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To improve the performance of double clad high power fibre lasers, inner cladding design plays a significant role.A triangular inner cladding and silica structure second cladding with large air holes to acquire high inner cladding numerical aperture are designed. Single mode and high power output of the fibre lasers need the double clad Yb doped fibre with large core. A fibre with annular refractive index distribution core and low numerical aperture to acquire a large mode area fibre core is designed and fabricated. Furthermore co-doping with aluminium (Al) has been used to improve the solubility of ytterbium (Yb) into silicate network, and the core absorption coefficients of two Yb doped fibres are compared with different Al concentration experimentally.