The waveguide structure of the proposed special fiber is shown in Fig.1. It consists of three layers: core, inner cladding and outer cladding respectively. The core and the outer cladding are both made of pure silica whilst the inner cladding is made of silica doped by fluorine element. Accordingly, the refractive index of the fiber core is equal to that of the outer cladding and larger than that of the inner cladding, just as depicted by Fig. 1.