An optical amplifier system for pulsed laser with short or ultrashort and energetic pulses, comprising: - an optical pumping source able to generate a pump wave, - an elongate amplifying medium comprising an input interface able to receive an optical signal to be amplified, said medium being able to amplify said optical signal propagating along said amplifying medium and able to extract an amplified signal, - an optical system able to couple the pump wave in the amplifying medium so as to pump said amplifying medium longitudinally, characterized in that: - the amplifying medium is rotationally symmetric, such as a doped fiber, and has a transverse dimension φ 3 greater than 50 μm and smaller than 1000 μm, - the amplifying medium is a gain medium able to guide said pump wave, said pump-guiding medium having a minimum transverse dimension φ 3 at the input interface, - the optical system is able to focus the pump wave inside the amplifying medium, said focused pump wave having at the focal point a transverse dimension φ 6 smaller than the minimum transverse dimension φ 3 and a numerical aperture smaller than the numerical aperture of the amplifying medium, in such a way that said pump wave propagates freely over a part of the amplifying medium, and that said pump wave propagates in a confined manner over a part of the amplifying medium by total internal reflection inside the guiding gain medium. - an optical pumping source able to generate a pump wave, - an elongate amplifying medium comprising an input interface able to receive an optical signal to be amplified, said medium being able to amplify said optical signal propagating along said amplifying medium and able to extract an amplified signal, - an optical system able to couple the pump wave in the amplifying medium so as to pump said amplifying medium longitudinally, characterized in that: - the amplifying medium is rotationally symmetric, such as a doped fiber, and has a transverse dimension φ 3 greater than 50 μm and smaller than 1000 μm, - the amplifying medium is a gain medium able to guide said pump wave, said pump-guiding medium having a minimum transverse dimension φ 3 at the input interface, - the optical system is able to focus the pump wave inside the amplifying medium, said focused pump wave having at the focal point a transverse dimension φ 6 smaller than the minimum transverse dimension φ 3 and a numerical aperture smaller than the numerical aperture of the amplifying medium, in such a way that said pump wave propagates freely over a part of the amplifying medium, and that said pump wave propagates in a confined manner over a part of the amplifying medium by total internal reflection inside the guiding gain medium. - the amplifying medium is rotationally symmetric, such as a doped fiber, and has a transverse dimension φ 3 greater than 50 μm and smaller than 1000 μm, - the amplifying medium is a gain medium able to guide said pump wave, said pump-guiding medium having a minimum transverse dimension φ 3 at the input interface, - the optical system is able to focus the pump wave inside the amplifying medium, said focused pump wave having at the focal point a transverse dimension φ 6 smaller than the minimum transverse dimension φ 3 and a numerical aperture smaller than the numerical aperture of the amplifying medium, in such a way that said pump wave propagates freely over a part of the amplifying medium, and that said pump wave propagates in a confined manner over a part of the amplifying medium by total internal reflection inside the guiding gain medium.