We propose a photonic crystal fiber with a high refractive index ring, which supports up to 180 orbital angular momentum (OAM) modes without any high order radial modes from 1.5 um to 1.7 um. The finite element method is employed to numerically study the properties of the designed fiber. Further research shows that the refractive index differences of all the eigenmodes are above 2 ×10 ^{- 3} , which indicates that the corresponding eigenmodes can be well-separated and ensures the stable transmission of OAM modes. In addition, it is quite interesting that the high order radial modes are significantly suppressed via increasing the radius of the cladding in the design, while most of the OAM modes are without phase distortion, which is beneficial for the (de)multiplexing of the OAM modes. Particularly, the mode quality of all the eigenmodes is higher than 94.9% and the chromatic dispersion is flat with minimum value of the dispersion variation is 0.36 ps/(km·nm). Moreover, the designed fiber possesses various advantages such as low nonlinear coefficient (<; 0.7 /W/km) and confinement loss (on the order between 10 ^-12 and 10 ^{- 7} dB/m). The proposed fiber has shown great potential for high capacity OAM mode division multiplexing communications.

In this paper, we propose a metamaterial structure for realizing the electromagnetically induced transparency effect in the MIR region, which consists of a gold split-ring and a graphene split-ring. The simulated results indicate that a single tunable transparency window can be realized in the structure due to the hybridization between the two rings. The transparency window can be tuned individually by the coupling distance and/or the Fermi level of the graphene split-ring via electrostatic gating. These results could find significant applications in nanoscale light control and functional devices operating such as sensors and modulators.

In this paper, we numerically and theoretically study the tunable plasmonically induced transparency (PIT) effect based on the graphene metasurface structure consisting of a graphene cut wire (CW) resonator and double split-ring resonators (SRRs) in the middle infrared region (MIR). Both the theoretical calculations according to the coupled harmonic oscillator model and simulation results indicate that the realization of the PIT effect significantly depends on the coupling distance and the coupling strength between the CW resonator and SRRs. In addition, the geometrical parameters of the CW resonator and the number of the graphene layers can alter the optical response of the graphene structure. Particularly, compared with the metal-based metamaterial, the PIT effect realized in the proposed metasurface can be flexibly modulated without adding other actively controlled materials and reconstructing the structure by taking advantage of the tunable complex surface conductivity of the graphene. These results could find significant applications in ultrafast variable optical attenuators, sensors and slow light devices.

We propose a temporal manipulation scheme for the inner-mode interactions in graded-index multimode fibers (GRIN-MMF). In this scheme, a so-called Pearcey-Gaussian (PeG) wave packet is constructed for the pulse profile, which allows the manipulation of the self-compressing control by the second- and third-order dispersions. Our simulations show that such linear compressing cooperates with the nonlinear compressing of the Kerr effect when the wavelength is close to the zero-dispersion point of the fiber, which enables the manipulation of the nonlinear energy transfer from the higher modes to the fundamental one. Such cooperation of the linear and nonlinear effects triggers a Stokes soliton in the fundamental mode while the Raman red-shifts in the higher modes are negligible. After the proper filter in the spectrum, a nearly perfect Raman self-cleaning with the energy of the fundamental mode reaching 97.2 % is observed.