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Trapped in the stringent adiabatic transmission condition of high-order modes, low-loss fused biconical taper mode selective coupler (FBT-MSC) has long been challenging to achieve. We identify the adiabatic predicament of high-order modes to stem from the rapid variation of the eigenmode field diameter, which is caused by the large core-cladding diameter difference of few-mode fiber (FMF). We demonstrate that introducing a positive-index inner cladding in FMF is an effective approach to address this predicament. The optimized FMF can be used as dedicated fiber for FBT-MSC fabrication, and exhibits good compatibility with the original fibers, which is critical for the wide adoption of MSC. As an example, we add inner cladding in a step-index FMF to achieve excellent adiabatic high-order mode characteristics. The optimized fiber is used to manufacture ultra-low-loss 5-LP MSC. The insertion losses of the fabricated LP01, LP11, LP21, LP02 and LP12 MSCs are 0.13 dB at 1541 nm, 0.02 dB at 1553 nm, 0.08 dB at 1538 nm, 0.20 dB at 1523 nm, and 0.15 dB at 1539 nm, respectively, with smoothly varying insertion loss across the wavelength domain. Additional loss is less than 0.20 dB from 1465.00 nm to 1639.31 nm, and the 90% conversion bandwidth exceeds 68.03 nm, 166.68 nm, 174.31 nm, 132.83 nm, and 84.17 nm, respectively. MSCs are manufactured using commercial equipment and a standardized process that takes just 15 minutes, making them a potential candidate for low-cost batch manufacturing in a space division multiplexing system.

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It is the basic characteristic of pure vortex light that there is a phase singularity at the origin. Such a singularity may be multiple degenerate, which determines the order of vortex light. Singularities splitting phenomenon means that singularities no longer concentrate at the origin but distribute around the space, usually occurring in impure vortex light. In this paper, we demonstrate the singularities splitting phenomenon and propose an analysis method, based on which one may rapidly estimate the modal components of impure vortex light. As two common singularity discrimination methods, the spiral and fork wire interference patterns are compared in distinguishing splitting singularities. The most widely used spiral interference pattern is revealed to be the worst form because of the low resolution. Instead, the fork wire interference pattern is with higher and easily adjusted resolution. 1‰ impurity is still able to be distinguished through fork wire interference patterns in the experiment.

All-fiber mode converters for generating orbital angular momentum (OAM) beams have many applications in optical communications, optical sensing and lasers. Currently, it is a great challenge to use a long-period fiber grating (LPFG) to broadband excite high-order OAM modes above the second-order. Here, we demonstrate a preset-twist LPFG fabrication method, which introduces asymmetry in the refractive index modulation area, for efficient generation of third-order modes. Through optimization, the generation of third-order OAM modes with 99.55% conversion efficiency, 0.81 dB insertion loss, and over 99% purity is achieved with only 40 pitch number. In addition, a multi-period preset-twist LPFG is proposed and demonstrated to achieve the excitation of broadband third-order mode with conversion efficiency of more than 99%, insertion loss of less than 1 dB, and mode purity of more than 90%. The 15 dB bandwidth (96.8% conversion efficiency) of the LPFG is 109 nm in the wavelength range from 1475 nm to 1584 nm, and the 20 dB bandwidth (99% conversion efficiency) of the LPFG is 92 nm from 1488 nm to 1580 nm. To the best of our knowledge, this is the first time to generate efficient and broadband third-order mode using a long-period fiber grating.