Flat-topped hollow laser beams can be obtained by using the multi-core photonic crystal fiber mixed with GeO2

In this paper, we design a photonic crystal fiber (PCF) which can generate a flat-topped hollow laser beam. The PCF is considered to add six special fiber cores doping with GeO2. The six special fiber cores arrange in hexagon structure. By solving Maxwell's equations with the perfect matched layer (PML) boundary conditions, we find the beam can be adjusted to some extent. Through simulation analysis we can obtain the following points: if we change the inner and outer fiber cores of the PCF or the molar percentage of doping GeO2, the laser intensity distribution of the flat-topped hollow laser beam can be influenced, but there is little influence on the size of dark spot (DSS) and the width of annular beam (Wr). While if we change the size and spacing of cladding air holes of the PCF, the DSS and Wr of flat-topped hollow laser beams can be influenced. Such result well meets our designed needs to gain high quality flat-topped laser beam with various DSS. Namely the flat pattern of obtained flat-topped hollow laser beams has a lower peak intensity in comparison with Gaussian model, and in the red part of the visible and near infrared wave band, the limit losses of the proposed PCF is low, and it also has the independent characteristics of six fiber cores, these properties help flat-topped laser beam effectively reduce the heat effect when trapping the active biological molecules. This doped PCF has simply structural design and is more easily prepared than the liquid filling one, so it has stronger practical applications.