[Enlarge image]A 3D model of the waveguide structure. Depending on which single-mode waveguide is coupled in at the input facet light, the light will be transformed into a different OAM state until it reaches the output facet (or vice versa).
A light beam carrying orbital angular momentum (OAM) has a cross section in which the orbit around the beam center acquires a phase of a multiple of 2π. The fact that beams with different OAM are orthogonal allows the OAM to be used as an identifier of different channels, opening the possibility of transporting an increased amount of information through a single fiber.1 To exploit the mode space of OAM in this way, however, requires an efficient, compact device to create, superimpose and decompose OAM modes.2
We recently designed a waveguide structure that provides a solution.3 The structure we developed transforms the eigenmodes from spatially separated, single-mode waveguides adiabatically into modes of a ring waveguide carrying OAM. In a proof-of-principle experiment, the structure successfully multiplexed OAM modes with |OAM| ≤ 2; however, the system can be scaled up to multiplex higher OAM modes as well.
In an adiabatic evolution, the population of the eigenmodes remains constant while the eigenmodes change according to the system. Our waveguide structure uses two mechanisms to keep the propagation constants of each individual mode consistently spaced during the propagation through the structure. First, individual waveguides are detuned by changing their radius. And second, an artificial magnetic field is introduced by twisting the structure. This artificial magnetic field distinguishes between modes with positive and negative OAM and lifts their degeneracy.4
The inherent tolerance of an adiabatic evolution allows the device to operate effectively across a wide spectrum of wavelengths. Besides that, it can also be used as a demultiplexing structure if the adiabatic evolution is run backward. We demonstrated these capabilities both with beam-propagation method simulations and with experiments involving a polymer waveguide structure fabricated via direct laser writing.3 The experiments on the fabricated structures were in good qualitative agreement with the simulations, providing a proof of principle for the structure’s applicability for the generation or decomposition of OAM modes.
Researchers
Julian Schulz, Rheinland-Pfälzische Technische Universität (RPTU) Kaiserslautern-Landau, Kaiserslautern, Germany
Georg von Freymann, RPTU Kaiserslautern-Landau and Fraunhofer Institute for Industrial Mathematics ITWM, Kaiserslautern, Germany
References
1. Y. Li et al. Appl. Sci. 9, 11 (2019).
2. Z.S. Eznaveh et al. Opt. Express 26, 30042 (2018).
3. J. Schulz and G. von Freymann. Adv. Opt. Mater. 12, 2302597 (2024).
4. P.S. Russell et al. Philos. Trans. R. Soc. A 375, 20150440 (2017).