Search Results (90)

Few-mode fiber (FMF) grating sensing exploits the mode-dependent responses of multiple spatial modes to external perturbations, enabling vector and dual-parameter measurements. This paper reviews the research progress of FMF grating sensors from the perspectives of fundamental mechanisms, fabrication techniques, signal demodulation, and representative sensing applications. First, the mode propagation characteristics, coupled mode theory, and phase matching conditions of FMF gratings are discussed. Then, the fabrication strategies of FMF gratings are summarized, followed by an analysis of signal demodulation methods. Particular attention is given to vector sensing and dual-parameter sensing. Finally, the remaining challenges related to mode excitation stability, spectral resonance overlap, fabrication repeatability, packaging reliability, and long-term calibration are discussed, and future opportunities in mode engineering, multidimensional demodulation, and intelligent sensing systems are outlined. Full article

This paper presents a novel integration of quadrature amplitude modulation (QAM)-based fast optical orthogonal frequency-division multiplexing (F-OFDM) with machine learning (ML)-based equalization in spatial division multiplexing (SDM) applications, using few-mode fibers (FMFs). The FMFs support four LP modes, resulting in a total of 12 orthogonal modes, each accommodating two polarizations. A digital multiple-input multiple-output channel equalizer is employed at the receiver’s digital signal processing (DSP) unit to effectively mitigate channel crosstalk. The study harnesses supervised ML-DSP techniques, in particular recurrent neural networks (RNNs) and deep neural networks (DNNs), achieving substantial reductions in bit error rates (BERs). In addition, higher-complexity architectures, namely convolutional neural networks (CNNs) and long short-term memory (LSTM) networks, are evaluated to assess the impact of advanced spatial and temporal feature extraction. It is shown that F-OFDM demonstrates superior performance over conventional optical OFDM, particularly when supported by ML techniques. Simulation results reveal that RNNs achieve a BER of 0.0019 over 15 km at 12 Gbaud (worst-case selected channel), showcasing a remarkable 52.5% improvement compared to linear equalization. DNNs achieve a BER of 0.0025, reflecting a 37.5% enhancement. While RNNs perform better, their computational demands pose challenges for real-time applications, and the more complex models (CNN and LSTM) do not provide additional performance gains. The paper also explores cyclic prefix management and subcarrier number strategies in F-OFDM to optimize performance, paving the way for future advancements in SDM networks. Full article

This study proposes and numerically investigates the design and characterization of a ring-assisted (RA) fiber supporting 11 LP mode groups and a concentric-ring-assisted (CRA) fiber supporting 13 LP mode groups. Based on the relationship between the normalized frequency and the number of LP modes, a step-index (SI) fiber capable of supporting 13 LP mode groups is first designed. By leveraging the overlap between the high-index ring-assisted structure and the LP₂₂ mode, the effective index difference (Δn_eff) between the LP₂₂ and LP₀₃ modes is enhanced. The resulting RA 11-LP mode fiber achieves a minimum effective index difference Min|Δn_eff| of 0.78 × 10⁻³, comparable to that of a standard SI 4-mode fiber, and a minimum effective area Min|A_eff| of 164 μm², which effectively suppresses nonlinear effects. Furthermore, by introducing a second ring structure to form a CRA design, we realize a 13-LP mode fiber. This structure selectively increases the effective index of the LP₆₁ mode through overlap with its power distribution, while leaving the effective index of the LP₁₃ mode unaffected. The CRA 13-LP mode fiber exhibits highly stable effective indices across the C band. It demonstrates a Min|Δn_eff | of 0.55 × 10⁻³, which ensures effective mode separation and reduced inter-mode crosstalk. The Min|A_eff| is 131 μm²—still above 100 μm²—thereby mitigating nonlinear impairments. With support for 46 spatial modes in total, this fiber significantly enhances transmission capacity. 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

Based on mode crosstalk theory, this paper develops a spontaneous Raman scattering (SpRS) model for the quantum-classical coexistence system using few-mode fiber (FMF) integrated with wavelength-division multiplexing (WDM) and spatial-division multiplexing (SDM). Through numerical calculations, the influence degrees of three factors (mode coupling, the number of modes and wavelengths) on SpRS are analyzed. The investigation identifies the dominant contributors to SpRS and reveals their relative impact magnitudes. Based on these results, a ring-assisted FMF is proposed to mitigate noise impacts on quantum signals. The numerical results show that the optimized FMF enhances quantum signal transmission distance by up to 41.5%. 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

To support mode-division multiplexing with reduced inter-modal crosstalk, we propose a novel polarization-maintaining few-mode fiber design with a uniform doping profile and no air holes. The fiber employs two placed low-index inclusions to lift modal degeneracy and achieve strong birefringence while maintaining compatibility with standard MCVD and OVD fabrication processes. A genetic algorithm is used to optimize the geometrical and refractive index parameters. Finite element simulations show that the optimized design supports ten guided modes with a minimum effective index difference exceeding $3.8\times {10}^{-4}$ across the C+L band. The fiber exhibits moderate dispersion and strong modal isolation. Tolerance analysis confirms good robustness against index fluctuations and moderate sensitivity to dimensional variations. These features suggest that the proposed PM-FMF is a promising candidate for short-reach spatial-division multiplexing applications where intrinsic polarization and mode separation are desired. Full article

A nested semi-tube hollow-core anti-resonant fiber (HC-ARF) that can support the high-purity transmission of a few polarization-maintaining modes is designed in this paper. By employing bi-thickness hybrid silica/silicon anti-resonant tubes, the birefringence of the orthogonal polarized modes is significantly improved, and the weak coupling condition of the five lowest-order polarization maintaining modes, including the LP_{01_x}, LP_{01_y}, LP_{11a_x}, LP_{11b_x}, and LP_{11a_y}, can be met. The effective refractive index difference between each pair of the supported adjacent modes is larger than 1.0 × 10⁻⁴. With hybrid multi-layer nested semi-tubes, the confinement losses of the supported modes are all less than 1.50 × 10⁻¹ dB/m within a transmission band from 1.530 to 1.620 μm. The minimum confinement losses of the LP_{01_y}, LP_{01_x}, LP_{11a_y}, LP_{11a_x}, and LP_{11b_x} modes are 3.71 × 10⁻⁴ dB/m, 1.61 × 10⁻³ dB/m, 2.00 × 10⁻² dB/m, 1.30 × 10⁻¹ dB/m, and 4.20 × 10⁻² dB/m, respectively. Meanwhile, the unwanted higher-order modes are filtered out well to guarantee the modal purity. The minimum higher-order-mode extinction ratio of the lowest-loss LP₂₁ mode to the highest-loss LP₁₁ mode remains larger than 139 from 1.545 to 1.615 μm. The numerical results highlight the potential of the proposed polarization-maintaining few-mode hollow-core anti-resonant fibers in many application fields, such as short-range and high-capacity data transmission networks, fiber sensing systems, quantum communication systems, and so on. Full article

We propose a dual-groove few-mode fiber surface plasmon resonance sensor that exploits the LP₁₁ mode for enhanced plasmonic sensing. The device incorporates two physically separated grooves with distinct metallic coatings, enabling dual-channel operation via wavelength-division multiplexing. Finite element method simulations show that the optimized design achieves a maximum sensitivity of 14,800 nm/RIU within the RI range of 1.33–1.40. The introduction of a TiO₂–Au bilayer enhances mode coupling and ensures complete spectral separation, thereby improving stability and reducing environmental interference. Biosensing simulations at 37 °C further confirm the practicality of the proposed architecture. Channel 1, filled with ethanol as a temperature-sensitive medium, provides temperature monitoring, while Channel 2 successfully distinguishes between normal and tumor cells, reaching a sensitivity of up to 9428.57 nm/RIU for Jurkat cells. Overall, the TiO₂-enhanced dual-channel FMF-SPR sensor combines ultra-high sensitivity, spectral independence, and biosensing capability, demonstrating strong potential for next-generation fiber-optic sensing and biomedical applications. Full article

We propose and experimentally demonstrate an Integrated Multiband-Mode Multiplexing Photonic Lantern (IM3PL) that enables the selective excitation of high-order modes and stable modal preservation across multiple wavelength bands. As a proof-of-concept configuration, the IM3PL integrates a custom-designed input fiber array composed of three 980 nm single-mode fibers (SMFs) and two few-mode fibers (FMFs) operating at 1310 nm and 1550 nm, respectively. Simulations verify that 980 nm input signals can selectively excite $L{P}_{01}$, $L{P}_{11a}$, and $L{P}_{11b}$ modes at the FMF output, while the modal integrity of high-order linear polarized modes is preserved at 1310 nm and 1550 nm. The fabricated IM3PL device is experimentally validated via near-field pattern measurements, confirming the selective excitation at 980 nm and low-loss, mode-preserving transmission at the signal bands. This work offers a scalable and reconfigurable solution for multiband high-order-mode multiplexing, with promising applications in mode-division multiplexed fiber communication systems and multiband high-mode fiber lasers. Full article

Full control of the light field at the tip of the fiber holds the possibility of producing structured illumination patterns such as LG-beams or vector light fields, which have important applications in different fields such as imaging and quantum technologies. In this work, we show how, by measuring the transmission matrix (TM) and shaping the input of a few-mode fiber, we are able to produce cylindrical vector beams at the fiber output. We use singular value decomposition (SVD) to analyze the TM and use the singular vectors as the basis for beam shaping. We demonstrate the method in three different commercially available fibers supporting 6, 12 and 16 modes each. Full article

This review explores various advanced photonic-assisted techniques for microwave frequency measurement, highlighting their distinct advantages and challenges in detecting multi-tone and broadband microwave frequency signals. Different optical processing techniques for instantaneous frequency measurement, including frequency-to-time mapping techniques, are discussed in detail. The application of multicore and few-mode fibers, artificial intelligence-enhanced, and complex modulation techniques are also discussed. These recent advances collectively push the boundaries of microwave frequency measurement, offering robust and scalable solutions for various applications. Full article

With the widespread use and increasing importance of few-mode fibers (FMFs), comprehensive dispersion measurement for FMFs with large mode numbers is in urgent demand. Among existing methods, spatially and spectrally resolved (S²) imaging technique offers distinct advantages for measuring differential mode group delay (DMGD) and chromatic dispersion (CD) parameters. However, it suffers from several limitations such as uncontrollable mode excitation and an inability to measure absolute CD. In this study, we enhance the traditional S² method, making it possible to effectively measure the complete dispersion for high-mode-count FMFs. By introducing a mode multiplexer (MMUX), selectively and proportionally mode excitation can be realized. Combined with a tunable delay line array, the misalignment of the MMUX’s fiber pigtail lengths is canceled. Additionally, with the help of a reference path capable of generating planar light, the measurement of the absolute CD is enabled. Based on the enhanced MMUX-assisted S², a simultaneous DMGD and absolute CD measurement for an FMF supporting up to six LP modes is conducted, which has not been previously demonstrated with a single S²-based system. The proposed paradigm significantly expands the mode number of FMF measurable by S², enriches the parameters that S² can cover, and even has great inspiration for other measurement methods. Full article

The optical spectrum resource in the C-band has been used up due to dense wavelength division multiplexing (DWDM). Because of devices’ compatibility with both the C-band and the L-band, the L-band is a good choice for further capacity expansion. Meanwhile, the mode division multiplexing (MDM) method has been applied to increase the number of channels. However, the few-mode erbium-doped fiber amplifier must be redesigned to overcome the power differences among channels. In this work, a few-mode erbium-doped fiber (FM-EDF) is optimized and manufactured. Then, an in-line gain-equalized L-band FM-EDFA is constructed. The experimental results show that the FM-EDFA works well in the wavelength range between 1575 nm and 1610 nm. The minimum differential modal gain (DMG) is 0.54 dB, and the maximum modal gain is 22.22 dB. Due to the excellent performance of the L-band FM-EDFA, a DSP-free transmission scheme in the L-band is demonstrated. The bit error rates (BERs) of each channel are below 1 × 10⁻⁵ with a DSP-free receiver. Full article

Mode division multiplexing (MDM) techniques provide significant enhancement of the capacity of optical intensity modulation and direct detection (IM/DD) short-reach communication systems, like the datacenter interconnection scenarios. While the introduction of multiple modes leads to mode coupling that will extremely deteriorate the received signals, two approaches have been explored to address this issue: one involves the application of all-link weakly coupled components to suppress modal crosstalk, while the other utilizes optical multiple-input–multiple-output (MIMO) equalizers based on optical devices for signal decoupling. However, pure digital signal processing (DSP)-based schemes for mode decoupling in IM/DD MDM systems with strong mode coupling remain unexplored. In this paper, we propose to use a frequency-domain MIMO equalizer for compensating the modal interference in the strong-coupling linear-polarized (LP) MDM IM/DD system. The signal recovery capability of the proposed method is verified through numerical simulation. Finally, we successfully experimentally demonstrate the transmission of on–off-key (OOK) signals in a six-LP-mode strong-coupling MDM IM/DD system over a 10 km few-mode fiber, employing a pair of strong-coupling mode multiplexers/demultiplexers. The experimental results indicate that, with the frequency-domain MIMO equalizer, OOK signals from all modes can be recovered with an 11% hard-decision forward error correction threshold of 8.3 × ${10}^{-3}$. The proposed method facilitated by flexible DSP software offers an alternative for short-reach communication systems and has the potential to advance the practical application of MDM techniques in the future. Full article