System stability and robustness are highly desired in a mode division multiplexing (MDM) transmission system. The matching of the active fiber and transmission passive fiber effectively reduces the transmission impairments and is conducive to transmission stability. In this paper, we proposed and fabricated a well-matched triple-layered-core few-mode erbium-doped fiber (FM-EDF) for low differential modal gain (DMG) with multilayer ions doping adjustment. Based on the fabricated FM-EDF, the amplifier is constructed experimentally. By a simple LP ₀₁ pump mode, the modal gains are higher than 24 dB, and meanwhile, the DMG of 1.6 dB is obtained. The compatibility between the FM-EDF and the passive transmission FMF is verified in a 75.6 km MDM transmission system with a matching degree higher than 83%. Results show that after 75.6 km transmission and amplification, the difference in the output signal power is lower than 1.8 dB at the receiver. The fabricated FM-EDF is beneficial for practical MDM systems with high mode stability, excellent fiber linkage integration, and signal channel equalization, which shows much potential for long-haul MDM transmission and multistage amplification.

An optical channel distortion equalization method based on silicon photonic reservoir computing (PhRC) structure with particle swarm optimization (PSO) algorithm is proposed. The weights training scheme of the photonic readout layer of PhRC is trained by PSO algorithm towards a lower bit error rate (BER) and a better eye diagram, simultaneously. The self-organizing evolutionary PSO algorithm enables fast convergence and iterative optimization for training optical weights. Without the necessary the reservoir signal states obtained from the monitors, it eliminates the noise introduced by the optical monitors. We implement a system-level simulation, including RC structure, training algorithm, and fiber communication construction. Because of the excellent performance of the PSO training algorithm, a BER of 9.15 × 10 ⁻⁵ is obtained with the 25 Gb/s on-off keying (OOK) input signal in the case of 25 km single-mode transmission. This BER result is three orders of magnitude lower than that before the equalization. This equalizer mitigates the processing bandwidth limit and provides a distorted signals equalization method in the optical domain with simple and effective training strategy. This method shows much potential in high speed optical communication channel equalization in the future.

A novel physics- and data-driven deep-learning (PDDL) method is proposed to execute complete mode decomposition (MD) for few-mode fibers (FMFs). The PDDL scheme underlies using the embedded beam propagation model of FMF to guide the neural network (NN) to learn the essential physical features and eliminate unexpected features that conflict with the physical laws. It can greatly enhance the NN’s robustness, adaptability, and generalization ability in MD. In the case of obtaining the real modal weights (ρ²) and relative phases (θ), the PDDL method is investigated both in theory and experiment. Numerical results show that the PDDL scheme eliminates the generalization defect of traditional DL-based MD and the error fluctuation is alleviated. Compared with the DL-based MD, in the 8-mode case, the errors of ρ² and θ can be reduced by 12 times and 100 times for beam patterns that differ greatly from the training dataset. Moreover, the PDDL maintains high accuracy even in the 8-mode MD case with a practical maximum noise factor of 0.12. In terms of adaptation, with a large variation of the core radius and NA of the FMF, the error keeps lower than 0.43% and 2.08% for ρ² and θ, respectively without regenerating new dataset and retraining NN. The experimental configuration is set up and verifies the accuracy of the PDDL-based MD. Results show that the correlation factor of the real and reconstructed beam patterns is higher than 98%. The proposed MD-scheme shows much potential in the application of practical modal coupling characterization and laser beam quality analysis.

Few-mode erbium-doped fiber amplifier (FM-EDFA) is a key device to achieve power compensation in long-haul mode division multiplexing (MDM) optical system. The gain characteristics of FM-EDFA are related to the number of modes and multiplexing mode selection. In this paper, the gain characteristics of four modes and six modes FM-EDFA with various mode combinations and different wavelengths are analyzed in single and dual-pump cases. The results show that when the azimuthal-dependent pump is selected, different degenerate mode (odd and even mode) multiplexing will arise DMG. In the replacement of different degenerate modes at the same modal order, the DMG of signal modes LP _11o,s and LP _21o,s is 1 dB (single-pump: LP _01,p ) while the DMG between signal modes LP _11e,s and LP _21e,s is 8 dB (dual-pump: LP _01,p +LP _11e,p ). Compared with single-wavelength amplification, both the overall gain and the DMG decrease in the multi-wavelength case. The above conclusions can provide a theoretical basis for the selection of FM-EDFA in the wavelength division multiplexing (WDM)-MDM system.

Refractive index profile (RIP) determines the property and effectiveness of optical fibers. Accurate measurement of the RIP is essential for fiber fabrication, test and calibration. At present, computer tomography (CT) method is widely used in RIP measurement. However, in the measurement of the optical fiber with a complex RIP, the CT method is much time-consuming due to its dependence on small rotation angle step usually with 2° (performing 90 steps of rotation for angle range from 0° to 180°). In this paper, the total variation (TV) method is proposed to fast reconstruct the complex RIP with both two-dimension and three-dimension in transverse as well as longitude direction. The RIP of a seven-core trench-assisted optical fiber and a twisted polarization maintaining optical fiber are measured experimentally as examples. Experimental results demonstrate that the proposed TV method has higher accuracy of 4×10-4 and three times faster than that of traditional CT method

A ring-core few-mode erbium-doped fiber (FM-EDF) with step refractive index is designed and proposed to realize the modal gain equalization of the amplifier. The layered-doping method is used to reduce the mode gain difference (DMG), and the doping radius and doping concentration are adjusted to obtain the best FM-EDF structure. When it is applied to the few-mode erbium-doped fiber amplifier (FM-EDFA), the DMG at 1550 nm is less than 0.44 dB. The gains of each mode in the entire C-band are greater than 16 dB. To further improve the gain equalization characteristics of the designed fiber, the DMG can be reduced to 0.03 dB by adding trenches at appropriate positions on the basis of the same core structure. The tolerances in the optical fiber manufacturing process are considered to verify the reliability of the FM-EDFA performance. The doping radius and doping concentration are changed by ± 15 % based on the precise value to consider the tolerances in the fiber manufacturing process to verify the reliability of the FM-EDFA performance. Generally speaking, doping radius changes have a greater impact on DMG, so higher production accuracy is required. To explore the experimental structure and procedures of FM-EDFA, a preliminary experiment is carried out. The experimental results obtained have great reference significance for subsequent experiments. The characteristics of the FM-EDFA, such as all-fiber and low DMG, are difficult to implement by other methods.