In this paper, we propose and experimentally verify a method for optimizing the fault detection sensitivity of few mode fiber (FMF) link based on high-order spatial mode trend filtering. The employment of high-order mode trend filtering as a signal processing tool identifies meaningful level shifts from FMF optical time-domain reflectometer (FMF-OTDR) profile, which is associated with the problem of the minimization of the intrinsic random noise and modal crosstalk impact on the acquired data. A FMF link fault detection system is built, and the proposed method is utilized to detect the fault loss characteristics of 7.2 km 6-mode fiber with three fusion splice points with different fusion quality, and the detection results of each mode are compared with the results obtained by FMF-OTDR. The experimental results show that our proposed method can effectively improve the low fault detection sensitivity of high-order spatial mode caused by random noise and mode crosstalk.

We propose a novel monitoring technique based on multi-mode transmission reflection analysis for a long-reach few-mode fiber (FMF) based mode division multiplexing system. By launching unmodulated continuous-wave optical light modes into the corresponding spatial modes of the FMF, the transmitted and reflected or backscattered optical powers can be measured and quantitatively analyzed to accurately characterize and locate the fault. The influences of the capture fraction, attenuation coefficient, and Rayleigh backscattering coefficient are discussed, and simulation results show that the proposed method can realize the fault location of the FMF link. Moreover, considering the influence of mode crosstalk on localization accuracy, it is clear that using the monitoring combination modes LP₀₁ and LP₂₁ gives a high precision of 3.58 m.

In this paper, an analytical model of the fusion splicing loss of few-mode fiber (FMF) under the condition of dynamic crosstalk is proposed. Under different dynamic spatial modal crosstalk caused by fusion mismatches and rotation angles, we analyze the variation trend of the fusion splicing loss characteristics of FMF with L P 01 , L P 11 a , and L P 11 b , respectively. Simulation results show that under the condition of axial misalignment introducing modal crosstalk, the splice loss of the L P 11 a and L P 11 b modes is higher than that of L P 01 . On the other hand, the modal crosstalk caused by rotation angles makes the fusion splice loss of L P 01 present symmetrical distribution, and the loss of fusion splice of L P 01 is maximum at 45°. When rotating from 0° to 90°, for L P 11 a and L P 11 b modes, the fusion loss of L P 11 a decreases gradually, while the loss of L P 11 b increases gradually. Both are in a symmetrical distribution. Meanwhile, we build an FMF fusion splice loss measurement system based on photon lanterns and FMF circulators. The experimental results show that it is consistent with the simulation results, and the experimental and simulation results confirm the feasibility of the fusion splice fault loss analysis modal under dynamic crosstalk conditions.

Induction motors are widely applied in motor drive systems. Effective temperature monitoring is one of the keys to ensuring the reliability and optimal performance of the motors. Therefore, this paper introduces a multiplexed optical temperature sensing system for induction motors based on few-mode fiber (FMF) spatial mode diversity. By using the spatial mode dimension of FMF, fiber Bragg grating (FBG) carried by different spatial modes of optical paths is embedded in different positions of the motor to realize multipoint synchronous multiplexing temperature monitoring. The paper establishes and demonstrates a photonic lantern-based mode division sensing system for motor temperature monitoring. As a proof of concept, the system demonstrates experiments in multiplexed temperature sensing for motor stators using the fundamental mode LP₀₁ and high-order spatial modes LP₁₁, LP₂₁, and LP₀₂. The FBG sensitivity carried by the above mode is 0.0107 nm/°C, 0.0106 nm/°C, 0.0097 nm/°C, and 0.0116 nm/°C, respectively. The dynamic temperature changes in the stator at different positions of the motor under speeds of 1k rpm, 1.5k rpm, 2k rpm with no load, 3 kg load, and 5 kg load, as well as at three specific speed–load combinations of 1.5k rpm_3 kg, 1k rpm_0kg, 2k rpm_5 kg and so on are measured, and the measured results of different spatial modes are compared and analyzed. The findings indicate that different spatial modes can accurately reflect temperature variations at various positions in motor stator winding.