A new variant of staircase codes, termed block-wise staircase codes, is proposed for high-throughput hard-decision forward error correction in optical communication systems. By incorporating block-wise concatenation into the staircase structure, the proposed construction enables the use of stronger component codes with larger error-correcting capability, thereby significantly reducing the probability of miscorrections and improving overall error-rate performance. The block-wise structure also preserves an error-confinement property, which enables an odd-even One-Step-Ahead-aided (OSA-aided) iterative hard-decision decoding algorithm to further suppress dominant stall events without a noticeable increase in decoding complexity. Closed-form analytical expressions are derived to characterize the error-floor behavior of the proposed codes, enabling accurate error-floor estimation without time-consuming Monte Carlo simulations. Performance evaluations show that, at a target output bit error rate of 10$^{-15}$, the proposed block-wise staircase code achieves a net coding gain of 9.54 dB, corresponding to a 0.13 dB improvement over the conventional staircase code, while the proposed odd-even OSA-aided decoding algorithm further increases the net coding gain to 9.60 dB. In addition, the proposed scheme exhibits a steeper waterfall region and a lower error floor than conventional staircase codes, while maintaining comparable decoding complexity and latency.
A polarization-maintaining Yb-doped crystal-derived silica fiber (PM-YCDSF) was fabricated, exhibiting a gain coefficient exceeding 5.5 dB/cm at 1030 nm, a birefringence of 6.31×10−5, and a polarization extinction ratio (PER) variation of only 2.1 dB during the temperature range of 20–200 °C. Using only 0.6 cm of active fiber, a short-cavity distributed Bragg reflector (DBR) single-frequency fiber laser (SFFL) was constructed. The laser achieved a slope efficiency of 65% and a PER of 31.5 dB. Its output power is up to 672 mW. To the best of our knowledge, this is the first DBR SFFL exceeding 670-mW linearly polarized output with only 0.6-cm active fiber. Furthermore, the laser exhibited a threshold power as low as 3 mW and an optical signal-to-noise ratio exceeding 80 dB. The power fluctuation in the mode-hop-free operation was below 0.17% (RMS) over 570 min, and the relative intensity noise stabilized at approximately −159.8 dBc/Hz above 8.8 MHz. Additionally, by employing a homemade all-fiber quarter-wave plate, a circularly polarized light with an ellipticity angle of approximately 43.58° could be maintained during the 120 min. And its output power also reached 583 mW. These results show that the PM-YCDSF is a promising gain medium for the compact and high-power-density SFFL sources applied in atomic physics, quantum sensing, biomedical imaging, precision measurement, and beyond.
Digital twin (DT) technology has emerged as a cornerstone for autonomous optical networks; however, its predictive fidelity is critically dependent on the accuracy of the underlying physical parameters—particularly the fiber attenuation, component insertion losses (ILs), and amplifier gain profiles. These parameters are often impractical to measure directly in field-deployed systems. To address this challenge within practical constraints, we propose a two-stage, data-driven multi-parameter self-refinement framework for C+L-band transmission systems, capable of jointly refining fiber parameters and erbium-doped fiber amplifier (EDFA) gain spectra without requiring additional monitoring hardware, intrusive link reconfigurations, or service disruption. In the first stage, a neural network is constructed to learn the mapping between variations in fiber parameters (attenuation coefficient and ILs) and the corresponding changes in stimulated Raman scattering (SRS)-induced band powers measured by built-in photodetector at each EDFA, enabling fiber parameter refinement. The second stage extracts an equivalent optical multiplex section (OMS)-level EDFA gain spectrum via a bisection-based iterative process, utilizing the received power profile (RPP) difference monitored by optical channel monitor (OCM) at the reconfigurable optical add-drop multiplexer (ROADM) node. Comprehensive simulations on an OMS C+L-band system are conducted, and the results show that the proposed method reduces the generalized signal-to-noise ratio (GSNR) estimation error to a mean absolute error (MAE) of 0.10 dB and a maximum absolute error of 0.14 dB. The calibrated DT model demonstrates strong generalization capability, maintaining accuracy across 100 heterogeneous test links with diverse parameter sets, under varying launch power profiles, and in dynamic loading scenarios including partial channel cuts and full L-band shutdowns. Furthermore, experimental validation on a three-span C+L-band transmission system confirms the method's practical effectiveness, achieving GSNR MAE of 0.18 dB and a maximum absolute error of 0.23 dB after full refinement. The proposed framework established a viable pathway toward self-calibrating DT that maintain high fidelity throughout the network lifecycle. By achieving accurate parameter estimation using only existing measurement infrastructure, it proved potential applicability in current optical networks within measurement contraints.
Fiber Kerr nonlinearity remains a major factor limiting the transmission performance of long-haul coherent optical communication systems. Although digital subcarrier multiplexing (DSCM) provides improved processing flexibility and enhanced tolerance to transmission impairments, effective nonlinear equalization is still required in long-haul wavelength-division-multiplexed (WDM) systems, where both compensation capability and computational complexity are critical concerns. In this paper, we propose a DSCM-oriented staged nonlinear equalization framework, termed Synergistic SAGE, for long-haul coherent optical transmission. The proposed scheme combines a first-stage subcarrier-aware gated equalizer (SAGE) for dominant structured nonlinear compensation with a second-stage selective-update decision-directed least-mean-square (SU-DDLMS) equalizer for adaptive tracking of residual slowly varying impairments. Experimental results in a seven-channel 64-GBaud 16-QAM WDM system over 1600-km standard single-mode fiber show that the proposed framework consistently outperforms conventional DBP-based baselines under 2-, 4-, and 8-subcarrier configurations. Among the investigated subcarrier configurations, the 4-subcarrier case provides the best overall performance, achieving a 0.56-dB $Q^{2}$-factor improvement over chromatic-dispersion compensation (CDC) at 1600 km and extending the transmission reach by up to 700 km relative to the single-carrier baseline at the 20% forward error correction threshold. In addition, Synergistic SAGE surpasses DBP-32 while reducing the computational complexity by 98.4%. These results indicate that the proposed staged framework provides an effective and low-complexity solution for nonlinear equalization in future long-haul DSCM coherent optical systems.
Two dimensional (2D) optical wireless communications (OWC) with a large-angle coverage are eagerly being pursued by wireless and mobile applications of internet of things, high-resolution image/video transmission, 6G/beyond wireless communications and network, remote conference and diagnosing, etc. Silicon photonics based optical phased array (OPA) can provide the large-angle beam scanning for OWC, benefitting from the near-half-wavelength OPA antenna spacing. However, the coupling loss of the laser source to the OPA chip is too high, meanwhile the sensitivity for state-of-the-art OWC receiver is too poor to meet the practical requirements for OPA-based OWC. The additional optical amplification is inevitably required to enhance the transmitted power and the received power of the OWC system. In this work, we present the large beam-scanning angle of OPA by simultaneously phase-controlling multi-wavelength beams. A compact wavelength-division-multiplexing (WDM) emitter provides an efficient and broadband source of 1270 nm-1610 nm to trigger parallel emissions off the OPA grating antennas, covering the whole optical communication band. Furthermore, we enhance the sensitivity to -40 dBm for OWC using a highly sensitive avalanche photodiode without additional optical amplification. Finally, the 2D OPA-based OWC with the coverage angle of 48°×50° is achieved by actively controllable phase tuning of WDM-radiated 18 beams.
With the goal of evaluating connectivity, we propose a novel method to measure the rotation angles of multi-core fibers (MCFs) based on the guide hole (GH) centers at the end faces of a multi-fiber push-on (MPO) connector. The proposed method derives the rotation angle by processing the image of the fiber core as captured by a microscope. We first clarify the angular resolution required to adequately evaluate the connectivity of MCF connectors and then describe the procedure for obtaining the rotation angle using image processing. To capture the two GH centers and fiber cores with high resolution and high accuracy in an MPO connector with an oblique end face, we use focus stacking and image concatenation. We calculate the rotation angles by core recognition utilizing template matching. The results of measuring the relative rotation angle of paired MPO connectors holding 12 weakly coupled 4-core fibers through repeated experiments confirm that our proposed method attains the required measurement accuracy of 0.1 degrees. We also show that our approach is effective for connector inspection in terms of connection loss and for ensuring reliable connectivity among multi-fiber MCF connectors.
Spectral beam combining is an effective approach to overcome the single-fiber output power limitation of fiber lasers. For spectral beam combining systems, high-performance narrow-linewidth fiber laser sources play a crucial role in determining the output power, combining efficiency, beam quality, thermal management, and compactness. In spectral beam combining systems, a widely adopted scheme for narrow-linewidth fiber lasers is to broaden the spectrum of a single-frequency seed through phase modulation, followed by power amplification of the phase-modulated seed. This approach offers strong capability in spectral control while maintaining a narrow linewidth. In this work, the modulation waveform is optimized to reduce the spectral power density. In addition, a low-NA large-mode-area gain fiber is adopted in the main amplifier stage to further mitigate stimulated Brillouin scattering. Experimentally, a 5 GHz-level narrow-linewidth fiber laser with an output power of 3 kW is achieved. The beam quality reaches M2 = 1.26/1.21, and no transverse mode instability is observed. To the best of our knowledge, this represents the highest output power reported for a 5 GHz-level narrow-linewidth fiber laser within the full gain bandwidth of ytterbium-doped fiber. The results of this work are expected to provide a higher performance laser source for spectral beam combining systems.
As passive optical networks (PONs) evolve toward 200 Gb/s to support bandwidth-intensive services, intensity modulation with direct detection (IM-DD) faces challenges from limited optoelectronic bandwidth and chromatic dispersion. While coherent detection offers superior performance, its high hardware complexity is not suitable for cost-sensitive optical network units (ONUs). In this paper, we propose and experimentally demonstrate a linear phase-diverse direct detection (LPD-DD) receiver architecture without signal-signal beat interference (SSBI) for 200G PON. The proposed scheme employs three carrier phase-switching branches with a narrowband phase-shift configuration (i.e., θ, 0, and -θ). We experimentally demonstrate that LPD-DD receiver achieves a 35-dB optical power budget over 20-km standard single-mode fiber (SSMF) with 240-Gb/s 16-QAM signals at the 15% soft-decision forward error correction (SD-FEC) threshold. The proposed receiver requires only low-complexity linear DSP and operates at an optimal CSPR of 0 dB. Furthermore, to facilitate the practical implementation of low-cost ONUs, we theoretically investigate feasibility of realizing narrowband phase shifts using integrated silicon photonic all-pass filters (APFs) based on racetrack resonators and Mach-Zehnder interferometer (MZI)-coupled micro-ring resonators (MRRs). The LPD-DD architecture provides a high-speed and cost-effective solution for next-generation very-high-speed PON (VHSP) evolution.
We propose two fully digital frequency offset estimation (FOE) techniques, a frequency-window method and a pilot-tone approach, for coherent time-division multiplexing (TDM) passive optical networks (PONs), enabling ultra-wide carrier frequency offset (CFO) estimation and compensation. The proposed methods support CFOs up to ±20 GHz, significantly exceeding the ±3 GHz limit of conventional 4th-power-based FOE. Experimental demonstrations using super-rated 30 GBd DP QPSK and DP-16QAM signals confirm stable downstream continuous-mode and upstream burst-mode transmission over a 50 km optical distribution network with minimal performance penalty under large CFO conditions. Both methods operate entirely in the digital domain and require no additional hardware, facilitating the use of low-cost, frequency-unstable lasers and providing a practical pathway toward cost-efficient high-speed coherent PON deployment.
Graphene stands as an ideal membrane material for fiber-based Fabry-Perot (FP) microelectromechanical systems (FP-MEMS), courtesy of its exceptional properties—including atomic-scale thickness, ultralow mass density, and compatibility with fiber-integrated fabrication processes. However, its practical implementation has been fundamentally hindered by inadequate hermeticity: specifically, the persistent challenge of achieving reliable diaphragm sealing, particularly for large-area graphene membranes. Here, we report the first hermetically sealed, high-sensitivity graphene-based FP sensor arrays, which overcome this long-standing limitation through a pre-tension-assisted transfer technique combined with laser-assisted hermetic sealing. The device demonstrates an ultrahigh pressure sensitivity of 487 nm/kPa while maintaining perfect hermeticity—a performance combination that has hitherto remained unattainable in practical flow-sensing applications. When integrated into a five-hole aerodynamic probe and paired with a custom-developed demodulation system, the device achieves a pressure resolution of 0.049% full scale (F.S.) over the range of 0–3000 Pa and has been fully calibrated for ±30° vector flow fields in a subsonic wind tunnel. The probe successfully captures two-dimensional vector dynamics during fan oscillations and within cylinder wake flows, thereby validating its capability for real-world aerodynamic measurements. This work addresses the critical sealing bottleneck in graphene optical fiber sensors and paves a new pathway for ultra-sensitive fiber-optic probes in extreme-environment exploration.
A cascaded single-mode-multimode-hollow-core fiber (SMF-MMF-HCF) optical sensor based on Fabry-Pérot interferometer (FPI) is proposed for ultra-sensitive copper ions (Cu(II)) detection. The cascaded structure provides an unfilled-air reference cavity and an air-material hybrid sensing cavity with slightly detuned free spectral ranges, and by precisely tailoring their lengths, the Vernier effect is generated to intrinsically amplify the sensitivity. The terminal HCF section is partially loaded with an ion-imprinted polymer to provide specific Cu(II) recognition. The specific binding of Cu(II) changes the effective optical path of the material section, thereby inducing a distinct spectral shift of the Vernier envelope. Experimental results demonstrate that the sensor exhibits an ultra-high average sensitivity of -66 nm/μM and a remarkably low limit of detection (LOD) of 0.98 nM within a broad dynamic range. Furthermore, the sensor shows exceptional selectivity against interfering ions and excellent reusability. The practical feasibility of the sensor was validated by successfully detecting Cu(II) in tap water and biological fluid samples with high recovery rates, confirming its reliability against complex matrix interference and highlighting its great potential for trace heavy metal monitoring in environmental and biomedical applications.
Co-time co-frequency full-duplex (CCFD) doubles spectral efficiency for next-generation wireless networks, but its deployment is fundamentally limited by strong multipath self-interference (SI), which conventional electrical cancellation schemes cannot address due to narrow bandwidth and poor tunability. Some microwave photonic solutions, while offering ultra-wideband advantages, are restricted by their dependence on optical filters for phase tuning, limiting operating bandwidth and operational robustness. This work presents a filter-free wideband microwave photonic phase shifter using a dual-polarization dual-parallel Mach-Zehnder modulator (DP-DPMZM), which realizes continuous 360° phase tuning over 2–40 GHz through DC-bias control. Building on this core module, we construct a correlated-reference dual-path SI cancellation system, in which the phase shifter provides common phase pre-alignment before wavelength separation, while the subsequent variable optical attenuator (VOA) and optical tunable delay line (OTDL) branches provide path-dependent amplitude and delay control. Experimental results show an average cancellation depth of 37.19 dB for a 220-MHz dual-path SI signal, with a standard deviation of 0.18 dB across repeated trials using 31 hardware evaluations, and successful recovery of a 16-ary quadrature amplitude modulation (16-QAM) signal with an error vector magnitude of 10.06%. The proposed framework provides a robust, practical solution for high-depth SI cancellation in CCFD systems.
All-fiber nitrogen-vacancy (NV) diamond sensors are attractive for noncontact current measurement, but their practical performance is constrained by fluorescence collection efficiency and low-current nonlinear response. Here, we demonstrate an all-fiber integrated NV-diamond current probe based on side-pumped 532 nm excitation, a large-core high-numerical-aperture collection fiber, and an optimized miniaturized parabolic reflector. The decoupled excitation/collection architecture increases the collection etendue, while the reflector redirects backward and large-angle NV fluorescence into the collection channel. The optimized design achieves a simulated fiber-coupled fluorescence efficiency of 5.681%, and the packaged probe collects approximately 700 μW of red fluorescence under 50 mW excitation, enabling a noise-equivalent magnetic-field sensitivity of approximately $\rm{1}~\rm{nT}/\sqrt{\rm{Hz}}$ over the frequency range of 10-100 Hz. For current sensing, the probe is positioned so that the current-induced magnetic field at the diamond is approximately along the [100] direction. In this configuration, the four $\langle 111 \rangle$ NV orientations have projection components of $\pm B/\sqrt{3}$, and their optically detected magnetic resonance (ODMR) responses merge into two overlapped resonance branches. The current is retrieved from the frequency splitting between these branches. A square-root nonlinear calibration model derived from transverse strain/electric-field splitting of the $m_{s}=\pm 1$ manifold fits the measured splitting with an $R^{2}$ of 0.999994. For currents from 0 to 1600 A, the reconstructed current exhibits a root-mean-square error of 1.37 A and a mean relative error of 0.919%. Twelve-hour dual-branch PID tracking experiments at 50, 500, and 1600 A further show that the ratio-error fluctuations remain within approximately 1%, 0.05%, and 0.02%, demonstrating the capability of the proposed all-fiber NV-diamond probe for compact and noncontact current monitoring.
Conventional radar cross‑section (RCS) measurements rely on large anechoic chambers, making characterization of complex electromagnetic structures costly and space‑intensive. RF‑photonics‑based approaches offer compact and broadband alternatives; however, prior implementations based on bulk optics and fiber interferometers are limited by instability and system complexity. In this work, we present a photonic‑integrated‑circuit (PIC)–based RF‑photonic RCS measurement system that emulates a conventional microwave RCS range within a compact and stable platform. Cross‑range profiles are obtained from rotation‑induced Doppler frequency shifts, while down‑range information is recovered through autocorrelation of broadband optical noise. Proof‑of‑concept measurements using 3D Nanoscribe‑fabricated scaled targets demonstrate accurate reconstruction of scattering signatures. The proposed approach enables stable, compact, and wavelength‑scaled RCS measurements for complex electromagnetic structures using integrated photonic technology.
Narrow-linewidth fiber lasers operating in the 2-μm waveband demonstrate considerable potential for applications such as eye-safe coherent detection and atmospheric monitoring. In this work, a ring-cavity narrow-linewidth thulium-doped fiber laser (TDFL) is proposed and experimentally validated. This laser achieves wavelength locking through a uniform fiber Bragg grating, and combines the multi-peak filtering characteristics of Fabry–Pérot fiber Bragg grating with the Vernier effect in a compound ring cavity to achieve single longitudinal mode (SLM) selection. Simultaneously, the incorporation of ultra-high numerical aperture (UHNA) fiber provides weak distributed feedback, further compressing the laser linewidth. Experimental results demonstrate that, without self-injection feedback, the laser achieves stable SLM operation with an optical signal-to-noise ratio exceeding 70.90 dB. Over a continuous operation period of ten hours, the center wavelength and the peak power exhibit fluctuations of less than 0.03 nm and 0.89 dB, respectively. The relative intensity noise is suppressed to below −124.23 dB/Hz at frequencies above 1 MHz. By employing 100 m of UHNA fiber as a weak feedback element, the output linewidth is compressed from 12.56 kHz to 0.59 kHz at an integration time of 0.001 s. These results provide an effective technical pathway for high-performance narrow-linewidth fiber lasers.
This paper reviews optical beamforming approaches for the steering of microwave phased array antennas (PAAs). We highlight the use of true-time-delay (TTD) to form squint-free beams over wide instantaneous bandwidths. Optical beamforming approaches that leverage the switching of delay lines, optical dispersion, and integrated optical ring resonators (ORRs) are reviewed. We provide, in addition, a discussion on how recent advancements in photonics and its unique attributes could impact future developments of beamforming feeds for PAAs.
This study proposes a subcarrier time-interleaved Discrete Fourier Transform-spread-Orthogonal Frequency Division Multiplexing (DFT-s-OFDM) scheme for free-space optical transmission. This scheme is designed to mitigate turbulence-induced fading by introducing time diversity at the subcarrier level without sacrificing spectral efficiency. The proposed method redistributes subcarrier components over multiple time instances and recombines them at the receiver, enabling the effective averaging of channel fluctuations. The scheme was validated through simulations and a proof-of-concept experiment with 25 Gbps 16QAM coherent optical transmission, demonstrating significant bit error rate improvement over conventional coherent OFDM under turbulent conditions.
