少模光纤熔接点模式耦合特性的改进空间光谱分辨成像分析

Modified Spatially and Spectrally Resolved Analysis of Mode Coupling Characteristics at the Splicing Point of Few-Mode Fibers

基于改进的空间光谱分辨成像法,探究熔接参数对不同类型光纤熔接点处模式耦合特性的影响,通过调整步进来改变放电参数,量化分析影响熔接点处模式耦合的关键指标。结果表明,小包层光纤熔接的模式耦合特性受放电持续时间的影响较大,高阶模激发含量与放电时间呈正相关;当包层尺寸不同的光纤熔接时,角向模式耦合主要受到细光纤切割角度影响,而径向模式耦合受熔接参数影响较小;由于放电中心偏移和电极放电特性波动,包层尺寸不同的光纤熔接时热效应对熔点模式特性的影响具有随机性,包层尺寸差异越大熔接一致性越差;相反,大包层同尺寸光纤熔接对放电参数的变化不敏感,模式耦合特性在较大的放电参数范围内保持稳定。

Objective Fiber splicing is a fundamental procedure in constructing allfiber communication networks or fiber laser systems.Mode coupling occurring at the splicing point is a major cause of modal crosstalk and beam degradation in fewmode fibers.The performance of the splicing equipment significantly influences the mode coupling characteristics.Testing mode degradation at the splicing point is essential for determining optimal splicing parameters.In fewmode fiber splicing,mode coupling occurs simultaneously at the launching end and the splicing point,leading to a reciprocal mode coupling regime along the cascade fiber path.The traditional timedomain measurement methods require long transmission paths to accumulate total time delays of different modes and cannot distinguish or determine highorder modes,making it challenging to analyze mode coupling at the splicing point.Currently,fiber laser systems frequently involve fewmode fiber splicing,making it difficult to differentiate multiplemode coupling events.To achieve a comprehensive understanding of mode properties at the splicing point,it is necessary to explore a modified method that can decouple and quantify mode coupling characteristics in fewmode fiber architectures.Methods We propose a modified spatially and spectrally(M-S^(2))resolving method that accounts for the reciprocal mode coupling regime in fewmode fiber splicing.This method allows for the separation and decoupling of different origins of mode coupling at the splicing point,facilitating quantifiable analysis of specific highorder modes.Numerical simulations are conducted with different mode coupling events and modal weights are conducted to evaluate the method’s effectiveness,showing strong potential for analyzing mode coupling characteristics at the splicing point.An experimental setup,as shown in Fig.4,is constructed to investigate splicing.A tunable source,operating between 1070 nm and 1090 nm,injects singlemode laser beams into the fibers under test with a 0.05 nm wavelength interval.Each wavelength’s excited modes propagate through the fiber path and experience mode coupling at the splicing point.A 10 bit camera records the output field at each wavelength,generating an image sequence for mode property analysis.Finally,the effects of splicing parameters(e.g.,arc power or duration)and fiber cleaving angles on mode coupling properties are studied for various fiber types.Investigated samples include fibers with identical or different cladding dimensions:1)Splicing of 125μm fibers,with dimensions of 10/125μm and 15/125μm and numerical apertures of 0.080 and 0.076.2)Splicing of 125μm and 250μm fibers,with dimensions of 10/125μm and 20/250μm and numerical apertures of 0.080 and 0.112.3)Splicing of 125μm and 400μm fibers,with dimensions of 10/125μm and 20/400μm and numerical apertures of 0.080 and 0.065.4)Splicing of 400μm fibers with identical parameters,including dimensions of 20/400μm and a numerical aperture of 0.065.Results and Discussions Numerical simulations demonstrate that the MS2 resolving method effectively decouples and quantifies mode coupling at the splicing point.In the first case,the modal weights of E1,E2 and E4 are 99.0%,0.5%,and 0.5%,respectively,corresponding to a theoretical multipath interference(MPI)of−23.0 dB for E2 and E4.The MPI tested by the MS2 method is−24.8 dB.In the second case,the modal weights of E1,E2,and E4 are 99.0%,0.8%,and 0.2%,respectively,corresponding to theoretical MPIs of−20.9 dB and−27.0 dB for E2 and E4.The MPIs tested by the MS2 resolving method are−22.8 dB and−28.7 dB,showing good agreement with theoretical values.For 125μm fibers,the LP11 mode content at the splicing point correlates strongly with ARC duration,increasing from−20.4 dB to−15.8 dB as arc duration extends from 1000 ms to 6000 ms.For 125μm/250μm fibers and 125μm/400μm fibers,the excited contents of azimuthal or radial modes show little correlation with arc parameters,while LP11 mode evolution is highly correlated with the cleaving angle of the 125μm fiber.For 400μm fibers,mode coupling characteristics remain stable across a wide range of arc parameters,showing minimal sensitivity to arc parameter variation.Conclusions Using the modified spatially and spectrally resolving method,we investigate the influences of splicing parameters on mode coupling characteristics at the splicing point for various fiber types.By altering arc parameters at equal intervals,we quantitatively assess the key factors affecting mode coupling.Results indicate that for fibers with thin cladding thicknesses,mode coupling is significantly influenced by arc duration,which positively correlates with highorder mode content.For fibers with different cladding parameters,angular mode coupling is mainly affected by the cleaving angle of the thin fiber,while radial mode coupling is less affected by splicing parameters.Due to the offset of the discharge center and the fluctuation of arc behavior,the thermal effects on splicing point mode characteristics show random properties and greater cladding size differences lead to poorer splicing consistency.In contrast,thick fibers with identical cladding parameters exhibit less sensitivity to the variation of arc parameters,with stable mode coupling characteristics across a wide range of arc parameters.This work is of great significance for decoupling and quantitatively evaluating mode coupling characteristics at fiber splicing points and is particularly useful for mode properties analysis and beam quality optimization in highpower fiber laser systems.