Scan imaging of optical fiber Brillouin scattering spectra by a fixed intermediate frequency conversion for distributed temperature sensing

Light propagating in optical fiber will generate Brillouin scattering spectra (BSS), and the center frequency shift of the BSS is linearly changing with the temperature fluctuation of the light scattering position, so by combination with time domain reflecting technology it can be used to measure and locate the temperature information along the optical fiber. In this paper a three dimensional BSS scan imaging method is proposed and experimentally demonstrated, which is based on digital down converter (DDC) module to shift the frequency points in BSS to a 63MHz intermediate frequency (IF) by frequency scanning. By this method the BSS is converted to two Brillouin peaks, and the spectrum shape of each is nearly the same with the original BSS. And it provides a much convenient choice for the extraction of the center frequency of BSS, because the center frequency of the original BSS is at the center between the two Brillouin peak points. Therefore, to seek the peak points and determine their frequencies from the three dimensional Brillouin scattering spectra, the center frequency of the original BSS can be obtained, which is time-saving for distributed temperature measurement compared with conventional data fitting scheme, such as Lorentzian fitting algorithm, Polynomial fitting algorithm, shape correlation operation and so on.