Study On The Characteristics Of Ge-doped Photonic Crystal Fibers

Photonic crystal fibers (PCFs) have a microscopic array of air-holes that run along the fiber length. Due to their flexible structure design, PCFs possess remarkable features which cannot be achieved in conventional optical fibers, such as endless single mode, high nonlinearity, high birefringence, controllable dispersion, large mode area, etc. The applications in optical communications, nonlinear optics, and optoelectronics have also become the hot topic of international optoelectronic industry.Germanium doped in the core region can improve fiber material's photosensitivity which is the basis of making fiber Bragg grating. The effective refraction index of transmission mode in the waveguide will be changed when the core radius or concentration is modified, and at the same time the optical fiber photosensitivity is improved. It also will have effects on the coupling between different grating modes. So the research on the effects of germanium doping in the core region of PCFs can provide theoretical basis for fiber grating design.In this paper, we firstly study the effects of Ge-doping in the core region on the guided modes and effective refractive index of PCFs, through changing the radius of doped area or doped concentration. Then, the effects of structure on the guided modes are observed through changing the PCF's cladding air-holes. Finally, we study the effects of Ge-doping on the characteristics of PCFs, such as the mode cut-off, nonlinear effect, dispersion and the splice loss with SMF-28.We find that different Ge-doped radius has great effect on the effective refractive index of fundamental mode, and the guided mechanism is modified for larger doping radius. The larger radius and concentration have impact on the characteristics of PCFs, such as the mode cut-off, nonlinear effect and dispersion. For example, larger doping radius leads that the cut-off wavelength is red-shifted and the cut-off relative apertures will become smaller; higher doping concentration significantly increases the nonlinear coefficient; the effects of cladding air holes on dispersion become less when the doping radius is increased. Finally we find the doping radius and concentration range which manifests PCFs better characteristics, such as high nonlinear effect and low splice loss.