Mechanism and characteristics of a tunable dispersion-compensating dual-ring microstructure fiber for different orbital angular momentum modes.

A tunable dual-ring microstructure fiber that can support stable transmission for different orbital angular momentum (OAM) states and possess ultrahigh dispersion coefficients and low confinement losses is proposed and theoretically investigated. The proposed fiber is composed of two high-refractive-index rings and a double-cladding structure. Owing to the central air core and outer cladding, the dual-ring structure can support stable transmission for the OAM states. The mode fields of different OAM states in the inner ring can spread to the outer ring under certain conditions, which leads to high absolute values of dispersion around the coupling wavelengths. By tuning the refractive indices of the dual rings, the proposed fiber can achieve dispersion control for different OAM modes. Moreover, the specially designed two-layer air holes in the inner cladding can affect the mode-coupling coefficients, which are characterized by the effective mode areas and the overlap integral of the electric fields between the resonant ring modes. Therefore, the dispersion curves and operating wavelengths of the OAM modes can be modulated by regulating the physical parameters (the radius of the two-layer air holes or the infiltrated functional materials) of the inner cladding. We built a theoretical model and analyzed the modulation method and mechanism of the dispersion curves based on the coupled mode theory. The theoretical results indicate that the proposed fiber is flexible and has potential dispersion-compensating applications in fiber OAM systems.