. In contrast, employing a two-dimensional (2-D) modal set-i.e., modes characterized by two spatial indices-may enable a larger number of orthogonal modes, thereby supporting more parallel data channels and higher link capacity. In this work, we experimentally demonstrate a reconfigurable PD array-based MDM FSO receiver capable of supporting 2-D spatial mode sets. The multiplexed data channels are demultiplexed and recovered using multiple-input multiple-output (MIMO) digital signal processing (DSP). We demonstrate MDM links employing different types and numbers of 2-D spatial modes, including (i) a 60-Gbit/s link multiplexing six 2-D Laguerre-Gaussian (LG) modes detected by ten PD elements, and (ii) a 40-Gbit/s link multiplexing four 2-D Hermite-Gaussian (HG) modes detected by seven PD elements. In both cases, all channels achieve bit-error rates (BERs) below the forward error correction (FEC) threshold of 3.8e-3. We further demonstrate the capability of the PD array-based receiver to mitigate atmospheric turbulence effects in a 40-Gbit/s link multiplexing four 1-D orbital-angular-momentum (OAM) modes. With turbulence-compensating MIMO DSP, the error vector magnitudes (EVMs) of all four channels are reduced to below 35%, whereas they exceed 50% without DSP-based compensation.

We extend the recently-proposed pairwise transmission to wavelength domain, enabling dispersion-tolerant WDM multilane IM-DD systems. C-band 110Gb/s/λ 80km and 200Gb/s/λ 30km SMF transmissions using commercial WDM Mux/DeMux and low-complexity digital equalizers are demonstrated.

A mobile free-space optical communications system is experimentally demonstrated based on a high-speed, high-sensitivity avalanche photodetector array with spatial diversity reception. Receiving power sensitivity of -3 dBm with 26.7° field-of-view is realized in the 2.1-m line-of-sight link at 10.7°/sec.

Terahertz (THz) signal processing is crucial for enabling key applications in the THz band. However, conventional electronics-based solutions face several limitations, largely due to the limited roll-off characteristics of electronic devices, which make multiplexing and demultiplexing densely THz signals challenging. Photonic technology offers unique advantages, including wide bandwidth, low loss, steep roll-off, and high flexibility. In this paper, we present a photonic approach for simultaneous THz signal reception, demultiplexing, and downconversion by converting multiplexed THz signals into the optical domain and leveraging advanced photonic signal processing. As a proof-of-concept demonstration, we validate the system for millimeter-wave signals in the W band and THz signals in the 300-GHz band, achieving aggregate capacities of 150 Gb/s and 220 Gb/s, respectively. We also discuss the applicability and outlook of the proposed system for key THz-band applications, including multiple radio access, multi-user access networks, and integrated access and backhaul. The proposed system can pave the way for THz communications in 6G and beyond.

A "pairwise transmission" technique is discussed for dispersion-limited multilane IM-DD systems. C-band optical-amplification-free 112Gb/s/lane over up-to-40km SMF (similar to 640ps/nm dispersion) with low-complexity digital equalizers is demonstrated, showcasing 4-fold reach enhancement beyond conventional IM-DD.

We present a photodetector-array-based reconfigurable mode-division-multiplexing coherent receiver assisted by MIMO DSP. Without an optical demultiplexer, we show a 60-Gbit/s FSO link multiplexing six spatial modes varying two indices. Turbulence mitigation is further demonstrated.

Recent advances in the carrier-less computational coherent detection technique based on phase retrieval (PR), called the PR coherent detection, are reviewed including its application for optical performance monitoring. The pilot-aided PR allows the characterization of the amplitude, the phase as well as the non-linear responses of coherent optical components only by a monitor photodetector.

This special issue contains 13 papers, of which 5 are invited, relating to hot topics in the area of optical networks, systems, and technologies for future radio access. These topics are gaining increasing importance in mobile network evolutions and related radio systems and could represent relevant elements of innovation in this evolution.

We demonstrate a pairwise transmission concept which breaks the fiber dispersion barrier in multilane IM-DD systems by joint optoelectronic coding across multiple lanes. Low-complexity C-band 224Gb/s (112Gb/s/lane) up-to-80km transmissions with different link configurations are investigated.

We demonstrate IM-DD transmissions of 200Gbps/lane over 35.5km multicore fiber (MCF) and 128Gbps/lane over 85.4km MCF in the challenging C-band based on a “pairwise SDM transmission” technique, achieving a record per-lane bitrate-distance product with a simple pairwise transceiver and Rx-side linear equalizers.

We experimentally demonstrated a mobile free-space optical link by a three-stage high-speed optical beam stabilizer and 2-D photodetector array based diversity reception. The spatial diversity reception by photodetector array and maximum-ratio combining based digital signal processing effectively mitigated the impacts of inevitable residual misalignment and wavefront distortion in a typical free-space optical system, which provided an 11.5°/s angular moving speed over a 2.1-m free-space distance with 40-Gbps data transmission. Compared with typical free-space optical systems, the proposed approach provided a wide field-of-view with mobility support, such as human support robots in future 6G indoor applications.

To enhance the resilience of network-cloud ecosystems, we establish a data governance framework for sharing optical testbed data across organizations and fostering machine learning research of optical networks. We further introduce multientity cooperation for efficient network-cloud recovery with open and policy-based information sharing among entities.

A direct-detection based simultaneous monitoring and modeling of I/Q-dependent frequency responses and Volterra-type high-order nonlinearities in an optical I/Q modulator is proposed and demonstrated experimentally in a 10-Gbaud 128-QAM system.

We demonstrate the first photonics-enabled THz signal demultiplexing and downconversion using an ultra-broadband TFLN modulator and a stable two-tone optical signal. Densely multiplexed THz signals in the 300-GHz band are successfully demultiplexed and downconverted to the 100-GHz band, achieving a line rate exceeding 220 Gb/s. (c) 2025 The Author(s)

We propose a new method for sub-THz signal demultiplexing and downconverting using photonic technology. We demonstrated demultiplexing and downconversion of three multiplexed signals with a total capacity of 150 Gb/s in the W band to prove the proposed method.