A method of fabricating a waveguide in a substrate by means of a femtosecond laser system, said waveguide having a birefringence axis tilted at a fixed angle for at least a segment, comprising: - preparing a substrate in which the waveguide has to be fabricated, the substrate having a free surface upon which a pulsed laser beam is impinging; - positioning an immersion lens apt to focus the laser beam in the substrate; - introducing a first optical element; - making the laser beam impinge on the first optical element, which directs the laser beam towards the immersion lens; - focusing the femtosecond pulsed laser beam at a given depth inside the substrate, in order to induce a refractive index change in a volume of said substrate around the focus, wherein the laser beam propagates in the substrate along a direction, said focusing including making the laser beam impinge on said-immersion lens in a first position; - varying the propagation direction of the laser beam in the substrate, maintaining the beam focused, translating or rotating the first optical element, in order to operate a translation of the laser beam, to modify the impinging position of the laser beam on the immersion lens, in order to obtain a first propagation direction forming a first non-zero angle (θ t) with respect to the normal direction to the free surface of the substrate; - translating the laser beam focus with respect to the substrate in order to create a first waveguide segment, defined by adjacent volumes in which the refractive index has been modified, wherein the birefringence axis of the formed first waveguide segment is at said fixed angle, which is a function of the first angle (θ t) describing the obtained first propagation direction of the laser beam in the substrate. - preparing a substrate in which the waveguide has to be fabricated, the substrate having a free surface upon which a pulsed laser beam is impinging; - positioning an immersion lens apt to focus the laser beam in the substrate; - introducing a first optical element; - making the laser beam impinge on the first optical element, which directs the laser beam towards the immersion lens; - focusing the femtosecond pulsed laser beam at a given depth inside the substrate, in order to induce a refractive index change in a volume of said substrate around the focus, wherein the laser beam propagates in the substrate along a direction, said focusing including making the laser beam impinge on said-immersion lens in a first position; - varying the propagation direction of the laser beam in the substrate, maintaining the beam focused, translating or rotating the first optical element, in order to operate a translation of the laser beam, to modify the impinging position of the laser beam on the immersion lens, in order to obtain a first propagation direction forming a first non-zero angle (θ t) with respect to the normal direction to the free surface of the substrate; - translating the laser beam focus with respect to the substrate in order to create a first waveguide segment, defined by adjacent volumes in which the refractive index has been modified, wherein the birefringence axis of the formed first waveguide segment is at said fixed angle, which is a function of the first angle (θ t) describing the obtained first propagation direction of the laser beam in the substrate.