A light beam with the helical wavefront (ejlϕ) enables it carry the orbital angular momentum (OAM) [1]. The OAM per photon is lh/2π, where l is the topological charge and h is the Planck’s constant. Compared with the spin angular momentum, the OAM per photon can be theoretically infinitely large. So the OAM beams have the extensive applications, such as multidimensional micro-manipulation [2], super-resolution microscopy [3], high-capacity optical communications [4], and nonlinear optical area [5]. Multiple OAM beams with different topological charges, polarization states and wavelengths are exploited simultaneously in these applications. So how to conveniently sort the different OAM beams with the high integrity and high purity has become a basic problem for expanding these OAM applications.

There are four main approaches to sort the OAM beams in free space. Firstly, the specific OAM beam can be converted to the fundamental mode though the spatial light modulator, and be further sort out by the single-mode fiber [6], [7]. Z. Wang et al. designed an on-chip OAM sorter, which consists of several the arc-shaped waveguide grating coupler for coupling different spatial vortex beams into the specific waveguides [8]. Secondly, based on the plasmonic gratings, metalens, and Bloch surface wave structures, the different OAM mode will be focused into spatially separated subwavelength region [9], [10], [11], [12]. For example, X. Zhao et al. have demonstrated a beam splitter with the metallic nanoslit array for distinguishing both the spin and orbital components of the OAM beams [13]. Thirdly, by employing a Cartesian to log-polar transformation, the OAM beams can be mapped to a rectangular-shaped plane light with a transverse phase gradient and be spatially separated by optical lens [14], [15], [16]. R. Fickler et al. proposed a phase transformations scheme for sorting OAM beams, which consist of the phase elements designed by the evolutionary optimization algorithms [17]. All these three methods have been widely used in OAM detection devices and demultiplexing systems. However, the separated beams cannot be further applied in OAM systems because the helical phase structures, annular intensity distribution, and central singularities of the original OAM beams have been severely damaged. Fourthly, by using the Dove prisms and the Mach–Zehnder interferometer, or a beam splitter and two Porro prisms, or the cascaded tunable resonators, the OAM beams are totally preserved and sorted into different subsets [18], [19], [20], [21]. Q. Jia et al. also propose a nondestructive OAM beams sorting technique based on a pre-designed translation operator, which is numerically solved by the gradient descent algorithm [22]. Such non-destructive schemes are excellent for the further OAM application, for example, the quantum communication. However, those free-space optical systems are complicated in aligning and are incompatible with the requirement of integration and miniaturization.

Compared with bulk-optic elements, fiber devices have emerged as promising alternatives to engineer OAM light with high mode purity, low loss, compact structure, and good stability [23]. There are three types of OAMs fiber filter, namely helical fiber gratings, photonic lanterns, and fiber mode selective coupler. Based on the helical fiber gratings, the OAM modes have been flexible generated, converted, and exchanged [24]. For example, J. Tu et al. experimentally demonstrated the generation of OAM modes based on a helically twisted hollow-core antiresonant fiber [25]. G. Wu et al. used three long-period ring-core fiber gratings for generating the 1-, 2- or 3-order OAM modes [26]. Y. Cui et al. proposed a helically photonic crystal fiber for filtering the OAM modes with the same handedness [27]. However, this shame is difficult to further nondestructively split the OAM mode with the specific topological charge. The OAM photonic lantern, consisted of a multimode waveguide and multiple single-mode waveguides, convert the OAM mode into several discrete single-modes [28], [29], [30]. A. Alarcon et al. demonstrate an all-fiber OAM sorter by using a few-mode photonic lantern, a multiport beam splitter, and phase modulators [31]. The mode selective coupler based on a single-mode fiber and an OAM fiber have been proposed and fabricated for converting the OAM mode to the fundamental mode in a certain fiber port [32], [33]. X. Xue et al. fabricated an OAM mode selective coupler by the side-polished technology for converting the fundamental mode in single mode fiber and the OAM modes in ring-core erbium-doped fiber [34]. The above two OAM fiber sorters both damaged the annular shape and helical phase of the OAM modes. The nondestructive fiber sorter can be realized by matching the effective refractive indices of the same OAM fiber modes, instead of the fundamental mode. However, the close proximity with the effective refraction indices of the modes in the degenerated group will cause the serious crosstalk, which limited the separated beams to be used in the further OAM applications.

In this paper, we proposed an all-fiber asymmetrical structure for nondestructively sorting the OAM modes. The OAM modes with different topological charges, polarization states and wavelengths can be separated with the high purity (>90%). The multiple modes sorter can be realized through the multistage cascaded structure. The proposed all-fiber OAM mode sorter can be a potential multiplexer for the OAM fiber communication networks, OAM fiber lasers, sensors, and other systems.