Method for measuring a temperature and/or a strain in an optical fiber, comprising the steps of: - Calibrating the optical fiber by measuring, for at least one longitudinal position inside the fiber, a signal (21, 22, 23, 24, 27) sensitive to an intensity and a phase of a Rayleigh backscattering signal of the optical fiber generated by injecting a laser beam in the optical fiber at a known state of temperature and a known state of strain, for various conditions of frequencies or wavelengths or wavenumbers of the backscattered signal by: ∘ moving the laser central frequency for each scan in the frequency domain among several scans in the frequency domain, or ∘ changing the laser frequency, - Measuring the signal sensitive to an intensity and a phase of a Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain, for various frequencies or wavelengths or wavenumbers of the backscattered signal and for at least one longitudinal position inside the fiber, - Based on a shift of frequency or temporal period or wavelength or wavenumber or a correlation between: ∘ the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain, for various conditions of frequencies or wavelengths or wavenumbers of the backscattered signal and/or of known states of temperature and/or known states of strain, and ∘ the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain, determining, by technical means, the absolute value of the unknown state of temperature and/or the unknown state of strain, for at least one longitudinal position inside the optical fiber the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain being done for more frequencies or wavelengths than the measurement of the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain. - Calibrating the optical fiber by measuring, for at least one longitudinal position inside the fiber, a signal (21, 22, 23, 24, 27) sensitive to an intensity and a phase of a Rayleigh backscattering signal of the optical fiber generated by injecting a laser beam in the optical fiber at a known state of temperature and a known state of strain, for various conditions of frequencies or wavelengths or wavenumbers of the backscattered signal by: ∘ moving the laser central frequency for each scan in the frequency domain among several scans in the frequency domain, or ∘ changing the laser frequency, ∘ moving the laser central frequency for each scan in the frequency domain among several scans in the frequency domain, or ∘ changing the laser frequency, - Measuring the signal sensitive to an intensity and a phase of a Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain, for various frequencies or wavelengths or wavenumbers of the backscattered signal and for at least one longitudinal position inside the fiber, - Based on a shift of frequency or temporal period or wavelength or wavenumber or a correlation between: ∘ the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain, for various conditions of frequencies or wavelengths or wavenumbers of the backscattered signal and/or of known states of temperature and/or known states of strain, and ∘ the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain, ∘ the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain, for various conditions of frequencies or wavelengths or wavenumbers of the backscattered signal and/or of known states of temperature and/or known states of strain, and ∘ the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain, determining, by technical means, the absolute value of the unknown state of temperature and/or the unknown state of strain, for at least one longitudinal position inside the optical fiber the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain being done for more frequencies or wavelengths than the measurement of the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain.