Optoelectronic distributed measuring device based on brillouin scattering

A method for digitally processing a signal from an optoelectronic distributed measuring device based on Brillouin scattering comprising a continuous light source (1) emitting a continuous light signal at a first frequency ν0, a coupler (2) able to divide said continuous light signal into two identical signals distributed into two arms, the first arm comprising a device (30) for generating frequency shifted pulses comprising at least one acousto-optic modulator (3) able to transform said continuous signal into a pulsed signal, with a frequency νp, intended to be injected into an optical fiber (5) to be tested so that it emits in return a signal by spontaneous Brillouin backscattering at a frequency νF equal to νp−νBz, where νBz is the Brillouin frequency to be measured at every point z of said optical fiber (5), and the second arm forming a local oscillator (16) emitting another light signal intended to be mixed with said return signal emitted by Brillouin backscattering by said optical fiber (5) to be tested to allow the frequency of said return signal to be lowered, so that a detection module (9) is able to detect said Brillouin shift frequency νBz at every point z of said optical fiber and that a processing module (12) is able to link this Brillouin shift frequency νBz at every point z of said optical fiber to a temperature value and a strain value, wherein the local oscillator (16) comprises a reference optical fiber (18) having a Brillouin frequency identical or close to that of the optical fiber (5) to be tested, said reference optical fiber (18) emitting a signal by spontaneous Brillouin backscattering, in response to said continuous light signal emitted in said second arm by said light source (1), said Brillouin backscattering signal being emitted at a frequency νOL=ν0−νBRef, where νBRef is the Brillouin frequency of the reference fiber without strain and at a reference temperature;

• characterized in that the process comprises the following steps:
• a) digitizing a signal corresponding to the beat between a backscattered signal from an optical fiber (5) to be tested and a backscattered signal from a reference optical fiber (18), and detected by a photodetector (9),
• b) slicing said digitized signal into a plurality of slices (T1 . . . Ti . . . TN) by applying a sliding time window of the rectangular, or Hamming, or Hann, or Blackman-Harris window type, each slice having a width equal to the time width of a half-pulse of the pulsed signal injected into the optical fiber (5) to be tested, the width of each slice further being centered around a date t corresponding to a point of coordinate z of said optical fiber (5) to be tested,
• c) computing, by using a fast Fourrier transform FFT algorithm, the frequency spectrum of each slice (T1 . . . Ti . . . TN) of said digitized signal;
• d) repeating steps a), b) and c) and averaging the spectra obtained for each point z of said optical fiber to be tested;
• e) using the results obtained in the previous step, plotting a graph of the distributed frequency variation measured as a function of the backscattering round-trip time tz,
• f) applying, on the one hand, a temperature sensitivity coefficient and, on the other hand, a strain sensitivity coefficient, on said measured distributed frequency variation graph obtained, in order to obtain a result in terms of temperature distributed measurement or a result in terms of strain distributed measurement, respectively.