A process for delaying a useful optical signal (P 1 ) having a wavelength value λ comprised between 0.2 μm and 3 μm, with respect to a reference optical signal (P 2 ) having the same wavelength value λ, comprising the following actions which are performed simultaneously: - causing the useful optical signal (P 1 ) to propagate in an optical fiber , between two opposite ends of said optical fiber, whereas the reference optical signal (P 2 ) does not propagate in said optical fiber; - causing at least a part of the useful optical signal (P 1 ) that has propagated in the optical fiber without change in the wavelength value of said part of the useful optical signal, to mix with the reference optical signal (P 2 ), so that a stationary optical intensity produced by the optical signal mixing depends on a delay which is effective for the part of the useful optical signal due to propagation time in the optical fiber; and - adjusting a length (L) of the optical fiber through elastic stretching of said optical fiber, so as to produce a selected value for the delay due to the propagation time of the part of the useful optical signal (P 1 ) in the optical fiber, or so as to obtain a selected value for the stationary optical intensity produced by the optical signal mixing, characterized in that a portion of the optical fiber which is comprised between both ends of said optical fiber, is a tapered fiber portion having a diameter (d 10 ) comprised between λ/3 and 10 μm, so that a ratio (ΔL/L)/T, the inverse of the Young's modulus, is comprised between 20 %/N and 4·10 5 %/N, where L is a length of the tapered fiber portion, T is a tension applied to the optical fiber between both ends thereof for stretching elastically the tapered fiber portion, ΔL is an elastic lengthening of said tapered fiber portion caused by the tension T, and ΔL/L being the lengthening ratio expressed as a percentage value. - causing the useful optical signal (P 1 ) to propagate in an optical fiber , between two opposite ends of said optical fiber, whereas the reference optical signal (P 2 ) does not propagate in said optical fiber; - causing at least a part of the useful optical signal (P 1 ) that has propagated in the optical fiber without change in the wavelength value of said part of the useful optical signal, to mix with the reference optical signal (P 2 ), so that a stationary optical intensity produced by the optical signal mixing depends on a delay which is effective for the part of the useful optical signal due to propagation time in the optical fiber; and - adjusting a length (L) of the optical fiber through elastic stretching of said optical fiber, so as to produce a selected value for the delay due to the propagation time of the part of the useful optical signal (P 1 ) in the optical fiber, or so as to obtain a selected value for the stationary optical intensity produced by the optical signal mixing, characterized in that a portion of the optical fiber which is comprised between both ends of said optical fiber, is a tapered fiber portion having a diameter (d 10 ) comprised between λ/3 and 10 μm, so that a ratio (ΔL/L)/T, the inverse of the Young's modulus, is comprised between 20 %/N and 4·10 5 %/N, where L is a length of the tapered fiber portion, T is a tension applied to the optical fiber between both ends thereof for stretching elastically the tapered fiber portion, ΔL is an elastic lengthening of said tapered fiber portion caused by the tension T, and ΔL/L being the lengthening ratio expressed as a percentage value. <EOS>