Utilizing free-running laser and passive envelope detector (ED)-based mmWave generation and detection, seamlessly converged fiber-mmWave transmission systems can be implemented to enable continuous flows of 6G-originated heterogeneous signals across different network segments (fiber and free space). However, due to the absence of intermediate signal distortion compensation and inherent nonlinear operations of the EDs, such transmission systems suffer from ED-induced signal-to-signal beating interference (SSBI) and accumulated signal distortions along the entire transmission link. To effectively address these challenges, this paper theoretically investigates and experimentally demonstrates the global linearization of a fiber-mmWave converged transmission system by employing an iterative SSBI mitigation technique of low complexity. The optimal number of iterations required for effective SSBI suppression is analytically derived, based on which comprehensive performance evaluations are undertaken experimentally of a ${\gt}3.556\,\,\text{Gb}/\text{s}$ transmission system comprising a 25 km fiber link and a 5 m, 36 GHz wireless link. Experimental results show that the optimal iteration number can be accurately estimated using key transmission system design parameters, including the carrier-to-signal power ratio, modulation index, and scaling factor employed in the iterative SSBI mitigation process. Furthermore, compared with Kramers–Kronig receivers, the iterative SSBI mitigation technique reduces the required multiplier and adder counts by ${\gt}44.4\%$ and ${\gt}53.7\%$, respectively, and eliminates the need for additional lookup-table memory, while still maintaining similar transmission performance and excellent mmWave frequency tunability, flexible bitrate variation, and adaptive mmWave radio coverage.

Future local area networks require seamlessly converging legacy fiber and future indoor optical wireless (OW) transmission systems, enabling continuous signal propagation across the entire fiber-OW links without intermediate optical-electrical/electrical-optical conversion and/or digital signal processing. As 850 nm is the most imperative wavelength for indoor OW applications, existing ITU-T G.652.D standard single-mode fibers (SSMFs) at such a wavelength support two propagation modes (LP 01 and LP 11 ), and the resulting differential mode delay restricts the achievable 3-dB bandwidths to <250 MHz·km. To effectively address the technical challenge, under strong mode-coupling conditions emulated by multiple cascaded optical connectors, this paper experimentally demonstrates, for the first time, a fiber-OW-converged transmission system incorporating 850-nm two-mode separators, achieving continuous 850-nm signal propagation across ∼1 km SSMF and 2.5 m OW links at bitrates of 23.5 Gbit/s. The two-mode separators are constructed using commercially available 1550-nm polarization controllers and optical couplers, offering effective LP 01 mode extractions with a purity of >99%. With such mode separators, the achievable 3-dB bandwidths of the fiber-OW-converged transmission systems are increased to 7.4 GHz, resulting in 29-fold bandwidth enhancements. Moreover, 850-nm signal propagation characteristics are thoroughly characterized experimentally and numerically in ∼1 km SSMF links with multiple cascaded optical connectors. The experimental and numerical results show that even a single additional optical connector can lead to relatively strong mode coupling, which can be further intensified by increasing the number of optical connectors.

Iterative SSBI mitigation-enabled global linearization is first experimentally demonstrated in 3.556Gb/s, 25km fiber-mmWave-converged systems, offering >51.4% DSP complexity reductions and similar transmission performances, mmWave frequency tuning ranges and radio transmission coverages, compared to Kramers-Kronig receivers.

Secure data transmission is experimentally demonstrated in a 1.67 Gb/s seamlessly converged fibre-millimeter wave (mmWave) network using the authors' newly proposed chaotic digital filter (CDF)-based physical layer security (PLS) technique. The CDF-based encryption/decryption operates by introducing noise-like, key-dependent phase variations to conventional filter impulse responses. Validation is performed in a seamlessly converged network comprising a 25 km standard single-mode fibre (SSMF) link and a 5m 36GHz mm Wave wireless link, utilizing cost-effective photonic-based mmWave generation and envelope detector-based reception. Experimental results show that the demonstrated PLS technique allows the encrypted signals to continuously flow between the fibre and radio frequency (RF) domains. The PLS technique also supports simultaneous optical and radio frequency access with almost identical BER transmission performances, and power penalties of <1 dB. To gain an in-depth understanding of the measured results, the CDFs' characteristics, including their chaotic nature, sensitivity to security keys and optimum CDF design parameters, are explored both theoretically and experimentally in detail. The optimum security key properties and CDF's filter lengths are identified, which are independent of the transmission media and major characteristics of the encrypted signals. The CDF-based PLS technique offers salient advantages of 'security-by-design', 'openness-by-design', 'dynamic security at the traffic level', and 'universal network compatibility'.

We demonstrate improved performance using active learning for both GPR and hybrid models to predict SNR using experimental data from a 15-channel WDM system over 1000km. Physical model interpreted GPR agrees with interpreting measured data.

Point-to-multipoint (P2MP) optical transceivers are promising for meeting the diversified requirements of emerging applications in the 6G era. However, for intensity modulation and direct detection (IMDD) passive optical networks (PONs), the previously reported P2MP optical transceivers cannot effectively accommodate a large number of diversified applications due to their limited flexibility, high digital signal processing (DSP) complexity and latency, as well as relatively high sensitivity to transmission system impairments. To cost-effectively address the challenges, this paper proposes a novel P2MP flexible optical transceiver based on a new parallel multi-channel aggregation/de-aggregation operation and an advanced extended Gaussian function (EGF)-based orthogonal digital filter bank. In comparison with the previously reported transceivers in a >50Gbit/s@25 km IMDD upstream PON with two optical network units (ONUs), our experimental measurements show that for channel counts of 8 (>10), the proposed transceivers can enhance the maximum achievable upstream transmission capacities by >52% (>290%). The new design can also reduce the overall transmitter DSP complexity (transmitter maximum DSP latency) by >70% (>24%) and decrease the receiver multi-channel de-aggregation DSP complexity by >40%. In addition, for the proposed transceivers, the network operation flexibility and scalability are also experimentally explored in a 25 km, >52Gbit/s IMDD upstream PON with five ONUs supporting 70 independent channels of different bandwidths and latencies. It is shown that the proposed transceivers fully support flexible sleep-awake ONU operation, and for a specific ONU, doubling the activated ONU count can only increase its required minimum received optical powers by ∼1.7 dB. Furthermore, the proposed transceivers can also support the ONU count growth without considerably compromising the maximum achievable upstream transmission capacities.

To cost-effectively meet 6G latency requirements, concurrent upstream and direct inter-optical network unit (ONU) communication passive optical networks (PONs) based on flexible point-to-multipoint (P2MP) optical transceivers and intensity modulation and direct detection (IMDD) have been reported to enable direct communications among different ONUs within the same PON without passing data to the optical line terminal (OLT). However, the previously reported P2MP transceivers suffer from high DSP complexity for establishing ultra-dense connections. For such application scenarios, the PON’s remote nodes also have high inter-ONU signal power losses. To effectively solve these technical challenges, this paper experimentally showcases (a) new P2MP transceivers by utilizing parallel multi-channel aggregation/de-aggregation and advanced extended Gaussian function (EGF)-based orthogonal digital filter banks, along with (b) low inter-ONU signal power loss-remote nodes. By introducing these two techniques into a 27 km, >54.31 Gbit/s concurrent upstream and direct inter-ONU communication IMDD PON, comprehensive experimental explorations of the PON’s performances were undertaken for the first time. The remote node is capable of supporting 128 ONUs. The results show that the new P2MP transceivers lead to >75% (>40%) reductions in overall transmitter (receiver multi-channel de-aggregation) DSP complexity, and they can also equip the PONs with an enhanced capability of providing ultra-dense connections. The experimental results also show that the PON allows each ONU to flexibly change its upstream and inter-ONU communication channel count without considerably compromising its performance. Therefore, the PON outperforms those of previously reported works in terms of ensuring low DSP complexity, highly robust transmission performance, and enhanced capabilities of flexibly accommodating numerous applications with diverse requirements regarding traffic characteristics, thus making it suitable for ultra-dense connection application scenarios.

Employing free-running laser/envelope detection-based millimeter wave (mmWave) signal generation/detection at remote radio heads (RRHs)/user equipment (UE) offers a cost-effective solution for seamlessly integrating existing intensity modulation-direct detection (IM-DD)-dominated optical access networks and wireless networks. Such fiber-wireless convergence enables a continuous flow of signals with varying characteristics between the baseband unit (BBU) and UE across fiber and wireless network segments without the need for optical-electrical-optical (O-E-O) conversions and digital signal processing (DSP) at intermediate nodes. In this paper, we extensively investigate the performance of such a fiber-wireless converged access network employing free-running laser/envelope detection-based mmWave generation/detection in an IM-DD-based 1.67 Gbit/s transmission system with 25 km standard single-mode fiber (SSMF) and 5 m @38 GHz mmWave wireless links. Experimental results demonstrate that both mmWave frequency tunability and adaptive mmWave network coverage are achievable by just dynamically and adaptively configuring the output wavelength and power of the RRH-embedded free-running laser. Additionally, envelope detection allows RRHs to use low-cost MHz-linewidth-level free-running lasers while maintaining excellent performance stability.

We introduce a sensing technique that tracks polarisation states from the equaliser tap-weights within an Alamouti-coded simplified coherent receiver used for PONs. Experimental results demonstrate downstream transmission at 100 Gb/s/lambda with a loss budget exceeding 35 dB alongside vibration sensing capabilities. (c) 2025 The Author(s)

For implementing next-generation radio access networks (NG-RANs) supporting services/applications in the beyond-5G (B5G) era, seamless fiber-wireless network convergence is vital for enabling heterogeneous signals of various characteristics to continuously flow between the optical and electrical domains, i.e., the baseband unit (BBU) and user equipment (UE), without optical-electrical-optical (O-E-O) conversions or digital signal processing (DSP) at any intermediate nodes. To address such challenges, this paper proposes and experimentally demonstrates, for the first time, a cost-effective fiber-wireless converged flexible and dynamic access network based on intensity modulation and direct detection (IM-DD). The demonstrated network utilizes O-E-O conversion-free Soft-reconfigurable optical add/drop multiplexers (Soft-ROADMs) at remote nodes to dynamically establish connections between the BBU and the remote radio heads (RRHs) at the sub-wavelength level. In addition, free-running laser-enabled photonic millimeter-wave (mmWave) signal generation and passive electrical envelope detector-enabled mmWave down-conversion are also used, respectively, at the RRHs and UEs to achieve mmWave frequency tunability and adaptive wireless network coverage. The network performance and optimum network configuration are experimentally explored extensively in a fiber-wireless converged access network with 3 x 1.333 Gbps dynamic BBU-UE connections over a 10 km IM-DD fiber link and a 5 m, 38 GHz mmWave wireless link. The results show that wide mmWave frequency tuning ranges and adaptive mmWave coverages are achievable by just adjusting the RRH-laser frequency and output powers.

We demonstrate 200 Gb/s bidirectional coherent PON solutions using a simplified optical network unit (ONU) over 19 km of field-installed fiber. The ONU receiver is a single-polarization heterodyne detector with either a balanced or a single-ended photodetector and the transmitter is based on dual-polarization electro-absorption modulated laser (EML). The optical line terminal (OLT) uses standard dual-polarization coherent transceivers. The downstream solutions achieve 37 dB and 30.5 dB power budget for the receiver with balanced and single-ended photodiodes, respectively. For the upstream, two line rates have been investigated: 200 Gb/s/lambda for symmetrical transmission and 100 Gb/s/lambda for asymmetric transmission achieving a power budget of 30.1 dB and 40.9 dB respectively.

We demonstrate 200 Gb/s/λ bidirectional coherent PON solution with simplified ONU on field installed fiber. We achieve 30.5/37 dB power budget for the downstream transmission with single-ended/balanced photodiode and 30.1 dB for the upstream transmission.

In order to meet the diverse needs of different services and applications in the B5G era, seamless integration of fiber and wireless network segments of next-generation radio access networks (NG-RANs), with flexible/fine channel switching granularity and ultra-low latency, is vital for enabling dynamic and continuous flow of heterogenous signals of various characteristics across different network segments without optical-electrical-optical (O-E-O) conversions or digital signal processing (DSP) at intermediate nodes. To address these requirements cost-effectively, we experimentally demonstrate 3xl.333 Gbps flexible BBU-UE connections over 10 km standard single-mode fiber (SSMF) and adaptable millimeter-wave (mmWave) wireless reach up to 50 m, using free-running laser/envelope detection-based tunable mm Wave generation/detection, and soft-ROADM-enabled dynamic sub-wavelength-level channel switching. The proposed technique requires no O-E-O conversions or DSP at intermediate nodes and offers dynamic RRH-controllable mmWave transmission with large frequency tunability and adaptive wireless network coverage.

Driven by a large number of emerging diversified services, in the 5G and beyond era, concurrent direct inter-ONU and upstream communications inside a PON-based mobile access network are highly desirable to provide dynamic, ultra-dense, and fast ONU-to-ONU (without involving an OLT) and ONU-to-OLT connections. To cost-effectively deliver highly dynamic and low latency direct inter-ONU communications, this paper proposes and experimentally demonstrates novel concurrent direct inter-ONU and upstream communications in an upstream 27 km, >62.47 Gbit/s IMDD PON. For supporting inter-ONU communications between a large number of ONUs, an advanced passive remote node is also proposed. Based on different passive optical components, this remote node can be implemented using two approaches, which can, respectively, reduce the inter-ONU signal power losses by >12.2 dB and >16.6 dB (for 128 ONUs) in comparison with existing inter-ONU communication techniques’ remote nodes. In each ONU and OLT, a single pair of cascaded IFFT/FFT-based point-to-multipoint (P2MP) flexible optical transceivers are employed to simultaneously and dynamically establish multiple ONU-to-ONU and ONU-to-OLT communications according to actual users’ requirements. Experimental results show that the proposed network has excellent robustness against various transmission system impairments, including chromatic dispersion, the Rayleigh and Brillouin backscattering effects, and the channel interference effects. For each ONU, dynamic channel allocation can be made without compromising its overall performance.

Recently several machine learning methods have been proposed to estimate the SNR, based on launch data and other system factors. These data-driven methods typically require a large number of datasets for training and generally are not interpretable. In this paper, we propose an alternative approach that requires less data and is interpretable, specifically a hybrid algorithm combining a physical model with Gaussian process regression. We develop a measurement-informed physical model, systematically reducing the number of independent parameters based on the underpinning physics and improve the overall performance of the physical model marginally. The model is validated using measurements performed on a 15-channel wavelength-division multiplexed system propagating over 1,000 km of standard single-mode fiber. The proposed hybrid model is not only interpretable but also obtains better agreement with measurements than a Gaussian process regression model and a simple neural network model for a given number of training datapoints.

Digital coherent transceivers have developed to the stage that they can monitor the physical state of an optical network and thus are capable of generating data to build measurement-informed physical layer models. After reviewing the measurement capabilities of coherent transceivers, we discuss different modeling approaches including physics-based models, data-driven models as well as hybrid models that incorporate elements of both physics-based and data-driven models. Having reviewed both the measurement capabilities and the modeling methodologies, the salient features of building digital twins based on measurement informed models for optical fiber communication systems are discussed.

This paper provides an overview and recent advancement of emerging technologies including transceivers, flexibility features, optical sensing and physical layer security for next-generation passive optical networks (PON).

A simple single-polarization heterodyne optical network unit (ONU) receiver with Alamouti-coding based polarization diversity at the optical line terminal (OLT) side is a promising solution for passive optical networks (PON) beyond 50 Gbps. The special equalizer required for data recovery of such system has been demonstrated for 200 Gbps PON downstream using offline processing of serial data. In this letter, we extend such equalizer implementation with parallel processing that is required for real-time implementation. The proof-of-concept experiment for 200 Gbps downstream transmission is demonstrated to show the effectiveness of the proposed digital signal proccesing (DSP). The impact of phase noise and limitation of degree of parallelism are further investigated via numerical simulations.

We propose a data-driven erbium-doped fiber amplifier (EDFA) gain model utilizing Gaussian process regression (GPR). An additive Laplacian and radial-basis function kernel is proposed for the GPR and was found to outperform deep neural network (DNN) methods while additionally providing prediction uncertainty. Performance is measured using mean absolute error (MAE) averaged across five different EDFAs with three manufacturers. The GPR achieves an MAE of 0.1 dB using 30 training samples in contrast to the DNN that achieves an MAE of 0.25 dB using 3000 training samples. Additionally, we demonstrate that active learning can be used to improve robustness and repeatability of convergence.