A Brillouin-scattering distributed optical fiber sensor can measure strain or temperature along an optical fiber of tens of kilometers, and thus it is very effective in the integrity monitoring of large structures such as infra-structures, plants or conveyances. The techniques of signal-processing and performance enhancement for a Brillouin-scattering distributed optical fiber sensor were developed and presented in this paper. The theoretical simulation and the experimental results of strain measurement could verify the techniques. Additionally, the TRHEFPI (transmission/reflection-type hybrid extrinsic Fabry-Perot interferometric) optical fiber sensor was developed and designed which was one of the point-sensors for the local monitoring. The TRHEFPI optical fiber sensor was very effective in the distinction of measurement directions, and it was also verified through strain measurement.
The theory of stimulated Brillouin-scattering was analyzed, and the techniques of signal-processing for the scattering signals were presented. The theory could consider the Brillouin frequency distribution within smaller range than the spatial resolution in contrast to the conventional analysis. The pulse-base level of pump light was nonzero in the BOTDA (Brillouin optical time-domain analysis) system, and it induced the distortion of Brillouin spectra. The compensation methods using difference and ratio for the distortion were presented. The detection method of the Brillouin frequency in the small scattering signals was also investigated.
The strain-measurement characteristics of a Brillouin-scattering distributed optical fiber sensor were simplified as averaging of distributed strain over the spatial resolution range. The compensation method for location error and strain error near the boundaries of measurement range was presented, and the deflection of an 8 m-bending beam was measured with strain measurement at continuous locations.
The spatial resolution of a Brillouin-scattering distributed optical fiber sensor is known to be limited to few meters due to linewidth broadening and power decreasing with small pulse width of the pump light. The novel signal-processing technique was developed to overcome the limitation of the spatial resolution, and was verified through the simulation and the strain-measurement experiments. The technique could generate a BOTDA signal with very small spatial resolution from two BOTDA signals with different spatial resolution. The spatial resolution of the processed signal was same as the spatial resolution difference of two BOTDA signals. The sensitivity of detecting the Brillouin frequency could be kept good because the linewidth broadening was prevented even with small spatial resolution below 1 m by the signal-processing technique. The signal-to-noise ratio was also improved through the technique, which was more effective in case of smaller spatial resolution. The improving rate to the signal-to-noise ratio before the signal-processing was 302% in case of 1 m-spatial resolution. The measurement of distributed strain in a bending beam with 1 m-spatial resolution was demonstrated through the signal-processing technique.
The TRHEFPI optical fiber sensor could distinguish measurement directions by the phase-shift signs in the linear combination signal of the transmission-type and reflection-type sensor signals. The sensor was designed to set the phase shifts to ±π/2. The triangular and sinusoidal strain in an aluminum tension specimen was measured with the attached TRHEFPI optical fiber sensor, and the results were shown to be successful.
