A high power amplifier waveguide system for amplifying an optical signal comprising: a) a pump laser adapted for delivering power through multimode fibers or arrays of fibers bundled together, which pump laser operates at a wavelength λ pump and with a pump power exceeding 100 W; b) a coupling device ; c) a cladding pumped rare earth doped amplifier waveguide ; where the wavelength λ pump is resonant with an absorption band of said rare earth doping, said cladding pumped amplifier waveguide comprising • a signal core adapted to guide an optical signal at a signal wavelength, λ signal , said signal core having an effective refractive index n core-1 ; • an inner cladding region for guiding pump light, said inner cladding region comprising a first inner cladding having an effective refractive index n inner-clad-1 , said first inner cladding comprising at least one rare earth doped silica region, said rare earth element being Ytterbium (Yb); where n core-1 > n inner-clad-1 , where the area of said signal core is larger than 50 μm 2 , and wherein, in use, the overlap of said guided optical signal to said rare earth doped silica region(s), said overlap being defined as the fraction of the optical signal mode power that propagates in the rare earth doped silica region(s) relative to the total optical signal mode power, is 25 %, or less, preferably the signal wavelength, λ signal , of said optical signal is within the interval from 1020 nm to 1090 nm. a) a pump laser adapted for delivering power through multimode fibers or arrays of fibers bundled together, which pump laser operates at a wavelength λ pump and with a pump power exceeding 100 W; b) a coupling device ; c) a cladding pumped rare earth doped amplifier waveguide ; where the wavelength λ pump is resonant with an absorption band of said rare earth doping, said cladding pumped amplifier waveguide comprising • a signal core adapted to guide an optical signal at a signal wavelength, λ signal , said signal core having an effective refractive index n core-1 ; • an inner cladding region for guiding pump light, said inner cladding region comprising a first inner cladding having an effective refractive index n inner-clad-1 , said first inner cladding comprising at least one rare earth doped silica region, said rare earth element being Ytterbium (Yb); where n core-1 > n inner-clad-1 , where the area of said signal core is larger than 50 μm 2 , and wherein, in use, the overlap of said guided optical signal to said rare earth doped silica region(s), said overlap being defined as the fraction of the optical signal mode power that propagates in the rare earth doped silica region(s) relative to the total optical signal mode power, is 25 %, or less, preferably the signal wavelength, λ signal , of said optical signal is within the interval from 1020 nm to 1090 nm. • a signal core adapted to guide an optical signal at a signal wavelength, λ signal , said signal core having an effective refractive index n core-1 ; • an inner cladding region for guiding pump light, said inner cladding region comprising a first inner cladding having an effective refractive index n inner-clad-1 , said first inner cladding comprising at least one rare earth doped silica region, said rare earth element being Ytterbium (Yb); where n core-1 > n inner-clad-1 , where the area of said signal core is larger than 50 μm 2 , and wherein, in use, the overlap of said guided optical signal to said rare earth doped silica region(s), said overlap being defined as the fraction of the optical signal mode power that propagates in the rare earth doped silica region(s) relative to the total optical signal mode power, is 25 %, or less, preferably the signal wavelength, λ signal , of said optical signal is within the interval from 1020 nm to 1090 nm. <EOS>