A Brillouin scattering-based distributed optical fiber sensor can measure strain or temperature change along an optical fiber of tens of kilometers, and it is very effective for the integrity monitoring of large structures such as infrastructures, plants and conveyances. The most common Brillouin scattering-based optic fiber sensor is known as BOTDA (Brillouin Optical Time Domain Analysis). In this paper, characteristics of the BOTDA signal were analyzed, and spatial resolution enhancement techniques were developed and presented. Additionally, the temperature compensation techniques, prerequisite for the application of the BOTDA system to structural health monitoring, were demonstrated.
As the BGS (Brillouin gain spectrum) characteristics were observed with the variance of the width of the launched pulse light, the measured BGS's showed not only that the Brillouin frequency of the optical fiber was dependent on the width and power of the launched pulses, but also that it was very difficult to actualize the spatial resolution less than 1 m with a conventional pulsed pump and probe system used in BOTDA.
When the pulsed pump light was generated using an external intensity modulator, the intensity modulated pulse light had always a CW (continuous wave) component of a pulse base. The BGS distortion caused by the pulse base was investigated through theoretical simulations and experimental works. In the theoretical simulation, the theoretical Brillouin interaction model, which took the interaction between the pulse base and probe light into consideration, was proposed. In addition a few methods with no allowance of the BGS distortion caused by the pulse base effect were presented.
For the structural health monitoring with the BOTDA sensor system, an optical fiber was attached to the surface of a structure to measure the strain distribution of it. However, the Brillouin frequency shift caused by a temperature change of the structure could be misinterpreted as a strain change of the BOTDA sensor, and this was why the strain sensor should consider temperature compensation to measure the actual strain change of the structure. Two temperature compensation techniques were presented; one used a reference fiber for sensing the temperature, the other got the information on the changes of the Brillouin frequency and the Brillouin gain power for the temperature compensation. The application tests for the two techniques were conducted and the merits and demerits of them were presented.
Double-pulse technique and pulse base technique were proposed for the improvement of the spatial resolution of the Brillouin scattering-based sensor. In the BOTDA sensor system, the spatial resolution determined two significances; one was the accuracy in the measurement of the strain or temperature change, and the other was the ability to resolve the two adjacent events. The improvement of the former performance included the latter performance. To enhance the spatial resolution of the BOTDA sensor system, shorter pulses should be used, resulting in a reduced signal power, which caused decrease of the dynamic range. Furthermore, Brillouin frequency resolvable BGS could not be obtained with less than a 10 ns pulse (~ 1 m spatial resolution). The double-pulse technique could be used to enhance the latter performance of the two significances, and the pulse base technique, the former performance. Experimental results showed that the ability to resolve two adjacent events could be enhanced by about twice, by just introducing a double-pulsed pump light without severe decreases in the dynamic range. The pulse base technique, in which the pulse base was used to excite the medium for a long time, was proposed to actualize the higher spatial resolution than 1 m. The experimental works showed that the BGS, whose Brillouin frequency was resolvable, could be obtained with shorter pulses than 10 ns by the pulse base.
