A new iterative matrix algorithm has been applied to improve the precision of temperature and force decoupling in multi-parameter FBG sensing. For the first time, this evaluation technique allows the integration of nonlinearities in the sensor’s temperature characteristic and the temperature dependence of the sensor’s force sensitivity. Applied to a sensor cable consisting of two FBGs in fibers with 80 µm and 125 µm cladding diameter installed in a 7 m-long coiled PEEK capillary, this technique significantly reduced the uncertainties in friction-compensated temperature measurements. In the presence of high friction-induced forces of up to 1.6 N the uncertainties in temperature evaluation were reduced from several degrees Celsius if using a standard linear matrix approach to less than 0.5°C if using the iterative matrix approach in an extended temperature range between −35°C and 125°C.

A 20 mm long 3D-FEM model of a Panda fiber was developed to improve the understanding of multi-parameter sensing in attached or embedded polarization maintaining (PM) fibers. Previously unknown but for FEM modeling critical material parameters of the fiber's stress applying parts (SAP) were determined by a combination of measurements and simulations. The methodology gave evidence of a doping concentration of about 24 mole-% B2O3 in the SAPs for the specific type of PM fiber used. The model was evaluated for axial strains up to 1600 μstrain, axial stress up to 170 MPa and in a temperature range from -40°C to 160°C. The simulated changes in the Bragg wavelengths of the slow and the fast axes and the change of their separation due to applied axial strain, stress and temperature variations were in good agreement with measured data. A reliable 3D-FEM model of an FBG in a Panda fiber is now available that can be integrated into lager FEM models of embedded or surface-glued sensors fibers of type Panda. The model can be used to predict the influences of any arbitrary mechanical loads on an FBG in a Panda-type fiber at different temperatures.

A novel technique for strain and temperature decoupling with surface-glued fiber Bragg gratings (FBGs) is presented and applied for strain-independent temperature measurements in a temperature range between −30 °C and 110 °C with uncertainties below 4 °C over the entire measurement range. The influence of temperature-dependent glue-induced transversal forces on the fiber sensor could be eliminated with a sensor element consisting of two FBGs in identical polarization-maintaining fibers that were spliced perpendicular to each other. After aligning and gluing the sensor element with its optical axes parallel and perpendicular to the specimen, the averaged Bragg wavelength shifts of both FBGs were proven to be independent of the glue’s influence and therefore independent of any change in the material characteristics of the glue, such as aging or creeping behavior. For the first time, this methodology enables temperature measurements with surface-attached bare FBGs independently of arbitrary longitudinal and glue-induced strains. This is of great value for all applications that rely on a fully glued sensor design, e.g., in applications with high electromagnetic fields, on rotating parts, or in vacuum for space applications.