A fiber optic power laser device comprising: - a power laser diode emitting a pump wave, - an optical resonator comprising: ∘ a fully reflective end and a partially reflective end, ∘ an amplifying multimode optical fiber between the fully reflective end and the partially reflective end, the amplifying multimode optical fiber having a core with a diameter greater than 20 μm, and presenting an input face and an output face at the ends thereof, ∘ a first optical submodule comprising one or several lenses, said first optical submodule being formed between the input face of the multimode optical fiber and the fully reflective end, ∘ a second optical submodule comprising one or several lenses, said second optical submodule being formed between the output face of the multimode optical fiber and the partially reflective end, - optical coupling means (3, 11, 2) that couple said pump wave in the multimode optical fiber so as to produce a laser beam in the optical resonator, a part of said laser beam being transmitted outside the optical resonator through the partially reflective end and another part of said laser beam being reflected inside the optical resonator, characterized in that: - said one or several lenses of the first optical submodule are configured to reimage the fundamental mode issuing from the input face of the multimode optical fiber on itself on the input face, so that the first optical submodule reproduces, after a round trip of the laser beam in the first optical submodule, the amplitude and phase of the fundamental mode onto the input face of said multimode optical fibre; and - wherein the first optical submodule comprises a first single-mode fiber tip in contact with the input face of the multimode optical fiber; and - and the second optical submodule comprises a second single-mode fiber tip in contact with the output face of the multimode optical fiber. - a power laser diode emitting a pump wave, - an optical resonator comprising: ∘ a fully reflective end and a partially reflective end, ∘ an amplifying multimode optical fiber between the fully reflective end and the partially reflective end, the amplifying multimode optical fiber having a core with a diameter greater than 20 μm, and presenting an input face and an output face at the ends thereof, ∘ a first optical submodule comprising one or several lenses, said first optical submodule being formed between the input face of the multimode optical fiber and the fully reflective end, ∘ a second optical submodule comprising one or several lenses, said second optical submodule being formed between the output face of the multimode optical fiber and the partially reflective end, ∘ a fully reflective end and a partially reflective end, ∘ an amplifying multimode optical fiber between the fully reflective end and the partially reflective end, the amplifying multimode optical fiber having a core with a diameter greater than 20 μm, and presenting an input face and an output face at the ends thereof, ∘ a first optical submodule comprising one or several lenses, said first optical submodule being formed between the input face of the multimode optical fiber and the fully reflective end, ∘ a second optical submodule comprising one or several lenses, said second optical submodule being formed between the output face of the multimode optical fiber and the partially reflective end, - optical coupling means (3, 11, 2) that couple said pump wave in the multimode optical fiber so as to produce a laser beam in the optical resonator, a part of said laser beam being transmitted outside the optical resonator through the partially reflective end and another part of said laser beam being reflected inside the optical resonator, - said one or several lenses of the first optical submodule are configured to reimage the fundamental mode issuing from the input face of the multimode optical fiber on itself on the input face, so that the first optical submodule reproduces, after a round trip of the laser beam in the first optical submodule, the amplitude and phase of the fundamental mode onto the input face of said multimode optical fibre; and - wherein the first optical submodule comprises a first single-mode fiber tip in contact with the input face of the multimode optical fiber; and - and the second optical submodule comprises a second single-mode fiber tip in contact with the output face of the multimode optical fiber.