Achromatic Broadband Multi‐Layer Diffraction Mode Multiplexing.

Efforts have been made in recent years to achieve broadband mode‐division multiplexing (MDM) for wavelength and mode hybrid multiplexing optical interconnect. However, most multiplexers are suffering from either strong chromatic aberration or material dispersion, like planar lightwave circuits, mode selective couplers, and single‐layer diffraction elements. The multi‐layer diffraction structure provides a method for achieving both mode conversion and broadband properties. However, the reported multi‐layer diffraction MDM can usually only achieve high efficiency working at a specific narrow bandwidth due to the monochromatic coherent light design. Here, a general achromatic broadband multi‐layer diffraction model is proposed to compensate the dispersion by wavefront matching of discrete multi‐wavelength optical fields. Using this model, an achromatic orbital angular momentum beam generator and an achromatic linearly polarized mode multiplexer for coarse wavelength‐division multiplexing (CWDM) optical interconnections are designed. Compared with the monochromatic designs, the generator exhibits lower wavelength dependence and the multiplexer doubles the bandwidth with mode purity exceeding 91%. Additionally, signal transmission over a 5‐km fiber is demonstrated experimentally in the CWDM‐MDM hybrid communication system. This work represents a solid step toward practical implementation of achromatic MDM and may find applications in the design of various achromatic devices. [ABSTRACT FROM AUTHOR]