Analysis and optimization of few-mode fibers with low differential mode group delay by variational method

Mode-division multiplexing (MDM) technology based on few-mode fibers (FMFs) is the current researchhotspot of optical fiber communication system because of its ability to increase the transmission capacity severaltimes. When the number of multiplexed modes is large, the crosstalk between modes can be removed bymultiple input multiple output digital signal processing algorithm at the receiving end. The larger thedifferential mode group delay (DMGD, ), the more complex the algorithm is. Therefore, in order to reducethe complexity of the receiver, it is necessary to use FMFs with low DMGD. The variational method isproposed to analyze any FMFs with higher refractive index of core than that of cladding. The analyticalformula of the fundamental mode size, the normalized propagation constant for each of all guided modes, andDMGD relative to the fundamental mode are derived. Moreover, their relationship with the normalizedfrequency and other fiber manufacturing parameters are given. On this basis, the graded-index FMFs arestudied, and the fiber parameters are optimized. The optimization parameters are the difference between themaximum core refractive index and cladding refractive index n1 - n2 = 0.01, the core radius a = 14 mu m, and theparamenter of refractive index distribution a= 1.975. In the optimized FMF, 6 LP modes can be guided and|| is less than 15 ps/km within the C band and L band. In the end, the effects of the fiber manufacturing errors on DMGD are discussed