A dual-frequency fibre laser comprising a laser optical source emitting at least one main polarised optical wave at a first wavelength (λ 1), a fibre amplifier medium and means (11, 24, 201) for interfering with two primary optical waves (O 1, O 1') coming from said main polarised optical wave, said two primary optical waves being: ∘ the same optical frequency v 1 = c λ 1; ∘ the same polarisation state, said state being polarised along an Eigen axis e 1; e 1 and e 2 being the two own axes of the respective refraction indices n e and n o of the doped optical fibre; - said amplifier medium comprising a polarisation-maintaining doped optical fibre and being previously optically pumped to achieve a population inversion between two energy levels, said fibre being highly bi-refringent with bi-refringence Δn = n e -n o,; - the coupling of said two primary optical waves in the amplifier medium generating a diffraction grating within said fibre amplifier medium and thus generating a coherent gain grating over the entire length of the fibre and a DFB laser cavity at the frequency v 2 along the axis of the fibre corresponding to the refraction index no, the DFB laser cavity being injected at the frequency v 1; - some of the amplified spontaneous emission photons polarised along the axis e 2 being the source of a second oscillating wave (02, 02') at the optical frequency v 2 corresponding to the wavelength λ2 and being linked to the gain grating pitch by the equation Λ= λ2 /2no, with, moreover, A = being defined by the following equation: v 2 = n e v 1 n o v 2 v 1; - the pitch of the grating responding to the two equations: Λ = λ1/2ne and Λ=λ2=2no, - said dual-frequency laser emitting out of said amplifier medium at the optical frequencies v 1 and v 2, and said dual-frequency fibre laser comprising: an optical circulator, through which passes one of said two primary optical waves (O1') and said second oscillating wave, so as to generate the beat between said waves (O1',O2) at the frequency v 1 and v 2. ∘ the same optical frequency v 1 = c λ 1; ∘ the same polarisation state, said state being polarised along an Eigen axis e 1; e 1 and e 2 being the two own axes of the respective refraction indices n e and n o of the doped optical fibre; - said amplifier medium comprising a polarisation-maintaining doped optical fibre and being previously optically pumped to achieve a population inversion between two energy levels, said fibre being highly bi-refringent with bi-refringence Δn = n e -n o,; - the coupling of said two primary optical waves in the amplifier medium generating a diffraction grating within said fibre amplifier medium and thus generating a coherent gain grating over the entire length of the fibre and a DFB laser cavity at the frequency v 2 along the axis of the fibre corresponding to the refraction index no, the DFB laser cavity being injected at the frequency v 1; - some of the amplified spontaneous emission photons polarised along the axis e 2 being the source of a second oscillating wave (02, 02') at the optical frequency v 2 corresponding to the wavelength λ2 and being linked to the gain grating pitch by the equation Λ= λ2 /2no, with, moreover, A = being defined by the following equation: v 2 = n e v 1 n o v 2 v 1; - the pitch of the grating responding to the two equations: Λ = λ1/2ne and Λ=λ2=2no, - said dual-frequency laser emitting out of said amplifier medium at the optical frequencies v 1 and v 2, and said dual-frequency fibre laser comprising: an optical circulator, through which passes one of said two primary optical waves (O1') and said second oscillating wave, so as to generate the beat between said waves (O1',O2) at the frequency v 1 and v 2. - said amplifier medium comprising a polarisation-maintaining doped optical fibre and being previously optically pumped to achieve a population inversion between two energy levels, said fibre being highly bi-refringent with bi-refringence Δn = n e -n o,; - the coupling of said two primary optical waves in the amplifier medium generating a diffraction grating within said fibre amplifier medium and thus generating a coherent gain grating over the entire length of the fibre and a DFB laser cavity at the frequency v 2 along the axis of the fibre corresponding to the refraction index no, the DFB laser cavity being injected at the frequency v 1; - some of the amplified spontaneous emission photons polarised along the axis e 2 being the source of a second oscillating wave (02, 02') at the optical frequency v 2 corresponding to the wavelength λ2 and being linked to the gain grating pitch by the equation Λ= λ2 /2no, with, moreover, A = being defined by the following equation: v 2 = n e v 1 n o v 2 v 1; v 2 = n e v 1 n o v 2 v 1; - the pitch of the grating responding to the two equations: Λ = λ1/2ne and Λ=λ2=2no, - said dual-frequency laser emitting out of said amplifier medium at the optical frequencies v 1 and v 2, and said dual-frequency fibre laser comprising: an optical circulator, through which passes one of said two primary optical waves (O1') and said second oscillating wave, so as to generate the beat between said waves (O1',O2) at the frequency v 1 and v 2.