Abstract: A pulsed fiber laser oscillator comprising in succession: a pumping source (10) for providing pumping energy at a first wavelength to a multimode optical fiber; a first Bragg grating optical filter (11) reflecting a second wavelength and transparent to said first wavelength; an active optical fiber (20) having a predefined wavelength which emits radiation at said second wavelength; a switch (14) arranged to periodically interrupt said second wavelength to provide a pulsed laser beam; and a second Bragg grating optical filter (15) partially reflecting said second wavelength; characterized by comprising, following said active optical fiber (20), a reflector (21) of said first wavelength, such that said pumping energy undergoes two passages within said active optical fiber (20); said predefined wavelength being determined such that the absorption of said pumping energy at said first wavelength within said active optical fiber (20) is greater than 80% after undergoing said two passages.