Mode filtering and transmission performance in multimode doubly clad graded-index silica photonic crystal fibers at 850 nm

Multimode graded-index (GI) silica optical fibers are widely used for short- to medium-range data transmission but face bandwidth limitations. Achieving higher bandwidths requires innovative fiber designs that minimize modal dispersion and suppress higher-order modes. Here, we theoretically analyze a multimode doubly clad (W-type) GI silica photonic crystal fiber (SPCF) using the power flow equation to model its transmission characteristics. We demonstrated that for thinner inner cladding (δ = 0.24), the equilibrium mode distribution (EMD) and steady-state distribution (SSD) are achieved at fiber lengths of L_c = 36 m and z_s = 95 m, respectively, while for thicker inner cladding (δ = 0.48), these lengths are L_c = 40 m and z_s = 105 m. Increasing δ reduces leaky mode losses and increases the number of guided modes, thereby extending the lengths required to reach EMD and SSD. The sooner the EMD is achieved, the faster the bandwidth improvement occurs. The standard OM4 and OM5 multimode silica optical fibers with GI core distribution have a bandwidth of 4.7 GHz · km at 850 nm. The W-type GI SPCF investigated in this work achieves a much higher bandwidth-length product of 9.8 GHz · km at 850 nm. These findings suggest that such SPCFs can significantly enhance bandwidth performance in data centers and telecommunications, while offering greater design flexibility through air-hole geometry adjustments without complex doping processes.