Nonlinear Optical Effects In A Graphene- Covered Micro/Nano Fiber Composite Structure

Owing to its unique two dimensional structure feature and photoelectric properties, graphene has attracted a lot of interests since it has been found. Especially, graphene is a promising ultra-compact nonlinear optical material as it has very high nonlinear parameter. This thesis combines graphene and micro/nano fiber to form a graphene-covered micro/nano fiber composite structure and analyze its nonlinear optical properties through theoretical modeling and numerical simulation.Based on the research progress of the graphene waveguide, we build the theoretical model of nonlinear parameter process of a graphene covered micro/nano fiber composite structure combining the propagation properties of micro/nano fibers and the optical properties of graphene. According to the simulation calculations, the structure both have high nonlinear parameter and heavy losses, and based on which, a figure-of-merit (FOM) coefficient is used to evaluate the nonlinear performance. For the 1550-nm pump in the telecommunication band, the FOM is optimized to be 0.0083 W-1 in a 0.75-μm-thick graphene covered micro/nano fiber composite structure with a peak pump power of 10 W. Using such a graphene-covered tapered fiber, a maximum conversion efficiency of -25.2 dB is numerically obtained when the graphene length is only 26.2 μm. Moreover, a 3-dB bandwidth as broad as 400 nm can be realized in the 1550-nm telecommunication band, and the bandwidth can be further enhanced to 780 nm if it is pumped at 0.95 μm, near the zero-dispersion wavelength (0.98 μm) of the composite fiber. We can find that covering graphene around micro/nano fiber to form a graphene covered micro/nano fiber composite structure can improve its nonlinear performance effectively.This thesis analyzes the influence of the chemical change of graphene to the composite structure’s nonlinear performance. It turns out that when the graphene chemical is set to be 0.5 eV, the structure will present slit effect in 1550 nm waveband and the ability of graphene layer to restrict the light field is greatly enhanced. Under that condition, the required length of composite structure will be reduced to 2 μm, and the conversion efficiency will be enhanced to 1.6 dB, when the pump power is only 2 mW. However, the conversion bandwidth is ultra-thin, only 4nm due to the waveband which qualified the slit effect is thin.