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 high-order 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 short-reach MDM optical interconnects for data centers and high-performance computing.

Orbital angular momentum (OAM), emerging as an inherently high-dimensional property of photons, has boosted information capacity in optical communications. However, the potential of OAM in optical computing remains almost unexplored. Here, we present a highly efficient optical computing protocol for complex vector convolution with the superposition of high-dimensional OAM eigenmodes. We used two cascaded spatial light modulators to prepare suitable OAM superpositions to encode two complex vectors. Then, a deep-learning strategy is devised to decode the complex OAM spectrum, thus accomplishing the optical convolution task. In our experiment, we succeed in demonstrating 7-, 9-, and 11-dimensional complex vector convolutions, in which an average proximity better than 95% and a mean relative error <6 % are achieved. Our present scheme can be extended to incorporate other degrees of freedom for a more versatile optical computing in the high-dimensional Hilbert space.

Explosive growth in demand for data traffic has prompted exploration of the spatial dimension of light waves, which provides a degree of freedom to expand data transmission capacity. Various techniques based on bulky optical devices have been proposed to tailor light waves in the spatial dimension. However, their inherent large size, extra loss, and precise alignment requirements make these techniques relatively difficult to implement in a compact and flexible way. In contrast, three-dimensional (3D) photonic chips with compact size and low loss provide a promising miniaturized candidate for tailoring light in the spatial dimension. Significantly, they are attractive for chip-assisted short-distance spatial mode optical interconnects that are challenging to bulky optics. Here, we propose and fabricate femtosecond laser-inscribed 3D photonic chips to tailor orbital angular momentum (OAM) modes in the spatial dimension. Various functions on the platform of 3D photonic chips are experimentally demonstrated, including the generation, (de)multiplexing, and exchange of OAM modes. Moreover, chip-chip and chip–fiber–chip short-distance optical interconnects using OAM modes are demonstrated in the experiment with favorable performance. This work paves the way to flexibly tailor light waves on 3D photonic chips and offers a compact solution for versatile optical interconnects and other emerging applications with spatial modes.