Optical filter insensitive to polarisation centred on a wavelength (λ 0) measured in a vacuum, comprising: -- a first and a second waveguide (21, 21') each supporting a propagation mode; -- a first grating formed in or on the surface of the first waveguide, the first grating being periodic at least along a first axis Ox defining a first orthonormal base Oxyz; -- a second grating formed in or on the surface of the second waveguide, the second grating being periodic at least along a second axis Oy' defining a second orthonormal base Ox'y'z; - the first and the second gratings are arranged above each other, the Oz axis being normal to the surface of the waveguides, and are such that the first axis Ox and the second axis Oy' form an angle of π/2+ξ radians such that, when the first grating is illuminated by a light beam, the first and second propagation modes are excited and their fields are orthogonal for an angle of incidence of the determined light beam, - the directions of propagation V 1, V 2 of the orders of diffraction n and m, n and m being relative integers, of the first and second grating being defined by V 1 = sin φ cos φ − nλ d 1 x ^ + sin θ sin φ y ^ V 2 = sin θ cos π 2 − φ + ξ − mλ d 2 y ^ ʹ + sin θ sin π 2 − φ + ξ x ^ ʹ, with θ and φ being respectively polar and azimuth angles, and λ the wavelength corresponding to the incidental beam, the excitation of the first mode via the first grating being ∥V 1 ∥ ≈ n f1, the excitation of the second mode via the second grating being ||V 2 ∥ ≈ n f2, the orthogonality between the fields of the two excited mode being translated by V 1· V 2 =0 · -- a first and a second waveguide (21, 21') each supporting a propagation mode; -- a first grating formed in or on the surface of the first waveguide, the first grating being periodic at least along a first axis Ox defining a first orthonormal base Oxyz; -- a second grating formed in or on the surface of the second waveguide, the second grating being periodic at least along a second axis Oy' defining a second orthonormal base Ox'y'z; - the first and the second gratings are arranged above each other, the Oz axis being normal to the surface of the waveguides, and are such that the first axis Ox and the second axis Oy' form an angle of π/2+ξ radians such that, when the first grating is illuminated by a light beam, the first and second propagation modes are excited and their fields are orthogonal for an angle of incidence of the determined light beam, - the directions of propagation V 1, V 2 of the orders of diffraction n and m, n and m being relative integers, of the first and second grating being defined by V 1 = sin φ cos φ − nλ d 1 x ^ + sin θ sin φ y ^ V 2 = sin θ cos π 2 − φ + ξ − mλ d 2 y ^ ʹ + sin θ sin π 2 − φ + ξ x ^ ʹ, with θ and φ being respectively polar and azimuth angles, and λ the wavelength corresponding to the incidental beam, the excitation of the first mode via the first grating being ∥V 1 ∥ ≈ n f1, the excitation of the second mode via the second grating being ||V 2 ∥ ≈ n f2, the orthogonality between the fields of the two excited mode being translated by V 1· V 2 =0 · - the first and the second gratings are arranged above each other, the Oz axis being normal to the surface of the waveguides, and are such that the first axis Ox and the second axis Oy' form an angle of π/2+ξ radians such that, when the first grating is illuminated by a light beam, the first and second propagation modes are excited and their fields are orthogonal for an angle of incidence of the determined light beam, - the directions of propagation V 1, V 2 of the orders of diffraction n and m, n and m being relative integers, of the first and second grating being defined by V 1 = sin φ cos φ − nλ d 1 x ^ + sin θ sin φ y ^ V 2 = sin θ cos π 2 − φ + ξ − mλ d 2 y ^ ʹ + sin θ sin π 2 − φ + ξ x ^ ʹ, with θ and φ being respectively polar and azimuth angles, and λ the wavelength corresponding to the incidental beam, the excitation of the first mode via the first grating being ∥V 1 ∥ ≈ n f1, the excitation of the second mode via the second grating being ||V 2 ∥ ≈ n f2, the orthogonality between the fields of the two excited mode being translated by V 1· V 2 =0 ·