An optical-delay-line device providing a fixed or variable optical delay, said device comprising: - an optical input, - an optical output, - an optical assembly that directs a beam along an optical path from said optical input to said optical output, said optical assembly comprising: - a retroreflector that ensures a self-alignment of the direction of the output beam with the direction of the incident beam, the direction of the output beam, which is called the second direction, being opposite to that of the incident beam, characterized in that said optical-delay-line device further comprises - an optical element that is said to ensure positional invariance, comprising a first reflective surface, a second reflective surface and a third reflective surface, said first, second and third reflective surfaces being planar or substantially planar, said second and third reflective surfaces being arranged to make therebetween an angle of 45°, the beam that propagates along said optical path and that enters said optical element in said second direction, being reflected by said first surface with an angle of 90° toward said second surface, then being reflected in succession by said second and third surfaces, or reflected along an inverse optical path, in succession by the third surface, the second surface and the first surface, when it enters said optical element in said second direction, said beam is reflected first by said third surface, in order to exit from said optical element in a direction parallel to the direction of the beam incident on said optical element, said output beam having a constant positional offset Δ with respect to the incident beam, - said optical element being arranged in said optical assembly so as to steer said output beam onto said retroreflector, the return beam thus passing through said retroreflector. - an optical input, - an optical output, - an optical assembly that directs a beam along an optical path from said optical input to said optical output, said optical assembly comprising: - a retroreflector that ensures a self-alignment of the direction of the output beam with the direction of the incident beam, the direction of the output beam, - an optical element that is said to ensure positional invariance, comprising a first reflective surface, a second reflective surface and a third reflective surface, said first, second and third reflective surfaces being planar or substantially planar, said second and third reflective surfaces being arranged to make therebetween an angle of 45°, the beam that propagates along said optical path and that enters said optical element in said second direction, being reflected by said first surface with an angle of 90° toward said second surface, then being reflected in succession by said second and third surfaces, or reflected along an inverse optical path, in succession by the third surface, the second surface and the first surface, when it enters said optical element in said second direction, said beam is reflected first by said third surface, in order to exit from said optical element in a direction parallel to the direction of the beam incident on said optical element, said output beam having a constant positional offset Δ with respect to the incident beam, - said optical element being arranged in said optical assembly so as to steer said output beam onto said retroreflector, the return beam thus passing through said retroreflector.