Double-cladding (pure silica) passive photonic crystal fibre is an air-clad fibre with annular air holes that effectively restrict the pump beam transmission in the pure silica inner cladding. Double-cladding passive photonic crystal fibre has a very high numerical aperture in the cladding (due to the large refractive index difference between silica and air hole). The microstructure design of the inner cladding allows the single-mode fibre core to have a large mode field diameter, ensuring high-power laser transmission and high-quality laser beam.