Abstract
In this work, an all-fiber-based mode converter for generating orbital angular momentum (OAM) beams is proposed and numerically investigated. Its structure is constructed by cascading a mode selective coupler (MSC) and an inner elliptical cladding fiber (IECF). OAM modes refer to a combination of two orthogonal LPlm modes with a phase difference of ±π/2. By adjusting the parameters and controlling the splicing angle of MSC and IECF appropriately, higher-order OAM modes with topological charges of l = ±1, ±2, ±3 can be obtained with the injection of the fundamental mode LP01, resulting in a mode-conversion efficiency of almost 100%. This achievement may pave the way towards the realization of a compact, all-fiber, and high-efficiency device for increasing the transmission capacity and spectral efficiency in optical communication systems with OAM mode multiplexing.
Highlights
IntroductionOptical vortex beams (OVBs) possessing orbital angular momentum (OAM), i.e., OAM beams or OAM modes, with a helical phase wavefront indicated by the term exp(il ), where denotes the azimuth angle and l is an integer representing topological charges (TCs), have a phase singularity in the transverse phase center and a doughnut-shaped profile in the transverse intensity distribution[1]
Optical vortex beams (OVBs) possessing orbital angular momentum (OAM), i.e., OAM beams or OAM modes, with a helical phase wavefront indicated by the term exp(il ), where denotes the azimuth angle and l is an integer representing topological charges (TCs), have a phase singularity in the transverse phase center and a doughnut-shaped profile in the transverse intensity distribution[1].In recent decades, mode-division multiplexing has increasingly attracted attention as a method for increasing the transmission capacity and spectral efficiency of optical communication systems, with a special focus on multiplexing technology with OAM modes because of their infinite value of lћ per photon[2,3,4]
We theoretically studied the generation of first-order OAM modes[18] and high-order OAM (HO-OAM) modes by using a helical fiber Bragg grating[19]
Summary
Optical vortex beams (OVBs) possessing orbital angular momentum (OAM), i.e., OAM beams or OAM modes, with a helical phase wavefront indicated by the term exp(il ), where denotes the azimuth angle and l is an integer representing topological charges (TCs), have a phase singularity in the transverse phase center and a doughnut-shaped profile in the transverse intensity distribution[1]. The mode converter is developed by combining a mode-selective coupler (MSC)[32] and an inner elliptical cladding fiber (IECF). Based on the coupled-mode theory, the principle of MSC is to reach the phase-matching condition between the fundamental mode LP01 in the SMF and HOM LPl1 in the MMF. If the effective indices or propagation constants are equal, the phase-matching condition can be satisfied and the power of the LP01 mode in the SMF can be completely transferred to the LPl1 mode in the MMF with fiber lengths that are odd multiples of the half beat coupling length. In order to design the proper IECF length (LIECF) for generating different OAM modes, we calculate the Δneff using COMSOL software and the corresponding beat length (Λ) at a wavelength of 1550 nm, as listed, where Λ can be expressed as Λ = λ/Δneff. According to the coupling mode theory, the polarization states of the output LPlm modes are the same as that of the input LP01 mode[32,36]
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