engleski

Fiber-optic sensor cable for simultaneous distributed measurement of multiple physical quantities

Fiber optic sensor cable technology is a relatively new research area that combines a set of scientific and technical disciplines in order to meet distributed sensor systems needs and quality standards. This thesis discusses a particular fiber optic cable design with three tightly encapsulated fibers for multipurpose and multivalent sensing, presently commercialized for temperature, strain and acoustic measurements. Furthermore, it introduces a mechanism for birefringence change as a function of outer perturbations, such as pressure and cable bending, as a new capability and potential feature for future optical sensor products. The concept is based on using cable raw materials (optical fiber, stainless steel strips, matured manufacturing procedures) with standard geometries produced and commercialized in high volumes nowadays. It uses symmetrical elements only and it exploits the geometrical configuration of equilateral triangle with optical fibers in each vertex that are tight buffered within the stainless-steel tube. This results with unsymmetrical loading of the optical fibers when exposed to external hydrostatic pressure thus giving rise to the birefringence change in the optical fibers. In this way the hydrostatic pressure as external mechanical measurand is coupled with optical parameters inside the interior optical fibers hermetically closed inside the stainless-steel tube. The concept was first evaluated with the Finite Element Analysis (FEA) commercial software tool resulted in proving the concept and giving the insight into the magnitude of tube compression. The prototype of the cable 1.24mm in diameter was manufactured and tested up to 1200bar in a 24m long high-pressure chamber, especially designed for such purposes, enabling both mechanical and optical characterization of the sensor cable. The polarimetric method was chosen to prove the concept of changing birefringence properties of optical fiber. Although non-linear and irreversible, it demonstrated a strong change in Stokes parameters during both, pressure increase as well as pressure decrease cycles. Furthermore, the high-pressure facility is further used for evaluation of tube compression without optical fibers. The compression of the tube 2.1mm in diameter and steel wall thickness of 0.4mm, steel grade 316L was measured and thus exhibited linear characteristics for the pressure range from 0 to 1200bar. Due to off-the-center positions of optical fibers, the construction has the intrinsic capability to measure cable bending. The information on bending is always available since there is always at least one fiber in the compression zone and one in the extension zone. The sensor fibers were interrogated with the Brillouin Optical Time Domain Analysis (BOTDA) interrogation method. For testing purpose, a 20m long sample was produced and arranged in a coil of different diameters, and BOTDA measurements demonstrated shift in Brillouin peak frequency for all three observed fibers. In this way it was confirmed that optical fibers 0.455mm in diameter, placed off-the center inside the stainless-steel tube of 1.25mm in diameter, can be used for fiber optic distributed pressure and/or cable bending evaluation using stimulated Brillouin scattering technique.

optical fiber ; fiber optic measurements ; monitoring systems ; fiber in metal tube (FIMT) ; pressure sensors ; acoustic sensors ; strain sensors ; temperature sensors ; distributed optical fiber sensors ; Brillouin effect ; Brillouin Optical Time Domain Analysis (BOTDA)

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