[Amended] An apparatus for producing, through multiple frequency conversions, at least two optical radiations suitable to be used in a Brillouin analyser suitable for measuring the stimulated Brillouin frequency-shift in an optical fibre sensor, the apparatus comprising: at least one laser source of primary radiation with frequency f 0 and linewidth not greater than the Brillouin gain bandwidth, GBW B,BGM, in a first Brillouin gain medium, BGM (74, 105); and at least one first stimulated Brillouin frequency conversion system (72,102) comprising an optical fibre that comprises said first Brillouin Gain Medium BGM (74, 105) which is doped in Germanium and/or other substances and/or has a photo-acoustic coupling different from that of the optical fibre sensor so as to convert at least a fraction of the primary radiation into a first secondary radiation having a frequency shift of Δf 1 =Δf B,BGM equal to the Brillouin shift Δf B,BGM of the first gain medium so that |Δf B,SNS -Δf B,BGM |>2·GBW B,SNS, that is Δf 1 is different and far from the Brillouin shift Δf B,SNS of the optical fibre sensor at least twice the width of the Brillouin gain bandwidth GBW B,SNS of the optical fibre sensor; and at least one further frequency conversion system configured for modulating part of the primary radiation or, respectively of the first secondary radiation, so as to generate one or more modulation sidebands shifted of a frequency Δf 2 arbitrarily controllable such that |Δf 1 + Δf 2 | ≈ Δf B,SNS, wherein the first and the further frequency conversion systems are configured to generate radiations of frequency f 0 -Δf 1 and f 0 ±Δf 2 in case the first and the second frequency conversion systems are both directly fed with a fraction of the primary radiation of frequency f 0, or respectively f 0 and f 0 -Δf 1 ±Δf 2 in case the two frequency conversion systems are cascaded, such that, in any of the two above-mentioned cases, the first and the further frequency conversion systems generate a single pair of radiations, respectively f 0 - Δf 1 and f 0 + Δf 2 for the first case and f 0 and f 0 - Δf 1 - Δf 2 for the second case, having a relative frequency shift within the Brillouin gain bandwidth GBW B,SNS of the optical fibre sensor, so that each of the single pair of radiations is suitable for producing stimulated Brillouin scattering in the optical fibre sensor; the apparatus further comprising: - at least one optical heterodyne system configured for measuring Δf 1 and/or |Δf 1 ± Δf 2 | and for providing a feedback control on Δf 2 so as to stabilise the value of |Δf 1 +Δf 2 | or of |Δf 1 - Δf 2 | on the basis of said measured values of Δf 1 and/or |Δf 1 ± Δf 2 |. at least one laser source of primary radiation with frequency f 0 and linewidth not greater than the Brillouin gain bandwidth, GBW B,BGM, in a first Brillouin gain medium, BGM (74, 105); and at least one first stimulated Brillouin frequency conversion system (72,102) comprising an optical fibre that comprises said first Brillouin Gain Medium BGM (74, 105) which is doped in Germanium and/or other substances and/or has a photo-acoustic coupling different from that of the optical fibre sensor so as to convert at least a fraction of the primary radiation into a first secondary radiation having a frequency shift of Δf 1 =Δf B,BGM equal to the Brillouin shift Δf B,BGM of the first gain medium so that |Δf B,SNS -Δf B,BGM |>2·GBW B,SNS, that is Δf 1 is different and far from the Brillouin shift Δf B,SNS of the optical fibre sensor at least twice the width of the Brillouin gain bandwidth GBW B,SNS of the optical fibre sensor; and at least one further frequency conversion system configured for modulating part of the primary radiation or, respectively of the first secondary radiation, so as to generate one or more modulation sidebands shifted of a frequency Δf 2 arbitrarily controllable such that |Δf 1 + Δf 2 | ≈ Δf B,SNS, wherein the first and the further frequency conversion systems are configured to generate radiations of frequency f 0 -Δf 1 and f 0 ±Δf 2 in case the first and the second frequency conversion systems are both directly fed with a fraction of the primary radiation of frequency f 0, or respectively f 0 and f 0 -Δf 1 ±Δf 2 in case the two frequency conversion systems are cascaded, such that, in any of the two above-mentioned cases, the first and the further frequency conversion systems generate a single pair of radiations, respectively f 0 - Δf 1 and f 0 + Δf 2 for the first case and f 0 and f 0 - Δf 1 - Δf 2 for the second case, having a relative frequency shift within the Brillouin gain bandwidth GBW B,SNS of the optical fibre sensor, so that each of the single pair of radiations is suitable for producing stimulated Brillouin scattering in the optical fibre sensor; the apparatus further comprising: - at least one optical heterodyne system configured for measuring Δf 1 and/or |Δf 1 ± Δf 2 | and for providing a feedback control on Δf 2 so as to stabilise the value of |Δf 1 +Δf 2 | or of |Δf 1 - Δf 2 | on the basis of said measured values of Δf 1 and/or |Δf 1 ± Δf 2 |. - at least one optical heterodyne system configured for measuring Δf 1 and/or |Δf 1 ± Δf 2 | and for providing a feedback control on Δf 2 so as to stabilise the value of |Δf 1 +Δf 2 | or of |Δf 1 - Δf 2 | on the basis of said measured values of Δf 1 and/or |Δf 1 ± Δf 2 |.