Search Results (103)

Multimode fibers (MMFs) have been a key component in short-reach transmission systems for over 50 years and remain the predominant transmission medium for Vertical Cavity Surface-Emitting Laser (VCSEL)-based short links in data centers. To meet the growing demand for higher data rates, MMFs have continuously evolved to enhance bandwidth performance. This paper provides an overview of the fundamental properties of MMFs, with an emphasis on fiber parameters that influence bandwidth capabilities. We discuss trends in increasing data rates for MMF transmission systems in data centers and review recent progress in MMF technology aimed at boosting bandwidth. In particular, we highlight innovative fiber designs, including high-bandwidth 50 μm MMFs, large-core MMFs, long-wavelength MMFs, universal fibers, MMF bundles, and multicore fibers. Full article

The growing demand for high-fidelity, multi-parameter, distributed sensing in critical domains such as structural health monitoring, oil and gas exploration, and secure perimeter surveillance is pushing traditional optical fiber sensors (OFS) to their performance limits. Although conventional multiplexing techniques such as time-division and wavelength-division multiplexing (TDM, WDM) have been commercially successful, they are rapidly approaching fundamental bottlenecks in sensor density, spatial resolution, and data capacity. This review argues that the synergistic convergence of space-division multiplexing (SDM) and artificial intelligence (AI) represents a paradigm shift, enabling a new generation of intelligent, high-dimensional sensing networks. We comprehensively survey the state of the art in SDM-based OFS, detailing the operating principles and applications of multi-core fibers (MCFs) for ultra-dense sensor arrays and 3D shape sensing, as well as few-mode fibers (FMFs) for mode-division multiplexing and enhanced multi-parameter discrimination. However, the unprecedented spatial parallelism provided by SDM introduces significant challenges, including inter-channel crosstalk, complex signal demultiplexing, and massive data volumes. This paper systematically explores how AI, particularly machine learning (ML) and deep learning (DL), is being leveraged not merely as a tool but as an indispensable core technology to mitigate these impairments. We critically analyze AI’s role in digital crosstalk suppression, intelligent mode demultiplexing, signal denoising, and solving complex inverse problems for parameter estimation. Furthermore, we highlight how this AI–SDM synergy enables capabilities beyond the reach of either technology alone, such as super-resolution sensing and predictive analytics. The discussion is extended to include the critical supporting pillars of this ecosystem, such as advanced interrogation techniques and the associated data management challenges. Finally, we provide a forward-looking perspective on the trajectory of the field, outlining a path toward cognitive sensing networks that are self-calibrating, adaptive, and capable of autonomous decision-making. This review is intended to serve as a foundational reference for researchers and engineers at the intersection of photonics and intelligent systems, illuminating the pathway toward tomorrow’s intelligent sensing infrastructure. Full article

Beyond supporting ultra-high-capacity data transmission, metropolitan and access networks are expected to enable real-time infrastructure monitoring, driving the emergence of integrated sensing and communication (ISAC). Distributed acoustic sensing (DAS) has proven to be well-suited to urban sensing application requirements, yet its seamless integration into ISAC remains challenging—conventional high-peak-power sensing pulses in DAS induce nonlinear crosstalk in communication channels. DAS inherently suffers from interference fading due to single-frequency laser sources, which limits sensitivity. Here, we propose an ISAC architecture based on an electro-optic (EO) comb and a 7-core fiber, achieving nonlinearity-suppressed self-homodyne transmission and fading-suppressed DAS. Unmodulated comb lines and sensing pulses are polarization-multiplexed into orthogonal polarization states within the central core to minimize nonlinear crosstalk while delivering local oscillators (LOs) for wavelength division multiplexing (WDM) coherent transmission within six outer cores—achieving 10.56 Tbit/s capacity. In addition to supporting WDM transmission, the EO comb’s wavelength diversity is also exploited to enhance DAS performance. Specifically, a dual-pulse probe loaded onto four comb lines yields a 6 dB signal-to-noise ratio gain and a 64% reduction in fading occurrences, achieving a sensitivity of 1.72 $p\epsilon /\sqrt{\mathrm{Hz}}$ with 8 m spatial resolution. Moreover, our system supports simultaneous multi-wavelength backscatter detection in sensing and simplified digital signal processing in self-homodyne communication, reducing receiver complexity and cost. Our work presents a scalable, energy-efficient ISAC framework that unifies high-capacity communication with high-sensitivity sensing, providing a blueprint for future intelligent optical networks. Full article

The present study introduces a novel design for lensed, chemically etched optical fibers (LEOFs) designed for efficient coupling with multicore fibers (MCFs). Experimental characterization at a wavelength of 1550 nm yielded an average coupling loss of approximately 0.76 dB under direct physical contact and 0.40 dB when the fiber end was positioned at an optimal working distance. Moreover, it was experimentally demonstrated that LEOFs exhibit high tolerance to longitudinal displacement and minimal wavelength-dependent variation in coupling efficiency. Based on this approach, fiber-in–fiber-out (FIFO) multicore couplers were fabricated using bundles of LEOFs that had been aligned to MCF cores. Bidirectional measurements yielded average insertion losses of 3.23–3.30 dB in TX and 3.20–3.27 dB in RX transmission directions at 1550 nm, with core-resolved losses as low as 1.09 dB for well-aligned channels. The results confirm the viability of LEOF-based multicore free-space coupling, with further improvements expected from enhanced fabrication accuracy. Full article

We present a high-precision endoscopic shape-sensing method using only two calibrated outer cores of a multicore fiber Bragg grating (MC-FBG) array. By leveraging the geometric relationship among two non-collinear outer cores and the central core, the method estimates curvature and bending angle without relying on multiple outer-core channels, thereby reducing complexity and error propagation. On canonical shapes, the proposed method achieves maximum relative reconstruction errors of 1.62% for a 2D circular arc and 2.81% for a 3D helix, with the corresponding RMSE values reported for completeness. In addition, representative endoscope-relevant configurations including the α-loop, reversed α-loop, and N-loop are accurately reconstructed, and temperature tests over 25–81 °C further verify stable reconstruction performance under thermal disturbances. This work provides a resource-efficient and high-fidelity solution for endoscopic shape sensing with strong potential for integration into next-generation image-guided and robot-assisted surgical systems. Full article

We propose a vector bending sensor based on a tapered few-mode multi-core fiber (FM-MCF). A seven-core six-mode fiber is tapered with an optimized taper ratio, enabling bending sensing through power monitoring. When the tapered FM-MCF bends, coupling occurs between the central core and side cores in the tapered region. By monitoring the power of all cores and employing a power differential method, the bending direction and curvature can be reconstructed. The results show that within a curvature range of 2.5 m⁻¹ to 10 m⁻¹, the sensitivity of the ratio of the side core’s power to the middle core’s power with respect to curvature is not less than 0.14/m⁻¹. A deep fully connected feedforward neural network (DNN) is used to demodulate all power information and predict the bending shape of the optical fiber. The algorithm predicts the bending radius and rotation angle with mean absolute errors less than 0.038 m and 3.087°, respectively. This method is expected to achieve low-cost, high-sensitivity bending measurement applications with vector direction perception, providing an effective solution for scenarios with small curvatures that are challenging to detect using conventional sensing methods. Full article

The explosive growth of optical interconnection service traffic urgently necessitates the evolution of optical transponders and fibers. The core-division-multiplexed (CDM) transmission technique using weakly coupled multi-core fibers (MCFs) and beyond-1T optical transport network (OTN) transponders has emerged as an attractive solution to meet the bandwidth demands of future networks. In this paper, we demonstrate an ultra-high-speed OTN system using C+L-band 1.2 Tb/s OTN transponders with a weakly coupled seven-core fiber. The OTN transponders support two configurable modulation rates of 135 Gbaud and 155 Gbaud, along with a probability constellation-shaping 64-ary quadrature amplitude modulation (PCS-64QAM) format. The MCF exhibits characteristics comparable to those of SMFs and negligible inter-core crosstalk, providing a superior physical channel for ultra-high-speed CDM transmission. Fiber length and low-noise EDFAs are also chosen to enhance the transmission distance under the limited optical signal-to-noise ratio (OSNR) budget when using 1.2 Tb/s OTN transponders. Benefiting from the high-performance OTN transponders and MCF, we achieve real-time 0.672 Pb/s and 0.571 Pb/s 4 × 89 km CDM transmissions using 135 Gbaud and 155 Gbaud modulation rates, respectively. The performance of the two modulation configurations is also compared and discussed. This work demonstrates the feasibility of implementing 1.2 Tb/s OTN transponders with weakly coupled MCFs to achieve ultra-high-speed metro–regional transmission, presenting a promising solution for next-generation inter-city data center interconnections. Full article

We demonstrate an integrated core-pumped 4-core erbium-doped fiber amplifier (4C-EDFA) that achieves a record-low differential core gain of 0.5 dB across the whole C-band. This is enabled by utilizing a 4C-EDF with a minimal core-dependent absorption coefficient and passive devices with low core-dependent loss. The 4C-EDFA also exhibits an average gain of 15.50 dB, an average output power of 22.5 dBm, and a maximum noise figure of 4.91 dB. Furthermore, simulations on a 4-core fiber (4CF) transmission link confirm that the proposed 4C-EDFA can support transmission exceeding 10,000 km with a minimal inter-core Q² difference of only 0.5 dB. Here, Q² is defined as the ratio of the mean received signal levels to the corresponding noise variances. It is a critical metric in optical systems to quantify the signal quality, which highlights its potential for high-capacity and long-haul uncoupled 4CF systems. Full article

Traditional single-mode Brillouin optical time-domain analysis systems are inherently limited in terms of sensing capacity, susceptibility to bending loss, and spatial resolution. Multi-core fibers present a promising approach to overcoming these limitations. In this study, a seven-core fiber was utilized, with the central core and three asymmetrically positioned off-axis cores selected for sensing. The temperature coefficients of the four selected cores were experimentally calibrated as 1.103, 0.962, 1.277, and 0.937 MHz/°C, respectively. By employing differential pulse techniques within the Brillouin distributed sensing system, temperature-compensated bending measurements were achieved with a spatial resolution of 20 cm. The fiber was wound around cylindrical mandrels with diameters of 7 cm, 10 cm, and 15 cm. Experimental results demonstrate effective decoupling of temperature and bending strain, enabling accurate curvature reconstruction. Error analysis reveals a minimum deviation of 0.04% for smaller diameters and 0.68% for larger diameters. Cross-comparison of measurements conducted at varying temperatures confirms the robustness and effectiveness of the proposed temperature compensation method. Full article

Azimuth sensing plays a vital role in numerous industrial applications where tilt angle serves as a key parameter. To address the demand for accurate and reliable measurements, we propose an all-fiber two-dimensional inclinometer based on the Vernier effect in a multi-core fiber Fabry–Perot interferometer. The sensor is capable of simultaneously measuring both inclination and azimuth angles with high accuracy. A cascaded Fabry–Perot interferometer was inscribed in a seven-core fiber using a femtosecond laser plane-by-plane direct writing technique. By monitoring the wavelength shifts in two peripheral cores, we demonstrated the feasibility and performance of the proposed sensor. The experimental results showed that the inclinometer exhibited high sensitivity, with maximum values of −0.5272 nm/° for azimuth measurement (maximum measurement error: 7.33°) and −0.5557 nm/° for inclination measurement (maximum measurement error: 5.97°). The measurement ranges extended from 0° to 360° for azimuth and from –90° to 90° for inclination. Owing to its wide measurement range, compact structure, and high sensitivity, the proposed all-fiber two-dimensional inclinometer holds significant potential for practical applications. Full article

Multi-core fiber (MCF) has drawn increasing attention for its potential application in distributed acoustic sensing (DAS) due to the compact optical structure of integrating several fiber cores in the same cladding, which indicates an intrinsic space-division-multiplexed (SDM) capability in a single piece of fiber. In this paper, a dual-channel DAS integrated with MCF is presented, of which the equivalent self-noise characteristic is analyzed. The equivalent self-noise of the system can be effectively suppressed by signal superposition with the phase matching method. Considering that the noise correlation among the cores is not zero, the signal-to-noise (SNR) gain after signal superposition is less than the theoretical value. The dual-channel DAS system is set up by a piece of 2 km long seven-core MCF, in which the dual-sensing channels are constructed by a four-core series and three-core series, respectively. The total noise correlation coefficient of the seven cores is 11.28, while the equivalent self-noise of the system can be suppressed by 6.32 dB with signal superposition. An equivalent self-noise suppression method based on a linear delay phase matching scheme is proposed for noise decorrelation in the DAS MCF system. After noise decorrelation, the suppression of the equivalent self-noise of the system can reach the theoretical value of 8.45 dB with a time delay of 1 ms, indicating a noise correlation among the seven cores of almost zero. The feasibility of the equivalent self-noise suppression method for the DAS system is verified for both single-frequency and broadband signals, which is of great significance for the detection of weak vibration signals based on a DAS system. Full article

Secure communication for critical command nodes has emerged as a pivotal challenge in modern warfare, in particular considering the vulnerability of these nodes to electronic reconnaissance. To cope with the severe interference, this paper proposes a robust solution for long-distance secure command and control system leveraging phase-modulated microwave photonic links. Studies that analyze the impairing nonlinear distortions and phase noise stemming from different sources in optical phase demodulation during long-haul transmission has been carried out, unveiling their impairment in coherent transmission systems. To overcome these limitations, a linearized phase demodulation and noise suppression technique based on composite optical phase-locked loop and multi-core fiber is proposed and experimentally validated. Experimental results demonstrate a long-haul transmission over 100 km with an 81 dB suppression for third-order intermodulation distortion and a 27 dB improvement in noise floor at 5 MHz under closed-loop condition, verifying a significant enhancement in the fidelity in long-distance transmission. This method ensures a highly reliable secure communication for command and control systems in contested electromagnetic environments. Full article

We present a low-loss fan-in/fan-out (FIFO) device fabricated from a bundle of chemically etched optical fibers integrated within a standard FC/PC connector. The device demonstrates efficient coupling with insertion losses of 0.32 dB and 0.40 dB at wavelengths of 1310 nm and 1550 nm, respectively. Crosstalk and back reflection were measured to be below −41.8 dB and 51.3 dB, confirming high channel isolation and minimal signal degradation. This compact and connectorized solution offers a practical approach for scalable multicore fiber interfacing in advanced optical communication systems. Full article

Tethered unmanned aerial vehicles (UAVs) operating in terrestrial environments face critical safety challenges from power cable breaks, yet existing solutions—including fiber optic sensing (cost > USD 20,000) and impedance analysis (35% payload increase)—suffer from high cost or heavy weight. This study proposes a dual innovation: a real-time break detection method and a low-cost multi-core parallel sensing system design based on ACS712 Hall sensors, achieving high detection accuracy (100% with zero false positives in tests). Unlike conventional techniques, the approach leverages current differential (ΔI) monitoring across parallel cores, triggering alarms when ΔI exceeds I_rate/2 (e.g., 0.3 A for 0.6 A rated current), corresponding to a voltage deviation ≥ 110 mV (normal baseline ≤ 3 mV). The core innovation lies in the integrated sensing system design: by optimizing the parallel deployment of ACS712 sensors and LMV324-based differential circuits, the solution reduces hardware cost to USD 3 (99.99% lower than fiber optic systems), payload by 18%, and power consumption by 23% compared to traditional methods. Post-fault cable temperatures remain ≤56 °C, ensuring safety margins. The 4-core architecture enhances mean time between failures (MTBF) by 83% over traditional systems, establishing a new paradigm for low-cost, high-reliability sensing systems in terrestrial tethered UAV cable health monitoring. Preliminary theoretical analysis suggests potential extensibility to underwater scenarios with further environmental hardening. Full article

To address the demand of equivalent self-noise suppression in a distributed hydroacoustic sensing system, this study proposes a method to enhance the acoustic sensitivity and signal-to-noise ratio (SNR) using space division multiplexed (SDM) technology based on multi-core fiber (MCF). Specifically, a dual-channel demodulation system for distributed acoustic sensing is designed using MCF. The responses of different cores in MCF are almost consistent under external acoustic pressure, while their self-noise is inconsistent. Accordingly, the acoustic pressure phase sensitivity (APPS) and SNR gain based on the accumulation of dual-channel signals are analyzed, which are verified by experiments. It is shown that the self-noise correlation coefficient between the two cores is 0.11, increasing the noise power by 3.46 dB. The APPS is increased by 5.97 dB re 1 rad/μPa after the accumulation of two-core signals, which is close to the theoretical value (6 dB). The equivalent self-noise is reduced by 2.54 dB. The experimental results reveal that the enhancement of acoustic pressure phase shift sensitivity and SNR can be achieved by the space division multiplexing (SDM) of multi-core signals, which is of great significance for suppressing the equivalent self-noise of the system and realizing the acoustic pressure detection of weak underwater signals. Full article