Off-axis dispersion-managed metasurface for routing orbital angular momentum mode and wavelength multiplexing channels

Abstract

Offering the strengths of mode orthogonality and physical independence from wavelength dimension, optical orbital angular momentum (OAM) modes hold immense potential for enhancing communication capacity through multi-dimensional channel multiplexing. Although methods using angle-separated gratings have seen advances in multi-dimensional (de)multiplexing, routing these multiplexed channels is impeded by the resultant mode-wavelength dispersion resulting from Bragg diffraction of grating. This induces not only multi-level mode conversion but also confined wavelength separation scope, posing obstacles in spatial reallocation of both OAM modes and wavelengths for routing channels. To tackle these issues, we propose a wavelength-dependent multi-foci transformation solution for OAM mode-wavelength routing that utilizes an off-axis dispersion-managed metasurface. Incorporated with composite lens phases and helical modulations, the metasurface enables the precise manipulation of OAM mode-wavelength dispersion upon multi-foci Fourier transform without multi-level diffraction, facilitating both efficient mode conversion and versatile wavelength separation. Harnessing the multi-dimensional independence, this approach establishes a high-dimensional linear mapping relationship among OAM modes, wavelengths, and spatial positions, thereby achieving the routing of mode-wavelength hybrid channels. In a proof-of-concept simulation, we demonstrated the successful routing of 20 channels with five OAM modes and four wavelengths across four depth planes, with the average channel crosstalk below -15.2 dB. Additionally, 16-ary quadrature amplitude modulation signals carried by these 20 multiplexed channels were successfully routed, and the bit-error-rates approach 1 × 10^-5. Our method shows flexibility in spatial reallocation of both OAM modes and wavelengths, as further explored by altering the number of routing channels and their spatial positions, which may provide new insights for advanced OAM-based optical communications.