received by the International Bureau on 01 October 2015 (01.10.2015)

[Claim 1] An apparatus for interrogating at least one sensing optical fibre (34) by analysing the distribution of the peak frequency-shift of stimulated Brillouin scattering along the length of the sensing fibre (34) using a wavelength-scanning optical frequency-domain interferometric technique and characterized by the facts of comprising:

- at least one wavelength- swept primary source (12) of radiation characterized by a wavelength λ_Ρ, and by a spectral linewidth not greater than the spectral width of the Brillouin scattering in the sensing fibre (34) and characterized by a wavelength sweeping width Δλ_Κίνρ wide enough to typically achieve a resolution Δζ not worse than -10cm according to the Δζ » cX²/(2n_gAX_swp); and

- at least one secondary source (14) of wavelength λ_Ρ+Δλ instant- by-instant shifted with respect to the sweeping wavelength of the primary source λ_Ρ of a quantity Δλ that is constant during the wavelength sweeping process and that can be arbitrarily controlled within the limits required for the desired Brillouin analysis scope; and

- at least one sensing optical interferometer (27), having ordinary or modified Mach-Zehnder or Michelson topology, that is excited by one first of the two sources so that it is split into two fractions that are routed respectively into at least one reference arm (38) and one measurement arm that comprises the sensing fibre (34), and, after having travelled along such arms, are recombined to produce an optical differential interference output; and

- at least one mean for injecting the light of the second source in the measurement arm of the interferometer (27) so that to locally produce perturbations of the excitation light by stimulated Brillouin scattering;

- at least one mean (73) to measure the interference signal at least at one of the differential outputs of the interferometer (27) possibly with a balanced differential photodetector pair (40, 41); and

- at least one control and analysis mean (53) adapted to analyse the interference signal in the frequency domain versus the sweeping of the wavelength of the sources and also at multiple different wavelength shifts between the primary and secondary sources.

[Claim 2] An apparatus according to claim 1 characterized by the fact of

comprising at least one mean of commutation (57) of the optical circuit and/or controllable optical attenuator adapted to enable or disable the injection of the light of the second of the two sources in the measurement arm of the interferometer (27), in order to switch the working mode of the apparatus between a "Wavelength-Scanning Brillouin Optical Frequency Domain Analyser (WS-BOFDA)" mode and a (Rayleigh) "Optical frequency Domain Reflectometer (OFDR)" mode.

[Claim 3] An apparatus according to any of the preceding claims characterized by the fact that the secondary source comprises at least one Brillouin ring laser (14) or converter that is seeded by the primary source and adapted to produce a radiation with wavelength X_P+AX(T_b) shifted with respect to that λ_Ρ of the primary source of a quantity AX(T_b) that is controllable through the strain and/or temperature of the Brillouin gain medium (17) of the ring circuit(14).

[Claim 4] An apparatus according to claim 3 characterized by the fact of

comprising at least one modulation or suppression system for the mode -hopping of the Brillouin laser (14) comprising at least one mode mixer active or passive and/or a system (54, 21) adapted to vary the resonance length of the Brillouin laser cavity, possibly with feedback from the emission intensity (47, 52, 48, 53), so that it can be properly tuned during the wavelength-sweeping process.

[Claim 5] An apparatus according to the claim 3 characterized by the fact of

comprising at least one purging system (55) for the output spectrum of the Brillouin LASER acting on the ring-type optical circuit (14) in order to accelerate the quenching of the distortion of the spectrum of the light produced in the optical circuit consequently to the wavelength- sweeping process.

[Claim 6] An apparatus according to the claim 3 characterized by the fact that the

Brillouin gain medium (17) of the ring laser (14) comprises at least one silicon photonic suspended waveguide, ad/or at least one Photonic Crystal Fibre (PCF), and/or at least one optical fibre having reduced Mode Field Diameter (MDF), and/or at least one non-linear waveguide or optical fibre with enhancement of the overlapping between longitudinal photonic and phononic modes and possibly suppression of shear modes.

[Claim 7] An apparatus according to any of the preceding claims 1, 2 or 3 characterized by the fact of comprising at least one system for controlling the state of polarization, such as polarizer and/or polarization controller and/or de-polarizer.

[Claim 8] An apparatus according to any of the preceding claims 1, 2 or 3 char- acterized by the fact of comprising at least one mean of optical

switching adapted to switch between a working mode with the sensor connected in transmission and a working mode with the sensor connected in reflection.

[Claim 9] An apparatus according to any of the preceding claims 1, 2 or 3 characterized by the fact of comprising at least one reflector or absorber possibly partially reflective/absorbing even in a variable and/or controllable way, connected so that to induce a stationary propagation, or non-stationary even partially, in the measurement arm of the measurement interferometer.

[Claim 10] An apparatus according to any of the preceding claims 1 or 3 characterized by the fact of comprising at least one system for generating optical pulses connected so that to limit in the time and/or distance the interaction between the "pump" and "stimulus" radiations within the measurement arm of the measurement interferometer.