Design of weakly-coupled 3-mode 4-core fibers with low differential mode delay for the C + L band using a common-depressed layer.

We investigate weakly-coupled 3-mode 4-core fibers for the C + L band, comparing graded-index (GI) and dual-step-index (DSI) core profiles with a focus on differential mode delay (DMD) reduction. While both profiles are effective for minimizing DMD in 3-mode transmission, we found that it is difficult for GI profiles with a profile exponent of around 2 to satisfy both the cutoff and low-loss conditions for 3-mode transmission across the entire C + L band. In contrast, the DSI profile offers a higher degree of design freedom and superior controllability over the wavelength dependence of DMD. Furthermore, we demonstrate that a common-depressed cladding layer effectively relaxes cutoff conditions for both GI and DSI core profiles. By replacing a previously reported weakly-coupled 3-mode 4-core step-index fiber with the DSI profile, we achieved inter-core crosstalk (XT) < -50 dB/km while maintaining DMD < 60 ps/km across the entire C + L band.