Improvement of Resolution in Orbital Angular Momentum Decomposition Based on Beam Duplication by using a Sagnac Interferometer

Abstract

Recently, the spatial mode multiplexing based on optical vortices (OVs) attracts much attentions as a promissing technology for high-capacity optical communications. For such OV-based telecommunications, optical components for mode decomposition with respect to azimuth mode indices (topological charges) — so-called orbital angular momentum (OAM) sorters — are key devices, and various kinds of OAM sorters have been demonstrated such as computer-generated holograms as far. Among them, the geometric transformation (GT) technique has great advantages that the insertion loss is quite low and it supports inverse transformation (multiplexing) [1]. In earlier studies, GT-based OAM sorters have been realized by using spatial light modulators (SLMs) [1], and more recently, transparent refractive elements produced by using such as diamond machining [2] and Pancharatnam Berry phase [3] have been demonstrated. However, the GT-based method has the serious disadvantage of relatively large crosstalk between the neighboring modes. In order to overcome the problems, the beam copying technique by using diffractive optics was demonstrated [4]. The novel technique can greatly reduce the crosstalk between the OAM modes, but spoils the advantage that the OAM sorter can support the low-loss inverse transformation. In this paper, we demonstrate the new beam duplication technique by using a Sagnac interferometer, which can improve the resolution in the OAM mode decomposition and also support low-loss inverse transformation.