Finding the superior mode basis for mode-division multiplexing: a comparison of spatial modes in air-core fiber

Abstract

Diverse spatial mode bases can be exploited in mode-division multiplexing (MDM) to sustain the capacity growth in fiber-optic communications, such as linearly polarized (LP) modes, vector modes, LP orbital angular momentum (LP-OAM) modes, and circularly polarized OAM (CP-OAM) modes. Nevertheless, which kind of mode bases is more appropriate to be utilized in fiber still remains unclear. Here, we aim to find the superior mode basis in MDM fiber-optic communications via a system-level comparison in air-core fiber (ACF). We first investigate the walk-off effect of four spatial mode bases over 1-km ACF, where LP and LP-OAM modes show intrinsic mode walk-off, while it is negligible for vector and CP-OAM modes. We then study the mode coupling effect of degenerate vector and CP-OAM modes over 1-km ACF under fiber perturbations, where degenerate even and odd vector modes suffer severe mode cross talk, while negligible for highorder degenerate CP-OAM modes based on the laws of angular momentum conservation. Moreover, we comprehensively evaluate the system-level performance for data-carrying single-channel and two-channel MDM transmission with different spatial mode bases under various kinds of fiber perturbations (bending, twisting, pressing, winding, and out-of-plane moving). The obtained results indicate that the CP-OAM mode basis shows superiority compared to other mode bases in MDM fiber-optic communications without using multiple-input multiple-output digital signal processing. Our findings may pave the way for robust shortreach MDM optical interconnects for data centers and high-performance computing.

Mode-bases gain difference for Different phase profiles in Few-mode Erbium-doped Fiber Amplifiers

“…[26][27][28] Both the LP mode and OAM mode can be deduced from each other and synthesized from the linear combination of vector modes in few-mode fibers. 29 The modal field intensity distributions of the LP and OAM modes exhibit different shapes in space. 30 However, the mode spatial intensity distribution determines the photon spatial distribution, which determines the gain in FM-EDF.…”

“…30 However, the mode spatial intensity distribution determines the photon spatial distribution, which determines the gain in FM-EDF. Because of the difference in the mode spatial intensity distributions between the OAM mode and the LP mode within the same order of radial vector mode groups, 29,[31][32][33] there is differential mode-base gain (DMBG) during the amplification process. By controlling the mode phase distribution, [33][34][35] the amplified mode bases in FM-EDFAs are gradually switched between the LP mode and the OAM mode to generate DMBG, which can be utilized to dynamically adjust the DMG and increase the saturated output power with the different order modes of FM-EDFAs.…”