Numerical aperture of optical fiber with pure quartz core and fluorine-doped glass cladding

Optical fiber with large pure quartz core and fluorine-doped glass cladding is a new structure of large core fiber,the material of core is pure quartz glass,and the cladding is quartz glass doped with fluorine,because the fluorine impurity can reduce the refractive index of the quartz glass,so the core and cladding material with low refractive index can consist of the light waveguide.It is made by POD(plasma outside deposition) technic in abroad,we use creatively the rode in tube method to make it with the same structure in domestic,in order to promote the property of the fiber,we coat it with double coating material,i.e.in-coating with low refractive index and out-coating with high refractive index,we found that the numerical aperture of the fiber is influenced greatly,which changes from 0.1889 to 0.3152 with the coating length changes shorter and shorter.